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Neutron induced peaks in germanium detectors Gete, Ermias 1994

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NEUTRON INDUCED PEAKS IN GERMANIUM DETECTORSByERMIAS GETEB.Sc., University of British Columbia., 1992A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinTHE FACULTY OF GRADUATE STUDIESDepartment of PhysicsWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAOctober 1994© Ermias Gete, 1994In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference arid study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)________________Department of fk sThe University of British ColumbiaVancouver, CanadaDate 3 c+. i3 /9DE-6 (2/88)AbstractPeaks from fast and thermal neutron interactions in a HPGe detector have beenstudied using neutrons from 232Cf, the 28Si(1r,un) and the 209Bi(ir,n) reactions.In the latter case, it was possible to separate the neutron induced events using theTOF method.The peaks from (n,n’) interactions in the the detector crystal have a peculiartriangular shape of about 40 keV across due to the recoiling nucleus, The two major prominent peaks at 596 and 691 keV corresponding toT4Ge(n,n’) and72Ge(n,n’)have been studied for the above three neutron sources. The triangles have havebeen fitted to an exponential function to compare the peak shapes and a differenceof less than 20 % has been observed between the three different cases.In addition, many peaks resulting from (n,-y) and (n,n’) reactions in the materials surrounding the detector have been observed and identified. The ‘-y-rays from252Cf fission fragment isotopes and from the 209Bi(7r ,xn) have also been analyzedand identified.IIContentsAbstract iiList of Tables viList of Figures viiAcknowledgements viii1 Introduction 11.1 Detection Mechanism of Photons 21.1.1 Interaction of photons with matter 31.1.2 Detection and measurement of photons 41.2 7-ray Spectroscopy with Semiconductor Detectors 51.2.1 Physical properties of semiconductors 51.2.2 Detection of 7-rays using germanium detectors 91.2.3 Major features of germanium detectors as 7-ray spectrometers 111.3 Exotic Atoms . . 161.3.1 Muonic atoms and nuclear muon capture 161.3.2 Pionic atoms and pion absorption by the nucleus 191.4 Neutron energy spectrum from 252Cf 212 Experimental Methods 222.1 The TRIUMF Cyclotron 222.2 The Experimental Setup 242.3 Electronics 251112.3.1 Stopping telescope 252.3.2 Compton suppression 262.3.3 Ge electronics . . 272.3.4 Strobe 282.3.5 Timing logic 292.4 Experimental Set Up for the First 252Cf Run 292.5 Data Acquisition System. 303 Methods of Data Analysis 323.1 Introduction 323.1.1 Peak fitting 333.1.2 Energy calibration 333.1.3 Time of flight discrimination . . . 353.1.4 Background reduction 374 Results and Discussion 394.1 Introduction 394.2 7-Rays from ir Absorption on 209Bi 404.2.1 The total 7-ray spectrum 404.2.2 The prompt spectrum 444.2.3 The neutron spectrum 514.2.4 The delayed spectrum 534.3 Neutron Induced 7-rays from the 252Cf Run 554.3.1 Lines from the Ge(n,n’7) and Ge(n,7) reactions 554.3.2 The 7-rays from (n,n’) and (n,7) reactions on surroundingmaterials 594.4 Comparison of Neutron Triangles from , ir, and 252Cf Runs 62iv5 7-rays from 252Cf 705.1 Introduction 705.1.1 Nuclear fission 705.2 Transitions of the fission fragment isotopes observed from 252Cf . . . 726 Conclusion 87Bibliography 88VList of Tables3.1 Calibration energies used . ,.... 344.1 A list of 7-rays from ir209Bi run 464.2 Excited states of Ge isotopes observed 534.3 Abundance and capture cross-section of Ge isotopes 574.4 7-rays from Ge(n,’y)reactions 584.5 7-rays from (n,n’) and (n,7) reactions from the 252Cf run 604.6 Peak shapes of the 596 and 691 keV lines 655.1 A list of 7-rays observed from the 252Cf spectrum 755.2 A list of the even-A fission fragment isotopes observed 815.3 A list of the odd-A fission fragment isotopes observed 85viList of Figures1.1 Band structure of a) insulators, b) semiconductors . 61.2 Energy levels produced in semiconductors by impurity atoms . . . 81.3 A diagram of a planar Ge(Li) Detector ...... 111.4 A diagram of an HPGe detector. ..... . 122.1 The TRIUMP cyclotron 232.2 Experimental set up for the ir209Bi experiment . . . . 252.3 A block diagram for the pion telescope . . . . 262.4 A block diagram for the electronics of the Compton suppressor . . 272.5 A block diagram of the Ge electronics . . 272.6 A logic diagram for the strobe signal 282.7 Experimental set up for the 252Cf experiment 303.1 Deviation of the linear calibration energy from the literature energy 363.2 Deviation of the energy from eq. 3.4 from the literature energy . 363.3 Time spectrum of the ir on Bi 373.4 Compton suppressed and unsuppressed spectra from the 252Cf run 384.1 7-ray spectrum from the background run 414.2 7-ray spectrum from the ir on 209Bi with no timing cut 434.3 Prompt 7-rays from ir absorption on 209Bi (low energy) 454.4 Prompt 7-rays from r absorption on 209Bi (high energy) 454.5 The neutron induced spectrum from the ir on 209Bi 544.6 The neutron induced spectrum from the ir on 209Bi 544.7 The delayed 7-ray spectrum from the ir on 209Bi 554.8 A portion of the 7-ray spectrum from the 252Cf . . 574.9 7”Ge(n,n’) line from the irBi run 664.10 74Ge(n,n’) line from the [iSi run (Gel) 664.11 T4Ge(n,n’) line from the 1rSi run (Ge2) 674.12 T2Ge(n,n’) line from the irBi run 674.13 72Ge(n,n’) line from the 252Cf run 684.14 72Ge(n,n’) line from the 1uSi run (Gel) 684.15 72Ge(n,n’) line from the uiSi run (Ge2) 69viiAcknowledgementsI would like to express my sincere gratitude and appreciation to my supervisorProfessor David F. Measday for his direction, support and encouragement throughout the progress of this work.I would also like to thank my fellow graduate students; B, Moftah, M. Salibaand T. Stocki for their valuable assistance in the experiments.viiiChapter 1IntroductionGermanium detectors are commonly used to detect 7-rays in the energy range from50 keV to several MeV. In an experiment at TRIUMF which was detecting 7-raysfrom muon capture, a serious problem arose from neutron induced reactions in thedetector. Such effects cannot easily be distinguished from 7-ray effects and cancause serious systematic errors. Thus it became clear that it was necessary to studyneutron interactions in germanium crystals to understand these background effects.We shall now describe the muon capture experiment.The experiment on muon capture on 285i (E570) has been performed to measure the induced pseudoscalar coupling constant of the weak interaction, gp bymeasuring the angular correlation between the 1229 keV de-excitation 7-ray andthe neutrino following the reaction2SSi(t_,v)8A1* [6]. This angular correlationcould be measured by measuring the Doppler broadening of the de-excitation 7-rayusing a high resolution HPGe detector. The 7-ray lineshape is a function of theangle between the 7-ray and the recoiling nucleus (which is opposite to the directionof the emitted neutrino). Hence, the 7-ray lineshape is a function of the 7-21 angularcorrelation and is therefore a function of gp [6] - [10].In order to minimize the systematic error on the measured value of gp, it isessential to fully understand the origin of any background especially around theline of interest. This work was started in an effort to understand the plateau regionwhich existed in the 1210-1230 keV region (mainly caused by (n,n’) reaction in thedetector crystal itself). It was then extended to study of y-rays resulting from neutron inelastic scattering and thermal neutron capture inside the germanium crystalas well as its surrounding materials by using a 252Cf source and neutrons from pionabsorption on 209Bi. The latter experiment was performed since it was possible toseparate the neutron induced events by using the time of flight method. In additionto the neutron induced events, the x and 7-rays following r absorption onand the fission fragment 7-rays have also been analyzed and identified,The remainder of this chapter is devoted to the discussion of the basic idea of7-ray detection techniques using Ge detectors, and the fundamental properties ofmuonic and pionic atoms have also been given. A discussion about 252Cf has beenpostponed until chapter 5 where the 7—ray spectra will be explained.1.1 Detection Mechanism of PhotonsWhen 7-rays interact with matter, they produce high energy electrons. These electrons deposit their energy in the medium by ionization or excitation depending onthe nature of the medium. The detection and measurement of 7-rays is performedby measuring the ionization and excitation produced by these electrons. The basicinteraction mechanisms of photons with matter and their detection mechanisms arediscussed below,21.1.1 Interaction of photons with matterWhen passing through matter, photons can interact with the atoms in various ways.The main interaction processes relevant to 7-ray spectroscopy are:1. The photoelectric effect.2. The Compton effect.3, Pair production.The relative importance of these effects depends on both the photon energy and theatomic number of the absorbing medium.The Photoelectric effectIn this phenomenon the photon interacts with a bound electron and all of the photonenergy is absorbed. This results in the ejection of an electron with a kinetic energyequal to the difference between the the photon energy and the binding energy ofthe electron. The cross-section of this interaction is significant at low energies andit also increases rapidly with the Z value of the medium and is approximately givenby:Zn= const.- (1.1)where n varies between 4 and 5.The Compton scatteringIn the Compton scattering, the photon transfers a portion of its energy to theelectron and the remainder appears as a secondary photon. It is the predominantinteraction mechanism for 7-ray energies of about 0.5 to 5 MeV. The probability ofCompton scattering per atom of the absorber depends upon the number of electronsavailable as scattering targets, and is therefore proportional to Z.3Pair productionIn this phenomenon, the energy of the photon is converted in the nuclear coulombfield to an electron-positron pair. For the pair-production to occur, the incidentphoton must have an energy at least equal to the sum of the rest masses of theelectron and the positron i.e, 1.02 MeV, and this process dominates at high energies(greater than 5 MeV). The excess photon energy is transformed into the kineticenergy of the electron and the positron. For the pair production cross-section, nosimple expression exists but its magnitude varies approximately as the square ofthe absorber atomic number.1.12 Detection and measurement of photonsIn all the three cases discussed above, free electrons are generated, and as theseelectrons are slowed down on their path through matter, they create excited molecular states (for example scintillating crystals), electron-ion pairs (for example gases)or electron-hole pairs (for example semiconductor crystals) . In many photon detectors, one makes use of these information carriers; i.e, charge pairs or the lightemitted in the de-excitation of the molecular states either to detect the passage ofa photon or to determine its energy by measuring the quantity of charge produced.In all the cases of photon interaction discussed, the interaction cross-section isdependent on the Z value of the medium. Hence, materials with high Z are chosenfor the detection of photons above 100 keV. NaI(Tl) detectors are the most commondetectors and are often chosen due to the high Z-value of iodine.The other factor to consider for a detector material is the average numberof the information carriers generated per photon energy. For example, the aver4age energy required to produce an electron ion pair in gas ionization chambers isabout 30 eV whereas the average energy needed to produce an electron-hole pairin semiconductor detectors is 3 eV, an order of magnitude lower. Since the energyresolution of the detector is inversely proportional to the number of informationcarriers generated, semiconductor detectors have superior energy resolution.For 7-ray detection semiconductor (germanium) detectors are preferred overNaI(Tl) detectors because of their superior resolution. Germanium, however, mustbe operated at low temperature (typically 77K) and is more expensive, so it is usedonly when its advantages are needed.1.2 7-ray Spectroscopy with Semiconductor Detectors1.2.1 Physical properties of semiconductorsSemiconductors and insulators have the characteristic property that the highestfilled energy band of the electrons bound to the atom is separated from the band ofthe electrons not bound to the atom by an energy known as the band gap, Eg (Fig1.1). In semiconductors, the size of the band gap is small enough (about 1 eV) suchthat a few electrons are excited to the conduction band by thermal energy at roomtemperature. In insulators, the bandgap is large (about 6 eV) so that at normaltemperatures the electrons are all in the valence band since thermal energy is notsufficient to excite the electrons to the conduction band.At any non zero temperature, it is possible for an electron in the valenceband to be excited to the conduction band by thermal energy. The excited electronleaves a vacancy (called a hole) in the valence band. The combination of the two5(a) (b)Empty conduction bandConduction bandEj6eV E1eVValence BandFilled Valence BandFigure 1.1: Band structure of a) insulators, b) semiconductorsis called an electron-hole pair. When an electric field is applied across the crystal,the electron and the hole drift in an opposite direction. Thus, the motion of boththe electrons and the holes contribute to the conductivity of the material.The charge carrier concentration (the concentration of electrons or holes) ata given absolute temperature T is given by:n = AT3/2exp(2) (1.2)Where A is the proportionality constant of the material, k is Boltzman’s constantand Eg is the bandgap energy. From the above equation, it can be seen that the concentration of charge carriers is a strong function of temperature for a given material.Thus if the material is cooled to low temperature, the charge carrier concentrationdecreases drastically.6Under the action of an external electric field, both the electrons and the holesundergo a net migration parallel to the direction of the applied field. For low andmoderate electric field, the drift velocities of the electrons and the holes are proportional to the applied electric field, the proportionality constants being the mobilitiesof the electrons and the holes respectively. At higher electric field values, the driftvelocity increases more slowly with the field until a saturation value is reached.Many semiconductor detectors are operated with electric field values such that toresult in a saturated drift velocity which is of the order of 1O m/s.Effects of impuritiesIn a pure semiconductor crystal, the allowed energy levels are present only in thevalence or conduction bands. Such a material is called an intrinsic semiconductor.In practice, however, it is not possible to achieve perfect semiconductor crystalsfree of impurities, and the existence of these impurities perturbs the energy bandstructure by adding additional levels in the forbidden energy gap as shown in Fig1.2. If the energy levels created by the impurities are localized near the conductionor the valence band, they are called shallow impurities; whereas deep impuritiesproduce deep lying levels near the center.Deep lying levels are introduced by impurities belonging to transition metals.Such an impurity could capture an electron from the conduction band and thenit may capture a hole while still holding the electron, allowing them to annihilate.This process is called recombination. Hence such impurity sites are effectively recombination centers. The existence of these recombination centers could affect theperformance of the radiation detector since a charge loss could occur as a result ofrecombination which results in the degradation of the resolution.7Valence bandDonor levels — —/Deep lying impuritiesAcceptor levelsFigure 1.2: Energy levels produced in semiconductors by impurity atomsOn the other hand, some deep impurities are capable of capturing either onlyelectrons or only holes. Such centers hold the electrons or holes and release themafter a certain characteristic time. If this time is on the order of the charge collection time, charges will be lost and incomplete charge collection will result.Besides impurities, structural defects of the crystal, namely point defects anddislocations could also act as trapping and recombination centers. Structural defects could arise when growing the crystal or due to radiation damage.Impurities by elements with 3 and 5 valence electrons produce shallow levelsnear the conduction or the valence band respectively. The effect of impurities withthree valence electrons such as boron, aluminum, gallium or indium is the introduction of free holes within the crystalline structure. These are called acceptorimpurities, since the holes can accept electrons. Similarly, impurities with five vaConduction band8lence electrons, such as phosphorus, arsenic and antimony introduce free electrons.These are called donor impurities, since they donate electrons. Materials in whichthe acceptor impurities predominate are called p-type materials, those with primarily donor impurities are known as n-type materials.When donor and acceptor impurities are both introduced in a crystal lattice,the material is called compensated. The excess electron from the donor atoms nolonger find an absence of empty states, since states are available at the acceptorlevels. These will be filled preferentially by the electrons from the donor levels,as a result, the electrical effects of the impurities present is neutralized. Thus thematerial retains its intrinsic property.Thin layers of semiconductors that have an unusually high concentration ofimpurity are often given a special notation. Thus n and p designate heavily dopedn-and p-type layers that, as a result have very high conductivity. These layers areoften used in making electrical contact with semiconductor devices.1.2.2 Detection of 7-rays using germanium detectorsWhen a photon interacts in the crystal, it produces high energy electron(s) by oneof the processes discussed. These high energy electrons deposit their kinetic energyin the crystal by creating electron-hole pairs. If the secondary electrons are sufficiently energetic, they create additional electron-hole pairs. The over-all effect ofthis process is the production of many electron-hole pairs along the track of eachhigh energy electron created. These electron-hole pairs are then free to be collectedat the electrodes which are in electrical contact with the electrodes that generate astrong electric field along the crystal of the order of 1000 V/cm.9In practice, however, the presence of acceptor or donor impurities makes theoperation of 7-ray detectors more complex than discussed above. When an electricfield is applied across a semiconductor crystal with donor or acceptor impurity, anelectric current based on the electrons or holes from the impurity results. The statistical fluctuations in this current results in a noise which masks the pulse resultingfrom photon interaction.One way of reducing this steady-state current which results from the impuritiesis to reduce the impurity concentration by drifting lithium ions into the germaniumcrystal. Lithium, being an interstitial donor compensates the acceptor impuritiesand the resulting material has an electrical property similar to the intrinsic material. Germanium detectors made by this process are called Ge(Li) detectors. Thestructure of a planar Ge(Li) detector is given in Fig. 1.3. The excess lithium onthe upper surface resulted in a highly doped n+ layer which served as an electricalcontact, and a thin uncompensated layer remained on the opposite side. A majordrawback of Ge(Li) detectors is that the detector must be kept cold at all times inorder to prevent further migration of the lithium ions which will ruin the donor-acceptor compensation obtained.If the impurity concentration in the germanium crystal could be reduced toabout 1010 atoms/cm3,the intrinsic region can be achieved by creating a diodestructure. This structure is obtained by doping one surface of an ultrapure ptype germanium with lithium, creating an n+ layer on one side. Hence, the bulkof the crystal consists of a “p” region as well as a thin heavily doped + layer.When a reverse bias is applied to this ii+p region, electrons and holes are pulledout of an intermediate region called the depletion layer, and current cannot flowacross the junction except for some leakage current. The thickness of the depletion10+Uncompensatedn layer p—type Ge/7Figure 1.3: A diagram of a planar Ge(Li) Detectorlayer is related to to the applied voltage and the impurity concentration in thematerial. A detector made in this way is called an intrinsic or high purity germanium(HPGe) detector and is illustrated in Fig. 1.4. One major advantage of highpurity germanium detectors is that they can be stored at room temperature andcooled when they are in operation. Because of thermal noise, however, germaniumdetectors must be operated at 77 K.1.2.3 Major features of germanium detectors as 7-ray spectrometersThe most important parameters characterizing a 7-ray spectrometer are the resolution, linearity, efficiency and timing characteristic and these properties are discussedbelow:Energy resolutionThe major factor which makes germanium detectors the best 7-ray spectrometersis their excellent energy resolution (typically about 1.8 keV Full Width at Half Maxat 1.33 MeV 7-ray energy as compared to about 90 keV for NaI(Tl) at the same11P contact n contact/7Figure 1.4: A diagram of an HPGe detectorenergy.)The total energy resolution of a germanium detector is determined by thefollowing factors:1. The inherent statistical fluctuation of the charge carriers.2. The efficiency of the charge collection process.3. The electronic noise from the spectrometer system.The intrinsic energy resolution due to statistical fluctuations of the chargecollection process (wi) is given by:w = 2.35v’ (1.3)Where e is the average energy for electron-hole creation, F is the Fano factorand E is the 7-ray energy.The contribution to the energy resolution due to the charge collection effi12ciency (wv) is caused by trapping and recombination which result in the loss ofinformation carriers.The noise from the electronic system also contributes to the energy resolution(we). This contribution mainly depends on the leakage current and the capacitanceof the detector.The total energy resolution is given by the combination of the above three.(1.4)LinearityAnother major advantage of germanium detectors is their linearity over a wide energy range. Due to the non linearities in the electronics, however, a slight deviationfrom linearity can be observed during high precision measurements over a wideenergy range.EfficencyCompared to NaI(Tl) detectors, germanium detectors have a small efficiency sincethey are much smaller in size than NaI(Tl) detectors. The efficiency of germaniumdetectors is usually measured relative to 7.6 cm x 7.6 cm NaI(Tl) detector at 25 cmfrom the source. Typical germanium detectors have efficiencies ranging from 10 to50 % although 100 % detectors have been made recently by Ortec.Timing characteristicsThe timing resolution of germanium detectors is much worse than that of scintillation detectors. The pulse risetime of germanium detectors is of the order of 10013nsec and is restricted by the time required for the charge carrier collection. In addition, the detailed pulse shape of the rise from germanium detectors can vary fromone event to another depending on the site of the creation of the charge carriers.Hence, output pulses show variation in their leading edge. With special electronicprocessing the time of arrival of a 7-ray can be determined with a resolution of 8to 10 ns, but a NaI(Tl) can achieve 1 to 2 ns.Neutron interaction in Ge detectorsWhen germanium detectors are used to measure 7-ray spectra from reactions wherefast neutrons are produce as outgoing particles, peaks resulting from inelastic neutron excitation of the nuclei of the various isotopes of germanium appear in themeasured 7-ray spectra. These peaks have a peculiar triangular shape of about 40keV across because the recoiling Ge nucleus deposits its energy inside the detector,which adds to the energy of the 7-ray deposited. The presence of these peaks havefirst been recognised by Chasman et al. [24]. In addition, a further investigation ofthese peaks was done by Bunting and Kraushaar [21].In addition to peaks arising from neutron inelastic excitation thermal neutroncapture 7-rays could also be observed if thermal neutrons are produced as background. The thermal neutrons capture 7-rays are not broadened since the recoilingnucleus has an energy of the order of a few hundred electron volts.Since the purpose of this work is to investigate these various peaks, a detaileddiscussion will be given in Chapter 4.14Radiation damageAs a semiconductor detector, germanium detectors are sensitive to radiation damage. The transfer of energy from the incident radiation to the crystal lattice cancause lattice defects by knocking the atoms from their normal positions. The deeplying energy levels resulting from these lattice defects serve as recombination centersand hole traps. Hence, in a radiation damaged detector, some charge will be lostbefore collection due to trapping and recombination and this results in a subsequentdegradation of energy resolution and low energy tailing.Fast neutrons as well as heavy charged particles such as ce-particles couldproduce a significant radiation damage. On the other hand, 7-rays and electronsproduce very little radiation damage since they mainly interact with the atomicelectrons. A neutron fluence of about i0 n/cm2 could bring a significant deterioration of resolution, and a neutron fluence of about 1010 n/cm2 could make thedetector totally unusable [26].It is found that HPGe coaxial detectors fabricated from high-purity n-typegermanium crystals where electrons are the carrier type are much more resistantto performance (about 30 times) degradation when compared with HPGe co-axialsmade from high purity p-type germanium where holes are the carrier type [17]. Thisis because of the fact that the damaged sites preferentially trap the holes instead ofthe electrons. The n-type crystals also have the advantage of a thinner dead-layerfrom the electrical contact and so can be used for lower energy photons (down toa few keV where as the p-type do not work below 40 keV). Unfortunately, n-typecrystals are more expensive.151.3 Exotic AtomsWhen a negatively charged meson is slowed down in matter and eventually stopped,it can replace an electron in an atom forming a mesonic atom. The process involvedfrom the time the particle enters matter until the mesonic atom is formed can beexplained as follows:1.In the first stage, the free meson loses energy by collisions and it is slowed downto a few keV in about 10_il to i0 s.2. Now the meson has the same velocity as electrons in the atom. It interacts withelectrons and is eventually bound to a particular atom in a highly excited state.During this time, its energy is reduced from 2 keV to 0 in lO_15 to iO’ s.3. The meson then cascades down in series of “hydrogen like” bound states. Transition accompanied by Auger electrons initially and later by x-rays in iO’ s.The slowing down and capture of slow pions and muons is essentially the sameprocess for both. This is due to the fact that the slowing down and capture processdepends almost entirely upon the charge and mass of the meson. (The chargesare the same and the masses are only about 30 percent different). However, theabsorption processes of the muon and the pion are quite different and are discussedbelow.1.3.1 Muonic atoms and nuclear muon captureOnce a muonic atom is formed, the muon cascades down to lower lying orbitalsthrough Auger transition and x-ray emission. After the muon reaches the iS state,the weak interaction comes into play, and it can either decay as a free muon would,or it can interact with one of the protons in the nucleus resulting in the capture of16the ,u by the nucleus:r +A (Z) —* VIL +A (Z — 1)* (1.5)The elementary process for the above reaction is:(1.6)The nucleus is normally left in an excited state, and de-excites by emitting 7-rays, neutrons and sometimes protons or heavier particles. In heavy nuclei, however,because of the coulomb barrier it is almost always the neutrons that are emitted.The neutron energy spectra following r capture on nuclei have been measured by several authors [12-13]. In Fig 1.5 is given the neutron energy spectra frommuon capture on 0, Si, Ca and Pb [12]. Unfortunately the peak at a few MeV hasnot been measured accurately.This spectrum consists of: 1, Low energy neutrons (about 10 MeV or less)from de-excitation of the the nucleus formed after the capture i.e:(1.7)2. Intermediate Energy Neutrons (between about 10 to 25 MeV) which result fromthe elementary process itself. i.e. muon absorption on a single proton;(1.8)3. High energy neutrons (greater than 25 MeV) which result from r absorptionon correlated nucleon pairs. This is often hypothesized as resulting from a muontransforming into a virtual pion via inverse pion decay, then the ir interacts witha quasi-deuteron to form a pair of neutrons.1710112.00 SiI•Co>_____opb-0--4-. ,3—4--4-w±-510106 I0 20 40 60 80—+ E( Fv1V)Figure 1.5: Neutron energy spectrum from t on O,Si,Ca Pb From Ref. [12]181.3.2 Pionic atoms and pion absorption by the nucleusSimilar to muonic atoms, the pion cascades down from higher excited states tolower ones by emission of Auger electrons and x-rays. However, since the pion is aspinless particle, the fine structure observed in muonic x-ray spectra is not presentin the pionic x-ray spectra. In addition, unlike muonic atoms where the weak muon-nuclear interaction is quite negligible, the pion is quickly absorbed by the strongpion-nuclear interaction which produces an important effect on the intensity of thex-rays and the position and width of pionic atom energy levels. The strong interaction causes:1. The x-rays below the absorbing level to be missing.2. The energy of the last x-ray to be broadened due to the short lifetime of thelevel.3, The energy of the x-ray transitions to be shifted.Pion absorption by the nucleus occurs when the pion is captured by correlated np or pp pairs in the nucleus. Absorption of a ic on a single nucleon is highlyunlikely because to conserve energy and momentum, the Fermi momentum of thenucleon must be about 500 MeV/c, but such a high momentum cannot be obtainedfrom the Fermi motion of a single nucleon in the nucleus.The absorption thus occurs on correlated np or pp pairs:1. r 2p —* np.2. iC np —* nn.(but absorption on the np pair is the most probable). Hence, the pion absorption bythe nucleus results in one or two fast neutrons depending on the above reactions. Inaddition, neutrons from the de-excitation of the resulting nucleus are also produced.19A neutron energy spectrum from pion absorption on Pb is given in Fig 1.6 [14]. For’very light nuclei, the direct neutrons from the absorption process are more visibleas a shoulder at higher energies. In Fig 1.6 the extrapolation to zero energy (in theinset) is purely fictional; in reality the curve has to turn over somewhere, and go tozero probability at zero neutron energy. Similar to r capture the portion of thepeak in the probability function has not been measured.IoI —00 —I I•. 0 I 2 3EIMeVI.Ic0- LEAD..1.0 10 100 200E 1MeV)Figure 1.6: Neutron energy spectrum from ir absorption on Pb from Ref [14].201.4 Neutron energy spectrum from 252CfThe neutron energy spectrum from the spontaneous fission of 252Cf is peaked between 0.5 and 1 MeV and it extends up to about 10 MeV. A measured neutronspectrum from 252Cf source is given in Fig 1.7 [15]. This spectrum could be wellapproximated by a Maxwellian distribution given by:=dEWhere T is about 1.3 MeV [11].Figure 1.7: Neutron energy spectrum from 252Cf source from Ref [15](1.9)2 3 4 5NEUTRor ENERGY UileV)21Chapter 2Experimental MethodsThe aim of the experiments performed in this work is to study the background 7-raysresulting from neutron interactions inside the Ge crystal as well as its surroundingsusing neutrons from a 252Cf source and neutrons resulting from r absorption, andthen to compare them with the peaks in the muon capture experiment.The 252Cf run was performed in the M9B area during the shutdown period inSeptember 1993 and its experimental setup is described in Section 2.4. The pion runwas performed in March 1994 during the polarized beam period of the M13 Channelat the TRIUMF cyclotron using a 209Bi target. Another 252Cf was also performedduring this run. The M13 Channel and the experimental geometry are discussed insection 2.2. The electronics used in these experiments is given in section 2.3. Theexperimental set up for the 252Cf run performed in September 1993 is discussed insection 2.4.2.1 The TRIUMF CyclotronThe TRIUMF facility (Fig. 2.1) is a variable-energy isochronous cyclotron whichaccelerates H ions to produce protons of peak energy of 520 MeV. The H ionsare stripped of their two electrons by passing the ion beam through a thin22REMOTE_____________HANOIINGFACILITY—SERVICEERIDGE______________bP%UCTION$-iL4(P)VAULTBL2A(P)BLIB(P)_______CYCLOTRONITYM1301/u)HALLM9(fl/p)cILLAHE*X\\EXTENSIONIIHIONSOURCERATHOTHERMALMESONHALLIONSOURCE3[1NHPOLARIZEDM15(ii)LYISERVICEIONSOURCEBEAMLINESANDEXPERIMENTALFACILITIESEXISTINGPROPOSEDfoil of carbon There are two stripper foils which are 180 degrees apart and each ofthem can be adjusted to provide its own beam of protons with independent energiesup to 520 MeV, and currents up to 140 tA in 5 nsec bunches with a separationtime of 43 nsec, corresponding to the 23 MHz cyclotron frequency. The first protonbeam is used for proton-induced reactions while the other one is directed to a pionproduction target (usually carbon or beryllium).2.2 The Experimental SetupA schematic of the experimental setup for the pion run is shown in Fig. 2.2 Si,52, S3 and S4 are thin plastic scintillators used to define the pion stop signal. Thedimensions of these scintillators are 15 cm x 20cm x 0.31 cm, 11.3 cm x 0.31 cm, 10cm x 10 cm x 0.31 cm, 19.4 cm x 13.3 cm x 0.31 cm respectively. In order to slowdown the pions coming from the channel , a 2 mm thick Al degrader was insertedbetween Si and S2, and a 2.5 cm thick Al degrader was inserted between S2 andS3. The optimum degrader thickness was determined by adjusting the thickness ofthe Al degrader so that the number of pions stopped in the target was maximized.A 10 % HPGe with a resolution of 2 keV full width at half maximum (FWHM)at 1.33 MeV was used to observe the 7-rays. The Ge detector was surrounded by aNaT annulus to detect 7-rays Compton scattered from the Ge detector.The Ge detector was placed at 50 cm from the target in order to have atime of flight separation between the neutrons and the 7-rays reaching the detector.Typical flight times over this distance for neutrons of 1 to 100 MeV are about 60and 7 ns respectively compared with 1.7 ns for photons. The time resolution of theGe-detector is about 7 ns FWHM so that the neutrons can effectively be separated24S=ScintillctorT=TargetD=Oegroder7VSi Dl S2 D2 S3Figure 2.2: Experimental set up for the ir209Bi experimentin the time spectrum.2.3 ElectronicsThe essential features of the electronics and data acquisition system are illustratedin Figs. 2.3-2.6. The electronics is composed of the beam telescope, the Comptonsuppression and the germanium detector. Part of the electronics resided in theexperimental area and the other part in the counting room.2.3.1 Stopping telescopeSignals from the telescope scintillators Si, S2, S3 and S4 were taken from the experimental area to the counting room through 50 ohm cables. The signals were then fedto a leading edge discriminator.The threshold of the discriminator corresponding toNolS425D.B. = Delay BoxD=Leading Edge DiscriminatorFigure 2.3: A block diagram for the pion telescopeSi was set just above the electronic noise so that every incoming charged particlethat is incident on Si could be detected. S2 was adjusted to trigger mainly onpions by adjusting the threshold of its discriminator in such a way that it ignoredparticles with a lower energy loss, like electrons and muons. The time of a pionstop was defined by S3, which also mainly triggered on pions. A veto signal fromS4 completed the telescope logic Si.S2.S3. defining a pion stop. This signal wasthen fed to a CAMAC TDC after being delayed by 384 nsecs. The time intervalbetween the stopping of the pion in the target and the detection of the 7-rays inthe detector is then determined by looking at the spectrum from this TDC slot2.3.2 Compton suppressionFor each of the 6 PMTs to the NaT annulus, the signals were fed to an amplifier andthe amplifier’s output was fed to CFDs whose threshold was set at about 150 keV,just at about the difference between the Compton edge and the full energy peak.The output of the CFD was then fed to the CAMAC TDC after 80 nsec delay.To TDC(384 us)26NaI(TI)________To TDC Stop(80 ns)Figure 2.4: A block diagram for the electronics of the Compton suppressorPA. = PreamplifierSA. = Spectroscopic AmplifierT.F.A.= Timing filter amplifierCFD = Constant fraction discriminatorGe—E::::EzLTo AD:(To Strobe)Figure 2.5: A block diagram of the Ge electronics2.3.3 Ge electronicsThe two signals from the pre-amplifier of the germanium detector were sent to aspectroscopic amplifier and a timing filter amplifier to provide energy and timingsignals respectively. The output of the spectroscopic amplifier was then fed to aspectroscopic ADC. The ADC was gated by a 12 is signal which was generatedby the strobe. The germanium energy electronics were in the experimental area toavoid a distortion of the pulse amplitude due to electronic noise.The timing signal from the TFA output was sent to the counting room whereit was fed to a Constant Fraction Discriminator (CFD). The output of the CFDwas then used to generate the strobe.27G.G = Gate generator2.3.4 StrobeThe strobe signal was generated by the anti-coincidence of the “Ge.t” signal andthe INHIBIT signal. (The INHIBIT signal stops any further event processing untilthe data acquisition from the previous event is completed). The INHIBIT signalwas generated by the computer busy signal, the strobe and a 1 msec. protectiongate between the strobe and the generation of the computer busy signal. The strobewas then routed to:1. Produce a delayed gate which defined the Spectroscopic ADC’s digitizing window.2. The INHIBIT.3. A gate generator to generate a protection gate for the INHIBIT.4. Start TDCs in the CAMAC crates.5. The Starburst to start the data acquisition system after going through an LRS688 level adapter.To Spect.ADCS)[1 msec)INHIBFFComputer BusyFigure 2.6: A logic diagram for the strobe signal282.3.5 Timing logicThere are two main timing signals of interest incorporated into the electronics circuitry: the NaT time and the time between the pion stop signal and the Ge signal.The Nal time logicThe NaT timing signals described previously in the NaT Compton suppression logicsection were input to the CAMAC TDC after 80 ns delay. During the data analysis, a software cut of the Compton scattered events was made by selecting eventscorresponding to both the Ge and the NaT(Tl) firing at the same time.Time of pion logicThe S1.S2.S3. signal was sent to a TDC after being delayed by 384 nsecs. Thetime interval between the stopping of the pion in the target and the detection ofthe 7-ray in the detector is then determined by looking at the spectrum from thisTDC slot.2.4 Experimental Set Up for the First 252Cf RunThe tests with the californium source were run when the cyclotron was not running.The physical setup is shown schematically in Fig. 2.7. A 0.5 mCi (1.01 g) 252Cfsource with a yield of about 2.3 x 106 neutrons per sec. was contained in a bunkermade of concrete and wax. The bunker had a concrete collimator which was 27cm long and approximately 1.6 cm in diameter. The source was about 3 cm awayfrom this collimator and the Ge detector was placed at about 50 cm away from thecollimator.Several runs were taken by putting Pb, polyethylene, concrete and wax in front29Ge detector127 C2CfConcretebunker1.6 cm3 cmFigure 2.7: Experimental set up for the 252Cf experimentof the collimator to change the relative flux of neutrons and 7-rays coming throughthe collimator. This was done to discriminate events induced by neutrons against7-rays coming from the source. In one run the Ge detector was housed in the NaTannulus. This was done to see what effect this would have on the background lines.2.5 Data Acquisition SystemThe data acquisition system used in this experiment consisted of the VDACS dataacquisition program running on a DEC VAX computer, the Starburst (a PDP-11computer module in the main CAMAC crate), and the CAMAC modules containedin crate 1.The Starburst reads the data from the CAMAC modules in the crates andsends to VDACS. The VDACS then transfers the data to the VAX where it canbe written to VCR tapes or read by the online program running (Display). A userdefined TWOTRAN program which is compiled on the VAX and executed in theStarburst instructed the Starburst on reading events from CAMAC modules andprocessing the data.30The overall control of the acquisition was performed by the VDACS program.The user could interactively start, pause and stop acquisition; load and unload VCRtapes.31Chapter 3Methods of Data Analysis3.1 IntroductionThe data analysis of a 7-ray spectrum from a Ge detector is generally carried outby the determination of the channel positions, x of the observed peaks followed bydetermination of the energy vs channel relation, E(x). In this work, similar stepshave been taken to analyze the data obtained from the various runs. These stepscould be outlined as follows.1. Determination of the energy and intensity of the 7-rays obtained from the variousspectra.2. Identification of each 7-ray peak with a specific nuclear de-excitation and determination of the interaction responsible for the excitation.3. Analysis of the peak shapes resulting from neutron interactions inside the Gedetector from the r and 252Cf runs.The data obtained from the different runs were analyzed using the TRIUMFVax cluster of computers. The sorting analysis program Display was used to readthe data from the VCR tapes and sort the data into the respective energy andtime spectra. The energy spectra corresponding to different time windows werethen analyzed and fitted using the program Displot. PLOTDATA was also used for32fitting and producing the final spectra.3.1.1 Peak fittingWith the exception of the neutron induced peaks inside the Ge crystal, a majorityof the y-ray peaks were quite symmetric and they could be fitted by a gaussianfunction (Equation 3.1) whose height, width and position are allowed to vary,(xx )2F(x) = Ae 22 + Bx + C (3.1)The peak is represented by the first expression which is a gaussian with centroid atx0. The Full Width at Half Max of the peak (w) is determined from u using therelation:w = 2vo- = 2.35cr (3.2)The area under the gaussian peak is determined fromArea =Aw42 (3.3)The spectral background is represented by a straight line which is not allowed tovary. This line is generally chosen to have a zero slope although occasionally anon-zero slope is used for peaks below 300 keV. In addition, it was necessary to usedouble gaussian fits for closely spaced peaks.3.1.2 Energy calibrationThe energy calibration for the spectra from the ir and 252Cf runs performed inMarch 1994 was done using the strong background peaks from ‘52Eu and 60Co.These came from induced radioactivity in the experimental area and cannot beavoided. Although the appearance of these peaks in the spectra was undesirable,they proved to be very useful for calibration since they are well known lines which[19], [20] which cover a wide range of energy (Table 3.1). Moreover, it was possible33Table 3.1: Calibration energies used [19], [20]Radionuclide Energy(keV)152EU 344.277(2)778.903(2)964.043(7)1112.063(7)1408.000(7)60Co 1173.238(4)1332.502(5)to calibrate the spectrum under experimental conditions because of these peaks,hence avoiding the possible variations of the spectrometer system which could arisefrom conditions changing with time. For the spectrum from the j run, the calibration was done using muonic x-rays and the well known 41Ar line from air activation.The 511 keV annihilation line was not used since it has natural width of 1.8 keVdue to the Doppler effect arising from the electron motion at the annihilation point[22]Calibration relationFor most practical purposes, it is assumed that the relationship between the-ray energy and the channel position is linear. Due to nonlinearities in the detectionsystem, however, this relationship is, in general, not linear. A special large deviationfrom linearity was observed at low (below 300 keV) and at high energy (above 2MeV) . To account for the nonlinearity at the low energy side for the 252Cf run, thecalibration was done using the relation given by equation 3.1 as suggested by Dryak[21],34E(x)= a(l/x)+b+cx-{-dx2 (3.4)Where E is the energy, x is the peak position a,b,c and d are constants derived by least square fit of the well known calibration peak energies to the respectivechannels.A -y-ray spectrum of a radiothorium source was analyzed to test whether thedeviation of the detection system from linearity could adequately be compensatedby using equation 3.4 as calibration equation instead. Fig 3.1 and 3.2 show thedeviations of 7-ray energies obtained from calibrations using a linear calibrationequation and equation 3.4 respectively. In figures 3.1 and 3.2, the data points thatare marked by an asterisk are points which are used in calibrating the spectrumwhereas the square data points are those 7-ray energies which are not used forcalibration. The error bars represent only the statistical uncertainties. Comparingfig. 3.1 and 3.2, a deviation of as much as 1.2 keV was observed for the linearcalibration, whereas the maximum deviation observed in fig 3.2 is about 0.13 keV.For the ir and for the 1tr runs, a quadratic calibration was sufficient sincethe energy region of interest was 400 to 2000 keV.3.1.3 Time of flight discriminationA time spectrum representing the time between the 7t stop and a Ge signal isshown in Fig. 3.3. The relation between the channel position and time is 0.244ns/channel. The time resolution of the Ge detector is about 7 ns FWHM.The time spectrum consists of the following events:1. A “prompt” peak which results from the pionic x-rays and capture 7-rays occurring almost immediately after the pion stop.2. A tail-like continuum from neutron induced 7-rays, which take few nanoseconds350.6- I — ——————————— I I—‘>ci)Li00Li0,090.03L—0.09—0.150.4-0.2-0.0-—0.2 -—0.4 -20 500I I1000 1500 2000Energy (keV)2500 3000Figure 3.1: Deviation of the linear calibration energy from the literature energy0,151000 1500 2000Energy (keV)Figure 3.2: Deviation of the energy from eq. 3.4 from the literature energy363000- I—___ _______Prompt2400- NNeutrons1800-° 1200-Background Delayed600--_________________ __ __ __ __U I I0 100 200 300 400 500Time (ns)Figure 3,3: Time spectrum of the ir on Bito reach the Ge-detector and its surroundings after being produced from r absorption in the target.3. A wavy continuum from delayed 7-rays, “slow” neutron and background events.The waviness is caused by the R.F. structure of the proton beam4. The continuum to the left of the prompt peak which is due to random backgroundevents not related to the pion absorption.3.1.4 Background reductionAs described in chapter 2, one of the factors that determine the accuracy of a fitto a 7-ray peak is the signal to noise ratio. In addition to distorting the peaksthe background could obscure peaks which are relatively weak. A reduction ofthe continuum resulting from Compton interactions in the Ge crystal was doneby rejecting the events from the NaT annulus surrounding the Ge detector which37CD0C.)Figure 3.4: Compton suppressed and unsuppressed spectra from the 252Cf runare in coincidence with the signals from the Ge crystal. As shown in fig. 3.4a,it was possible to reduce the Compton background by a factor of 3 practicallywithout affecting the full energy events. It is also evident from fig. 3.4b that somepeaks which were not observed in the unsuppressed spectrum appeared clearly inthe suppressed spectrum. This technique also suppresses single and double escapepeaks which are very prominent for high energy 7-rays (but also act as a usefulenergy calibration). Another useful aspect of Compton shields is that backgroundlines that are part of a cascade also get suppressed.2400 2500 2600 2700 2800 2900 3000Energy (key)38Chapter 4Results and Discussion4.1 IntroductionAs discussed in Chapter 1, the main purpose of this work is to determine the neutroninduced events in the Ge detector as well as the surroundings. The 252Cf run wasperformed for the purpose of accomplishing this aim. However, there were manyshortcomings associated with the data from this run:1. The 7-rays from the fission fragment isotopes produced a background.2. Most of the neutrons from the source were thermalized by the concrete shield.3. There were not sufficient statistics to observe the relatively weaker transitions inthe Ge isotopes, especially the 1204 and the 1216 keV peaks.The pion run was performed in the hope of obtaining a spectrum which wasmainly due to neutron interactions in the Ge detector and its surroundings by using the time of flight discrimination technique. Spectra of the prompt, delayed andneutron events from the ir absorption on 209Bi target, as well as the background7-rays associated with many different origins in the area, have been studied extensively and are given in section 4.2. The neutron induced peaks from the excitedstates of the various Ge isotopes and the surrounding materials are also studied in39this section. Since one of the major goals of this work is to examine closely the1200-1230 keV region, some detailed study of this region has also been given.Most of the 7-rays from (n,-y) reactions in the Ge crystal and the surroundingmaterials have been observed from the September 1993 252Cf run. These 7-rayshave been identified and explained in section 4.3. Because the 7-ray spectrum fromthe 252Cf source is quite complex, a full analysis of the 7-ray spectrum and theidentification of the peaks is given in Chapter 5.A comparison between the peak shapes for the neutron induced Ge lines fromthe 252Cf, ir and the jr runs was made in order to determine whether there is adependence of peak shape on the neutron energy spectrum. This investigation willbe discussed in Section 4.4.4.2 7-Rays from ir Absorption on 209BiConsistent with the explanation given in Section 1.3, the pionic cascade via emission of x-rays is followed by a prompt absorption by the nucleus, resulting in theemission of neutrons, charged particles, and nuclear 7-rays. The total 7-ray energyspectrum as well as the energy spectra corresponding to different windows in thetime spectrum have been analyzed separately and are given in the following sections.4.2.1 The total 7-ray spectrum.This spectrum was obtained by imposing no condition on the timing. Many peaksassociated with the ir absorption as well as neutron induced 7-rays have beenobserved in this spectrum. Since the 7-rays resulting from ir absorption in thetarget and neutron interaction will be discussed shortly, only the background peaks40£LUV6016501100mEUEuz4Na.rEu I/I /KJj3EJJJ__820 1020 1220 1420 1620keVFigure 4.1: 7-ray spectrum from the background runwill be discussed here. Many of the lines which have been identified as backgroundhave also been observed in the spectrum obtained from background runs whichwere performed before and after the ir runs. A portion of the spectrum from thebackground run is given in Fig. 4.1, and the total 7-ray spectrum from r on 209Biis given in Fig. 4.2. A complete list of the observed 7-rays in the prompt, neutron,delayed and general spectrum is given in Table 4.1. The 7-ray energies determinedfrom a fit are given to two decimal places with the errors indicating the statisticaluncertainties only. For the peaks which were too small to be fitted, the energies aregiven to one decimal place only and the corresponding errors are estimated to be ofthe order of 0.5 keV. In the last column of this table is indicated in which spectrumthe particular 7-ray 1S observed.keVL0C0041The strong lines at 778.9, 867.4, 964.1, 1085.8, 1112.1 and 1408.0 indicatedin the general spectrum as well as some other peaks given in Table 4.1 have beenidentified with the radioisotope‘52Eu, In addition, the weak peaks at 723.4, 756.8,996.4 and 1596.58 keV have been assigned to the radioisotope‘54Eu. These isotopesare produced from the (n,7) reaction in the concrete blocks surrounding the M13area. The aggregate from which these concrete blocks are made contains a smallamount of europium which has an isotopic composition of 47.9% ‘51Eu and 52.1%‘53Eu. Since the area is near the target where the pions are produced, these blocksare exposed to a large flux of thermal neutrons. Due to the large neutron capturecross-section of 151Eu ( 5800 barns) ,‘52Eu is produced by the (n,7) reaction. Because of the relatively small cross-section of 153Eu (380 barns), a smaller amountof ‘54Eu is produced (7% of‘52Eu). ‘52Eu(t112t= 3.2 years) /3-decays tol52Gd*and also /9+decays tol52Sm*; ‘54Eu(t112=8.55 years) 9-decays tol54Gd*, Theseexcited isotopes de-excite by 7-ray emission.The very strong lines at 1173 and 1333 keV belong to 60Co. The productionmechanism for these isotopes is probably via spallation reactions on copper in themagnetic coils. In addition, the positron annihilation line at 511 keV; the 40K, the41Ar and the ‘H(n,’y)2 lines at 1460, 1293 and 2223 keV respectively have beenobserved. The lines at 583.14 and 2614.4 keV belong to 208T1 which is a daughterisotope of 228Th. These lines originate from a 228Th source which was containedand shielded in the area.7-rays from ir interactions in the Al degrader have also been observed inthis spectrum. These are the 472, 1274 keV peaks which are produced from27Al(7r ,p2n)24Na; the peak at 1368 keV is from 27A1(ir ,3n)24Mg; and the peaks at984 keV and 1633.5 keV are from27A1(7r,3p4n)0Fand27A1(ir,2p5n)0Nerespec42Figure 4.2: 7-ray spectrum from the ir on 209Bi with no timing cuttively. All these peaks were not observed in the prompt and neutron spectra.The peaks at 843, 1014 and 2211 keV belong to the first, second and thirdexcited states of 27A1 respectively. These peaks result from the excitation of theAl nuclei by the ir in the aluminum degrader, and they are also prominent in theneutron spectrum. The peaks appearing in the neutron spectrum, however, are dueto (n,n’) reactions. It is interesting to compare the widths of the 2211 and 2223keV peaks (Fig. 4.2). The peak at 2211 keV is Doppler broadened due to its shortlifetime (28 fs), but the hydrogen capture line is not broadened.The strong peak at 1808 keV is from27A1(ir,irp)6Mg. This peak was alsoobserved in the prompt spectrum. The reason why it was observed in the prompt50043spectrum is from the pions which escape the degrader and fire the ir stop signal.The peak at 1778 keV, is a result of28Si(n,n’) interaction in the concrete blocks.4.2.2 The prompt spectrumMost of the 7-rays in the prompt spectrum have been identified with 200207Pb isotopes. These isotopes are produced from the reaction209Bi(ir,xn)()Pbfor xfrom 2 to 9. The primary reaction proceeds on correlated np pairs forming2o7Pb*which de-excites by emission of a 7-ray or a few neutrons. Proton and charged particle emission are strongly inhibited because of the large Coulomb barrier associatedwith heavy nuclei.The 7r-atomic transitions (5g —* 4f), (6g —* 4f), (7g —* 4f) have been observedin the prompt spectrum. The strong peak at 589 keV which belongs to the (5g—*4f)transition is notable because of its linewidth, due to the pion’s very short lifetimein the 4f level as a consequence of the r absorption by the strong interaction.The strong line at 718.5 keV and the peak at the 1021.8 keV are identifiedwith 10B which is produced from‘2C(ir,2n)’°B in the styrofoam which was usedto support the target.The peaks at 896.4, 992.1 and 1608.6 keV results from2O9Bi(n,n)i* reaction in the target. Although these 7-rays result from neutron interaction, theyappear in the prompt spectrum because the reaction occurs in the target itself.44Figure 4.3: Prompt 7-rays from r absorption on 209Bi (low energy)2400Figure 4.4: Prompt 7-rays from r absorption on 209Bi (high energy)640-(I) 480-z8320-1601800120keV160, 1200° 80keV1900 2000 2100 2200 230045Table 4.1: A list of 7-rays from ir209Bi run.N= Neutron SpectrumP= Prompt SpectrumD= Delayed SpectrumG= Spectrum with no timing requirementA= Observed in all spectraPeak Identification Lit. value Transition Observed411.4 ‘52Eu ,152Gd* 411.126(3) 755,4 —* 344.3 G412.7 G418.10(5) 127j 417.95(10) 418.0 —* 0 N,D,G422.3 202Pb 422.13 1382.8 — 960.7 N,D439.99(1) 23Na 439.991(10) 439.9 —* 0 N,D,G443.99(2) 152Eu 152Sm* 443.965(4) 1529.9 —* 1085.8 G458.1 N,D462.4 200Pb 462.34(13) 1488.4 —+ 1026.2 N,D,G472.3 24Na 472.207(9) 472.2 — 0 G477.64(2) 7Li 477.605(3) 477.6 — 0 G506.30(4) N,P,G511.00(1) annh. 511.0034(14) A516.3 206Pb 516.18 2200.2 —* 1684.1 G541.2 G562.5 76Ge 562.93 562.9 —+ 0 N564.1 152Eu 152Sm* 563.983(4) 684.8 —+ 121.7 G569.64(6) 207Pb 569.65 569.6 —* 0 N,G583.1 208T1 ÷2O8pb* 583.191(2) 3197.7 —+ 2614.6 D,G589.87(2) irBi 589.89(.06) 5g —* 4f A593.08(6) 127j 593.3(2) 650.9 — 57.6 N596.0 T4Ge 595.8 595.8 —* 0 N600.2 D,G604.77(3) G609.5 G46Table 4.1 (continued)Peak Identification Lit, value Transition Observed618.31(6) 127j 618.5(2) 618.5 —* 0 N,D,G628.69(3) 127J, 201Pb 628.6(2),628.2 628.6 — 0 A628.2 —* 0636.4 G657.6 ‘271,0F 658.90(11),656.0 716.5 —* 57.6 N,P,G656.0 —* 0666.39(5) G669.9 P683.77(9) P,D,G688.8 ‘52Eu Z152Sm* 688.674(6) 810.5 — 121.8 G,D692.6 72Ge 691.3 691.3 —* 0 G,D703.4 205Pb 703.4 703.4 —* 0 P718.26(5) ‘°B 718.29(9) 718.3 —* 0 N,P,G723.4 154Eu _154Gd* 723.356(22) 1719.6 —* 996.3 G744.70(4) 127j 744.70(1) 744.7 —* 0 N,G756.6 154Eu _*154Gd* 756.808(22) 1127.8 — 371.0 G760.01(7) P778.929(8) ‘52Eu 152Sm* 778.920(4) 1123.2 —* 344.3 G785.6(1) P786.9 202Pb 786.95 2169.8 —* 1623.1 G794.8 N795.74(3) P,G803.1 206Pb 803.1 803.1 —* 0 P810.3 G825.2 203Pb (6.1 s) 825.21 825.2 —* 0 G47Table 4.1 (continued)Peak Identification Lit. value Transition Observed834.93(1) D,G836.3 N838.62(6) A843.76(3) 27A1 843.76(3) 843.7 —* 0 A846.77(3) 56Fe 846.754(20) 846.8 —* 0 A866.87(13) P867.43(2) ‘52Eu 152Sm* 867.390(6) 1233.8 —÷ 366.5 A873.45(5) ‘54Eu .+154Gd* 873.230(18) 996.3 —* 123.0 G881.03(8) 206Pb 881.0 1684.1 —* 803.1 P,G888.8 A896.39(6) 209Bi,3Pb 896.2(1),896.85 896.2 —> 0 A896.85 —* 0899.1 204Pb 899.15(10) 899.2 —* 0 A904.88(8) irBi 904.82(6) 6g —*4f A911.6 204Pb (65.9 m) 911.7 2185.5 — 1273.9 G912.65(11) A914.6 D,N916.6 D,N917.2 P960.75(5) 202Pb 960.67 960.7 —+ 0 P964.09(1) ‘52Eu 152Sm* 964.055(4) 1085.8 —* 121.8 A977.70(9) P,N,G984.21(12) 204Pb ,20F 984.0,983.8 2257.8 —÷ 1273.9 P,G983.8 —÷ 048Table 4.1 (continued)Peak Identification Lit. value Transition Observed987.5 205Pb 987.7 987.7 —* 0 P,G992.5 209Bi 992.6(1) 2601.2 —* 1608.6 P996.38(6) ‘54Eu _154Gd* 996.329(18) 996.3 —* 0 G1005.03(3) ‘52EU 152Sm* 1005.06(12) 1371.6 —* 366.45 G1014.42(2) 27A1 1014.45(3) 1014.4 —* 0 A1021.8 1021.78(14) 1740.2 —* 718.3 P1026.47(4) 200Pb 1026.2 1026.2 —ì 0 A1044.38(15) 127J 1044.2(2) 1044.0 —÷ 0 N1040 70Ge 1040.6(5) 1040.6 —* 0 N1063.55(12) 207Pb 1063.64 1063.6 — 0 A1085.87(1) 152Eu Z152Sm* 1085.80(8) 1085.8 —* 0 G,D,N1089.79(5) ‘52Eu _152Gd* 1089.767(14) 1434.2 —* 344.3 G1094.61(14) 127j 1094.40(12) 1094.4 —* 0 N1108 76Ge 1108.41(8) 1108.4 —* 0 N1112.08(1) ‘52Eu 152Sm* 1112.087(6) 1233.8 —* 121.8 D,G1120.1 G1147.32(11) D1173.254(3) 60Co 6ONj* 1173.238(4) 2505.7 —* 1332.5 G,D,N1208.0 N1212.97(5) 152Eu Z152Sm* 1212.89(9) 1579.5 —* 366.45 G1218.0 127j 1218.4(2) 1218.4 —* 0 N1228.5 127J 1228.9(2) 1228.9 —* 0 N1238.7 56Fe 1238.255(26) 2085.1 —* 846.8 N,G1257.3 G49Table 4.1 (continued)Peak Identification Lit. value Transition Observed1267.3 G1274.54(1) 22Na 1274.542(7) 1274.5 —* 0 D,N‘54Eu 1274.54(3) 1397.5 —* 123,11293.46(4) 41Ar 41K* 1293.609(8) 1293.6 —* 0 G1299.15(4) 152Eu 1s2Gd* 1299.152(9) 1643.4 —* 344.3 G1332.521(4) 60Co _*60Ni* 1332.501(5) 1332.5 —* 0 D,N,G1368.60(5) 24Mg 1368.675(6) 1368.7 —* 0 N,G1401.7 27J 1401.6(2) 1401.6 —* 0 N1408.038(8) 152Eu 152Sm* 1408.011(14) 1529.9 — 121.8 G,N,D1413 127J 1413.4(2) 1413.4 —* 0 N1457.6 ‘52Eu 152Sm* 1457.619(15) 1579.5 —+ 121.8 G1460.82(1) 40K 40Ar* 1460.859(5) 1460.9 —+ 0 G,D1528.5 152Eu 152Sm* 1528.106(19) 1650.2 —* 121,8 G1547.3 209Bi? 1546.7(1) 2442.8 — 896.2 P1596.61(6) ‘54Eu _154Gd* 1596.582(20) 1719.6 —* 123.1 G1608.66(10) 209Bi(n,n’) 1608.6(1) 1608.6 —* 0 P,G1633.5 20Ne 1633.8 1633.8 —* 0 G1658.54(15) G1764.45(6) 205Pb 1764.30(10) 1764.3 —÷ 0 G1778.84(5) 28Si 1779.030(11) 1779.0 —* 0 N,G1808.74(12) 26Mg 1808.70(6) 1808.7 — 0 P,N2211.0 27A1 2211.1(6) 2211.1 —* 0 P,N,G2223.08(23) H(n,-y) 2223.247(17) 2223.2 — 0 G2614.42(7) 208T1 2O8pb* 2614.533(13) 2614.5 —* 0 G2753.7 24Mg 2754.030(14) 4122.8 —* 1368.6 G504.2.3 The neutron spectrumThe neutron spectrum mainly consists of 7-rays produced by (n,n’) reactions on70’246Ge, 127J, 27Al and 56Fe, The various excited states of these isotopes observedin the neutron spectrum are discussed below.70Ge(n,n’)Referring to Fig. 4.6, the triangular peak at 1040 keV belongs to the first excitedstate of 70Ge, In the complicated structure existing in the 1200-1240 keV region,the edge below the 1218 keV 127J peak is identified with the second excited state of70Ge (1216 keV). It is this peak which initiated this enquiry.72Ge(n,n’)The first excited state of 72Ge (693.6 keV) has not been observed in the neutronspectrum. However, this peak is very prominent in the delayed spectrum (Fig 4.7).and is marked by its triangular shape. This peak is a result of the production ofthe 0+ first excited state of 720e by (n,n’) reaction, and it can only de-excite byemission of conversion electrons. The peak appears at the full transition energybecause the energies of the conversion electrons and subsequent x-rays are summedin the detector and both are detected with 100 percent efficiency. This peak is verydominant in the “delayed” spectrum and it is not seen in the neutron spectrum dueto the long lifetime (0.42 us) of this state.The triangular peak which lies underneath the cluster of 7-ray peaks in the830-850 keV region belongs to the second excited state of T2Ge (833.95 keV).74Ge(n,n’)The prominent triangular peak observed at 596 keV (Fig. 4.5) belongs to the first51excited state of 74Ge . The second excited state (1204 keV) could also be seen atthe edge of the 1200-1240 keV region. The peak at 608 keV which belongs to the(1204 —* 596 keV) transition could not be observed distinctly in the neutron spectrum since it is hidden beneath the tail of the 596 keV peak. However, this peakis observed from 73Ge(n,) reaction in the 252Cf run (Fig. 4.8). It is also evidentthat the triangular 596 keV peak is wider than the peak at 691 keV because it is acombination of the 596 and 608 keV triangles (Sect. 4.4).76Ge(n,n’)The peak at 563 keV, which belongs to the first excited state of 76Ge has been observed clearly. The bump at about 1108 keV (Fig. 4.6) could be due to the secondexcited state of 76Ge (1108.4 keV).All the excited states of the Ge isotopes observed from (n,n’) reaction havebeen summarized in table 4.2.27Al(n,n’)The strong peaks at 843 and 1014 keV as well as the peak at 2211 keV, which havealso been observed in the prompt spectrum (section 4.2.2), appear in the neutronspectrum as well. These peaks are due to neutron interaction in the aluminum canssurrounding the Nal annulus as well as in the aluminum degrader.56Fe(n,n’)The strong peak at 846.8 keV belongs to the first excited state of 56Fe. The relatively weak peak at 1238 keV also corresponds to the transition of 56Fe from thesecond to the first excited state.52Table 4.2: Excited states of Ge isotopes observedIsotope Energy (keV) Lit. value Reference70Ge 1040 1040.6(5) [37]1216 121672Ge 691 691.3 [30]835 834.474Ge 596 595.8 [36]608 608.41204 1204.376Ge 563 562.93 [18]1108 1108.4‘271(n,n’)There were many strong peaks observed which are identified with the excited statesof 127J which came from the NaT surrounding the detector. There are strong peaksoccurring at 593, 618, 628, 658, 744 keV. In addition, weaker peaks at 1094,1218,1228 1401 and 1413 keV were observed.4.2.4 The delayed spectrumAs shown in Fig 4.7, most of the peaks due to (n,n’) reactions which were observed inthe neutron spectrum appear in this spectrum too. This is because of the interactionof slow and delayed neutrons. The dominant peak at 692 keV from72Ge(n,n’) hasbeen discussed in Section 4.2.3. Many of the strong background lines which wereobserved in the general spectrum have also been observed in the delayed spectrum,but not all of them, because the general spectrum has better statistics, so somelines are hard to identify in the delayed spectrum.53C,)cD000,D0(-)0,CD0C)Figure 4.5: The neutron induced spectrum from the ir on 209Bi80602001000 1050 1100 1150 1200 1250Energy (keV)1O0Figure 4.6: The neutron induced spectrum from the ?t on 209Bi54800 511/Ce(n,n)i, 600:o500 600 700 800 900 1000keV600 I I I:::1000 1100 1200 1300 1400 1500keVFigure 4.7: The delayed 7-ray spectrum from the ir on 209Bi4.3 Neutron Induced 7-rays from the 252Cf RunThe data from the 252Cf has essentially two major parts. The first one is the-ray emission from de-excitation of the fission fragment isotopes, and the second isthe 7-ray spectrum from (n,n’) and (n,7) reactions. The 7-rays from the fissionfragments will be discussed in in detail in Chapter 5.In this section, only the 7-rays from (n,n’) and (n,’y) reactions observed in the252Cf 7-ray spectrum will be discussed.4.3.1 Lines from the Ge(n,n’7) and Ge(n,7) reactionsOne special feature of the data from the 252Cf source is the observation of excitedGe isotopes resulting from (n,n’) and (n,-y) reactions. It was possible to make thedistinction between the two reactions which result in the excited states of the same55isotope (for example 73Ge(n,7) and 74Ge(n,n’)) because the 7-rays produced following inelastic neutron excitation are noticeably broadened on the high energyside, producing a peculiar triangular shaped peak, whereas the thermal neutroncapture 7-rays appear to be oniy slightly broadened since the recoiling nucleus hasan energy of less than a keV. Most of the excited states of 70’24Ge isotopes from(n,n’) reactions that were observed from the r run were also observed in the 252Cfrun. Because of the relatively high capture cross-section of T3Ge (Table 4.3), manylines from 73Ge(n,7) reactions were also observed even though it is one of the lesscommon isotopes. The peaks from the (n,n’) reaction were enhanced in the runswhere the Pb shield was used, while peaks originating from the (n,7) reaction weremuch stronger in the runs where polyethylene and wax were used as shields of thecollimator hole (Chapter 2). A portion of the 7-ray spectra from the 252Cf sourcewhen 5cm lead and 4 cm polyethylene were used to shield the collimator is givenin Figs. 4.8a and 4.8b. As could be seen from fig 4.8a the triangular peaks at 596and 691 keV are evident whereas in Fig. 4.8b they are much weaker.Ge(n,n’)The peaks at 596, 691 and 1040 keV which belong to the first excited state of72’04Ge were observed. In addition, the peak at 835 keV (second excited state of72Ge) was observed.Ge(n,-y)The thermal capture 7-rays from 70’3T4Ge are listed in Table 4.4. The strongestpeaks observed were seen from capture on 73Ge. This is because of the relativelyhigh capture cross-section of 73Ge. The narrow peaks at 493, 595 608, 868 and 1204keV are due to the 73Ge(n,)reaction. The peaks at 174.9, 198.5, 326.5, 499.8 and56Abundance and capture cross-section of Ge isotopesAbundance ( %) cr (barns) Ac’- CEnergy (key)Figure 4.8: A portion of the 7-ray spectrum from the 252CfTable 43:Isotope(relative intensity)T0Ge 20.5 3.25 66.6372Ge 27.4 1.0 27.4T3Ge 7.8 15 11774Ge 36.5 0.52 19.076Ge 7.8 0.16 1.2357Table 4.4: 7-rays from Ge(n,7)reactionsReaction Energy (keV) Lit. value Reference70Ge(n,7) 174.9 174.88(5) [31]198.5 198.35(7)326.5 326.0(2)499.8 499.85(6)708.5 708.16(8)831.1 831.3(1)1094.8 1095.8(3)1298.8 1298.8(3)73Ge(n,7) 492.7 492.936(6) [36]595,7 595.847(6)608.2 608.353(5)868.2 867.898(6)961.7 961.055(10)1101.2 1101.267(12)1204.0 1204.208(12)740e(n,-y) 139.7 139.2(1.0) [32]574.9 574.7(10)708.5 due to 70Ge(n,’y) reactions and the peak at 139.7 keV due to 74Ge(n,’y) havebeen reported in the work by Bunting and Kraushaar [23].584.3.2 The 7-rays from (n,n’) and (n,7) reactions on surrounding materialsThe major components in the experimental area for the first 252Cf run were the concrete (the shield and the collimator) and the detector system. Concrete is mainlymade of water, Si02 and CaCO3. The detector system has several components madeof Al and Fe; while the Ge crystal has an indium layer beneath it to which the highvoltage is applied. Many 7-rays which were attributed to (n,n’) and (n,-y) reactionson H,Al, Si, Fe, In and I were observed. The strong peak at 2223.3 keV is due to the‘H(n,’y) reaction. It has been seen in many other experiments where there are thermal neutrons . The lines at 471.6 keV and 1368.4 keV arise from27A1(n,a7)4Na.The strong peaks at 843 keV and 1014.3 keV due to (n,n’) reactions on 27A1 havealso been observed in the 252Cf spectrum. The peak at 1778.8 keV arises from thereactions27A1(n,7)81.+28Si and 28Si(n,n’). The lines at 1273.3 keV and 2093.5keV are from the 28Si(n,7) reaction.The peaks at 1942.6, 2001.2 and 2010 keV result from (n,7) reactions on 40Ca.There is little doubt about the identification of these lines as the intensity ratiosagree with reference [39]. The peaks at 847 and 1238 are from (n,n’) reactions on56Fe, In addition the 1613 and 1726 lines arising from 56Fe(n, ) reaction have beenobserved.The most intense lines from‘15n(n, ) observed in this experiment are listedin table 4.5. In the work of Schaller et al., [41] some lines from 115n(n, 7)reactionhave been reported as background lines resulting from thermal neutron capture inIn.59Table 4.5: 7-rays from (n,n’) and (n,7) reactions from the 252Cf runIsotope Energy (keV) Lit. value Reaction Reference‘H 2223.3 2223.247(17) ‘H(n,7)227Al 472.2 472.207(9) 27A1(n,a7)4N [28]843.9 843.76(3) 27Al(n,n’)1014.5 1014.45(3) 27A1(n,n’)1368.3 1368.633(6) 27A1(n,a)4Na—+ 24Mg1778.5 1779.030(11) 27Al(n,’y)8l —* 28Si28Sj 1273.3 1273.398(11) 28Si(n,7) [28]1778.5 1779.030(11) 28Si(n,n’)2092.7 2093.027(12) 28Si(n,-y)40Ca 1942.3 1942.61(11) 40Ca(n,-y) [28]2001.1 2001.49(31) 40Ca(n,-y)2009.5 2009.8(2) 40Ca(n,7)56Fe 847.2 846.764(6) 56Fe(n,n’) [18]1612.5 1612.70(10) 56Fe(n,-y)1724.6 1725.05(10) 56Fe(n,7)“51n 272.8 272.9 “51n(n,7) [39]417.1 417.2 1n(n,7)819.2 819.3 “51n(n,7)1097.2 1097.0 1n(n,7)1293.3 1293.4 115n(n,7)1507.3 1507.5 “51fl(fl,7)2111.9 2112.1 ‘15n(n,7)127J 133.4 133.6 ‘271(n,-y) [39]202.5 202.94(10) 271(n,n’7) [35]417.7 417.95(10) ‘271(n,n’7) [35]442.9 442.9 ‘271(n,7) [39]618.0 618.5(2) 271(n,n’7) [35]628.5 628.6(2) ‘271(n,n’y) [35]745 744.70(10) 271(n,n’7)60The various peaks resulting from 127(n, y) and 127(n, n’) were observed onlyin the run where the NaT suppressor was in place, a further confirmation that theselines come from this reaction. The peaks from‘271(n, n’) have also appeared moreclearly in the spectrum from the ir run and it was possible to identify the weakerand higher lying transitions from this spectrum (Sec. 4.2.3).614.4 Comparison of Neutron Triangles from ,u, ir,and 252Cf RunsIn this section, the 596 keV and the 691 keV peak shapes from the first excitedstates of74Ge(n,n)e* and72Ge(n,n)e* have been studied. A comparison ofthese peak shapes from the 28Si(r,nv), 252Cf and 209Bi(ir,xn) spectra has beenmade. This comparison is based on the different neutron energy spectra arisingfrom these reactions. The average neutron energy emitted from 252Cf is about 2MeV. The muon capture reaction produces neutrons ranging in energy from about1 MeV to 10 MeV. The neutrons from the pion absorption could have an energy ashigh as 60 MeV or more, but also peak at a few MeV.The purpose of doing this comparison is to determine whether there is anydependence of the peak shape on the neutron energy. This was needed for the1216 keV region for which knowledge of the shape of the neutron background is ofparticular importance. Unfortunately, however, this comparison could not be donefrom the existing data for the 1216 keV region, due to lack of sufficient statistics.This comparison was made by fitting the peaks to the following function.Y(x) =a1ERFC[(x x1)]EXP [(x x1)J+E [_(x — X)2]+FO2 i=1(4.1)In the above equation, the first expression corresponds to a complementaryerror function which basically determines the edge of the Ge(n,n’) peak. The secondexpression determines the tail of the peak which was assumed to be exponential.The expression inside the summation corresponds to a Gaussian function whichcorresponds to any symmetric peak which might be sitting in the region of inter62est. There were up to three peaks sitting on the tail which are related to differentsources. The last expression corresponds to a background which was assumed to beflat for that region.A least squares fit was done by allowing a1, a, u2, x1, x0 to vary inthe above expression. The parameter which was compared is a2 since it determinesthe characteristic of the decaying exponential tail.The various fits are given in Figs. 4.9-4.15. The values of cr2 for the 596 and691 peaks are given in Table 4.6. The errors given in Table 4.6 are the statisticalerrors only obtained from the fit. Although statistical errors obtained from the fitare less than 7 % , systematic errors arising from the lack of knowledge of the precise shape of the background, as well as due to other possible peaks existing in theregion of the fit, contribute to the uncertainty in the results obtained. This can beseen in Fig. 4.9 where a fit was done using different regions of the same peak and adifference of 20 % in the value of a2 was observed. In addition, it was observed thatincluding weak peaks which exist in the region of the fit could give results whichare different by more than 10 % (Fig 4.13).A discussion of the results from the two peaks is given below.691 keVThe values of a2 found for the 252Cf and the7r209Bi are very similar (15.2 and15.4 keV respectively) whereas the values for 28Si are different by about 15 %For the 28Si study, there was a 7-ray peak at 691 keV which was superimposed onthe 691 keV triangle. In addition there were two more peaks at 718 and 731 keV.63qOn the other hand, the 209Bi and 252Cf spectra are cleaner (Figs 4.12-4.15). Thedifferences observed could mainly be due to the systematic errors discussed. Hence,we conclude that no measurable difference has been observed.In the work of Skoro et al., [42] they fitted the 691.3 keV peak from thefollowing neutron sources to a similar function to ours.1. Environmental neutrons using a 10 cm lead shield.2. Environmental neutrons using 20 cm lead shield.3. Neutrons from a 252Cf source.The values of cr2 they found are 20.8, 12.0 and 20.4 keV respectively. They explainedthe significantly different result in the second case to be caused by the differentneutron energy spectra resulting from the different thickness of the shielding used.However, these differences could probably be caused by the poor statistics and poorfit to the background as well as due to the contribution of small 7-ray peaks whichwere not included in the fit.596 keVAs given in Table 4.6, the values of cr2 are higher than for the 691 keV. Thisis because this peak is composed of the 596 keV and the 608 keV peaks which areboth from72Ge(n,n’) reactions. Due to background lines, it was not possible to fitthe 596 keV triangle from the 252Cf.64Table 46: Peak shapes of the 596 and 691 keV lines.Note: Errors include statistical errors from the fit only.Neutron Source u2(596 keV) u2(691 keV)(keV) (keV)252Cf * 15.2(1.0)209Bi(7r ,xn)209Pb 19.2(1.1) 15.4(L0)28Si(.c ,xnv)28A1 (Gel) 23.0(1.0) 18.8(0.5)28Si(,xnv)A1(Ge2) 22.9(1.0) 17.5(0.4)* Not fitted due to background and poor statistics.652= 21.0 keVo = 17.4 keV590 600 610 620 630Energy (key)640 650 660C/)C:300100009000 -8000 -7000 -6000 -5000-—590Figure 4.9: 74Ge(n,n’) line from the 7rBi run660Figure 4.10: 74Ge(n,n’) line from the 1cSi run (Gel)271(nn)30025020015010050590 600 610 620 630 64Energy (keV)30025020015010050•0I I I I I127(n,n)11271 ( n , n’)I I600 610 620 630 640 650keV66I I14000 -13000 -12000 -- 11000-C10000 -9000 -8000 -Cl)C00127(n,n’)I I600 610 620 630 640 650keVFigure 4.11: 74Ge(n,n’) line from the 1cSi run (Ge2)6607000-—-590160140120100806040200Figure 4.12: 72Ge(n,n’) line from the irBi run067I I I I I 500 I I I I4504003503002502006500450400350300250200 —690ZJ1 I I I I660 690 700 710 720 730Energy (key)1400012000 -_10000 -080006000 -Figure 4.13: 72Ge(n,n’) line from the 252Cf run4000-—680I I I690 700 710 720 730 740keV750Figure 4.14: 72Ge(n,n’) line from the 1iSi run (Gel)450400350300250 -(C)0 695 700 705 710 715 720 7:Energy (key)514 3. keVzUut680 690 700 710 720 730 740 REnergy (key)500(d450400Ji.2IçlkeV.350•300250695 700 705 710 715 720 725Energy (key)740 75010BJ68I I I1600014000 -120OO -0100008000 -.6000-—680Figure 4.15: 72Ge(n,n’) line from the 1rSi run (Ge2)10B690 700 710 720 730 740 750keV69Chapter 57-rays from 252Cf5.1 IntroductionCalifornium-252 is a fissioning radionuclide which has many applications in nuclear,radiological and medical sciences and technology. It is made by leaving 238U in areactor for a long time where the 238U nucleus undergoes 14 consecutive (n,7) reactions and some /3-decay.5.1.1 Nuclear fissionNuclear fission results mainly from the competition between nuclear and Coulombforces in heavy nuclei. Fission can occur spontaneously as a natural decay processor it can be induced by adding energy to the nucleus either through the absorptionof a relatively low-energy particle such as a neutron or a photon or from a highenergy charged particle such as a proton or a pion. This produces excited statesor compound nuclear states that are high enough in energy to overcome or moreeasily penetrate the barrier. Nuclear fission is a very complex process. Althoughcertain aspects can be explained by various theoretical models, there is, at present,no comprehensive theory that explains all aspects.70The sequence of the process of fission and the time scales involved could beoutlined as follows:Instantaneous neutron emission occurs during the fission process (scission neutrons) and very shortly after it (less than 10—13 secs). Neutron emission is followedby emission of 7-rays, conversion electrons and x-rays in from 10_14 secs to iOsecs.Some of the fragment isotopes could de-excite by ,6-decay. This process ismuch slower compared to neutron and 7-ray emission with the half lives beingbetween about 10_i secs to 102 sec. Delayed neutron emission proceeds with half-lives ranging from about 10_i secs to 102 secs. Delayed neutrons are those emittedpromptly from the nuclides after a delayed /3 decay. The resulting nucleus is in anexcited state above the neutron binding energy and the observed neutron activityfollows the half-life of its /3-decay precursor.Mass distributionThe fission of a nucleus into two fragments does not occur uniquely, and the fragmentmass distribution covers a wide range. The mass distribution of fission of heavynuclei have been studied extensively using radiochemical and mass spectrometrictechniques. Spontaneous fission and fission that is induced by low-energy particlesare less likely to split into equal masses, while fissions induced by high energyparticles are more likely to split into equal-mass fragments.Energy distributionThe total energy release in fission is very large, about 200 MeV, and is simplyequivalent to the difference in mass between the fissioning nucleus and the sum ofthe fragment masses. The energy is manifested in the kinetic and excitation energies71of the fragments. The excitation energy is dissipated through neutron and ‘-y-rayemission and relates directly to the shape of the fragments at scission.Neutron and 7-ray emissionFission is accompanied by instantaneous neutron emission, and these neutrons arecalled prompt neutrons. The average number of prompt neutrons emitted per fission is called v, and is characteristic of the particular fission process. For 252Cf,v=3.9. In addition to prompt neutrons, there are delayed neutrons emitted after,8-decay of the fission products. The number of delayed neutrons emitted is about1 per 100 fission.A relatively large average number of 7-rays are emitted per fission, and these-rays are not isotropically distributed about the emitting fragment. This anisotropicdistribution of the 7-rays signifies the presence of fragment angular momenta whichcorrelates with Coulomb induced rotation of the distorted fragments. Measurementsof the transition energies and lifetimes verify the existence of high fragment spinsand identify the spins with a particular fragment isotope. Hence, a study of 7-raysfrom fission gives information about states which normally are not populated inradioactive decay or nuclear reactions. A limitation is that the 7-ray spectrum isvery complex because it consists of several 7-rays emitted from a great number offission fragment isotopes.5.2 Transitions of the fission fragment isotopesobserved from 252CfThe total 7-ray spectrum from a 252Cf source is very complex because of severalfission fragments having too close 7-ray energies to be resolved by the detector.72This complexity has at times made it difficult to make an accurate determinationof the energy of the 7-rays and hence to make a proper identification.A list of the 7-rays observed in this work is given in table 5.1. The 7-raysconstitute:1. Background.2. Neutron induced.3. Direct fission fragments and beta delayed products.Since the background and the neutron induced peaks have been discussed inthe previous chapter, only the 7-rays from the de-excitation of the fission fragmentswill be considered here.The fission fragment 7-rays have two origins.1. Prompt 7-rays resulting from the de-excitation of the fission fragments.2. 7-rays from ,6-decay of fission fragment isotopes.In the present experiment these cannot be distinguished, but there have beenexperiments in which a coincidence with a fission fragment is demanded. It is thuspossible to make the identification if needed. In some cases, a particular 7-ray maycome from both effects.Most of the fission 7-rays have been identified with the 2 —÷ 0 transitionof the even A isotopes. In some cases it was possible to observe up to 6+ * 4+transitions. A list of these isotopes and the respective 7-rays is given in Table 5.2.A help in the identification of these isotopes comes from the work of Chieftz et. al[48], [47] and also other works [49], [46], [44].The /3-delayed 7-rays have been identified with the help of the information73obtained from the fission fragment isotopic yield [43], and from [50]. A majorityof the -de1ayed 7-rays are identified with odd A isotopes and are given in Table 5.3.74Table 5.1: A list of 7-rays observed from the 252Cf spectrum.BG = Background, N Neutron induced,F = Fission fragment 7-ray,Peak Identification Lit, value Comments75.69(2) 152Nd, ‘56Sm 76.0,75.9 F77.9110.1121.98(1) 152Eu 121.7824(4) BG123.3 154Eu 123.070(4) BG126.1133.8 ‘44Pr 133.544(5) F138.2 ‘°8Tc 138.4 F140.3 99Tc, ‘°4Zr 140.50,140.3 F145.5 141Pr 145.4441(14) F150.4 ‘°2Zr, 85p 151.9, 151.18 F158.98(3) 148Ce 158.7 F162.6 154Nd 162.4 F171.9 106Mo, 171.7 F174.9 74Ge(n,7) 174.89(5) N181.1 ‘46Ba 181.0 F185.3 65Cu(n,7) ? 185.91 N190.2 141Ba 190.2 F192.4 ‘°4Mo 192.3 F197.47(4) 136Xe 197,33 F199.22(5) 144Ba 199.3 F211.71(2) ‘°°Zr, ‘34Te 212.7,210.8 F218.0 ‘46Pr 218.3 F228.6 132j 228.6(6) F231.8 ‘42La 231.52 F240.83(4) ‘10Ru 240.8 F242.29(4) ‘°8Ru 242.3 F244.81(3) 152Eu 244.6989(10) BG249.78(2) ‘35Cs 249.79 F255.3 ‘42La 255.12 F75Table 5.1 (continued)Peak Identification Lit. Value Comments258.56(2) 146Ce,’38s 258.7,258.3 F263.0268.4269.87(3) ‘°6Ru 270.3 F275.6 ‘41La 277.0 F283.0293.2 143Pr 293.26 F295.98(1) ‘°2Mo,148Ce 296.0,295.7 F302.7 ‘°7Pd 302.8 F304.1 14La 304.2 F306.74(2) ‘°‘Tc,’°5Pd? 306.86, 306.1 F312.0 133j 312.1 F314.3 147Nd 314.7 F316.59(3) ‘°5Rh , ‘‘6Pr 316.4, 316.8 F319.1 105Pd 319.2 F321.7324.6326.59(3) 102Zr 326.6 F330.77(4) ‘44Ba 331.0 F332.80(3) “4Pd, 146Ba 332.9, 332.7 F336.0340.0 116Pd,‘51Pm 340.4, 340.07 F343,4 ‘‘1La 343.7 F344.20(1) ‘52Eu 344.2811(19) BG348.56(3) ‘12Pd 348.8 F350.5 ‘°6Mo 350.8 F351.7 100Zr, 95Sr 352.6, 352.2 F357.78(4) ‘°4Ru 357.8 F359.37(5) 142Ba,‘271(n,n’)? 359.7,360.3 F,N364.1 131Xe 364.480(20) F368.3369.5 104Mo 368.8 F373.9 110Pd 127(n,n’)? 373.8(5), 374.9 F,N76Table 5.1 (continued)Peak Identification Lit. value Comments376.3 ‘40Xe 376.8 F381.2 ‘36Xe 381.5 F387.99(2)397.17(2) ‘44Ce 397.3(2) F400.19(4) ‘°2Mo 400.0 F407.4 133j 407.9 F409.6 146Ce, 152Eu?? 409.7, 411.126(3) F,BG417.71(3) ‘271(n,n’7) 417.95(10) N422.6 ‘°8”°Ru 423.0, 423.1 F426.4431.2 144Ba 431.7 F433.9 8Cs, 134Te 434,5, 434.8 F439.78(3) 23Na(n,n’) 439.9 N443.8 ‘52Eu 443.965(4) BG446.7 ‘°2Mo?? 447?? F453.6 131Te,‘46Nd 452.4, 453.8 F457,3 ‘40Xe 457.8 F462.6 138Ba 462.785 F465.7469.0 ‘°5Ru 469.2 F472.2 27A1(n,7a) 472.207(9) N474.9 ‘2Ba 475.0 F477.5 477.605(3) N481.6483.3 ‘38Xe 483.7 F486.97(4) “8Cd,‘40Ce 487.8, 487.018(9) F496.93(2) ‘°3Rh 497.080(13) F499.8 74Ge(n,-y) 499.85(6) N510.96(3) annihilation 511.0 BG519.49(5)527.6 140Ba 528.26 F530.0 133Xe 529.872 F535.1 ‘36Te 535.8 F537.3?? 142La 537.27 F540.97(4) 144Ce 541.1 F546.4 “°Pd, 138Ba 546.3, 546.9 F571.1 ‘42Cs 571.66(2) F77Table 5.1 (continued)peak identification Lit. value Comments574.9 74Ge(n,) 575.0(8) N582.7584.9 ‘44Ce 585.0 F588.76(4) 138Xe 588.9 F596.3 74Ge(n,n’-y) 596.3 N602.06(4) 140Ba 602.2 F608.5 74Ge(n,n’-y) 608.4 N618.2 127(n,n”-y) , 112Cd 618.5(2), 617.4 N625.5628.5 ‘271(n,n’-y) 628.6(2) N631.1 ‘42Ba 632 F641.19(4) 142Ce 641.17 F647.9658.2 ‘271(n,n’), 9TMo 658.90(11), 657.92 N,F661.532(8) ‘37Cs 661.660(3) F667.6 132Xe 667.73(5) F676.6 676.4 F692.9 72Ge(n,n’7) 691.3 N696.9 132Te 696.8 F706.0708.5 T4Ge(n,7) 708.2(1) N724.04(3) ‘°5Rh,‘54Eu 724.199(5), 723.356(22) F,BG729.6738.9743.2 ‘271(n,n’)?, 97Nb 744.70(10), 743.36 N,F756.5 95Nb 756.729(12) F765.6 95Mo 765.807(6) F772.6 132Xe 772.68(5) F778.83(2) ‘52Eu 778.920(4) BG798.6815.2 96Sr, 140Ce 815.5, 815.766(4) F831.4 T4Ge(n,n’),’38Xe 831.2(1), 830.8 N835.03(6) 72Ge(n,n’7) 834.4 N844.43(20) 27Al(n,n’) 144Ce 843.76(3), 844.9 N846.95(4) 56Fe(n,n’7), 134J 846.764(6), 847.02 N,F867.47(5) 74Ge(n,n’7)??,‘52Eu 867.8, 867.390(6) N,BG78Table 5.1 (continued)peak identification Lit. value Comments873.4 ‘54Eu 873.230(18) BG875.7 ‘38Xe 875.3 F884.16(6) 134j 884.09 F894.9 ‘42La 894.9, 894.85 F912.6919.0 94Zr 918.8 F925.0 ‘40Ce 925.2 F948.6 ‘42La 948.8 F964.06(2) ‘52Eu 964.055 BG974.32(6) 132Te 973.9 F996.5 154Eu 996.329(18) BG1004.90(10) ‘54Eu 1004.775(21) BG1009.84(10) ‘38Ba 1009.7 F1014.45(6) 27A1(n,n”y) 10 14.5(3) N1040.4 70Ge(n,n’7) 1039.6 N1085.85(3) ‘52Eu 1085.842(4) BG1089.8 ‘52Eu 1089.767(14) BG1094.8 74Ge(n,7) 1095.7(2) N1112.03(2) ‘52Eu 1112.087(6) BG1139.3 74Ge(n,-y) 1139.3(2) N1173.262(8) 60Co 1173.238 BG1180.41213.06(10) 152Eu 1212.970(13) BG1220.01222.81261.2 135Xe 1260.4 F1274.56(2) 22Na,152Eu 1274.542(7), 1274.54(3) BG1279.25(8) ‘34Te 1279.1 F1294.11298.8 74Ge(n,7)?? 1298.3(2) N1299.3 ‘52Eu 1299.152(9) BG1313.04(5) 136Xe 1313.0 F1332.530(8) 60Co 1332.501 BG1408.09(2) 152Eu 1408.022(4) BG79Table 5.1 (continued)peak identification Lit, value Comments1428.21435.03(18) 138Cs 1435.7 F1435.89(10) 138Ba 1435.9 F1457.93(27) 152Eu 1457.619(15) BG1460.91(6) 40K 1460.895(5) BG1596.31(4) ‘40Ce, 154Eu?? 1596.182(20), 1596.582(20)?? F,BG1612.4 56Fe(n,’y) 1612.70(10) N1632.91778.9 28Si(n,n’7) 1779.030(11) N2016.12217.52223.21(5) 1H(n,7)2 2223.247(17) N2239.62397.75(16) 142Ce 2397.7 F2613.76(27) 232Th 2614.533(13) BG2639.02777.82789.65(25)80Table 5.2: A list of the even-A fission fragment isotopes observedIsotope Energy (keV) Lit. value Transition94Zr 919.01 918.8 2 —* 0‘°°Zr 211.9 212.7 2 —* 0352.4 352.6 4 ,‘ 2102Mo 295.9 296.0 2 —* 0400.19 400.0 4+ —+ 2‘°2Zr 152.8 151.9 2 —*326.59 326.6 4+ —÷ 2‘°4Mo 191.9 192.3 2 —*‘°4Ru 357,78 357.8 2 — 0‘°4Zr 140.25 140.3 2 —* 0‘°6Mo 171.88 171.7 2 —+ 0351.66 350.08 4+ —* 2‘°6Ru 269.87 270.3 2 — 0108Tc 138.22 138.4 2 —* 0108RU 242.29 242.3 2 —* 0“°Ru 240.83 240.8 2 —*81Table 5.2 (continued)Isotope Energy (keV) Lit. value Transition“°Pd 373.89 373.8 2 —* 0546.35 546.3 4+ ‘ 2112Pd 348.56 348.8 2 —* 0‘12Cd 618.21 617.4 2 — 0“4Pd 332.80 332.9 2 —*118Cd 486.97 487.8 2 —* 0132Te 974.32 973.9 2 —* 0696.97 696.8 4+ —* 2132J 228.65 228.2 ‘32Te —* 132J‘32Xe 667.58 667.68 132j —p‘34Te 1279.25 1279.1 2 —* 0‘34Xe 846.95 847.02 134j —884.16 884.09 134j —+136Te 535.07 535.8 2 —+‘36Xe 1313.04 1313.0 2 —* 0381.22 381.5 4 —* 2197.47 197.33 6 —* 4138Ba 1435.89 1435.9 2 - 0462.63 462.785 4 —* 21009.84 1009.8 ‘38Cs —* ‘38Ba82Table 5.2 (continued)Isotope Energy (keV) Lit, value Transition‘38Xe 588.76 588.9 2 —483.33 483.4 4+ .‘ 2831.38 830.8 138J — 138Xe875.67 875.3 138J —*‘38CS 258.56 258.31 ‘38Xe —*433.88 434,49 38Xe —*‘40Xe 376.30 376.8 2 —* 0457.33 457.9 4 —÷ 2140Ba 602.06 602.3 2 —*527.60 528.26 4 —+ 2‘40La 537.3 537.27 140Ba —* °La140Ce 1596.31 1596.49 2 —*486.97 487.03 4+ —* 2‘42CS 571.06 571.66 ‘42Xe —*142Ba 359,4 359.3 2 —+475.2 475.0 4 —* 2631.14 632 6 —+ 4142La 231.85 231.52 ‘42Br —‘42La255.28 255.12 142Br —* 142La894.96 894.9 ‘42Br —*948.55 948.8 ‘42Br —÷ 142La142Ce 641.19 641.17 ‘42La —*2397.75 2397.7 142La —* ‘42Ce83Table 5.2 (continued)Isotope Energy (keV) Lit. value Transition‘44Ba 198.5 199.4 2 —* 0330.9 331.6 4 .‘ 2431.17 431.7 6 ,‘ 4‘44Ce 397.17 397,3 2 —540.97 5411 4 —* 2844.43 844.9584.90 585.0 6 .‘ 4+‘46Ba 181.12 181.0 2 —p332.80 333.0 4 —+ 2‘46Ce 258.6 258.7 2 —*409.59 4097 4 —* 2‘46Pr 218.0 218.3 ‘46Ce —*316.59 316.8 146Ce —* ‘46Pr‘46Nd 453.59 453.9 6Pr —* 146Nd‘48Ce 158.98 158.7 2 —295.98 295.7 4 —÷ 2‘52Nd 75.69 75.9 2 —* 0‘54Sm 72.7 72.8 2 —+ 0154Nd 72.7 72.8 2 —* 0162.2 162.4 4 — 2‘56Sm 75.69 76.0 2 —+ 084Table 5.3: A list of the odd-A fission fragment isotopes observedIsotope Energy (keV) Lit. value Transition85Rb 150.37 151.18 85mJ(r — 5Rb95Nb 724.0 724.18 95Zr — 95Nb756.48 756.71 95Zr —* 95Nb95Mo 765.63 765.79 95Nb —* 95Mo97Nb 743.17 743.36 97Zr —* 97Nb97Mo 658.20 657.92 97Nb —* 97Mo99Tc 140.25 140.51 99Mo —* Tc‘°1Ru 306.74 306.80 101Tc —+103Rh 496.93 497.08 103Ru —*105Rh 316.59 316.4 105Ru —÷ 105Rh469,03 469.2 ‘°5Ru —* ‘°5Rh724.04 724.2 ‘°5Ru — 105Rh105Pd 306.74 306.1 105Rh —*319.13 319.2‘°TPd 302.69 302.8 107Rh—* 107Pd‘31Xe 364.05 364.47 131J —* ‘31Xe133j 312.0 312.1 ‘33Te —* 133j85Table 5.3 (continued)Isotope Energy (keV) Lit, value Transition133Xe 530.04 529.872 133j —‘33Xe135Xe 1260.1 1260.4 135j 135)(‘35Cs 249.78 249.79 135Xe —141La 190.21 190.2 141Ba —* ‘41La275.57 277.0304.14 304.2343.38 343.7141Pr 145.52 145.44 ‘41Ce —+‘43Pr 293.19 293.26 143Ce —* ‘Pr147Nd 314.3 314.67 ‘47Pr —+ 147Nd151Sm 340.03 340.08 151Pm —*‘51Sm86Chapter 6ConclusionIn this work, the nature of several peaks from (n,n’) and (n,7) reactions in a Gecrystal has been studied when the germanium detector was submitted to a mixedflux of neutrons and 7-rays. An identification of these peaks is very useful whengermanium detectors are used to detect 7-rays in an environment where fast andthermal neutrons are present as a background. In addition, several peaks from the(n,7) reaction in the NaT crystal surrounding the detector and the other materialsin the experimental area have been analyzed and identified.By fitting the (n,n’) peaks at 596 and 691 keV from three different neutronsources, it was found out that no significant energy dependence was observed for thepeak shape. This is surprising and was not expected. Unfortunately the neutronspectrum from the three tests is not known precisely.The 7-rays from ir absorption on 209Bi, the fission fragment 7-rays and thebackground 7-rays m the area will be useful as a reference for future work on similaraccelerators.87Bibliography[1] E.H.S. Burhop, High Energy Physics, 111:115, (1969)[2] C. S. Wu and L. Wilets, Ann. Rev, of Nucl. Sci., 19:527, (1969).[3] C.T.A.M De Laat et al., Nucl. 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