Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Egoriti, Luca
Doctor of Philosophy - PhD
University of British Columbia
This thesis reports on the development of techniques to better investigate elemental transport phenomena in accelerator-based targets for RIB production through the ISOL method. This development has been carried out at TRIUMF, Canada's particle accelerator centre and home to ISAC-TRIUMF, a leading RIB facility where constant development is key to enable cutting-edge experiments performed in user facilities where the radioactive beams are delivered. Elemental diffusion and effusion in ISOL targets are frequently the major bottleneck for delivering RIBs with desired properties; this thesis focuses on developing systems to improve our understanding of their basic release phenomena, as well as enhance target performance. A major component of this thesis comprises the conceptualization, design, testing and irradiation of a spallation-driven two-step target for isotope production. Unlike every target irradiated for the past 25 years at ISAC, where protons induce nuclear reactions by directly impinging on a target material, this prototype produces spallation neutrons which subsequently induce fission in an annular uranium carbide target, and the reaction products are then extracted as a RIB. Besides unlocking new science with radioisotope beams having previously unavailable characteristics, this methodology brings several advantages for studying transport mechanisms at high-temperatures and in mixed particle fields. Such new features and capabilities provide a new parameter space for better understanding transport phenomena in harsh environments. Additionally, this thesis reports on radiotracer experiments performed on table-top systems and using activated materials, to systematically study the high-temperature diffusion and effusion phenomena in exotic materials and complex geometries. First studies were conducted to characterize the release performance of ISOL target samples, so that the most suited material could be irradiated in the ISAC station. Moreover, the first experimental investigations of the effusion phenomenon in ISOL targets are reported, which provide a better understanding of its principles and offers further ground for access to fundamental transport properties in these materials. Such approaches build on previous established work and extend it further to shed more light through systematic investigations. In turn, this will enable new target development activities based on experimental data and dependable models for predicting isotope transport and release.
Transport Phenomena in Radioisotope Beam Production TargetsbyLuca EgoritiB.Sc. Politecnico di Milano, 2013M.Sc. Politecnico di Milano, 2016A THESIS SUBMITTED IN PARTIAL FULFILLMENTOF THE REQUIREMENTS FOR THE DEGREE OFDoctor of PhilosophyinTHE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES(Chemistry)The University of British Columbia(Vancouver)December 2022© Luca Egoriti, 2022The following individuals certify that they have read, and recommend to the Faculty of Graduateand Postdoctoral Studies for acceptance, the thesis entitled:Transport Phenomena in Radioisotope Beam Production Targetssubmitted by Luca Egoriti in partial fulfillment of the requirements for the degree of Doctor ofPhilosophy in Chemistry.Examining Committee:W. Andrew MacFarlane, Professor, Department of Chemistry, UBCCo-supervisorAlexander Gottberg, Professor, Department of Physics & Astronomy, UNIVERSITY OF VICTORIACo-supervisorDoug Bryman, Professor, Department of Physics & Astronomy, UBCUniversity ExaminerMark Thachuk, Professor, Department of Chemistry, UBCUniversity ExaminerPierre Delahaye, Research Scientist, GRAND ACCE´LE´RATEUR NATIONAL D’IONS LOURDSExternal examinerAdditional Supervisory Committee Members:Carl A. Michal, Professor, Department of Physics & Astronomy, UBCSupervisory Committee MemberGrenfell N. Patey, Professor, Department of Chemistry, UBCSupervisory Committee MemberTom Troczynski, Professor, Department of Materials Engineering, UBCSupervisory Committee MemberiiAbstractThis thesis reports on the development of techniques to better investigate elemental transport phe-nomena in accelerator-based targets for RIB production through the ISOL method. This developmenthas been carried out at TRIUMF, Canada’s particle accelerator centre and home to ISAC-TRIUMF,a leading RIB facility where constant development is key to enable cutting-edge experiments per-formed in user facilities where the radioactive beams are delivered. Elemental diffusion and effusionin ISOL targets are frequently the major bottleneck for delivering RIBs with desired properties; thisthesis focuses on developing systems to improve our understanding of their basic release phenom-ena, as well as enhance target performance.A major component of this thesis comprises the conceptualization, design, testing and irradi-ation of a spallation-driven two-step target for isotope production. Unlike every target irradiatedfor the past 25 years at ISAC, where protons induce nuclear reactions by directly impinging on atarget material, this prototype produces spallation neutrons which subsequently induce fission inan annular uranium carbide target, and the reaction products are then extracted as a RIB. Besidesunlocking new science with radioisotope beams having previously unavailable characteristics, thismethodology brings several advantages for studying transport mechanisms at high-temperatures andin mixed particle fields. Such new features and capabilities provide a new parameter space for betterunderstanding transport phenomena in harsh environments.Additionally, this thesis reports on radiotracer experiments performed on table-top systems andusing activated materials, to systematically study the high-temperature diffusion and effusion phe-nomena in exotic materials and complex geometries. First studies were conducted to characterizethe release performance of ISOL target samples, so that the most suited material could be irradiatedin the ISAC station. Moreover, the first experimental investigations of the effusion phenomenon inISOL targets are reported, which provide a better understanding of its principles and offers furtherground for access to fundamental transport properties in these materials. Such approaches buildon previous established work and extend it further to shed more light through systematic investi-gations. In turn, this will enable new target development activities based on experimental data anddependable models for predicting isotope transport and release.iiiLay SummaryParticle accelerators have contributed to advances in fundamental and applied sciences, enablingdiscoveries that revolutionized our understanding of the laws of physics and improved our day-to-day livelihoods. At TRIUMF, Canada’s particle accelerator centre, scientists impinge high-energyparticles onto targets for the production of radioisotopes, which are used to further advance disci-plines such as nuclear physics and materials science. The most interesting radioisotopes for thesestudies are however produced with low rates and have average half-lives of few milliseconds, mak-ing it very challenging to produce a significant number of them to enable their study.This work focuses on the science and technology of radioisotope production targets, to improvethe quality and quantity of radioisotopes that are produced in these core components. The resultspresented have enabled scientific experiments that had previously failed, and improved key metricsthat now allow for quicker and more effective research activities.ivPrefaceThis section aims at giving an overview of the breakdown of the collaborative contributions pre-sented in this thesis, divided by chapter.The development of a spallation-driven two-stage target presented in chapter 3 started as a col-laboration between TRIUMF and ISOLDE a few years before I arrived at TRIUMF. Scientist P.Bricault and my supervisor A. Gottberg at TRIUMF developed an initial concept that is presentedin this thesis and took inspiration from previous literature. My contribution started by taking own-ership of the project at this stage, developed its concept further and went through its detail designin consultation with A. Gottberg and P. Bricault at TRIUMF, J.P. Ramos and T. Stora at CERN,D. Houngbo, L. Popescu and M. Diercks at SCK CEN. All particle simulation, thermo-mechanicalsimulations and bench top experimental tests were performed by me, as well as the detail shapeoptimization of both the target and converter bodies, in consultation with A. Gottberg. The researchproposal submitted to the Experimental Evaluation Committee (EEC) was written by me and A.Gottberg, and I analyzed all the presented on-line data from the beam times. The data from theexperimental campaigns where gathered by two teams of ∼a dozen people each (the Target and IonSources (TIS) R&D group and TRIUMF’s Ion Trap for Atomic and Nuclear science (TITAN) group),who would take yield measurements with the ISAC yield station and mass measurement using theTITAN Multi Reflection Time-of-Flight (MR-TOF) system.The implementation of radiotracer experiments presented in chapter 3 conceived by A. Gottbergand myself, and all the simulation presented were performed by me, in consultation with A. Gottberg.The Isotope Production Facility (IPF) facility has been used for irradiating material samples, and L.Moskven designed the target containers that enabled this irradiation in IPF. CHI stand has been de-signed and built as a split project by myself and M.Sc. student M. Au as described in the text, bothof us contributing for∼50% of the work and in consultation with A. Gottberg. The full release char-acterization of the graphite samples was carried out in by me and M. Cervantes, both contributingfor ∼50% of the work including experimental work and data analysis, and in consultation with A.Gottberg. The release characterization of the SiC samples and the release tests through macroscopicstructures presented in section 4.2 was performed by me in consultation with A. Gottberg.vTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiLay Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ixList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiList of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviiiAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Description of the ISOL method . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.1 Primary beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.2 Target material considerations . . . . . . . . . . . . . . . . . . . . . . . . 71.1.3 Ion sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.4 Mass separation and transport . . . . . . . . . . . . . . . . . . . . . . . . 121.1.5 User experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2 RIB production facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.2.1 ISAC at TRIUMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.2.2 ISOLDE at CERN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181.2.3 ARIEL at TRIUMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191.2.4 Catcher facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.2.5 In-flight facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211.3 Transport phenomena relevant for ISOL targets . . . . . . . . . . . . . . . . . . . 221.3.1 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221.3.2 Effusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281.4 Exploring transport phenomena in ISOL targets . . . . . . . . . . . . . . . . . . . 30vi1.4.1 Overview of past offline release activities . . . . . . . . . . . . . . . . . . 321.4.2 Mathematical methods to model release in ISOL targets . . . . . . . . . . 381.5 Thesis formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.1 Yield station and its detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.1.1 Gamma spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.1.2 Beta scintillator detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.2 Thermal equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482.2.1 CHI stand furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482.2.2 ARIEL Thermal Test Stand (ATTS) . . . . . . . . . . . . . . . . . . . . . 492.2.3 Temperature measurement methods . . . . . . . . . . . . . . . . . . . . . 502.3 Sample activation in the IPF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532.4 Simulation tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.4.1 The FLUKA Monte Carlo software . . . . . . . . . . . . . . . . . . . . . 572.4.2 The ANSYS Workbench FEA software . . . . . . . . . . . . . . . . . . . 582.5 TITAN MR-ToF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603 Development of a spallation-driven two-step radioisotope production target . . . . . 613.1 Introduction and motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.1.1 Historical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.1.2 Motivation for developing a proton-to-neutron converter at TRIUMF . . . . 643.2 Goals and concept development for a new proton-to-neutron converter . . . . . . . 683.2.1 Goals and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.2.2 Concept design iterations . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.3 Proton-to-neutron converter detailed design . . . . . . . . . . . . . . . . . . . . . 783.3.1 Target design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783.3.2 Converter engineering design . . . . . . . . . . . . . . . . . . . . . . . . 803.4 Online experiment: irradiation of target-converter units . . . . . . . . . . . . . . . 823.4.1 Preparation and planning . . . . . . . . . . . . . . . . . . . . . . . . . . . 833.4.2 Yield results from the online converter irradiations . . . . . . . . . . . . . 863.4.3 MR-ToF results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 983.4.4 Post irradiation examination . . . . . . . . . . . . . . . . . . . . . . . . . 983.5 Discussion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024 Radiotracer release studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044.1 Release tests from pellets and discs . . . . . . . . . . . . . . . . . . . . . . . . . . 1044.1.1 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044.1.2 Thermal considerations in CHI stand . . . . . . . . . . . . . . . . . . . . 1064.1.3 Single pellet experiments and results . . . . . . . . . . . . . . . . . . . . . 109vii4.2 Release tests through macroscopic structures . . . . . . . . . . . . . . . . . . . . . 1134.2.1 Methodology and rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.2.2 Temperature benchmarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 1154.2.3 Experimental parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 1164.2.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1194.3 Discussion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129A Additional proton-to-neutron information . . . . . . . . . . . . . . . . . . . . . . . . 151A.1 Target engineering design considerations . . . . . . . . . . . . . . . . . . . . . . . 151A.2 Converter engineering design simulations . . . . . . . . . . . . . . . . . . . . . . 156A.3 Uranium carbide production for the proton-to-neutron target . . . . . . . . . . . . 165A.4 Additional online plot results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170A.5 MR-ToF results table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173B Additional release data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174B.1 Target material description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174B.2 HPGe uncertainty determination . . . . . . . . . . . . . . . . . . . . . . . . . . . 175B.3 Single pellet release: additional data . . . . . . . . . . . . . . . . . . . . . . . . . 177B.4 Additional macroscopic effusion results . . . . . . . . . . . . . . . . . . . . . . . 181B.5 Technical information of CHI stand . . . . . . . . . . . . . . . . . . . . . . . . . 184viiiList of TablesTable 1.1 Work function for some most used ion source materials . . . . . . . . . . . . . 11Table 1.2 List of RIB facilities around the world and their principal parameters. Theirgeographical location is shown in figure 1.5. . . . . . . . . . . . . . . . . . . . 14Table 1.3 Lattice and physical properties of three refractory metals [1]. . . . . . . . . . . 33Table 1.4 Summary of the major particle release model and their characteristics. A = An-alytical models. MC = Monte Carlo models. (*) = Model developed for pressedpowder pellets and relies on knowledge of ideal pore parameters. . . . . . . . . 41Table 3.1 ISOL target material used in ISAC, and their relevant physical properties forradioisotope production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Table 3.2 Comparison of activities at 100 days after End of Beam (EOB) from two of themost produced alpha emitters. The numbers refer to a simulations with 20 µAon the ISAC target, and 100 µA on the proton-to-neutron target, after the sameduration of proton irradiation of ∼500 h. . . . . . . . . . . . . . . . . . . . . . 67Table 3.3 Figures of merit of the first converter concept and conventional ISAC-UCx target. 70Table 3.4 Beam window material and location calculated from the center of the proton-to-neutron target container . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Table 3.5 Fission rates induced by neutrons (N-fiss) or protons (P-fiss) in the proton-to-neutron target material in the geometries outlined in figure 3.5. “Conv.” standsfor converter, “coll.” stands for collimator. . . . . . . . . . . . . . . . . . . . . 72Table 3.6 Rb isotopes and their half lives. . . . . . . . . . . . . . . . . . . . . . . . . . . 83Table 3.7 Cs isotopes and their half lives . . . . . . . . . . . . . . . . . . . . . . . . . . 83Table 3.8 Neutron rich improvement factor (NRIF) for rubidium isotopes (left) and cesiumisotopes (right), to compare the performance of the proton-to-neutron target ver-sus the selected ISAC targets UCx#1, UCx#2, UCx#3,UCx#7 and their FLUKAexpected values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Table 3.9 Average atomic percentages of elements in the phases labelled in figure 3.32. . . 99Table 4.1 Average properties and characteristics of the graphite samples used for releasestudies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110ixTable 4.2 Main isotopes of interest in the effusion setups. The “FLUKA” column is thesimulated activity, while D7/5 and R7/5 are the activity fractions in pack 7 vspack 5, for the D-shape and round geometries respectively. Column “Avg.” isthe average of the activity fractions R7/5 / D7/5. . . . . . . . . . . . . . . . . . 123Table A.1 Summary of the major parameters recorded during the proton-to-neutron targetoffline thermal testing. Inner TC corresponds to the reading from the thermocou-ple whose tip is positioned at the centre of the empty target volume while outerTC is the more external one. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154Table A.2 Von Mises maximum stresses (σV M [MPa]), maximum elastic (εel [MPa]) andplastic (εpl [MPa]) strains in the tungsten converter and copper alloy clampscomputed in the two situations mentioned, pre-tension only (PT) and pre-tensionplus thermal (PT+T). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159Table A.3 Intensity ratio for some isobaric species from the proton-to-neutron convertertarget, for both experimental data and from FLUKA simulations . . . . . . . . . 173Table B.1 Counts difference resulting from a radial misplacement of the sample by 5 mm,considered the maximum possible radial displacement when aligning the sampleon the detector jig “by hand”. . . . . . . . . . . . . . . . . . . . . . . . . . . . 176Table B.2 Irradiation parameters and properties for single pellet release samples. . . . . . 177Table B.3 Summary of all measurements made on three different graphite-based materials,and calculated release fraction (RF) with uncertainty. All samples have been keptat temperature for about one minute. . . . . . . . . . . . . . . . . . . . . . . . 178Table B.4 Summary of all measurements made on two SiC samples, and calculated releasefraction (RF) with uncertainty. The counting time was always 900 s for all samples.179Table B.5 Yield and activity of isotopes produced in a single SiC pellet irradiated in IPFfor 5 minutes with 5 µA proton beam current at 450 MeV . . . . . . . . . . . . 180Table B.6 Yield and activity of 7Be in a single 0.5 mm thick graphite pellet irradiated inIPF for 5 minutes with 5 µA proton beam current at 450 MeV . . . . . . . . . . 180Table B.7 Yield and activity of isotopes produced in a single tantalum foil irradiated in IPFfor 5 minutes with 5 µA proton beam current at 450 MeV . . . . . . . . . . . . 181xList of FiguresFigure 1.1 Nuclide chart, colour coded according to the major decay channel of the isotope.Adapted from [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 1.2 Schematic representation of the ISOL production method. . . . . . . . . . . . 4Figure 1.3 Production plots in units of isotope produced per second computed by FLUKAwhen a 1 mm-thick UCx ISOL pellet is bombarded with a 100 µA proton beamof different energies: 70 MeV, 100 MeV, 500 MeV and 1400 MeV from top leftto bottom right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Figure 1.4 Resonance ionization pathways used by the ISOLDE RILIS, taken from [3] . . 12Figure 1.5 RIB facilities around the world, some are still in construction, refer to table 1.2for details. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 1.6 Schematic top view of the accelerator facilities at TRIUMF. . . . . . . . . . . 16Figure 1.7 Core components of the ISAC target stations. . . . . . . . . . . . . . . . . . . 17Figure 1.8 Images of the ISOLDE target vessel, adapted from [4] and [5]. . . . . . . . . . 19Figure 1.9 AETE production map at 30 MeV and 100 kW . . . . . . . . . . . . . . . . . 20Figure 1.10 ISAC target transport sequence schematically presented. Red lines representdiffusion and green lines effusion both inside and between the porous pellets. . 22Figure 1.11 Schematic representation of radiation damage in materials. . . . . . . . . . . . 26Figure 1.12 Residual doses from ISAC targets measured at different decay times after endof bombardment (EoB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 2.1 Schematics of the ISAC yield station, adapted from Peter Kunz’s slides. . . . . 44Figure 2.2 Gamma spectrum recorded from TRIUMF’s 152Eu calibration source. Thesharp peaks represent specific gamma rays corresponding to transitions betweennuclear states. In addition, there is a broad background. . . . . . . . . . . . . . 45Figure 2.3 These plots only pertain the efficiency of the HPGe spectrometer in detectingγ-ray photons. The plot on the right includes in red the fitted efficiency curveεHPGe shown in equation 2.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Figure 2.4 Experimental data and fitting curve for isotope 145Cs (T1/2 = 0.587 s), where thecontributions of its daughter 145Ba (T1/2 = 4.31 s) and 145La (T1/2 = 24.8 s) areshown as dark red, bright red and blue lines respectively. . . . . . . . . . . . . 48xiFigure 2.6 Rendering of the ATTS stand with an ARIEL target installed in it. . . . . . . . 50Figure 2.7 Top left: cut away view of an IPF target. Top right: picture of the hot cellduring the retrieval of a target. The target is handled through a single stickmanipulator, while in the background one can notice the drive chain assembly.Bottom: layout of the Isotope Irradiation Facility . . . . . . . . . . . . . . . . 53Figure 2.8 Top view of the IPF station FLUKA geometry . . . . . . . . . . . . . . . . . . 56Figure 2.9 External dose-rate equivalent from a target loaded with 10 tantalum foils ofcombined thickness 0.254 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . 56Figure 3.1 ISOLDE target converter unit. The right figure shows the detrimental effect of1.4 GeV protons on a test tantalum converter rod used for target UC2-C-183 [6]. 63Figure 3.2 Dedicated prototype of proton-to-neutron assembly tested at ISOLDE in 2014 [7] 64Figure 3.3 Bottom: Theoretical comparisons between the in-target production rates of tinand cesium isotopes from a conventional ISAC target and from a spallation-driven two-step target. Top: ratio between tin and cesium rates in the two re-spective configurations. The advantage of two-step targets resides in the spreadin this ratio plot between blue stars and red dots, accounting for up to a factor∼50 in purity gain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Figure 3.4 Initial proton-to-neutron converter geometry implemented in FLUKA, with OuterDiameter (OD)=43 mm and Inner Diameter (ID)=18mm. . . . . . . . . . . . . 69Figure 3.5 Proton and neutron fluences when a tungsten converter is placed inside or down-stream of the target container . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Figure 3.6 Proton flux distribution due to scattering on the upstream windows described intable 3.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Figure 3.7 Proton and neutron spectra impinging on the target material for two possibleconverter locations: inside (W-in) or downstream (W-out) of the target container 72Figure 3.8 Fission rates induced by both protons and neutrons, and power deposition in thetungsten converter are plotted against several parametric lengths of a tungstencylindrical converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74Figure 3.9 Left: overlap between the gaussian proton beam (in red) and the inner fringesof the UCx target (in light green). Right: number of fission events per secondinduced by neutrons and protons for a UCx target with OD=33 mm and IDranging from 10 to 20 mm, and the final ID was chosen as 15 mm as in thedashed line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75Figure 3.10 Proton (left) and neutron (centre) fluxes shown across the P2N target middleplane. On the right is the target and converter geometry modelled in FLUKA. . 75Figure 3.11 Simulated in-target production rates in the P2N target material at 100 µA ofincoming proton beam intensity. . . . . . . . . . . . . . . . . . . . . . . . . . 76xiiFigure 3.12 Simulated in-target production rates of all isotopes in the proton-to-neutron con-verter irradiated with 100 Ip = µA (top left) and in a conventional ISAC-UCxirradiated with Ip = 20µA (top right) targets. Below is the ratio of the two,“converter/conventional”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77Figure 3.13 Cut-away view of the proton-to-neutron target and converter assembly (left),and of a conventional ISAC target (right). . . . . . . . . . . . . . . . . . . . . 78Figure 3.14 Left: Cut view of the finalized proton-to-neutron target assembly with graphitecartridge and UCx. Right: Simulation results of the minimum and maximumtemperatures in the annular UCx target material domain, with resistive heatingonly or with full beam delivered to the assembly. . . . . . . . . . . . . . . . . 79Figure 3.15 Cut view of the proton-to-neutron converter assembly, axially clamped by fourM5 screws and water cooled all around the copper-alloy brackets. . . . . . . . 80Figure 3.16 Nuclide chart snapshot in the vicinity of isotopes 134Cs and 134Sn, downloadedfrom [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Figure 3.17 Release efficiency from the ISOLDE proton-to-neutron converter prototype ir-radiated in 2014 [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84Figure 3.18 Proton beam size measurements taken with fast wire scanners after centeringon the 26Na yield minimum, see text for explanation. . . . . . . . . . . . . . . 86Figure 3.19 Bottom: simulated in-target production and measured yields of Rb isotopesfrom this converter and from conventional ISAC-UCx targets [8]. Top: simu-lated Rb in-target production ratios and measured Rb yield ratios between iso-topes from this converter and from conventional ISAC-UCx targets. NRIF is theNeutron Rich Improvement Factor. . . . . . . . . . . . . . . . . . . . . . . . . 88Figure 3.20 Bottom: simulated in-target production and measured yields of Cs isotopesfrom this converter and from conventional ISAC-UCx targets [8]. Top: sim-ulated Cs in-target production ratios and measured Cs yield ratios between iso-topes from this converter and from conventional ISAC-UCx targets. . . . . . . 89Figure 3.21 Efficiency of Rb isotopes from the proton-to-neutron target at a heater current of1150 A, compared to the ISAC yields collected from targets UCx#1 and UCx#3.More comprehensive plots are in appendix A, in figures A.18 and A.20. . . . . 90Figure 3.22 Efficiency of Cs isotopes from the proton-to-neutron target at a heater current of1150 A, compared to the ISAC yields collected from targets UCx#2 and UCx#7.More comprehensive plots are in appendix A, in figures A.19 and A.21. . . . . 91Figure 3.23 Proton beam size measurement taken towards the end of the P2N beam time(June 23rd). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Figure 3.24 Comparison and ratios between Rb yields taken with a proton beam FWHM of7 mm and 4 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92Figure 3.25 Comparison and ratios between Cs yields taken with a proton beam FWHM of7 mm and 4 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92xiiiFigure 3.26 Lower plot: Zn in-target production calculated by FLUKA and measured onlineduring this experiment and compared to all other measurements from ISAC-UCx targets in the yield database [8]. Upper plot: ratio between the simulatedin-target production and measured yields values, in green and black respectively. 93Figure 3.27 Efficiency of Zn isotopes from the proton-to-neutron target at different targetheater (P2NHT) currents, compared to the yields collected from two UO2 ISACtargets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Figure 3.28 Two data sets measured in consecutive days, showing the effect of proton beamintensity on the yields of four Cs isotopes. The top two neutron-rich isotopes(140Cs, 141Cs) show linear trends with increasing Ip, while the bottom twoneutron-deficient (124Cs, 126Cs) deviate from linearity likely because of posi-tion instabilities of the proton beam. . . . . . . . . . . . . . . . . . . . . . . . 95Figure 3.29 Bottom: Sn in-target production calculated by FLUKA and measured online atP2NHT=1150 A during this experiment and compared to measurements fromISAC target UCx#33 [8]. Top: ratio between the simulated in-target productionsand measured ratio between the yield values, in green and black respectively.Both plots are preliminary, since thorough data analysis is still needed. . . . . . 96Figure 3.30 Efficiency of Sn isotopes from the proton-to-neutron target at nominal heatercurrent (1150 A), compared to the ISAC yields collected from targets UCx#33.A standard laser ionization scheme was used to ionize these atoms. This plot ispreliminary, since thorough data analysis is still needed. . . . . . . . . . . . . 97Figure 3.31 Post-irradiation examinations (PIE) of core components of the target assembliesirradiated over the three beam times. . . . . . . . . . . . . . . . . . . . . . . . 100Figure 3.32 EDS mapping of two uranium carbide flakes extracted from the online materialbefore irradiation (left) and after irradiation (right). The two grey-scale picturesare magnifications of the oxidized UCx matrix (green) from their respectiveimage underneath. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Figure 4.1 Schematics of the overall procedure used to perform release studies, where thegreen disc represents the sample under investigation. . . . . . . . . . . . . . . 105Figure 4.2 The solid black line represents the exponential trend of a general activity of anisotope in the samples undergoing the release procedure. The sudden drop inthe middle represents the partial escape of such radioisotope during the heatingphase, and the dashed line indicates what their activity would be if no heatingoccurred. Areas S1 and S2 represent the activity integral before and after heat-ing respectively, while S3 is the integral activity which left the sample duringheating and is therefore not detected. . . . . . . . . . . . . . . . . . . . . . . . 106Figure 4.3 Single-pellet release assembly: 3D CAD model (left) and picture of the mountedassembly without copper heat shields for clarity (right) . . . . . . . . . . . . . 107xivFigure 4.4 Simulated temperature distributions in the tantalum heater (left), copper sup-ports and shields (right). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Figure 4.5 CHI stand temperature calibration using reference wires made out of vanadium(Tm=1910 ◦C) and titanium (Tm=1668 ◦C). Left: assembly being heated. Cen-ter: graphite foil held by its tantalum retainer foils after the calibration, withmelted wires as evidence. Right: example of a calibration curve where theblack dots are optically measured heater temperatures. The highlighted areasshow approximately where it was noticed that the titanium wire (blue shade)and vanadium wire (green shade) melted. . . . . . . . . . . . . . . . . . . . . 108Figure 4.6 Images of these three materials taken with a Scanning Electron Microscope byMarla Cervantes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Figure 4.7 Release fractions measured at two temperatures for the three graphite materialsunder investigation. Each point is a measurement. . . . . . . . . . . . . . . . . 110Figure 4.8 SEM pictures of two different SiC materials Left: conventional ISAC SiC com-posite before heating. Centre: conventional ISAC SiC composite after heating,with some neck between particles in evidence (both adapted from [9]). Right:the new nanometric electrospun material (adapted from [10]) where SiC grainsare suspended on a carbon fibre matrix. The material tested is the central one. . 111Figure 4.9 Release fractions of 7Be and 24Na measured at two temperatures and differentheating times for the two equivalent SiC samples. . . . . . . . . . . . . . . . . 112Figure 4.10 Assembly pictures for macroscopic release tests. The assembly is shown mountedin (a) and being resistively heated in (b),(c) demonstrating how the non-activeD-shape tantalum foils are put together in packs of 20 foils kept in place bytantalum wire, before sliding them into the resistor (d). (e) is a magnificationof (b) highlighting the foils through the side hole of the heater, where the blackcircumference is the pyrometer scope aim area. f) is a schematic cut-view rep-resentation of the assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . 114Figure 4.11 Simulated temperature distribution achieved with 450 A of Ohmic current, us-ing an emissivity value of 0.2 across all tantalum surfaces and a heater thicknessof 0.2 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Figure 4.12 Experimental calibration with the pyrometer focused on the foils, through theion source hole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116Figure 4.13 D-shape pack on the HPGe for γ-ray spectroscopy . . . . . . . . . . . . . . . 116Figure 4.14 Recorded variables during the heating phase of the tube for macroscopic releasetests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117Figure 4.15 Left: γ-ray spectrum recorded from a 20-foil pack. Centre and right: examplesof peaks and their fit performed with the InterSpec software [11]. . . . . . . . 119xvFigure 4.16 Activity distributions estimated from different gamma lines in the D-shape (left)and Round (right) foil assemblies for 165Tm and 170Lu. These two isotopes areshown as examples of the typical contamination trends seen in the foil stacksduring the analysis, and additional isotopes can be found in section B.4. . . . . 120Figure 4.17 Activity distributions estimated from different gamma lines in the D-shape (left)and Round (right) foil assemblies for 160Ho and 173Hf. . . . . . . . . . . . . . 121Figure 4.18 Comparison between the average activities from D-shaped vs round pellets . . 122Figure A.1 All colour scales are in [◦C] . . . . . . . . . . . . . . . . . . . . . . . . . . . 152Figure A.2 Built prototype of the model shown in figure A.1f:before heating (left), afterheating (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Figure A.3 Proton-to-neutron test target assembly mounted in the ATTS vacuum furnace. . 154Figure A.4 Temperature profiles during thermal tests number 3, 4, 5. . . . . . . . . . . . . 156Figure A.5 Voltage at the power supply terminal and temperature behaviour during thermaltest #1 when the current was stable at 1070 A . . . . . . . . . . . . . . . . . . 156Figure A.6 Temperature distributions in [◦ C] across the proton-to-neutron converter assembly159Figure A.7 Creep rate vs applied stress for a variety of copper alloys [12] . . . . . . . . . 160Figure A.8 Simulation result showing Von Mises stresses [Pa] in the tungsten converter(left) and copper brackets (right) . . . . . . . . . . . . . . . . . . . . . . . . . 160Figure A.9 Simulation result showing elastic strain in [m/m] in the tungsten converter (left)and copper brackets (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161Figure A.10 Simulation result showing plastic strain in [m/m] in the tungsten converter (left)and copper brackets (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161Figure A.11 Converter TIG test: Model of the 3D printed version of the modified converterfor TIG tests (top left), and pictures of the assembly during and after testing. . 162Figure A.12 Temperature results from the offline converter tests against torque applied to thefour bolts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Figure A.13 Cut-away view of the heat shield with the proton-to-neutron target and converterassemblies in their nominal position. . . . . . . . . . . . . . . . . . . . . . . . 163Figure A.14 P2N material production steps and tools . . . . . . . . . . . . . . . . . . . . . 166Figure A.15 Vacuum furnace parameters during the proton-to-neutron target material reduc-tion. The first plot (pressure) gives hints for when the organic binders evap-orated from the UO2, and when the reduction took place producing copiousamounts of carbon monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . 167Figure A.16 P2N target body - assembly ITD0685 . . . . . . . . . . . . . . . . . . . . . . 168Figure A.17 P2N target assembly with electrical terminals - assembly ITD0685 . . . . . . . 169Figure A.18 Efficiency of Rb isotopes from the P2N target at different target heater (P2NHT)currents, compared to the the two ISAC targets UCx#1 and UCx#3. . . . . . . 170Figure A.19 Efficiency of Cs isotopes from the P2N target at different target heater (P2NHT)currents, compared to the the two ISAC targets UCx#2 and UCx#7. . . . . . . . 170xviFigure A.20 Efficiency of Rb isotopes from the P2N target at different target heater (P2NHT)currents, compared to the maximum and average ISAC yields collected fromseveral targets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171Figure A.21 Efficiency of Cs isotopes from the P2N target at different target heater (P2NHT)currents, compared to the maximum and average ISAC yields collected fromseveral targets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171Figure A.22 Lower plot: Rb in-target production calculated by FLUKA and measured onlineduring this experiment and compared to all measured ISAC UCx targets [8].Upper plot: ratio between the simulated in-target productions for both targets,and the measured ratio between their yield values, in green and black respectively.172Figure A.23 Lower plot: Cs in-target production calculated by FLUKA and measured onlineduring this experiment and compared to all measured ISAC UCx targets [8].Upper plot: ratio between the simulated in-target productions for both targets,and the measured ratio between their yield values, in green and black respectively.172Figure B.1 Statistical uncertainty estimated by the spectrum analyzer software (Uncer-tainty) and from the standard deviation (STD) from 6 equal measurements. . . 175Figure B.2 Difference of counts in various peaks when the calibration source is movedaxially with respect to the benchmark position of 5 cm away from the detector.The label also shows the number of counts recorded in the benchmark peak at5 cm distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176Figure B.3 Distribution of most other isotopes measured during the effusion experimentswhich are not shown in the main text for the sake of brevity, in complement tofigures 4.16 and 4.17. The D-shape samples are on the left and round on theright. This procedure was possible only for this group of 8 isotopes due to theirhigh residual activity and recognizable gamma lines. . . . . . . . . . . . . . . 182Figure B.4 Distribution of 166Tm isotopes measured during the effusion experiments whichare not shown in the main text for the sake of brevity, in complement to figures4.16 and 4.17. The D-shape samples are on the left and round on the right. . . . 183Figure B.5 Comparison between the average activities from D-shaped vs round pellets forsome isotopes which were not captured in the main text for the sake of brevity. 183Figure B.6 CHI stand PID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184xviiList of AbbreviationsADC Analog to Digital ConverterAETE ARIEL Electron Target EastALARA As Low As Reasonably AchievableAMU atomic mass unitAPTW ARIEL Proton Target WestARIEL Advanced Rare IsotopE LaboratoryATG Advanced Technology GroupATTS ARIEL Thermal Test StandBL1A Beam Line 1ABL2A Beam Line 2Aβ -NMR β -detected Nuclear Magnetic ResonanceCANREB CANadian Rare-isotope facility with Electron-Beam ion sourceCFD Computational Fluid DynamicsCHI Chamber for Heating InvestigationsCNSC Canadian Nuclear Safety CommissionCNT Carbon NanotubeCPET Cooler Penning TrapCPU Central Processing Unit.CSV comma separated valueCW Continuous WaveDAQ data acquisitionDC Direct CurrentDPA Displacement Per AtomDRAGON Detector of Recoils And Gammas Of Nuclear reactionsDTL Drift Tube LinacxviiiEBIT Electron Beam Ion TrapEDS Energy-dispersive X-ray spectroscopyEEC Experimental Evaluation CommitteeEMMA ElectroMagnetic Mass AnalyserEOB End of BeamEVAP1 Evaporator oneFEA Finite Element AnalysisFEBIAD Forced Electron Beam Induced Arc DischargeFLUKA FLUktuierende KAskadeFWHM Full Width Half-MaximumGAPS Graduate Student and Postdoc SocietyGEANT4 GEometry ANd TrackingGRIFFIN Gamma-Ray Infrastructure For Fundamental Investigations of NucleiHPGE High Purity GermaniumHRIBF Holifield Radioactive Ion Beam FacilityHV High VoltageID Inner DiameterIGISOL Ion Guide Isotope Separation On-LineINFN National Institution of Nuclear PhysicsIPF Isotope Production FacilityIPNO Institut de Physique Nucleaire OrsayISAC Isotope Separator and AcceleratorISOL Isotope Separation On-LineISOLDE Isotope Separator On Line DEviceISS ISOLDE Solenoidal SpectrometerITE ISAC Target station EastITW ISAC Target station WestLINAC-2 Linear accelerator 2LISOL Leuven Isotope Separator On-LineLN2 Liquid NitrogenMPET Measurement Penning TrapxixMR-TOF Multi Reflection Time-of-FlightNEW Nuclear Energy WorkerNRIF Neutron Rich Improvement FactorOD Outer DiameterP2NHT Proton-to-Neutron Target HeaterPKA Primary Knock-on AtomPIE Post Irradiation ExaminationPSB Proton Synchrotron BoosterRED Radiation Enhanced DiffusionRFQ Radio Frequency QuadrupoleRIB Radioactive Ion BeamRIBS Radioactive Ion BeamsRIBO Radioactive Ion Beam OptimizerRILIS Resonance Ionization Laser Ion SourcesSAP-EEC Subatomic Physics Experiments Evaluation CommitteeSAR Safety Analysis ReviewSCK CEN Research Centre for the Applications of Nuclear EnergySEM Scanning Electron MicroscopeSIS Surface Ion SourceTIG Tungsten Inert GasTIGRESS TRIUMF-ISAC Gamma Ray Suppressed SpectrometerTIS Target and Ion SourcesTITAN TRIUMF’s Ion Trap for Atomic and Nuclear scienceTNDP Thermal Neutron Depth ProfilingTRIUMF TRI University Meson FacilityUV Ultra VioletxxAcknowledgmentsThis PhD has been a long learning adventure across many different levels, and several people haveplayed a huge part in making it a great and fun experience.My research supervisor and mentor Alex Gottberg has been a constant source of insightfulinput and support, not only guiding me through unfamiliar terrain but also teaching me how toguide my own way. Countless invaluable discussions have been a great source of inspiration andmotivation, making this PhD an enjoyable learning experience. Thank you for teaching me how tothink critically on every level, inside and outside of the box, and giving me the opportunity to exploreand learn from experience. I am also grateful to my academic supervisor Andrew MacFarlane forguiding me through the academic milestones, and especially for supporting me during the finalwriting sprint.Tom Day Goodacre, Carla Babcock, Jens Lassen, Peter Kunz and Anders Mjos have providedmuch support to this PhD with their experience and knowledge of this ISOL field, the ISAC facilityand all the equipment needed to make TRIUMF a great radioisotope laboratory. They have seenmy lowest lows when nothing seemed to be working, and my highest highs when we achieved greatresults together. Thank you for your time, support and knowledge you bring to the table.A big thank you goes to Mia Au for teaming up and building together the CHI stand, andto Marla Cervantes for participating in the release experiments at CHI stand over long days - teamwork is dream work! To them and our fellow students Fernando Maldonado and Ferran Boix-Pamiesgoes my gratitude for sticking together and sharing discussions, ideas, jokes, volleyball games andlong beam time shifts. Nolan Esplen has been a fantastic companion throughout the FLASH project,a big thank you goes to him for guiding me through the medical physics world and for working hardside by side to successfully irradiate live mice - a very much unexpected outcome of these past fewyears.Thank you to our colleagues overseas at ISOLDE-CERN and SCK CEN. Joa˜o Pedro Ramosand Donald Houngbo have been fundamental for the development of the proton-to-neutron target, Iappreciate all the discussions and ideas that led to successful irradiations. My gratitude goes to allother also involved in this collaboration: Lucia Popescu, Sebastian Rothe, Thierry Stora and MarcDierckx.I need to also thank a big number of people for the continuous support provided through theyears in several areas that contributed to my PhD work, such as: Ricardo Dos Santos Augusto forhis deep FLUKA knowledge and guidance, Aaron Schmidt for his infinite resources in handling,xxiassembling and testing targets, Darwin Ortiz Rosales and John Wong for their help in the coldlab, Mike Wicken, Bob Welbourn and Tim Goodsell for making my mad mechanical designs intophysical objects in the machine shop, Arthur Leung for his power supply expertise and support, JohnLangrish and Quinn Temmel for their help from the design office, Brandon Humphries and ThomasPlanche for their support with any e-linac business, Louis Moskven and the ATG group for theirtireless help during the IPF irradiations, and Chad Fisher, Matt Gareau, David Wang, Frank Song,Maico Dalla Valle and Dan McDonald for their technical support in the ISAC facility. A big thankyou goes to all my fellow trailer mates including Kevin, Brendan, Navid, Norman, Sundeep, Mike,Bill, Allon, Roger and Michele, with whom I shared lots of fun moments during lunch, volleyballor table tennis games. My thanks especially go to Sam Varah and Josh Smith for being excellentoffice mates and friends. Moreover, I thank all the members of the TITAN group for their supportduring the online beam times which helped us show the potential of this work. Finally, a big thankyou goes to the members of the Graduate Student and Postdoc Society (GAPS) committee for theirenergy, ideas and good will to constantly improve the experience of students at TRIUMF, I am verygrateful for meeting you and for sharing important moments through our quest.I am also grateful for having shared incredible moments outside of TRIUMF, especially in theBC mountains, with several good friends: Nick, Vincent, Arthur, Mirtha, Jack, Amy, Nathan, Dani,Yukiya for the fantastic ski days, Boyan, Marybel, Jacob, Remi, Vincent, Mary, Tiffy and Jose forthe summer hikes and beach volleyball games. A special thank you goes to Freek, for being presentin most moments and adventures, and for constantly pushing my limits - best wishes for a speedyrecovery. Since my initial days in Canada I have always felt in a safe space thanks to my homiesSimran, Raj, Rohan, Majd, Gowtham, Karan, Zubby, Manu and Ryan, sharing countless memorablemoments in the comfort of our millionaire houses.Un grazie di cuore va ai miei genitori Beppe e Paola, e a mia sorella Roberta per avermi suppor-tato e sopportato da un altro continente che ho sempre chiamato casa, per avermi dato la possibilita´di esplorare le mie curiosita´ senza mai metterle in discussione. Agli instancabili amici di ON-OFFDami, Ross, Eli, Ghido, Chiara e Simo per la loro positivita´, e a tutti gli altri amici da una vita, Ale,Caglia, Pito, Maggio, Marcio, Punki, Giombe, Pane, Gio´ and Michi per essere presenti a distanza.And lastly, a huge acknowledgement goes to Jenny, caring partner in life, crime and adventure,for keeping me grounded and giving perspective to all this.TRIUMF receives federal funding via a contribution agreement with the National ResearchCouncil of Canada. ARIEL is funded by the Canada Foundation for Innovation (CFI), the Provincesof AB, BC, MA, ON, QC, and TRIUMF. I acknowledge additional support from the NSERC CRE-ATE IsoSiM fellowship and the New Frontiers in Research Fund (NFRF).xxiiChapter 1IntroductionThis thesis focuses on the study of targets which are employed to produce radioactive ion beamsthrough the Isotope Separation On-Line (ISOL) method. This chapter provides an initial overview ofthis radioisotope production technique and describes some of the main research facilities employingthis methodology. One key aspect affecting the efficiency of such targets involves the transportphenomena that these radioisotopes undergo after being produced by nuclear reactions inside thetarget. Special focus will be directed towards this topic through a general overview of transportphenomena and by a more comprehensive review of past activities that have been carried out in thisfield.The atomist theory proposed by the Greek philosopher Democritus had stood as a successfulspeculation since the 5th century BC, postulating that matter is composed of indivisible atoms.Although this theory had been attacked by other philosophical streams, it stood the test of time forcenturies and its building blocks were ordered according to their properties and atomic number Z byDmitri Mendeleev in his table of elements, which has seen constant additions until the current times.By the turn of the 20th century, scientists such as Antoine-Henri Becquerel, Ernest Rutherford, Pierreand Marie Curie discovered and described the phenomenon of radioactivity in some of its forms,which leads to the transformation of one atom into another [13]. During this first year of workon radioactivity, it was measured experimentally that the decay rate of a radioactive substance wasfollowing an exponential law and suggested the statistical nature of the process for which the decayrate of a given substance is proportional to the number of atoms in it N, according to the equationdNdt=−λN (1.1)where t represents time, and λ is the decay constant. The exponential solution of this equation isthenN(t) = N(0)exp−λ t (1.2)where N(0) is the initial number of radioactive atoms in that substance at time t=0. It is usuallyconvenient to define the half-life t1/2 of an atom as the time such that N(t1/2)=N(0)/2, and the1radioactive activity as the number of disintegrations every secondA(t) = λN(t). (1.3)If the radioactive isotope (parent) decays into another radioactive isotope (daughter), then the num-ber of atoms of the n-th radioactive generation can be written using the Bateman equationsNn(t) = N1(0) ·(n−1∏i=1λi)·n∑i=1e−λit∏nj=1, j 6=i(λ j−λi)(1.4)The discovery of the atomic nucleus in 1911 by Rutherford led to the realization that atoms areindeed divisible, and do not represent the fundamental building block of nature, unlike their namesuggests. Further investigation into atomic nuclei led to the discovery of subatomic particles suchas protons and neutrons in 1919 and 1932 respectively, giving de facto rise to the new disciplineof nuclear physics. By this time, Mendeleev’s table was not enough to capture the properties ofall atoms - or better, nuclides - but needed a second dimension to it; in fact, atoms with sameatomic number Z presented equivalent chemical properties, yet they could have a different numberof neutrons N and would be defined as isotopes of the same element. The Segre´ chart (named afterscientist Emilio Segre´) was then proposed and developed throughout the late 1930s to present acoherent and structured version of all nuclides, focusing on their nuclear - rather than chemical -properties as shown in a contemporary version in figure 1.1. In this plot, each square representsa nuclide, and their colour is associated to the most prevalent decay mode or characterizing itsstability. The three main decay types, mediated by the weak nuclear force, can be written asAZXN −−→ AZ+1XN−1+ e−+ ν¯ (negative β−decay, pink) (1.5)AZXN −−→ AZ−1XN+1+ e++ν (positive β+decay, light-blue) (1.6)AZXN+ e− −−→ AZ−1XN+1+ν (orbital electron capture, light-blue) (1.7)where X is a general element of atomic number Z, with N number of neutrons and mass A=Z+N,e− and e+ are the symbols for electrons and positrons respectively, while ν and ν¯ are (electron)neutrinos and antineutrinos [13]. Many heavy nuclides can undergo alpha decay, in yellow in figure1.1, which results in the expulsion from the mother nucleus of an α particle following the reactionmechanismAZXN −→A−4Z−2 XN−2+α. (1.8)This process is due to the Coulomb repulsion forces present in nuclei with high atomic number,which counteracts the attracting potential from the strong nuclear force. In nuclei where these twoopposite contributions reach a similar intensity, the most likely outcome is that an α particle, avery stable nuclear configuration with high binding energy, undergoes a tunnelling process and isemitted with a certain defined energy. The initial research on radioactivity from Marie and Pierre2Figure 1.1: Nuclide chart, colour coded according to the major decay channel of the isotope.Adapted from [2]Curie involved the study of uranium and its major decay products, most of which emit α particles.Given the long half-life of 238U , it is still possible to find in nature all daughters of the long decaychain, including 226Ra and 218Po which were discovered by the French couple and yielded them aNobel Prize [14]. All of the work on radioactivity until the early 1930s was based on the discoveryand understanding of naturally occurring radioactive sources, which were also used to producenew stable nuclei [15]. Irene Curie and her husband Frederic Joliot became the first to artificiallytransform an element into another radioactive nuclide. They placed a polonium sample close toan aluminum foil, bombarding the latter with the α particles coming from the decaying poloniumatoms. After removing the polonium sample from the foil vicinity, they noticed that the aluminumfoil kept being radioactive for an additional time: they had produced and observed the first human-made radioactive material [16]. The field of particle accelerator science and technology was alsoborn in the early 1930s, and led to the creation of the first cyclotrons and linear accelerators, openinga highway to discover many new nuclides, including the most recent oganesson (Og, Z=118) [17]only a few years ago.1.1 Description of the ISOL methodSince these early ages of accelerator science, it was common practice to irradiate a sample and eitheranalyze the reaction products directly inside the target, or by applying radiochemical techniques toseparate the wanted element from the cocktail of nuclides present. This last methodology has beenapplied if the reaction products had a half-life long enough to allow for the chemical procedures,3Figure 1.2: Schematic representation of the ISOL production method.but some of the interesting short-lived nuclides would disappear before any such analysis couldtake place. Otto Kofoed-Hansen and Karl-Ove Nielsen were the first ones that, in the early 1950s,could successfully separate reaction products online, meaning that the separation was happeningwhile irradiating the target material. At the Niels Bohr Institute in Copenhagen, they accelerateddeuterons onto a beryllium target to produce a neutron field, part of which would interact with amixture of 10 kg of UO2 mixed with baking powder [18]. The interactions between neutrons anduranium nuclei would trigger fission reactions, and some of the gaseous reaction products would becarried out of the target by the CO2, H2O and NH3 from the decomposition of the baking powder.After being ionized and accelerated to 50 kV by an electrostatic potential, the extracted nuclideswere mass separated and sent to a station for dedicated recoil measurements of Kr and Xe nuclideswhich were never possible before [19].The great results obtained by the Danish setup and the promising prospects paved the wayfor building the Isotope Separator On Line DEvice (ISOLDE) facility at CERN with the scope ofdeveloping this technique further to improve the production of radioactive ion beams [20]. At thepresent day the ISOLDE facility at CERN and the Isotope Separator and Accelerator (ISAC) facilityat TRIUMF are the reference facilities in the field of Isotope Separation On-Line (ISOL). In an ISOLsystem, an incoming particle beam with intensity I and energy E interacts with a target material andtriggers nuclear reactions with its nuclei depending on the reaction cross section at that energyσi(E). The reaction products usually retain a high kinetic energy from the recoil, and are eitherstopped in a catcher material which can be separate from the target (such as a gas catcher) or arethermalized in the target material itself. In this latter case, the target is then kept at high temperatureto promote the product’s migration by atomic diffusion in the solid material and effusion throughthe microscopic and macroscopic porosity, a process which is described in depth throughout thisthesis. These isotopes are usually migrating outside of the target container through a transfer lineand directed to an ion source where one or more electrons are stripped from them, as shown in4a schematic view in figure 1.2. Since the target and neighbouring components are connected tohigh voltage (HV), the ions are extracted into a beam line section provided with the necessaryelectromagnetic optics to shape the effluent ions into a Radioactive Ion Beam (RIB). Since high-energy particle beams usually produce a number of different reactions when colliding with the targetnuclei, the initial beam will be composed of a diverse cocktail of stable and radioactive nuclides.The RIB then passes through a mass separator, a dipole magnet equipped with slits that select thewanted range of A/q, where q is the ionic charge of the nuclide, and delivers it through a beam lineto a downstream end user which would use it for a variety of experiments in many fields of studyincluding, but not limited to, nuclear physics, material science and biology [21].For now, it is important to note that once the nuclear reaction occurs in the target material, all thesubsequent steps are characterized by an efficiency: εR for release, including diffusion and effusionsteps, εI for ionization, εMS for mass separation and εT for transport. All these play a role in definingthe final RIB intensity (or RIB current) at the experiment IRIB according to equation [22]IRIB =∫lσi(E) ·Ntarg · I · εdxwith ε = εR · εI · εMS · εT ,(1.9)where Ntarg is the target material atomic density and l is its length. Each of the parameters justintroduced have an impact on the RIB current delivered to users, and will be analyzed one by onein the following pages. The target-related release efficiency is especially crucial for the delivery ofRIBs, since it can reduce the yields to users by several orders of magnitude. This thesis will takea deep dive especially into the release efficiency εR with analysis and proposals for improving theRIB yields in specific cases.1.1.1 Primary beamThe characteristics of the primary beam that impinges on the target nucleus with intensity I andenergy E are of fundamental importance to trigger the production of the desired isotope. The twomajor operational ISOL facilities in the world, ISOLDE-CERN and ISAC-TRIUMF, utilize protonsmostly because of their ability to produce a wide range of radionuclides through nuclear reactions.The type of primary particle and its energy affects the choice of target material, and it is in generalfair to assume that high energy protons (with E>100 MeV, approximately) can cover a wide rangeof nuclides when coupled to the proper target material.Protons and heavier atomic nuclei accelerated at high energies interact with matter through theelectromagnetic and strong forces, both of which are very relevant when considering their effectson an ISOL target. When protons are directed towards a target, their electromagnetic interactionwith the sea of electrons leads to energy transfer between the two which reduces the proton’s initialenergy by a quantity −(dE/dx) called stopping power, describing the rate of energy loss dE over5the spacial dimension x through the Bethe-Block formula [23]−(dEdx)= NZ4pie4z2m0v2·[ln(2m0v2I)− ln(1− v2c2)− v2c2](1.10)where N and Z are the number density and atomic numbers of the material atoms, m0 is the electronrest mass, e is the electronic charge, I is the average excitation and ionization potential of thetarget, v is the charged particle velocity and c is the speed of light. This energy transfer is themain mechanism responsible for the beam power deposition into the material, of core interest forthe future chapters. This continuous slowing process induced by electrons can be accompaniedby additional sporadic elastic events where heavy charged particles feel the Coulomb repulsionof a nucleus in the material, a process referred to as Coulomb or Rutherford scattering where thetrajectory of the incoming proton is deviated by an angle Ω with a cross section σS(E,Ω) accordingto equation [13]dσS(E,Ω)dΩ=(zZe24piε0)2( 14E)2 1sin4(Ω/2)(1.11)where ze is the projectile charge, Ze is the target atom charge, ε is the material dielettric permittivityand E is the kinetic energy of the proton. During such interactions, the projectile can be deflectedfrom its initial direction hence transferring a significant amount of energy to the lattice atom whichcan be ejected from its site creating a Primary Knock-on Atom (PKA). If the charged projectile hassufficient energy, the Coulomb potential barrier of the target nucleus can be overcome and nuclearreactions can take place with a certain cross section σi(E), of which two general reactions are ofspecific interest for this thesis: spallation and fission.The spallation reaction involves the dislodgement of a few nucleons from the initial target nu-cleus, resulting in the production of a lighter isotope. This reaction channel becomes available onalmost any nucleus, provided the incoming particle energy is high enough to trigger it, and can beinduced on uranium nuclei by protons above ∼100 MeV. When the de Broglie wavelength of theincoming particle approaches the size of individual nucleons (protons and neutrons) inside the targetnucleus, a first intra-nuclear cascade happens in a time frame on the order of 10−22 s. In this phase,individual nucleons interact with their neighbours and some of them might be ejected most likelyin the forward direction, following mechanisms expected from reaction kinetics. Once the excita-tion energy spreads over to the whole nucleus, some nucleons might be ejected isotropically in aprocess called evaporation with a Maxwellian energy spectrum and average kinetic energy around1-2 MeV [24]. Depending on the types of fragments ejected in the processes described above, theproduct(s) of the spallation reaction can be distributed in several areas of the nuclide chart, andfigure 1.3 hints at the general trends seen at some relevant proton energies in the ISOL field. Whenthe target nucleus has atomic number approximately higher than 73, fission can take place whenthe beam energy is higher than ∼1-70 MeV, depending on the target nucleus. The reaction channel238U(p, f iss) becomes in fact available with cross section of approximately 1 barn (1 · 10−24 cm2)[25], independent of the beam energy. The nucleus as a whole has enough excitation energy for6Figure 1.3: Production plots in units of isotope produced per second computed by FLUKAwhen a 1 mm-thick UCx ISOL pellet is bombarded with a 100 µA proton beam of differ-ent energies: 70 MeV, 100 MeV, 500 MeV and 1400 MeV from top left to bottom right.two or more sub-units, with masses usually around A∼95 and A∼135 AMU, to tunnel out of eachother’s Coulombian barrier.Another consideration for the choice of primary particle concerns the kind of particles that ourcurrent technologies can effectively and efficiently produce as high energy beams. Neutral particlessuch as neutrons and photons can also be employed (and in fact, they are) to trigger the desirednuclear reactions in target materials. However, they usually need to be produced by acceleratingfirst a charged particle, i.e. a proton, deuteron or an electron, and impinging it onto a convertermaterial that generates the uncharged particle. Unless there is a substantial gain in some propertyof the specific reaction that needs to be triggered, this conversion process represents in most cases afurther loss in efficiency as stated in equation 1.9 and is usually not employed. Notable exceptionsare the ARIEL-AETE target and the proton-to-neutron converter target, presented in section 1.2 andchapter 3 respectively.1.1.2 Target material considerationsThis section outlines some important principles to understand the choices and optimizations that areconsidered when specific materials are chosen to be used in ISOL targets.7Target nucleus and material choice A core component of the whole ISOL system is the targetmaterial, where the primary beam interacts with the nuclei inside it to create, among many others,the nuclide required by the end user. The choice of target nucleus aims at maximizing the reac-tion product, first by examining its expected production rate using cross section tables as a guidingtool (i.e. [25]) and then refining these estimations for a more comprehensive understanding usingnumerical simulation tools such as the Monte Carlo code FLUktuierende KAskade (FLUKA) [26].A second aspect for consideration is the physical state of the target material, since its operationalparameters are often quite extreme: ISOL targets are usually connected to at least the downstreamRIB line, which operates at vacuum pressures around 10−6 Torr, and are usually heated to temper-atures up to 2000 ◦C to speed up isotope transport phenomena. The vapour pressure and propertiesof the target material must be such that it shall not significantly evaporate, melt nor go through anyphysical or phase transformation, effectively limiting the candidates to highly refractory elementsor compounds. Throughout the past few decades of ISOL method, a broad variety of material classeshave been tested in online systems as summarized in [22], and few notable examples can be lookedmore in detail.Uranium carbides of general nomenclature UCx have been extensively used in several ISOL fa-cilities [27, 28, 7] and will play an important role in future ones [29, 30] because of their versatilityin producing a wide variety of products thanks to the high atomic number of uranium. It is in facta common choice to operate carbide targets such as SiC, ZrC and ThC2 over their oxide or metal-lic counterparts due to their refractory nature that usually yields lower vapour pressure and highermelting point, which overall enable its improved performance thanks to its increased operationaltemperature. Solid metal targets made of thin, refractory material foils such as tantalum or niobiumare also widely used for their broad range of spallation products they produce with high-energyprotons such as rare-earth elements, and evaporation-specific nuclei like 8Li. In addition, the com-bination of high melting point and thermal conductivity are is a very favourable property in thesehigh-power facilities where the primary beam carries enough power to heat up the target material;in these situations, refractory metals could be the only choice if solid targets are required.Molten targets of several pure metallic elements or eutectic combinations have been explored inthe past, and their advantage relies on the creation of convective flows that effectively could reducethe diffusion lengths by orders of magnitude. Moreover, the liquid state facilitates the heat removalprocess if that is a main challenge. Molten targets are used only in few facilities because of theirpeculiar handling and operational procedures which are not easily compatible with most existingISOL target infrastructures, which are mostly tailored for solid targets.Material effects on its release properties The macro and micro-structure of the target material isalso an important parameter to take into consideration when the nuclear reaction products need tobe extracted from the target container. It plays a central role in the amount of isotope current IRIBextracted from the target due to the release efficiency εR. The overall volume in which the targetmaterial is located and where the nuclear reactions happen, also represents the space that atoms8can migrate through. A large volume filled with a great amount of material would yield higherin-target production rates, however it might translate into a significant reduction of RIB current dueto release losses [31]. Moreover, refractory target materials in ISOL containers have usually somespecific configurations to promote the fast release of radionuclides, with the intent of minimizingthe diffusion lengths from the nuclear reaction location to the closest surface. This usually leadsto the use of very thin metal foils (between 2 and 25 µm thick) which are dimpled to effectivelycreate macroscopic void structures in between foils and arranged in the target container in specificconfigurations [32, 33]. Such geometrical arrangements and stacking solutions are not fully appliedto ceramic target materials, where a maximization of their release properties is usually achievedby shaping their microstructure during the early stages of synthesis, by means of grinding initialmaterial and then pressing it into pellets or casting on backing foils [34, 30]. The characteristic pathlength that the nucleus needs to diffuse through in order to be extracted is in this case representedby the material particle radii, while their effusion geometry is represented by intricate and intercon-nected pores which extend all the way to the outer surface of each pellet. All mentioned geometricaldimensions play a role in determining the release efficiency εR, a crucial variable in equation 1.9which has the highest impact on the overall RIB current extracted from targets.Temperature considerations The promotion of transport phenomena inside the target happens bothfrom a material engineering perspective as mentioned in the previous paragraph, but also by care-fully adjusting the target material temperature. Most ISOL facilities that irradiate solid targets tendto insert material pellets, foils or structures into a cylindrical tantalum container, ∼2 cm in diameterand 20 cm long, which is coaxial with the driver beam. Direct current is applied to the target con-tainer ends, resulting in resistive heating of the container which warms up the inner target materialto a supposedly homogeneous temperature, since the heat source in this scenario is only comingfrom the outer target edge. The nominal operational temperature is set mostly according to thephysico-chemical properties of the target material to avoid a change in physical state and phase, atoperating vacuum pressures around ∼ 10−6 Torr. Additional considerations are included in deter-mining the nominal target temperature, such as the potential sintering effects that might increasethe grain size of the microstructure or seal the porosity between metal foils with detrimental effectsduring the effusion phase. This known sintering effect is carefully considered while designing newISOL target materials [35, 36, 37, 30], and some approaches to limit it in carbide targets, such asthe addition of over-stoichiometric carbon amounts and the use of a nano-structured material, havebeen proven successful in online tests [22]. The temperature profile in the target container howevercan drastically change if the driver beam deposits significant power into the target assembly. Inthese situations the temperature gradients need to be minimized across the material volume, so thatthe maximum temperature will not induce any sintering or melting, and the minimum temperatureis high enough to still effectively promote isotope release.The considerations brought up in this section are at the core of this thesis work, which focuseson transport phenomena in ISOL targets. A deeper dive into the theoretical aspect of transport9properties is given later in sections 1.3, while this next section focuses on what happens to thecocktail of nuclides after it exits the target container.1.1.3 Ion sourcesThe objective of an ISOL facility is creating radioactive nuclides which need to be transportedthrough beam lines until their final user experiment, which might be up to a few hundred metersaway from their production location. The most efficient way to carry out this operation is to turnthe radioactive atoms into ions, so that they can be quickly and efficiently transported to the userexperiment by accelerating them with an electrostatic potential, and precisely delivering them to theend location using electromagnetic steerers and focusing elements. An ion source is a componentthat generates either positive or negative ions starting from the equivalent atomic state of the sameelement, by removing from or adding electrons to the outer shell(s). In most ISOL applicationsonly positive and singly-charged ions are requested, and the most used techniques to provide theseare surface, laser or electron-impact ionization, all relying on the excitation of one electron from anouter shell which detaches from the atom if its energy overcomes the ionization potential Ei.ISOL target containers are usually sealed on every side except for a hole∼3 mm in diameter lo-cated at the centre of their cylindrical surface that allows the isotopes to migrate into the ion source,a component dedicated to produce mostly singly-ionized species, ready for further acceleration.In its simplest configuration, this component is consisting of a tantalum tube resistively heated athigh temperature to promote the ionization through contact between the atom and the hot surface.The fundamentals of this Surface Ion Source (SIS) process are usually described in terms of theSaha-Langmuir equationnisn0s=gig0exp[φ −EikBT](1.12)which predicts the relative concentrations of ions nis and neutral atoms n0s when they are in ther-modynamic equilibrium with a surface at temperature T . φ is the work function of the material, kBis the Boltzmann constant while gi and g0 are the statistical weights of the ionic and atomic groundstates [3, 38]. Atoms that are ionized have to escape the ionizer region, usually a few cm long, andthe resulting overall ionization efficiency can be expressed asεI =βsγκ1−βs(1− γκ) (1.13)taken from [39], where κ is the mean number of collisions of atoms with the walls, γ is the probabil-ity of an ionized particle to leave the cavity as an ion, and βs is the local surface ionization efficiencydefined asβs =nisnis+n0s. (1.14)This hot surface ion source mechanism is applied in most ISOL facilities for elements whose ion-ization potential Ei is comparable or lower than the work function of the surface material - some ex-amples are given in 1.1 - so that the exponential term in equation 1.12 becomes significant. Lithium,10rubidium, cesium and the lanthanides are examples of elements that ISOL facilities usually ionizeusing a hot surface ion source alone.Material Work function [eV] [40, 41]Tantalum 4.25Tungsten 4.55Rhenium 4.96Table 1.1: Work function for some most used ion source materialsElements with ionization potential higher than∼6-7 eV are rarely ionized with significant yieldsusing the method described above, and Resonance Ionization Laser Ion Sources (RILIS) are em-ployed to resonantly excite their electrons, exploiting specific features of their atomic energy levels.The laser wavelengths are precisely tuned so that the photon energies match successive electrontransition energies of an ionization scheme as shown in figure 1.4, which can lead to the ionizationof atoms whose potential is as high as 10 eV. Two or three laser frequencies are needed to make surethat subsequent transitions happen in series, and fundamental differences between schemes exist de-pending on the nature of the final atomic level that is populated, which can be an auto-ionizing state,a Rydberg state or belong to the energetic continuum above the ionization potential. Auto-ionizingstates are bound atomic states that lie slightly above the ionization energy and, if an electron isexcited precisely into these, it would consequently redistribute its extra energy resulting in its emis-sion from the atom. Similarly, Rydberg states can also be accessed by electrons if lasers are finelytuned to access these, and their final energy state lies slightly under the ionization threshold; if smallextra energy is delivered to these electrons due to e.g. thermal excitation, this process leads also toionization. Both kind of schemes need fine tuning of the laser parameters, but lead to much greaterionization efficiencies if compared to the third process of ionization to a non-resonant, “continuum”value above the threshold. The great advantage of implementing a resonant laser ionization schemeinvolves the highly selective nature of the process, which can in some cases select even betweenisotopes of the same element. However, both methods presented above are not applied to elementswhose ionization potential is higher than 10 eV. In these scenarios, electron impact ionization ionsources have been employed since the 1970s to ionize elements such as noble gasses.The Forced Electron Beam Induced Arc Discharge (FEBIAD) is a kind of ion source that workson the electron impact principle [42], and replaces the SIS tube with a chamber (anode) equippedwith a hot cathode positioned towards the target and a series of extraction electrodes further intothe RIB line direction [43]. When the cathode is heated to ∼2000◦C it emits electrons that areaccelerated into the anode volume and impact with the neutral radionuclides escaping the target,ionizing them. The extraction electrodes attract the newly created ions out of the anode volume,which are further accelerated into the RIB line by electrostatic fields.11Figure 1.4: Resonance ionization pathways used by the ISOLDE RILIS, taken from [3]1.1.4 Mass separation and transportSingly charged positive ions coming out of the ion source are extracted by a series of electrodes justoutside of the ion source, which are kept at a potential of∼2 kV with respect to the target assembly,which is itself held at∼30-60 kV with respect to ground (see figure 1.2). These potential differencesare defined by design so that a RIB from an ISOL target can be delivered with some ten thousandkeV to an experimental station at ground potential, so that the user experiments do not need to holdany voltage by design. Electromagnetic lenses are used to steer and focus the beam towards a massseparator dipole magnet which is usually capable of selecting a mass unit but without the resolutionrequired for separating isobaric components - isotopes with the same atomic mass number. Aftermass separation, the RIB can either be directed to low energy experiments, or be sent for furtheracceleration - usually carried out by a linear accelerator - reaching energies up to tens of MeV pernucleon. It is noticed that a charge breeder may be needed for preparing ions in the right chargestate for re-acceleration.1.1.5 User experimentsThe purpose for going through all the steps described above is delivering radioactive ion beams toexperimental user stations installed at endstation of facilities, all competing with each other to getthe most beam time and perform experiments that are possible only by using these unstable nucleithat are found only in a handful of places around the globe. Nuclear physics experiments that aimat measuring specific properties of the nuclides produced, such as their mass and decay modes, arecommonly found in the lower-energy RIB sections, immediately after the mass separator.One notable example relevant for this thesis is the TITAN experiment at TRIUMF, equipped12with a Multi Reflection Time-of-Flight (MR-TOF) spectrometer that can resolve and separate evenisobaric contaminants from the beam of interest, with a resolution power of m/σm ∼ 105. Fromthis MR-ToF, radioactive isotopes can be sent to a Penning trap able to measure the mass of singleions with a whopping precision of m/σm ∼ 108 thanks to their ability to detect cyclotron resonantfrequencies of ions, which is used to calculate its mass [44]. TITAN and other mass measuringfacilities such as ISOLTRAP at CERN fit perfectly in the low-energy section of the RIB beamlines, since they are mostly interested in properties of the nuclear masses when at rest, as well asthe Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei (GRIFFIN) [45]. More-over, several other experiments profit from accelerated radioactive projectiles to study the reactionsbetween unstable RIB nuclei with stable isotopes. This is the case for experiments such as theDetector of Recoils And Gammas Of Nuclear reactions (DRAGON) [46] and the ElectroMagneticMass Analyser (EMMA) [47] at TRIUMF or the ISOLDE Solenoidal Spectrometer (ISS) at ISOLDE[48, 49], which aim at measuring reaction rates happening in stars and supernovae events, whosefuel-burning cycles are still not fully understood given the many uncertainties that still exist in ourknowledge of certain reaction rates. In the accelerated beam areas are also found spectrometer ar-rays such as the TRIUMF-ISAC Gamma Ray Suppressed Spectrometer (TIGRESS) [50] at TRIUMFand MINIBALL at ISOLDE [51], aiming at measuring the γ-ray emitted by RIB nuclei and theirreaction products with stable targets to infer their properties, such as shape, nuclear energy levels,moments and half-life.Nuclear physics is surely the main focus of the experimental programs that ISOL facilities serve,nonetheless the sensitivity levels that can be achieved by nuclear techniques have advanced manyother fields of science in the last few decades. One striking example are the β -detected NuclearMagnetic Resonance (β -NMR) facilities at both TRIUMF and ISOLDE, which are able to improvethe sensitivity of the standard NMR technique by ten orders of magnitude, and provide a depth-resolved probe of magnetism and local electronic environment on length scales from 2-200 nm byimplanting polarized 8Li RIBs in the material of interest [52, 53]. At TRIUMF alone, this facilityhas often been used to investigate ion diffusion for energy-storage applications, superconductivityin metals (i.e. niobium) relevant for particle accelerator science, solvent molecular dynamics andcoordination chemistry.The numerous experiments delivering cutting-edge science in several fields using radioactiveion beams are all sources of motivation for continuous effort by the ISOL target community to keepenabling these experiments. Since experimental stations are usually overbooked, extra pressureis put to the field of ISOL targetry to improving the quality and quantity of the RIBs delivered,enabling more effective experimental campaigns.13Label Facility name Type Driver beam Operational1a ISAC (TRIUMF) ISOL P+ 500 MeV - 100 µA Y [33]1b ARIEL (TRIUMF) ISOL P+ & e− 35 MeV - 10 mA N [33]2 ISOLDE (CERN) ISOL P+ 1.4 GeV - 2 µA Y [54]3 ALTO (IPNO) ISOL e− 50 MeV - 10 µA Y [55]4a SPIRAL (GANIL) ISOL Ions 95 AMeV Y [56]4b SPIRAL2 (GANIL) ISOL D+ 40 MeV - 5 mA N [57]5 LISOL (CRC) ISOL Ions 110 MeV/u - 10 µA Y [58]6 MINERVA (SCK CEN) ISOL P 100-600 MeV - 500 µA N [59]7 MAFF (FRM-II) ISOL th. neutrons - 1014 n/s N [60]8 SPES (INFN-Legnaro) ISOL P+ 70 MeV - 500 µA N [61]9 EXCYT (INFN-Catania) ISOL Ions 100 MeV/u - 5 µA Y [62]10 FAIR (GSI) In Flight Ions 11 GeV/u - 5·1011/cycle N [63]11 IGISOL (U. of Jyva¨skyla¨) ISOL Ions 130 MeV/u Y [64]12 Twinsol (U. of ND) In Flight Ions 10 MeV/u - 1µA Y [65]13 FRIB (ORNL) In Flight Ions 200 MeV/u - 400kW Y [66]14 ATLAS/CARIBU (ANL) In Flight+252Cf fiss. Ions 50 MV Y [67]15 RIBRAS (U. of Sa˜o Paulo) In Flight Ions 52- MeV/u Y [68]16 RAON (IBS) ISOL/In Flight P+ 70 MeV - 750 µA / Ions 200 MeV/u - 8.3 µA N [69]17 RIKEN In Flight Ions 345 MeV/u - 1 µA Y [70]18 BRIF (Beijing) ISOL P+ 100 MeV - 200 µA Y [71]19 ACEB (iThemba Labs) ISOL P+ 66 MeV - 50 µA Y [72]20 HIRFL-CSR (Lanzhou) In Flight Ions 750 MeV/u - 1 µA Y [73]Table 1.2: List of RIB facilities around the world and their principal parameters. Their geographical location is shown in figure 1.5.141.2 RIB production facilitiesSeveral facilities for radioactive beam production are scattered around the world as summarized infigure 1.5, and their most relevant parameters are captured in table 1.2. Some of these facilities arebased on the ISOL method, of which ISAC and ISOLDE are the two major ones in operation andwill be described in more detail here. In-flight facilities and ISOL facilities operating with catchermaterials will be briefly touched later in this section.1.2.1 ISAC at TRIUMFThe TRI University Meson Facility (TRIUMF) is Canada’s particle accelerator research centre,initially sponsored by three local universities located in British Columbia (University of BritishColumbia, Simon Fraser University and the University of Victoria) and hosting the world largestcyclotron in size, see figure 1.6. One key feature of this machine is its ability to accelerate H−ions at currents up to 400 µA which can be split and delivered simultaneously to four differentbeam lines, each carrying a maximum of 100 µA. Carbon stripping foils are positioned in differentlocations of the cyclotron and can be lowered into the H− beam to strip two electrons from eachion, changing its curvature radius so that it is extracted into one of four beam lines with energiesup to 500 MeV. In its early days, the high-energy protons were used to produce mesons and otherelementary particles for high-energy physics experiments. A decision was taken in the mid 1990sto shift the main purpose of the research centre to nuclear physics, and the ISAC facility was built,taking the first beam in 1999.One of the beam lines, Beam Line 2A (BL2A), delivers accelerated proton beams from thecyclotron to either one of the two ISAC target stations - the ISAC Target station East (ITE) andISAC Target station West (ITW) - which are almost a mirrored image of each other. Since theradiation levels coming from both stations during irradiation are very high, all components close tothe proton and RIB path were placed at the bottom of tightly shielded concrete canyons. The targetassembly and its supporting electrical infrastructure are mounted at the bottom of the target module(figure 1.7c), a ∼4 m tall structure that is craned into the target station, and into the hot cell forinstallation and retrieval of target assemblies. Its top service cap is its only portion that is exposedto air and allows connecting the various services, such as vacuum equipment, water cooling andelectrical lines to heat up the target and keep it at high voltage.A cut-view render of the target assembly is available in figures 1.7a and 1.7b, made out of twotantalum tubes (target and ionizer) connected perpendicularly to each other through a centre block.A heat shield is made up of two water-cooled copper plates which surround the target and preventsoverheating nearby components. The target and heat shield assembly is prepared in the lab by atechnician, and it is inserted into the hot cell to be mounted on the target module by connecting itselectrical and water services with the help of two telemanipulators.Ions escaping the ionizer tubes are accelerated by the electrostatic potential applied to the targetmodule, and go through a 60 ◦ pre-separator magnetic dipole bender followed by the ISAC mainseparator magnet to reach a maximum resolving power of m/∆m = 2000. The RIB line is then15Figure 1.5: RIB facilities around the world, some are still in construction, refer to table 1.2 fordetails.Figure 1.6: Schematic top view of the accelerator facilities at TRIUMF.16(a) ISAC target (b) ISAC heat shield assembly(c) Render of the ISAC target module.Figure 1.7: Core components of the ISAC target stations.directed up to the ground floor where all user experiments are built. The ISAC-I experimental hallhosts the low-energy user facilities, where the RIB energy can be tuned from few tens of kV up to1.5 MeV/u by combining the accelerating power of the ISAC Radio Frequency Quadrupole (RFQ)plus a Drift Tube Linac (DTL). For user facilities located in the ISAC-II hall, the RIB energy can befurther pushed to ≈40 MV thanks to a superconductive linac made out of 40 quarter-wave cavitiesthat can accelerate the RIB beyond the Coulomb barrier and enable a greater variety of experiments[33].ISAC’s strength relies in its capability of producing a great variety of radioisotopes throughhigh-energy nuclear reactions on a handful of target nuclei, covering the production of most isotopesin the known nuclide chart 1.1. Until now, ISAC and ISOLDE are the only existing ISOL facilitieswith beam energies above 100 MeV which enable the broadest production of exotic nuclides as17presented in figure 1.3, leading RIB research through this method and attracting users from aroundthe world. Research and development of ISOL targetry has been several times approached as acommon goal between TRIUMF and ISOLDE, exchanging expertise, knowledge and complement-ing each other’s capabilities given the boundary conditions of both facilities. The proton-to-neutronconverter development project described in chapter 3 is a clear example of such collaborations be-tween the target groups at both facilities and at the Research Centre for the Applications of NuclearEnergy (SCK CEN), a future player in this space.1.2.2 ISOLDE at CERNISOLDE at CERN is the main other ISOL facility that is the most comparable to TRIUMF asfar as RIB intensities and production capabilities, although there are several differences betweenthe two that put them apart. ISOLDE is located in the CERN accelerator complex and uses 40-50% of the protons that are accelerated to 1.4 GeV by the Linear accelerator 2 (LINAC-2) and theProton Synchrotron Booster (PSB); this higher beam energy with respect to TRIUMF’s 500 MeVgives ISOLDE some advantage in production cross sections for some isotopes, as can be seen infigure 1.3. Its pulsed time structure delivers up to 3.33·1013 protons within a 2.4 µs beam pulse,resulting in an average beam current of ∼2 µA, fifty times lower than the maximum ISAC driverbeam Continuous Wave (CW) current [74]. The protons can be delivered to either one of two targetstations, the general purpose or high resolution separators (GPS or HRS, named after their massseparator) where RIBs are produced with the ISOL technique and are then optically treated by aRFQ cooler-buncher to improve the isotope signal to noise ratio at the experimental station [21].The core components of the ISOLDE infrastructure is also a 20-cm long tantalum heater insidewhich pellets of several materials are installed to be heated and irradiated by the proton beam forradioisotope production. The instantaneous beam power of ∼ 3 ·109 W delivered by a single protonpulse at ISOLDE triggers an instantaneous temperature rise in the affected volume, and heat isthen transfer to neighbouring areas by conduction and radiation. The different time scale for beamheating in ISOLDE adds some complexities in terms of materials survival as investigated by Lettryet al. [75], but has also several beneficial consequences such as the high signal-to-noise ratio,an improved release efficiency on liquid metal targets due to a shaking effect, and an intrinsicadvantage to perform target development activities, since time-dependent isotope release profilescan be measured when single proton pulses are sent [76].The pot-style aluminum vessel that surrounds the target container (figure 1.8) also differs sig-nificantly from the ISAC heat shield since this was designed especially to act as primary vacuumenclosure connected to the RIB line. This difference changes the entire paradigm of how servicesare delivered to the target and how the remote handling and regular operations are carried out, sinceonly the vessel and its inner components are moved. One of the main factors that allowed this de-sign were the lower prompt radiation fields generated by the lower average proton beam current,whose direct consequences are the availability of rubber as vessel sealing material - which was notpossible at ISAC - and much quicker target turnaround times with respect to ISAC since there is no18Figure 1.8: Images of the ISOLDE target vessel, adapted from [4] and [5].heavy shielding component that needs to be moved by cranes to exchange target units. All the abovedifferences enable ISOLDE to operate up to 30 different target units every year instead of the ≈12now available at ISAC, and allows for a much quicker target exchange if unexpected issues cameup.1.2.3 ARIEL at TRIUMFTRIUMF has embarked in an effort to triple its radioactive ion beam production by building thenew ARIEL facility (see figure 1.6), the experimental complex consisting of a superconductivelinear electron accelerator (e-linac), a fourth proton beam line from the existing TRIUMF cy-clotron, their respective target stations to produce RIBs through photofission (ARIEL Electron Tar-get East (AETE)), spallation and fission (ARIEL Proton Target West (APTW)), and the CANadianRare-isotope facility with Electron-Beam ion source (CANREB) delivery system to transport RIBsproduced in the ARIEL target stations to the ISAC experimental halls [33]. The proton target as-sembly is very similar to its ISOLDE counterpart, where a tantalum cylindrical heater is placed onthe proton beam line axis and is enclosed in a round aluminum vessel equipped with feedthrough forwater cooling, current heating and insertion of mass markers. The proton beam can also be stirredand rastered by a magnet a few meters upstream of the station to allow for more flexibility in targetdesign and operation (i.e. using a neutron converter).The biggest discontinuity from the past is definitely the AETE station, where the target vesselincludes an electron-to-photon converter so that 30 MeV electrons impinging on it generate a highflux of gamma Bremmstrahlung photons, which in turn interact with uranium carbide material in anISOL target. Photofission reactions 238U(γ, f iss) on uranium nuclei are triggered and their productsare mostly neutron rich fission fragments which will be extracted through the ISOL method, seefigure 1.9. The converter development has been conceptually completed [77, 78], while efforts are19Figure 1.9: AETE production map at 30 MeV and 100 kWstill needed to configure the target for operation at a nominal beam power of 100 kW.Both ARIEL target stations and their core components are being designed by merging the bestand necessary aspects of ISAC and ISOLDE. The development of new polymer-based (PEEK) ra-diation resistant seal technologies allows for a ISOLDE-like target vessel that reduces the overallpumping volume for a much faster target exchange and drastically lowers the chances of water-vacuum leaks which are currently very hard to prevent in ISAC. Each ARIEL target is remotelyinstalled with a crane on the station front end, which hosts all the required services connections tothe target vessel and is operated at a voltage up to 60 kV; this assembly will be hanging from thetop using ∼0.5 metre-long ceramic insulating rods, designed to support the load and to maintaina minimum distance between high-voltage equipment of ≈30 cm to prevent discharges in the airsurrounding these assemblies. The amount of services such as water and current that can be deliv-ered to an ARIEL vessel has been improved, allowing for more optimized heating and cooling oftarget/vessel components and including the added capability of delivering mass markers, vapoursof different elements, to the target-ion source assembly for more standardized quality control andimprovement development opportunities. Both AETE and APTW stations are under construction atthe moment, and their completion is expected in 2024 and 2026 respectively.1.2.4 Catcher facilitiesA second type of radioactive ion beam facilities that work under a different extraction method arethe ones such as the Leuven Isotope Separator On-Line (LISOL) [79] and the Ion Guide Isotope Sep-aration On-Line (IGISOL) mass separator [80], where protons or ion beams impinge onto a targetfoil to trigger nuclear reactions inside it. The target is designed thin enough (i.e. few mg/cm2 ofmaterial for an incoming 20 MeV proton beam) so that the recoil energy of the reaction products isenough to be ejected out of the foil into the catcher, usually a noble gas at pressures of hundreds ofmbar. Because of the large recoil velocities, the reaction products are in a high charge state when en-tering the gas cell, their energy is reduced after each impact with the catcher atoms and their charge20is lowered by charge-exchange collisions. These systems are usually optimized to electrostaticallyextract 1+ or 2+ ions from a series of holes kept at progressively increasing potential, which alsohelp reducing the neutral gas pressure towards RIB transport section.An advantage of these facilities is their ability to extract almost any element out of a target-catcher system regardless of their specific chemistry, while the cm-long diffusion and effusion pathsin other ISOL systems has so far prevented the extraction of highly refractory elements. The greatcombination of target materials and beams can create the right conditions to produce the requiredreaction product and pure from isobaric contamination, yet the production rates in most cases aremuch lower than for solid target ISOL facilities [21].1.2.5 In-flight facilitiesSimilarly to catcher facilities, the in-flight method relies on the acceleration of ions at high energies(up to 440 MeV/u at FRIB) that impinge on a thin foil to trigger a specific nuclear reaction. Theproducts, which are usually a mix of several isotopes at different energies and charge states, willbe ejected out of the foil in a narrow kinematically-focused cone in the forward direction. Withoutbeing stopped or slowed down, the fragments then go through a series of dipole magnets and energydegrader sections. The former bends the whole beam so that the isotope of interest can make itsway through a pair of slits, following the dependence of the magnetic rigidity Bρ ∝ (p/e), whereB is the magnetic field intensity, ρ the particle curvature radius, p its momentum and e its charge.Additional contamination will also pass through the slits because of the different charge states, hencethe energy degrader sections are added to create an atomic number (Z) dependence on the isotopeselection process which makes it possible to better purify the cocktail and measure the isotope ofinterest at the end of the separator beam line [81].These in-flight facilities do not usually deliver RIB currents comparable to ISOL-made RIBsfor the great majority of isotopes. On the other hand, their strength is the extremely fast isotopetransport times from their production location inside the target until their detection (∼100 ns atRIKEN [82]), since they never have to thermalize but are always in-flight at high velocity. Thisallows these facilities to detect and discover isotopes with sub-µs half lives, an important verificationagainst theoretical models in the field of exotic nuclei [83].211.3 Transport phenomena relevant for ISOL targetsIsotopes produced by nuclear reactions throughout the ISOL target material could be millimetresto centimetres away from the ion source, where they can be turned into ions and be acceleratedby electric potentials and quickly delivered to the user experiments. There is however no potentialthat guides these atoms from their initial production location towards the ion source, and randomthermal motion is their only chance to make it. Two different fundamental kinds of transport thathappen to isotopes in ISOL targets are schematically presented in figure 1.10. One is diffusiontransport in the solid target material, from the location where the reaction products thermalize tothe a material-vacuum interface, and the second is effusion transport from this surface through thevacuum volumes of the target. These two processes happen in series and their combined durationrepresents the total release time which, if shorter than its half life, will more likely result in asuccessful ionization and delivery to the user station.This section aims at discussing transport phenomena in the context of ISOL facilities, hencefocusing on tracer elements produced by nuclear reactions whose concentration is much lower thanthe host lattice atoms.1.3.1 DiffusionThe diffusion phenomenon was first investigated and reported in 1829 when botanist Robert Brownnoticed that pollen particles in a liquid solution were translating and rotating in ways that appearedrandom, and it was not until 1905 that an atomistic description of the process was given by Einstein[84]. The main thesis of his work involved the definition of a diffusion coefficient of a tracer in aliquid medium, which led to the determination of atomic dimensions by Perrin [85], confirming thevalidity of a molecular-kinetic theory of heat. The Einstein-Smoluchowski relationD =λ 2D6τD(1.15)Figure 1.10: ISAC target transport sequence schematically presented. Red lines represent dif-fusion and green lines effusion both inside and between the porous pellets.22in a 3D geometry was then confirmed, where D is the diffusivity or diffusion coefficient of theparticle in its medium, λD is the arithmetic mean-squared displacements, and τD is the displacementtime. The validation of the molecular-kinetic theory of heat and diffusion, initially studied in liquids,opened possibilities for better describing the same phenomenon in solids by considering a traceratom performing a “jump” between its current location towards an allowed site within the matrixlattice. Equation 1.15 can then be re-written in atomistic terms asD =d2J ncω6(1.16)where dJ is the jump length, nc‘ is the coordination number of sites where a jump can occur, and ωis the resulting hopping frequency for the tracer atom towards a certain site [86].The difference in migration Gibbs free energy GM between the initial position and the saddle-point of the jump can be used to describe the diffusion kinetics since it was shown [87] that thehopping frequency ω could be written using statistical thermodynamics asω = ν0 exp(−∆GmkBT)= ν0 exp(∆SMkB)exp(−∆HMkBT), (1.17)where ∆HM and ∆SM are respectively the enthalpy and entropy of such hopping in thermal equilib-rium defining the Gibbs free energy∆GM = ∆HM−T∆SM, (1.18)ν0 is the vibration frequency around the equilibrium position in the direction of the reaction path,T the lattice temperature and kB is the Boltzmann constant. Equation 1.16 can then be re-writtenusing expression 1.17 to highlight the temperature dependence of the diffusion coefficientD = D0 · exp(−∆HMkBT)(1.19)where the exponential term is usually referred to as an Arrhenius-type dependence and D0 as pre-exponential factor, usually measured experimentally, and theoretically defined asD0 =d2J nc ν06· exp(∆SMkB). (1.20)The Arrhenius dependence in equation 1.19 is a big part of the reason why solid ISOL targetsare operated at very high temperatures, in order to increase the transport rate of radioactive speciesout of the target material before they decay. These quantities D0 and ∆HM are ultimately used to ex-perimentally measure and characterize diffusion processes across materials at different parameters.This macroscopic description is mathematically governed by Fick’s laws which in the mid 1800swere purely phenomenological and only later were backed up by the atomistic understanding of theprocess. The first Fick’s equation links the flux J of a diffusing particle to the gradient of its con-23centration C, where the proportionality factor is the diffusion coefficient (or diffusivity) introducedbeforeJ =−D ∇C, (1.21)which is the basis for Fick’s second law∂C∂ t= ∇(D∇C), (1.22)which are valid under the assumption of isotropic diffusivity.Defects role in diffusion In a perfect crystal lattice, the tracer atoms move between interstitialsites of the lattice atoms, introducing strains that deforms the original structure around the newlyinserted tracer. Real-world materials however can present several features that distort the perfectscenarios which were presented so far, and that usually would increase the mobility of tracer atomsunder consideration. Point defects in the lattice such as vacancies, interstitials or their combinationas Frenkel/Schottky defects can significantly alter the diffusion speed of the tracer species in theimmediate vicinity of the defect, providing a drastic reduction of the Gibbs free energy for thetransition. Taking vacancies as an example, corresponding to a “vacant” lattice atom in one of itssites, their presence can be thermally enabled and their equilibrium concentration can be describedbyCv = exp(−∆GFvkBT)= exp(∆SFvkB)exp(−∆HFvkBT). (1.23)where the quantities ∆SFv and ∆HFv denote entropy and enthalpy for the formation of a vacancy. Thevacancy-enabled hopping rate ω totv of a tracer atom can be then written asω totv = ωv ·Cv = ν0 exp(∆Sv+∆SFvkB)exp(−∆Hv+∆HFvkBT)(1.24)where ωv is the exchange rate between an atom and a vacancy, and ∆Hv, ∆Sv are the associatedenthalpy and entropy for the transition [86]. It should also be noted that the tracer specie occupyinga vacancy could also be significantly hindered from further diffusing in the material. The casewhere two vacancies come together to form a divacancy is not discussed here, although it followsan analogue reasoning.Additionally to point defects, real materials also present more extended defect that extend in onedimension (1-D) such as dislocations, and 2-D such as grain boundaries and surfaces. The lowerfree energy on these features makes it such that the total hopping rate increases at comparable tem-perature with respect to bulk interstitial diffusion, which can be intuitively interpreted by the moreopen structure of these features which provides more room for migration, thus giving rise to loweractivation barriers. For example, experimental evidence points to the enthalpy of grain boundarydiffusion to be∼40% to 60% lower than for its lattice counterpart, while the pre-exponential factorsare closer to the bulk [88, 89]. In general, it is accepted that the diffusion enthalpies in the lattice24are higher than in dislocations (Hd), grain boundaries (Hgb) and surfaces (Hs) according to [86]∆H > ∆Hd > ∆Hgb > ∆Hs (1.25)which leads to the following equivalent relations between diffusion coefficientsD < Dd < Dgb < Ds. (1.26)These considerations also apply for ISOL target materials, where it would be preferable to haveas many small grains as possible in target materials to decrease their bulk diffusion distances. Thethermalized atom is however generally produced in the bulk material, and will need to undergolattice diffusion before potentially encountering some diffusion-enhancing features. The very highoperational temperatures in ISOL targets make it hard for a polycrystalline structure to maintain thesame amount of grain boundaries as in its room temperature configuration, since phenomena likegrain growth are observed in such materials employed at ISAC-TRIUMF which tend to minimizethe surface area of each grain [9]. A common procedure to suppress grain growth in ISOL carbidetargets is to add over-stoichiometric amounts of fine graphite powders which prevents excessiveparticle growth and sintering [30, 27].Radiation damage effect on diffusion ISOL target materials are constantly bombarded by high-energy particles which interact with the nuclei to trigger nuclear reactions that produce radioisotopesfor delivery to experiments. These interactions also lead to lattice damage which can manifest inseveral forms and affect radioisotope transport in the solid material.When a nucleus in the material lattice undergoes Coulomb scattering or a nuclear reaction witha high-energy particle, this event results in one or more heavy nuclei ejected at high energy as PKA.This event not only leaves behind a vacancy in the lattice, but creates high-energy, high-mass projec-tiles that in turn will travel through the material following comparable slowing down and scatteringevents according to equations 1.10 and 1.11 respectively. It becomes quite difficult to analyticallyrepresent how these and other processes interplay and affect each other due to further considera-tions on electron screening effects which alter the effective Coulomb potential that projectiles see,their electron pick-up likelihood along their tracks and potential nuclear reactions they might trig-ger. Some general considerations can nonetheless be made to assess the total number (νd) of atomsdisplaced by these cascades using the formula [90]νd(E) =(E2Ed)·ξ (E,Z,rs) (1.27)where E is the energy of the PKA or reaction product, Ed (∼ 20-50 eV [91]) is the required amountof energy to knock the lattice atom out of its site and create a displacement, ξ (E,Z,rs) is the Lind-hard partition function which depends on the beam energy E, the atomic number Z and the screeningradius rs of the target nuclei [90]. The ballistic process mentioned here of binary collision and col-25Figure 1.11: Schematic representation of radiation damage in materials.lective displacement finish within the first 10−12 s, after which another fast process called thermalspike takes place; here, the hot and compressed volume of atoms surrounding the orange areas offigure 1.11 reach temperatures in excess of 104 ◦C for less than 10−10 s, followed by a quick quench-ing that usually freezes vacancy clusters and single reaction products in the centre of this depletedzone. From here onwards, interstitials and defects would start behaving according to the diffusionmechanisms described at the beginning of this subsection 1.3.1, driven by thermal agitation andfavoured by those features such as vacancies, grain boundaries and surfaces which promote fastermotion [92, 90, 91].In accelerator facilities, there are several components that receive a significant amount of doseover the course of their lifetime, and Displacement Per Atom (DPA) is the unit that quantifies theaverage number of displacements that each atom in the material undergoes, due to the radiation-induced processes mentioned here. Beam-intercepting devices such as targets, beam windows, beamdumps and collimators are receiving some of the highest DPA amongs all accelerator components[93, 94, 95, 96] and are at times in operation for very long time periods. In these devices, whichcould also be operated at high temperature and under cyclic loads, the dose received is usually wors-ening the thermo-mechanical properties of their components, and careful analysis of these effectsis needed while designing them in order to prevent or minimize failure. For ISOL target materi-als, which are usually replaced every 1-4 weeks at ISOLDE-CERN and ISAC-TRIUMF, receivingradiation damage should play only a positive role in improving the diffusion speed.At ISAC-TRIUMF, this Radiation Enhanced Diffusion (RED) effect was reported to be seenboth by systematically increasing the proton beam current and measuring an over-linear trend ofthe extracted RIB, and by tracking the RIB current at identical parameters over days of operationand measuring a RIB increase [97, 98]. Another interesting observation on radiation-dependentdiffusion was made at ISOLDE-CERN by Ramos et al. [36], when argon RIBs were extractedfrom a nanometric calcium oxide target with average grain size of ∼40 nm. Upon reducing thetarget temperature, the RIB yields were only a factor 2-3 lower than at operational temperature,26orders of magnitude higher than what was expected from an Arrhenius dependence. The partialexplanation given by the authors involved the recoil energy of argon atoms produced by the reaction40Ca(p,[3p+3n])35Ar which carry an average energy of 9.2±1.8 keV, correspondent to an ion rangeof 10.9 nm. A significant fraction of these moving recoils would be able to escape the target materialgrain only by means of their recoil kinetic energy, making it very likely to be released from the fulltarget container body, explaining this incredibly high yield. However, the recoil nucleus would beimplanted into the following grain with its remaining kinetic energy, hence nullifying the importanceof its ejection from the previous grain.Uncertainties on the causes of these observations and on the effects of radiation enhanced diffu-sion in ISOL target materials are still under discussion, and likely depend on a variety of combinedfactors which have not yet been investigated systematically.Porosity effect on diffusion One feature of materials which greatly affects the diffusion rate is thepresence of a network of pores inside the material, more or less interconnected with each other.One useful way to describe its effect on the overall transport properties is to describe its diffusioncoefficient D as [99]1D=1Ds+1Dp(1.28)where Ds and Dp are respectively the diffusion coefficients in the solid bulk of the material and inthe pore network. In sub-µm pores, it is usually the case that free molecular diffusion (or effusion,expanded in the next section) is the main driving mechanism, and its effective value can be describedby the equationDp =εpτpDsp (1.29)where εp is the pore fraction of the whole material, Dsp is the diffusivity of the pore if it were astraight cylinder, and τp is the tortuosity of the pore, a geometrical factor taking into account theinefficiencies of transport in a non-straight channel.A great percentage of materials irradiated in ISOL facilities are ceramics, mostly carbides, cho-sen for their superior thermal properties which allow for the use of several target nuclei (i.e. ura-nium, zirconium and silicon) that would not perform as well in other chemical forms (i.e. metals oroxides). In order to increase the diffusion coefficient in ceramic target materials, a porous matrix isvery frequently adopted [37, 36, 34, 35, 100, 10], and experiments pointed to open porosity as themain contributor to the overall diffusion performance, where inner pores have a direct connectionto the outside surface of the pellet [100]. It should be noted that although pore diffusion in ISOLmaterials relies on the same effusion phenomenon explained in the next section, it is assimilatedto the diffusion stage since it happens within each target material pellet and does not fall into thearbitrary classification of an effusion process.271.3.2 EffusionEffusion in ISOL targets is usually [101, 102, 103, 104] described as the transport phase whereatoms move in the void spaces within the target container in a flow regime known as Knudsendiffusion when the mean free path of the gas λg is greater than the geometrical dimension where theflow happens, i.e. the hydraulic diameter lh [105]. Since ISOL targets are subjected to an extensivepump-down phase down to pressures in the order of∼ 10−6 Torr and heated at nominal temperaturebefore irradiation, it is assumed that these flow conditions apply in all voids within the target domainwhere the mean free path is ∼100 m [106, 107].The effusion phase for an isotope of interest consists in a series of events - a desorption, a freeflight and an adsorption - until the particle is either ionized in the ion source or decays into itsdaughter nucleus. When an atom diffuses or is emitted onto a surface, electromagnetic interactiveforces between the surface and the atom itself can have different strengths depending on the na-ture of the couple adsorbate-adsorbant. Van der Waals forces can arise if the electronic fluctuationscreate a dipole moment that can cause meaningful electrostatic attraction, are characterized by alow adsorption enthalpy ∆Hads and are usually relatively weak and easily broken by thermal agita-tion. Alternatively, stronger bonds can arise when atoms get closer to the surface and their orbitalsoverlap, forming chemical bonds of much higher strengths. The desorption rate can be written asrdes = ν0 exp(−∆HdeskBT)(1.30)where ν0 is a frequency factor related to the vibrational entropy and ∆Hdes is the activation energy fordesorption. Since ∆Hdes is in the order of tens of meV for physisorption and eV for chemisorption,desorption rates for the two processes can be vastly different [108]. Instead of referring to desorptionrates, it is more practical to express the average time for which an atoms stays stuck to the surfaceover which it was deposited, or sticking time τs as [109, 110]τs =1ν0exp(∆HdeskBT). (1.31)Once the thermal motion overcomes the attractive potential and allows the atom to be desorbed, itis ejected most likely following a cosine angular distribution [111] and will travel in that directionuntil it meets another surface and is adsorbed on it. Throughout this transport phase, millionsof interactions can happen between the effusing atom and material surface, which can lead to asubstantial release delay if a strong interaction leads to a long sticking time [104]. The presenceof chemical impurities could further hinder the fast mobility of some atomic species, yet the highoperational temperature of ISOL targets (T>1600 ◦C) favours their evaporation during the earlyconditioning stages, allowing for the retention of only few refractory compounds.At thermal agitation energies, it is most likely that the atom will not penetrate inside the materialbut it will rest on its surface and continue in its effusion journey. At this stage, even if the atomdiffuses onto another neighbouring site on the surface, it still has the chance to be desorbed from28that equivalent site, without disturbing the effusion phenomenon. In contrast, the re-diffusion intothe material bulk does create an additional delay with respect to the previous case, since the atomneeds to keep diffusing until the next available surface before even having the chance to be desorbedagain. This last scenario is however much less likely due to the differences in Gibbs free energybetween the surface versus bulk lattice sites, which favours atomic segregation on surfaces [86] aswell as the aforementioned diffusion coefficient trends in equation 1.26.291.4 Exploring transport phenomena in ISOL targetsThe production of RIBs using the ISOL method has been successfully carried out since the early1950s, when the first proof of principle [19] successfully produced noble gases from a “cold” ura-nium oxide target. Baking powder was added to the uranium mixture, which would decompose ina low-pressure atmosphere, carrying along the volatile noble gases produced from the nuclear reac-tions [18]. This first demonstration set the stage for a new era of precise measurements on exoticnuclear species, starting at the ISOLDE facility in 1967. Even before the start of ISOLDE opera-tion, it was clear that the future ISOL targets needed to have “considerable thickness” [112] in orderto produce enough radioisotopes for the users. As Andersen et al. put it, “diffusion is apparentlythe only process capable of transporting the active atoms to the surface of the target.”[112]. Atthis point, it was already understood that the biggest bottleneck of the whole ISOL process was thetransport of the atomic species from their production location to the ion source, where they wouldbe ionized. From here, they could proceed in their journey towards the experiment being carefullyguided by the electromagnetic field of the RIB beam line instead of wandering around the targetdriven only by thermal motion. Since the early days of the ISOL field, both online and offlinestudies were performed in order to systematically investigate the diffusion performance of severalelements in target materials. The word online is used to refer to experiments carried out through theISOL method as explained in section 1.1, while offline refers to experiments where the transport of aspecific isotope is studied separately from the RIB extraction system, as carried out here in chapter4.The early availability of high-energy proton beams at ISOLDE-CERN has been fundamentalto perform online release studies of radioactive species produced by several different reactions in avariety of materials. Notable examples of such comprehensive studies are reported by Carraz et al.[113] and Ravn et al. [114, 20], where continuous improvements in the target and ion source unitswere yielding every year new elements and isotopes. Through the decades, incremental upgradesin target science and technology have contributed to an improvement in the overall availabilityof isotopes, with the latest major developments happening for example in the nanomaterial field[36, 37, 115, 10]. Yield measurements of online targets are the major test bench to experimentallyprove the efficacy of a target configuration in delivering exotic isotopes to the final users. It isotherwise difficult to recreate the exact same operational conditions of an ISOL target, where severalthermal and electrical considerations come into play to create a unique set of conditions.Although online experiments give a good idea of the performance of the target, they are not ingeneral the best benches to develop new target materials and perform systematic studies. Carrazet al. were already pointing this out in 1978 when referring to new target materials: “Only a fewsuch targets have been used in the past since prediction of their properties, and therefore the designof appropriate systems, has been difficult owing to the lack of diffusion parameters and surfacedesorption energies for systems of interest for nuclear studies.”; and again: “This situation has beenrapidly changing over recent years...” [113]. The systematic studies referred here were performedindeed offline [116, 117], where iterations on target materials and trial-error procedures have no30impact on the facility operation and require less overhead. In the present days, similar rationales arestill supporting systematic developments on offline benches.The first reason involves the value of beam time at ISOL facilities: in the past few years atISAC-TRIUMF, ∼5% of the total shifts were dedicated to operating development targets, whichusually comes down to one target each year. Moreover, comparing yield data between differenttargets is a non-straightforward process, especially in a high power facility as ISAC-TRIUMF wherebeam power deposition and resistive heating both contribute to locally increasing the target materialtemperature in different regions. In addition to the challenges of online operation, the residual doseof long-lived decay products is another main reason for the scarcity of experiments and tests thatcan be performed on targets after irradiation. Figure 1.12 shows the residual dose recorded fromISAC targets after a certain decay period, much higher than the dose rate that TRIUMF NuclearEnergy Worker (NEW) personnel can receive in one hour (6.25 µSv/h), if they were to work inthat environment even just one hour a day. This calls for the use of dedicated hot cells, wheremanipulators are the only tools that can be used to handle irradiated targets, but even operations thatmight seem simple (such as extracting one sample of target material) are technically challengingand time demanding.Figure 1.12: Residual doses from ISAC targets measured at different decay times after end ofbombardment (EoB)These challenges all together have called in the past for more offline development, which trans-lates into more systematic studies, more testing availability and in general less constraints. Thefollowing section presents a comprehensive overview of offline release studies that have been per-formed until the present days, which were translated into useful information for the next onlinetarget.311.4.1 Overview of past offline release activitiesPrior to the ISOLDE startup, rare-earth nuclides were (and still are) of great interest to the nu-clear physics community, but their volatility at room temperature was deemed to be too low to beefficiently extracted from any material. This realization gave the start to a series of experimentalcampaigns that investigated release properties of several elements in refractory metals, a variety ofceramics and other niche materials at high temperatures.Release from metal targets The Arrhenius dependence of the diffusion process shown in equa-tion 1.19 called for high operational temperatures, hence the first release experiment was done withtantalum metal because of its high melting point of around 3000 ◦C [118, 119]. Andersen et al.irradiated tantalum samples of thickness 0.2 mm for 7 hours in the CERN synchrocyclotron to pro-duce nuclear reaction products, which were then detected via a gamma spectrometer [112]. Thesesamples were then transferred to an offline electron bombardment oven where they were progres-sively heated up to a certain temperature, for a set amount of time. The residual activity detectedby the gamma spectrometer could be used to quantify the speed of the diffusion and surface evap-oration happening at each temperature/time combination. This first qualitative approach lead to therealization that significant fractions (above 30%) of such rare earth atoms as europium, ytterbium,lutetium and terbium could have been extracted in the temperature range between 1500 - 2300 ◦ C.A first systematic analysis of rare earth elements release from refractory metals was performedby Latuszynski et al. in 1974 [117]. Tantalum, rhenium and tungsten discs of thickness 0.1 mm wereactivated by 660 MeV protons, and then transferred to a hot surface ion source which would also heatup the irradiated sample. The fraction of isotopes released from the discs (ηD) was experimentallymeasured similarly to Andersen et al., as the ratioηD =IM− I′MIM(1.32)where IM is the intensity of the gamma line of a particular isotope before heating, and I′M is thesame, but after heating. Applying this methodology, they could prove that tantalum was moreeffective than the other two metals (Re and W) at releasing all the elements they compared (Ga,Tm, Yb and Lu), and a summary of their main lattice and physical properties is available in table1.3. A comparison of diffusion times of most rare earth elements from tantalum was performed,investigating the effects of thinner and thicker samples (0.05 mm and 0.1 mm), heated for shorter orlonger times (from 3 to 15 minutes) and at temperatures of between 1700 and 2100 ◦C.Beyer et al. took a step further for these release experiments [120, 121]. It was pointed outhow the particle escape process from the metal samples was not only related to diffusion in the bulkof the material, but also linked a second evaporation process from the surface of the sample thatneeded to take place. Because of the flat shape of the samples analyzed, it was assumed that thetime-determining step was only due to diffusion inside the material, since a single evaporation wasneeded to eject the particle. Thanks to this assumption and to the thin-foil geometry of these discs,32Property Ta Re WCrystal structure bcc hcp bccLattice constant, a [A˚] 3.3074 2.7609 3.16524Lattice constant, c [A˚] - 4.4583 -Melting temperature [◦C] 3270 3308 3650Table 1.3: Lattice and physical properties of three refractory metals [1].the experimental observable known as fractional release (F) was able to be expressed asF [%] = 100− 800pi2∞∑n=01(2n+1)2· exp[−(2n+1)2pi2Dtd2](1.33)where t is the release time, d is the foil thickness, and D is the diffusion coefficient of the element inthe bulk material which follows an Arrhenius temperature dependency. The validity of 1.33 standsif the concentration of the diffusing elements is homogeneously distributed in the pellet volume,which is the case in most of the measurements performed using a 600 MeV proton beam whichinteracts throughout the full thickness of the metal foils. This experiments shed light on somediffusion trends seen in tantalum metal, such as the trace element diffusion coefficients could beordered asDY > DAs > DLanthanides > DSr > DBa > DHf > DRb > DCs. (1.34)This trend is also seen of a confirmation of the diffusion dependence on the trace element ionicradius [121].Element-dependent diffusion behaviour is very recognizable, and represents a first approxima-tion of atomic escape from a single metal foil. However, hundreds of foils are present in an ISOLtarget, all facing each other so that one desorbed species from one foil is very likely to be ad-sorbed on another adjacent foil, over and over again. The importance of the adsorption/desorptionphenomenon included in the effusion phase could then have a great importance depending on thenumber of surface collisions an average isotope undergoes before entering the ion source. Beyer etal. took on this investigation [122] by measuring the release fraction of radioactive species whichwere implanted at low energy (45 keV) on a tantalum foil surface, only few nm into the material.For a given temperature, the release fractions of elements of interest were FRb > FSr > FY, oppositeto their diffusion behaviour when implanted deeper into the same material, and compatible withtheir boiling point trend. [119].Some of these conclusions were confirmed from online studies by Ravn et al. [123] who com-pared the release of Rb from two different targets loaded with 20 µm Nb powder target, but indifferent macroscopic configurations. Regardless whether the powder was sintered into 4 mm thickpellets or just loaded as it was in the heater, Rb release performance was unchanged, which pointsat the diffusion step from inside the powder grain to its surface being the bottleneck for the wholeprocess. In a more general study on powder targets, it was confirmed from online measurements33that [116]:1. Release from elements in groups 1A, 8A and possibly 7A is controlled by intra-particle dif-fusion, with their inter-particle path characterized by Knudsen diffusion.2. Release from groups 2A, 6A, 2B and some lanthanides is still controlled by intra-particlediffusion, but their inter-particle diffusion is dominated by lingering/sticking time3. The release of refractory elements is very delayed by the effusion through the pore structure.Unfortunately, the release properties of a few elements cannot be investigated with the offlinemethodology presented above, since all unstable isotopes have very short half lives compared to thepractical times for irradiation, gamma spectroscopy and heating. One of the most requested ions atISAC-TRIUMF is 8Li [52], which is the longest-lived radioactive lithium isotope with a half-life of839.4 ms only. It is one of the flagship isotopes for the β -NMR experiment in ISAC, which relies onhigh intensity 8Li beams for probing local magnetic properties of condensed matter or functionalityof biological molecules. High energy protons usually impinge on a thin-foiled tantalum target sothat 8Li is produced from spallation and evaporation, hence the interest of measuring its diffusioncoefficient in refractory metals. Vacik et al.[124] tried to measure for the first time this parameterby using stable 6Li atoms impinging on tantalum at a defined energy, correspondent to an aver-age implantation depth. After annealing the foil at a certain temperature and time, the sample wasbombarded with neutrons which would trigger the reaction 6Li(n, 3H) 4He to measure its spatial dis-tribution using the Thermal Neutron Depth Profiling (TNDP) technique, capable of finely resolvingthe position isotope position with enough accuracy. Although this test was well performed withvery little experimental error, the results in tantalum were inconclusive since the distribution wasfollowing a non-Fickian trend (i.e. was not gaussian) and no diffusion coefficient was measured.Potential causes for this strange behaviour were attributed to the radiation damage created by thebeam itself in the material, hence creating trapping centres for lithium, or unexplained propertiessuch as non-homogeneous diffusion coefficients.Tantalum foils - along with some other refractory metals - have been used as targets for decadesand will keep playing a big role as ISOL materials because of their high thermal conductivity, theirhigh melting points and chemical compatibility with the heater container made from the same mate-rial. However, the smallest foil thickness ever operated online at ISAC-TRIUMF is ≈25 µm, whichcould be regarded as too thick for some diffusion-limited isotopes. Moreover, sintering betweenadjacent discs could develop at the hottest locations of the discs, effectively further increasing thediffusion path for some isotopes. One of the most-employed ways to cope with this issue is thesynthesis of porous, micro- or nano- structured ceramic materials where the average particle sizedetermines the diffusion length required for release.Release from ceramics Carbides and oxides of several elements have been widely used in the ISOLmethod since its inception because of their physical properties compatible with high temperature34operation, and their internal structure and synthesis procedures have been optimized for the specificapplication. Hence, comparing the absolute performance of ceramic targets over a wide range ofliterature references is not straightforward because of their different stoichiometry, microstructureand filler/backing material. On the other hand, the production of ceramics with different metalelement (U, Si, Zr, etc) can be achieved through very similar procedures, yielding comparablemicrostructures [9, 36]. Uranium carbide is the reference material for most ISOL facilities world-wide [22] because of its broad range of fission or spallation reaction products created with eitherprotons, neutrons or gamma rays. Initial investigations aiming at enhancing the release for ISOLrelated studies were carried out on a variety of samples. For example, the release behaviour ofuranium carbide powder of two different sizes (1.3 and 20 µm) sintered at 1800 - 2000 ◦C wasinvestigated offline by Carraz et al. [116] following the same methodology from Latuszynski [117]and Beyer [120]. The data show that the release speed of various elements follows the order I,Te,Sb, Sn > Cs > Ba, pointing to in-grain diffusion as the time limiting step for the release out of thepellet, since the desorption energies of I, Cs and Ba should follow the trend ∆Hdes(Cs) > ∆Hdes(I) ∆Hdes(Ba) noticed on refractory metals [125, 126].Graphite cloths were also impregnated with uranium carbide and tested offline [116], showingeven better release performance which especially favours isotopes with shorter lifetime. This studyis particularly interesting since this target was also operated online and release data is available byCarraz et al. [113], where it was confirmed that the intra-particle diffusion is the release bottleneckfor elements within the IA group (Rb, Cs, Fr). Moreover, a better performance was confirmed onlinefor the graphite-impregnated target, a conclusion that speaks for the importance and significance ofoffline testing, especially when access to beamtime is precious and the target turnover is limited.Similar studies from uranium carbide were performed by Hoff, where it was shown that the additionof CF4 to the target matrix could greatly enhance the release of some lanthanides due to the forma-tion of molecular ions with lower sticking times [127]. Moreover, this study offers a good insightof release properties of several other materials following the same offline testing procedure.More recently, a series of extensive systematic release studies have been performed at the ALTOfacility at the Institut de Physique Nucleaire Orsay (IPNO), France. Neutrons are generated fromthe interaction of 27 MeV deuterons with a graphite converter, and induce fissions in two uraniumcarbide pellets from the same production batch. The activity of both samples is measured withgermanium detectors at the same time after irradiation and after one of the two samples has beenheated up in a vacuum furnace. The cold sample is not heated, which should prevent any isotopefrom escaping the volume, providing a reference/normalization value for both measurements onthe heated sample. This work performed by Hy et al. [128] gives insights on the method andprovides some preliminary release performance results for some samples. A more comprehensiveand systematic approach was carried out by Tusseau-Nenez et al. [100] where several sampleswere characterized and their release properties measured. Results show an overall trend of higherrelease for the Carbon Nanotube (CNT) sample, where the starting material presents finer grains andmore open porosity. At high temperature, the effectiveness of the CNT structure decreased probably35due to sintering which increased the grain size. The development of microstructure stabilizationtechniques was therefore recommended. The decrease of release efficiency with increasing pelletthickness was confirmed by Guillot et al. [129], also noticing that high release of fission productscorrelates with total porosity and small-size pores and grains [130, 131]. The number of releasestudies performed on uranium-carbides is much broader than for other carbides since fission canbe achieved with many high-energy particles, including protons, neutrons and gamma rays. Morefacilities, not only proton accelerators, can therefore study its release performance, as the case ofIPNO [128].Silicon carbide has been first investigated by Hoff et al. [127] for the release of Be and Na, butwas initially discarded due to poor performance compared to other carbide materials. Regardlessof the low expectations, SiC has been successfully implemented for online radioisotope productionboth at ISOLDE [132] and TRIUMF [98] and its release performance was later on studied offline byFernandes [133]. This extended study aimed at measuring the release fractions for several isotopesof Be, Na and Mg from eleven different SiC matrices using the standard proton activation combinedwith gamma spectroscopy, which yielded higher release fractions than Hoff’s. Moreover, the ionimplantation technique was also adopted for measuring fluorine diffusion in only one of the materi-als, which also was tested online. These results suggest that diffusion of Mg and Na atoms might bedominated by an aggregate vacancy mechanism, beryllium diffusion is mostly through interstitialsites due to its small radius while fluorine fast diffusion could be explained by the grain boundarymechanism.Among other materials of interest for ISOL targets, Hoff et al. [127] investigated several optionswhich were sometimes adopted in facilities such as ISOLDE-CERN and ISAC-TRIUMF. It alsotrue that release properties alone do not always drive the decision of which material should betested online. At TRIUMF, only few oxide targets have been operated in the past such as ThO,CaO, Al2O3 and UO2, since the more suitable thermal properties of carbide targets make theselatter more appealing for online operation [9]. For example, uranium oxides have a low thermalconductivity and higher vapour pressure at temperatures beyond 1500 ◦[27] which would limit itsoperation to 2 µA [134], instead of the maximum allowed current of 40 µA. A similar motivationis applicable for the choice of ZrC over its oxide counterpart, hence the carbide has been employedseveral times even if measurements from Hoff et al. [127] show that ZrO2 has a much better releaseperformance.Release from catchers Metal foils are extensively used in ISOL facilities such as TRIUMF [135,136] and ISOLDE [20, 32] as the target material. Additional research on diffusion and release timesfrom thin refractory foils is performed in the framework of catcher research for in-flight facilities[137], or when isotopes in their ionized state need to be neutralized for downstream experiments[138, 139]. For such applications, charged reaction fragments or a pre-made ion beam , usuallyradioactive, are delivered and stopped/implanted in a thin layer of metal foil. Their stopping depthdepends on the material physical properties and the particle charge and energy. In the context of36neutral beam production, diffusion properties of radioactive 37K in several refractory metal foilswere investigated at TRIUMF [140]. Zirconium was finally chosen the neutralizer material since itallowed good diffusion of potassium at lower temperature with respect to other catcher materials.More refractory matrices are usually chosen as catchers for secondary ion beam production, sinceon top of stopping the incoming beam, it is quite helpful to use them as a hot cathode for producinglow energy ion beams. Release studies for efficiency determination from thin tantalum foil catcherswere carried out by Klepper et al. [141], while more involved studies were carried out by Kirchneret al.[142, 101], which also include guidelines for catcher-ion source design. As for general purposecatcher material, tantalum seems the most advantageous for its refractory nature and best diffusionproperties compared to its neighbour elements, but has a low work function which makes it not thebest surface ionizer substrate [39]. As a potential substitute, tungsten could be used to overcome theionization issue, but its low diffusion characteristics make it useful only in very high temperatureenvironments.Graphite is also used in solid catcher applications because of its refractory nature and desorptionenthalpy close to commonly chosen metals such as Ta, Mo, Nb [142]. Different graphite grades havebeen shown to perform differently: Kirchner [101] reports faster gold release in smaller grains andwith more open porosity. However, the lack of control on the microstructure of commercially-available graphite turned out to be counterproductive. A study has shown worse release from a4-µm-grain graphite rather than just a flat 5 µm foil, which could be explained by the presenceof mostly closed porosity which significantly slowed the release of silver and noble gases [101].This shows how proper quality control on the material microstructure should be implemented whendealing with granular materials where the pore and grain distributions are key factors for successfulexperiments.Offline testing of release, diffusion and desorption of various elements from different matriceshave proven to be effective methods to select ISOL target materials based on their performance,compare their efficiency at high temperatures (1500-2200 ◦ C) and measure their diffusion coef-ficients. Moreover, recent investigations performed on uranium carbides clearly show which arethe most effective microstructures and production methods that result in high release and betterlong-standing performance. It was also recognized that in order to make quantitative comparisonsbetween different matrices, it is important to build an understanding of the different processes hap-pening inside these target materials. The next section will provide an overview of the mathemati-cal methods proposed to categorize the processes happening within ISOL targets, compare perfor-mances of different materials and structures, and optimize future targets to maximize radioisotopeyields.371.4.2 Mathematical methods to model release in ISOL targetsThe two main transport processes that make up for the extraction inefficiencies of ISOL targets,diffusion and effusion, were well known since the beginning of the studies reported in the sectionabove, therefore several attempts of categorizing the obtained results was usually included for dis-cussion and comparison. Both diffusion and effusion rates were known to be highly temperature de-pendent via Arrhenius-type relations [86, 108] and from kinetic gas theory [143], even though mosttimes the actual experimental parameters at those high operational temperatures were not known.For an ISOL pellet where the two processes supposedly happen in series, one after the other, it isnot trivial to distinguish between the two phenomena.A first approach to model the release path of radioisotopes in powder pellets, pressed and sin-tered, was presented by Carraz et al. [116], where the two transport processes were described intwo separate forms. The first diffusion stage takes place from the bulk of powder grains to theirsurface, which can be modelled as perfect spheres with diameter equal to the average grain size.The fractional activity - percentage of the element of interest present inside the solid sphere at acertain heating time t - can be expressed asFS(t) =6pi2∞∑k=11k2e−k2µst (1.35)where the diffusion parameter is linked to the particle size and diffusion coefficient through µs =pi2D/r2, with D the diffusion coefficient of the elemental species in the medium, and r the sphereradius [144]. Once the diffusing particles arrive at the edge of the grains, transport through thepore structure takes place as Knudsen diffusion and its fractional activity is modelled similarly toequation 1.35 as:FP(t) =8pi2∞∑k=11(2k+1)2e−(2k+1)2µpt . (1.36)The pore diffusion parameter here is expressed as µp = pi2De/r2 where De is an effective diffusioncoefficient linked to the pore structure properties by the relationDe = ε2(L/Le)Dt (1.37)where ε is the porosity value and Le/L is the ratio of the actual diffusion length to the straight-linedistance through the solid, while Dt is the diffusion coefficient through a cylindrical capillary ofradius ρ . According to Fick’s second law of diffusion [145], the average time t¯ for a particle toescape the pore network can be expressed as [146]t¯ =L26Dt(1.38)or can also be written ast¯ =L24ρu+L2τ8ρ2(1.39)38where u is the particle velocity through the pores, while τ represents the average sticking/lingeringtime of that particle adsorbed on the inner pore surface. Combining 1.38 with 1.39, the diffusioncoefficient through the pore structure can be re-written as a function of the sticking time or poretravel velocity.This mathematical representation of the particle path in a powder pellet aims at quantifying thetransport process by looking for the bottleneck step in the release. Following the same structure laidout in section 1.4, if the time-limiting step is diffusion-driven, the fractional activity in the pelletwould be represented by equation 1.35. On the other hand, if the effusion ends up being the limitingprocess, the fractional activity would have the shape of 1.36, where the diffusion parameter is eitherµp = pi2 2ε2L3L3eρu if the limiting step is the flow in the pore structure or µp = pi2 4ε2ρ2L3L3eτfor high stickingtimes.Carraz’s approach is enough to describe the release path while labeling the time-defining stepfor radioisotope extraction. The distinction between three mechanisms is sometimes beneficial toeffectively model some classes of elements, such as volatile species, which would be mostly subjectto diffusion delays as presented by Liu et al. [147]. However, simplification comes with a lack ofcomprehensive description of the release in those cases where both diffusion and effusion make upa significant fraction of the total release time. Moreover, it is to be pointed out that the effusionmodel, although it relies on physical quantities such as velocity, pore size and length, is not easilyapplicable because of difficulties in estimating these parameters.While studying thin-foil gas catchers, Kirchner [101] suggested a way to describe diffusion andeffusion as in-series processes taking places at intervals [0,t’] and [t’,t] respectively, so that theircombined release function could be described by the convolution:p(t) =∫ tt ′=0pD(t ′)pE(t− t ′)dt ′. (1.40)The thin-foils arrangement of the catcher allowed modelling the solid diffusion through the delayfunction pD in planar geometrypD(t) =8µ0pi∞∑n=0e−µnt , (1.41)while the effusion curve was phenomenologically presented aspE(t) = νe−νt (1.42)where ν is a frequency parameter linked to the mean flight time between collisions (τ f ), the meansticking time τa and the number of collisions χ through the relationν =1χ(τa+ τ f ). (1.43)This very elegant description comes only at the expense of having a couple of free parameters, τ fand χ , which are usually unknown, since they depend on the specific geometry under consideration.39A very important caveat of this and other models, which was pointed out by Kirchner [101], lies inthe description of diffusion and effusion as series processes. In fact, the model does not account forparticles which, after a partial effusion step, re-diffuse into the material. In practice, this conditionshould stand for volatile elements with desorption enthalpy ∆Hd < 5 eV, but rarely applies to morerefractory elements, whose sticking times could be much longer. However, the probability of re-diffusion should be hindered by the higher surface diffusion likelihood which would keep the atomavailable on the surface for desorption, just on a neighbouring site as discussed in 1.3.A similar analytical approach for describing element release in the RIST target [148, 149, 150],made up of thousands of annular thin-foil tantalum discs, was presented by Bennett [151]. He sug-gests applying the second Fick’s law not only for the diffusion step inside the solid foils, but alsoto model the effusive flow in the macroscopic voids of the target container and transfer line. Sincethe discs are positioned progressively further away from the ion source (in the middle of the tube),the effusion step would take on average longer for a particle leaving from the end of the discs stack,rather than if it started from its middle disc. Bennett took this into account by integrating the effu-sion starting position over the target tube length, and proceeding with the convolution of diffusionand effusion similarly to equation 1.40 from Kirchner. These analytical models had played a majorrole in the first few decades of experiments on target material for ISOL, and have provided stan-dardized methods for comparing the release performance of different materials, microstructures andmacroscopic foil arrangements. Few other similar examples of analytical treatments were developedby Zhang et al. [152] and Liu et al. [147] at Oak Ridge.The need to further understand the release mechanisms inside ISOL targets and to optimize theirgeometries for maximizing release has been more and more possible with the growth of Central Pro-cessing Unit (CPU) power which allowed the development of Monte Carlo methods. Densham et al.[103] developed a probabilistic approach to describe separately the diffusion and effusion processes,and combine this information to obtain the release time distribution. The diffusion distribution func-tionpD(t) =8piDd2∞∑n=0exp(−pi2D(2n+1)2td2)(1.44)returns the probability that particles homogeneously distributed in a thin disc of thickness d diffuseout of the disc at time t, where D is the particle diffusion coefficient. By sampling this function,the value tD can be extracted which represents the diffusion time for one of the particles inside thedisc. The average sticking time (τs) for a particle on a particular surface can be calculated usingthe equation 1.31. As far as the effusion stage, the authors built a 3D geometrical model of theRIST target and implemented it within a Monte Carlo code which was purposefully written for thisscope. One particle at a time was generated on one of the disc surfaces, and its “free flight distance”across the geometry was recorded, until it escaped from the ion source orifice. Its travel time tE isthen computed from sampling its velocity from the Maxwell distribution [153], and added to thediffusion time tD and sticking time τs so that the release time is computed astR = tD+ τs+ tE . (1.45)40If the sampling operations and the 3D Monte Carlo effusion simulation are performed many times,the release distribution can be plotted and directly compared to experimental data from pulsed-beam facilities such as ISOLDE, providing a parameter-free approach to model release. This studyconfirms the accuracy of the sticking times obtained from Kirchner’s measurements [142, 154],but at the same time it overestimates diffusion coefficients measured by earlier offline studies fromBeyer [120] by up to several orders of magnitude.Following similar procedures laid out by Densham et al., several other examples of Monte Carloapproaches for modelling release were published. At Argonne National Laboratory, Mustapha andNolen adapted the particle tracking simulation code GEometry ANd Tracking (GEANT4) - com-monly used for nuclear physics simulations - to model effusion [104] and optimize the geometryof next-generation ISOL targets. To combine the diffusion and effusion components, a very similarapproach to Densham’s [103] was implemented. At Oak Ridge National Laboratory, Zhang andAlton developed a similar GEANT4 utility kit to show that the target being developed for the Ho-lifield Radioactive Ion Beam Facility (HRIBF) [147, 155] presented effusion times about two ordersof magnitude faster than the ISOLDE targets [156, 157]. A further step forward in the ISOL targettransport description is presented by Santana Leitner in his Radioactive Ion Beam Optimizer (RIBO)code [158, 159], a software which treats effusion with a Monte Carlo approach, and includes ioniza-tion and ionic transport processes for a complete computation of the ion yield into the RIB transportsystem. Moreover, it also allows modelling diffusion both analytically using Fick’s laws, and nu-merically for special cases such as thick pellets or non-homogeneous initial conditions. Lastly,another approach to model the full atomic transport in the liquid droplets LIEBE target was devel-oped at SCK CEN by Houngbo [160], and Egoriti et al. [161] similarly applied it to thin metal foils.Author Method Free param. Target materialCarraz et al. [116] A 0(∗) Metal powder pelletsKirchner et al. [101] A 2 (χ,τ f ) Metal structuresBennett [151] A 1 (E) Thin metal foilsDensham et al.[103] MC 0 Thin metal foils (RIST)Mustapha et al. [104] MC 0 Thin metal foilsZhang al. [157, 156] MC (Eff. only) 0 PelletsZhang et al. [152] A 1 (l) Thin pelletsSantana (RIBO) [159] MC (+A) 0 Thick pelletsHoungbo [160] A + MC 0 Liquid dropletsEgoriti et al. [161] A + MC 0 Thin metal foilsTable 1.4: Summary of the major particle release model and their characteristics. A = Ana-lytical models. MC = Monte Carlo models. (*) = Model developed for pressed powderpellets and relies on knowledge of ideal pore parameters.411.5 Thesis formulationThe core thesis of this manuscript revolves around the necessity of developing meaningful onlineand offline development programs, aimed at the understanding of transport phenomena in ISOLtargets. Both approaches are undertaken within this work and briefly outlined here.Chapter 3 presents the development of a new online ISOL target, a spallation-driven proton-to-neutron converter, developed by the author and successfully irradiated at ISAC-TRIUMF. Its mainfundamental objective is to enable accurate online release investigations by carefully controlling thethermal environment within the target. This was achieved by minimizing the proton beam powerdeposition into the target material, while still inducing nuclear reactions inside it using neutronsgenerated by a spallation-driven converter. This controlled thermal environment created the properboundary conditions to perform the systematic investigations presented in section 3.4, while moreis expected in the future. In addition, this target unit was also optimized to provide end userswith a RIB of neutron-rich fission products which are much cleaner from isobaric neutron-deficientcontamination than usual. This peculiarity, driven by the prevalence of neutron-induced fissionevents, is very appealing for end users and is a key reason for the prioritization of this developmentactivity at the facility level.The offline components of this thesis are presented in chapter 4, where transport phenomenain ISOL targets are investigated through a table-top setup that requires prior activation of targetmaterial samples in a proton beam. The chapter contains two main components. First, the releaseproperties of some ISAC target material samples were measured and compared in ways compatiblewith the literature references given in section 1.4.1. Secondly, a new experimental setup is proposedand successfully employed to study the macroscopic effusion phenomenon alone in ISOL targetswhich is, to our best knowledge, a first-of-its-kind. It was experimentally demonstrated how particletransport through effusion is affected by the geometry of the target material foils, which opens upthe possibility of further target optimization and benchmark of transport models.The procedure and experimental setup for carrying out these offline experiments were not availableat TRIUMF before this project started. The procedure to irradiate material samples, safely handlethem and carry out these experiments had to be developed from scratch by the author, as well as thedesign, assembly, commissioning and operation of the vacuum furnace test stand where all theseinvestigation took place, the CHI stand. These descriptions will be presented through chapter 2 and4.42Chapter 2MethodsThis chapter provides an overview of the main experimental and simulation tools which were usedin the following chapters 3 and 4.2.1 Yield station and its detectorsMost experiments at TRIUMF ultimately rely on the detection, identification and quantification ofspecific isotopes by looking at the α and β particles which are emitted by their decay as mentionedin equations 1.7 and 1.8. Moreover, the decay product nucleus from alpha or beta decays is usuallyleft in an excited state, for which the most common way to be de-excited is to emit a γ-ray withinnanoseconds and can also be considered a unique fingerprint of an isotope’s decay.One of the main purposes of the target and ion source department at TRIUMF is to operateISOL targets, making sure that the expected quantities of isotopes are extracted and delivered touser stations upon irradiation. The main tool used to characterize each target performance involvesthe extraction of its radioactive reaction products into an ion beam and sending it to the ISAC yieldstation [162], a part of the low-energy beam line section in the ISAC-I experimental hall which isequipped with a Faraday cup for measuring current intensity, as well as α , β and γ-ray detectors.Figure 2.1 shows a schematic representation of a vertical cross section of the yield station, where themass separated RIB is coming from the bottom and can be intercepted by an retractable Faraday cup.This is used to measure intensities of stable beams or with long half life if there is enough confidencethat there is only one major component to the beam. Since Faraday cups do not distinguish betweenatomic species, it is important to have awareness of what measurement one is making. Laser ionsources can help in this assessment: if the laser beam is tuned to ionize one specific isotope, its yieldcan be determined by measuring the full RIB current with lasers ON and subtracting the backgroundreading when the laser is turned OFF (by blocking it with an optical shutter) which allows in thebeam all other particles except the one of interest. The lower detection limit fo the Faraday cupemployed at the yield station is around ∼ 107− 108 ions/s, below which it is common practice touse a Channeltron [163], an electron multiplier device which serves the same purpose as a Faradaycup but with sensitivity range between 1 - 106 ions/s or slightly higher.43Figure 2.1: Schematics of the ISAC yield station, adapted from Peter Kunz’s slides.If the Faraday cup is not inserted, the Radioactive Ion Beam (RIB) can be deposited on analuminized Mylar tape which can be rolled to move the collected activity away from the centralsensitive region of the yield station. In order to send only a fixed amount of activity to the tape,a kicker magnet upstream of the yield station is used to allow a fixed bunch length to control theimplantation time. The kicker controller is also communicating with the data acquisition systemsoftware (MIDAS, developed at TRIUMF [164]) for the full detector system detector, so they canstart acquiring data when the bunch is being sent.2.1.1 Gamma spectroscopyThe most versatile detector is arguably the High Purity Germanium (HPGE) crystal (Ortec GMX40),used to perform gamma-ray spectroscopy and identify nuclides through their most available particleemitted, γ-rays. The detector is located just outside of the yield station, in air, and can be slid onrails either manually or through an engine controlled by the MIDAS data acquisition (DAQ) system.The main crystal is surrounded by an aluminum cup and positioned in a box in contact with the coldfinger, a copper rod that extends all the way to a Liquid Nitrogen (LN2) dewar that keeps the crystalat cryogenic temperatures.When high-energy electromagnetic rays interact with the Germanium atoms through either pho-toelectric, Compton or Pair production effects, a number of electrons-hole pairs are produced pro-portionally to the total energy of the incoming particle. Under the influence of an electric field,electrons and holes travel to the electrodes, resulting in a pulse that is split into a timing branchfor gating and separate signal with pre-amplification and pulse-shaping units. Subsequent digitiza-tion is performed with a 8-channel peak-sensing Analog to Digital Converter (ADC) (Caen V1785)[162]. The result of the digitization is a plot similar to figure 2.2, where a 152Eu calibration sourcewas used for energy and efficiency calibration. The objective, to characterize a sample implantedfrom the ion beam, is a similar energy spectrum of the emitted γ-rays which can then be used as afingerprint of the specific radioisotope(s) present.44Figure 2.2: Gamma spectrum recorded from TRIUMF’s 152Eu calibration source. The sharppeaks represent specific gamma rays corresponding to transitions between nuclear states.In addition, there is a broad background.Calibration procedure Energy calibration is carried out when one wants to find the linear orquadratic relation between each Analog to Digital Converter (ADC) channel number and an energyvalue. The use of one or more calibration source is needed to have reference peaks as benchmarksand carry out the fit between the two parameters which, for the case presented in figure 2.3a, resultedin an almost linear relationy =−5 ·10−7x2+0.6796x−19.231, (2.1)where the ADC channel number is shown in the abscissa and the peak energy is in the ordinate. Forthis calibration, three sources were used: 152Eu, 56Co and 60Co in order to cover a broad energywindow.Detection efficiency calibration has also been performed with this data set combining the threesources used for energy calibration. For this purpose, the net area under each peak needs to beextracted, and one out of two methods can be used to do this. TRIUMF’s ROOT-based analyzer toolrequires manual selection of the peak boundaries, and returns the integrated number of counts ineach peak, corrected by a linearly-interpolated background. Alternatively, a more user friendly soft-ware called InterSpec developed at Sandia National Laboratories, USA [11] uses peak-recognitionfeatures to automatically fit peaks, saving some time for every peak analyzed and providing a handycomma separated value (.CSV) report of all properties of peaks in the file. Once the number ofevents in each peak is calculated and converted into counts per second Cm, the detection efficiencyat that peak energy is given by εHPGe =Cm/Ca where Ca is the known activity of the whole calibra-tion source at that time. The efficiency of each peak can then be plotted against their energy, to fit45(a) HPGe energy calibration(b) HPGe efficiency calibration curveFigure 2.3: These plots only pertain the efficiency of the HPGe spectrometer in detecting γ-ray photons. The plot on the right includes in red the fitted efficiency curve εHPGe shownin equation 2.2.an efficiency curve for a broad energy window as shown in figure 2.3b, where the fitted equation isεHPGe = aEb exp[−(Ec)d]. (2.2)Measurement of activity or RIB yield Once the yield station gamma detector is calibrated, it canbe used to detect γ-rays originating by a nearby source. To investigate offline release propertiesof material as described in chapter 4, activated samples were mounted at a fixed distance from thedetector and the data acquisition was started for a time period between ∼10 minutes and few hours.Alternatively, if an online experiment was performed, the RIB would be implanted on the yieldstation tape which is positioned at least ∼15 cm away from the HPGe detector. By accuratelytuning this collection time tcoll and recording the number of events in a certain peak recorded by thedetector, the yield of each isotope can be calculated. The formuladNdt= IRIB−λN (2.3)describes the evolution of the number N(t) of radioactive isotopes implanted on the yield stationtape, where IRIB is the RIB current, λ is the isotope decay constant, and its solution isNon(t) =IRIBλ(1− e−λ t). (when beam is ON) (2.4)After the RIB implantation is interrupted at time t = tcoll , the number of radioactive isotopes ofinterest on the tape during the subsequent decay time tdec isNo f f (t) =IRIBλ· (1− e−λ tcoll ) · e−λ t . (when beam is OFF) (2.5)The total number of decay events on the tape within tdec can be calculated by integrating the activity46A(t) through the decay time tdec, see equation 1.3:∫ tdec0A(t)dt =∫ tdec0λNo f f (t)dt =IRIBλ(1− e−λ tcoll )(1− e−λ tdec). (2.6)The number of counts measured by the detector under a single gamma peak isCm = ε ·BR ·∫ tdec0A(t)dt (2.7)where BR is the branching ratio of that specific γ-line and ε is the detector efficiency at that energyE. Hence, merging equations 2.6 and 2.7 the RIB intensity IRIB or yield can be calculated asIRIB =Cm ·λε ·BR · (1− e−λ tcoll ) · (1− e−λ tdec) (2.8)2.1.2 Beta scintillator detectorsFour plastic scintillators (Saint-Gobain BC-408) are arranged around the implantation spot on thetape and usually implemented as electron/positron scintillation detectors. When a light charged par-ticle interacts with these materials, the electronic configuration of the scintillator molecules changesfrom a lower-energy singlet state to a higher-energy triplet state, after which de-excitation happensthrough fluorescence, a phenomenon releasing visible and Ultra Violet (UV) light. These photons arethen sent via plastic light guides to photo-multipliers (Hamamatsu H3165-10) outside the vacuumchamber that convert a light signal into an electronic signal, then processed by a VME multichannelscaler (SIS3820) which acts as particle counter, providing an integer number of events recorded ineach time bin.The information is stored and analyzed with a processing tool that fits the measured data pointsover time with the solution of the Bateman equation 1.4 graphically shown in figure 2.4. It is theexperimenter’s responsibility to select the appropriate fitting parameters (i.e. isotopic half lives andfirst-guess values) depending on the expected isotopes in the extracted mass range. If the properinputs are given, a pre-exponential coefficient for the isotope of interest N0(0) is returned by thefitting routine, so that the yield (IRIB) can be calculated with equation 2.8. In this case, ε is the scin-tillator geometrical efficiency (can be ∼50% or ∼5% depending on which one of two scintillatorsare used), tdec is the decay time and Cm corresponds to N0(0) for the isotope of interest.47Figure 2.4: Experimental data and fitting curve for isotope 145Cs (T1/2 = 0.587 s), where thecontributions of its daughter 145Ba (T1/2 = 4.31 s) and 145La (T1/2 = 24.8 s) are shown asdark red, bright red and blue lines respectively.2.2 Thermal equipmentISOL targets at TRIUMF are heated to nominal temperatures around∼1600◦C in SiC up to∼2300◦Cfor some areas of tantalum metal foil targets. In order to reproduce these environments, test standsare built with the proper equipment and services. This section gives an overview of two such piecesof equipment where release tests at high temperatures were performed (CHI) and where some pro-totypes of proton-to-neutron converter targets were tested.2.2.1 CHI stand furnaceThe Chamber for Heating Investigations (CHI) stand is a vacuum furnace designed and built by theauthor and Mia Au, a master student in our group at that time. Its main body consists of a custom-designed stainless steel six-way cross manufactured by MDC Vacuum, allowing the installation ofone of two modular experimental configurations:• Release setup: this configuration includes a water cooled resistive furnace, able to heatcentimetre-sized samples up to ∼2000 ◦C which is mainly used for isotope release experi-ments described in detail in this chapter;• Electron gun setup: this configuration allows for testing thermal properties of actinide ma-terials by bombarding samples with an electron beam with a predefined time structure whilemeasuring its temperature with a pyrometer. Since this was the main thesis topic of Mia Au[165], it will not be described further here.Both setups are mounted on a ConFlat® flange which can be coupled to the vacuum chamber flange,by positioning either one of them on top of the table on rails allowing for an easier sliding motionduring sample installation and retrieval, hosting water and electric feedthroughs to deliver theseservices to the inner vacuum components. The rendered 3D model and a picture of the test standshown in figures 2.5a and 2.5b also show the custom-built glove box which allows for handling48(a) Picture of CHI stand (b) Rendered CHI stand imageactivated samples in a non-restricted TRIUMF area (ISAC-I experimental hall). Two pairs of glovesare installed on either of the long sides of the long acrylic panels allowing up to two people at a timeto collaborate in the sample handling procedures, while the front panel is connected to the sampleflange by means of a latex rubber rolled sheet. The box panel facing the vacuum chamber is alsoconnected to it by means of a latex rubber sheet, glued to the acrylic and sealed to the chamberthrough a pressed silicone o-ring. This glove box behaves as an accordion when the sliding sampleflange is moved on its rails, so that the atmosphere in the experimental hall is never in contact withthe glove box nor the vacuum chamber atmosphere. In picture 2.5a can be noticed the presenceof water hoses (in red) and electrical cables (in black) which are connected to the inner vacuumcomponents using rated feedthroughs, and on the background the blue rack hosting several powersupplies is visible. A LAMBDA EMS 10-500 with output rated up to 10 V and 500 A is used forresistively heating components for both setups, while a XP GLASSMAN KR5R600 provides highvoltage up to 5 kV and 600 mA to accelerate electrons and heat samples through bombardment.The high vacuum atmosphere is provided by a LEYBOLD TMP361c turbo pump providing pumpingspeeds of∼300 l/s, and works in series with a dry Adixen-ACP15 backing pump to provide pressuresas low as 10−7 Torr.2.2.2 ARIEL Thermal Test Stand (ATTS)The ARIEL Thermal Test Stand (ATTS) is also a vacuum steel furnace, built at TRIUMF to performa variety of target thermal tests and shown in figure 2.6. Its inner dimensions 50x50x50 cm3 enablethe installation of a variety of the high-temperature components used for RIB production. Twopairs of water-cooled pipes can deliver a DC current up to 2400 A to one pair of copper electrodes(positive and negative polarity), equipped with features to bolt the target components for testingonto them. A turbomolecular pump with pumping speed of 1000 litres/s is bolted to the top flange,49Figure 2.6: Rendering of the ATTS stand with an ARIEL target installed in it.and is capable of evacuating the box down to pressures in the 10−7 Torr range. Two thermocouplefeedthrough flanges enable measuring temperature with up to four type-K and type-C thermocouplessimultaneously (see next section for details), while three glass windows located on different surfacesallow for visual access to the inner components for inspection and pyrometer measurements.2.2.3 Temperature measurement methodsThe strong dependence on temperature of release processes calls for an accurate understanding andbenchmark of temperatures during offline and benchtop tests, where measurement devices can beused to experimentally verify the expected temperature values from simulations or estimations. Thetwo main tools used to measure temperatures are through thermocouples and pyrometer.Thermocouples Thermocouples working principle is the Seebeck effect, involving the creation ofan electromotive force emf when two dissimilar metals are put in contact at a point referred to as thehot junction and heated to a certain temperature. At the other end of both wires, referred to as thecold junction, the emf will generate a potential difference ∆V (with a range of few mV) that can bemeasured by a volt meter and proportional to the temperature difference ∆T between hot and coldjunction through the Seebeck coefficient defined asCs =∆V∆T(2.9)In our experiments and tests, two different kind of thermocouples are used, both purchased fromOmega Sensing Solutions ULC [166]: Type-K thermocouples leads are made out of wires of Ni-Cr50(positive lead) and Ni-Al (negative lead), and can measure temperatures in the range (-200◦C to1250◦C). These thermocouples are relatively cheap (a few dollars each) and could be repeatedlyused for a handful of times each to perform offline temperature measurements, i.e. in the P2Nconverter tests presented in chapters 4 and A. Silver conductive thermal paste from Alpha Aesar[167], was used to improve the contact resistance between the component and the thermocouple hotjunction, especially if the temperature was to be measured in a drilled hole. For other uses on flatsurfaces not exceeding ∼400◦C, the thermocouple was usually taped with either electrical or Kap-ton ® tape in case the test was performed in air or vacuum respectively. Temperature measurementswere for example run to make sure that the CHI stand copper components close to the heater wouldnot get close to melting temperatures, see chapter 4.Type-C thermocouples are made out of refractory alloys tungsten(5%)-rhenium (positive lead)and tungsten(26%)-rhenium (negative lead), allowing extreme measurements in the range (0 to2320◦C). These high temperatures are usually reached in vacuum in resistively heated tantalumparts making up ISOL targets, and in offline test components to reproduce similar behaviour. Sinceconductive paste and tape is not possible due to vacuum and temperature constraints, the hot junc-tion has been at times precisely spot welded or crimped to the component in order to maintain properthermal contact. In enclosed bodies, such as tantalum cylinders with no aperture to the outside otherthan a small hole, thermocouples have been inserted through this hole by embedding them first in ahafnia sleeve (HfO2) to prevent touching the tantalum component and shorting it. The average tem-perature of inner components that are not in direct contact with the cylinder walls is then measuredfrom the black-body thermal radiation emitted.Although thermocouples provide a consistent temperature measurement without any free pa-rameter to manually input, they are not always the best choice, for example where the attachment isnot easy, where temperature measurements are to be taken in many points during the same exper-iment or when their thermal inertia would significantly affect the measurement. In these cases, anoptical sensor might be more favourable and provided the needed flexibility.Pyrometers Pyrometers are optical sensors capable of providing a temperature measurement of abody by reading the intensity of light emitted by the body at certain wavelengths.Every body at temperatures higher than absolute zero emits some thermal radiation. Ideal emit-ters are called black bodies, which emit electromagnetic radiation in discrete packets, called quanta,so that their spectral radiance follows Planck’s radiation law where the energy per unit volume perunit wavelength range is expressed as [168]p(λ ,T ) =8pihcλ 51ehc/λkBT −1 (2.10)where h is the Planck constant, c the speed of light, λ the light wavelength, bB the Boltzmann con-stant and T the body temperature on the absolute Kelvin scale. The total emissive power densityemitted by a black body is expressed as eb = σT 4 by the Stefan–Boltzmann law, where σ is the Ste-51fan–Boltzmann constant. For real parts that do not behave as black bodies and are instead partiallyreflective, their emittance or emissivity ελ is defined as the ratio of their emissive power comparedto the emissive power of a black bodyελ (λ )≡eλ (λ ,T )eλb(λ ,T )(2.11)so that their emitted power density is expressed aseg = ελσ(T 41 −T 42 ), (2.12)where T1 and T2 are the temperature of the body under consideration, and of the environment aroundit, respectively.The METIS M311 pyrometer from Process Sensors corp. was used in high-temperature mea-surements in chapter 3 and 4, and uses a dual-wavelength technique to measure the temperature of abody. It measures the intensity of the light signal received at two infrared wavelengths (0.99 µm and0.78 µm for the specific model used), and based on the knowledge of the emissivity ratio betweenthe intensities at these two wavelengths and the Planck spectrum in equation 2.10, the pyrometercomputes the temperature of the material.An extensive campaign conducted by co-op student Cassidy Donaldson at TRIUMF was setup in order to experimentally determine the emissivity slope [169]. This approach relies on tightlywrapping a piece of wire of a specific element around a tantalum resistor strip, where resistivecurrent is flown through. The pyrometer is pointed at a central area of the tantalum material to readits temperature while the Ohmic current is increased progressively, until the wire is seen melting.At this specific point, the temperature of the tantalum strip and wire must be equivalent to themelting temperature of the strip, and the emissivity slope is adjusted until the pyrometer readingmatches such value. Wires of Vanadium (Tm = 1910◦C), Pt90/Ir10 alloy (Tm = 1800◦C) and Silver(Tm = 962◦C) were used for the benchmark, so that a slope value was obtained on a fairly broadtemperature range. The slope values at the two higher temperatures were close to 0.93, hence thisnumber was used in all pyrometer measurements when tantalum was employed, and its temperaturewas expected to be approaching 2000◦C.522.3 Sample activation in the IPFIn the context of this thesis it was necessary to find a facility at TRIUMF where samples of severalmaterials could be activated and retrieved in a relatively short time to perform offline release ex-periments described in chapter 4. It is ideal if the same proton beam used for irradiation of ISACtargets is also used for these investigations, however the ISAC target station is not suited for rapidsample retrieval. The Isotope Production Facility (IPF) is the only location where samples can beroutinely irradiated with a proton beam whose characteristics are effectively equivalent to the ISACtarget stations. For reference, figure 1.6 shows the IPF facility location on Beam Line 1A (BL1A)Figure 2.7: Top left: cut away view of an IPF target. Top right: picture of the hot cell duringthe retrieval of a target. The target is handled through a single stick manipulator, while inthe background one can notice the drive chain assembly. Bottom: layout of the IsotopeIrradiation Facility.where protons arrive after interacting with muon production targets T1 and T2. The proton energy isdegraded to an average of 450 MeV while its intensity can drop down to 50% the injected quantity,however the T1 and T2 targets are usually retracted during these test irradiations to improve theoverall proton transmission. The lower portion of figure 2.7 gives an overview of the IPF facility53itself where the important components for the experiment are:• IPF targets are vacuum-tight containers inside which material samples can be inserted to belater irradiated, see top left image in figure 2.7. They were designed by TRIUMF’s commer-cial partner for medical isotope production Advanced Technology Group (ATG), and wereadapted to be reusable. Their steel windows and outer frame can hold rough vacuum pres-sures thanks to C-seals and rubber O-rings so that material samples within them would avoidcorrosion in air.• The hot cell itself is the shielded environment where IPF targets are loaded and extracted froma port in the back, and installed on the drive chain assembly using a single stick manipulator;• The chain drive assembly is the mechanism that can move vertically the targets from the hotcell to the beam line location, 9 m below.The IPF targets can be manually loaded with material samples as shown in figure 2.7 (top left).The air trapped inside the target is evacuated from a side port to maintain a rough vacuum aroundthe samples; this suction force pulls the thin beam window inward, touching the front face of thefirst sample loaded, providing a heat removal path. The loaded targets can be inserted in the hot cellthrough a port in the back, mounted on the chain drive and driven down to their irradiation locationin line with the proton beam coming from BL1A. These IPF targets are submerged in a water bath,with forced flow directed at them to convectively remove the heat deposited by the proton beam intothe samples and target components.Proton beam irradiations can be carried out with current intensity Ip ∼5 µA for a time length ∆tof about 5 minutes for this specific application, and extensions of either parameter can be requestedif safety precautions and procedures are in place. Once the irradiation is finished, all steps arerepeated in reverse to remove the IPF targets out of the hot cell. All operations interfacing the hotcell are carried out by trained and certified technicians, while once the target is retrieved, it is theexperimenter’s responsibility to handle it properly. Since their outside surfaces are contaminatedwith activated water and other particulates, the targets are decontaminated using water, soap and anultrasonic cleaner, and are then carried into a fume hood for opening. Once the sealing screws areundone, the front face of the assembly is removed and the inner samples can be retrieved and carriedaround in bags.For each material sample, several parameters define how much activity is produced in it byirradiation:• The proton beam fluence (Φp): this quantity is the product of the proton beam current Ip andthe total irradiation time ∆t. The activity of a “long-lived” isotope will be proportional to Φpif the condition ∆t T1/2(i) is valid, where the second term corresponds to the half-life ofthe isotope i under study - and this is usually the case.• The number of atoms of interest per unit area Nk making up the sample. For samples madeup of compounds, only one atom is usually important to trigger the wanted reaction, such54as uranium atoms in its carbide compound. Moreover, these thin targets do not significantlydegrade the energy spectrum of ∼500 MeV protons, hence the particle intensity throughoutthe sample thickness remains the same.• The nuclear reaction cross sections σk(E), which depend on the energy (E) of the protonbeam and the elements (k) in the sample.The quantity of a decay product isotope Ni produced just after irradiation can be written asNi(t = 0) =∑k∫∆tσk(E) · Ip ·Nk dt (2.13)when the assumptions stated above are valid. In case where the nuclear reaction produces both theisotope of interest N2 and its parent N1, the quantity of N2 over time is described by a combinationof the solutions to the Bateman equation:N2(t)︸ ︷︷ ︸N2 over time= N2(0) · e−λ2t︸ ︷︷ ︸Decay of N2 produced by beam+N1(0) · λ1λ2−λ1 · (e−λ1t − eλ2t)︸ ︷︷ ︸Decay of N2 generated from N1 decay(2.14)where N1(0) and N2(0) are the initial production quantities of the daughter and parent isotope de-scribed in equation 2.13.The same equations and considerations are valid for the nuclides created inside the IPF targetcontainer metal structures such as windows, holder and frame, whose mass was minimized duringthe design process to lower their activity after irradiation. Their activation is the main contributingfactor to the external dose received by experimenters. The FLUKA Monte Carlo code was used tocompute the nuclear reaction products in several target materials, by implementing the IPF geometryas shown in 2.8 including all relevant elements of the station such as the material samples of interest,target container and the water regions. The isotope inventories extracted from simulations haveproved very important to assess the feasibility of this study and to foresee the radiation exposurethat experimenters would receive. The doses received by handling radioactive material should beminimized according to the As Low As Reasonably Achievable (ALARA) principle, knowing thatthe threshold dose that a Nuclear Energy Worker (NEW) at TRIUMF can receive in a single day totheir body is 0.5 mSv. Plots in figure 2.9 are important to assess the dose rates (in mSv/h) received bythe experimenters, which coupled to estimations of the time needed to clean one of these containers(about 1 hour each) can give an initial estimation of how much cooling time one has to wait beforestarting to work with them, in order to receive a body dose much less than the TRIUMF daily limit.It was established that the handling operations of the whole IPF container could begin only if thedoses from containers are lower than 1 mSv/h at 10 cm distance, and lower than 100 µSv/h at 50 cm,corresponding to an estimated wait time of at least 8 hours after End of Beam (EOB). More detailsare available in the internal TRIUMF safety document [170].55Figure 2.8: Top view of the IPF station FLUKA geometryFigure 2.9: External dose-rate equivalent from a target loaded with 10 tantalum foils of com-bined thickness 0.254 mm.2.4 Simulation toolsSome of the main components that were designed in this thesis to achieve scientific goals neededto be carefully vetted and optimized to make sure they would be able to carry out their intendedpurpose. The starting point for any initial consideration on a design always passes through a back ofthe envelope calculation in order to obtain an intuitive order of magnitude estimates for the processor component to be optimized. The next stage has been to start a single simulation or a full campaigndepending on the objective of the study. Two main broad simulation tools have been adopted. First,the Monte Carlo code FLUKA [26] was used to simulate particle behaviour when impinging ontotargets, and several quantities of interest were scored, such as in-target production, power densitydeposition, prompt and residual dose from an irradiated component and particle fluence. Second,several modules of the Finite Element Analysis (FEA) code ANSYS Workbench [171] were used tosimulate distributions in the domain of several variables such as temperature, volumetric resistiveheating, stresses and strains. Some tasks required iterations between the two codes, or between two56ANSYS modules, in order to pass variables and results to the next iterative step.2.4.1 The FLUKA Monte Carlo softwareThe Monte Carlo FLUKA code has been developed by a core group of developers based in particleaccelerator labs, with the aim of building a realistic, semi-open and supported platform to enablereliable particle-tracking simulations [26]. Frequent updates are rolled out by the development team,and most of the results presented in this thesis were obtained with version Fluka2020.0 or earlier.As all other particle-tracking Monte Carlo models, the FLUKA code attempts at solving theBoltzmann transport equation for a large number of primary particles, and then integrates all resultsto reduce statistical uncertainties in each result. The general equation can be written in an integro-differential form in phase space coordinates (~r,~Ω) abbreviated as psc [172, 26]∂N∂ t︸︷︷︸1)time= Q︸︷︷︸2)source− v~Ω∇N︸ ︷︷ ︸3)part. flux− σvN︸︷︷︸4)react.+∫∫σ ′v′N′ f (~r;(~Ω′,E′)→ (~Ω,E))d~Ω′dE′︸ ︷︷ ︸5)particle generation in space phase(2.15)where ~r is the particle position vector, ~Ω is the unit vector in the direction of motion, E is theparticle energy considered, t is the time instant considered, v is the absolute value of the velocityof the particle, N ≡ N(~r,~Ω,E,t) represents the particle angular density, σ represents a reaction crosssection, Q ≡ Q(~r,~Ω,E,t) represents the particle production rate density from external sources, f isthe probability density function that a particle in~r with direction ~Ω and energy E ′ will undergo aninteraction with probability σ ′ so that its direction and energy after the event are (~Ω,E). Each termin the equation has a specific meaning, i.e. 1) the time variation of the number of particle N foreach phase space variable is equal to 2) the production rate from an external source of particles withthose psc, minus 3) the spacial flux of particles out of those psc, minus 4) the disappearance rate ofparticles from the psc due to nuclear reactions of any kind plus 5) the creation rate of particles in thepsc due to their reactions from other (prime ′) psc. The likelihood for any nuclear reactions to occur,including simple elastic scattering, is contained in probability density functions which are evaluatedusing random number generators to estimate e.g. scattering angles, interaction lengths and natureof the outgoing particle.Practically speaking, all calculations happen behind the scenes in a relatively inaccessible envi-ronment built and maintained by experts in the area based at CERN, at the Italian National Institutionof Nuclear Physics (INFN) and several other research and academic institutions. Users have to inputinformation as a FLUKA input file, containing:• Beam properties, such as beam size and particle type are required; beam intensity and irradia-tion time are instead needed only if cumulative properties such as residual dose are requested.• Geometry and materials: building a representative geometry for one’s simulation is usuallythe most time consuming part of the whole set up, since the structures cannot easily be im-ported from CAD tools, but need to be built by combining basic geometrical shapes such as57infinite cylinders, infinite planes, cubes and spheres using combinatory logic statements. Anykind of material can be assigned to each of the 3D regions created.• Physics packages. Unlike other simulation tools, there are only a limited number of physicspackages that can be used in FLUKA, preventing users from implementing their own physicsfor the sake of consistency. The new evaporation model, with heavy fragment evaporation,the coalescence and electro-magnetic dissociation models are added to all the inputs, in orderto get a precise estimation of isotope production through nuclear reactions, i.e. our main goal.Additionally, photonuclear interactions are activated between nuclei and γ-rays, electrons andpositrons. As far as transport, the group-wise treatment of neutrons is always activated downto 10−5 eV, the photon transport thresholds are set to 100 keV and to 300 keV for all otherparticles, as specified in the defaults.• Detectors are the objects added to a simulation in order to score quantities of interest, such aspower density deposition, isotope production rates in specific regions, particle fluxes, residualdoses after the beam has been shut off, and many other possible quantities that can be addedto the list. The output files are usually formatted in table form and are not easily read by thehuman eye. They can be processed into plots within FLAIR, FLUKA’s graphical user inter-face, or by exporting and processing them with user-written code. A relevant and importantexample is the export of the USRBIN detector, where power deposition density can be scoredin units of [GeV cm−3 primary−1]. It can then be processed by an external code to turn theoutput into a usable .txt file with units of [W cm−3 s−1], that can be imported as input file intothe ANSYS thermal module to simulate temperature profiles.• Biasing information (if needed). Weights can be applied to some areas in the geometry,making it artificially more likely for particles to reach and interact in certain areas. Thisbiasing technique is used to improve the statistics in areas which otherwise would not befrequently populated by the algorithm.2.4.2 The ANSYS Workbench FEA softwareThe Finite Element Analysis (FEA) method is used to approximate the exact solution of a contin-uous mathematical problem by the superposition of several test functions on subsets of the initialdomain, called elements, in order to satisfy an equivalent version of the initial problem. The varioussets of initial differential equations are written in their integral (weak) form, where the variable is re-placed by a linear combination of test functions weighted by coefficients. The system of differentialequations is then turned into a linear algebra problem where the aim is to solve for these coeffi-cients, and after having found a solution can be evaluated beside their test functions to visualize theapproximate solution through the domain [173].The geometries imported were all created within the SolidWorks [174] software, edited to avoidfeatures such as singularities and penetrations which could affect the solution stability, and then58imported into the ANSYS Workbench environment [171]. Three main classes of problems weresolved in the following chapters: purely thermal, thermal-electric and structural. For all of them,the solver choice was left program controlled, which usually resulted in the automatic choice ofa sparse matrix direct solver for the first two cases, and in a distributed preconditioned conjugategradient for the structural module. For each major simulation result presented in this thesis, asummary of the main boundary conditions and inputs is given in the text.One important feature that is key for high-temperature problems is the surface-to-surface ra-diative exchange, which is at times implemented in thermal target heater simulations to capturetemperature profiles in a more realistic way. When two hot surfaces are placed close to each other,the local thermal radiation emitted from every point of their surface radiates with a certain angulardistribution and could be partially absorbed by the second surface. The surface-to-surface featureis an algorithm that computes the view factors between points of the two geometries and enablesthe radiation heat flux calculation between nodes. This feature was frequently included in thermal-electric simulations, especially on the inner surfaces of cylindrical components that shine on eachother. When this feature is applied to outer surfaces facing “the environment” at a constant tem-perature, it was found much harder for these simulations to converge. In these latter situations, theboundary conditions are usually set for emission only, according to equation 2.12, without account-ing for any incoming heat flux, which is a reasonable approximation when the ambient temperatureapproaches 300 K or lower.592.5 TITAN MR-ToFThe TITAN MR-TOF is an instrument which was used during the online experiment of the proton-to-neutron converter (chapter 3), hence this paragraph provides some background on how the instru-ment works and its capabilities.The TITAN experiment is one of the busiest experimental user facilities in the ISAC halls, whichreceives RIBs without any post acceleration with the intent of precisely measuring masses of ra-dionuclides. Its experimental apparatus is made up of few major components including an RF cool-er/buncher that takes a continuous beam and restructures it in time as a pulsed beam, an ElectronBeam Ion Trap (EBIT) to breed the ions to higher charge states (2+, 3+, · · · ) and a Cooler PenningTrap (CPET) to lower the kinetic energy of higher charged states. The final measurement device isa Measurement Penning Trap (MPET), performing precision mass measurements employing a sta-ble magnetic field coupled to an RF field which excites the cyclotron frequency of these ions. Theinteraction between these fields and the nucleus under study enables measuring their mass m withprecision on the order of ∆m/m∼ 5 ·10−8 through the equationνC =q2pimB (2.16)where q is the ion charge, B the magnetic field intensity and νC the cyclotron frequency [175].More recently, a Multi Reflection Time-of-Flight (MR-TOF) separator was also installed be-tween the RF cooler-buncher and the MPET, now an integral part of the apparatus [176, 44]. Thisinstrument has the outstanding capability to separate isobaric contaminant from the radioactive ionbeam of interest and selectively send them to the rest of the TITAN apparatus thanks to its massresolving power on the order of ∆m/m∼ 2.5 ·10−6. In fact, the ISAC mass separator is not capableof separating isobaric species due to its limited ∆m/m = 2000, so most measurements would beotherwise heavily contaminated by other species. Moreover, the MR-TOF also enables measuringmasses with unprecedented resolution for some very exotic radioactive beams even without sendingthem to MPET. This separator consists of two electromagnetic mirrors, spaced by a distance l, whichallow a packet of isobaric nuclides to be reflected back and forth for a number of turns nr, so that intotal they would cover a flight distance d f = nr · l. Having the same energy, lighter nuclides wouldtravel a bit further following the relationmq=2Ueff(l)l2t2tof (2.17)between their time of flight ttof, mass m, charge q and effective electric potential Ueff(l). However,even the MR-TOF has an upper boundary on the maximum RIB intensity that it can accept (∼ 106ions/s). This means that for RIB with high isobaric contamination, only a fraction of the RIB canbe delivered to the experiment, effectively reducing the number of ions of interest and increasingthe experiment length. This consideration further highlights the importance of beam purity in thecontext of the proton-to-neutron converter presented in chapter 3.60Chapter 3Development of a spallation-driventwo-step radioisotope production target3.1 Introduction and motivationThis chapter describes the development carried out in order to irradiate the first spallation-driven,two-step target at ISAC-TRIUMF, which is also commonly called a proton-to-neutron converter inthe ISOL field. Its design is radically different compared to all targets irradiated so far in ISAC forthe past ∼20 years, offering a new set of possibilities to investigate transport phenomena in ISOLtargets and to produce purer neutron-rich Radioactive Ion Beams (RIBS).3.1.1 Historical backgroundIn this section I review the previous milestones in ISOL target development that led to starting thisproject. The very first RIBS were produced at the Niels Bohr Institute in Copenhagen by a two-stepprocess, through the fission reactions within 10 kg of UO2 - the target - induced by neutrons [19].These neutrons were themselves produced by the interaction of a deuteron beam with a berylliumtarget - the converter - and then thermalized in paraffin before reaching the UO2 mass. To Kofoed-Hansen and colleagues, this methodology was enough to produce the required quantities of Krisotopes for their studies of recoils from beta decays [18]. However, at this time, nuclear instrumentswere not in an advanced state, hence the quest to produce radioisotopes for different experimentalstudies did not take off until the sixties. During this decade, advancements in nuclear spectroscopyand theoretical improvements in the understanding of nuclear structure, were fundamental driversfor pushing the discovery and experimental investigation of more and more isotopes [20]. Fromthis renewed interest, the ISOLDE facility was born at CERN with the intent of continuing thedevelopment and production of radioactive isotopes. In the first three years of operation, alreadyfifty targets were irradiated online, and isotopes from twelve different elements were extracted withthe ISOL method [20]. Among these, one molten lead target was effectively used as the secondproton-to-neutron converter in ISOL history.61Molten lead targets have been widely used since the early times of ISOLDE for the productionand extraction of mostly mercury isotopes [177]. The low melting point of lead (Tmelt = 327.5 ◦C)makes it a perfect candidate for being operated in its liquid state as an internal proton-to-neutronconverter [178] for the purpose of studying the 207Hg → 207Tl decay. At first, 600 MeV protonsimpinge on the molten lead, inducing nuclear reactions such as Pb (p,3pxn) Hg which give offprotons, neutrons and several isotopes. These neutrons can interact themselves within the samelead bath to produce the secondary reaction 208Pb (n,2p) 207Hg, which undergoes the 207Hg −→β−+207Tl decay under study with a half life of T1/2 = 2.9 minutes.In an effort to decouple the high primary beam power deposition in the ISOL target, a novelconverter-target concept was proposed by Nolen [179]. He suggests the use of a primary acceleratordelivering 200 MeV, 0.5 mA deuterium beam to a light converter made out of Li, Be or graphite andproduce forward-peaked neutrons mostly through the following reactions:• 7Li (d,2n) 7Be; 7Li (d,n+α) 4He; 6Li (d,n) 7Be [180]• 9Be (d,2n+d) 7Be; 9Be (d,n+t) 7Be;9Be (d,3n+p) 7Be [181] used by Kofoed-Hansen• 12C (d,n) 13N; 12C (d,n+d) 11C;12C (d,2n+p) 11C [181]The primary particles are fully stopped in the converter, depositing a large fraction of their 100 kWbeam power, which can be removed by an active cooling system. The outgoing neutrons interactwith the downstream uranium carbide ISOL target, where temperature homogeneity can be moreeasily achieved.Around the turn of the 21st century, the demand for neutron-rich radioisotopes increased [182],and ISOLDE responded with some initial studies and successful tests of their proton-to-neutronconverter targets [6, 183]. This assembly consisted of a regular UCx target where a tungsten ortantalum rod was installed underneath the target itself, as shown in figure 3.1, where spallationreactions on these heavy metals (with Z=73, 74) generate a number of neutrons emitted isotropically- see introductory section 1.1.1. These converter materials are more efficient at producing neutrons,up to ∼8 neutrons for each incoming proton, with respect to the deuteron-induced reactions abovewhich produce a maximum of three per incoming particle [25]. The proton beam at ISOLDE canbe steered from the centre of the target, to the centre of the converter. This allows extra flexibilityfor deciding which proton beam position is the best for the current isotope to be extracted. Initially,the bottom converter was installed as an add-on to the target, and protons were sent on the converterpart only towards the end of the beam time to reduce the level of neutron-deficient isotopes whichcontaminate exotic, neutron-rich beam desired by particular user experiments. Eventually, theseassemblies were operated more and more frequently as dedicated converter assemblies to extractmore pure neutron-rich beams.Different configurations of this assembly were irradiated online, and experience was gained bythese tests. It was noticed experimentally and proved through Monte Carlo simulations that tungstenperformed better as converter compared to tantalum due to its lower resonant neutron absorptioncross section [24]. Moreover, post-irradiation examinations of the target assembly showed that the62(a) Before irradiation (b) After irradiationFigure 3.1: ISOLDE target converter unit. The right figure shows the detrimental effect of 1.4GeV protons on a test tantalum converter rod used for target UC2-C-183 [6].tantalum converters were not withstanding the shocks and thermally-induced stresses induced bythe proton beam, creating electrical issues during their operation. Most importantly, these testsproved that it was possible to increase the purity of neutron-rich fission products by using thisconverter configuration. For some experiments which are done with these RIBs, the drop in neutron-deficient background is indeed the discriminator between a successful or failed experiment, andtarget development is necessary to achieve this goal.Several conceptual designs of future ISOL converter configurations were proposed, includingan annular target geometry surrounding a central tungsten converter body, first envisioned by Nolen[184]. The core idea of the annular geometry was the maximization of the view factor from theconverter body to the target, in order to increase the geometrical likelihood for the neutrons toimpact the target. This idea was subsequently backed up by following publications [185] and designstudies [186] [187] where annular targets (UCx or BeO for the production of fission products and6He respectively) are envisioned, at times surrounded by a BeO layer as neutron reflector material[188].An important milestone in the proton-to-neutron converter development for ISOL purposescame in 2012, when a realistic conceptual design was laid out by Luis et al. [189] and experimen-tally tested by Gottberg et al. [7]. One of the major goals of this prototype was to further reduce theneutron-deficient contaminants by suppressing the amount of protons which interact with the targetmaterial after scattering inside the converter body. The converter part was hence shifted towards theend of the assembly to minimize the impact of these scattered protons reducing the proton-inducedfission contribution as shown in figure 3.2. Moreover, the UCx target was shortened and its diameterwas increased to cover more of the neutron solid angle. This new geometrical arrangement wasquite successful in reducing yields from contaminating isotopes in the proton-rich region, resultingin the enhancement of the ratio between the desired species versus its contaminant up to a factor200 (for 80Zn/80Rb) and pointed to the need for a more thermally optimized design.This ISOLDE converter prototype from 2014 showed that neutron-rich RIB purity could indeed63Figure 3.2: Dedicated prototype of proton-to-neutron assembly tested at ISOLDE in 2014 [7]be enhanced for several species by up to two or even three orders of magnitude, which is of extremerelevance for experiments which require a very low background of isobaric contaminants. It alsoshowed that the absolute yields of the species of interest are lower than the standard ISOLDE-UCxtargets. This is partially due to the intrinsic inefficiencies in view factor between the converterand the target, which covers only a portion of the solid angle. Moreover, the lower operationaltemperature of the ISOLDE-2014 P2N target container (Tmax = 1750 ◦C) - caused by the limitedresistive heater current - slowed down the release of some species, especially the less volatile ones,with respect to standard ISOLDE targets where temperatures up to 2000 ◦C are reached [7]. In thishistoric context, I now move to our development of the proton-to-neutron converter at TRIUMF,which builds on all previous concepts and prototypes developed.3.1.2 Motivation for developing a proton-to-neutron converter at TRIUMFThe common interest of ISOL facilities for developing new targets drove the birth of the p2n col-laboration between TRIUMF, ISOLDE and SCK CEN for the development of two different proton-to-neutron converters to be tested in the first two facilities. ISOLDE wanted to leverage the positiveresults from previous converter configurations and develop a newer version which would be opti-mized for increasing the intensity of neutron-rich beams, while still reducing the contamination fromneutron-deficient isobars. This has resulted in a successful irradiation of a new converter prototypein 2018 [28]. On the other side, TRIUMF wanted to develop its first proton-to-neutron converterprototype which would be focused on improving the purity of neutron-rich fission products, whiledelivering similar yields compared to the ISAC-UCx standard targets which have been limited to amaximum proton beam intensity of 20 µA due to safety considerations.The success of ISOLDE prototypes [6, 7] confirms how many benefits it brought to the facility,in terms of delivering more pure neutron-rich isotopes. For TRIUMF, the advantages would notstop here, but have positive implications with respect to the high-power nature our proton beam,64and offers a unique opportunity to investigate release phenomena systematically.Neutron-rich pure beams ISAC users requesting a specific isotope for their experiments usuallyhope to have a minimum number of particles delivered to their setup in order to gather enoughstatistics during the limited beamtime allocated to them. Along with other important parameterssuch as beam energy and ionic charge, the presence of other isobaric species in the beam (usuallyreferred to as contaminants) can sometimes be a show-stopper for the experiment. The TRIUMFmass separator system, comprised of a pre-separator section followed by a high-resolution sepa-rator, has a resolving power of m/δ m = 2000 which is enough to distinguish between low massisobaric species (below ∼30 AMU) but cannot separate the higher isobaric components of the beam[190]. Most often, the contaminants are elements such as alkali and alkaline earth metals which arereleased from ISOL target with high efficiency because of their high vapour pressure and are alsoionized with significant efficiency [177].Some of the experiments which suffer the most from neutron-deficient contamination in theRIB are especially related to the nuclear astrophysics field, where accurate measurements of nuclidemasses, decay time and energy levels are necessary to reduce the nuclear physics uncertainties ofr-process models [191]. To perform these experiments, neutron-rich (n-rich) isotopes with massesbetween 70-140 AMU are usually requested at ISAC user facilities [192, 193, 194], which can beproduced at ISAC by spallation/fission reactions in a uranium carbide target. If neutron-deficientcontaminants are present, they can lead to much longer measurement times [192] or an impossibilityto perform some of these measurements [195]. This high request and low supply of pure, n-richbeams was one of the main reasons for investigating a new spallation-driven two-step target design,built to improve the purity of these requested beams. A preliminary hint of the in-target productionpurity gains up to a factor 50 is shown in figure 3.3, where FLUKA simulation results of tin andcesium are compared.Temperature uniformity and control The release properties of atomic species from ISOL targetsare temperature dependent through an Arrhenius-like function. A general and practical way to speedup their release from the target is to heat up the material and its container to their maximum opera-tional temperature. Table 3.1 shows the estimated operational temperature in ISAC compared to thephysical properties of the material. For some of the lighter targets such as SiC and ZrC, the mainconstraint is their vapour pressure at high temperature, which ideally should be lower than 10−5 Torrto avoid dissociation and evaporation under the operational vacuum conditions. Refractory metalscan withstand much higher temperature, but their operational temperature is not pushed close totheir physical limits. This is due to concerns about the structural integrity of the tantalum con-tainer/heater, which at times shows signs of recrystallization which embrittles the container [196].This effect, potentially caused by a combination of conditions such as high-temperature operationand presence of carbon, can induce cracks in the tantalum container and decrease dramatically theisotope yield or even prematurely end the experiment.65Figure 3.3: Bottom: Theoretical comparisons between the in-target production rates of tin andcesium isotopes from a conventional ISAC target and from a spallation-driven two-steptarget. Top: ratio between tin and cesium rates in the two respective configurations. Theadvantage of two-step targets resides in the spread in this ratio plot between blue starsand red dots, accounting for up to a factor ∼50 in purity gain.Material TISAC [◦C] Vap. Press.[torr](Tre f ) Tre f [◦C]SiC ∼1600 2.6·10−4 1600 [197]ZrC ∼1800 4.5·10−6 2330 [198]UCx ∼1950 7·10−6 2000 [27]Ta ∼2300 2·10−6 2300 [199]Nb ∼2100 1·10−2 2700 [119]Table 3.1: ISOL target material used in ISAC, and their relevant physical properties for ra-dioisotope production.ISAC targets are also designed and operated in order to minimize the temperature differencesacross the whole container. When a low-power beam impinges on a target, the main temperaturedifferences across the container and target material geometry are only due to Ohmic heating whichinduces relatively small temperature gradients. However, in high power targets, the power depositedby the impinging proton beam generates a steep temperature gradient and as a consequence theexternal portion of the target material is at temperatures hundreds of degrees colder than its centralhot spots [97]. For materials with low volatility, this might mean some of the produced isotopeswould condense in these cold spots and decay before making their way out of the container [200],since on average every desorbing particle would go through hot and cold regions [104].66The proton-to-neutron converter target would be able to overcome this issue related to tempera-ture gradients induced by beam power deposition because the neutron flux deposits negligible heatin comparison to the proton beam. The converter body would in fact dissipate the whole beampower, while the target would be heated up by resistive heating only. This way, the temperaturedifference between hot and cold spots in the UCx target material would be close to ∼50 ◦C (seefigure 3.14b) regardless of the proton beam intensity, while for ISAC-UCx targets this grows ∼6 ◦Cfor every additional µA of proton beam current, making it very challenging to withstand the fullpotential of the beam delivered by the cyclotron.Operating license for ISAC targets Another limitation to increasing the proton beam current onUCx target is due to the license issued by the Canadian Nuclear Safety Commission (CNSC). Limitson residual activity in the target after irradiation are imposed by taking into account the amount ofalpha emitters, gaseous and volatile species which are produced in the target material during a threeweeks operation period. On a practical level, this introduces a limitation to the maximum protonbeam fluence that can be delivered to the target [201].Although it uses the same UCx target material as a regular ISAC target, the CNSC license foroperating the proton-to-neutron converter target was extended in 2020 [202] to allow operation upto a proton beam current of 98 µA. This extension is due to the better thermal performance of theproton-to-neutron target at high proton beam currents, and also to its much lower production ofalpha-emitters compared to standard ISAC-UCx targets, see table 3.2 and figure 3.12c. More detailson these topics are present in TRIUMF internal technical documents such as [203].Material ISAC [Bq]) [201] P2N [Bq]228Th 1.22·109 1.18 ·107227Ac 1.33·107 1.41 ·105223Ra 1.20·109 1.16 ·107Table 3.2: Comparison of activities at 100 days after End of Beam (EOB) from two of the mostproduced alpha emitters. The numbers refer to a simulations with 20 µA on the ISACtarget, and 100 µA on the proton-to-neutron target, after the same duration of protonirradiation of ∼500 h.673.2 Goals and concept development for a new proton-to-neutronconverterThe design and prototyping phase of this proton-to-neutron converter concept has been a nonlinearjourney where creativity had to meet several constraints in order to come up with a full proton-to-neutron target and converter assembly that would make sense irradiating. This section summarizesthe early stages of the process where some initial goals have been defined and guided the develop-ment of a conceptual design which had to take into account nuclear physics considerations as wellas initial thermo-mechanical and practical engineering constraints.3.2.1 Goals and objectivesThe required purity of neutron-rich beams was set initially by looking at the isobaric couple 134Sn,considered a n-rich beam of interest, and 134Cs, considered its main non-exotic contaminant. Thesetwo isotopes will coexist in the RIB coming out of the mass separator because their mass differenceis not resolved by the ISAC mass separator resolving power m/∆m ∼ 2000. The ratio of in-targetproduction for 134Sn/134Cs in a standard ISAC-UCx target is ∼ 7.9 ·10−3, while in the new proton-to-neutron prototype we were aiming at keeping this ratio lower than the conventional design byone to two orders of magnitude. Our latest FLUKA [26] simulation suggested a ratio of ∼ 3.4·10−1which was considered satisfactory. Moreover, the absolute in-target production of 134Sn in the newproton-to-neutron design at a proton beam current of Ip =100 µA (∼ 2.3 · 108 at/s) is also higherthan the standard ISAC target design at Ip =20 µA (∼ 1.25 ·108 at/s).In order to release most of the isotopes used in RIB experiments, the proton-to-neutron targetshould be able to be operated reliably at high, homogeneous temperature of ∼ 1900 ◦C. It wasdecided that the heating should be only provided by the resistive current flowing through the tan-talum heater, minimizing the impact of other sources of heat such as the incoming proton beampower deposition. Hence, this target should be operated at nominal thermal condition with anyincoming proton beam currents, i.e. from 0 to 98 µA, unlike standard ISAC targets which canbe efficiently operated only on narrower beam current ranges because of the thermal impact ontheir performance. On top of providing more flexibility and ease of operation during RIB delivery,the almost full decoupling between target temperature and beam current sent to the assembly en-ables a first-of-a-kind study at a high-power ISOL facility. For example, the presence or absenceof the Radiation Enhanced Diffusion (RED) phenomenon can be directly tested for the first timein a temperature-independent environment, giving better insights on this phenomenon which wasreported earlier [98, 97], but is still controversial (see section 1.3.1).3.2.2 Concept design iterationsProton-to-neutron converters have been irradiated online at ISOLDE for several years, and recentlysome improvements in their design has been suggested [189] and tested online [7]. Following themain takeaways from these studies, the ideal shape for a uranium carbide-based target is annular, to68increase the geometrical view factor of the spallation neutrons produced from an in-beam converter.Another critical geometrical feature consists in the position of the converter body, which can bepositioned either at the centre of the annular UCx target material, to improve the neutron view factorto trigger fission reactions, or downstream of the target body as in ISOLDE’s prototype in figure3.2, to limit spallation-like reactions induced by scattered protons. Following Luis’s proposal [189],my supervisor Dr. Alexander Gottberg initially suggested a concept where a tungsten converter isplaced after a conical target. A copper heat sink was then coupled to this converter as a placeholderfor a more refined cooling concept as shown in figure 3.4, and at this point in time my involvementin this project started.The target material Inner Diameter (ID) was set to 1.8 cm as first guess (more details are providedlater) to prevent most of the protons to directly interact with the uranium carbide, the length andconical angles were assigned arbitrarily while the Outer Diameter (OD) dimensions were a result ofthermal considerations. From initial estimations of the current delivery capabilities of the electricalinfrastructure, it was assessed possible to increase the resistive current delivered from 600 A to amaximum of ∼1300 A, taken as an approximate limit. A back of the envelope calculation aimingat keeping constant the electrical current density (per unit area) suggests a maximum target outerdiameter of ∼43 mm, keeping the tube thickness at 0.38 mm as in conventional ISAC targets. Aswill be shown later, these dimensions will change significantly.Figure 3.4: Initial proton-to-neutron converter geometry implemented in FLUKA, withOD=43 mm and ID=18mm.A particle tracking simulation was performed with this geometry and its fission rates are sum-marized in table 3.3, showing a ratio between neutron-induced and proton-induced fissions of ∼21and compared with an ISAC-UCx performance. In these initial considerations, the ratio betweenneutron-induced and proton-induced fission reactions has been taken as a proxy for the RIB purity,since accurate predictions are reached more quickly in Monte Carlo simulations. Once this initialconcept was put through a practical preliminary design study, a few issues were raised. First, theconical target surface would have been difficult to manufacture, weld and assemble because of itsthin and and sharp central window features. A second point of concern is the close proximity of the69Configuration P-fissions N-fissions N− f issP− f iss- [s−1] [s−1] -Converter (100 µA) 4.59·1010 9.56·1011 2.08·101Conventional ISAC-UCx (20 µA) 3.08·1012 6.53·1010 2.12·10−2Table 3.3: Figures of merit of the first converter concept and conventional ISAC-UCx target.Window # Material Upstream distance1 Tantalum 10.1 cm2 Copper 37.9 cm3 Aluminum 119.6 cmTable 3.4: Beam window material and location calculated from the center of the proton-to-neutron target containertwo bodies, target and converter, which should not touch under any circumstance and would other-wise affect the resistive heating distribution. Considering that the target heater is usually wrappedin three layers of 25 µm thick tantalum foil to keep it homogeneously warm, it would be neces-sary to move the two bodies further apart to avoid any electrical contact. Introducing an arbitrary9 mm of distance would consequently reduce the fission rates induced by both particles by ∼30%.These practical considerations led to a first major decision to do away with the conical target featurein an attempt to reduce manufacturing complexity and failure modes, at the expense of improvedperformance.The simplification of the target assembly led to a reconsideration of the converter position withrespect to the annular target material. A tungsten converter cylinder of arbitrary diameter and lengthdimensions of 1 cm and 2 cm respectively is placed either inside the annular target geometry ordownstream of it, in order to assess the relative contributions of neutron-induced and proton-inducedfission. It should be noted that independent thermal considerations had restricted the target OD downto ∼36 mm, as discussed later in section 3.3. Figure 3.5 shows the proton and neutron fluxes inboth configurations, where the distribution of both particles points to a much more pronouncedneutron flux in the annular target region when the converter is positioned inside it. The proton fluxfigures 3.5c and 3.5d show only a slight difference at their entrance inside the target volume, whichis due to beam scattering which happens in three beam windows positioned upstream of the target,see table 3.4. These are part of the proton beam line and ISAC target module infrastructure, andfigure 3.6 captures their effect in scattering protons along a ∼1.5 m section of it. Their effect onthe spectrum of both particles within the target material region in both configurations is presentedin figure 3.7, where the high spike at ∼500 MeV in proton beam flux is indeed representative ofthis low-angle primary beam scattering and is very comparable between the two configurations70(a) Neutron flux - converter inside (b) Neutron flux - converter outside(c) Proton flux - converter inside (d) Proton flux - converter outsideFigure 3.5: Proton and neutron fluences when a tungsten converter is placed inside or down-stream of the target containerFigure 3.6: Proton flux distribution due to scattering on the upstream windows described intable 3.4.with converter inside the annular target region (W-in) and outside (W-out). The remaining energydistribution of both proton spectra is instead very different between the two configurations due tothe significant scattering and energy degradation that happens within the converter. Ultimately, thefission rate induced by both particles in the two configurations is shown in table 3.5, which arepresented alongside other ideas explored. The third row in the table shows the fission rates whena tungsten collimator is added upstream of the target within the unit, to degrade the proton beamenergy and prevent some of the high-energy spallation-like events. Moreover, lower rows explore71Figure 3.7: Proton and neutron spectra impinging on the target material for two possible con-verter locations: inside (W-in) or downstream (W-out) of the target containerConfiguration Conv. Material N-fiss P-fiss N−fissP−fissConv. inside target W 9.05·1011 2.84·1011 3.19Conv. downstream W 1.45·1011 1.37·1011 1.06Conv. downstream + coll. upstream W 1.47·1011 6.99·1010 2.10Conv. downstream Ta 1.35·1011 1.36·1011 0.99Conv. downstream Pb 1.16·1011 1.37·1011 0.85Conv. downstream U 1.94·1011 1.38·1011 1.41Conventional ISAC-UCx U 6.53·1010 3.08·1012 2.12·10−2Table 3.5: Fission rates induced by neutrons (N-fiss) or protons (P-fiss) in the proton-to-neutron target material in the geometries outlined in figure 3.5. “Conv.” stands for con-verter, “coll.” stands for collimator.different converter materials in the downstream configuration as explained later.Another major decision was taken with respect to the position of the converter. As far as purefission rates, the inner converter concept seems more appealing due to a higher ratio N−fissP−fiss as shownin table 3.5. However, the beam power deposited inside this body would need to be removed insome way through conduction, convection or radiation. The only realistic way to cool this “insideconverter” - given the ISAC infrastructure - is to allow for either conduction or radiation exchangewith the target body, which would impact the temperature profile of the target body and requireactive adjustment of the Ohmic heating supplied to the target when the incoming beam current ischanged. Moreover, this internal converter concept would add future constraints in the detailedthermal design of the target/converter assembly or limit the maximum proton beam current to avoid72overheating. These considerations are not aligned with the initial thermal goals set above, and itwas decided to focus on implementing a converter assembly positioned downstream of the targetand thermally independent from it.As far as the other proposed configurations outlined in table 3.5, the introduction of a collimatoris beneficial to the fission rates and its presence would be transparent to all other target and converterrelated considerations. Its implementation involves however some minor engineering challengessuch as added stress to the remote handling infrastructure due to its extra weight, and its mechanicaland electrical connections to the heat shield. It was hence decided not to pursue this feature andleave it as an addition for the future to further reduce the proton-induced fission by a factor ∼1.371011/6.991010 = 1.96.Regarding the converter material, tungsten has been used as first guess due to its common useas spallation source material [24, 28, 204]. Tantalum is also frequently used as ISOL target contain-er/heater for its high melting point and its ductility compared to tungsten, which makes it a goodcandidate for small and thin geometries that would need to comply with a variety of mechanical andthermal stresses during installation and heating. Nonetheless, for solid and compact bodies such asa cylinder for neutron production, tungsten was preferred due to its lower price and slightly higherfission-inducing performance from neutrons. A liquid lead converter has a slightly lower neutronyield than for W and Ta, yet it could be a good choice due to its high power removal qualities comingfrom convection. As we shall see below, the converter geometry was optimized to be able to dissi-pate the full proton beam power without major concerns, hence lead was discarded until its superiorheat extracting capabilities will be deemed of utmost necessity. As the last case study, a uraniumconverter presents significant gains in terms of neutron production, inducing a ∼34% higher fissionrate with respect to tungsten and improving the ratio between neutron and proton-induced reactions.On the more practical side however, proton impacting on uranium would produce many more of theα-emitting, radiotoxic nuclides which this proton-to-neutron design aims at reducing, so its imple-mentation would be undermining a major safety goal. Moreover, both uranium melting point andthermal conductivity are both a factor 2-4 lower than tungsten in their full solid temperature range[1], which would affect its heat removal performance and constrain the maximum proton beam cur-rent that could be delivered to the proton-to-neutron target assembly. It was hence decided to locatea tungsten converter downstream of the target container, at the shortest and safe distance of ∼9 mmthat would conservatively prevent the target and converter bodies from touching during operation.This safe spacing is required since their contact would create a current return path through the con-verter and would disturb the nominal Ohmic heating of the target container. This value of 9 mmwas chosen to also account for the presence of three layers of ∼25 µm thick tantalum dimpled foilswhich would act as heat shields to keep the target at its nominal temperature, as homogeneous aspossible.The one converter parameter that needs further optimization is the converter length. The twomain parameters affected by this length are the fission rates in the uranium carbide and the protonbeam power deposition in the converter assembly, which are shown in figure 3.8. The power depo-73Figure 3.8: Fission rates induced by both protons and neutrons, and power deposition in thetungsten converter are plotted against several parametric lengths of a tungsten cylindricalconverter.sition curve steadily and linearly increases with converter length, meaning that the proton flux doesnot attenuate but continues to deposit the same power per unit length across the whole part. This isexpected from a 500 MeV proton beam, as predicted by equation 1.10 presented in section 1.1.1.Similarly, the neutron-induced fission rate monotonously increases with converter length althoughthe second derivative of this curve is negative, meaning that the later tungsten sections contributeless to the fission rate. Since the proton-induced fission events are not affected by the converterlength, this dimension was restricted to 2 cm, resulting in a ∼7 kW power dissipation due to prac-tical considerations of beam power deposition in the converter assembly as will be shown later. Infact, an effective and safe cooling strategy was proved achievable at this power level, which will bepresented in section 3.3.Ultimately, the last important parameter which was optimized through this physics-based ap-proach is the Inner Diameter (ID) of the uranium carbide target material. This dimension stronglyaffects the proton-induced fission rate in the target material since the proton beam “extremities”would overlap with the UCx as schematically represented in the left image of figure 3.9, triggeringmore undesired spallation-like reactions. The first step taken in this direction was to evaluate ifthe proton beam could be further reduced in size. The usual proton beam sent to ISAC targets hasa 7 mm Full Width Half-Maximum (FWHM) at the target location, and beam physicists confirmed[205] that a 4 mm FWHM was indeed achievable by changing the settings of the focusing elements(quadrupoles) of the last section of beam line 2A. The right side of figure 3.9 shows the variation ofthe neutron-induced and proton-induced fission rates for increasing ID values and a 4 mm FWHMbeam. The proton-induced fission rate curve presents a steeper trend especially at small inner diam-74Figure 3.9: Left: overlap between the gaussian proton beam (in red) and the inner fringesof the UCx target (in light green). Right: number of fission events per second inducedby neutrons and protons for a UCx target with OD=33 mm and ID ranging from 10 to20 mm, and the final ID was chosen as 15 mm as in the dashed line.Figure 3.10: Proton (left) and neutron (centre) fluxes shown across the P2N target middleplane. On the right is the target and converter geometry modelled in FLUKA.eter values, due to the greater overlap between the gaussian beam fringes and the UCx target. On theother hand, the neutron induced fission rate is less sensitive, since its value should be only linearlyproportional to the volume of target material, and independent of its overlap with the proton beam.These last statements are better understood by looking at figure 3.10, which show how protonand neutron fluxes across a later iteration of the proton-to-neutron target geometry implemented inFLUKA. As a starting point, it was decided to pick the ID equal to 15 mm so that the proton-inducedand neutron-induced fission rates would be approximately equal. Nonetheless, future ID changescan be easily accommodated if experimenters wish to improve the neutron-rich isotope purity andaccept a reduction in yields.This concept yields a neutron-induced and proton-induced fission rates of 1.45·1011 s−1 and1.36·1011 s−1 respectively, far from the initial numbers presented in table 3.3 based on a first guessdesign. When production rates of specific isotopes need to be considered, it is helpful to look at theisotope production plot mapped onto the Segre` chart as shown in figure 3.11. The top images are theisotopes produced by nuclear reactions triggered by protons in figure 3.11a and neutrons in figure3.11b. First, figure 3.11a presents a broader range of isotope production compatible with spallationreactions and with almost identical distributions, yet lower rates, to the ISAC conventional targets.Areas around (Z,N)=(15, 25) and (75,105) are populated, while their neutron-induced counterparts75(a) P-induced production rate (b) N-induced production rate(c) Ratio: N-induced VS p-induced (d) Induced by all particlesFigure 3.11: Simulated in-target production rates in the P2N target material at 100 µA ofincoming proton beam intensity.are not present. Moreover, proton-induced reactions provide higher chances for producing isotopesslightly lower than uranium with (Z,N)∼(87,135), many of which are α-emitters and need carefulsafety assessments because of their radio-toxicity. Finally, the more important feature for the scopeof this project is a much higher production of neutron-rich isotopes centred around the fission peaksat (Z,N)=(38, 58) and (54, 85), with respect to the proton-induced plot which shows a broader pro-duction area, more skewed towards the stability line (black dots). The relative ratio of the reactionproducts triggered by protons vs neutrons is given in figure 3.11c, while figure 3.11d captures allreaction products in a single plot, regardless of the particle triggering it.A comparative look at the isotope production map from the proton-to-neutron target at 100 µAof incoming proton current versus an ISAC-UCx target at its nominal proton intensity of 20 µA isshown in the two figures 3.12a and 3.12b. The same density and composition of the uranium carbidetarget material is implemented in the two geometries, with an average density of 1.048 g/cm3, andmasses of 18.43 g and 14.60 g for the proton-to-neutron and conventional geometries respectively.To better compare these two images, their ratio is computed and shown in figure 3.12c, highlightingthe potential for order-of-magnitude gains in purity that are achievable on dozens of RIBs extractedfrom the proton-to-neutron target, especially around (Z,N)=(38, 52) and (Z,N)=(52, 85). This plotrepresents the major “selling point” of this proton-to-neutron target development activity for de-livering much purer RIBs to ISAC-TRIUMF user facilities. These purity gains are in fact crucialto enable some user experiments as presented in 3.1.2; at TITAN, the improved beam purity from76(a) Proton-to-neutron target at Ip=100 µA. (b) ISAC-UCx target at Ip=20 µA.(c) Production plot ratio between proton-to-neutron target at 100 µA and ISAC-UCx target at 20µA proton beam currentFigure 3.12: Simulated in-target production rates of all isotopes in the proton-to-neutron con-verter irradiated with 100 Ip = µA (top left) and in a conventional ISAC-UCx irradiatedwith Ip = 20µA (top right) targets. Below is the ratio of the two, “converter/conven-tional”.operating the first two proton-to-neutron converter targets has already led to first-ever mass mea-surements of ten new isotopic masses - see results sections 3.4.2. It is difficult to find publishedreferences of experiments which were unsuccessful at measuring an isotope due to low RIB inten-sity or excessive contamination, so this information was collected from our physics colleagues inprivate communications.773.3 Proton-to-neutron converter detailed designThe physics-based approach presented in the previous section was adopted to drive the design ofthis proton-to-neutron converter target, and engineering challenges would be approached afterwardsonce the physics trends were clear. However, some heating, cooling and manufacturing constraintswere kept under consideration throughout conceptual design to always come up with a potentialsolution that could be realized within a doctoral student timeline. Two clear examples of this ap-proach regard the constraint on the annular uranium carbide Outer Diameter (OD), which neededto be constrained by minimum temperature considerations, as well as the need for positioning theconverter downstream of the target assembly to decouple their thermal constraints. The engineeringchallenges, process and solutions implemented to meet the initial goals for this project are describedin this section with reference to appendix A, where simulation and prototyping steps are discussedwith more detail.Figure 3.13: Cut-away view of the proton-to-neutron target and converter assembly (left), andof a conventional ISAC target (right).3.3.1 Target designWhen a recent proton-to-neutron converter prototype with an external diameter was tested online atISOLDE in 2014 [7], one of the main drawbacks was the low target temperature which preventedextraction of some important benchmark radioisotopes of tin and zinc. Hence, the first optimizationfor this target at TRIUMF involved simulating its thermal profile at early stages to make sure it couldreach ∼1900 ◦C. Initial thermal considerations for designing the target container started when itsconical shape was recently abandoned, hence a cylindrical geometry was taken to start with.Some major remarks will be made here on the overall process to optimize this target containerand its main features. First, temperature profiles were simulated using the reliable FEA softwareANSYS [171], yet the extreme conditions reached in these targets challenge some of the materialproperties and assumptions used for simulations. As an example, the thermal conductivity of theUCx target material at high temperature is not known, so its low temperature values were used [165].78A combination of simulation results and offline thermal testing of some target prototypes was thenused to assess a nominal Ohmic heating current to be flown through the heater assembly duringirradiation.Secondly, throughout optimization, one major step involved the definition of target “legs” ge-ometry (see figure 3.13) which lead the Ohmic current into the assembly through four attachmentpoints in each of the two flat faces and improve the temperature homogeneity throughout the assem-bly. This new heating paradigm, combined with three tantalum “heat shields” which are wrappedaround the target container, further increase the uncertainties in assessing a precise temperature. Asa consequence, considerations and yield values presented later are reported at a certain Proton-to-Neutron Target Heater (P2NHT) value instead of at a specific temperature. The relative gradientssimulated within the target assembly have been considered representative of the real situation andare reported in figure 3.14.Additionally, an important feature of the target assembly is the presence of thin, 0.1 mm thick“windows” which should allow the proton beam to pass through them almost undisturbed. Theworking assumption for this engineering design was that the assembly should be air tight exceptfor the ion source hole, hence these tantalum windows were deemed important for the successfulrelease of radioisotopes. Their effectiveness is discussed in the Post Irradiation Examination (PIE)section 3.4.4 with respect to online yield measurements observed during the online irradiations. Adescription of the major simulation steps that led to the final target design is presented in sectionA.1.(a) Target temperature at P2NHT=1300 A (b) UCx temperature rangesFigure 3.14: Left: Cut view of the finalized proton-to-neutron target assembly with graphitecartridge and UCx. Right: Simulation results of the minimum and maximum tempera-tures in the annular UCx target material domain, with resistive heating only or with fullbeam delivered to the assembly.This target optimization process was undertaken with the scope of minimizing temperature gra-dients throughout the target material domain, which should be well characterized by the FEA soft-ware and represented in figure 3.14a. The absolute temperature resulting from these simulations isprobably a lower boundary due to the heat shield considerations above and offline testing resultspresented in section A.1. The online yield measurements have the final word in assessing the tar-79get thermal performance as a consequence of the planned systematic investigations, which will bepresented in the next section 3.4.3.3.2 Converter engineering designThe engineering design of the converter assembly started by brainstorming the potential clampingand cooling solution to secure the tungsten converter in a defined location, and at the same time coolit properly. One proposal was to radiatively cool the tungsten, i.e. let it glow until the heat emittedby radiation matched the power deposition. A back-of-the-envelope calculation can already providean average surface temperature assuming a converter length of 2 cm and radius of 4 cm which couldfit in the heat shield:P = εσA · (T 4−T 40 ) (3.1)where P is the power deposited by the beam in the tungsten (∼ 7 kW), ε is the emissivity oftungsten at high temperature (∼ 0.28 [1]), σ is the Stefan-Boltzmann constant equal to 5.67 ·10−8 [W/m2/K4], A is the area of the cylinder of dimensions mentioned above, T0 is the temperatureof the surrounding media (at most ∼100 ◦C) and T =∼ 2490 ◦C is the average calculated temper-ature of the converter external surface. At this high temperature, several unknowns would arise,including predicting the tungsten thermo-mechanical properties and finding a realistic supportingstructure. Moreover, the intense thermal shine from the converter would be partially absorbed bythe target body, adding more thermal gradients in the uranium carbide material and reducing therelease efficiency as a consequence. Although some strategies to mitigate these effects might beconsidered, i.e. adding asymmetric cooling fins to promote heat transfer in one direction only, thisradiative cooling concept was abandoned in favour of a more practical solution.The second proposed cooling concept which was successfully implemented is water-cooling,where water can circulate into cooling channels drilled into two copper-alloy brackets which actboth as structural support for the tungsten converter and its heat sink. Figure 3.15a shows an op-(a) CAD model (b) Thermal simulation resultFigure 3.15: Cut view of the proton-to-neutron converter assembly, axially clamped by fourM5 screws and water cooled all around the copper-alloy brackets.timized version of the converter which was subsequently manufactured and tested both offline and80online. The outer square dimensions are constrained by the container in which this converter wouldbe installed online, the cooling channel diameter was maximized to avoid flow restrictions, and theirposition was the outermost feasible from a machining perspective. The tungsten converter part canbe viewed as the union of two cylinders, the first one 2 cm long and 1.8 cm in diameter, the second0.9 cm long and 4 cm in diameter. The latter outermost part is intended to provide enough surfacearea for axial heat exchange, allowing for good contact between the tungsten and the two brack-ets made out of the creep-resistant copper alloy CuCrZr (C18150). These two brackets are axiallycompressed by means of four screws, nuts and spring washers to always maintain the parts in goodthermal contact. In order to minimize any radiation-induced effect in the screws, nuts and washers,these components were manufactured from Inconel-718 and Inconel-625 superalloys from ExtremeBolt and Fasteners [206], which have proved reliable performance under extended irradiation con-ditions [207, 208]. Another practical constraint on this geometry includes the position of the firstflat face of the tungsten part, which should be flush with the first flat copper bracket to generateneutrons as close as possible to the target assembly. Moreover, the size of the inner converter diam-eter was set to 1.8 cm to intercept all the protons which pass through the annular target hole, a greatmajority of the beam. With these constraints in mind, initial thermal simulations were conducted tofirst make sure it was sound from a thermal perspective. Progressive adjustments on some delicatedimensions were performed, mostly involving the interface between the tungsten converter bodyand its copper alloy cooling brackets. The boundary condition details and the thermo-mechanicalresults from the latest geometry are presented in appendix section A.2.The general outcome of this converter design phase has given confidence in the ability of this as-sembly to withstand the harsh thermal, mechanical and radiation-dense conditions that characterizehigh-power radiation targets, and the resulting thermal simulation results are shown in figure 3.15b.Some uncertainties on the thermo-mechanical properties of tungsten at high temperature imposesome limits on the structural simulation conclusions. However, a risk evaluation was performed bythe TRIUMF accelerator division safety committee with a positive outcome, granting permissionfor irradiation.813.4 Online experiment: irradiation of target-converter unitsThe approval to proceed with an online experiment was granted thanks to promising simulationresults and the urgent user-based requests for purer n-rich radioactive ion beams. Initially, the targetand ion sources R&D group at TRIUMF submitted a proposal to the Subatomic Physics ExperimentsEvaluation Committee (SAP-EEC) to operate such an ISOL unit as development target, which wasaccepted with high priority at the beginning of 2018. The simulation, prototyping and testing phaselasted until 2019, and it was only after some further infrastructure tests done in 2020 that the proton-to-neutron R&D team could schedule the experiment, accounting for the long manufacturing timesfor some of the most critical components, i.e. the tantalum target assembly. An internal SafetyAnalysis Review (SAR) meeting was also hosted, where safety experts and TRIUMF’s acceleratordivision leaders granted their permission to irradiate the development target [203].A first proton-to-neutron unit was mounted for online irradiation in October 2020, howeverextensive electrical issues in the target module prevented any meaningful test, and its few resultswill be discussed in section 3.4.4. The next best available slot for operating the proton-to-neutrontarget was at the beginning of the 2021 operational period of the ISAC facility, which starts everyyear around May/June when the yearly cyclotron maintenance finishes. A total of 36 shifts of 8hours each was then assigned in the period between June 16th and June 29th, 2021, when the targetcould be irradiated and RIBs could be sent to the ISAC yield station and to the TITAN’s MultiReflection Time-of-Flight (MR-TOF) (see sections 3.4.3 and 2.5). Moreover, the promising resultsfrom the 2021 measurement campaign led to another online irradiation of a converter in June 2022,and some of the results shown later in this section are taken from that irradiation, even though acomplete analysis of all measurements still needs to be performed.Figure 3.16: Nuclide chart snapshot in the vicinity of isotopes 134Cs and 134Sn, downloadedfrom [2]823.4.1 Preparation and planningAmong the several beam time objectives, the first and most pressing was to identify if the proton-to-neutron target could produce purer beams of certain elements than usually produced by ISAC-UCxtargets. Ideally, one would measure the yield for one isotope of a certain element with the yieldstation, along with its so-called contaminants that come with the isotope of interest in the samebeam. However, this approach cannot be applied in many cases, since the isotope of interest or itscontaminant(s) might be either stable or too long-lived to be detected by the particle detectors in theyield station, measuring α , β or γ radiation. Some relevant cases can be found looking at figure 3.16,where if i.e. 134Sn is the isotope of interest with T1/2=1.05 s, the same beam from a UCx target wouldalso carry 134Cs in it, with half life T1/2 = 2.06 y, which is too long to be detected with any availableparticle detector. Nonetheless, 134Cs is still considered a contaminant of the beam of interest sinceits presence in the radioactive ion beam will affect measurements at several experimental stations.A second issue, intrinsic to the way beam composition is measured, regards the kind of informationthat is extracted by the measurement, even if both, the isotope of interest and its contaminant, couldbe detected through decay particle detectors. It involves the fact that several elements detected haveusually very different physico-chemical properties, and would release with different average timesout of the target material, hence having a different release efficiency. Although the information ontheir ratio could be interesting for practical purposes, this proton-to-neutron experiment also aims atbenchmarking simulation results, which relies on having a better understanding of the efficienciesinvolved.Isotope 80Rb 82Rb 91Rb 92Rb 93Rb 94RbT1/2 33.4 s 1.26 m 58.2 s 4.48 s 5.84 s 2.70 sIsotope 95Rb 96Rb 97Rb 98Rb 99Rb 100RbT1/2 377 ms 203 ms 169 ms 115 ms 54 ms 52 msTable 3.6: Rb isotopes and their half lives.Isotope 124Cs 126Cs 140Cs 141Cs 142CsT1/2 30.9 s 1.64 m 63.7 s 24.8 s 1.68 sIsotope 144Cs 145Cs 147Cs 148Cs 149CsT1/2 994 ms 587 ms 230 ms 146 ms 113 msTable 3.7: Cs isotopes and their half livesThe methodology proposed and used to systematically benchmark the neutron-rich purity of thistarget consists in measuring yields of most isotopes of the same element, and comparing those withsimilar half life. This approach allows removing the release efficiency from the comparison, since83isotopes of the same element with similar half life have comparable release efficiency, see figure3.17 as experimental evidence from the ISOLDE converter prototype [7]. From past experience,elements such as caesium (Cs) and rubidium (Rb) are known to be released from and ionized byISAC targets, with overall efficiency close to 80% when the isotope half life is higher than a fewseconds [209]. More systematic investigations were then conducted on these two elements, andtables 3.6 and 3.7 show the main ones measured. 80Rb, 82Rb, 124Cs and 126Cs are consideredneutron deficient isotopes of their two respective elements, and their half life is a few tens of seconds.Among the neutron-rich isotopes, 91Rb, 140Cs and 141Cs also have comparable half lives with theirrespective n-deficient counterpart, which suggests that their release times and efficiency εR shouldbe also comparable.Figure 3.17: Release efficiency from the ISOLDE proton-to-neutron converter prototype irra-diated in 2014 [7].A second goal involved the extraction of a radioactive ion beam which would have as its maincontaminants the same neutron-deficient Rb and Cs isotopes that are to be extracted in this experi-ment. Zinc and tin were chosen because their atomic number, Z=30 and Z=50 respectively, locatesthem in the two different fission islands so that n-rich Zn isotopes (i.e. Zn with A>78 a.m.u.) haveas main contaminants n-deficient Rb isotopes (i.e. 80,82Rb), and the same is valid for n-rich Snisotopes (i.e. Sn with A>130 a.m.u.) and their associated n-deficient Cs contaminants. Moreover,Zn was used as reference isotope in ISOLDE’s proton-to-neutron converter prototype experimentfrom 2014 [7] and hence provides further benchmarking grounds, while n-rich Sn isotopes are gen-erally considered a good benchmark for new ISOL facilities being built for their rich science cases.An important side note for the choice of Sn as a RIB for this experiment regards its relatively lowvapor pressure [119] which makes it a difficult element to release out of an ISOL target if thistarget is operating at relatively cold temperatures, i.e. lower than 1700 ◦C. The final yield of Snisotopes would provide a good idea if the proton-to-neutron target is at nominal temperature of84around 1900 ◦C under the designed resistive heating conditions. To add another layer of techni-cal complexity, radioactive ion beams for these two elements would additionally require dedicatedionization techniques since their ionization potential of 9.39 eV and 7.33 eV for Zn and Sn respec-tively [1] are beyond the reach of hot cavity surface ion sources [3]. A combination of three laserbeams need to be directed to the inner tube of the ion source to interact with the Zn and Sn atomsescaping the target cylinder, and ionize them through three excitation steps, following the generalscheme presented in 1.4, where the three laser wavelengths are (213, 463, 1064) nm for Zn, and(286, 811, 823) nm for Sn, a scheme adapted from Scheerer et al. [210], with a modified first step[211]. Adding these laser-ionized candidates would strengthen the case for the proton-to-neutrontarget project, showing its capabilities of extracting beams which require extra ionization steps andwould prove its readiness for delivering a wide range of elements to user experiments.The maximum proton beam current on uranium carbide targets have also been a major limitationfor the operation of the ISAC facility, and even if currently the limit has been lifted, the averagecurrent is still kept at values lower than 25 µA on most UCx targets. The proton-to-neutron targetcould be the first “high-power” ISOL target in circumventing this limitation. An important figureof merit of this proton-to-neutron target test involves monitoring isotope yields while increasing theproton beam current up to 80 µA, and verify if the yields measured follow a linear trend. This testshould be both element and isotope independent, hence one isotope of either Rb or Cs are pickeddepending on practical considerations.Another interesting set of comparisons that will help understanding the effect of the proton beamsize, measured in terms of Full Width Half-Maximum (FWHM). Although the proton-to-neutrontarget is mostly be irradiated with a 4 mm FWHM proton beam, it is interesting to understand howmuch purity is gained by squeezing the beam from 7 mm to 4 mm, both to benchmark simulations inthese two conditions, and to assess if the extra effort in tuning the beam down to 4 mm is worthwhilefor future beam times. It is worth noting that the maximum beam current achievable with a protonbeam FWHM of 4 mm is 80 µA since the beam windows in the beam line would otherwise besubject to stresses higher than usual, while during operation with a 7 mm FWHM it can be as highas 98 µA.Most of these objectives will be carried out in progressive temperature and beam power steps,starting taking data when the target is heated only with 800 A and with a proton beam current of only5 µA. Progressively, these two main parameters will be raised to highlight the expected changes ofyield, especially with temperature (i.e. heater current, called P2NHT). Radioactive ion beams havebeen delivered to two different end stations: the yield station [212], where most of the data presentedbelow are from, and TITAN’s MR-TOF, sporadically used in some instances to accurately measurethe isobaric composition in the beams extracted by the target. Both apparatuses are described inmore detail in 2.5.The few days prior to the start, the full proton-to-neutron assembly was remotely installed ontothe target module in a hot cell, and was then brought to the ISAC Target station East (ITE) irradiation85Figure 3.18: Proton beam size measurements taken with fast wire scanners after centering onthe 26Na yield minimum, see text for explanation.station using the ISAC target hall crane. Pumping was started as soon as possible, and once thevacuum pressure reached values of around 1 · 10−6 Torr after a couple of days, power supplieswere turned on to apply High Voltage (HV) to the whole target module, and to start heating up itscontainer through the target heater power supply (P2NHT). When the proton beam was sent ontothe proton-to-neutron target for the first time, the first RIB extracted and separated was 26Na whichwas delivered to the yield station to measure its intensity. Sodium is an element that is usuallyextracted with high yield from uranium carbide targets, and its isotope with mass A=26 and half-life of 1.07 s is usually chosen to test the performance of the conventional ISAC-UCx targets, andespecially to position the proton beam where the highest yields of 26Na are seen. In the proton-to-neutron case, however, FLUKA simulations show (see figure 3.11) that 26Na is only produced byproton-induced reactions in the uranium carbide annular material. For this reason, the proton beamaligning procedure was performed with the intent of minimizing the 26Na signal at the yield station,instead of maximizing it as usual. A last proton beam size measurement was then performed asshown in figure 3.18, before starting with the experimental yield measurement campaign.3.4.2 Yield results from the online converter irradiationsDuring both online irradiation periods in June 2021 and June 2022, two teams of around a dozenpeople each were enlisted to cover a total of forty shifts, each 8-hours long, to take measurementsof the radioactive ion beams at the yield station and at the TITAN’s MR-TOF. In COVID-19 pan-denic times, the teams were mostly remotely connected to the instruments at the lab, and wouldcommunicate through video calls to TRIUMF’s control room operators - both in the cyclotron and86ISAC control rooms - who would accommodate any reasonable request from experimenters. Theresults presented below mostly come from the June 2021 irradiation, and measurements from theJune 2022 campaign will be specifically pointed out.The error bars associated to the measured yields reported in the following sections are includedin the thickness of their respective markers and are not explicitly drawn. At times, a number ofmeasurements of the same quantity might be taken at different times throughout the beam time,yielding significantly different results, hence all measurements have been reported to provide insighton the data scatter observed.Cesium and Rubidium isotopes In order to give a proper overall picture regarding the results, datafrom the recent proton-to-neutron converter experiment is presented alongside data from the ISACyield database [8] to allow for immediate comparisons and further discussion. It is important toremark that a choice was made to not directly compare the yield of a neutron-rich isotope (i.e.134Sn) with the yield of a neutron-deficient isotope of the same mass number (i.e. 134Cs). Thisis because the release efficiency for these two elements and isotopes is massively different, andno systematic comparison between these two isotopes can be made for a full understanding of thebehaviour of this two-step target. Instead, neutron-rich isotopes are compared with neutron-deficientisotopes from the same element in order to remove chemistry-related effects. Figures 3.19 and3.20 show a comprehensive performance comparison between the proton-to-neutron target, whenheated with a resistive current of P2NHT=1150 A, and specific ISAC-UCx targets where yields weremeasured for several isotopic masses. The bottom part of the figures show the FLUKA simulationresults (red and blue dots) of in-target production in units of isotope produced every second bynuclear reactions, per µA of proton beam current, while the measured yields at the yield stationare shown as purple and light-blue triangles for proton-to-neutron converter (P2N) and ISAC-UCxmeasurements respectively. Note that the amount of uranium loaded inside each ISAC-UCx target issubject to some variability; the last 11 ISAC-UCx targets have been loaded with an average uraniumthickness of 10.649 g/cm2± 0.86, and this average was taken as reference value for comparisonin simulations. The red dots (ISAC in-target production) in both figures present a maximum intheir distributions in the centre of the mass range, around mass A=90 and A=135 for Rb and Csrespectively, while the corresponding blue distributions (proton-to-neutron in-target production) hasthe same maximum shifted a few atomic mass unit (AMU) towards higher masses, corresponding tothe more favourable production cross sections of neutron rich species, expected inside the proton-to-neutron target. Also, aside from some specific cases better highlighted in figure 3.12c, the proton-to-neutron target is usually under-producing most isotopes with respect to standard ISAC targets.This is true comparing these production values per unit of incoming proton beam, which has alwaysbeen the accepted trade-off to extract purer n-rich beams. However, notice that this target can beirradiated with four times more proton intensity, which can make up for at least part of the RIBintensity gap.Comparing the FLUKA-simulated values with their respective measured ones, it is clear that87Figure 3.19: Bottom: simulated in-target production and measured yields of Rb isotopes fromthis converter and from conventional ISAC-UCx targets [8]. Top: simulated Rb in-targetproduction ratios and measured Rb yield ratios between isotopes from this converter andfrom conventional ISAC-UCx targets. NRIF is the Neutron Rich Improvement Factor.the two distributions are approaching each other in correspondence with the central mass values,representing the stable or more long-lived isotopes. On the fringes, the measured yields differgreatly from the in-target production values because of the short half-lives of the more exotic iso-topes, many of which decay before being released. A summarizing snapshot of these trends isavailable in the top part of both figures 3.19 and 3.20, where the ratio of the simulated values forthe proton-to-neutron converter and ISAC-UCx target is shown in green dots, highlighting the morebiased neutron-rich contribution in the proton-to-neutron production plot, compared to ISAC-UCx.The “P2N/ISAC measured” data sets show the ratios between all “Measured P2N” and “MeasuredISAC” points, which overall follow the general distribution of the ratio taken between simulated val-ues (green dots), giving confidence in the general trends foreseen and also allowing extrapolationof these simulated results in other areas. One powerful feature of this ratio plot is the independenceof the measured isotope from its half-life, since the ratio is performed on the same isotopes ex-tracted from different targets. For both figures 3.19 and 3.20, the measured ratio is largest relativeto the simulated value, which points to a better efficiency of the proton-to-neutron target containerin releasing these two isotopes.For quantifying the performance of the proton-to-neutron target, it is useful to define two con-ceptually different mass regions: the neutron-rich region to the right of masses A=90 and A=136for Rb and Cs respectively, and the “flat” region to the left of this mass threshold. Looking at thetop plots in figures 3.19 and 3.20, a Neutron Rich Improvement Factor (NRIF) can be defined as the88Figure 3.20: Bottom: simulated in-target production and measured yields of Cs isotopes fromthis converter and from conventional ISAC-UCx targets [8]. Top: simulated Cs in-targetproduction ratios and measured Cs yield ratios between isotopes from this converter andfrom conventional ISAC-UCx targets.ratio of either quantity (experimental or simulated P2N/ISAC ratio) on the right-hand side, dividedby its correspondent left-hand side quantity. The experimental NRIF value is graphically capturedby the blue ellipses highlighted in these yield plots, showing the average spread in the ratio betweenthe neutron-rich isotopes and the neutron-deficient ones. Measured and simulated NRIF for specificISAC targets and isotope mass combinations can be found in table 3.8, which quantitatively con-firm the neutron-rich production and release improvement in the proton-to-neutron assembly. Asmentioned, these plots are comparing ISAC data from only two UCx targets at a time, since a longchain of Rb and Cs isotopes was only measured for these targets UCx#1, UCx#2, UCx#3 and UCx#7.More comprehensive and denser plots including all data collected through the years from the yielddatabase can be found in appendix A (figures A.22 and A.23).A more detailed representation and comparison of the overall release efficiency of differentisotopes of rubidium and cesium is found in figures 3.21 and 3.22 respectively, plotted as a functionof isotope half-life. The proton-to-neutron points almost in all cases present comparable or higherefficiencies to their ISAC counterpart, likely due to a much more homogeneous temperature in theproton-to-neutron target heater, which explains why the “measured ratio” in figures 3.19 and 3.20was higher than the simulated values. Comparisons with the maximum efficiencies from any ISACtarget are available in figures A.20 and A.21 which show that across ∼30 ISAC targets, only a fewselected measurements have demonstrated a release efficiency higher than the proton-to-neutrontarget for such elements.89Table 3.8: Neutron rich improvement factor (NRIF) for rubidium isotopes (left) and cesiumisotopes (right), to compare the performance of the proton-to-neutron target versus theselected ISAC targets UCx#1, UCx#2, UCx#3,UCx#7 and their FLUKA expected values.Rb n-rich mass Cs n-rich mass91 94 95 96 141 142 144 145n-deficientmassUCx#1n-deficientmassUCx#280 1.9 7.1 14.5 18.5 124 11.2 18.5 52.3 28.682 45.2 170 348 447 126 5.7 9.2 26.7 14.6UCx#3 UCx#780 N/A 5.7 6.1 5.7 124 0.43 0.5 2.2 1.882 N/A N/A N/A N/A 126 2.6 2.9 13 10.7FLUKA FLUKA80 1.9 7.7 6.5 9.6 124 7.2 12.9 22.6 16.982 2 8.1 6.9 10.1 126 7.7 13.8 24.3 18.2Figure 3.21: Efficiency of Rb isotopes from the proton-to-neutron target at a heater current of1150 A, compared to the ISAC yields collected from targets UCx#1 and UCx#3. Morecomprehensive plots are in appendix A, in figures A.18 and A.20.Interestingly, in both efficiency plots it can be noticed that some of the neutron-deficient isotopes(124Cs, 126Cs and 80Rb) present an efficiency higher than 100%. Moreover, taking into account thatthe ionization efficiency for Rb and Cs in a standard ISOL surface ion source is in the order of75% - 80% [209], it seems fair to assume that the FLUKA simulations were underestimating theamount of n-deficient nuclides by a factor ∼2. Initially, this effect was thought to be linked toa higher production of these n-deficient nuclides whose production is boosted by proton-inducedreactions. The causes for this unexpected - albeit proportionally small - production increase ofneutron-deficient isotopes was better investigated during the June 2022 beam time, when the protonbeam position was monitored and re-adjusted on a daily basis. These corrections of proton beam90Figure 3.22: Efficiency of Cs isotopes from the proton-to-neutron target at a heater current of1150 A, compared to the ISAC yields collected from targets UCx#2 and UCx#7. Morecomprehensive plots are in appendix A, in figures A.19 and A.21.position were responsible for a significant reduction of proton-induced reaction products by up to afactor 5, explaining the higher than expected yields of neutron-deficient isotopes.Figure 3.23: Proton beam size measurement taken towards the end of the P2N beam time (June23rd).Yield dependence on proton beam FWHM The proton beam size was one of the parameters whichwas carefully controlled throughout the experiment, and for most of the time the incoming beamhad a FWHM of ∼4 mm. During one of the yield measurement shifts, it was decided to increasethe beam size to 7 mm and compare data from some reference Rb and Cs isotopes to the previous91data taken at nominal 4 mm. Figures 3.24 and 3.25 show the results of simulations and yieldFigure 3.24: Comparison and ratios between Rb yields taken with a proton beam FWHM of7 mm and 4 mm.Figure 3.25: Comparison and ratios between Cs yields taken with a proton beam FWHM of7 mm and 4 mm.data taken in these two beam size conditions, and suggest a reasonable agreement with predictedvalues for n-rich, Rb and Cs fission products; however, for the neutron-deficient side of the Rb andCs plots (left-hand side), these data suggests that these proton-induced contributions are present92but not as pronounced as expected. This mismatch could still be explained by the misalignmentnoticed on the proton beam position during the measurements performed at FWHM 4 mm, whichled to an increase of the n-deficient contributions. As the FLUKA data show in both plots, theneutron-deficient products for the 7 mm proton beam are already much higher than their 4 mmcounterparts, which makes these 7 mm measurements much less sensitive to proton beam position.One point worth mentioning is that these comparative yield measurements between different protonbeam sizes were taken while the target heater current was set to 900 A, hence the yield numbers aredifferent than the nominal temperature data presented in 3.19 and 3.20.Zinc isotopes Yields were collected in the mass range around mass A=75, focusing mostly on zincisotopes.Figure 3.26: Lower plot: Zn in-target production calculated by FLUKA and measured onlineduring this experiment and compared to all other measurements from ISAC-UCx targetsin the yield database [8]. Upper plot: ratio between the simulated in-target productionand measured yields values, in green and black respectively.At nominal target heater current, the zinc yields were compared to the historical records fromtwo uranium oxide FEBIAD targets and shown in figure 3.26, since zinc has never been extractedfrom a uranium carbide target at ISAC. The normalized yields (yield/µA) are in the range of his-torical data, although the ionization method implemented for these ISAC targets was a FEBIADion source, rather than lasers, that yield an ionization efficiency of about 5% [213]. These obser-vations, coupled with the heater-dependent efficiency plot in figure 3.27 point again to a very highproton-to-neutron target release efficiency with respect to standard ISAC targets, and its potential tobe coupled to a laser ion source. However, direct comparisons with previous targets in the literatureis not possible because of such different target configurations. In figure 3.27 one can note that thehalf life of the isotope extracted is longer than 2 seconds, hence all efficiencies at a heater current93Figure 3.27: Efficiency of Zn isotopes from the proton-to-neutron target at different targetheater (P2NHT) currents, compared to the yields collected from two UO2 ISAC targets.of 1150 A are quite flat and comparable with each other; yet, for a lower heater setting (∼950 A)the efficiency reduction trend is more evident when the isotope half-life drops below 10 seconds,although the scarcity of data does not allow for conclusive statements.Yield measurements at increasing proton beam intensity So far, all the yield data was presented inunits of particles per second per µA of incoming protons, assuming that the RIB yields are linearlyproportional to the proton beam intensity. This linear relation between isotope yield and protonbeam intensity up to 80 µA has been measured during the June 2022 beam time for four isotopes ofcesium, two neutron-rich (140Cs and 141Cs) and two neutron-deficient (124Cs and 126Cs). The choiceof these isotopes was just driven by their simple decay characteristics and their relatively high rateswhich allow for several yield measurements in a short time, approximately 8 hours. Figure 3.28shows the two sets of measurements taken during two consecutive days where a linear fit is added.It can be noticed that the series in the two top plots, 140Cs and 141Cs, follow a more linear trend,while for 124Cs and 126Cs the trend seems to disappear for proton beam currents of 50 µA or higher.This behaviour is most likely attributed to a movement in proton beam position when increasingits current, which improves the yields of neutron-deficient 124Cs and 126Cs if more protons interactwith the annular uranium carbide. This sub-optimal steering has little effect on the neutron-richyields of 140Cs and 141Cs, whose production is dominated by neutron-induced fission. Discussionswith cyclotron experts has confirmed that the beam can indeed change shape and “move” when theintensity is changed due its extraction methodology from the cyclotron tank described in section1.2.1. This is, however, not an issue for operating this target in the future, since the absolute beamposition can be tuned while high currents are delivered to it, and can be left untouched until furtheradjustments are needed.94Figure 3.28: Two data sets measured in consecutive days, showing the effect of proton beamintensity on the yields of four Cs isotopes. The top two neutron-rich isotopes (140Cs,141Cs) show linear trends with increasing Ip, while the bottom two neutron-deficient(124Cs, 126Cs) deviate from linearity likely because of position instabilities of the protonbeam.Tin isotopes On-line measurements of the tin isotopic chain were performed during the June 2022beam time, and the results from one are shown in figure 3.29. These are compared with expectedsimulated values and with previous measurements from conventional ISAC-UCx targets. In thetop plot of figure 3.29, the ratio clearly shows a rise in correspondence with neutron-rich isotopes,compared to conventional UCx targets, a key feature of this converter target which gives again con-fidence for enabling more pure neutron-rich experiments in the future. Differently to the Rb andCs isotopic chains extracted by both successful beam times, the release efficiency for Sn isotopes islower with respect to other conventional UCx targets as observed in figure 3.30, where this convertertarget underperforms in almost all isotopes. This observation paints a different picture than whatwas observed in figures 3.22 and 3.21, and potential explanations are suggested here. One involvesthe temperature distribution in both assemblies and ties to the different bottleneck release mecha-nism for these species, i.e. diffusion should be the limiting stage for Rb and Cs while higher stickingtimes would affect more the release of Sn during its effusion stages [116]. A different temperatureprofile in ISAC-UCx targets as compared to this converter-target might still favour release in theformer if higher temperatures are achieved locally, even if gradients are much larger. This couldmean that the average proton-to-neutron heater temperature should be further increased to provide95Figure 3.29: Bottom: Sn in-target production calculated by FLUKA and measured online atP2NHT=1150 A during this experiment and compared to measurements from ISAC tar-get UCx#33 [8]. Top: ratio between the simulated in-target productions and measuredratio between the yield values, in green and black respectively. Both plots are prelimi-nary, since thorough data analysis is still needed.comparable release efficiency to ISAC-UCx targets. A second explanation might involve the changein UCx microstructure due to heating, which might affect the release properties of the material.One would expect that sintering would mostly affect the release of diffusion-limited species; how-ever, the amount of open porosity might also be compromised, increasing the overall effusion timeas well. In addition, the different annular target geometry might increase the release time of thiseffusion-limited element, hence systematic comparative studies of these two geometries could pro-vide a hint for this discrepancy seen. A further hypothesis could be that the ionizer was partiallyblocked by a target material flake, potentially hindering the laser beam from effectively ionizingthe atoms. In addition to the already small release, a gradual decrease in performance was noticedthrough the beam time, reducing the normalized yields by a factor 10x. These measurements stillneed to be carefully looked at, but the general trend might again point to an ageing of the targetassembly over time.Beam time end Most of the information and experience gained from the online irradiation ofproton-to-neutron converter targets comes from the successful June 2021 and June 2022 beam times,providing many measurements for the Target and Ion Sources (TIS) and TITAN teams.The 2021 converter beam time was characterized by a ramp-up in our understanding of the target96Figure 3.30: Efficiency of Sn isotopes from the proton-to-neutron target at nominal heater cur-rent (1150 A), compared to the ISAC yields collected from targets UCx#33. A standardlaser ionization scheme was used to ionize these atoms. This plot is preliminary, sincethorough data analysis is still needed.parameters, and our attention was mostly focused on exploring parameter space for cesium andrubidium isotope chains, with some measurements performed on zinc. A cyclotron issue discoveredin the middle of this time slot prevented delivering proton beam currents higher than 50 µA, henceyield measurements up to 80 µA had to be postponed to the following year. Moreover, this beamtime finished four days earlier than expected and prevented the extraction of tin isotopes, plannedfor the last few days. The cause was an electrical short circuit that developed across cables in thecurrent delivery infrastructure (within the target module), hence the power had to be shut off earlierpreventing further RIB extraction. This event has been related to mechanical vibrations in theselines, which have happened during the operation of previous targets as well, and was not tied to thetarget assembly in any way. Similarly, the October 2020 converter beam time was also cancelled dueto similar issues in the current delivery infrastructure that hindered operation since the beginning ofthat online time slot.The 2022 converter beam time built on previous experience, it allowed to confirm some resultsfrom 2021 and accurately monitor and control the proton beam position. Moreover, the isotoperepertoire extracted from converter targets at TRIUMF has been expanded to include tin isotopes atdifferent target heater settings. Finally, the cyclotron proton beam current was able to be raised untilits maximum value of 80 µA (for a proton FWHM of 4 mm) and allow the measurement of yieldsproportionally to the incoming proton beam current.973.4.3 MR-ToF resultsDuring the online beam time, the RIB was at times directed to the MR-ToF spectrometer in order tomeasure more accurately the isobaric components of the beam. The MR-ToF can only accept a max-imum of 106 particles per second due to space charge and safety limitations, hence some strategiesare adopted to reduce the RIB intensity before entering the apparatus (i.e. deploying RIB attenua-tors and closing the mass separator slits). These strategies affect the total beam intensity and makeit hard to calculate the absolute yield of each isotope, but still provide an accurate measurement ofthe relative percentages of isobaric species. Some of the data collected over the two online experi-ments is shown in table A.3, where intensity ratios are presented relatively to the intensity of the first(Iso1) column. One important value obtained from the MR-ToF is the ratio 134Sn/134Cs = 3.18·10−4,which was measured towards the end of the beam time when a ∼ 10× reduction in Sn yield hadbeen recorded with respect to the first few days. It is then foreseen that a fresh target should yielda ratio closer to 134Sn/134Cs= 1 ·10−3. It is important to notice that this number cannot be directlycompared with the in-target production value given in section 3.2.1 because of release efficienciesreduce n-rich tin rates much more significantly than long-lived Cs isotopes.This online beam time collaboration with TITAN was very fruitful in both directions, since itwas agreed that they would have taken some shifts to attempt some science measurements whichhad not been possible in the past due to excessive beam contamination. Their team was able toperform first-ever measurements of the masses of several isotopes during the 2021 and 2022 beamtimes: 83Zn, 149Cs, 150Cs, 151Cs, 151Ba, 152Ba, 136Sn, 137Sn, 138Sn and 86Ga. These achievementsreinforce the strength of this proton-to-neutron converter target for future operation, since it hasproven its potential for a real experiment.One unforeseen point of strength of this target stemmed from its ability to operate at homo-geneous temperature, regardless of the proton beam power sent to the unit. This enabled the op-eration of this target in “temperature-controlled mode”, reducing the heater to colder temperatures(P2NHT∼900 A) while irradiating protons at high intensity. This strategy allowed thermally sup-pressing the release of some stable lanthanides (i.e. Sm, Eu, Pr, Nd around mass A∼150) by afactor ∼1000, while cesium was still able to be extracted with meaningful intensities, independentof the proton beam intensity. This operational mode is so far only possible in this proton-to-neutronconverter target due to its thermal decoupling between beam power deposition and target tempera-ture. This feature will be exploited again in the future to investigate temperature-dependent releasephenomena and will be applied for selectively releasing these and other interesting elements bypotentially adjusting the target configuration.3.4.4 Post irradiation examinationAfter irradiation of all three target assemblies in October 2020, June 2021 and June 2022, the targetunit was retrieved in the hot cell to inspect its core components. In all cases, most of the targetcontainer parts looked in very good condition, with only few signs of embrittlement on its flat facesas shown in 3.31 as expected [196]. Significant differences were however seen in the health of98the thin tantalum “windows” on the target body. In the 2021 target, which received a maximumof 50 µA proton intensity, both entrance and exit windows were in good shape without tears orholes evident (figure 3.31b). However, in both the 2020 and 2022 target bodies, at least one ofthe windows in each assembly showed holes and tears as shown in picture 3.31a and 3.31c. Thesewindows of thickness 0.1 mm and 0.2 mm were conservatively introduced to seal the cylindricaltarget volume and prevent the escape of radioisotopes, but so far there is no evidence in favour oragainst their usefulness. In fact, the graphite cartridge could be a sufficiently impermeable mediumfor containing the escape of radioisotopes in all directions, and funnelling them through the ionsource hole only. More evidence is still needed to claim the need for these windows, which mightcome from additional analysis of the 2022 run data or from offline effusion studies (see chapter 4)or during future runs where the windows might be changed or removed completely.The health assessment of the converter assembly was very important for the safety case ofthis proton-to-neutron target unit, since the steep thermal gradients and high temperatures in thetungsten part could have triggered unforeseen behaviours. All three converter assemblies wereinspected in the hot cell after irradiation and no signs of melting, cracking or other effects wereseen. After successfully irradiating three converter units, two at high proton intensity and one atmedium intensity, this design has been confirmed fit for routine irradiation.Not irradiated IrradiatedC O Co W U C O Co W U39.3 24.6 2.1 1.8 32.3 Part. ox. UCx 36.6 28.2 0.03 0.3 35.019.8 54.1 0.9 11.6 13.6 Part. ox. WUC2 54.2 9.5 0.02 16.8 19.487.0 5.4 1.9 0.6 5.2 Carbon 94.1 3.4 0.02 0.2 2.3Table 3.9: Average atomic percentages of elements in the phases labelled in figure 3.32.An involved post-irradiation examination of the uranium carbide material from the October 2020irradiation was performed by breaking the target heater in the hot cell and extracting a small fractionof an annular UCx disc. This sample was sealed in an aluminum enclosure equipped with a rubberseal, loaded in a shielded type-A container for radioactive shipments and was sent to our Belgiancollaborators at SCK CEN along with another non-irradiated UCx sample coming from the samematerial batch. Even if no isotopes were meaningfully extracted from this target, it was kept atnominal heating around 1200 A for ∼48 h, and the assembly was irradiated for approximately oneday to stress test the converter. Hence, radiation and temperature effects would still be expected tobe evident by comparison of the two materials.SCK CEN has the capability of performing several kinds of microscopy on radioactive actinidesamples, and it was decided to exploit their capabilities to compare the microstructure and compo-sition of the two UCx samples before and after irradiation. Figure 3.32 compares the SEM scansfrom both samples, and overlays a colour-coded map representative of the main elements identifiedby an Energy-dispersive X-ray spectroscopy (EDS) detector. From visual comparison, sintering and99(a) Target, Oct 2020(b) Target, June 2021(c) Target, June 2022 (d) Converter, Oct 2020Figure 3.31: Post-irradiation examinations (PIE) of core components of the target assembliesirradiated over the three beam times.grain growth of both UCx and WUC2 grains are evident effects, where sub-micrometric featuresgrew by approximately one order of magnitude, likely due to the high operational temperature.The terrace-like shape of these uranium carbide grains which have grown after irradiation is pecu-liar, and different from more rounded shapes noticed on other proton-irradiated ISAC-UCx targets.Similar studies have observed the growth of this structure when carbon atoms diffuse into under-stoichiometric UC2 mixtures to form cubic β -UC2 [118], which presents a faster sintering rate thanthe former [214]. In the future, more analysis might be conducted to further investigate if thesedifferences might be linked to the nature of the predominant particle flux interacting with theseparticles, i.e. neutrons or protons.The average composition of the two colour-coded images in figure 3.32 is presented in table 3.9,100Figure 3.32: EDS mapping of two uranium carbide flakes extracted from the online materialbefore irradiation (left) and after irradiation (right). The two grey-scale pictures aremagnifications of the oxidized UCx matrix (green) from their respective image under-neath.where high contents of oxygen were detected on both the uranium-based matrix and the tungsten-based grains. Comparable concentrations of elements were detected on irradiated and non-irradiatedsamples coming from this target and from two other ISAC targets, hinting that the oxidation is likelyhappening during the handling, transport or analysis phases, when the sample’s atmosphere is notfully inert nor controlled.1013.5 Discussion and outlookThe goal of operating an optimized version of a proton-to-neutron converter target at TRIUMFhas been reached, and most of the objectives set out at the beginning of the project have beenaccomplished.The online experiment provided a large set of data that gives lots of confidence in the capa-bility of this proton-to-neutron target converter to produce fission reaction products more biasedtowards the neutron-rich side of the stability valley, peaking its production plot in the centre of thetwo fission islands. Conclusive statements can be made for the improvement of the neutron-richpurity of this target, with average enhancements of a factor 5-20 in line with the expectations fromsimulations. The approach implemented involved the yield comparison within the same isotopicchain of the same element extracted from the proton-to-neutron target versus similar measurementsfrom single past ISAC targets. The objective of this approach was to compare “apples to apples”by i.e. controlling the release efficiency dependence on the specific half-life and reducing the widerscatter and systematics that could have been introduced by merging together data from several ISACtargets. Very useful isotopic chains were retrieved for some ISAC targets (UCx#1, UCx#2, UCx#3,UCx#7 and UCx#33) that confirm the value of time dedicated to target development, not only for theinformation extracted for the specific target, but also in light of future experiments and benchmarks.Linear yield trends have been measured as a function of proton beam current delivered to theconverter target assembly, which matches the expected outcome since the annular target materialtemperature is minimally affected by the beam power deposition. This observation performed bysystematically increasing only the proton beam intensity between 5 µA and 80 µA had never beenperformed in ISAC targets, for which Ohmic heating needs to be adjusted when the beam intensitychanges, significantly altering the thermal conditions of the target. Moreover, this linear trendsalso suggest that the Radiation Enhanced Diffusion (RED) effect is not detected when neutrons withfluences or around ∼ 1013 s−1 interact with the annular target material. It should be noted that REDeffects had been reported in ISAC targets in the past [200, 97], but these experiments were performedon a narrower beam intensity window and were changing the heating power as mentioned before,likely affecting the temperature profile. Observations at ISOLDE also suggest some RED effects intheir targets [36], but the intertwined radiation and beam power effects make it again challenging tosuggest a realistic mechanism.The release efficiency of the proton-to-neutron target when heated at nominal current has provedto be on average higher than standard ISAC targets, when extracting Cs, Rb and Zn isotopes. Onthe other hand, lower efficiencies were measured for Sn isotopes, but more time needs to be stilldedicated to perform a thorough data analysis before drawing conclusive statements. The releaseefficiency is an important parameter to monitor for target development activities, and even if Snrelease was unexpectedly low, the more controlled conditions of this target will allow for morecomparable measurements in the future. For example, it is foreseen to look more closely at tinyields, at more and even higher temperatures than done so far.The successful yield measurement data have been proven to follow the expected theoretical102trends, validating the particle-tracking simulation-based approach followed which has been an in-valuable tool to come up with an optimized geometry given the constraints. Moreover, the engineering-based design approach has also delivered a solid thermo-mechanical concept which matched opti-mal performance expectations. Several radioactive ion beams have been delivered by this targetwith comparable release efficiencies with respect to conventional ISAC targets, which TRIUMFhas operated and optimized for the past ∼25 years. Minor further optimizations are foreseen in fu-ture versions of this target assembly, especially addressing the beam-induced holes in the tantalumtarget window. The converter assembly has so far proved to be very reliable in withstanding thehigh-power proton beam, so all safety and operational concerns have been retired. It is worth topoint out here again that conventional ISAC targets are rarely irradiated with a proton beam cur-rent higher than 60 µA due to intrinsic material properties (i.e. melting point) and yield reductionsat high proton beam intensities due to temperature gradients in the target material. Instead, thisproton-to-neutron converter assembly can accept the full proton beam power without compromis-ing on target performance, a so-far unmet requirement within the field of high-power RIB productiontargets.The collaboration between TRIUMF, ISOLDE and SCK CEN resulted in the first two annularUCx ISOL targets, the ISOLDE converter prototype [28] and this work. One interesting questionthat arose when developing the proton-to-neutron annular target geometry, involved the differencesin release times between the conventional geometry (cylindrical) and the annular one. The questfor answering similar questions has been one of the motivations to develop a dedicated procedureand experimental setup to test hypotheses about transport phenomena in ISOL targets, which willbe presented in the following chapter 4.103Chapter 4Radiotracer release studiesIn ISOL systems, the isotope release from the target material is the main contributor to the ultimateoverall performance, so its characterization and understanding has driven several research programson this topic as discussed in section 1.4. This chapter presents the offline release studies that werecarried out through new procedures and using new equipment at TRIUMF, specifically developedwithin this thesis project to perform release experiments. The term offline identifies studies whichare performed by first activating the material by means of a radiation source, retrieving it, and thencarrying out investigations in separate experimental areas. The facility and apparatuses used here(i.e. IPF and CHI stand) are described in detail in chapter 2, and will only be mentioned wherenecessary.4.1 Release tests from pellets and discsSystematic understanding of the release properties of an ISOL target material has usually beenperformed through offline testing, where the release performance of the pellet/disc can be probedin isolation from all other factors. This section builds on the past experience of other RIB facilities,summarized in section 1.4.1, which gives an overview of the experimental procedure developed tocarry out such studies at TRIUMF, and presents the first results from offline experiments on ISACtarget materials.4.1.1 Experimental procedureA schematic version of the procedure is shown in figure 4.1, where three core elements are needed.First, a high-energy proton beam induces the desired nuclear reactions in ISOL target materialsamples and produces the radioisotope(s) desired for the specific study. The layout of the IPFfacility which has been employed in this work for activating the samples is described in section2.3. Second, a γ-ray spectrometer such as a High Purity Germanium (HPGE) detector (see 2.1) isemployed to measure the activity of specific radioactive isotopes in the sample after irradiation. Forthe majority of this study, the ARIEL yield station HPGE detector was used, which is the same modelas at the ISAC yield station described in section 2.1. Third, a vacuum furnace is required to heat104Figure 4.1: Schematics of the overall procedure used to perform release studies, where thegreen disc represents the sample under investigation.the sample at a certain temperature to promote the thermally-enabled release of the radioisotopesout of the sample, through an initial diffusion and subsequent effusion/desorption from the externalsurface. The release flange configuration of CHI stand was used as furnace, as described in section2.2.1. After the heating process, the activity of the same isotope in the heated sample is measured,which would have decreased through two processes:1. Release: thermally activated diffusion and effusion result in migration of radioactive speciesoutside of the sample, which leads to a reduction of detectable activity.2. Radioactive decay: the activity of each isotope over time changes depending on its own decayrate, and the one of its parents. Decay processes are precisely understood and quantified forthe isotopes employed in this work. The main uncertainties reside in the initial quantificationof isotopes through γ-ray spectroscopy and their extrapolation at later times when parent-daughter decay chains are present.For a specific γ-ray energy line detected by the HPGE spectrometer, a release fraction can be calcu-lated by simply recording the total number of counts before and after heating through the formula:RF = 1− S2S2+S3(4.1)105where S2 is the recorded activity after heating. The quantity S2 +S3 is calculated by exponentiallyextrapolating the measured activity before heating (S1) from time T1 to time T2, through the equationS2+S3 = S1 · exp[− ln(2)tT1/2]. (4.2)The quantity (S2+S3) is hence the activity that would have been recorded if no heating was appliedto the target, and a visual representation of these variables is given in figure 4.2.Figure 4.2: The solid black line represents the exponential trend of a general activity of anisotope in the samples undergoing the release procedure. The sudden drop in the mid-dle represents the partial escape of such radioisotope during the heating phase, and thedashed line indicates what their activity would be if no heating occurred. Areas S1 andS2 represent the activity integral before and after heating respectively, while S3 is theintegral activity which left the sample during heating and is therefore not detected.Prior to performing routine experiments following this procedure, the TRIUMF accelerator divi-sion safety committee gathered to review the experimental proposal, and gave permission to performa first short study with a silicon carbide target sample. After confirming all steps went nominally,another safety committee approved the procedure could be carried out with several other samples ofISOL target materials [170].4.1.2 Thermal considerations in CHI standThe core of the release setup consists of a resistive heating assembly, whose main body is madeout of two copper brackets that clamp a water pipe, where Ohmic heating is applied by flowing aDirect Current (DC). Two thin tantalum foils bridge between the two copper brackets and act asheaters, and a ceramic insulator rated up to 10 kV was brazed between two sections of the pipeso that the current can be routed only through the heating assembly as shown in figure 4.3. Thetarget material sample under investigation has a mass of few hundred milligrams and approximatedimension of 2 cm in diameter and ∼0.5 mm thick. It is either held by a thin annular holder foil,or directly placed on top of the tantalum resistors to heat up the material pellet. The sample will106Figure 4.3: Single-pellet release assembly: 3D CAD model (left) and picture of the mountedassembly without copper heat shields for clarity (right)heat up either by absorbing thermal radiation while supported by an insertion tray, or by conductionfrom the bottom foil. Radioactive isotopes will be released from the material through diffusion andeffusion at high temperature, and will be either pumped into the nuclear ventilation system throughthe turbo and backing pump, or will stick to the cold surfaces within the CHI stand vacuum vessel.Either way, part of the activity would leave the sample under investigation and this quantificationreveals the material release performance for different elements.Some initial thermal-electric simulations were performed using ANSYS [171] to assess thesample heating performance, as well as to check against the potential risks of melting the copperparts of the assembly. The heat shields are meant to improve the heating performance by reflectingthe thermal radiation back towards the resistors, as well as protecting the vacuum chamber from theexcessive thermal shine that might otherwise require more extensive cooling capacity. This analysiswas performed with conservative resistor parameters, i.e. with wider and thicker dimensions thanforeseen (30 mm and 0.076 mm respectively) and high currents (500 A). After few iterations on theclamp and shield geometries, the simulated resistor foils were able to reach a satisfactory 2400 ◦Cwhile the copper temperature stayed lower than 700 ◦C as shown in figure 4.4, which was considereda safe temperature to advance towards the next testing phase.Figure 4.4: Simulated temperature distributions in the tantalum heater (left), copper supportsand shields (right).In addition to ANSYS simulations, temperature measurements were performed on the heatedassembly to establish a calibration curve between the current flowing through the heater assemblyand the sample temperature. During each calibration, a dual-colour METIS-M3 pyrometer with107spatial resolution of ∼3 mm (see section 2.2.3) was employed to monitor the temperature of afixed component of the assembly (i.e. the side of the tantalum heater foil). Meanwhile, a roundgraphite foil was sandwiched between two tantalum annular retainers, and was laid on the bottomheater foil. Pictures of the heater setup during and after heating are shown in figure 4.5, alongsidea reference plot with temperature recorded by the pyrometer and power computed from the powersupply current and voltage values.Figure 4.5: CHI stand temperature calibration using reference wires made out of vanadium(Tm=1910 ◦C) and titanium (Tm=1668 ◦C). Left: assembly being heated. Center: graphitefoil held by its tantalum retainer foils after the calibration, with melted wires as evidence.Right: example of a calibration curve where the black dots are optically measured heatertemperatures. The highlighted areas show approximately where it was noticed that thetitanium wire (blue shade) and vanadium wire (green shade) melted.This calibration allows one to estimate the current for which the top of the graphite samplereaches a temperature of 1668 ◦C and 1910 ◦C, the melting temperatures of titanium and vanadiumwires respectively. These two temperatures are for now enough to characterize the release, yet morewire materials can be implemented as benchmarks in the future, potentially also allowing for morecomfortable extrapolation to other temperature regimes.The edges of the bottom resistor sheet are usually bent upwards by∼2 mm creating a lip so thatthe retainers can be laid on one of these edges to leave some space between the sample lower surfaceand the bottom resistor itself. This configuration was implemented so that the pellets are not in directthermal contact with the much hotter resistor, which could induce high temperatures locally on thesample. Moreover, this lip allows for a tantalum surface perpendicular to the pyrometer line of sight,making its measurement more consistent. The pyrometer measurements can be used to estimate theerror bar that should be applied to the temperature measurement, by taking the pyrometer valuescorresponding to 10 A below and above the wire melting current range as shown in figure 4.5.Other than for error estimation, this temperature recorded optically is only a reference and is usedfor later comparing the temperature when activated samples are heated in the release setup, withouthaving the reference wires. Since these resistors are usually cut by hand, different temperaturecalibration curves need to be measured every time a new resistor is installed, which was happeningquite frequently at the beginning since a thin 25 µm resistor foils were mounted to keep the resistivecurrent low. Once the temperature of the release assembly has been characterized and the workingparameters are understood, the full procedure was carried out on specific sets of materials.108(a) Pellet (b) Foil (c) compositeFigure 4.6: Images of these three materials taken with a Scanning Electron Microscope byMarla Cervantes4.1.3 Single pellet experiments and resultsGraphite discs The target department at TRIUMF has recently proposed the irradiation of graphite-based targets for the production of selected RIBs such as 7Be, 8Li and 11Li. This choice results froman attempt to increase the yield of the mentioned species compared to tantalum targets. Moreover,the residual dose of irradiated graphite targets is a factor ∼5 lower than for tantalum counterparts,reducing the dose to personnel and expediting their disposal process (see figure 1.12). In addi-tion, the lower stopping power of 500 MeV protons in graphite and its high melting point allowfor increasing the proton beam current without reaching a thermal limit, as opposed to all otherconventional ISAC targets.Few candidate graphite structures were proposed to be placed inside ISAC online targets. Baregraphite foils of thickness 0.13 mm are the immediate first candidate, since they already constitutethe backing foil for composite ceramic targets at TRIUMF, and can be easily cut and loaded in targetheaters. A second proposed option involved slicing SIGRAFINE-R7510 ®graphite rods down to athickness of∼0.5 mm, as benchmark for common isostatically pressed graphite. The third option isa composite material developed at TRIUMF by Marla Cervantes [215], where a similar productionprocess to other ISAC composite carbide materials is employed. Their characteristics and physicalproperties are summarized in table 4.1, and Scanning Electron Microscope (SEM) images of allthree samples are available in figure 4.6, where particles on the order of ∼10 µm are present in thecomposite material only. These three materials were irradiated in the IPF facility over four differentirradiations, usually once every three weeks during some opportunistic time window.Most of the important measurement parameters and results for the three kinds of graphite sam-ples are presented in table B.3, where the result of every measurement is shown in the “counts”column. For some samples, more than one HPGE measurement was performed to further confirmthe statistical uncertainties which are characterized in figure B.1. The calculation of the releasefraction (RF) for each temperature is performed through equation 4.1; since the measurements areperformed a few hours apart, and given that the half life of 7Be is approximately 53 days, it can beassumed that S1∼ S2+S3, i.e. no significant decay happens over a few hours time span.109Sample material Thickness Bulk density Porosity- [mm] [g/cm3] [%]Graphite foil 0.13 1.15 43.54Graphite pellet 0.54 1.81 12.44Graphite composite 0.35 0.77 61.99Table 4.1: Average properties and characteristics of the graphite samples used for release stud-ies.Figure 4.7: Release fractions measured at two temperatures for the three graphite materialsunder investigation. Each point is a measurement.The uncertainty in the number of γ-ray photons detected has been determined by the InterSpecsoftware [11] assuming a Poissonian distribution, and averaged in case multiple measurements wereperformed. Moreover, additional uncertainties were added to the activity measurements for thecomposite pellets to account for the positional uncertainty during their measurement, resulting fromloose clamping and coarser alignment on the measurement jig due to their fragility. This factoraccounts for an additional ∼1.4% and 6% uncertainty in radial and axial position respectively, andmore information on this is available in the appendix section B.2 (table B.1 and figure B.2).The composite material outperforms its commercial counterparts at both temperatures, with re-lease efficiency improvements by a factor up to 10 as summarized in figure 4.7. An additionalconsideration to be made regards the structure of these composite samples; their backing substratemakes up for approximately half of the composite disc mass and is the same “C-foil” which is shownto have release efficiency lower than 8% at the temperatures under consideration. This means thatthe release efficiency of the compressed graphite powder mixture is even higher than quoted, how-ever, a number can not be assigned for the moment because of the big uncertainty in measuringthe mass of these samples. Although graphite composite presents the highest porosity from table4.1, this quantity does not seem fully correlated with the release fraction since graphite foil samplesdo not perform significantly better than the pellets. Instead, the fine microstructure of the compos-ite samples could alone explain its high performance compared to the two commercially-sourced110Figure 4.8: SEM pictures of two different SiC materials Left: conventional ISAC SiC compos-ite before heating. Centre: conventional ISAC SiC composite after heating, with someneck between particles in evidence (both adapted from [9]). Right: the new nanometricelectrospun material (adapted from [10]) where SiC grains are suspended on a carbonfibre matrix. The material tested is the central one.samples.The composite graphite discs have been irradiated online in ISAC targets twice by now, and haveshown better yields than standard tantalum targets for a limited - yet very crucial - set of isotopessuch as 8Li and 7Be. This online result fully validates the methodology and instrumentation whichhas been developed within this project and allows for effective use of online beam time by choosingthe best materials. It should be noticed here that release measurements were performed only for aberyllium isotope (7Be) because none of the lithium isotopes has a proper half life, either being tooshort (below one second) or stable (6Li, 7Li).Silicon carbide A common material in ISAC targets is a silicon carbide composite, produced atTRIUMF through the same slip casting procedure described in [9]. After tantalum and uraniumcarbide targets, silicon carbide is the third most irradiated target at ISAC because of its high perfor-mance in delivering RIBs of low atomic number elements (Z<13) coupled to its refractory natureand relatively high operational temperature of approximately 1600 ◦C.In the past years, an effort was started as part of John Wong’s master thesis [10] dedicated toproducing SiC-based nanofibrous material for ISOL production, as a proof-of-principle for the po-tential replacement of the slip casting procedure with an electrospinning method. As with someother initiatives [36, 35, 216], the new morphology and nanometric dimensions of these materi-als could enhance the release efficiency of exotic nuclei, which is usually the bottleneck for theirextraction [34]. A comparison of the microstructures of standard ISAC SiC material and the newelectrospun morphology is presented in figure 4.8 showing SEM images of both samples. Afterheating the standard ISAC SiC material for a few days at nominal operational temperatures, SiCparticles show signs of grain growth and the start of neck formation, decreasing the particle surfaceto volume ratio and increasing the isotope diffusion length. In contrast, in the nanometric mate-rial they are suspended on a carbon fibre backbone which prevents such thermal sintering effects,yielding a more consistent material microstructure throughout the target heating period.Release tests were performed on two standard SiC target discs following the same procedureexplained above for graphite materials, by looking at the release of two isotopic species, 7Be and111Figure 4.9: Release fractions of 7Be and 24Na measured at two temperatures and differentheating times for the two equivalent SiC samples.24Na. The same samples SiC1 and SiC2 were exposed to progressively higher heating times andtemperatures, by measuring their gamma activity in between heating stages. The results summariz-ing their release fractions are shown in figure 4.9 and table B.4. Exposing this material at the samelower temperature of ∼1668 ◦C for one minute or 10 minutes had comparable release fractions,suggesting the release process saturates within one minute. In addition, higher release fractionswere measured at 1910 ◦C even if the nominal operational temperature of SiC targets is limited to1600 ◦C. Both samples were irradiated on Sept 21st , 2021 and were heated two days after because ofpractical dose rate and personnel limitations which did not allow earlier processing. Unfortunately,it was not possible to fit the nano-SiC samples in the busy irradiation schedule, but there are plansto test these in the near future and compare the two.The procedure and experimental apparatuses developed for performing these experiments hasbeen validated with a second material class (SiC), and comparable release results were obtainedacross the two samples when similar heating conditions were applied. Previous offline studies atISOLDE on SiC matrixes [109] have shown higher release fractions for Na both Be isotopes inmicrostructured materials similar to the ISAC samples. Their analysis of several different samplemorphologies concluded that parameters such as microstructure and carbon content were signifi-cantly affecting the release results, which makes for a difficult comparison. Moreover, the presenceof graphite backing foil in the ISAC samples introduces geometrical constraints for diffusion andtriggers the production of the 7Be isotope in the carbon foil, making it difficult to compare our re-lease fractions to other samples. A common feature observed in all ISOLDE and ISAC SiC samplesis the reduction in release rate over time, which could be explained by high-temperature annealingof radiation-induced defects or material evolution such as sintering and densification [109]. Moregranular data on the ISAC samples would however be required to reach definitive conclusions onthe mechanisms at play.1124.2 Release tests through macroscopic structuresAs mentioned in section 1.4, the 60s and 70s have seen a wealth of release experiments to charac-terize radioisotope transport out of the material sample where they were produced. This approachfocuses on an important building block of an ISOL system i.e. the single material discs/foils/pellets,and the release measurements provide insights on the diffusion efficiency (εD) in these materials.One could argue that, for porous discs/foils/pellets, these measurements offer more than a measureof the diffusion process, since the isotope travel path is also characterized by flow through porestructures; yet, the information is limited to the boundaries of one single building block of an ISOLtarget.In order to understand how the macroscopic release process works throughout the stack of disc-s/foils/pellets of an ISOL target, several kinds of simulations have been performed and analyticalmodels have been developed (see section 1.4.2), and some good agreement with experimental on-line data were reached for some selected cases. However, to model the real effusion process, onehas to make several assumptions on parameters such as sticking time τs and processes such as thelikelihood for a travelling particle to be re-adsorbed on another surface and undergo subsequentdiffusion into the material. Moreover, ideal geometrical and operational assumptions such as tem-perature homogeneity and perfect target geometry are frequently taken as given, yet they may besignificant in changing the simulated results. Despite these uncertainties, no experimental inves-tigation of this macroscopic effusion step alone has ever been performed in an ISOL geometry toour knowledge, and this section aims at providing a description of the first tests in this directionperformed at TRIUMF.4.2.1 Methodology and rationaleThe experimental procedure to perform such macroscopic offline measurements has been developedwithin my PhD work and is conceptually similar to the methodology in the previous section, but witha few key differences. ISOL material samples would still be irradiated at the Isotope ProductionFacility (IPF), and instead of measuring their activity on a HPGE detector, they are loaded into ashort version of an ISAC target heater/container as shown in figure 4.10. This assembly wouldalready be loaded with more than one hundred other foils of the same shape and material, so theactivated sample(s) can be placed at one end of the stack. Then, two round tantalum end-caps areinserted on either side of the stack to seal the inner target volume, in an identical fashion to thepreparation of online targets. Once the assembly is heated, isotopes will diffuse out of the activatedsample(s) and start their effusion path through the stack of foils until being released out of thecentral hole where the ion source (IS) is usually connected to the target. After heating the assemblyfor an arbitrary (but set) time, it will be cooled down to room temperature and all of its foils will beremoved. The activity of each individual foil - or each stack of an equal number of adjacent foils- will be measured with an HPGE detector to assess the activity distribution throughout the heaterlength, which should provide the first insights on the effusion process in ISOL systems.The first (and so far only) experiment carried out with this setup is meant to assess how geomet-113Figure 4.10: Assembly pictures for macroscopic release tests. The assembly is shownmounted in (a) and being resistively heated in (b),(c) demonstrating how the non-activeD-shape tantalum foils are put together in packs of 20 foils kept in place by tantalumwire, before sliding them into the resistor (d). (e) is a magnification of (b) highlightingthe foils through the side hole of the heater, where the black circumference is the py-rometer scope aim area. f) is a schematic cut-view representation of the assembly.rical configurations affect effusion phenomena in ISOL targets. Practically, this involves assessingthe radioisotope activity in all foils across the heater length, and compare it across the two targetconfigurations which employ different sample geometries. Tantalum foils of standard 25 µm thick-ness and cut in two shapes, D-shaped and round, were chosen for this initial experiment for differentreasons.Firstly, all ISOL targets irradiated in the ISAC facility are made up of D-shape pellets or foilswhich are thought to promote faster effusion towards the ion source, which would ultimately im-prove the yield especially for short-lived species. On the opposite side is the fact that a round foilgeometry would increase the in-target production rate of radioisotopes just by employing more ma-terial per unit length, as well as improving the thermal conduction path to the higher portion of thetantalum container. However, no experimental study has been performed to compare such effusionmechanisms, and this work intends to fill this gap. Secondly, tantalum metal foils were chosen for114this first test since they are easier to handle, unlike carbide materials used in ISOL targets which tendbreak into smaller pieces. Lastly, the high spallation cross section for tantalum results in the pro-duction of several isotopes with different half lives and elements with diverse chemical properties,increasing the chances to more accurately quantify their activity in the foil stack.4.2.2 Temperature benchmarksThe target heater used for these effusion investigations was designed so that the temperature condi-tions would be equivalent to standard ISAC targets operated at low power, since it is impractical toreproduce the temperature distribution generated from the proton beam power deposition. The corepart of this heater assembly is a 20-cm long tantalum cylinder with lateral thickness of 0.38 mm,where Ohmic currents up to 580 A are applied to bring its temperature close to 2000 ◦C. Since themaximum operating current in CHI stand is limited to 500 A, it was decided to reduce its thicknessdown to 0.2 mm so that a similar power distribution could be achieved. Temperature simulationsFigure 4.11: Simulated temperature distribution achieved with 450 A of Ohmic current, usingan emissivity value of 0.2 across all tantalum surfaces and a heater thickness of 0.2 mm.of a modified target geometry were first performed to assess if the new geometry could reach thedesired ∼1900 ◦C. Since this container would be wrapped in tantalum heat shields, an effectiveemissivity of 0.2 was applied to all tantalum surfaces following the calibrations performed in [97].The simulated temperature plot of the geometry described with an input current of 450 A and with-out any target material foil inside, is shown in figure 4.11, where its central areas exceed the desiredtemperature.The heater components were assembled into the CHI stand oven to characterize their tempera-ture using the METIS two-colour pyrometer, by looking directly at the tantalum samples throughthe ion source hole as pictured in figure 4.10e. Temperature measurements along with the powervalue recorded by the supply are shown in figure 4.12, where temperatures higher than 1900 ◦Cseem achievable at currents of 450 A or beyond, compatible with expectations from simulations.115Figure 4.12: Experimental calibration with the pyrometer focused on the foils, through the ionsource hole.Figure 4.13: D-shape pack on the HPGe for γ-ray spectroscopy4.2.3 Experimental parametersTwo tantalum foils of each shape (D and round) were irradiated in the same IPF container, and eachpair of equal shape were mounted in one of two effusion heater assemblies shown in figure 4.10.These were previously loaded with seven packs of twenty non-activated foils each, one containingonly D-shape, and the other only round foils, so that their equivalent irradiated samples were addedat one of the extremities as shown in the schematic figure 4.10f.Once each assembly was individually loaded into the vacuum chamber, the heater current wasgradually increased to 200 A and left constant for ∼ 10 minutes to let any residual water and othermolecules outgas until the system pressure stabilized in the 10−5 Torr range as shown in figure4.14. An experimental measurement using the pyrometer was not possible, indicating temperatures<1000 ◦C which would likely prevent any meaningful transport of elements of interest. More-over, ANSYS simulations at this current showed a maximum temperature of the heater assembly of840 ◦C, compatible with the pyrometer. Then, as shown in figure 4.14, the current was manually116increased to 450 A within 30 seconds, and a few more seconds were needed until the voltage stabi-lized, indicating that a steady temperature profile was reached. The heater current was kept constantfor an additional 60 seconds, at the end of which the power was abruptly brought to zero, allowingthe assembly to cool rapidly below 1000 ◦C.Figure 4.14: Recorded variables during the heating phase of the tube for macroscopic releasetests.The intense outgassing noticeable from the pressure rise during the heating phase needs to beconsidered as a potential factor changing the gas flow conditions inside the target. The Knudsennumber Kn = λg/lh gives an estimation of the gas flow regime by comparing the mean free pathof the gas λg to the characteristic length of the geometry under consideration lh. One could arguethere are two separate characteristic lengths in the targets assembled where atoms can flow, i.e. thesub-millimetre spaces between foils and the inner diameter of the tantalum tube, and the upper voidportion of the D-shaped discs shown in figure 4.10, whose hydraulic diameter Dh is 5.82 mm whereDh = 4A/P, with A = 50.7 mm2, P=34.8 mm. To achieve molecular flow, the Knudsen numberKn > 10, hence the mean free path of the inner gas should be greater than ∼6 cm. Using expression[217]λg =kBT√2Ppid2(4.3)where d is the atomic cross section ∼0.5 nm for a large atom, kB the Boltzmann constant and Tthe absolute temperature, the maximum pressure which would allow for molecular flow conditionsinside the target assembly is 3.4 · 10−3 Torr. During the heating phase shown in figure 4.14, themaximum recorded pressure at the pressure gauge located ∼50 cm away from the target was oneorder of magnitude smaller than this computed value and could result in the onset of a transitionregion between molecular and laminar flow regimes. Further considerations about the flow regime117implications on the experimental results will be discussed in section 4.3.All seven foil packs in each heater were extracted using hammer and pliers where needed, sincesome of these foils sintered in some spots to the hot tantalum container surfaces. Each pack wasthen put in two ziplock bags and transported to the HPGE detector for gamma measurement. Due tothe very small amount of activity migrating onto these foils, it was necessary to fix these samples incontact with the outer surface of the detector window as shown in figure 4.13 to achieve meaningfulstatistics while making sure that the activity from all packs could be measured within roughly a dayand capturing the same radioisotopes.In order to compare activities across all fourteen packs with each other it is necessary to applya similar procedure to the one described in section 4.1, but with some key differences:• The activity in each pack is measured after heating only, since before heating none of thepacks were active.• The comparison that can be made is between D-shape and round packs positioned at the samereciprocal position with respect to the active pellets.• All activities measured shall be extrapolated to one arbitrary time, in this case set as when thesecond assembly was heated• Some gamma background peaks were interfering with the measurements hence a further cor-rection needed to be applied, and in some cases the interfering peaks were discarded.• Gamma lines at different energy and from the same isotope need to be compared, hencethe detector efficiency εHPGe and the self-attenuation efficiency εSA (see equation 4.5) in thesample packs should be accounted for.After measuring the activity of each line in the γ-ray spectrum, the activity for each isotope iscalculated from each gamma line with the formulaAs =ADBRi exp(− ln(2)∗(tmeas−tre f )T1/2)εHPGeεSA(4.4)where εHPGe is the γ-ray detection efficiency defined in equation 2.2, BRi is the branching ratiofor the decay of isotope i under inspection, (tmeas− tre f ) represent the time gap between when thegamma measurement happened and the heating time of the second foils stack (taken as referencetime), and T1/2 is the isotope half life. The self-attenuation efficiency is computed asεSA =∫ L0 Al · exp(− µρ (L− x))dx∫ L0 Aldx=ρµ[1− e µρ L]L(4.5)where (µ/ρ) is the energy-dependent total mass attenuation coefficient as defined in [218], L is themass thickness of the sample expressed as L = ρt where ρ=16.6 g/cm3 is the tantalum density and118t=0.05 cm is the thickness of all combined tantalum 20 foils in each pack. In such geometry, thisself-attenuation coefficient accounts for at most a 20% reduction in transmission. Al is the activitydensity, which is here assumed to be uniformly distributed across the thickness of each 20-foil pack.4.2.4 ResultsAn example of a γ-ray spectrum recorded by the HPGE detector from one of the 20-foil packs isshown in figure 4.15, where several gamma lines were identified and fitted through the InterSpecsoftware. The calculation of the activity for each isotope in the foil stacks enabled computing theirFigure 4.15: Left: γ-ray spectrum recorded from a 20-foil pack. Centre and right: examplesof peaks and their fit performed with the InterSpec software [11].relative distribution throughout the two loaded targets. Figures 4.16 and 4.17 show this activity dis-tribution of four isotopes throughout the foil stacks, measured from every γ-ray energy. It should benoted that the activities as low as∼20 Bq were measured, which is hence considered as approximatelower limit for activity detection, given the maximum measurement time for each sample of around30 minutes.In order to more accurately compare the isotope profiles from the two geometries, a weightedaverage of the activities from each gamma line is taken, and the comparison between these averagedactivity profiles in D-shaped discs and round discs is shown in 4.18, where a couple of features canbe noticed. Firstly, the round pellets do not allow for any activity detection after the fifth stack offoils, corresponding roughly with the middle of target where the ion source hole is. This likely pointsto the preferential path for effusing isotopes to be directed out of the∼3 mm ion source opening tuberather than continuing through the small openings between the round discs and the inner surface ofthe heater tube. The activity distribution in D-shaped foils only slightly drops between the 5th and1st pack, likely due to the bigger cross sectional area that allows effusion through the void portion ofthe assembly, rather than being extracted from the ion source hole. This potentially obvious resultsdoes not however provide any quantifiable quantity of the different effusion properties betweenthe two assemblies, but still proves that a significant qualitative difference exists. A quantificationis possible by comparing the activity ratios between foil pack#7 and pack#5 of both geometries,119(a) 165Tm in D-shaped foils (b) 165Tm in Round foils(c) 170Lu in D-shaped foils (d) 170Lu in Round foilsFigure 4.16: Activity distributions estimated from different gamma lines in the D-shape (left)and Round (right) foil assemblies for 165Tm and 170Lu. These two isotopes are shown asexamples of the typical contamination trends seen in the foil stacks during the analysis,and additional isotopes can be found in section B.4.since it was always possible to detect a meaningful activity in these. To compare the two ratiosbetween geometries, the quantity (R7/5/D7/5) is calculated for all gamma lines and shown in table4.2, which provides a quantification of how much steeper the activity drop in the round discs appearswith respect to D-shape discs. The ratio (R7/5/D7/5) is calculated for each isotope by averaging theactivities of each gamma line and weighting them according to their branching ratio, and shown incolumn “Avg.” in table 4.2. These ratios span between 2.5 and 16, with an average of 6, meaningthat the D-shape geometry allows for approximately a six-fold more effective effusion betweenpack#7 and pack#5.Some clarifications are necessary to capture the significance and limitations of these plots. First,for isotopes 160Ho and 173Hf, significant discrepancy is present between the activities calculatedwithin the same foil stack with different gamma lines; possible explanations for the mismatch mightbe due to the presence of a noisy background which are usually subtracted only if there is a no-ticeable peak in them, close to the peak of interest. Another explanation could be linked to anunderestimation of the statistical uncertainty due to the low gamma energy of some of these lineswhich could be attenuated more than foreseen by the tantalum foils themselves in case an asym-metric contamination was present in the foils, resulting in enhanced attenuation. Moreover, since120(a) 160Ho in D-shaped foils (b) 160Ho in Round foils(c) 173Hf in D-shaped foils (d) 173Hf in Round foilsFigure 4.17: Activity distributions estimated from different gamma lines in the D-shape (left)and Round (right) foil assemblies for 160Ho and 173Hf.capturing the trend across packs is more important than an absolute activity comparison betweengeometries, it was decided to exclude some gamma lines during the computation of the averageactivity shown in figure 4.18; focus was instead put on the lines which were detected in most packs(excluding gamma lines E = 296, 306 and 311 keV for 173Hf) and with significant branching ratio(excluding line at E = 872 keV for 160Ho).Another custom treatment has been dedicated to some isotopes that are significantly producedinside the samples by decay of a long-lived parent, if their own half life is much shorter than the timedifference between reference and measurement times (T daughter1/2 < 3 (tmeas−tre f )). This is the case ofthe parent-daughter pair 160Er, 160Ho (T parent1/2 = 28.6 h and Tdaughter1/2 = 25.6 m), for which the half-lifeof the parent nuclide (Er) was used in equation 4.4 to extrapolate the daughter activity at time tre f .It should be noticed that FLUKA simulations are predicting the production of 160m1Ho isomer witha yield ∼30 times lower than for 160Er, which decays into the 160Ho ground state with T1/2=5.02 h.Since Ho and Er should have similar effusion properties due to their similar boiling points (2300◦Cand 2600◦C respectively [119]), it can be expected that most of these gamma rays (a factor ∼30more) were produced by the 160Ho atoms coming from the decay of 160Er rather than from thedecay of 160m1Ho. The same procedure was also applied to the parent-daughter couple 166Yb, 166Tm(T parent1/2 =56.7 h and Tdaughter1/2 =7.7 h respectively), although the assumption that Tdaughter1/2 < 3(tmeas−121(a) 165T m (b) 170Lu(c) 160Ho (d) 173H fFigure 4.18: Comparison between the average activities from D-shaped vs round pelletstre f ) is no longer fully valid since (tmeas− tre f ∼ T1/2(166Tm)=7.7 h. Plots of the 166Tm distributionand other isotopes detected across packs are all available in appendix B in figures B.3 and B.5.One last comment should be made on the potential influence of the high vacuum pressure inthe chamber while Ohmic heating was applied to the target containers. If transition or viscous flowregimes were present in the target, especially in the upper void section of the D-shaped foils, mostof the radioactive isotopes coming from the activated side of the foil stack would be preferentiallydriven out of the central ion source hole by the gas load, rather than moving towards the otherclosed end of the target. The detection of gamma lines of all isotopes in packs #1 through #4 in theD-shaped foils hints at the presence of molecular flow regime that allows isotopes to explore anyregion of the void target volume rather than being driven out of the biggest aperture (the ion sourcehole in the middle) by pressure gradients.122Nuclide FLUKA γ-En. BR D7/5 R7/5R7/5D7/5Avg.R7/5D7/5- [Bq] [keV] - - - - -Ho160 6.90E+05 645.2 4.5 52 144 2.8 3.6728.2 9.0 50 N/A N/A872.0 1.5 65 N/A N/ATm165 1.36E+06 242.9 35.5 43 292 6.8 6.0806.4 9.5 46 137 3.0Tm166 1.09E+06 184.4 16.2 28 135 4.9 4.3705.3 11.1 43 74 1.7778.8 19.1 51 233 4.6785.9 10.0 43 230 5.4875.7 4.1 57 N/A N/A1176.7 9.6 52 N/A N/A1273.5 15.0 51 207 4.01374.2 5.7 51 N/A N/A1867.9 4.1 49 N/A N/A2052.4 17.4 46 202 4.42079.5 6.4 53 293 5.5Tm167 3.59E+05 207.8 42.0 35 261 7.5 7.5Lu169 2.04E+06 191.2 18.7 39 N/A N/A -960.6 21.2 48 N/A N/A1449.7 9.0 57 N/A N/ALu170 1.47E+06 1225.7 4.8 N/A N/A N/A 2.61280.3 7.9 62 N/A N/A2041.9 8.4 68 174 2.62364.1 1.5 40 N/A N/A2748.2 2.1 N/A 198 N/ALu171 4.73E+05 739.8 48.7 58 192 3.3 3.3Lu172 1.96E+05 1093.6 63.0 51 N/A N/AHf173 3.12E+06 123.7 83.0 24 394 16.2 16.2297.0 33.9 N/A N/A N/A306.6 6.4 N/A N/A N/A311.2 10.7 N/A N/A N/ATable 4.2: Main isotopes of interest in the effusion setups. The “FLUKA” column is the sim-ulated activity, while D7/5 and R7/5 are the activity fractions in pack 7 vs pack 5, for theD-shape and round geometries respectively. Column “Avg.” is the average of the activityfractions R7/5 / D7/5.1234.3 Discussion and outlookThe two kinds of experiment explained and performed in sections 4.1 and 4.2 have proved that thedeveloped methodologies and capabilities of CHI stand are appropriate for testing hypotheses tobetter understand phenomena of diffusion and effusion in ISOL targets at large, representing themain bottleneck for delivering intense RIBs to ISOL users.The experiments explained in section 4.1 provided for the first time a quantification of the dif-fusion and release properties of ISAC target materials, in order to gain deeper understanding andpredictive capability of the role of microstructure in release phenomena. These have already provedvery useful in confirming that the graphite composite material has been the best choice for onlineoperation. Moreover, testing and comparison of several other materials are already planned, espe-cially comparing ceramic samples produced with different techniques, i.e. the standard ISAC slipcasting procedure against its improved 1-step casting for producing uranium carbide samples [219]or comparing standard SiC discs with their nano-SiC equivalent.More experiments following this methodology are foreseen in the future, building on the experiencegained in the first few complete runs. The temperature benchmarking methodology developed asa combination of simulations, pyrometry and looking at the melting point of materials has yieldedconsistent results. More granular data should now be collected, by performing these tests aftermore heating times and at more temperatures. Some upgrades for speeding up the transport anddecontamination of the samples are also foreseen, to further minimize the dose received by the ex-perimental team and hence increase the number of samples available for testing in each irradiation.This could also allow for higher activity in the samples, improving the statistical significance of thedata collected.The macroscopic effusion experiments performed in a fully stacked target presented in section4.2 are the first direct offline measurements which quantify the effusion phenomenon in ISOL tar-gets. From both qualitative and quantitative information, it was proved that the D-shape foils providethe conditions for enhancing the effective transport lengths of isotopes through the voids within thetarget assembly by the addition of an upper volume which provides an “accelerated pathway” for ef-fusion. From a practical perspective, this study gives immediate input for future optimization of thepacking structure of ISOL targets. In fact, promoting “long-distance” effusion through a tantalumcontainer should be a priority for the foils located furthest from the central ion source hole, but itmight be best to also add a round pack of foils in correspondence of the ion source hole. The centralround foils should in fact improve the ability of extracting more efficiently short half-life isotopesproduced closer to the ion source hole, without affecting the long-distance transport. Moreover, thishybrid structure would create two mirroring halves of the target volume, which should not add anysignificant delay in the effusion stage of isotopes created in packs further away from the centre, foran overall increase of performance.Several other experiments can be envisaged, such as performing a similar procedure with porouscarbide discs, or comparing significantly different geometries such as ISAC D-shaped pellets to theannular pellets used for the proton-to-neutron target, or to the SPES target geometry [220] and to124the ISOLDE rolled foils [221]. Another question very relevant for the future development of theP2N target assembly is to determine if the graphite cartridge acts in any meaningful way as an iso-tope container, or if it is instead effectively “transparent” to isotopes, which could go through itsporous structure without any meaningful delay. Systematically testing these hypotheses offline canbe foreseen, giving more room for exploration of target design ideas where the tantalum containerdoes not have to be hermetically welded as long as a tight graphite cartridge is present.125Chapter 5ConclusionThe most challenging aspect for the delivery of Radioactive Ion Beams (RIBS) through the IsotopeSeparation On-Line (ISOL) method involves the process of transporting radioisotopes out of thetarget material through diffusion and effusion. Elements with low vapour pressure or with shorthalf-lives in the order of less than a few milliseconds have not been successfully extracted fromsuch targets, preventing a wealth of experiments from happening in user facilities such as ISAC.Although technical improvements of ISOL targetry have been ongoing in several facilities, less ef-fort has been directed to better understanding some core transport behaviours of isotopes in ISOLtarget materials. Hence, this thesis presents the conception and successful implementation of twodevelopment projects aimed at experimentally studying key processes and mechanisms, which ulti-mately improved the intensity and quality of certain important Radioactive Ion Beams (RIBS). Thishas enabled more successful user experiments and has provided additional insights to fundamentalmechanisms governing the diffusion and effusion at high temperature and high radiation fields.Two main approaches to the same underlying question were undertaken, investigating target per-formance during proton irradiation in the main target station (online) and in a much more controlledlab environment where certain parameters are subject to less uncertainty (offline). This double-sidedapproach that keeps fundamental development activities closely linked to user needs has proved tobe a winning strategy thanks to the high prioritization of these activities.The spallation-driven, two-step target development project presented in chapter 3 is the mostevident example of the successful partnership between the target team and user facilities, wherean overlap in interests was identified and exploited to satisfy both needs. Firstly, the careful con-ceptual design that included particle-tracking simulations and thermo-mechanical analysis allowedfor a minimal interaction between the incoming proton beam and the annular target material, sup-pressing unwanted isobaric contaminants by a factor 5 to 20. Secondly, its resulting homogeneoustemperature distribution for any given proton beam intensity enabled performing several parametricinvestigations which are otherwise not possible in conventional ISAC-TRIUMF targets. As a result,release efficiencies of tens of isotopes were characterized at different heating conditions, confirmingexpected trends for all isotopes. Moreover, yield measurements were found to be linearly correlatedto the proton beam intensity, suggesting that Radiation Enhanced Diffusion (RED) is negligibly126important for this target system. Such effects are usually very difficult to study systematically inconventional ISOL targets due to the beam power deposition that results in non-homogeneous andmore unpredictable temperature distributions. In addition, the irradiation for the first time of a con-ceptually different target geometry (annular) provided a new parameter for investigation of transportphenomena in ISOL systems.After two successful irradiations of this spallation-driven two-step target, it is likely that it willbecome routinely operated in the ISAC facility, as well as in the new ARIEL Proton Target West(APTW). Some technical and operational questions are still to be answered, i.e. if target assemblywindows are necessary or if the measured tin efficiencies can be improved at higher target tem-perature. Nonetheless, a very promising overall picture has been established and much room forfurther development is available. As examples, changing the annular target material dimensions canfurther improve the RIB purity, as well as the implementation of an upstream collimator to suppressscattered protons. On the opposite side, a higher neutron fission rate can be achieved by includingthe converter body within the annular target material.A new experimental apparatus to perform off-line radiotracer studies has been developed andimplemented at TRIUMF as shown in chapter 4, where two methodologies were carried out toinvestigate transport phenomena in RIB production targets. The single pellet methodology has beendeveloped and has been validated through several experiments which have already been proveduseful in driving the existing target material development projects in the TRIUMF Target and IonSources (TIS) department. The main strength for conducting such studies at the Isotope ProductionFacility (IPF) at TRIUMF involves the broad range of radioisotopes which are accessible duringmaterial activation thanks to the high energy proton beam sent to this irradiation station. As a result,graphite composite materials were identified as far better candidates for RIB production comparedto other commercial materials, and their great performance has been proved in two on-line targets.Materials already foreseen to undergo similar systematic tests in the near future are all carbides ofsilicon, uranium and tantalum. Beyond the characterization of ISOL target materials, this systemwill allow quantitative contributions to high-temperature diffusion properties and mechanisms thatare required on the path to developing models that can describe elemental transport processes incomplex materials.The second kind of radiotracer study presented and validated in chapter 4 has been used to gain abetter understanding of the effusion process in ISOL targets. So far, analytical models and MonteCarlo simulations have been used as tools to qualitatively understand effusion times in selectedbut limited configurations, however no offline experimental validation of such systems has beenattempted at ISOL conditions. This thesis presents the first experimental study conducted with theintent of measuring the distribution of radioactive species effusing throughout the whole ISOL targetvolume. In general, this procedure will be used for gaining a better understanding of the effusionprocess in such targets, to optimize their geometries, to benchmark transport models, and also toanswer future questions on isotope permeability through materials at high temperature.To summarize, this thesis has aimed at developing online and offline capabilities to better un-127derstand transport phenomena in RIB production targets. The successful experiments performedonline and offline have contributed to opening up new concepts and parameter spaces for investiga-tion not only at TRIUMF, but also across the ISOL target and high-temperature elemental transportcommunities.128Bibliography[1] G.V. Samsonov. Handbook of the PHYSICOCHEMICAL PROPERTIES of the ELEMENTS.Springer, Boston, MA, 1 edition, 1968. ISBN 978-1-4684-6066-7.doi:https://doi.org/10.1007/978-1-4684-6066-7. ix, 33, 73, 80, 85[2] Brookhaven National Laboratory. 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ISSN 00223131.doi:10.1080/18811248.1982.9734137. 165150Appendix AAdditional proton-to-neutroninformationThis section includes additional information on the data from the proton-to-neutron converter chap-ter.A.1 Target engineering design considerationsAn extensive simulation campaign was started with the objective of both optimizing the thermalperformance of the target, i.e. minimizing temperature gradients and achieve container temperaturesclose to 1900◦C, and specifying the needed resistive current to achieve these conditions. Figure A.1shows cut views of six proton-to-neutron target geometries which were investigated with the FiniteElement Analysis (FEA) software ANSYS [171], along with their temperature distribution whenan arbitrary amount of resistive current (mentioned in each caption) is forced from the terminals,also called “legs”. The geometries here simulated at the beginning were simplified by not includingthe connection tube to the ionizer volume, neglecting the ∼5 cm long connections to the lowerwater-cooled current connectors and not including the UCx target material.All target heating simulations were set up with a defined current input (i.e. 1000 A) on one leg,and a voltage constraint (usually 0 V) on the other leg so that the current would flow between thesetwo parts, while passing through the target heater. The ANSYS Thermal-Electric module wouldthen compute the volumetric Joule heating that results from the current flowing in the tantalumtube. As far as heat exchange, a perfect thermal contact is always assumed between each part giventhey are all electron-beam welded together, and that the target “cap” is press fit with tight tolerances.The elevated temperatures of the heater enable high thermal exchange from their surfaces throughradiation, which was modelled here. For the surfaces facing a closed volume of the geometry, i.e.the inner faces of the tantalum heater, the thermal exchange through radiation was implementedthrough the surface-to-surface routine available in ANSYS (see section 2.4.2). The outer surfacesof ISOL target heaters are usually wrapped in three layers of tantalum heat shields to reflect thethermal radiation inward and make the temperature profile more homogeneous. To represent this151(a) Current=1000A(b) Current=1000A(c) Current=1000A (d) Current=1500A(e) Current=1500A(f) Current=1500AFigure A.1: All colour scales are in [◦C]situation, the outer surfaces of the target heater were assigned a constant emissivity value of ε=0.2according to test previously performed at TRIUMF in comparable situations [97]. It should be notedthat effective emissivity value might not capture the temperature smoothing effect resulting fromthe radiation exchange, hence the real situation should result in a more homogeneous temperatureprofile with respect to the simulated profile.Figure A.1a shows a first-trial geometry made by simply connecting two 3-mm thick legs to thebases of the target cylinder. Although a good maximum temperature of 2244 ◦C is reached, the coldspot on the graphite cartridge is several hundred degrees colder than its hottest spot. To mitigatethe non-homogeneity, the same cartridge was inserted in a longer tantalum heater figure A.1b, withthe intention of spreading the current and temperature throughout the lateral surface and achieveform factors similar to standard ISAC targets (see figure 1.7a). The wanted gradient effect wasindeed achieved, however the average temperature across the graphite dropped significantly. This152suggests that spreading the resistive power deposition on the container could achieve the wantedeffect only with higher currents. A second major issue of the presented geometry in figure A.1b isthat the extension of the tantalum tube does not allow for placing a water-cooled tungsten converterassembly very close to the cartridge, and the concept was then dismissed. Figures A.1c and A.1dshow instead two examples of an attempt of re-directing the current to the cylindrical heater throughtwo contact points from each side, which is creating an aperture in the middle of each “leg”. Theformer presents the first attempt, presenting a small hole which results in little improving effect,while the latter presents a better temperature distribution, although with a very hot spot on theleg. Figure A.1e shows an attempt at mitigating the hot spot issue on the leg by increasing itswidth, although a high temperature was still present. More importantly, it was tried to achieve fourresistive contact points between the leg and the heater, a concept which was named “elephant ears”and improved even more the temperature gradients across the graphite heater. In figure A.1f, the legsthickness was increased everywhere to 3 mm, except close to the cylinder where a small it was leftat 2 mm to increase the current density and resistive heating only close to the cylinder. Moreover,the outer diameter of the heater was reduced from 40 mm to 36 mm in order to help increasing thetemperature of the main body.After reaching a satisfactory simulated result, this assembly was prototyped and built for thermaltesting in the ATTS stand (see section 2.2.2), to understand if it could survive at high temperaturefor extended periods of time, although a hot spot of >2400 ◦C was present in simulations on theelephant ear legs. After 24 hours at high temperature, the tantalum connection points between thelegs and the cylinder broke and generated a runaway effect which completely separated one leg fromthe rest of the assembly as shown in figure A.2. The radius of curvature of the lower leg-to-heaterconnection was then increased to avoid stress concentration, along with few other curvature changesin the legs which resulted in lower leg temperatures in the latest iteration in figure 3.14a, where theuranium carbide is also present. The latest version of the target container was then manufactured andheated up to online conditions in the ATTS stand, monitoring its behaviour and compare measureddata to simulations.Figure A.2: Built prototype of the model shown in figure A.1f:before heating (left), after heat-ing (right)The target main cylinder was wrapped in three layers of tantalum heat shields as mentioned ear-153lier, covering both the lateral surfaces and the two round bases of the assembly in order to decreaseits effective emissivity and achieve higher temperatures. Two type-C thermocouples were insertedin the assembly as shown in figure A.3 to monitor the temperature of the cylindrical surface underthe tantalum shields (outer TC), and one dangling inside the hollow target body to capture an aver-age temperature of its walls radiating inward (inner TC), approximating the closed target enclosureas a black-body (see section 2.2).Figure A.3: Proton-to-neutron test target assembly mounted in the ATTS vacuum furnace.Test # # cycles Heat time Current Inner TC Outer TC[s] [A] [◦C] [◦C]1st 1 3 hours 1070 1990 - 1997 2047 - 20512nd 1 4 days 1070 1925 - 1980 1950 - 20303nd 1 16 hours 1050 1540 - 1620 1928 - 19334nd 10 1 day 1150 1845 - 1880 2015 - 20355nd 1 1 day 1150 1840 - 1850 1985 - 20256nd 1 3 days 1275 1970 - 2000 2065 - 2129Table A.1: Summary of the major parameters recorded during the proton-to-neutron targetoffline thermal testing. Inner TC corresponds to the reading from the thermocouple whosetip is positioned at the centre of the empty target volume while outer TC is the moreexternal one.A total of 15 thermal cycles from ambient temperature up to ∼2000 ◦C were performed, anda total of ∼10 days at nominal conditions were achieved. This thermal test proved very useful in154assessing the capability of this target design of withstanding the electrical load and maintainingtemperatures compatible with online operational conditions. Figure A.4 shows graphically the tem-perature trends recorded during tests 3 through 5 from table A.1, where two long periods of heating(test 3 and 5) are shown, with a series of 10 cycles in between (test 4). One weird observation is theinner temperature plot, which significantly drops during test 3 but then seems to recover, followingthe trend of the other outer thermocouple. Other characteristic features of high temperature com-ponent testing are also evident in figure A.5, where voltage across the assembly and temperaturebehaviour are shown when the current was left constant at 1070 A during test #1. Here, a downwardvoltage trend was noticed, being reduced by ∼2% over half hour, which is a typical observation atthe beginning of a heating cycle due to sintering of components together. Also, voltage spikes canbe noticed, whose “tail” is visible for up to few minutes after the initial rise, and which are likely re-lated to material properties and configuration changing while at high temperature. The temperatureresponse of both thermocouples is also shown, presenting ordinary fluctuations that are usually seenover half hour runs or longer periods, which affect the width of the temperature ranges mentionedin table A.1. The absolute temperature measured by the two thermocouples is always such that theinner thermocouple is colder than the outer one. Since the inner probe should capture an averagetemperature of all inner walls, including the colder cylinder bases/faces, it is reasonable that thelower reading is always the inner temperature.The results from this tests were deemed positive since the target withstood the increasinglyhigher temperature conditions and thermal cycling, unlike its previous prototype. Moreover, themeasured temperatures reached compatible values with the online operation of a conventional ISAC-UCx target, and were higher than the simulated case with the target material present inside. Thegeneral design features of this target, especially the material thickness and elephant legs specificshape, were considered adequate and implemented in the full assembly detailed design.The tantalum target heater defines the outer envelope of the container, and in turn defines theouter radius of the UCx target material. Taking into account the graphite cartridge that shall beinserted into the heater, the maximum uranium carbide target diameter is around 33 mm. On theother hand, the inner diameter of the annular UCx target material has been optimized by means ofparticle simulations as presented in section 3.2.155Figure A.4: Temperature profiles during thermal tests number 3, 4, 5.Figure A.5: Voltage at the power supply terminal and temperature behaviour during thermaltest #1 when the current was stable at 1070 AA.2 Converter engineering design simulationsThe conceptual development and overall rationale of the converter assembly has been discussed inchapter 3, while here more details are provided on the thermo-mechanical simulations performed toassess its capability to withstand a high-power proton beam irradiation.Converter thermal boundary conditions The heat input into the system is made out of two contri-butions: the beam power deposition in the full converter geometry from an incoming 100 µA protonbeam imported from FLUKA is 7525 W, including the power deposited in the copper brackets; sec-ond, the radiation shine coming from the hot target container 9 mm away from the front face ofthe converter is 698 W, and the surface to surface radiation module is enabled to emit some of thispower back to the target. The remaining heat is fully extracted by the water running in the channels156around the converter, which is routed out of the target station and cooled down by a heat exchangedbefore being reinserted at a lower temperature and start cooling again. A crucial parameter to assessthe amount of power that can be extracted by fluid convection is the heat transfer coefficient whichcan be computed analytically from the Dittus-Boelter formulaNu = 0.023Re0.8Pr0.4, (A.1)with the formula defining the Nusselt numberNu =hLk f(A.2)where h is the heat transfer coefficient expressed in W m−2 K−1, L is the characteristic length ofthe geometry - in our case the hydraulic diameter of the pipe is 7.7 mm, k f is the thermal con-ductivity of the fluid - which for water at 30 ◦C is 0.62 W m−1 K−1. The adimensional numbersRe (Reynolds) and Pr (Prandtl) describe the ratio of inertial to viscous forces, and the ratio of mo-mentum diffusivity to thermal diffusivity respectively [222]. Considering a flow rate of 6 liters perminute, the resulting heat transfer coefficient is 1.04 · 104 W m−2 K−1 which could be inserted inthe ANSYS simulations to compute the overall temperature distribution. However, the presence ofsharp turns in the geometry makes the fluid flow less developed and the predictions of the aboveequations become less accurate; a Computational Fluid Dynamics (CFD) simulation was performedusing ANSYS CFX module to confirm these results.To set up the CFD simulation, the pipe geometry was extracted directly from the CAD modeland meshed with up to 7 inflation layers, for a total of 1.79 ·105 nodes and 3.98 ·105 elements. Thephysical quantities of mass flow and temperature were set as boundary conditions, and the modelwas run using smooth wall and adiabatic approximations for the wall surfaces, an approximate con-stant temperature for the fluid of 40 ◦C. The simulation returned a 3D heat transfer map with valuesbetween 6 ·103 and 2.9 ·104 W m−2 K−1 which were imported in the ANSYS thermal simulation ofthe converter assembly instead of the analytical value, to better capture the more realistic 3D natureof the case study.The last condition that still needs to be considered is the contact between the tungsten converterand the copper brackets. This thermal conductance value is directly proportional to the torqueapplied to the clamping screws and results in approximately 2.5 ·104 W m−2 K−1 when a torque of30 lb/in is applied to the bolts. This calculation involves converting torque into axial force throughequationF = c ·T ·D, (A.3)where c is an adimensional friction coefficient with values approximately 0.3 for Inconel alloys, thescrew and nuts material [223], T is the torque applied to the bolts, D is the major bolt diameter andF is the axial force. The thermal conductance can be found by calculating the contact pressure from157the force calculated above, and applying equation [224]hg(p) = 1.55ksmsσ[√2PEgm]0.94(A.4)where ks is an effective thermal conductivity between the two materials, ms and σ are interfaceparameters related to surface roughness, P the pressure applied and Eg the Young modulus of thesoftest material (copper alloy C18150).Thermal simulation results All thermal inputs and outputs for simulating the thermal profile insidethe converter have now been defined, and focus can be directed on the intent of this simulation,which is meant to assess the capability of the converter assembly to withstand the thermal load, givenits input/output conditions discussed above. The results of this thermal simulation are presented infigure A.6. The first important figure of merit to keep under control is the maximum temperatureof the water channel surfaces ∼ 149◦ C shown in figure A.6d, which is lower than the boilingtemperature of water at the nominal cooling loop pressure Tboil(∼ 6.8 bar) = 166 ◦ C. Althoughthese temperature are quite close to each other, this was not considered to be an issue; in fact, inthe event that the water approaches a phase transition, the phenomenon of nuclear boiling wouldincrease the local heat transfer coefficient by up to one order of magnitude, preventing bulk boilingand the consequent catastrophic failure of the converter assembly [222]. Figure A.6a shows that thetemperature profile at the contact between the converter and brackets is not continuous, due to thenon-perfect thermal conductance resulting from the pressure contact calculation. The beam powerhence finds some impediment in flowing from the tungsten towards the copper brackets, whichincreases the temperature in the tungsten (Tmax = 1740 ◦ C), and lowers it in the copper brackets(Tmax = 360 ◦ C) with respect to a perfect contact situation. The maximum temperatures in bothbodies are far below their respective melting temperature, and the copper alloy CuCrZr was chosenspecifically to limit the onset of the creep phenomenon [12], which could compromise the abilityof the material to hold the clamping force on the tungsten assembly. Moreover, the maximumbracket temperature is much lower than the alloy annealing temperature of ∼500 ◦C [225], anothergood benchmark when keeping in mind degradation of material properties at high temperature.Although the thermal conductivity of alloy CuCrZr is ∼20% lower than for pure copper hencereducing its cooling abilities, its improved mechanical properties seemed most suited for preventingfailure scenarios and was ultimately chosen as preferred material. It should be noted that the specificshape and dimensions of the contact interface between tungsten converter and copper brackets is aresult of few and minor geometry iterations aimed at minimizing the maximum temperatures of bothcomponents.Converter mechanical analysis The thermal profiles obtained by the steady state thermal simu-lation were then used as input profiles for mechanical simulations, where the focus was to assessstress and strain values in the converter assembly. In the first place, the initial stress situation was158(a) Full converter assembly (b) Tungsten converter(c) CuCrZr brackets (d) Wet surfacesFigure A.6: Temperature distributions in [◦ C] across the proton-to-neutron converter assem-blyComponent Condition Max σV M [MPa] Max εel [m/m] Max εplW-Converter PT only 136 3.3·10−4 0W-Converter PT+T 902 2.4·10−3 1.15·10−2Cu-Converter PT only 65 5.6·10−4 0Cu-Converter PT+T 308 2.5·10−3 2.8·10−3Table A.2: Von Mises maximum stresses (σV M [MPa]), maximum elastic (εel [MPa]) and plas-tic (εpl [MPa]) strains in the tungsten converter and copper alloy clamps computed in thetwo situations mentioned, pre-tension only (PT) and pre-tension plus thermal (PT+T).computed by assuming the only forces acting on the converter body were the tension coming fromthe four screws bolted to a pretension Fp = 2.3 kN computed from a torque of 30 lb/in, and the resultsof this initial situation are reported in table A.2. Once the beam hits the converter and its temper-ature increases, the assembly expands in the screw direction by ∼ 0.034 mm, number computedfrom an unconstrained ANSYS simulation where only the thermal load was applied to the convertergeometry. The Belville washer elastic constant is 10944 N/mm obtained from the manufacturer159Figure A.7: Creep rate vs applied stress for a variety of copper alloys [12]Figure A.8: Simulation result showing Von Mises stresses [Pa] in the tungsten converter (left)and copper brackets (right)[206] and was used to compute the extra axial thermal force Ft = 372 N applied by the screws dueto the thermal dilatation. Another ANSYS simulation is then run including the total force appliedby every screw, Ftot = Fp +Ft = 2641 N, and the results from this final simulation are reported infigures A.8, A.9, A.10 and table A.2. Plastic strains are present in both in the tungsten converter andin the inner portion of the copper bracket that faces the proton-to-neutron target as shown in figureA.10. This plastic behaviour region in the copper alloy is caused by deformation-induced stressescoming from the converter thermal expansion, combined to the high temperature present in this re-gion which lowers the yield stress of the material. Nonetheless, heat is still effectively transportedout of the tungsten body into the water-cooled bracket due to the even tighter contact pressure be-tween the two bodies. This situation is analogous to the interaction between a hot nuclear fuel pelletand their cladding tube when the phenomenon of gap closure happens, which is very much allowed160Figure A.9: Simulation result showing elastic strain in [m/m] in the tungsten converter (left)and copper brackets (right)Figure A.10: Simulation result showing plastic strain in [m/m] in the tungsten converter (left)and copper brackets (right)and guarantees a safe operation of nuclear reactors [90]. In case the proton beam is shut down andthen is brought back at nominal values, the spring washers would keep the components in the samereciprocal position and the same temperature and stress distribution would be reached, without anycompromise to the integrity of the converter assembly.Converter offline tests A first benchmark of these presented simulations was performed by manu-facturing a full converter prototype and performing thermal tests on it while recording temperaturesat some specific locations shown in figure A.11 using type-K thermocouples. The main objective ofthis test was to define a proper value for the clamping screw torque which determines the thermalconductance parameter between tungsten and copper brackets, fundamental for heat removal. Thiswas achieved by systematically increasing the torque from the four clamping screws, and recordhow temperatures in both copper and tungsten bodies changed compared to simulations. The ther-mal input power was provided by Tungsten Inert Gas (TIG) welding torch, which can deliver a161power up to 2 kW to a small area of any surface within few millimetres from the tungsten electrodetip by discharging electrons through an electric arc. The torch is also provided with an argon nozzleto saturate the local atmosphere with inert gas, aiming at preventing unwanted oxidation at hightemperature. The assembly and torch were secured on a table with clamps, the TIG tip was insertedin a conical feature drilled in the tungsten converter shown in figure A.11, and water was flownthrough the assembly at a rate of ∼ 0.6 lpm.Figure A.11: Converter TIG test: Model of the 3D printed version of the modified converterfor TIG tests (top left), and pictures of the assembly during and after testing.A summary of the data recorded during the tests with power input of 1100 W is compared tothe equivalent simulation values in figure A.12, which shows that measured values of temperaturein both components (blue and green line for W and Cu respectively) reached a steady value withtorques higher than a threshold of 20 lb/in. The simulated lines are shown to give an appreciation ofhow, although the mismatch between their absolute values, the asymptotic behaviours are reachedat the same torque value of 20 lb/in. One way to investigate further the origin of the mismatch couldhave been by inputting more power in the assembly to increase the temperature profile in the copperbrackets; however, it was decided to not push further this test to prevent further growth of a tungsten162Figure A.12: Temperature results from the offline converter tests against torque applied to thefour bolts.Figure A.13: Cut-away view of the heat shield with the proton-to-neutron target and converterassemblies in their nominal position.oxide layer in the assembly centre, which developed during these high temperature tests despite theinert gas blowing towards the hottest areas.A final assessment on the converter cooling efficacy and structural integrity will be made duringits online irradiation and the final Post Irradiation Examination (PIE) to be performed in the ISAChot cell. If any unforeseen surface contact loss and consequent overheating were developing, thismight create cracking on the tungsten converter surface or induce local melting with a potentialreduction of the neutron-generating capability. This scenario would be critical for the success of163the experiment, but was not deemed as a threat for personnel nor for maintaining the target stationfunctionality, and was given approval for irradiation [203], and the PIE tests performed after irradi-ation have confirmed the predictions. The last version of the proton-to-neutron converter assemblyinstalled in the ISAC heat shield is shown in the cut-away 3D model rendered in figure A.13.164A.3 Uranium carbide production for the proton-to-neutron targetThe use of uranium-based targets in ISOL facilities is quite common, because of the large number ofnuclei that are available as reaction products from the selected reaction, be that spallation, fission,evaporation or generic scattering reactions. At ISAC-TRIUMF, the use of uranium targets onlystarted in 2008 with the first uranium oxide assembly, and two years later the first uranium carbidetarget was produced as mentioned by Kunz et al. in [27]. The material synthesized for this proton-to-neutron run is produced by following a newer methodology - yet very similar to the older one asfar as final material properties are concerned - described by Cervantes et al. in [30], which involvesslip casting a slurry of uranium oxide (UO2), excess graphite powder, binders and solvents onto agraphite foil. Once the slurry has dried overnight, annular discs are cut with a custom made toolshown in figure A.14a and loaded into a graphite cartridge (figures A.14b and A.14c) made to fitthe new proton-to-neutron target. Once closed, the cartridge is mounted in Evaporator one (EVAP1)(figure A.14d), the vacuum furnace for actinide materials which is routinely used to reduce theuranium oxide into its carbide through the reactionUO2+5.6C∆−−→ UC2+1.6C+2CO (A.5)at temperatures usually higher than 1800 ◦C, which yield a relatively fast reduction when kineticsdata i.e. from Suzuki et al. [226] is considered. A resistive current up to 1000 A is delivered from aDC power supply to the terminals of a tantalum heater, built with a similar structure as the proton-to-neutron tantalum target container, and presented in figure A.14d. The cylindrical surface of this“proton-to-neutron target reduction assembly” is made by crimping a 25 µm-thick tantalum foil toyield temperatures as high as 2000 ◦C through resistive heating, because of the limited amperagesupply. The heating parameters recorded while conditioning the material for the June 2021-P2Nrun are shown in figure A.15, where the pressure plot (upper) shows a couple of main features thatare usually observed while the amperage (lower plot) is continuously increased during the uraniumoxide reduction into its carbide. The first few peaks are associated to the burn-out of water, organicbinders and plasticizers which are added to the initial mixture to make a slurry. A plateau is thensettling at pressures lower than 1·10−3 Torr until the pressure starts rising again, sign attributed tothe carbon monoxide production from the desired reduction reaction, at which point the resistivecurrent is kept constant. Once the pressure starts dropping significantly, more current is forcedthrough the resistor and since the pressure kept going down, this is usually taken as a sign thatthe full volume has reached the proper reduction temperature. In the late stages of the reducingreaction, the power supply had a glitch around noon that day which abruptly reduced the outputto zero. After recovering from the technical issue, the current was increased again to its pre-triplevels, and it was kept there until the pressure went gradually below 1·10−3 Torr, at which point theheater was cooled down. Once the target is cooled, The cartridge assembly was then brought to aglovebox with argon atmosphere and mounted inside the proton-to-neutron target heater, which wasthen closed and mounted in its copper heat shield, ready to be brought to the irradiation station.165(a) Cutting the green slurry into annular discs (b) P2N graphite cartridge(c) P2N graphite cartridge loaded with UCx pellets(d) Reduction station for actinide materialsFigure A.14: P2N material production steps and tools166Figure A.15: Vacuum furnace parameters during the proton-to-neutron target material reduc-tion. The first plot (pressure) gives hints for when the organic binders evaporated fromthe UO2, and when the reduction took place producing copious amounts of carbonmonoxide167Figure A.16: P2N target body - assembly ITD0685168Figure A.17: P2N target assembly with electrical terminals - assembly ITD0685169A.4 Additional online plot resultsFigure A.18: Efficiency of Rb isotopes from the P2N target at different target heater (P2NHT)currents, compared to the the two ISAC targets UCx#1 and UCx#3.Figure A.19: Efficiency of Cs isotopes from the P2N target at different target heater (P2NHT)currents, compared to the the two ISAC targets UCx#2 and UCx#7.170Figure A.20: Efficiency of Rb isotopes from the P2N target at different target heater (P2NHT)currents, compared to the maximum and average ISAC yields collected from severaltargets.Figure A.21: Efficiency of Cs isotopes from the P2N target at different target heater (P2NHT)currents, compared to the maximum and average ISAC yields collected from severaltargets.171Figure A.22: Lower plot: Rb in-target production calculated by FLUKA and measured onlineduring this experiment and compared to all measured ISAC UCx targets [8]. Upper plot:ratio between the simulated in-target productions for both targets, and the measuredratio between their yield values, in green and black respectively.Figure A.23: Lower plot: Cs in-target production calculated by FLUKA and measured onlineduring this experiment and compared to all measured ISAC UCx targets [8]. Upper plot:ratio between the simulated in-target productions for both targets, and the measuredratio between their yield values, in green and black respectively.172A.5 MR-ToF results tableTable A.3: Intensity ratio for some isobaric species from the proton-to-neutron converter tar-get, for both experimental data and from FLUKA simulationsExp/FLUKA mass Rb SrFLUKA 100 1 5.91·101Exp 100 1 8.14·10−1FLUKA 98 1 2.02·101Exp 98 1 9.05·10−2FLUKA 99 1 2.68·101Exp 99 1 1.80·10−1Exp/FLUKA mass Cs In Ba SnFLUKA 124 1 2.51·102 2.76·10−2 4.61·102Exp 124 1 3.70·10−1 3.79·10−3 1.89·10−3 lasers OFFFLUKA 125 1 9.13·101 - -Exp 125 1 7.29·10−2 - -FLUKA 126 1 2.19·101 5.25·101 -Exp 126 1 1.41·10−2 7.92·10−2 -FLUKA 134 1 - 8.50·10−1 3.38·10−1Exp 134 1 - 1.17·10−1 3.18·10−4Exp/FLUKA mass Cs Sm Nd Eu BaFLUKA 147 1 1.34 7.76 3.93·10−1 3.96·101Exp 147 1 4.83·10−1 1.31 2.52·10−1 2.60·10−2173Appendix BAdditional release dataB.1 Target material descriptionThe material samples under examination are called discs, pellets or foils in TRIUMF’s slang de-pending on their physical properties and production method, regardless of the specific compositionof material they are made of. Their final physical dimensions are similar to each other, consistingof thin round discs, ∼19 mm in diameter, and with thickness between 25µm and 0.5 mmCarbide discs are the most common material irradiated in ISAC and consist of a porous carbidelayer of thickness between ∼0.2 and 0.4 mm deposited on top of a ∼130 µm-thick graphite foilsubstrate. This configuration is achieved by casting a slurry of ground ceramic mixed with organicbinders and plasticizer, on top of a∼30 x 30 cm2 graphite backing foil to act as substrate. The slurryis then cut into round or “D-shaped” discs by means of a shard steel die, packed together into acylindrical tantalum container and resistively heated in vacuum to evaporate the organic compoundsin the slurry and resulting in a porous carbide composite. This procedure is applied to produceSiC, ZrC and composite graphite [9], while UCx target discs are thermally reduced starting fromtheir oxide form (UO2) mixed with excess graphite powder, yielding UC2 through the reaction inequation A.5 [30, 27].Graphite composite materials have been irradiated online in October 2020 (M. Cervantes et al.,in preparation) and high yields of 8Li and 7Be were measured, giving confidence on the capabilitiesof this material. Its production process, comparable to the other carbide targets, takes an averageof ∼ 5 days which could have been shortened if commercial graphite discs showed comparablerelease performance. It is proposed in chapter 4 to perform release studies on two other graphitematerials, i.e. the bare graphite ∼130 µm-thick graphite foil that is used as substrate for all othertarget materials, and 0.5 mm-thick slices of SIGRAFINE R7510 ® graphite rods provided by SGLCarbon and cut to this thickness to prevent breaking the disc frequently while cutting.174B.2 HPGe uncertainty determinationThis section provides more insights on how the gamma detector spectrometer uncertainties wereestimated by using TRIUMF’s 152Eu calibration source.Firstly, the uncertainty estimated by the analyzer software for each gamma peak was plotted(black dots in figure B.1) and compared to the standard deviation from a set of six measurementfrom the same sample with equal acquisition time and distance from the detector (red dots in figureB.1). As expected, the uncertainty reduces when the number of counts in the peak increase, andboth datasets show comparable uncertainty values. During the experimental campaigns, more mea-surements were performed when possible to reduce uncertainties; however, in some cases this wasnot possible due to the half life of the isotopes (i.e. 24Na in SiC samples) or to the several samplesthat needed to be measured (i.e. in section 4.2).Figure B.1: Statistical uncertainty estimated by the spectrum analyzer software (Uncertainty)and from the standard deviation (STD) from 6 equal measurements.Secondly, most samples could be mounted by hand on their measurement location with fairlygood precision, by using some alignment features as guides. However, the composite sampleswere extremely fragile and needed to be positioned using a holder which could result in someradial (∼5 mm) and axial (∼2 mm) misalignment with respect to the HPGe detector position. Themisplacement characterization - still performed using the 152Eu source - are shown in figure B.2(axial) and table B.1 (radial) and result in additional uncertainties of ∼ 6% and 1.4% in the tworespective directions.175Figure B.2: Difference of counts in various peaks when the calibration source is moved axiallywith respect to the benchmark position of 5 cm away from the detector. The label alsoshows the number of counts recorded in the benchmark peak at 5 cm distance.Energy r = 0mm r = 5mm Difference(keV) - - (%)121.78 83228 82952 0.3244.70 12612 12446 1.3443.96 3449 3431 0.5867.38 2276 2398 -5.4964.06 8251 8223 0.31085.84 6733 6615 1.81112.08 7367 7311 0.81408.01 9073 9117 -0.5Table B.1: Counts difference resulting from a radial misplacement of the sample by 5 mm,considered the maximum possible radial displacement when aligning the sample on thedetector jig “by hand”.176B.3 Single pellet release: additional dataSample Date irr Date heated p+ [µC]Cfoil1 Nov 2nd, 2021 Nov 12th, 2021 1600Cfoil2 Nov 2nd, 2021 Nov 12th, 2021 1600Cpel1 Sept 7th, 2021 Sept 9th, 2021 1500Cpel2 Sept 21st, 2021 Oct 6th, 2021 1500Cpel3 Nov 23rd, 2021 Dec 21st, 2021 1500Ccomp1 Sept 21st, 2021 Oct 7th, 2021 1500Ccomp2 Sept 21st, 2021 Oct 7th, 2021 1500Ccomp3 Sept 7th, 2021 Sept 9th, 2021 1500Ccomp4 Nov 2nd, 2021 Nov 13th, 2021 1600Sample Date irr Date heated p+ [µC]SiC1 Sept 21st, 2021 Sept 23st, 2021 1500SiC2 Sept 21st, 2021 Sept 23st, 2021 1500Table B.2: Irradiation parameters and properties for single pellet release samples.177ID T [°C] Count t [s] Counts Counts err RF RF err [%] # meas.Cfoil1 RT 3600 15182 153 - - 4Cfoil1 1668 3600 14547 150 0.042 1.4% 2Cfoil1 1910 3600 14015 149 0.077 1.5% 2Cfoil2 RT 3600 12215 143 - - 2Cfoil2 1668 3600 11862 142 0.029 1.7% 2Cfoil2 1910 3600 11720 142 0.040 1.7% 2Cpel1 RT 900 6717 87 - - 2Cpel1 1668 900 6578 86 0.021 1.85% 1Cpel1 1910 900 6433 85 0.042 1.85% 1Cpel2 RT 300 3336 60 - - 2Cpel2 1668 300 3344 61 -0.003 2.57% 2Cpel2 1910 300 3113 59 0.067 2.62% 2Cpel3 RT 3600 114707 355 - - 3Cpel3 1668 3600 112943 352 0.018 0.44% 3Cpel3 1910 3600 111699 350 0.029 0.44% 3Ccomp1 RT 900 1433 99 - - 2Ccomp1 1668 900 955 71 0.334 10.2% 2Ccomp1 1910 900 511 47 0.643 11.4% 5Ccomp2 RT 900 606 52 - - 2Ccomp2 1668 900 396 42 0.346 13.6% 2Ccomp2 1910 900 254 32 0.581 15.4% 3Ccomp3 RT 900 1592 108 - - 1Ccomp3 1668 900 1391 96 0.126 9.7% 1Ccomp3 1910 900 674 55 0.577 10.6% 1Ccomp4 RT 3600 9069 563 - - 2Ccomp4 1668 3600 5988 378 0.340 8.9% 2Ccomp4 1910 3600 4346 284 0.521 9.0% 2Table B.3: Summary of all measurements made on three different graphite-based materials,and calculated release fraction (RF) with uncertainty. All samples have been kept attemperature for about one minute.178ID Temp. Heat time Isotope γ en. Counts Counts RF RF # meas.[°C] [s] [keV] uncert. uncert.SiC1 - - 24Na 1369 2581 64 - - 1SiC1 1668 60 24Na 1369 2185 61 0.109 0.004 1SiC1 1668 600 24Na 1369 1955 58 0.135 0.005 1SiC1 1910 60 24Na 1369 1897 57 0.099 0.004 3SiC1 - - 24Na 2754 1433 42 - - 1SiC1 1668 60 24Na 2754 1244 38 0.086 0.004 1SiC1 1668 600 24Na 2754 1181 37 0.059 0.003 1SiC1 1910 60 24Na 2754 1021 36 0.102 0.005 3SiC1 - - 7Be 477 2686 77 - - 1SiC1 1668 60 7Be 477 2470 74 0.080 0.003 1SiC1 1668 600 7Be 477 2464 74 0.081 0.003 1SiC1 1910 60 7Be 477 2198 68 0.144 0.006 3SiC2 - - 24Na 1369 3962 77 - - 1SiC2 1668 60 24Na 1369 3408 70 0.067 0.002 1SiC2 1668 600 24Na 1369 3019 69 0.108 0.004 1SiC2 1910 60 24Na 1369 2696 68 0.154 0.006 3SiC2 - - 24Na 2754 2248 52 - - 1SiC2 1668 60 24Na 2754 1820 46 0.122 0.004 1SiC2 1668 600 24Na 2754 1691 44 0.120 0.004 1SiC2 1910 60 24Na 2754 1631 42 0.098 0.003 3SiC2 - - 7Be 477 5122 99 - - 1SiC2 1668 60 7Be 477 4645 97 0.092 0.003 1SiC2 1668 600 7Be 477 4588 93 0.103 0.003 1SiC2 1910 60 7Be 477 4459 89 0.127 0.004 3Table B.4: Summary of all measurements made on two SiC samples, and calculated releasefraction (RF) with uncertainty. The counting time was always 900 s for all samples.179Yield Activity [Bq]A Name [atoms/p+] 1d EoB24 Na 1.4E-07 2.0E+047 Be 7.4E-06 1.0E+0428 Al 5.7E-07 2.9E+0228 Mg 7.3E-09 2.9E+023 H 1.2E-05 2.0E+0222 Na 2.5E-06 2.0E+0218 F 1.0E-06 1.2E+02Table B.5: Yield and activity of isotopes produced in a single SiC pellet irradiated in IPF for5 minutes with 5 µA proton beam current at 450 MeVYield Activity [Bq][atoms/p+] 1d EoB7 Be 1.1E-04 7.9E+04Table B.6: Yield and activity of 7Be in a single 0.5 mm thick graphite pellet irradiated in IPFfor 5 minutes with 5 µA proton beam current at 450 MeV180B.4 Additional macroscopic effusion resultsYield Activity [Bq]A Name [atoms/p+] 1d EoB 10d EoB173 Hf 2.7E-06 3.1E+05 6.9E+02171 Hf 8.9E-07 2.5E+05 8.3E+01170 Hf 3.6E-06 2.5E+05 1.0E+02169 Lu 1.4E-06 2.0E+05 3.0E+03175 Ta 4.6E-06 2.0E+05 9.2E-01176 Ta 3.9E-06 1.7E+05 5.6E-01180 Ta 5.7E-06 1.7E+05 1.8E-03170 Lu 1.0E-06 1.5E+05 1.3E+04177 Ta 4.8E-06 1.4E+05 1.0E+04165 Tm 7.2E-08 1.4E+05 2.4E+03165 Er 1.7E-08 1.3E+05 3.1E+03166 Yb 9.3E-07 1.2E+05 9.3E+03166 Tm 7.0E-08 1.1E+05 1.1E+04160 Er 3.5E-07 9.4E+04 8.3E+02160 Ho 2.6E-08 6.9E+04 9.6E+02149 Eu 5.5E-09 6.2E+04 5.8E+04171 Lu 9.6E-07 4.7E+04 3.0E+04157 Dy 2.6E-08 4.4E+04 2.3E+02161 Ho 6.6E-09 4.2E+04 5.2E+02155 Dy 8.1E-08 3.8E+04 1.3E+02167 Tm 7.9E-08 3.6E+04 2.0E+04152 Tb 2.9E-08 2.4E+04 4.7E+01172 Lu 4.0E-07 2.0E+04 9.0E+03151 Tb 8.7E-08 1.7E+04 5.0E+01161 Er 1.8E-07 1.5E+04 2.9E+02Table B.7: Yield and activity of isotopes produced in a single tantalum foil irradiated in IPFfor 5 minutes with 5 µA proton beam current at 450 MeV181(a) 167Tm in D-shaped foils (b) 167Tm in round foils(c) 169Lu in D-shaped foils (d) 169Lu in round foils(e) 171Lu in D-shaped foils (f) 171Lu in round foilsFigure B.3: Distribution of most other isotopes measured during the effusion experimentswhich are not shown in the main text for the sake of brevity, in complement to fig-ures 4.16 and 4.17. The D-shape samples are on the left and round on the right. Thisprocedure was possible only for this group of 8 isotopes due to their high residual activ-ity and recognizable gamma lines.182(a) 166Tm in D-shaped foils (b) 166Tm in round foilsFigure B.4: Distribution of 166Tm isotopes measured during the effusion experiments whichare not shown in the main text for the sake of brevity, in complement to figures 4.16 and4.17. The D-shape samples are on the left and round on the right.(a) 167T m (b) 169Lu(c) 171Lu (d) 166T mFigure B.5: Comparison between the average activities from D-shaped vs round pellets forsome isotopes which were not captured in the main text for the sake of brevity.183B.5 Technical information of CHI standFigure B.6: CHI stand PID184
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Chemistry
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MacFarlane, W. Andrew
Gottberg, Alexander
Transport phenomena in radioisotope beam production targets
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