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The effects of 50 kilovolt x-rays on the alkali metal borohydrides Walker, Leonard George 1959

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THE EFFECTS OF 50 KILOVOLT X-RAYS ON THE ALKALI METAL BOROHYDRIDES by Leonard George Walker B.A., University of Brit i s h Columbia, 1957 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS POR THE DEGREE OF MASTER OF SCIENCE in the Department of • CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1959 ABSTRACT The marked physical changes in potassium boro-hydride such as decrepitation and the development of a deep blue coloration when the solid compound i s exposed to ionizing radiation stimulated a study of the effects of 50 Kvp X-rays on the a l k a l i metal borohydrides to determine the nature of the radiation induced changes such as color center formation and chemical decomposition. Methods were developed to prepare the a l k a l i metal borohydrides in a form suitable for radiation studies. Solvents studies showed that anhydrous hydra-zine was an exceptionally good solvent for potassium borohydride, the solubility being 2S.3 grams KBH^ per 100 grams a t c» T h e handling of hydrazine as a solvent required the construction of special appar-atus. A study of the use of hydrazine as a solvent for other ionic borohydrides and/or the growth of crystals suitable for spectroscopic work i s incomplete. Therefore, the spectroscopic studies on radiation induced absorption bands was done mainly with thin pressed pellets. The borohydrides of rubidium and cesium were prepared by metathesis reactions from potassium borohydride via a sulfonium borohydride. The preparation of the previously unreported trimethylsulfonium borohydride i s described. Color center formation was studied by spectroscopic methods only and the F and U type centers have been tenta-tively identified. The thermal s t a b i l i t y and optical bleachability of some of the radiation induced absorption bands were examined. Chemical studies of radiation damage in potassium borohydride fa i l e d to show the presence of free a l k a l i metal. Gaseous boron hydrides were also undetectable. Mass spectrometric examination of gaseous . material evolved during irradiation showed only hydrogen to be present. No gas was evolved when heavily irradiated samples of potassium borohydride were dissolved in liquid ammonia• A discussion of methods and apparatus character-i s t i c to the radiation studies such as the X-ray generator, radiation vessels, vacuum system, and a section on radiation dosimetry i s included in the thesis. The intensity of X-rays generated by the Machlett OEG-60 X-ray tube was determined by the application of the included dosimetry data together with a calorimetric measurement of the output flux of the tube. At 50 Kvp and 28 milliamperes the intensity output was found to be 0.220 cal.min. cm. * at the tube port. Some suggestions for further work are outlined at the end dT the study. In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the' requirements f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t permission f o r e xtensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver Canada. Date /?-&rtUsL , / ^ T ? .  J ACKNOWLEDGMENTS I wish to express my appreciation to Dr. H. G. Heal for his guidance and encouragement during the course of this work and especially for the opportunity he has given me to learn something of the effects of ionizing radiation in solids, I wish to thank Professor C. A. McDowell for many valuable discussions. Thanks are due to Professor J. A. Harris for informa-tion on the syntheses involved in this work and also for samples of cesium compounds. I also wish to thank Dr. B. A* Dune 11 for samples of compounds used in the experimental work. The many discussions I have had with Mr. J. Pringle were of great benefit. I should also like to thank Messrs. Hawkins, Muehlchen, and Sawford for their assistance in the design and construction of apparatus. TABLE OF CONTENTS Page LIST OF TABLES i i i LIST OF FIGURES i v LIST OF PLATES v i i Chapter I. INTRODUCTION 1 A. The Problem . .- 1 B. Methods of Investigation 2 C. The Interaction of Ionizing Radiation with Matter 4 D. Radiation Induced Lattice Imperfections . . 7 II. APPARATUS AND METHODS CHARACTERISTIC OF THE RADIATION STUDIES ON THE ALKALI METAL BOROHYDRIDES 13 A. The X-Ray Generator 13 B. Radiation Vessels 24 C. The Vacuum System and Ancillary Equipment . 29 D. Radiation Dosimetry 34 III. EXPERIMENTAL 51 A. The Preparation of the Alkali Metal Borohydrides 52 B. Galorimetric Determination of the Radiation Flux from the 0EG-60 X-ray Tube 72 C. Spectroscopic Examination of the Irrad-iated Borohydrides 77 1. Visible and ultraviolet absorption spectra 77 2. Infra-red spectra 98 3• Paramagnetic resonance spectrum . . . . 98 D. Examination of Irradiated KBRV for Chemical Changes 100 IV. DISCUSSION 104 BIBLIOGRAPHY 115 LIST OF TABLES Table Page 1. Characteristics of the AEG-50 and the OEG-60 X-ray tubes 15 2. High tension primary operating cir c u i t components 17 3. High tension secondary c i r c u i t components. . . 22 4* Mass absorption coefficients of selected elements 40 5. Mass absorption coefficients of the a l k a l i metal borohydrides 44 6. Parameters for the De Waard Formula and the Klein-Nashina Formula 48 7. Summary of solvent study 53 8. Assay of some of the ionic borohydrides. . . . 66 9. Energy flux for samples in the radiation vessel for chemical studies. 76 10* Application of the Ivey Formulas to the radia-tion induced changes in the a l k a l i metal borohydrides • 108 i i i LIST OF FIGURES Figure Page 1. Defects in a simple l a t t i c e . The a l k a l i metal borohydride lat t i c e 8 2. Color centers in the a l k a l i metal halides . . . 10 3. Schematic view of AEG-50 X-ray tube 14 4* Schematic view of OEG-60 X-ray tube 14 5. Angular distribution of intensity from AEG-50 X-ray tube 16 6. Angular distribution of intensity from OEG-60 X-ray tube 16 7. High tension primary operating circuit 18 8* High tension secondary c i r c u i t 19 9. Radiation vessel for chemical studies 25 10* Radiation vessel for spectroscopic studies. . . 27 11. Carriage for mounting spectroscopic radiation vessel in recording spectrophotometer 28 12. Vacuum system used in radiation studies • . • • 30 13. Klein-Nashina scattering factor 43 14. De Waard parameter, Bp 47 15. Energy distribution of Bremsstrahlung for various f i l t r a t i o n s 50 16. Infra-red spectrum of ammonia 56 17. Infra-red spectrum of hydrazine 56 18. Infra-red spectrum of KBHA extracted with liquid ammonia 56 19. Infra-red spectrum of commercial KBH^ 57 i v Figure Page 20. Infra-red spectrum of KBH^ extracted with anhydrous hydrazine 57 21. Infra-red spectrum of trimethylsulfonium iodide 57 22. Infra-red spectrum of trimethylsulfonium borohydride. 59 23. Infra-red spectrum of vacuum pyrolised product . 59 24. Infra-red spectrum of a i r pyrolised product. . . 59 25. Infra-red spectrum of Reinecke's salt 68 26. Infra-red spectrum of rubidium Reineckate. . . . 68 27. Infra-red spectrum of ammonium iodide 68 28. Infra-red spectrum of rubidium borohydride . . . 70 29. Infra-red spectrum of cesium borohydride . . . . 70 30. Cooling curve for silver disc i n calorimetric flux determination 74 31. Radiation induced changes in lithium borohydride 80 32. Radiation induced changes i n sodium borohydride 82 33. Radiation induced changes in potassium borohydride crystal. 84 34* Radiation induced changes in potassium borohydride crystal (continued) 86 3,5. Radiation induced changed in potassium borohydride pressed pellet 88 36. Radiation induced changes in potassium borohydride pressed pellet (continued) 90 37* Radiation induced changes in rubidium borohydride. 92 v Figure Page 38. Radiation induced changes in rubidium borohydride (continued) • • . 94 39* Radiation induced changes in cesium borohydride 96 40. Electron spin resonance spectrum of irradiated KB(powdered) 99 41* Calibration graph for small amounts of hydrogen collected in chemical decomposition studies 103 v i LIST OF PLATES Plate Page I. Liquid ammonia metathesis apparatus 32 II, Low temperature colorimeter for use with liquid ammonia 35 III. Apparatus for crystal growth from anhydrous hydrazine. . . . 36 v i i 1 I. INTRODUCTION A. The Problem The effects of high energy radiations on matter in the condensed phase has assumed, in recent years, a very practical importance as indicated by the interest in radia-tion damage to engineering structural materials (1,2) and to biological systems (3,4,5,6), and in the production of isotopes for military purposes (7,8), industrial operations and research programs (9,10,11)• Concurrent with this growth of interest in the gross effects of radiation, physical chemists have turned their attention to a study of radiation damage at the molecular level to correlate chemical effects with the mode of interaction of high energy particles in matter, the state of aggregation of the irrad-iated material, and the reactivity of resulting excited and ionized molecules and ions (12,13,14)* This study i s concerned with the effects of 50 kilovolt (Kvp) X-radiation on some ionic borohydrides. H. G. Heal (15) observed that solid potassium borohydride developed an intense blue coloration on short exposure to 50 Kvp X-rays at room temperature. On dissolving crystals of the irradiated material in cold water a vigorous evolution of gas occurs indicating the possibility of chemical decomposition products in the solid. This work attempts to elucidate the nature of 2 t h e c h e m i c a l d e c o m p o s i t i o n and t h e p h y s i c a l c h a n g e s s u c h a s c o l o r c e n t e r f o r m a t i o n i n t h e a l k a l i m e t a l b o r o h y d r i d e s on e x p o s u r e t o X - r a y s . However , no t h o r o u g h s t u d y on k i n e t i c a s p e c t s o f t h e r a d i a t i o n e f f e c t s was u n d e r t a k e n . The m a j o r p o r t i o n o f t h e e x p e r i m e n t a l work i n v o l v e d d e v e l o p -i n g methods f o r t h e p r e p a r a t i o n and p u r i f i c a t i o n o f i o n i c b o r o h y d r i d e s u s e d i n t h e r a d i a t i o n s t u d i e s . B . M e t h o d s o f I n v e s t i g a t i o n I n s t u d y i n g t h e e f f e c t s o f 50 K v p X - r a y s on t h e i o n i c b o r o h y d r i d e s s e v e r a l methods o f a n a l y s i s were a p p l i e d t o d e t e r m i n e t h e k i n d and q u a n t i t y o f change i n d u c e d . These w e r e : 1) I n f r a - r e d a b s o r p t i o n s p e c t r a o f i r r a d i a t e d m a t e r i a l s t o d e t e r m i n e t h e n a t u r e o f t h e p r i m a r y d e c o m p o s i t i o n p r o d u c t s i n t h e s o l i d . 2) U l t r a - v i o l e t and v i s i b l e a b s o r p t i o n s p e c t r a (2000 X t o a p p r o x i m a t e l y 7000 2) t o d e t e r m i n e t h e n a t u r e o f t h e c o l o r c e n t e r s . 3) C o l o r i m e t r i c e s t i m a t i o n o f n o n - v o l a t i l e d e c o m p o s i -t i o n p r o d u c t s ( e . g . , p o t a s s i u m m e t a l ) and q u a n t i t a t i v e measurement and a n a l y s i s o f g a s e o u s p r o d u c t s r e s u l t -i n g f r o m i r r a d i a t i o n . 4) P a r a m a g n e t i c r e s o n a n c e s p e c t r a o f i r r a d i a t e d c r y s t a l s t o h e l p e l u c i d a t e t h e n a t u r e o f p r i m a r y d e c o m p o s i t i o n p r o d u c t s i n t h e i o n i c c r y s t a l s . 3 Methods were developed for the preparation, purifica-tion and crystallization of the ionic borohydrides. During the period of t h i s study, the only borohydride commercially available was the potassium salt. A simple procedure for the preparation of other a l k a l i metal borohydrides from the potassium salt and other easily available materials was devised. An extensive search for suitable solvents to purify and to crystallize the ionic borohydrides was under-taken. The most satisfactory solvents found were anhydrous liquid ammonia and anhydrous hydrazine. The use of these materials as solvents entailed construction of special apparatus as described in Chapter II, Section G. Dosimetry data have been worked out for the radiation source, a Machlett OEG-60 beryllium windowed tube, which was used for the chemical decomposition studies. Although l i t t l e use was made of this information in the present study, i t has been included because of i t s possible value in later quantita-tive work using this source. A discussion of X-radiation dosimetry has also been included for reference. The interpretation of changes in the optical properties induced in the borohydrides by X-radiation i s based mainly on models developed from studies of radiation damage in the simpler a l k a l i metal halides. The remaining sections of this introduction w i l l outline the theory of radiation inter-actions in matter and survey some lattice imperfections and their properties. 4 C. The Interaction of Ionizing Radiation with Matter (16,17) Ionizing radiations are high energy emanations which can be generally classified as electromagnetic or corpus-cular. Because of the wave-particle duality of matter this division is not precise. Photons, however, can be distin-guished from other ionizing radiations by their property of zero rest mass. The interaction of two particles, in the rigorous sense, should always be treated in terms of the electromagnetic fields set up by the particles and the effect of these fields on the particles themselves. Electromagnetic interactions, in view of the corpuscular nature of the radiation field, should be described as processes of photon absorption and emission. Whenever a charged particle passes in the vicinity of an atom, one of several distinct effects may occur. If the distance of closest approach is large compared to the dimensions of the atom, the atom (or molecule) as a whole reacts to the field of the passing particle result-ing in excitation or ionization. When the particle approaches within distances of the order of atomic dimensions the inter-action involves the passing particle and one of the atomic electrons, the latter being removed with considerable energy from the bound state. If the binding energy of the emitted electron is small in comparison to the energy obtained in the "knock-on" process, then the interaction may be considered as occurring between the bombarding particle and a free electron. 5 In such close c o l l i s i o n processes the magnetic moments and spins of the interacting particles cannot be neglected (18). When the distance of closest approach becomes smaller than the atomic radius, the deflection of the passing particle by the f i e l d of the nucleus becomes the most important effect. As a result of the particle's deflection, numerous low energy photons known collectively as "Bremsstrahlung" are generated. The interaction of high energy photons with matter produces results analogous to those of charged particles. The interaction of the photon with the atom (or molecule) as a whole gives the photoelectric effect in which the photon i s annihilated, i t s energy appearing as electronic energy of the atom (or molecule) and possibly partly as kinetic energy of an emitted electron. This effect i s important only at low energies ( <50 Kev). The interaction of a photon with a free electron leads to the Compton effect in which the photon transfers only a part of i t s energy and momentum to the electron which i s considered at rest. The interaction of a photon with a coulomb f i e l d can lead to pair production whereby the photon i s annihilated with the simultaneous formation of a positive and a negative electron. This process occurs only when the energy of the photon exceeds the rest energy of the two electrons (1.02 Mev). Irradiation of matter with neutrons produces ionization only by secondary processes, the primary effect being momentum transfer to the 6 nucleus of the colliding atom. However, the f i n a l results of neutron capture may be a fission reaction or an energetic gamma emission which can cause tremendous structural damage to the molecule or in the soli d . Whenever high energy radiation i s absorbed in matter secondary electrons w i l l result from ionization processes which w i l l produce secondary ionizations which frequently swamp the effects arising from primary absorption. This i s always the case with high energy photons. Primary penetra-tion processes for high energy radiation absorption has been excellently outlined by Platzman (19). The absorption of ionizing radiation and subsequent transformations in a material can be studied conveniently in a time sequence consisting of three stages (20): a) Physical stage involving the highly inhomogeneous dissipation of the radiation in the system within a period of 10*^5 second. b) Physi co chemical stage lasting for 10""*2 second during which thermal equilibrium i s established. c) Chemical stage leading to the establishment of chemical equilibrium through diffusion and chemical reaction of reactive species. This requires a time greater than 10~ 1 2 second for macroscopic systems and may be of extreme duration especially in condensed systems. During continuous irradiations a l l three stages w i l l be in progress. 7 D. Radiation Induced L a t t i c e Imperfections An i d e a l c r y s t a l i n which the l a t t i c e has perfect p e r i o d i c i t y i n the occupation of l a t t i c e s i t e s does not exist i n r e a l i t y at temperatures above absolute zero. In r e a l c r y s t a l s certain defects can exist i n thermal e q u i l i b -rium within the l a t t i c e . These defects include vacant l a t t i c e points and i n t e r s t i t i a l atoms or ions. There are two ways i n which these l a t t i c e vacancies and i n t e r s t i t i a l atoms or ions can a r i s e . A vacancy and an i n t e r s t i t i a l are generated simultaneously when a Frenkel defect i s produced. Figure 1(a) shows the formation of t h i s type of defect. The second method of generating vacancies arises from Schottky defects. The l a t t e r process i s shown i n figure 1(b). Thus a Frenkel defect consists of a vacant l a t t i c e point and an atom or ion i n an i n t e r s t i t i a l position while a Schottky defect consists of a l a t t i c e vacancy only. The presence of vacancies gives r i s e to e l e c t r o l y t i c conductivity i n i o n i c c r y s t a l s . Schottky defects w i l l , i n general, be more numerous than the Frenkel type. An impurity atom or ion i s another kind of point defect. Besides point defects we may have other c r y s t a l imperfections such as an extra row of atoms or ions, a row of vacancies, or a row of impurity atoms or ions. These l i n e imperfections are known as di s l o c a t i o n s (21). Line and point imperfections probably play an important role i n color center formation (22). The removal of an electron from an atom or ion i n the l a t t i c e r e s u l t s i n the formation of a p o s i t i v e "hole" i n the (o) A FRENKEL TYPE DEFECT IN A SIMPLE LATTICE • • • • • • • • • • • • • • • • • • • • (b) A SCHOTTKY TYPE DEFECT IN A SIMPLE LATTICE BOROHYDRIDE ION ALKALI METAL ION FIGURE I + - - + + HYDROGEN ATOM UP - HYDROGEN ATOM DOWN (c) TWO SUCCESIVE LAYERS IN AN ALKALI METAL BOROHYDRIDE LATTICE (fact ctnttred cubic structure) 9> band of electronic energy levels. The electron removed by ionization w i l l contribute to the electronic conductivity of the material in much the same way as "free" electrons in a metallic crystal. The "hole" in the valence band of the crystal w i l l also contribute to the electronic conduc-t i v i t y . With insulators a more intimate association of the electron and positive hole exists in the type of center known as an exciton ( 2 3 ) . Excitons can be produced by absorption of photons of sufficient energy to transfer the electron from an anion to the cation. Exciton formation does not result in photoconductivity. Ionization of excitons or formation of positive holes with transfer of electrons to a conduction band results in photoconductivity. Point and line imperfections may act as "traps" for electrons which have been removed in excitation of the valence band electrons. The l a t t i c e defects arising from the association of positive holes or electrons with l a t t i c e vacancies and inter-s t i t i a l s are collectively known as color centers. Their properties have been comprehensively reviewed by Seitz (24) • Figure 2 shows the structures which have been tentatively assigned to explain the properties of a number of the important radiation induced color centers. These models are based on results obtained for the centers in the a l k a l i metal halides. Centers of the type F and F 1 are the simplest electron excessive centers. Complexes involving these simple 1 0 11 centers with positive and negative ion vacancies give rise to R and M type centers. V type centers are electron deficient. The H-center i s the complement of the F-center, i.e., a region of undamaged la t t i c e i s formed when an H-center and an F-center combine. Some of the electron deficient centers have been closely examined by Kanzig et a l . (25) by the paramagnetic resonance technique. The U-center involving the hydride ion (26,27) i s an important color center in radiation induced imperfections of the a l k a l i metal boro-hydrides. A model for the U-center in agreement with i t s known properties consists of a hydride ion, H-, occupying substitutional^ the position of the normal negative ion, e.g., halide, borohydride, etc. The various kinds of color centers have definite electronic structures and states. Transitions between allowed energy states give rise to the characteristic absorption bands, from which the name of these la t t i c e imperfections i s derived, and also fluorescence in the case of some of the centers. Ivey (2S) has developed empirical formulas which f i t the wavelengths of maxima of the absorption bands for electron excessive centers. For the F-center and the U-center these are: F: Xmax • 703d 1 * ^ Angstroms U: Amax - 6 l 5 d 1 # 1 ^ Angstroms where d i s the interionic distance between cation and anion in the ionic crystal. These formulas w i l l be applied in the 12 discussion of the radiation induced absorption bands obtained with the a l k a l i metal borohydrides in the spectroscopic studies outlined in the experimental work of th i s thesis. 13 II. APPARATUS AND METHODS CHARACTER1STIG OF THE RADIATION STUDIES A. The X-ray Generator Radiation in the X-ray region of the electromagnetic spectrum w i l l result whenever sufficiently energetic electrons interact with the strongly bound inner electrons of atoms or ions. In practice, X-radiation i s obtained by accelerating electrons through a high voltage in vacuum and then allowing them to impinge on a target. Thus two main components of any X-ray generator are: 1) a vacuum tube containing an "electron gun" and target, and 2) a high D.C. or re c t i f i e d voltage to accelerate the electrons from a filament to the target. In this study two types of high intensity beryllium windowed tubes were used. Figures 3 and 4 show cutaway diagrams of the Machlett OEG-60 and the Machlett AEG-50 tubes, respectively. The AEG-50 tube was used in conjunc-tion with the radiation vessel for absorption spectra work, while the vertically mounted OEG-60 tube was used with the irradiation vessel for chemical decomposition studies. Table 1 shows some important characteristics of the two tubes. Polar diagrams showing angular depence of radiation intensity are given in Figures 5 and 6. Figure 3 Schematic View in Section of AEG-50 Tube. CATHODE SHOCKPROOF SHIELD TUNGSTEN TARGET-BERYLLIUM WINDOW -COOLING WATER INLET Figure 4 Schematic View in Section oj OEG-60 Tube. TABLE 1 CHARACTERISTICS OF THE AEG-50 AND OEG-60 X-RAY TUBES AEG-50 0EG-60 Target Angle 20° 45° Focal Spot Size 5 mm2 6 mm2 Cathode Line focus with tungsten filament Line focus with tungsten filament Inherent Filtration 1,0 mm Beryllium 1.0 mm Beryllium X-ray Coverage 22° from central ray 30° from central ray Maximum Voltage 50 Kvp 60 Kvp Maximum Load Current (Single phase pulsating potential) 50 mamps 50 mamps Filament Characteristics 6.5 volts, 4*2 amps for 50 milliamps at 50 Kvp 6.5 volts, 4#2 amps for 50 milliamps at 50 Kvp Figure 5 — Angular Distribution of Intensity in Beam of Radiation from AEG-50 Tube. 17 The high tension for the tubes was generated by a modified General Electric KX-8 transformer with a primary voltage and operating circuit designed by H. G. Heal, Figures 7 and 8 give the wiring of the primary and secondary circ u i t s , respectively. Tables 2 and 3 describe components of the ci r c u i t s with some notes on their operation. TABLE 2 HIGH TENSION PRIMARY OPERATING CIRCUIT COMPONENTS S 220 volt unstabilized mains supply. F,F Two 25 ampere fuses. S-L D.P.S.T, main switch. S 2 S.P.S.T. X-ray filament switch. Si and S 2 are located on the control panel. Filament voltage stabilizer, consists of two trans-formers with their primaries i n series. The one which supplies most of the output voltage operates with a high flux density in i t s iron core, i.e., i t i s oversaturated. It i s also partly resonated by means of a capacitor, C^lO^afd. The other trans-former operates with a low flux density and in out-of-phase relationship to the f i r s t transformer. The potential between line C and the tap in use at the stabilizer i s 150 volts. T^ Autotransformer. Only two input connections are presently intact. S^  Multipoint switch which taps the autotransformer at various voltages to supply the primary current for the o i l immersed high voltage transformer. This switch i s located on the control panel with contacts lettered A to H. When S^ i s on contact 1, contact A gives 60 volts contact B gives 90 volts contact C gives 110 volts contact D gives 130 volts contact E gives 150 volts contact F gives 165 volts contact G gives I85 volts contact H gives 205 volts. M ca 20 TABLE 2 continued S/,, Multicontact switch similar to S3. Located on control panel with contacts numbered 1 to 8. Each contact increases the voltage obtained on S3 by an increment of approximately 2 volts, e.g., contact 2 increases S3 voltage by 3 volts contact 3 increases S3 voltage by 5 volts contact 4 increases S3 voltage by 7 volts, etc. T2,T3 Two 110 v.-6 v. stepdown transformers. The trans-former primaries are connected in series to the 220 v. li n e . The transformer secondaries are connected in parallel. These transformers supply current at 6 volts for the operation of the safety relays. T^T/j Two ganged Powerstat variable transformers type 116-20. S5 Safety switch ensuring that there i s no voltage across T - T when S7 i s closed. This switch must be closed before safety relay system w i l l operate. Contacts close only when variable transformers are f u l l y counterclockwise. S$ Water switch. Metropolitan Vickers Relay M6. This switch ensures that cooling water i s flowing through X-ray tube target before any high tension i s applied to the tube. Since this switch operates on the bellows principle, i t can open under excess flow of cooling water. Only when the circ u i t i s grounded through this switch w i l l the relay system operate. Sy "High Tension" switch located on control panel. V Volt meter which i s calibrated to read kilovolts across X-ray tube. A value of 63 Kvp on this meter corresponds to 50 Kvp across the X-ray tube. R]_ A current dissipating resistance. D.P. Relay. This relay switches the primary voltage on the T - T lines. This voltage i s applied to the primary of the o i l immersed transformer. This relay w i l l not operate unless one of either S5 or the S.P. relay i s closed to ground. In any case, however, S^ must be closed to ground. 21 TABLE 2 continued S.P. Relay. This relay ensures that the relay ci r c u i t i s grounded for any position of the coupled variable transformers TA and T5. Se must be closed to ground before this relay w i l l be actuated, .  L Variahle inductance with iron core. Potential across line between lines X and C i s 130 volts. I X-ray tube filament current meter. R 2 Shunt for X-ray tube filament current meter. T,T Current carrying lines for the primary circuit of the o i l immersed transformer. C Current carrying line common to X-ray tube filament circuit and Kenotron filament c i r c u i t . X Current carrying line used with line G for operat-ing X-ray tube filament transformer. Potential across lines X - C i s 130 volts. K Current carrying line used with line C for operat-ing the Kenotron transformers. Potential across lines K - C i s 145 volts. Timer An electric clock has been suitably placed across resistor Ri to supply 110 volts operating voltage. 22 TABLE 3 HIGH TENSION SECONDARY CIRCUIT COMPONENTS The secondary circuitry of the high voltage supply i s o i l immersed in a large steel tank. This container has several terminals on i t s cover, two of which are of the large porcelain type. One of the porcelain insulated terminals i s grounded while the other contains a socket with two leads from the X-ray tube filament transformer T5. These latter two leads are at the potential of the output from the second-ary circuit of the high voltage transformer, General Electric type KX-8, Although the two high voltage wires used to operate the X-ray tube filament are of different colors, viz., black and white, i t i s immaterial how they are connected to the X-ray tube filament terminals. The remaining terminals on the tank l i d are located on a terminal board. Terminals K, C, X, Tj_, and T2 are connected to the corresponding lines of the primary operating c i r c u i t with the reservation that T^ and T 2 need not be distinguished and thus may be simply connected to the lines T - T of the control c i r c u i t . Terminal G i s grounded while terminal P remains unconnected. Terminal M i s connected to the X-ray filament current meter on control panel, the other terminal on this meter being connected to ground potential. 23 TABLE 3 continued I,C,K,T - T lines are discussed as components of the operating primary c i r c u i t , Kenotrons KR-6 - General Electric r e c t i f i e r tubes for use to a maximum of 140 kilovolts. T I » T 2 » T 3 I T A - Kenotron transformers with primary windings in parallel across K - C lines at a voltage of 145 v. Secondaries of these transformers supply voltage to kenotron filaments. Tc I-ray tube filament transformer. Primary winding supplied with 130 volts from X • C lines. The secondary winding of this transformer supplies current to the tube filament. The voltage across this secondary winding i s approximately 8 v. when the tube i s operating at 50 Kvp and 2 8 milliamps. X-ray tube. Two types of tubes have been used in the present study. Their characteristics being dis-cussed in Table 2. The tube filament i s connected to the output of the secondary high tension trans-former which i s normally operated at 50 Krp above ground. The tube anode (target) i s grounded. High Tension Current Meter, I. This meter i s in the secondary winding of the high voltage transformer. One of the meter terminals has been grounded. High Tension Rectifying circuit was modified from the original form in order to make use of X-ray tubes with grounded anodes. This has been done by grounding the midpoint of the secondary winding of the high tension transformer along with the removal of two kenotrons. The resulting high tension i s f u l l wave with only half the peak voltage. 24 B. Radiation Vessels The radiation vessels used in this work were of a metal evacuable type based on the principle of the common Dewar flask. The vessels had 1.0 mm beryllium f o i l windows for sample irradiation. The vessel used in the chemical decomposition studies i s shown in Figure 9. It had a removable top flange to which was attached a sample tray mount (copper) by means of a glass tube (5 1/2 inches) which formed a thimble for coolant. Temperatures obtainable from that of boiling liquid nitrogen to slightly above room temperature were measured by a copper constantin thermo-couple which was connected from the two insulated leads i n the top flange to the small stud on the sample tray holder. The tray holder provided for seven samples to be irradiated simultaneously, each at a different intensity, by the use of suitably placed aluminum f i l t e r s . The various samples in an exposure were used with f i l t e r s of the following total nominal thicknesses of aluminum: Sample 1 Sample tray + 0 thousandths inches Sample 2 Sample tray + 19 thousandths inches Sample 3 Sample tray + 4 thousandths inches Sample 4 Sample tray + 12 thousandths inches Sample 5 Sample tray + 8 thousandths inches Sample 6 Sample tray + 21 thousandths inches Sample 7 Sample tray + 40 thousandths inches. Sample trays were made from aluminum f o i l (approximately 0.5 thousandths inches; 0.003 gm cm^ ) by punching out discs and shaping them in a specially made die. It i s to be noted 25 26 that the samples received radiation which had been "hardened" by more f i l t r a t i o n than that associated with the sample tray and the f i l t e r s attached to the sample tray holder. The total f i l t r a t i o n i s tabulated in figure 15, Chapter II, Section D. The radiation vessel was bolted to the port of the vertically mounted OEG-60 tube. The radiation vessel for spectroscopic studies of small crystals or pressed pellets i s illustrated in figure 10. For irradiations i t was bolted to the horizontally mounted AEG-50 tube. It has a flange and valve to allow evacuation and subsequent removal from the X-ray tube so that i t can be placed in the c e l l compartment of the recording spectro-photometer. Figure 11 shows the carriage used to position the vessel accurately in the Cary spectrophotometer. The material examined in the spectroscopic studies was cemented to the support on the base of the coolant thimble. Provision was also made for thermocouples as with the vessel for chemical decomposition studies. To irradiate the sample the material had to face the beryllium f o i l port. To obtain absorption spectra the sample was then rotated through 90° by turning the whole upper coolant reservoir at the metal tapered joint. This placed the material in a suitable posi-tion between the quartz windows to obtain the spectra. t RING METAL BELLQWS/fl THERMOCOUPLE CONNECTIONS DETAILS OF VALVE (ENLARGED) CRYSTAL MOUNT (COPPER) BERYLLIUM WINDOW -THERMOCOUPLE CONNECTION , , RADIATION VESSEL WITH ALL-METAL DEWAR FIGURE 10 RADIATION VESSEL FOR SPECTROSCOPIC STUDIES CONSTRUCTION MATERIAL: BRASS 0 1 2 3 - J I I I INCHES SET SCREW r w i BASE SUPPORTED BY THE HORIZONTAL RODS IN CELL COMPARTMENT OF 'CARY' RADIATION VESSEL MOUNT CARRIAGE POSITIONING ROD FIGURE II RADIATION VESSEL CARRIAGE FOR USE WITH THE CARY RECORDING SPECTROPHOTOMETER 0 I 2 INCHES 29 C. The Vacuum System and Ancillary Equipment A vacuum system was designed and constructed to carry out several operations connected with the radiation studies. It was designed with the following operations in mind: 1) The use of ammonia, deutero-ammonia and hydrazine as solvents in the purification of starting materials and the solution of the irradiated borohydrides. 2) The use of boron hydrides in attempts to study their radiation chemistry and as intermediates in the preparation of covaient and ionic borohydrides. This necessitated the use of mercury float valves. 3) The collection of gaseous decomposition products from the irradiated borohydrides upon solution i n a suitable solvent. 4) The evacuation of radiation vessels and the collec-tion of gases evolved during irradiations. Figure 12 shows the basic layout of the vacuum system. Pressures were measured by the thermal conductivity method using a Pirani gauge (Edwards 7-2A) with sensitivity of ±0.05/* in the range 0 - 5/JL . A second range on the instrument per-mitted pressure measurements from 0 - 500/t. A mercury diffusion pump was placed in the system for circulating gases through a series of traps or for collecting gases for storage in a two l i t r e bulb. This pump could operate up to a pressure AMMONIA STORAGE TO ATMOSPHERE TO MECHANICAL . PUMP AMMONIA PURIFICATION TRAIN V A C U U M S Y S T E M U S E D I N R A D I A T I O N S T U D I E S FIGURE 12 [_*>_ rTO FLOAT VALVE OPERATING SYSTEM RADIATION VESSEL FOR CHEMICAL STUDIES RADIATION VESSEL FOR SPECTROSCOPIC STUDIES -SILICA GEL 31 difference of approximately 100/*• The two Pirani gauge heads were placed across the diffusion pump, i.e., one on the high pressure side and the other on the low pressure side. Gas collection was considered complete when the low pressure gauge head read less than Zjx after one-half hour of pumping into the storage bulb. The quantity of collected gas could be determined by reference to a calibra-tion graph obtained for the storage system. Several pieces of auxiliary equipment were used with the vacuum system. These were: 1) A non-aqueous solvent metathesis vessel for reactions in liquid ammonia based in principle on a similar type of apparatus described by Parry et a l . (29) who described i t s operation. Plate I shows the construction of the apparatus used in the present work to prepare and purify some ionic borohydrides. 2) A low temperature colorimeter for use with liquid ammonia based on the "light piping" properties of lucite rod. It consists of a light source, a U-shaped lucite "light pipe," photo c e l l used in a bridge c i r c u i t , a colorimeter c e l l with s t i r r e r , and an electromagnet to actuate a small glass enclosed n a i l acting as a s t i r r e r . The apparatus was designed for estimating quantities of a l k a l i metal by measuring the optical density of the blue coloration produced in liquid ammonia when these metals dissolve in this solvent. Provision was made to prepare small amounts of the a l k a l i metals for calibration purposes by thermal decomposition of suitable compounds (e.g., alka l i metal azides) on a hot filament inserted into the evacuated colorimeter c e l l . The a l k a l i metal coloration could not be obtained with sodium in liquid ammonia unless mercury vapor from the vacuum system was prevented from condensing along with the ammonia as early noted by Jannis (30). The problem was solved by transferring sufficient ammonia to the trap at the top of the colorimeter line with a liquid nitrogen bath, then allowing the ammonia to be d i s t i l l e d to the colorimeter tube (now immersed in l i q u i d nitrogen) from the trap at a temperature provided by a chloroform slush bath (-64° C.). The colorimeter unit was then placed over the colorimeter c e l l and a dry ice-alcohol bath put in place to preserve the solution during optical density measurements. Since no a l k a l i metal was detected i n the irradiated potassium borohydride this method was not further studied. It i s mentioned here only because of i t s possible value in estimating a l k a l i metals in irradiated systems where they are known to be produced (e.g., the a l k a l i metal azides). Plate II shows the major components of this apparatus. 3) A crystallization apparatus for use with non-aqueous solvents in particular. It i s a modified version of an apparatus used by Dreyfus and Levy ( 3 D for the growth of al k a l i metal azides crystals from aqueous solution. The apparatus used in this work i s illustrated in Plate III. It was intended for growing crystals of the ionic borohydrides from anhydrous hydrazine. However, as this work i s not complete, no further mention w i l l be made of thi s apparatus in this study. D. Radiation Dosimetry Radiation dosimetry involves the determination of the amount of energy absorbed by a material on exposure to a radiation source (32). This energy may be determined in a number of ways. For example, the intensity of the trans-mitted radiation may be deducted from* that incident on the sample of material to give the amount of energy absorbed per second. In this study use was made of semi-empirical formulations describing the energy output of the radiation source together with attenuation factors (also calculated from semi-empirical formulas) for the incident radiation. 36 PLATE Iff 37 Thus the energy received in the samples was estimated by calculating both the intensity output of the X-ray tube and the portion of this incident flux which was attenuated by the sample. If I0(A) i s the incident intensity of the radiation for wavelengths between X and A + dX , then the energy dissipated per unit time in the sample i s : I0(A)= l - e -M^-'X p where e^/CJ^-Lx <j> i s the fraction of the incident intensity transmitted by the sample. The total energy, E ( A ) , absorbed by the sample for an exposure time, t, i s : E ( A ) = I D ( X ) [ 1 - e-/^/jp-pxjt I 0 ( A ) . t . yU/p'J>x for small x. The fraction of the incident radiation of wavelength X which i s absorbed and which can be expressed by 1 - e~/^}C7£ arises from the various kinds of interactions that exist between the quanta of the radiation and the electrons in the material. Macroscopically these interactions determine the value of^W/jp ~^5^'5^whereJU/p i s the mass absorption coefficient, m e X i s the thickness of the irradiated sample, and p i s the density of the sample. The absorption c o e f f i c i e n t ^ . , has two components such that/* = T + <y where qr i s the component due to photoelectric 38 interaction andcr'is that due to scattering. For quanta of fixed energy,T and <f depend upon the kinds of atoms in the irradiated substance because the number of electrons and their binding energies in a particular atom depends on the atomic number, Z . The photoelectric factor depends greatly on the energy of the quanta. absorption coefficients have been calculated for various wavelengths following the method of Victoreen (33). The calculated values agree within 1 per cent of experimental values by the use of the following semi-empirical formula: where C and D are constants for a particular element, 0~Q i s the scattering factor per electron as determined by the Klein-Nashina equation, - ~ i s the ratio of the atomic number to atomic weight, and N 0 i s avagadro's number. The absorption coefficients of several elements which were used as f i l t e r s or were present in compounds used in irradiation studies have been calculated for the wavelength range of 7 interest using values of C, D, and —=r— given by Victoreen (33). The mass absorption coefficients of these elements for several wavelengths in the range of interest are given in Table 4. The compton scattering factor, , was calculated for several wavelengths from the Klein-Nashina formula: For the dosimetry data used in the present work, 8 7 \ e + •1 1 + oe0 [ e(i + oL„) 3TT\C Z 4 < *1 L 1 + Zoc, j _ J _ ( L a ( l + 2oc^ l+3oc 0 aoc0 i ( l + 2 c c j a 39 where e i s the electronic charge m i s the mass of the electron c i s the velocity of light and a 0 = 7TIC ^where h i s Planck's constant and i s the frequency of the incident radiation. The values of (3^ for wavelengths of interest are plotted in figure 13. To determine the mass absorption coefficient of each a l k a l i metal borohydride the calculated mass absorption coefficient of each particular element in the compound was used in the ratio of i t s mole fraction. For example, the mass absorption coefficient of KBH^ i s given by: KBK 5 4 . 5 The mass absorption coefficients for the a l k a l i metal borohydrides have been tabulated for several wavelengths i n Table 5. TABLE 4 MASS ABSORPTION COEFFICIENTS OF SELECTED ELEMENTS Wavelength in H L i Be B N ftngstroma C 0 0.250 0.336 0.14S O.I56 0.167 0.188 0.201 0.219 0.260 0.339 0.150 0.157 0.170 0.191 0.207 0.228 0.270 0.342 0.152 0.159 0.172 0.195 0.213 0.235 0.280 0.345 0.153 0.161 0.174 0.198 0.220 0.243 0.290 0.347 0.155 0.163 0.177 0.203 0.226 0.252 0.300 0.349 0.156 0,166 0.169 0.179 0.207 0.231 0.261 0.320 0.351 0.158 0.184 0.215 0.244 0.280 0.340 0.353 0.160 0.172 0.189 0.224 0.25S 0.302 0.360 0.355 0.162 0.176 0.194 0.234 0.274 0.326 0.3 SO 0.35S O.I64 0.179 0.200 0.245 0.292 0.353 0.400 0.360 0.166 0.184 0.206 0.257 0.311 O.383 0.420 0.361 0.168 0.188 0.213 0.271 0.332 0.415 0.440 0.362 0.170 0.192 0.220 0.285 0.355 0.451 0.460 0.364 0.173 0.197 0.228 0.300 O.38O 0.490 0.480 0.365 0.175 0.202 0.236 0.316 0.408 0.532 0.500 O.366 0.17S 0.209 0.246 0.335 0.437 0.577 0.520 0.367 0.180 0.214 0.255 0.354 0.470 0.627 0.540 0.368 0.184 0.221 0.267 0.376 0.504 0.680 0.560 0.369 0.186 0.228 0.27S 0.39S 0.542 0.73S 0.5S0 0.371 0.190 0.236 0.290 0.432 0.582 0.799 0.600 0.373 0.194 0.245 0.304 0.44S 0.625 0.865 0.700 0.379 0.214 0.293 O.3S3 0.605 0.884 1.26 0.800 0.335 0.242 0.356 0.486 0.811 1.23 1.79 0.900 O.389 0.274 0.434 0.618 1.07 1.66 2.56 1.000 0.393 0.315 0.533 0.781 1.40 2.21 3.30 1.100 0.400 O.366 0.653 0.982 1.80 2.87 4.33 1.200 0.407 0.426 0.799 1.22 2.28 3.67 5.56 1.300 0.415 0.496 0.969 1.50 2.84 4.60 1.400 0.424 0.577 1.17 1.83 3.50 5.69 8.66 1.500 0.434 0.672 1.39 2.21 4.42 6.95 10.6 TABLE 4 continued Wa ivelength in Angstroms F Na Al S CI K 0.250 0.229 0.294 0.388 0.608 0.675 0.907 0.260 0.239 0.312 0.417 0.664 0.741 1.00 0.270 0.249 0.331 0.448 0.722 0.810 1.10 0.280 0.259 0.351 0.482 0.788 0.886 1.21 0.290 0.271 0.372 0.518 0.855 0.966 1.32 0.300 0.283 0.395 0.557 0.929 1.05 1.44 0.320 0.309 0.444 0.640 1.09 1.24 1.72 0,340 0.339 0.500 0.734 1.28 1.45 2.02 0.360 0.372 0.562 0.839 1.48 1.69 2.36 0.380 0.409 0.632 0.957 1.71 1.95 2.74 0.400 0.450 0.709 1.09 1.96 2.24 3.16 0.420 0.494 0.793 1.23 2.24 2.57 3.62 0.440 0.544 0.886 1.39 2.51 2.92 4.13 0.460 0.597 0.987 1.56 2.86 3.31 4.68 0.480 0.656 1.10 1.75 3.21 3.73 5.28 0.500 0.719 1.22 1.95 3.61 4.18 5.93 0.520 0.787 1.34 2.17 4.04 4.67 6.63 0.540 0.861 1.49 2.41 4.50 5.20 7.39 0.560 0.940 1.64 2.66 4.99 5.77 8.20 0.580 1.02 1.80 2.93 5.41 6.38 9.07 0.600 1.12 1.97 3.23 6.07 7.03 10.0 0.700 1.66 3.02 4.99 9.47 11.0 15.6 0.800 2.26 4.40 7.32 13.9 16.1 22.9 0.900 3.33 6.16 10.2 19.5 22.6 32.0 1.000 4.48 8.35 13.9 26.5 30.6 43.3 1.100 5.89 11.0 18.4 34.9 40.4 56.8 1.200 7.59 14.2 23.7 44.9 52.0 72.8 1.300 9.56 17.9 29.9 56.4 64.9 91.1 1.400 11.9 22.2 37.0 69.9 80.5 112. 1.500 14.5 27.2 45.2 85.O 97.5 136. TABLE 4 continued Wavelength in Sngstroma Rb Gs Br I 0.250 5.20 14.3 4.50 13.2 0.260 5.82 15.8 5.02 14.6 0.270 6.45 17.5 5.58 16.1 0.280 7.14 19.2 6.18 17.7 0.290 7.87 20.9 6.80 19.3 0.300 8.65 22.7 7.48 21.1 0.320 10.4 26.6 8.72 24.9 0.340 12.2 30.8 10.6 28.9 O.36O 14.3 6.38 12.5 33.2 0.380 16.6 7.43 14.5 6.65 0.400 ' 19.2 8.60 16.7 7.72 0.420 21.9 9.2 19.1 8.86 0.440 24.8 11.3 21.7 10.2 O.46O 28.0 12.7 24.6 11.6 0.480 31.4 14-6 27.6 13.1 0.500 35.0 I6.4 30.8 14.8 0.520 38.9 18.2 34.2 16.5 0.540 43.0 20.2 38.0 18.4 0.560 47.3 22.4 41.7 20.3 0.580 51.8 24.8 46.0 22.3 0.600 56.6 27.2 50.2 24.5 0.700 83.7 41.9 75.0 37.8 0.800 116. 6O.7 105. 55.9 0.900 22.6 83.0 139. 77.6 1.000 30.8 i l l . 20.5 100. 1.100 40.4 143. 27.0 130. 1.200 52.0 181. 34.7 165. 1.300 65.1 222. 43.5 202. 1.400 80.5 268. 53.9 245. 1.500 98.0 317. 65.6 305. 6-6CH — i 1 1 1 1 1 i 0.6 0.8 J.O 1.2 1.4 1.6 1.8 WAVELENGTH (A) TABLE 5 MASS ABSORPTION OF THE ALKALI METAL BOROHYDRIDES Wavelength in fingstroms LiBH 4 NaBH^ KBH4 RbBH^ CsBH^ NaB(0) 4 0.250 0.193 0.262 0.716 4.47 12.9 0.185 0.260 0.195 0.274 O.784 5.02 14.2 0.188 0.270 0.197 0.287 O.856 5.54 15.7 0.193 0.280 0.199 0.300 0.940 6.13 17.2 0.197 0.290 0.201 0.313 1.02 6.77 18.8 0.202 0.300 0.203 0.328 1.11 7.43 20.4 0.206 0.320 0.206 0.354 1.31 8.88 23.9 0.215 0.340 0.210 0.396 1.53 10.48 27.7 0.226 0.360 0.214 0.434 1.77 12.3 5.74 0.238 0.380 0.218 0.47S 2.06 14.2 6.67 0.252 0.400 0.222 0.528 2.37 16.4 7.72 0.265 0.420 0.226 0.57S 2.71 18.7 8.26 0.310 0.440 0.230 0.639 3.06 21.2 10.1 0.327 0.460 0.235 0.703 3.46 24.0 11.4 0.34S 0.4S0 0.240 . 0.774 3.S9 26.9 13.1 0.368 0.500 .0.246 O.850 4.37 29.9 14.7 0.394 0.520 0.252 0.924 4.S8 33.2 16.3 0.418 0.540 0.259 1.02 5.41 36.8 18.2 0.444 0.560 0.266 1.12 6.01 40.4 20.1 0.476 0.5S0 0.274 1.21 6.66 44.2 22.3 0.514 0.600 0.282 1.32 7.34 4S.4 24.4 0.540 0.700 0.328 1.9S 11.4 71.5 37.s 0.745 0.800 0.390 2.85 16.7 9S.6 54.5 1.02 0.900 0.468 3.96 23.3 19.3 74.6 1.23 1.000 0.560 5.34 31.6 26.2 100. 1.63 1.100 0.675 7.00 41.3 34.3 129. 2.10 1.200 0.817 9.00 53.0 44.2 163. 2 . % 1.300 0.976 11.4 66.3 55.4 200. 3.66 1.400 1.17 14.1 81.5 68.3 241. 4.52 1,500 1.40 17.2 99.0 83.2 285. 5.64 45 In order to determine the energy that i s absorbed in a sample of the irradiated substance, one must have, besides the attenuation factors, a knowledge of the output intensity of the source. For a given voltage drop across the X-ray tube a distribution of wavelengths of varying intensity i s obtained with the shortest wavelength being expressed by the Duane and Hunt Law: A p = ^ r J K - 12.39 where Ap i s the minimum wavelength occurring in the dis-tribution and y i s the tube voltage in kilovolts. K has been shown to be —|p where the symbols have their usual meaning. Providing the voltage drop i s not great enough to produce characteristic radiation from the target (69 k i l o -volts for the K series of tungsten), the distribution of intensity of the radiation i s continuous. Experimental and theoretical studies of the distribution of t h i s continuous radiation or bremsstrahlung have recently been published by Wang (34) and by Jennings (35) for sources similar to those used in this work. The results of Wang show that the distribution for the Machlett OEG-60 W target tube shows considerable L series of tungsten superimposed on the bremsstrahlung. As considerable aluminum f i l t r a t i o n has been used in the experimental work reported in this thesis, the characteristic radiation was assumed to be completely attenuated before reaching the samples of borohydride. 46 When alternating high tension i s applied across the X-ray tube, the i n t e n s i t y of X-rays, f o r a given wave-length, varies with time and thus an average i n t e n s i t y f o r each wavelength must be used i n the dosimetry c a l c u l a t i o n . De Waard (36) has calculated the i n t e n s i t y d i s t r i b u t i o n i n continuous Xi-ray spectra corresponding to d i f f e r e n t forms of high tension. In the present work, use was made of de Waard1s formula f o r single phase alternating current which i s : where i s the average i n t e n s i t y of the ra d i a t i o n produced at wavelength \ , L, i s the proportionality constant, 2 i s the current through the X-ray tube, Ap i s the minimum wave-length corresponding to the peak value of the voltage (for 50 kilovolts, Apis 0.254 Sngstroms). Q^ * i s a composite function defined by the following expression. 67 = £|siTlJ<-jJeOS0) where cos 0 = -^g. B# as a function of cos 0 has been A plotted i n figure 14. Table 6 gives the values of cos 0 f o r various wavelengths of radiation generated at 50 k i l o v o l t s . The Sommerfeld f i n e structure constant, ocQ , used i n the Klein-Nashina formula has also been included i n t h i s t a b l e . TABLE 6 COSINE 0 = - ^ e FOR 50 KVP AND THE SOMMERFELD FINE STRUCTURE CONSTANT, ©Co - — AT VARIOUS WAVELENGTHS W C 1 Wave Wave Wave Length I Cos 0 Length % Cos 0 Ot0 Length I Cos 0 0.248 1.000 0.097 0.760 0.326 0.032 1.500 0.165 0.016 0.250 0.992 0.096 O.78O 0.318 0.031 1.600 0.155 0.015 0.260 0.954 0.093 0.800 0.310 0.030 1.700 0.146 0.014 0.270 0.918 0.089 0.820 0.302 0,029 1.800 0.138 0.013 0.280 0.886 0.086 0.840 0.295 0.029 1.900 0.131 0.013 0.290 0.855 0.083 0.860 0.288 0.028 2.000 0.124 0.012 0.300 0.827 0.080 0.880 0.282 0.027 2.100 0.118 0.011 0.320 0.775 0.075 0.900 0.276 0.027 2.200 0.113 0.011 0.34© 0.729 0.071 0.920 0.270 0.026 2.300 0.108 0.010 0.360 0.689 0.067 0.940 0.264 0.026 2.400 0.103 0.010 0.380 0.653 0.063 O.96O 0.258 0.025 2.500 0.099 0.010 0 .400 0.620 0.060 0.980 0.253 0.024 2.600 0.095 0.009 0.420 0.590 0.057 1.000 0.248 0.024 2.700 0.092 0.009 0.440 0.564 0.055 1.020 0.243 0.024 2.800 0.089 0.009 0.460 0.539 0.052 1.040 0.238 0.023 2.900 0.086 0.008 0.480 0.517 0.050 1.060 0.234 0.023 3.000 0.083 0.008 0.500 0.496 0.048 , 1 .080 0.230 0.022 3 .100 0.080 0.008 0.520 0.577 0.046 1.100 0.225 0.022 3 .200 0.078 0.008 0.540 0.459 0.044 1.120 0.221 0.021 3.300 0.075 0.007 0.560 0.443 0.043 1.140 0.218 0.021 3 . 4 0 0 0.073 0.007 0.580 0.428 0.041 1.160 0.214 0.021 3.500 0.071 0 .007 0.600 0.413 0.040 1.180 0.210 0.020 3.600 0.069 0.007 0.620 0.400 0.039 1.200 0.207 0.020 3.700 0.067 0.006 0.640 O.388 O.O38 1.220 0.203 0.020 3.800 0.065 0.006 0.660 0.376 0.036 1.240 0.200 0.019 3 . 9 0 0 0.063 . 0.006 0.680 0.365 0.035 1.260 0.197 0.019 4.000 0.062 0.006 0.700 0.354 0.034 1.280 0.194 0.019 0.720 0.344 0.033 1.300 0.191 0.018 0.740 0.335 0.032 1.400 0.177 0.017 49 Figure 15 (1) shows the distribution of intensity for X-rays generated by 50 k i lovol t single phase rect i f i ed alternating current at a constant current i • The current was 28 milliamps in a l l irradiat ion experiments. Attenua-tion of the source by inherent f i l t r a t i o n and by beryllium windows has been taken into account. Measurement of the area under this curve gives a quantity proportional to the rate of energy output of the tube, i . e . , A calorimetric determination of the actual output of the 0EG-60 tube fixed the value of L i in the de Waard formula and thus related the area under curve 1, figure 15, to the absolute rate of energy output of the tube. Other graphs in figure 15 show intensity d i s tr ibu-tions after f i l t r a t i o n by various thicknesses of aluminum used in conjunction with the radiation vessel for chemical decomposition studies. As the rate of energy output of the OEG-60 tube (represented by the area under curve 1) has been determined in absolute units, the absolute inten-sity transmitted through each of the aluminum f i l t e r s i s direct ly proportional to the area under each of the curves representing a different amount of f i l t r a t i o n . These areas were determined by mechanical integration. The calorimetric calibration i s discussed in Chapter III . ENERGY DISTRIBUTION OF BREMSSTR AHLUNG AT 5 0 KVP.,SINGLE PHASE PULSED D.C W A V E L E N G T H (A) 51 I I I . EXPERIMENTAL The f i r s t s t e p t a k e n i n t h i s work was t h e p r e p a r a -t i o n o f t h e a l k a l i m e t a l b o r o h y d r i d e s i n a p u r e s t a t e s u i t a b l e f o r r a d i a t i o n s t u d i e s . P o t a s s i u m b o r o h y d r i d e was u s e d i n t h e p r e l i m i n a r y s o l v e n t s t u d i e s b e c a u s e o f i t s a v a i l a b i l i t y and s t a b i l i t y . S i n c e a l l t h e b o r o h y d r i d e s a r e h y d r o l i s e d i n aqueous s o l u t i o n , n o n - a q u e o u s s o l v e n t s f o r m e d t h e m a j o r i t y o f t h o s e s t u d i e d . The b o r o h y d r i d e s o f r u b i d i u m and c e s i u m were p r e p a r e d b y m e t a t h e s i s r e a c t i o n s i n s u i t a b l e s o l v e n t s . The p u r i f i e d b o r o h y d r i d e s were a s s a y e d b y a p e r i o d a t e t i t r a t i o n m e t h o d . I n f r a - r e d s p e c t r a were a l s o u s e d a s a c r i t e r i o n o f p u r i t y . C a l o r i m e t r i c d e t e r m i n a t i o n o f t h e e n e r g y f l u x f r o m t h e OEG-60 X - r a y t u b e was c a r r i e d out a s a p r e l i m i n a r y s t e p f o r more q u a n t i t a t i v e l a t e r w o r k . The vacuum s y s t e m u s e d f o r c o l l e c t i n g and m e a s u r i n g s m a l l amounts o f g a s e o u s p r o d u c t s r e s u l t i n g f r o m r a d i a t i o n i n d u c e d c h e m i c a l c h a n g e s was c a l i b r a t e d . C h a p t e r IV g i v e s t h e r e s u l t s o f t h i s c a l i b r a t i o n w o r k . ' I n f r a - r e d , v i s i b l e , and u l t r a v i o l e t a b s o r p t i o n s p e c t r a o f s i n g l e c r y s t a l s o r p r e s s e d p e l l e t s o f t h e b o r o -h y d r i d e s were e x a m i n e d i n an a t t e m p t t o d e t e r m i n e t h e k i n d o f p r o d u c t s r e s u l t i n g f r o m X - r a y a b s o r p t i o n . P a r a m a g n e t i c r e s o n a n c e s p e c t r a o f p o t a s s i u m b o r o h y d r i d e were a l s o t a k e n . 52 The mass spectrum of gaseous products evolved during the irradiation of potassium borohydride was examined. Infra-red spectra of solids were obtained by the K Br pellet technique. A. The Preparation of Al k a l i Metal Borohydrides 1. Solvent Studies with Potassiuip Borohydride Although the chemical literature (37) l i s t s a number of solvents for potassium borohydride, none mentioned was found suitable for producing crystals to be used in the radiation studies. Either the solvents reacted (e.g., HgO, CH^ OH) or the salt showed insufficient solubility. The search for a good solvent for potassium borohydride was also expected to solve the problem of producing the purified rubidium and cesium salts. The solvents were tested in most cases by placing a small amount of the potassium salt in 5 ml of the reagent grade solvent. If the salt was unaffected, the solvent was warmed on a steam bath. Table 7 summarizes the results. Unsym.-Dimethylhydrazine was prepared by the method of Fischer ( 3 8 ) . Sym.-Dimethylhydrazine was obtained by Folpmer's method ( 3 9 ) • Water made alkaline with potassium hydroxide, anhydrous liquid ammonia and hydrazine hydrate (85%) proved to be the most promising. However, each had disadvantages: the aqueous caustic solution and the hydrazine hydrate 53 TABLE 7 SUMMARY OF SOLVENT STUDY Solvent Water Water made basic with KOH HoO—KOH—glycerol Glycerol Ethylenediamine Isopropylamine N-butylamine N-methylformamide N,N—dimethylformamide Acetone Methyl Alcohol Ethyl Alcohol Pyridine Piperidine Dioxane Liquid ammonia 85$ hydrazine hydrate Anhydrous hydrazine Sym-dimethylhydrazine Unsym-dimethylhydrazine Liquid sulfur dioxide Liquid S0 2 + Thio-nylchloride 2-aminoethanol Diethanolamine Tetrahydro furan Fur an Remarks on Solvent Suitability Poor; considerable decomposition over a long period. Good. Poor. Insoluble. Insoluble in cold; slight solubility in warm. Insoluble. Insoluble. Insoluble. Insoluble. Insoluble. Poor, considerable reaction. Considerable reaction. Insoluble. Insoluble. Insoluble. Good. Soluble; slow hydrolysis. Good; 28.3 gm KBH4/IOO gm N2H/. at 18.5° C Slightly soluble. Slightly soluble. Slight solubility at b. pt.; only slight reaction. Vigorous reaction. Slight reaction. Considerable reaction. Insoluble. Insoluble. 54 showed extensive hydrolysis over the long periods required for crystallization, and liquid ammonia was found difficult to handle as a crystallizing solvent. Several attempts to grow crystals from liquid ammonia were unsuccessful. In the case of hydrazine hydrate, the solutions were evaporated over concentrated sulfuric acid in an evacuated desiccator, the insoluble hydrazine sulfate being easily recovered from the acid. As a result of the preliminary work with hydra-zine hydrate, it seemed promising to try anhydrous hydrazine because of its water-like properties and its basic character. The use of this material was delayed approximately one year during which time little progress was made in the purifica-tion and crystallization of the borohydrides. Anhydrous hydrazine was investigated as a solvent after the vacuum system and auxiliary equipment was con-structed to handle this easily oxidizable material. The solubility of potassium borohydride in anhydrous hydrazine at 1S.5° C was found to be 2S.3 gm/100 gm N2R4 (solubility of KBH4 in liquid ammonia at 30° C. is 20 gm/100 gm solvent and in water at 20° G. is 19.3 gm/100 gm H2O) . No decomposi-tion of the salt was noted even after two weeks in this solvent. The potassium salt did not appear to form a hydrazinate at the temperature studied whereas a very stable ammoniate was revealed by the infra-red spectra of material obtained from liquid ammonia. Figures 16 and 17 show the infra-red spectra of ammonia gas and hydrazine, 55 respectively, for reference purposes. The infra-red spectra of potassium borohydride extracted with liquid ammonia i s shown in figure 18. The infra-red spectra of the material extracted with anhydrous hydrazine i s shown in figure 20. Examination of the infra-red spectra of the commercial salt (Metal Hydrides Corp., 97$ minimum purity) revealed the presence of the ammoniate. The infra-red spectra of this material i s shown in figure 19» 2, Preparation of Al k a l i Metal Borohydrides The following four points were the guiding principles used in developing a method of preparing the ionic boro-hydrides suitable for the radiation studies: a) The borohydrides would be prepared by a simple method involving an intermediate borohydride which could be obtained easily in high yield and in high purity from the commercial potassium salt. b) The cation of the intermediate borohydride must be easily exchanged for an a l k a l i metal cation in a metathesis reaction. c) The purification of the product borohydride must be stringent yet simple. d) The synthetic method would depend to a large extent on the solubility properties of the boro-hydrides in various solvents particularly non-aqueous. AMMONIA gas phase 4000 3000 2000 1500 0.0 .10 CM-i j i i i i • i 1000 900 fig. 16 800 700 £ 2 0 < £.30 O £ 4 0 <50 .60 .70 1.0 oo 7 8 9 10 11 12 13 14 15 WAVELENGTH (MICRONS) HYDRAZINE liquid film 4000 3000 2000 1500 CM-i 1000 900 fig. 17 800 700 0.0 .10 £ 2 0 < S.30 O £.40 <50 .60 .70 1.0 oo null. J " * •.' 1 ' . i . i . i • ' i , -1 L. 1 J / 1 • 11 \ 11111 • 1 ") h \ / \ V \ 1 / \ 1 / \ 1 / 1 / Tf A r J V u ' 137 m 7 8 9 10 11 12 13 14 15 WAVELENGTH (MICRONS) K B H 4 extracted with liquid N H* 4000 3000 2000 1500 CM-T 1000 900 7 8 9 10 11 WAVELENGTH (MICRONS) 12 13 14 15 |<BH4 commercial material fig. 19 4000 3000 2000 1500 CM-i 1000 900 800 700 O.OF k M ^ l I I L T J / H J ^ / 1 1 \ 1 / 1 1 1 1 1 1 1 1 — — M M / 1 1 1 I I I I \ 1 / 1 I I I I I I I I 1 i l — 1 — — .10t w.20 £.30 O , £ 4 0 f <50 .60 f •70[ l.of WAVELENGTH (MICRONS) f<BH4 extracted with anhydrous N2 H4 fig. 2 CD 4000 3000 2000 1500 CM-' 1000 900 800 700 0.0 .10 • 11 •. 1 . 1 . • • • 1 1 1 L-l—1— . 1 / — « w / \ / \ / \ / \ / V \ / If f V *—1 1 •i 1 > r l 5 > 1 0 1 1 1 2 1 3 14 \i £.30 O 2-40 <50 .60 .70 1.0 WAVELENGTH (MICRONS) i<CH3)3 X 4000 3000 2000 1500 CM-i 1000 900 fig. 2li 800 700 0.0 .10 iu U.20 1.30 O £ 4 0 <50 .60 70 1.0 y ~-> ( A / V < r J f \ <*- \ I 1 y I 1 V / 1 r— - f IF ESAU 2E2 5S A discussion on "onium" salts by H, G. Heal (40) suggested suitable cations which could possibly be used as borohydrides for intermediates in metathesis reactions to obtain the alk a l i metal borohydrides. Thus, the ease with which salts containing the trimethylsulfonium ion, (CR^^S*, can be decomposed when combined with anions of high polariz-ing power was the basis of the following attempted method: S +(CH 3 )3 BH4" + RbF • 130° RbBH4 + S(CH 3) 2 + CH3F When a mixture of the sulfonium borohydride and rubidium fluoride (in slight excess) was pyrolized in vacuum, a very fast reaction occurred at approximately 1300 C. with large quantities of (CB^^S being liberated. The I.R. spectrum of the product i s shown in figure 2 3 . This spectrum shows the presence of 8 small amount of boro-hydride with larger quantities of an unidentified material. The pyrolysis was done by heating the sample tube (connected to a vacuum line) with a small electric furnace controlled by a variable transformer. Temperatures were measured with a copper constantin thermocouple. Pyrolysis of a similar mixture was done in air on a sand bath. A fast reaction occurred at a slightly lower temperature of approximately 90° C, The vapor above the mixture, upon reaction, spontaneously ignited with a b r i l l i a n t green flame indicat-ing the possible presence of boron hydrides in the gas. ^CH3)3BH4 4000 3000 2000 1500 CM-i 1000 900 fig. 22 800 700 0.0 .10 UJ £.30 O 2-40 ^50 .60 JO 1.0 - A \ \ —^ Y / h f > \ / > / / I / i L 1/ L u i FTJ? JUS WAVELENGTH (MICRONS) product from vacuum pyrolysis fig. 23 4000 3000 2000 1500 CM-i 1000 900 800 700 0.0 .10 UJ "20 £.30 O 2-40 <.50 .60 JO 1.0 uuk lull. ——». \ . 1 . .1 . pJ—L. _l 1 J . XUjJ '"1 •jJ-Ll / ] / \ r \ / "~ — \ \ / V \ 1 \ \ / V / Y \ \ \ J \ \ r A - I F FT17 WAVELENGTH (MICRONS) I product from air pyrolysis fig. 24 4000 3000 2000 1500 CM-i 1000 900 800 700 0.0 1 . 1 _i | 1 l l i 1 / r \ 1 \ / I / ! \ 1 f 1 \ j \ \ -A H j 5 i i r WA 8 iVELENC 9 5TH > (Ml i CRC 0 )NS 1 ) 1 I 2 i 3 14 15 .10 "20 £.30 O £40 <50 .60 .70 1.0 In one t r i a l this flame jumped approximately two feet igniting a large batch of mixed sulfonium borohydride and rubidium fluoride. The I.R. spectrum of the product from the air pyrolysis i s given in figure 24. Both prod-ucts showed considerable reducing power with alkaline permanganate and l i t t l e fluoride was detectable. Since the pyrolysis reaction of the sulfonium borohydride was of l i t t l e value in the preparation of alk a l i metal borohydrides, the possible use of such an onium salt for metathesis reactions in solution was con-sidered next. Depending on the kind of a l k a l i metal salt available, two different reactions involving the sulfonium borohydride were developed. Rubidium carbonate was converted to the reineckate which in turn was dissolved in a concentrated aqueous ammonia solution containing the onium borohydride. After removing the precipitated t r i -methylsulfonium reineckate the rubidium borohydride was isolated by lyophilyzing the ammoniacal solution. The sequence of reactions followed in the preparation of rubidium borohydride i s given by the following outline of equations: 2NH4 [Cr (CNS)4 (NH 3) 2J + Rb2C03 > R20;65°C 2Rb [ Cr (CNS)4 (NH 3) 2J + NHo + C0 o 61 Rb[cr (CNS)4 (NH 3) 2] + S(CH 3) 3 BH 4 cold, cone N H4 0 H.Rb+ + BH4" + S (CH 3) 3 R where R = [cr (CNS)4 (NH 3) 2] Cesium borohydride was conveniently prepared from cesium hydroxide. The stoichiometric amount of the sulfonium borohydride was dissolved in a minimum amount of N,N—dimethylformamide and this solution was then added to a concentrated methanol solution of the cesium hydroxide. On cooling the mixture in dry ice, cesium borohydride precipitated from the solution. Other onium borohydrides could possibly serve as a suitable intermediate such as the quaternary ammonium borohydrides reported by Banus (41). However, these latter borohydrides appear to have a f a i r l y high thermal s t a b i l i t y which i s a disadvantage in the preparation of pure a l k a l i metal borohydrides. Heating the prepared a l k a l i metal borohydrides from a sulfonium metathesis reaction to a temperature of 130° C. serves to decompose any occluded sulfonium compounds converting them to volatile or liquid ammonia insoluble products. Following a brief heat treatment to decompose any occluded sulfonium compounds, the crude a l k a l i metal borohydride was extracted with liquid ammonia in the special metathesis vessel described in Chapter II, Section C. 62 With some of the extracted borohydrides i t was necessary to pump under vacuum for considerable time (approximately 24 hours) in order to remove ammonia completely from the solid. In some cases the extracted borohydride was further* treated with anhydrous hydrazine in the metathesis vessel. In contrast to the tendency to form f a i r l y stable ammoniates, the borohydrides of potassium, rubidium, and cesium did not appear to occlude hydrazine in the l a t t i c e at room temperature. 2.1. The Synthesis of Trimethylsulfonium Borohydride Trimethylsulfonium borohydride was prepared by the following series of steps: a) l(CH 3) 2S + CH 3r *.S +(CH 3) 3I-b) S(CH 3) 3I + AgF i£°-^S(CH 3) 3F + Agl c) S(CH 3) 3F + KBH4 + S(CH 3) 3 BH4 a. The Synthesis of Trimethylsulfonium Iodide Methyl iodide (28.4 gms; 1/5 mole, Eastman white label) and dimethyl sulfide (12.4 gms; 1/5 mole, Eastman white label) were mixed in a 50 ml volumetric flask and the resulting mixture cooled by packing the flask in a Dewar vessel with ice. The Dewar vessel with the mixture was placed on a shaker set at a f a i r l y slow motion and shaken for 12 hours. 63 The excess reactants were then allowed to evaporate and the solid nonium n salt was dissolved in 50 mis of water. Although the heat of this reaction i s not large, i t i s advisable to cool the mixture of alkyl sulfide and alkyl iodide because of the volatile and abnoxious nature of these reactants. The yield was quantitative. Figure 21 shows the I.R. spectrum of trimethylsulfonium iodide recrystallized from anhydrous methanol. The purified material sublimed at 213° C. (uncorrected), literature ( 4 2 ) melting point, 20$°-213° C. b. The Preparation of Trimethylsulfonium Fluoride A silver fluoride solution was f i r s t prepared by dissolving silver nitrate (170 gms; 1 mole) in a minimum amount of cold water (approximately 150 ml). Sodium bicarbonate (100 gms; 1.25 moles) was dissolved in 5 0 0 ml of water in a 1000 ml beaker. The silver nitrate solution was added slowly with stirring to the bicarbonate solution and the yellow precipitate of silver carbonate washed three times by decantation using 200 ml portions of d i s t i l l e d water. The precipitate of silver carbonate was transferred to a 600 ml polyethylene beaker and hydrofluoric acid (k&f°) carefully added u n t i l the yellow precipitate was almost completely dissolved. The mixture was l e f t one-half hour to complete the reaction and then f i l t e r e d to remove insoluble material. The silver fluoride solution was stored in the dark. Silver fluoride solution was added to the sulfonium iodide (obtained in Section 2.1a) u n t i l Silver iodide was just completely precipitated. The mixture was then f i l t e r e d to remove silver iodide and the f i l t r a t e saturated with hydrogen sulfide to remove excess silver ion. The precipi-tate of silver sulfide was f i l t e r e d out. The f i l t r a t e was evaporated under vacuum at 60° C. to a volume of 30 ml. The concentrated solution of sulfonium fluoride was f i l t e r e d again, the f i l t e r paper being washed with a minimum amount of water. The remaining water in the sulfonium fluoride solution was removed by freeze-drying (lyophiliza-tion). Because of the extreme hygroscopic nature of this fluoride, i t was necessary to store i t in a vacuum desiccator over P2O5 and to handle i t in a dry box. The Preparation of Trimethylsulfonium Borohydride. To the trimethylsulfonium fluoride obtained in Section 2.1b ( i t remained slightly damp after lyophilization) was added potassium borohydride (10 gms) and the two reactants mixed well in the flask for 15 minutes. The mixture was l e f t over-night in the flask which was l i g h t l y covered with a beaker. The sulfonium borohydride was then extracted from the mixture with four 15 ml portions of N,N—dimethylformamide. The solvent was removed from the extract by vacuum evaporation at room temperature. The crude sulfonium borohydride was fi n a l l y extracted with liquid ammonia in the meta-thesis vessel shown in Plate I (p. 32) to obtain a f a i r l y pure product. The assay of two samples of this material i s l i s t e d in Table 8. The yield was 6.2 grams or 35 per cent of the theoretical yield based on the amount of dimethylsulfide used in Section 2.1a. The I.R. spectrum of the purified material i s shown in figure 22. Trimethylsulfonium borohydride i s very soluble, without reaction, in anhydrous liquid ammonia, anhydrous hydrazine, and N,N—dimethylformamide. It i s only slightly soluble in dioxane or n-butylamine. This borohydride i s rapidly hydolysed in aqueous solution. 2.2. The Preparation of Rubidium Borohydride Rubidium borohydride was prepared via the reineckate by the sequence of reactions outlined above. a. The Synthesis of Rubidium Reineckate The method of Palmer (43) was used to prepare ammonium reineckate. To ammonium reineckate (1G gms 0.025 mole) was added rubidium carbonate (5 gms; 0.030 mole) in 25 ml of water. The mixture was TABLE 8 ASSAY OF SOME OF THE IONIC BOROHYDRIDES Material Source Purity Potassium Borohydride Commercial material Sample #1 95.6% Potassium Borohydride Commercial material Sample #2 93.8% Potassium Borohydride R e c r y s t a l l i z e d from l i q u i d NH3 Sample #1 98.2% Potassium Borohydride Re c r y s t a l l i z e d from l i q u i d NH3 Sample #2 97.6% Potassium Borohydride R e c r y s t a l l i z e d from anhydrous ^H^ Sample #1 98.7% Potassium Borohydride R e c r y s t a l l i z e d from anhydrous N2H^ Sample #2 97.8% Potassium Borohydride R e c r y s t a l l i z e d from aqueous KOH Sample #1 88.8% Potassium Borohydride R e c r y s t a l l i z e d from aqueous KOH Sample #2 88.9% Trimethylsulfonium Borohydride Re c r y s t a l l i z e d from l i q u i d NH3 Sample #1 100.2% Trimethylsulfonium Borohydride R e c r y s t a l l i z e d from l i q u i d NHj Sample #2 98.0% ON ON 67 heated to 50° C. for one hour while a stream of air was bubbled through i t to remove ammonia and carbon dioxide. It was then allowed to cool to room, temperature and the precipitate of rubidium reineckate f i l t e r e d out. The resulting crude rubidium reineckate was recrystallized by dissolv-ing i t in 100 ml of water at 60° C , f i l t e r i n g the hot solution, cooling the f i l t r a t e i n ice and collecting the fine crystalline precipitate on a Buchner funnel. The product was f i n a l l y a i r dried at room temperature. The infra-red spectra of ammonium reineckate and rubidium reineckate are shown in figures 25 and 26, respectively. The I.R. spectrum of ammonium iodide i s shown in figure 27 for purposes of comparison. The reineckates are only slightly soluble in cold water, but are very soluble in liquid ammonia, anhydrous hydrazine and N,N—dimethylformamide. b. The Preparation of Rubidium Borohydride by Metathesis Rubidium reineckate (4 gms; 0.01 mole) was dissolved in concentrated aqueous ammonia (40 ml) in a 250 ml, conical flask. To this solution was added 10 ml of concentrated aqueous ammonia con-taining trimethylsulfonium borohydride (0.9 gms; 0.01 mole). The slurry was then transferred to a Buchner funnel and the f i l t r a t e used once to NH4 Cr(CNS)4(NH3)2 fig. 25> 4000 3000 2000 1500 CM-1 1000 900 800 700 ,i i i ,i i I, I I,I—h—i—i i i i—.1 i |II11J i • 111111 • 11 ••• I i i \ i i i i !• i 68 Rb Cr(CNS)4(NH3)2 4000 3000 2000 1500 CM-i 1000 900 fig. '26j 800 700 0.0 .10 UJ S.30 O £-40 <50 .60 .70 1.0 oo <-\ / f / j / V V f i J y y - t H— J2UH WAVELENGTH (MICRONS) 4000 3000 2000 1500 CM-i 1000 900 fig. 2*7 800 700 0.0 .10 I . I . I . I . i Luli '"1 I I I I I 1 1 l, , i i i, / \ / / i u II FIB £ 2 0 < S.30 O 2-40 <50 .60 .70 1.0 oo *4 WAVELENGTH (MICRONS) 69 wash the remaining precipitate from the flask. The precipitate of the f i l t e r was washed with a further 25 ml portion of concentrated aqueous ammonia. The f i l t r a t e ( i t s t i l l had a slight pink coloration which could not be discharged by the addition of a further amount of the sulfonium borohydride) was divided into two portions and each lyophilized. Only after a considerable amount of the ammonia had been pumped off could the freeze-drying operation work properly. The crude rubidium borohydride from the lyophilization was washed with N,N—dimethyl-formamide (approximately 10 ml) onto a f i l t e r paper. The residue was then allowed to air-dry on the paper. This residue was then extracted with liquid ammonia in the metathesis apparatus and an I.R. spectrum of the resulting product taken (figure 28). The yield was about 50 per cent. 2.3. The Preparation of Cesium Borohydride Because of the availability of cesium hydroxide, this borohydride was prepared by the direct metathesis of the sulfonium borohydride and the a l k a l i metal hydroxide in a methanol solution from which cesium borohydride precipitates in the cold. Cesium hydroxide (1.5 gms; 0.01 mole) was dissolved in methanol (25 ml, reagent grade) in a 50 ml volumetric flask. Trimethylsulfonium borohydride (0.9 gms; 0.01 mole) 70 71 was dissolved in N,N—dimethylformamide (10 ml Eastman White label). The dimethylformamide solution was then added to the methanol solution and the mixture thoroughly shaken for 5 minutes. Upon cooling the mixture in crushed dry ice, a fine white crystalline precipitate came down. The crystallites were rapidly collected on a sintered glass crucible and allowed to dry in a vacuum desiccator. The dried borohydride was placed in a heating tube attached to a vacuum system and kept at 110° C. for three hours by means of a small electric furnace. The crude cesium borohydride was f i n a l l y extracted with liquid ammonia in the metathesis vessel. The yield was 0.5 grams or 35 per cent. The I.R. spectrum of the purified cesium borohydride i s shown in figure 29. 3« The Assay of the Ionic Borohydrides Some of the ionic borohydrides (KBH^, S{CE^)^ BH^) were analyzed by the volumetric method of Lyttle et a l . (44). In brief, the method i s as follows: a) 0.5-0.6 millimoles of the borohydride i s dissolved in 25 mis of 0.5N NaOH. b) 35 ml of 0.25N (accurately known) KIO3 added immediately and the flask swirled for 30 seconds. c) 2 gms KI added and then 20 ml of 4N HgSO^ added. d) Flask placed in dark for 2-3 minutes. 72 e) Liberated iodine titrated with 0.10N Na 2S 20 3 with starch indicator. Table 8 summarizes the results of the analyses. Only representative results are shown. Liquid ammonia and anhydrous hydrazine yield a considerably purer product than any obtainable from an aqueous solvent. Trimethyl-sulfonium borohydride could be obtained in very high purity by a liquid ammonia extraction process. These results are confirmed by the I.R. spectra discussed in the previous sections. B. Calorimetric Determination of the Radiation Flux from  the OEG-60 X-ray Tube The calorimetric determination was carried out by measuring the maximum temperature of a silver disc approp-riately mounted by soldered silver wires in the radiation vessel for chemical work when the disc has reached thermal equilibrium in the X-ray beam. The energy output of the tube could be obtained by an experimental determination of the rate of cooling, the ambient temperature in the absence of radiant flux, and the application of Newton's law of cooling. By Newton's law, the rate of energy loss with time, i s -H t ' 1 S ' d| = _K(T-T0) dt where T i s the temperature of the silver disc at time, t . T Q i s the ambient temperature which in the f i r s t t r i a l was 22.3° C. Since dE = mcdT where mc i s the heat capacity of the silver disc and mounting material. Thus ^ 1 = ^1 (T-T 0) at mc Integrating, In (T-T 0) = ~ t + Constant, m m n -K/lIlCt or T-T 0 = Ce ' A plot of In (T-T 0) as a function of time enables one to calculate K from the slope of the graph (mc i s known). This plot for the experimental conditions used in this study i s shown in figure 30 . T h U S K - -mc < T- T° = _ n c 23.0 - In 8) lo.p = -mc 1»Q5° = 0.0571 m (cal.min.~1deg.~ 18.5 Now the heat capacity of the thermal radiator i s composite 1.504 gms of silver with specific heat of 0.0558 and 0.030 gms of solder (50/50 Sn-Pb) with specific heat of 0.0451. Thus the_heat capacity = mc = 1 . 5 0 4 (0.0558) + 0.030 ( o . 0 4 5 D = 0.0853 cal/C° Therefore K - 0.0853 x 0.0571 = 0.00487 cal.min.^deg." 1 75 Now at the maximum temperature of 46.3° C., when the rate of energy absorbed from the X-ray tube i s the same as that lost by thermal radiation, w i l l be: dE « 0.00487 (46.3°-22.3°) dt = 0.117 cal.min." 1 A second determination gave an energy flux value of 0.123 cal.min." 1. The energy output from the tube after f i l t r a t i o n by 0.662 gm cm"1 of beryllium and 0.081 gm cm"1 of aluminum can be taken as 0.120 ± 0.005 cal.min." 1 The mean diameter of the silver disc used as a thermal radiator was 1.09 cm with a corresponding area of 0.933 cm2. Therefore, the value of the radiant flux at 50 kvp and 28 milliamps for the f i l t r a t i o n noted above i s 0.129 cal.min." 1 cm"2. This intensity corresponds to the area under curve 2 of figure 15 (p. 50). The radiant flux for the other degrees of f i l t r a -tion can now be easily computed. The results are tabulated in Table 9. TABLE 9 ENERGY FLUX FOR SAMPLES IN THE RADIATION VESSEL FOR CHEMICAL . DECOMPOSITION STUDIES (Source: OEG-60 Tube at 50 kvp and 28 milliamperes) Irradiation No. of Intensity ' ' _ Area under Specific Energy Flux Sample No. Dist'n Curve of Filtration gm cm~d Curve of Figure 1 5 at 5 0 kvp & 28 m; Figure 1 5 (Sq. inches) (cal.min."1 cm' 1 2 0.081A1 + 0.662 Be 10.39 0.129 2 6 0.209A1 + 0.662 Be 7.01 0.087 3 3 0.102A1 + 0 . 6 6 2 Be 9.54 0.118 4 5 0.159A1 + 0 . 6 6 2 Be 7.98 0.099 5 4 0.138A1 + 0.662 Be 8.45 0.105 6 7 0.266A1 + 0.662 Be 6.12 0.076 7 8 O.36OAI + 0.662 Be 5.02 0 . 0 6 2 1 0.662 Be 17.74 0.220 ON 77 C. Spectroscopic Examination of Irradiated Borohydrides 1. Visible and Ultraviolet Absorption Spectra Only one crystal of potassium borohydride was obtained which was suitable for the spectroscopic work. Pressed pellets of potassium borohydride were used in the majority of experiments with this material. Pressed pellets of the other a l k a l i metal borohydrides were used exclusively. Pellets of the salts were prepared by placing approximately 10 to 15 milligrams of the borohydride in the die used to form KBr pellets for I.R. spectra studies. The loading of the die was done in a dry box. The die with salt was then placed under a 20,000 pound total load in a small laboratory hydraulic press. The resulting disc of borohydride was broken into three to four small sectors each of which could be used in a separate spectroscopic experiment. One of these small sectors of the pressed pellet was niounted on the crystal holder of the spectroscopic radiation vessel by means of rubber cement. The removal of the pellet from the die and the mounting operation had to be done in the dry box. The radiation vessel and i t s operation i s discussed i n Chapter II, Section B« In order to determine the radiation induced changes in the absorption spectra of the a l k a l i metal borohydrides i t was necessary to obtain the absorption spectra of the unirradiated materials. After the spectrum of the 78 unirradiated borohydride was obtained with the Cary Recording Spectrophotometer, the radiation vessel was transferred to the X-radiation laboratory and there mounted on the AEG-50 tube for exposure. The crystal or pellet had to be rotated through 90 degrees between the recording of the spectrum and the X-ray exposure. Fiducial markings located on the neck and body of the radiation vessel enabled the material to be aligned exactly during each exposure and spectroscopic run. The radiation vessel was connected to the vacuum system and pumped while mounted to the X-ray tube. The vacuum valve and flange joint allowed the vessel to be transported while under vacuum. Samples at such low temperatures as that of boiling nitro-gen were easily maintained with the evacuated vessel. A l l spectra in this study were recorded at liquid nitrogen temperature (-196° G. or 77° K). Generally, three exposures to X-radiation for varying times at 50 Kvp and 28 milliamps were taken on each sample. When bands of the spectrum had formed to a sufficient degree, the effect on each band of prolonged exposure to wavelengths of absorbed radiation (in the range of 2000 2) was studied to determine the bleachability of the bands and the interrelations between bands as shown by the changes in the over-all spectrum resulting from light absorption. Finally, the thermal stability of bands were studied in some cases as the sample 7 9 was warmed to dry ice or to room temperature. The spectra, however, were taken at liquid nitrogen temperature after such warming experiments. The results of these spectroscopic studies are shown in figures 31 to 39. The true absorption spectra of unirradiated materials were obtained by subtracting from the recorded spectra of unirradiated borohydrides, the "absorption spectrum" of the empty crystal mount ( s l i t ) . The spectra showing only radiation induced changes were obtained by subtracting from the recorded spectra at 100 2 Intervals (and at maxima and minima where possible), the recorded spectrum of the unirradiated material. When the optical density of the material was high, as i t often was near the absorption edge of the material, the optical density range of the instrument could be changed to higher values by placing a piece of fine wire mesh in the reference beam of the spectrophotometer. The results of these arith-metical operations were recorded in tables giving optical density increments for various wavelengths in ingstrom units. The values from these tables were then plotted as functions of the wavelength of absorbed radiation in units of recip-rocal centimeters or electron volts (both scales are included in the spectra reported in this work). Preliminary arith-metical work, difference tables, etc., have been omitted from this report. Only the f i n a l plotted spectra are included. The results of these spectroscopic studies are Figure 31* Radiation induced changes i n LiBH^ Curve 1. Absorption spectra of unirradiated L1BR4 pellet. Curve 2. Radiation induced change after 3 minutes exposure to X-rays at li q u i d nitrogen temperature• Curve 3» Radiation induced change after one hour exposure to X-rays at l i q u i d nitrogen temperature. Curve 4» Radiation induced change after 2 1/2 hours exposure to X-rays at liquid nitrogen temperature. Curve 5« Sample represented by Curve 4 exposed to light of 4500 &. wavelength for 1/2 hour at liquid nitrogen temperature. Curve 6» Sample represented by Curve 5 exposed to bright tungsten illumination for three seconds at liquid nitrogen temperature. / 81 W A V E L E N G T H (in Angstroms) z o r-0. o I -50H 1-25 4 l - 0 0 i 0-75 A 0 - 5 0 0-25 0 o O O O O O O o O ° ° ° o o O O O O O O © 0 0 0 0 ro * r> to s d gi o - N i o t i n CJ CJ CJ CM c j c J c J r o r o r o i o r o i p 1 i i i ' 1 I i I I — I — I — I — O o o o o m o o o o o o o O O O O o o m o « o >o y y y f 'r >r 0 0 0 1-00 0-75 -\ 0-5C-H 0-25-i Li BH 4 Pressed Pellet FIGURE 31 , 0 0 0 — r 5-5 E L E C T R O N V O L T S $2 Figure 32. Radiation induced changed in NaBH^ Gurve 1. Absorption spectra of unirradiated NaBH^ pellet• Curve 2. Radiation induced changes after 7 minutes exposure to X-rays at liq u i d nitrogen temperature. Gurve 3. Radiation induced change after 15 minutes exposure to X-rays at li q u i d nitrogen temperature• Curve 4» Radiation induced change after 30 minutes exposure to X-rays at liq u i d nitrogen temperature. Curve 5» Sample represented by Gurve 4 exposed to light of 2700 % wavelength for 5 minutes at liquid nitrogen temperature. Curve 6. Sample represented by Curve 5 exposed to X-rays for a further one hour at li q u i d nitrogen temperature. The sample has now received 1 1/2 hours exposure to X-rays. Curve 7. Sample represented by Curve 6 was exposed to light of 4750 % and then to light of 5400 £ for periods of 5 minutes at liquid nitrogen temperature without any noticeable change i n the absorption spectra. The sample was then exposed to bright tungsten illumination for 20 seconds at liquid nitrogen temperature. 83 WAVELENGTH (in Angstroms) UJ o 4 O o o ro O O O O O O O o O O O O O O O O O O O Q 0 0 0 0 • m to h-o)o>o — CJ 10 • in CJ CJ CJ cjcJCMrorororprprp • ' I I 1 J 1 1—I—I—I—L_ O o o o o 0 0 0 0 0 o o o o o o o •n o 10 o in o in y y y y y T T I • 25 H I -00-0-75 A 0-50-f 0-25 H </> 000 I -25 A o ' - 0 0 0-75 A 0-50 A 0-25 -t 0 0 0 Na BH 4 Pressed Pellet FIGURE 32 \ 5-5 50 ELECTRON VOLTS Figure 33. Radiation induced changes i n KBH/,, crystal Curve 1. Absorption spectra of unirradiated KBH^ crystal. Curve 2. Radiation induced change after 15 minutes exposure to X-rays at liq u i d nitrogen temperature. Curve 3* Radiation induced change after 30 minutes exposure to X-rays at li q u i d nitrogen temperature. Curve 4. Radiation induced change after one hour exposure to X-rays at l i q u i d nitrogen temperature• Curve 5* A sample represented by Curve 3 exposed to bright tungsten illumination for one minute at liquid nitrogen temperature. Curve 6. A sample represented by Gurve 5 warmed to dry ice temperature ( - 77° C.) for two hours. WAVELENGTH (in Angstroms) o o o o * m CM CJ O O O O o O ° ° ° O O O O o © 0 0 0 r~- as oi o — CJ ro * m CM CJ CJ to eo ro rp rp rp O o o o o to o O O O o O O O o m o m »p y y <p j L 0-504 0-25 H 0 00—r 5-5 E L E C T R O N VOLTS Figure 3 4 » Radiation induced changes in KBH^ crystal (continued) Curve 7 * Radiation induced change after 3 0 minutes exposure to X-rays at liquid nitrogen temperature. Curve 8 . A sample represented by Curve 4 warmed to dry ice temperature for two hours. 87 WAVELENGTH (in Angstroms) 5-5 50 45 4 0 3-5 30 2-5 2 0 E L E C T R O N VOLTS 88 Figure 3 5 . Radiation induced changes in KBH^ pressed pellet Curve 1. Absorption spectra of unirradiated KBH^ sample • Curve 2 . Radiation induced change after 3 minutes exposure to X-rays at l i q u i d nitrogen temperature. Curve 3 . Radiation induced change after 7 minutes exposure to X-rays at liquid nitrogen temperature. Gurve 4 . A sample represented by Curve 3 was exposed to light of 2775 8 wavelength for 30 minutes at liquid nitrogen temperature. Curve 5» Radiation induced change after 4 1/2 hours exposure to X-rays at liquid nitrogen temperature. 89 W A V E L E N G T H (in Angstroms) O o ro CJ i o o <• CM I O o m CM o o CM O O r-CM o o c6 CM I O o o o o o O o o o o o O — CM lO « • to ro ro ro ro ro ro J I I I I L _ O O o o o o o o o o o o o o o m o in o in o * i V m i 10 1 T r-i o o in I 2 5 K BH, Pressed Pellet 1 0 0 FIGURE 35 0-75 0-50H 0-25 H 0 0 0 1*25 < o i -Q. O i-ooH 0-75H 0-80-^ 0-25 A ® \ © 0 0 0 -5-5 - 1 — 5 0 3-5 E L E C T R O N V O L T S Figure 36. Radiation induced changes in KBH^ pressed pellet (continued) Gurve 6. Radiation induced change after 4 1/2 hours exposure to X-rays at li q u i d nitrogen temperature. This curve i s the same as Curve 5, f i g . 35. Gurve 7« A sample represented by Curve 6 was exposed to light of 57SO A* wavelength for 15 minutes at liquid nitrogen temperature. 9 1 WAVELENGTH (in Angstroms) I-25 H l-OOH 0-75 H oso H 0-25 A < u »-o. o 0 0 0 1-23-100-0 75-0-50-0-25-o o O O O O O O o O ° ° ° O O O O O O O O o 0 0 0 0 IO * in (O K C0 0> O — N nto CM OJ CVJ (M NNNMIOniQIOIp ' ' ' I I I I 1 1—I 1 I I o o o o o m o OOOOO O O O O O o o w o » o » p ? y * T *• KBH 4 Pressed Pellet FIGURE 36 0-00-L-, 55 — I — 50 — I 4 5 1 1 4 0 3-5 ELECTRON VOLTS 3-0 I 25 - 1 — 2 0 92 Figure 37» Radiation induced changes in RbBH^ Curve 1. Absorption spectra of unirradiated RbBH^ pressed pellet. Curve 2. Radiation induced change after 2 minutes exposure to X-rays at li q u i d nitrogen temperature. Curve 3• Radiation induced change after 8 minutes exposure to X»rays at l i q u i d nitrogen temperature• Curve 4* A sample represented by Curve 2 was exposed to light of 3100 % wavelength for 5 minutes at liquid nitrogen temperature. Curve 5. A sample represented by Curve 3 was exposed to light of 2400 1 wavelength for 3Q minutes at liquid nitrogen temperature. 93 WAVELENGTH (in Angstroms) O O O O O O O O o O O O O o o o o o o o o O O O O O O O O o O ° ° 0 0 o O O O O O o ro * m 10 s s) 01 o - N r t * i n o rn o in O in o in OJ CJ CJ oj o j c J o J r o r o r o r o r o r O * «• m ip ip lp S N E L E C T R O N VOLTS Figure 38. Radiation induced changes in RbBH^ (continued) Curve 6. A sample represented by Curve 3 was exposed to bright tungsten illumination for 2 minutes at liquid nitrogen tempera-ture. Curve 7. A sample represented by Curve 6 was warmed to room temperature for 16 hours* 95 WAVELENGTH (in Angstroms) >-(A z 0 * 0 0 - • UJ Q 1-25 -_i < o i 1 0 0 -0-75 5-5 5 0 4-5 4 0 3-5 3 0 2 5 2 0 E L E C T R O N VOLTS 96 Figure 3 9 . Radiation induced changes in CsBH^ Curve 1. Absorption spectra of unirradiated CsBH^ pressed pellet. Curve 2 . Radiation induced change after 15 seconds exposure to X-rays at liquid nitrogen temperature. Curve 3* Radiation induced change after 30 seconds exposure to X-rays at liquid nitrogen temperature. Curve 4 . Radiation induced change after one minute exposure to X-rays at liquid nitrogen temperature• Curve 5* A sample represented by Curve 4 was warmed to dry ice temperature for 5 minutes. Curve 6 . A sample represented by Curve 5 was warmed to room temperature for 5 minutes. 91 WAVELENGTH (in Angstroms) o o O O O O O O o O O O O o o O O O O O O o O O ° 0 ro * m to K a) cn o — N io t n CM CM CM CM CMCMCMK)IOr)IO(OK) o o o o o » o o o o o in o o o o O O O o o » o » I-50H in UJ o < o »-a. o IOOH 0-75 H o-soH 0-25 ^  o-oo-0-75H 0-50 H 0-25 H 1 1 1 1 1 1 1 —I 1—1—I—1—1 1 —1 1 1_ 1 1 1 L Cs BH4 Pressed Pellet FIGURE 39 A ®y \ V / S j 1 ®"* ' \ v . J © — J - i r= 1 — i 1 — i 1 1 i 5-5 50 4-5 4 0 3-5 30 2-5 2 0 E L E C T R O N VOLTS discussed in Chapter IV. Experiments were attempted in which potassium borohydride was intimately mixed with other ionic salts such as CaH2, NBYJBr, and CaBr2, the mixture being l e f t for several hours at room temperature under a to t a l load of 20,000 pounds. In a l l cases the pellets produced were too cloudy to use in absorption spectra and radia-tion studies. 2. Infra-red Spectra Samples of potassium borohydride which had received several hours exposure to X-rays were examined i n the infra-red region using the KBr pellet technique both with and without the KBr matrix in an attempt to learn something of the nature of the primary decomposi-tion products in the irradiated material. No differences between irradiated and unirradiated material was detect-able even when an unirradiated potassium borohydride pellet was placed in the reference beam of the infra-red spectrophotometer• 3. Paramagnetic Resonance Spectrum A paramagnetic resonance spectrum of a sample of powdered potassium borohydride that had been irradiated for four hours at liquid nitrogen temperature i s shown in figure 40. The spectrum was also taken at l i q u i d 3 fi »-Q. UJ O <n (0 H ATOM RESONANCE ABSORPTION Ul > I or UJ a or 5K> gouM between H- atom re«ooonce». FIELD STRENGTH H ELECTRON SPIN RESONANCE SPECTRA OF IRRADIATED KBH4 (POWDERED). FIGURE 4 0 H ATOM RESONANCE ABSORPTION \ MICROWAVE FREQUENCY: 9 1 KILOMEGACYCLES 100 nitrogen temperature. Irradiation and examination of an empty sample tube under the same conditions as the borohydride sample showed that the hydrogen atom peaks were associated with the sample tube. Exposure of a potassium borohydride sample for several hours to a high intensity of ultraviolet light from a mercury lamp failed to produce any sample coloration or paramagnetic resonance signal. D . Examination of Irradiated KBH^ for Chemical Changes Although electronic processes such as color center formation are now known to occur in potassium borohydride on exposure to 50 Kvp X-rays, a knowledge of possible chemical decomposition products and their yield for a given dosage of radiation would be desirable. Therefore, experiments were designed to determine the kind and quantity of some of the more l i k e l y products. These experiments were attempts to detect and estimate the amount of metallic potassium which may have been formed during irradiation, the collection and measurement of the quantity of gaseous material that may be evolved either during irradiations or on solution of irradiated samples, and the mass spectro-scopic examination of the collected gases. The experiments were only of limited success and the conclusions to be drawn from them are far from definite. The colorimetric system shown in Plate II and described in Chapter II, Section C was applied in the alkali metal estimation experiment. A sample of potassium borohydride (10 mgm) after 10 hours irradiation at room temperature was placed in a trap on the vacuum system and sufficient anhydrous ammonia to just dissolve the material was distilled onto the sample. No blue coloration was detectable in the solution. A sample of irradiated material with a similar dosage of X-rays, when left under vacuum and away from bright light, remained deep blue for about three days. No gas was evolved on dissolving the irradiated samples in anhydrous liquid ammonia. During the irradiation of samples (seven samples were irradiated simultaneously in the radiation vessel for chemical studies) large quantities of gas were evolved. The rate of evolution was small at the start of the irradia-tion period but after several minutes i t greatly increased and finally reached a value that persisted throughout the remainder of the irradiation period. As there were seven samples each of which received a different dosage of X-rays, no attempt was made to analyze the results of the rate study. However, the gas collection and storage unit of the vacuum system was calibrated for small amounts of gas by generat-ing known amounts of hydrogen in the system. This was done by taking a small amount of zinc dust (about 30 milligrams) and placing i t in the die used in making KBr pellets for I.R. studies and pressing the dust at 20,000 pounds total load. Small portions of zinc metal were then broken from the thin pellet and weighed on the Gohn electrobalance. A portion of zinc pellet weighing between 6 micrograms and 0.6 milligrams was transferred to a trap on the vacuum system and the latter pumped down. Sufficient dilute hydrochloric acid was then transferred ( i t had been placed in the vacuum system previously and the dissolved air removed by repeated transfers through a series of traps) onto the zinc metal and allowed to react completely. The circulating and collecting diffusion pump was then put in operation and the vapour cycled through traps cooled in liqu i d nitrogen to remove water and hydrogen chloride from the vapour. When the pressure had reached a constant low value the evacuated storage bulb (approximately 5 l i t r e s ) was opened to the system and the permanent gas transferred by means of the collecting diffusion pump. When the pressure on the intake side of the diffusion pump was less than 1JX the pressure in the storage system was measured. The results of this calibration experiment for small amounts of hydrogen generated from zinc dust (analar grade) are shown in figure 41* The gas collected during irradiation of potassium borohydride was examined in a mass spectrometer. Only hydrogen gas was detectable. Responses in mass regions associated with ions resulting from diborane or i t s frag-ments were absent. 40 IV. DISCUSSION Solvent studies designed to produce the a l k a l i metal borohydrides in a form suitable for the radiation studies showed that anhydrous hydrazine i s a very good solvent for the potassium, rubidium, and cesium borohydrides, although the sensitivity of the solvent to air makes the solutions d i f f i c u l t to handle. The use of anhydrous hydrazine as a solvent for lithium and sodium borohydrides was not attempted although the la t t e r salt would be expected to have a reason-able solu b i l i t y . Infra-red spectra and chemical assays of potassium borohydride samples extracted with anhydrous hydrazine show that no detectable reaction occurs with this solvent. Although liquid ammonia i s also a good solvent for potassium borohydride, the possibility of growing crystals that would be suitable for the spectroscopic studies would require more complex apparatus and closer control than the use of hydrazine, a solvent which i s quite similar to water in many of i t s physical properties (45). It i s hoped that further work with hydrazine solutions on a larger scale may produce crystals of the ionic borohydrides that are suitable for spectroscopic and other physical studies of radiation damage in these solids. 105 The methods used to produce the borohydrides of rubidium and cesium are cumbersome. It is hoped that these borohydrides become commercially available as products derived directly from the pure alkali metals much as the potassium salt is obtained. The sulfonium borohydride intermediate used in the present work is not too stable and its decomposition at room temperature becomes notice-able after a few days. However, the trimethylsulfonium borohydride method has advantages when the alkali metal hydroxide is available, the hydroxides of potassium, rubidium, and cesium being much easier to handle than the pure metals. In the present work the carbonate of rubidium was used in conjunction with the sulfonium borohydride method. The carbonate was converted to the reineckate which, in a sub-sequent metathesis reaction with the sulfonium borohydride in an ammoniacal solution, precipitates the very slightly soluble sulfonium reineckate, the alkali metal borohydride being removed from the aqueous solution by freeze drying. Methods were not developed to prepare the alkali metal borohydrides from a boron-containing compound other than an ionic borohydride. Methods are under study which would be used to prepare borodeuterides starting with compounds other than the borohydrides. The section on radiation dosimetry has been included for reference and for the possible use of such information 106 i n a later, more quantitative study of radiation damage in the borohydrides. The radiant flux determination for the Machlett OEG-60 X-ray tube gave the output value of 0.220 cal. -1 -2 min. cm. * at the port of the tube. The variation of this output with the age of the tube was not studied. Therefore, this value may change with the use of the source. Examination of irradiated potassium borohydride for chemical changes gave few conclusive results. Molecular hydrogen i s definitely produced. Elemental a l k a l i metal was not detected although the presence of trapped electrons i s almost certain. The removal of hydrogen atoms and electrons from borohydride ions through radiation damage can conceivably produce free molecular hydrogen, trapped electrons, and boron containing electron deficient molecules or molecule-ions. The absence of diborane in the evolved gaseous radiation products i s not known with complete certainty. The thermal disproportionation of diborane in the gas collecting diffusion pump may explain the absence of the diborane positive ion in the mass spectrometric study. If equivalent amounts of hydrogen and the electron deficient molecule, diborane, were formed and removed from the solid l a t t i c e , one would expect that an F band would be present in the absorption spectra of the irradiated borohydride that would be as persistent as that associated with the a l k a l i halides that have been heated in a l k a l i metal vapor. However, the radiation induced coloration in potassium borohydride 1 0 7 bleaches slowly in the dark at room temperature and the solution of heavily irradiated samples in liquid ammonia f a i l s to produce the blue coloration associated with free electrons or the equivalent amount of free a l k a l i metal. The Ivey formulas for electron excessive color centers appear to be applicable to the face-centered cubic a l k a l i metal borohydrides. The formulas for the F center and the U center are given on page 1 1 , and the results of their application are summarized in Table 1 0 . The observed values of the wavelength of maximum absorption, X max, for the various bands in the irradiated ionic borohydrides have also been included in Table 1 0 . The spectra shown i n figures 3 1 to 3 7 clearly show three major band systems. In Table 1 0 they have been denoted by I, II, and III. The set denoted by I appears to be F bands. A second clearly identifiable band system denoted by III seems to be U bands associated with the hydride ion. The band system denoted II has not been assigned although the properties of the band systems as a whole with considerations of the stoichiometry of the radiation induced decomposition to be discussed below indicate that band system II may be associated with an electron deficient center of the V or H type. The spectroscopic studies show that the number of U centers exceed the number of F centers although no quanti-tative estimate can be made without the knowledge of the TABLE 10 OBSERVED AND CALCULATED VALUES OF A^ax FOR RADIATION INDUCED ABSORPTION BANDS IN THE ALKALI METAL BOROHYDRIDES Sample Interionic Calculated Calculated Observed Observed Observed Distance d % m a x of F- X m a x of U- X m a x of X m a x of X m a x of in A (46,47) band in a band i n A Band System Band System Band System - - I i n ft II in ft III in A LiBRV. pressed pellet 2.63 NaBH^ pressed pellet 2 .95 KBH^ crystal 3»36 KBH4 pressed pellet 3*36 RbBH4 pressed pellet 3*51 CsBH^ pressed pellet 3 .72 4,170 5,150 6,530 6,530 7,250 7,890 1,740 2,020 2,330 2,330 2,450 2,610 4,300 3 , 0 0 0 5,350 6,900 6,400 3,050 2,950 2,350 not 2,850 recorded 6,500 3,000 2,410 3,250 2,590 1—• © 1 0 9 oscillator strengths of the two kinds of centers. It may be that the F centers are being developed from U centers* However, a more li k e l y explanation i s described below. The results suggest that hydride ions are stable products resulting from the overall energy dissipation of the absorbed X-ray quanta. Hydrogen production i s probably a result of dissociation of excited or ionized borohydride ions to atoms and other fragments with the subsequent atom reactions such as hydrogen abstraction during the early physicochemical stages. A possible scheme for the dissipation of the X-ray quanta at low temperatures i s the following: M+ + • M + + + e" BH^" + M + + ». M+* + BH^ -BH4- + f)V »» BH4' + e" M+* M+ + hi)' However, reactions and ionizations produced through secondary electrons w i l l greatly outweigh the effects of primary absorption noted above. e- + BH^ " BH4"* • e" BH^* + 2e~ BHj~ • H* BH^- ». BH3 + H* 110 R a d i c a l and atom r e a c t i o n s w i l l f o l l o w . C o m b i n a t i o n o f two s i m i l a r r a d i c a l s i s e x p e c t e d t o b e n e g l i g i b l e i n v i e w o f t h e " c a g e e f f e c t . 1 1 The r a d i c a l s and h y d r o g e n atoms a r e v e r y l i k e l y t o r e a c t w i t h t h e n e a r b y b o r o h y d r i d e i o n s . P o s s i b l e r e a c t i o n s f o l l o w : H. + B H 4 " BRj** + H 2 B H 3 * B H 4 - — B 2 H 6 + H" BH3- + B H ^ " — B 2 H 6 * • FT The s p e c i e s B2%" s h o u l d n o t b e t a k e n l i t e r a l l y a s a B2R*6 m o l e c u l e w i t h a n a t t a c h e d e x t r a e l e c t r o n . S u c h a s p e c i e s may p o s s i b l y be s t a b i l i z e d t o a h i g h d e g r e e b y n e i g h b o r i n g l a t t i c e v a c a n c i e s , a n d b y p o l a r i z a t i o n f r o m s u r r o u n d i n g c a t i o n s . Uncharged B 2 H ^ o c c u p y i n g a n o r m a l a n i o n s i t e w o u l d be e x p e c t e d t o t r a p a n e l e c t r o n . Now e l e c t r o n s removed i n t h e i n i t i a l i o n i z a t i o n e v e n t s may r e t u r n t o a s p e c i e s s i m i l a r t o t h e one t h a t was f o r m e d b y e l e c t r o n l o s s , o r t h e y may be t r a p p e d b y v a c a n c i e s , i m p u r i t i e s , o r d i b o r a n e i n t h e l a t t i c e • P r o v i d i n g t h e t e m p e r a t u r e i s l o w , e l e c t r o n s a r e p r e s u m a b l y t r a p p e d d u r i n g t h e i n i t i a l s t a g e s o f i r r a d i a t i o n b y t h e v a c a n c i e s p r e s e n t i n t h e r m a l e q u i l i b r i u m w i t h t h e l a t t i c e . The s p e c t r o s c o p i c s t u d y shows t h a t t h e e a s e o f f o r m a t i o n o f F c e n t e r s d e c r e a s e s w i t h i n c r e a s i n g e x p o s u r e t i m e . V a c a n c i e s may p o s s i b l y be p r o d u c e d b y t h e r a d i a t i o n (22) b u t a p p a r e n t l y n o t a t t h e r a t e a t w h i c h e l e c t r o n s a r e I l l released. However, as time goes on, the amount of B2H15 dispersed in the l a t t i c e may greatly a l t e r the rate of formation of F centers by the formation of B2i%" which now takes place at the expense of F-center formation. By the above scheme we would expect about twice the number of U centers as the number of F centers, assuming that a l l electrons removed i n primary ionizations of the borohydride ions become trapped i n anion vacancies to form F centers. Under such circumstances we would also expect the number of centers a r i s i n g from the B^R^" species to be the same as the number of F centers. The number of diborane molecules would also be the same as the number of F centers providing there are no losses from the l a t t i c e . Diborane w i l l not be spectroscopically detectable i n the v i s i b l e range of the spectrum. The number of major band systems observable i n the spectroscopic studies corresponds to the number of species which are produced i n large amounts and which are expected to absorb i n the v i s i b l e region of the spectrum. These are the F band, the U band, and a band denoted II which can be considered as originating from the trapped B2H6" species. The above scheme accounts for the kinds of absorption bands produced as well as t h e i r intensity. Depending on the perfection of the l a t t i c e , the r a t i o of the number of F centers to the number of centers arising from B2H5" i s expected to vary greatly. 112 The s t a b i l i t y o f t h e B2H6" s p e c i e s i s e x p e c t e d t o be l o w even i n c o n s i d e r a t i o n o f t h e p o l a r i z i n g e f f e c t s o f s u r r o u n d i n g v a c a n c i e s and c a t i o n s . Thus t h e i o n w o u l d be e x p e c t e d t o be e a s i l y i o n i z e d b y l i g h t i n t h e v i s i b l e r e g i o n o f t h e s p e c t r u m . A t h i g h t e m p e r a t u r e s i t i s e x p e c t e d t h a t i t w i l l " d e c o m p o s e " w i t h r e l e a s e o f c h a r g e . T h i s l a t t e r e f f e c t i s i l l u s t r a t e d by c u r v e 6 o f f i g u r e 3 3 f o r t h e c a s e o f t h e c e n t e r s p r o d u c e d i n c r y s t a l l i n e p o t a s s i u m b o r o h y d r i d e . The e a s e o f f o r m a t i o n o f t h e band s y s t e m d e n o t e d I I a p p e a r s t o i n c r e a s e f r o m t h e l i t h i u m s a l t t o t h e c e s i u m s a l t . W i t h l i t h i u m b o r o h y d r i d e t h e F band i s more i n t e n s e t h a n B a n d I I f o r a g i v e n d o s a g e . F o r sod ium b o r o h y d r i d e b a n d I I i s more i n t e n s e t h a n t h e F b a n d . T h i s l a t t e r o r d e r o c c u r s w i t h t h e p o t a s s i u m b o r o h y d r i d e s b u t n o t w i t h t h e r u b i d i u m s a l t . The F band o f c e s i u m b o r o h y d r i d e c o u l d n o t be r e c o r d e d . The p r o b a b i l i t y o f a s p e c i e s s u c h a s 8 2 % -o c c u p y i n g a l a t t i c e s i t e w i t h l e s s e n i n g s t r a i n w o u l d i n c r e a s e g r e a t l y f r o m t h e l i t h i u m s a l t t o t h e c e s i u m s a l t . Some i n f o r m a t i o n o f i m p o r t a n c e i n c o n n e c t i o n w i t h t h e t r a p p i n g o f s p e c i e s i n l a t t i c e s i t e s i s l i s t e d b e l o w : L e n g t h o f t h e d i b o r a n e m o l e c u l e 2 . 9 7 ft D i a m e t e r o f t h e b o r o h y d r i d e i o n 4 . 0 6 % D i a m e t e r o f t h e h y d r i d e i o n 4 . 1 6 ft D i a m e t e r o f t h e h y d r o g e n m o l e c u l e 2 . 3 4 & 113 The strong peak of the paramagnetic resonance spectrum shown i n figure 40 indicates that radicals and/or free elec-trons are produced in the irradiated samples at low temperature. Some fine structure i s also noticeable but no attempt has yet been made to analyze the spectrum. As the sample was a fine powder i t i s believed that few results can be obtained from i t . It i s hoped that single crystals of the a l k a l i metal borohydrides can be obtained on which a complete study of radiation induced changes can be studied by this valuable technique. Some suggestions for further work are now given. The following points can be considered as immediate extensions of t h i s study: 1. Development of methods to produce the borodeuterides and the growth of single crystals of these salts and the borohydrides to be used in more quantitative spectroscopic work. 2. Quantitative information on the properties of the band systems to check their properties against the known behavior of F centers, U centers, etc. Such properties as their ease of formation at extremely low temperatures, their optical breachability, thermal s t a b i l i t y , relations between bands, and the presence of photoconductivity during absorption in a particular band must be studied. Such information w i l l establish the kinds of color centers in the irradiated borohydrides. 114 3* Correlation of the intensity of the various centers produced at low temperatures with the amount of molecular hydrogen formed, 4* Electron spin resonance spectra with single crystals of borohydrides and borodeuterides to help elucidate the nature of the radiation induced l a t t i c e defects and chemical changes* 5* Quantitative information on the optical density of the various bands produced for a given dosage of radiation in each of the alk a l i metal borohydrides to determine the effects of lattice parameters on the stability of the color centers produced. 115 BIBLIOGRAPHY 1. U n i t e d S t a t e s A t o m i c E n e r g y C o m m i s s i o n , R e a c t o r H a n d -B o o k s . V o l . 4, M a t e r i a l s . M c G r a w - H i l l , New Y o r k , T5557 2. D i e n e s , G . , and G. H. 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