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A low energy beta-ray spectrometer and the beta-ray spectrum of Eul52-154 Ayers, Walter Revis 1953

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A LOW ENERGY BETA-RAY SPECTROMETER AND THE BETA-RAY SPECTRUM OF EU 15 2~154 by Walter Revis Ayers A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of PHYSICS We accept t h i s thesis as conforming to the standard required from candidates f or the degree of MASTER OF APPLIED SCIENCE Members of the Department of PHYSICS THE UNIVERSITY OF BRITISH COLUMBIA January, 1953 \ ABSTRACT Modifications have been made to a semi-circular focussing spectrometer to f a c i l i t a t e i t s operation and to improve i t s performance. Geiger counters f i l l e d with the saturated vapour of heptane kept i n an ice bath are used to detect the beta-particles. The windows of the counters are made of zapon films about 10 micrograms/cm2 i n thickness. The sources are mounted on s i m i l a r films and have a t o t a l thickness less than 100 micrograms/cm2. The combination of t h i n source and t h i n windows permits the measurement of beta-particle energies down to 2 Kev* An examination of the beta-spectrum of has been carried out. I t consists of 7 peaks corresponding to the energies 8 . 0 , 1 5 . 0 , 2 6 . 4 , 3 3 . 2 , 3 8 . 3 , 73 .1 and 7 4 . 8 Kev. The two upper peaks are assigned as K conversion l i n e s for gamma-rays of 121.5 and 123.2 Kev. The 3 3 . 2 and 38.3 Kev l i n e s are assigned as L and M Auger electrons i n Sm. The 8 . 0 Kev l i n e i s assigned as an M Auger electron i n Sm. The 15 .0 Kev peak i s assigned t e n t a t i v e l y as an M conversion l i n e corresponding to a gamma-ray of 16 Kev. The 26.4 Kev l i n e i s t e n t a t i v e l y assigned as either a K conversion l i n e corresponding to a gamma-ray of 73 Kev or an L conversion l i n e corresponding to a gamma-ray of 34 Kev. I ACKNOWLEDGEMENT The research described i n t h i s thesis was made possible by the award of a National Research Council Bursary and a National Research Council Summer Scholarship. I am deeply indebted to Dr. K. C. Mann for his invaluable advice and assistance and also to Dr. H. Brown for f a m i l i a r i s i n g me with the apparatus and the techniques involved i n i t s operation. Acknowledgement Is made also to Dr. K. Starke for hi s advice on the preparation of radio-aetive sources and to Messrs. J . Lees and A. Fraser for t h e i r technical assistance. TABLE OF CONTENTS Page I INTRODUCTION 1 I I THE SPECTROMETER 13 A. Design Considerations B. The Spectrometer Chamber C. The Magnetic F i e l d D. Mathematical Treatment of a Spectrometer I I I RESULTS 24 IV RECOMMENDATIONS 32 APPENDIX I 35 Counter Windows APPENDIX I I 37 Preparation of Sources APPENDIX I I I 40 The Counter F i l l i n g System APPENDIX IV 41 A u x i l i a r y E l e c t r o n i c Apparatus A. Current Regulator B. H. T. Regulation C. Pulse Amplifier and Scaler TABLE OF ILLUSTRATIONS Following FIGURE page 1 Semicircular Focussing 2 2 A Normal Beta Spectrum 2 3 Internal Construction of Spectrometer 19 4 Construction of the Magnet 20 5 Observed Varia t i o n of Magnetic F i e l d i n Spectrometer Region 20 6 Diagram of Electron Paths i n the Spectrometer 21 7 Theoretical Line P r o f i l e s 23 8 Proposed Decay Schemes for £^52-154 27 9 Beta Spectrum of Eul52-154 28 10 Block Diagram of Current Regulating System 42 11 C i r c u i t Diagram of Current Regulating System 42 I INTRODUCTION P a r t i c l e spectrometers were f i r s t introduced i n order to resolve the energy d i s t r i b u t i o n of charged p a r t i c l e s emitted by na t u r a l l y radio-active substances. A collimated beam of alpha-particles deflected by an e l e c t r i c or magnetic f i e l d was made to f a l l on a photographic plate or s c i n t i l l a t i o n screen. The energy of these p a r t i c l e s could be calculated from a measurement of the f i e l d and the defl e c t i o n of the p a r t i c l e . Baeyer and Hahn 1 b u i l t the f i r s t spectrograph of t h i s type f or the analysis of the energy spectrum of beta-p a r t i c l e s from radio-active n u c l e i . No use was made of any focussing properties, the r e s u l t being that most of the p a r t i c l e s emitted by the source were not permitted to enter the spectrograph, i . e . , only p a r t i c l e s with the same v e l o c i t y and the same i n i t i a l d i r e c t i o n would s t r i k e the same point on the photographic plate. To circumvent t h i s defect, focussing spectrometers of many types have been b u i l t . A focussing spectrometer i s an instrument i n which the resolution i s not c r i t i c a l l y dependent on the transmission (percent s o l i d angle subtended by the entrance s l i t at the source). Danysz 2 pointed out that two equal c i r c l e s drawn about points separated by a 1. G. Baeyer and 0. Hahn, Phys. Z e i t . , 11,488(1910). 2. J . Danysz, Comptes Rendus, 153,339,1066(1911). 2 distance small with respect to the radius would intersect at two points approximately diametrically opposite. Because the path of an electron i n a uniform magnetic f i e l d i s a c i r c l e , focussing can he obtained by allowing the electrons to t r a v e l through a semi-circle before s t r i k i n g a photo-graphic plate. F i g . 1 shows such an arrangement, i n which monoenergetic electrons leaving the source at point S w i l l t r a v e l through the b a f f l e and ar r i v e at point P on the photo-graphic plate. These electrons are said to be focussed since they w i l l a r r i v e near the point P so long as they are emitted within the angle A defined by the b a f f l e width. Electrons with higher energies w i l l be focussed to the right of P, those with lower energies to the l e f t . This arrangement, although an Improvement, i s not perfect since the central ray i n F i g . 1 does not s t r i k e the photographic plate at exactly the same point as the two outer rays. The radius of curvature of an electron i n a uniform magnetic f i e l d i s related to the momentum of the electron by the formula Hz»_ mvc ( l ) ' e where H i s the magnetic f i e l d i n gauss, g i s the radius of curvature i n cm., mv i s the momentum i n c.g.s. un i t s , c i s the v e l o c i t y of l i g h t and e i s the charge on the electron i n e.s.u. The momentum i s related to the energy of the p a r t i c l e by the r e l a t i v i s t i c formula BAFFLE F I 6 U H B 1 N 0 I M A L 8 £ r t S P E C T R U M 3 10V3 A/T(T + 1.02) (2) where T i s the k i n e t i c energy of the electron i n Mev. A spectrograph of the type i l l u s t r a t e d i n F i g . 1 has one objectionable feature. The photographic plate i s not equally sensitive to a l l energies of electrons. This d i f f i c u l t y may be overcome by placing a Faraday cage or a Geiger counter behind an e x i t s l i t as a detector, and varying the magnetic f i e l d to select the energy region under i n v e s t i g -ation. Using a spectrometer of t h i s type Ghadwi ck3 showed that i n addition to groups of electrons of discrete energies there was also a continuous d i s t r i b u t i o n of electrons, as I l l u s t r a t e d i n F i g . 2. As a r e s u l t of many experiments conducted during the past f o r t y years a comprehensive theory of beta decay has been developed. In addition to nuclear beta emission, atomic beta p a r t i c l e s are known to be emitted from atoms with excited n u c l e i , the l a t t e r phenomenom being known as i n t e r n a l conversion. A t h i r d process of interest i n beta-ray spectroscopy i s c a l l e d o r b i t a l electron capture and occurs when a nucleus absorbs one of the o r b i t a l electrons of the atom. 3. J . Chadwick, Vorh.d.D. Phys. Ges., 16,383(1914) 4 In nuclear beta emission a radio-active nucleus emits an electron thereby decaying to a daughter nucleus with atomic number one greater (negatron emission), or one less than (positron emission or o r b i t a l electron capture) the parent. The t o t a l energy involved i n such a t r a n s i t i o n i s discrete although the emitted electrons have a continuous d i s t r i b u t i o n of energies up to t h i s maximum t o t a l energy E 0. Furthermore experiment has shown that the electron does not necessarily leave i n the opposite d i r e c t i o n to the r e c o i l nucleus. I t therefore seems that neither energy nor momentum Is conserved i n beta decay processes, nor i s there conserv-ation of angular momentum. The electron, being a p a r t i c l e of i n t r i n s i c spin must carry away a h a l f - i n t e g r a l multiple of ii of t o t a l angular momentum since both parent and daughter are is o b a r i c , i . e . , either both have i n t e g r a l spins or both have h a l f - i n t e g r a l spins. A possible solution to the dilemma i s provided by the neutrino hypothesis of P a u l i . The neutrino hypothesis proposes the existence of a neutral p a r t i c l e (so f a r un-observed) of small, possibly zero mass and h a l f - i n t e g r a l spin. This p a r t i c l e , c a l l e d the neutrino, i s assumed to be emitted simultaneously with the observed beta-particle. The electron, neutrino and product nucleus share among them the energy, momentum and angular momentum available from the nuclear t r a n s i t i o n . The beta-particle has i t s maximum momentum when the neutrino i s emitted with zero momentum. 5 FermiA has developed a mathematical analysis of beta decay based on the neutrino hypothesis which results i n a t h e o r e t i c a l energy d i s t r i b u t i o n for the emitted beta-p a r t i c l e s which i s i n substantial agreement with experiment. The momentum spectrum of the beta-particles for the so-ca l l e d "allowed" or most probable t r a n s i t i o n s i s given by N(p)dp = CF(Z,E)p 2(E 0 - E)2 dp (3) where N(p)dp i s the number of beta-particles emitted with momentum i n the range dp at p, C i s a constant which i n general depends on the s p e c i f i c nucleus Involved i n the decay, F(Z,E) i s a r e l a t i v i s t i c Coulomb correction factor, E i s the energy of the beta-particle, and E 0 i s the t o t a l energy available from the nuclear t r a n s i t i o n . I t follows from (3) that a plot of (N(p)/Fp 2)i against energy E i s a straight l i n e i n t e r s e c t i n g the abscissa at E *» E 0. Such a plot i s ca l l e d a Kurie plot and has the advantage that i t may be used to f i n d E 0 by a straight l i n e extrapolation. A f t e r emission of a nuclear beta-particle the daughter nucleus i s often l e f t In an excited state. The exc i t a t i o n energy may be dissipated either by emission of a gamma-ray or by l i b e r a t i o n of an o r b i t a l electron. The l a t t e r process i s termed i n t e r n a l conversion. In order to determine the energy involved i n de-excitation by Internal conversion i t i s necessary to add the atomic s h e l l binding energy of the converted electron to i t s measured k i n e t i c energy. The most probable i n t e r n a l conversion process 4. E. Fermi, Z e i t s . fur Phys., 88,166(1934). 6 involves a K:shell electron. The K conversion c o e f f i c i e n t i a defined as the r a t i o of the p r o b a b i l i t y of K conversion to gamma emission. In the i n t e r n a l conversion process energy i s transmitted from the nucleus to the electron by the electromagnetic i n t e r a c t i o n of the nucleus and the electron. The main contribution comes from the e l e c t r o s t a t i c Coulomb in t e r a c t i o n . I t i s possible to make exact calculations of in t e r n a l conversion c o e f f i c i e n t s using the r e l a t i v i s t i c wave functions of the electrons. In general i n t e r n a l conversion c o e f f i c i e n t s increase as the atomic number and the multi-pole order of the t r a n s i t i o n increase and decrease as the energy of the t r a n s i t i o n increases. The multipole order of a t r a n s i t i o n i s a measure of the vector angular momentum change, i . e . , t r a n s i t i o n s involving a vector angular momentum change of -K are termed dipole, those involving a change of 2tt, quadrupole, etc. Exact calculations of K conversion c o e f f i c i e n t s have been made by Hulme et al5, Taylor^, Flak?, Rose et al$, Griffith.9, and ReitzlO. Experimental support of these 5. R\. R. Hulme, N. F. Mott, F. Oppenheimer, and H. M. Taylor, Proc. Roy. Soc. (London), A155,315(1936). 6. H. M. Taylor, N. F. Mott, Proc. Roy. Soc. (London), A142,215(1933). 7. J . B. Fisk, H. M. Taylor, Proc. Roy. Soc. (London), A146,178(1934). 8. M. E. Rose et a l , G. Goertzel, B. I. Spinrad, J . Harr, and P. Strong, Phys. Rev., 76,1883(1949). 9 . B. A. G r i f f i t h , J . P. Stanley, Phys. Rev., 75,534(1949). 10. J . R. Re l t z , Phys. Rev., 77,10(1950). calculations has been obtained by Waggoner 1 1, Petch 1^, and others. By measuring experimentally the K conversion co-e f f i c i e n t and r e f e r r i n g to the conversion c o e f f i c i e n t tables mentioned above, i t i s often possible to i n f e r the multipole order and thus the vector nuclear spin change involved i n the t r a n s i t i o n . The theory absolutely forbids quantum t r a n s i t i o n s of multipole order zero. In t h i s case the nucleus can de-excite only by i n t e r n a l conversion, i . e . , the Internal conversion c o e f f i c i e n t s are a l l i n f i n i t e . I t i s i n t e r e s t i n g to speculate on the possible effect that o r b i t a l electron capture might have on the mean l i f e of subsequent i n t e r n a l conversion t r a n s i t i o n s . The de-e x c i t a t i o n of a nucleus by i n t e r n a l conversion i s independent of the competing gamma-decay process. We may write where 2" I s "the mean l i f e of the excited state of the nucleus, 2p i s the mean l i f e f o r decay by photon emission, i s the mean l i f e f o r decay by Internal conversion i n the K s h e l l , and fL ffjf, etc. are respectively the mean: l i v e s f o r decay by i n t e r n a l conversion i n the L, M, etc. s h e l l . I t i s assumed that the p r o b a b i l i t y for emission of an Internal 11. M. A. Waggoner, M. L. Moon, and A. Roberts, Phys. Rev., 80,420(1950). 12. H. E. Petch, M. W. Johns, Phys. Rev.,80,478(1950). conversion electron by a p a r t i c u l a r excited nucleus i s d i r e c t l y proportional to the number of electrons i n the atomic s h e l l . This may be written \ M - 2 \l K where A ^ I s the p r o b a b i l i t y per unit time of emission of a K conversion electron from an atom with a f u l l K s h e l l , and X/^ i s the p r o b a b i l i t y per unit time of emission of a K con-version electron from an atom with only one K electron. Consider N n u c l e i at time t * 0 which have Just undergone a K capture t r a n s i t i o n to an excited state. I f Xx i s the pr o b a b i l i t y per unit time of f i l l i n g a vacancy i n the K s h e l l by an x-ray t r a n s i t i o n , we may write the p r o b a b i l i t y of there being one electron i n the K s h e l l at time t , PfK • e and the p r o b a b i l i t y for two K electrons, - X X t PZK -'/ - e The p r o b a b i l i t y per unit time of emission of a K conversion electron at time t i s then X/c(t) = XtK P2/C + \ t i c Pi/c \*K (/ - i e ' * ) The rate at which nuclei of t h i s species decay by K 9 conversion i s then given by which on integration y i e l d s o When In N/NQ s -1, only N0/e of the o r i g i n a l excited nuclei remain. We define the corresponding time T% as the mean l i f e f o r K conversion. Solving the previous equation yields By s u b s t i t u t i n g Cx~ \~x and rearranging we get T« - Tr* * (/ - e " ri ) which may be solved approximately i n three cases: fx (1) g « t : . T K - T I K + P -<5) x*. * ^ > * . *. 2 V * ? z * + *4 ~ [ ?4 - / } T2/C 10 Since the effe c t of a single misBing electron i n the L s h e l l i s r e l a t i v e l y small, the effect of K capture on the mean l i f e of the subsequent nuclear state, i f appre-ci a b l e , should be most e a s i l y detected by measurement of K/L r a t i o s . We may thus say with assurance that, so long as the mean l i f e f o r x-ray t r a n s i t i o n s to the K s h e l l i s greater than the mean l i f e f o r normal K conversion the K/L r a t i o for a nuclear process following K capture w i l l be smaller than that f o r the same process following beta emission. I f the mean l i f e f o r the x-ray processes which f i l l the K..shell i s shorter than the normal K conversion l i f e t i m e , and i f t h i s i n turn i s shorter than the mean l i f e for x-ray processes that f i l l the L s h e l l , then I t i s possible that the K/L r a t i o may become larger than that which would normally be expected. S i m i l a r effects to those noted above might be found i n cascade decay schemes where previous t r a n s i t i o n s involving i n t e r n a l conversion have robbed the atom of some of i t s o r b i t a l electrons. Beta-ray spectroscopy has provided a method for determining the energies of p a r t i c l e s and photons emitted by radio-active n u c l e i and has thus increased the knowledge of nuclear energy l e v e l s . Most spectrometers have been de-signed to cover the energy range from 0.1 to 3 Mev, the lower l i m i t being an instrumental one, caused by excessive scatter-ing of low energy electrons from b a f f l e s and re s i d u a l gas molecules, by the increasingly serious defocussing effect - due to the uncompensated earth's magnetic f i e l d , and by absorption of the low energy p a r t i c l e s both i n the source and i n the counter window. The upper l i m i t i s set by the power demands of the focussing magnet. A few spectrometers have been b u i l t to measure energies greater than 10 Mev and .some to extend the lower l i m i t to 10 Kev or l e s s . There are important reasons for the investigation of the low energy region. Cook and Langer 1^ and other experiment ers 14,15,16 report that i n a number of cases the Kurie plot deviates from a straight l i n e at low energies. More recent work!7,18,19 indicates that these result s were e n t i r e l y due to self-absorption i n the source. The presence of a low energy beta group would eause an apparent curvature In the Kurie plot at low energies. The presence of Auger and i n t e r n a l conversion electrons can so complicate the low energy beta spectrum as to make i t d i f f i c u l t to determine the shape of a low energy beta group. Low energy i n t e r n a l conversion l i n e s are very common. Lines at less than 50 Kev have been found i n the beta spectrum of almost a l l n a t u r a l l y 13. C. S. Cook and L. M. Langer, Phys. Rev., 73,601(1948). 14. A. W. Tyler., Phys. Rev., 56,125(1939). 15. J . L. Lawson, Phys. Rev., 56,131(1939). 16. A. A. Townsend, Proc. Roy. S o c , A1777,357 (1941). 17. R. D. Albert and C. S. Wu, Phys. Rev., 74,847(1948). 75,1107(1949). 18. L. Feldman and C..S. Wu, Phys. Rev., 76,697(1949). 19. L. M. Langer, J . W. Motz and H. C. Price J r . , Phys. Rev., 77,798(1950). 12 radio-active substances20 and i n some a r t i f i c i a l l y prepared i s o t o p e s 2 1 . In substances of low Z the flourescence y i e l d Is found to be s m a l l 2 2 ^ that i s to say, an atom with a missing K electron i s more l i k e l y to de-excite by emission of an Auger electron than by emission of an x-ray. I f a nucleus i s i n the ground state on capturing a K electron the soft x-rays and the Auger electrons offer the only means of detecting the t r a n s i t i o n . Many postulated decay schemes based on the measure-ment of high-energy gamma-rays and conversion electrons have energy l e v e l s within 50 Kev of each other. Most spectrometers now i n use cannot detect t r a n s i t i o n s between such l e v e l s . An instrument which could do so would be a valuable t o o l i n checking the proposed spin and p a r i t y values assigned to these states on the evidence of the high energy t r a n s i t i o n s alone. This research describes the redesign and use of a spectrometer f o r the measurement of beta p a r t i c l e s i n the energy range 1 to 100 Kev. For the reasons previously stated i t i s e s s e n t i a l to investigate t h i s energy region for a complete understanding of the mechanism of nuclear processes. 20. Radiations from Radio-active Substances. Rutherford, Chadwick and E l l i s , pp.360-380. 21. R. D. H i l l , Phys. Rev., 74,78(1948). 22. X-rays i n Theory and Experiment. Compton and A l l i s o n , pp.477-492. 13 I I THE SPECTROMETER A. Design considerations. ( l ) Detection of beta-particles. Beta-particles may be detected with photographic plates, and i n spite of the objection to a spectrograph, plates would be used i f they were sensitive to electrons of low energy. Unfortunately the s e n s i t i v i t y of even the best plates f a l l s off very r a p i d l y as the energy of the impinging electrons i s decreased 2^. The s e n s i t i v i t y of the s c i n t i l l -a t i o n c r y s t a l and photomultiplier combination f a l l s o f f greatly at low energies. By cooling the photomultiplier i n l i q u i d a i r i t i s possible to detect electrons with energies as low as 2 Kev, but the operation of a photomultiplier i s seriously effected by the focussing magnetic f i e l d . Electron m u l t i p l i e r s can be made with high e f f i c i e n c y for beta energies between 0.1 and 5 Kev. One of the better detection methods Involves post-acceleration of the electrons a f t e r they have passed through the e x i t s l i t of the spectrometer. This method shows every sign of more frequent use i n low-energy beta-ray spectrometry. In a spectrometer of the type described i n t h i s thesis post acceleration would be 23. L. Cranberg and J . Halpern, R.S.I., 20,641(1949). "impracticable. For our purpose a t h i n window Geiger counter seems best f i t t e d to the task. Geiger counters are known to be sensitive to electrons of almost n e g l i g i b l e energy and therefore the problem resolves I t s e l f into one of findi n g some method of Introducing the electrons into the sensitive volume of the counter. I f a window i s to be used i t must be s u f f i c i e n t l y t h i n to transmit electrons of low energy and yet strong enough to withstand the pressure of the f i l l i n g gas. Backus24 has described a method for making films as t h i n as a few micrograms/cm2. Windowless counters have been used with success by Langer, Motz and P r i c e l 9 with an equivalent window thickness due to d i f f u s i n g gas of about one microgram/cm2. The counters used i n the present equipment were b u i l t by Brown25 and u t i l i z e zapon f i l m of 3 to 5 micrograms/ cm2 covering an entrance s l i t 0.025 cm. wide. The combin-ation of the narrow window and the low gas pressure used i n the counter (1.1 em. Hg.) makes the use of such films e n t i r e l y s a t i s f a c t o r y . (2) The Source. The preparation of low energy beta-sources i s an extremely delicate operation since the optimum thickness of 24. J . Backus, Phys. Rev., 68,59(1945). 25. H. Brown, Ph.D. Thesis, University of B r i t i s h Columbia, (1951). 15 source plus backing i s vanishingly small. The thickness of the source can have a great Influence on the shape of the low energy beta spectruml9. Because of the f r a g i l i t y of the source i t s area should preferably be small. A small source poses an add i t i o n a l problem: since the resolution of the spectrometer i s i n general inversely proportional to the luminosity (transmission times source area), i t can be seen that once the resolution has been s p e c i f i e d , the o v e r a l l counting rate of the spectrometer i s then dependent e n t i r e l y on the t o t a l source strength. I t i s important that the o v e r a l l counting rate be reasonably high since only i n t h i s way can good s t a t i s t i c s be established quickly. A p r a c t i c a l lower l i m i t i n counting rate i s set by the cosmic-ray back-ground. Having sp e c i f i e d the resolution and thus the luminosity and having set a lower l i m i t on the useful count-ing rate, we may estimate the a c t i v i t y per unit area of the source necessary to produce t h i s counting rate. I f we now demand a source thickness which w i l l not too badly d i s t o r t the low energy end of the spectrum, we may then compute the minimum acceptable s p e c i f i c a c t i v i t y of the source material. (3) Limitations due to available material and techniques. The thinnest sources so f a r produced seriously broaden spectral l i n e s below 10 Kev. Because of t h i s fact there i s l i t t l e point i n designing a low energy spectrometer with a re s o l v i n g power better than one percent. The spectro-meter should be designed to u t i l i z e source material of that s p e c i f i c a c t i v i t y which may rea d i l y be produced. The s p e c i f i c a c t i v i t y and the size of the source are the factors 16 which- determine the minimum o v e r a l l size of the spectrometer. The magnetic f i e l d of the spectrometer should be large compared to the earth's f i e l d and the f l u c t u a t i n g magnetic f i e l d s of nearby e l e c t r i c a l equipment. Large f i e l d s produce short electron t r a j e c t o r i e s at low energies and since other considerations have placed a lower l i m i t on the size of the spectrometer, i t may therefore be necessary to effect a compromise. B. The Spectrometer Chamber. A semi-circular focussing type of instrument which best f u l f i l s the above conditions was designed and b u i l t by Brown25. The i n t e r n a l construction of t h i s spectrometer i s i l l u s t r a t e d i n F i g . 3. The radius of the curvature of t h i s instrument was kept small i n order to reduce the path length of the detected electrons and thus keep to a minimum scattering both from re s i d u a l gas molecules and from the walls and b a f f l e s . In order to have a large transmission t h i s spectrometer was designed to accept electrons from the source i n four di f f e r e n t d i r e c t i o n s . Geiger counters with zapon windows are used for detection, the pressure being kept constant by a dynamic f i l l i n g system. (See Appendix I I I ) The source material i s deposited i n a l i n e about 0.1 cm. wide and 1.5 cm. long on a t h i n zapon f i l m which i s supported on a l u c i t e holder. The holder i s so mounted that the source i s coincident with the axis of the spectro-meter. In Brown's arrangement, four sets of ba f f l e s were 17 placed so that electrons leaving the source at angles of 45° ^ 5 . 7 ° to the plane of the backing would t r a v e l a c i r c u l a r path of 3 .05 - .03 cm. radius to s t r i k e the window of one of four symmetrically placed Geiger counters. The ba f f l e s were cut on a lathe to the required radius and grooves cut i n the surfaces to reduce the r e f l e c t i o n of electrons into the counters. Each counter was made by d r i l l i n g a 0.625 inch diameter hole through a block of brass 1.5 inches long. The brass was then trimmed down with a shaper, p a r t i c u l a r care being taken with the face containing the window. This face was cut as smooth as possible to leave a thickness of 0.030 inches i n the center. Through the'narrowest part a l o n g i t u d i n a l s l o t about 5/8 inches long and 0.010 inches wide was cut. The inside of the counter was then polished thoroughly with emery paper and crocus clot h . Another hole of about the same size was d r i l l e d through the brass, p a r a l l e l to the f i r s t , f o r the purpose of f i l l i n g the counter, so that about 0.050 inches of metal separated the two. Small holes were d r i l l e d through t h i s separating wa l l to allow the f i l l i n g gas to enter the counter proper. The a u x i l i a r y hole was then plugged at one end with a brass plate and at the other with a copper tube to allow connection to the f i l l i n g system. The entire counter was then immersed i n b o i l i n g n i t r i c acid ( 0 . 1 Normal) f o r a few minutes u n t i l the sur-faces appeared clean. I t was then washed, f i r s t i n d i s t i l l e d water, and then i n absolute alcohol and dried. One end was closed with a Kovar seal and the other with a pyrex cap sealed with deKhotinsky wax. A 0.005 inch tungsten wire was used f o r the anode. The spectrometer chamber i s a brass cylinder of 6.5 inches inside diameter. The end plates are of 5/16 inch brass. Soft rubber rings are used as vacuum seals. In the bottom plate holes are d r i l l e d f o r the admission of the counter f i l l i n g gas f o r the admission of the high voltage lead for connection to the anode of the counters, and for connection to the vacuum system. The i n t e r n a l assembly, with the exception of the counters, i s mounted on a 1/8 inch brass base plate one inch above the lower end plate of the spectrometer chamber. The base plate was highly polished and the pos i t i o n of the source, the positions of the b a f f l e s and the location of the entrance s l i t s of the counters were marked on i t . The counters were aligned with the b a f f l e system by observing that the counter window, i t s r e f l e c t e d image, and the mark on the base plate, were i n one straight l i n e . The counters are connected to a four arm glass tee by lengths of.Tygon tubing to allow movement during alignment. A l l exposed metal surfaces are coated with a mater-i a l of low atomic number i n order to reduce scattering as much as possible. This was done by diss o l v i n g vacuum wax In carbon tetrachloride and applying several coats of the paint. In the course of the present work three modific-ations have been made to the inner spectrometer, two of which had been suggested by Brown 2^. The b a f f l e s have been redesigned to permit attachment of new windows without removal of the b a f f l e s and source from the instrument. The brass b a f f l e system was completely removed and replaced by four simple l u c i t e b a f f l e s made as i l l u s t r a t e d i n F i g . 3. These ba f f l e s were mounted on the base plate midway between the source and the counters, leaving the face of the counters f r e e l y accessible for attachment of windows. In order to reduce the background counting rate due to gamma-rays emitted by the source lead shields were i n s t a l l e d on the inner side of each counter. The thickness of these shields was dictated by the available space between the counters and the electron t r a j e c t o r i e s . (See F i g . 3) Because the counter anodes had been connected i n p a r a l l e l inside the spectrometer chamber the counters could not e a s i l y be tested i n d i v i d u a l l y , nor could f a u l t y ones he disconnected without terminating an experimental run. This f a u l t was r e c t i f i e d , by introducing four separate leads into the spectrometer chamber through a rubber vacuum seal. G. The Magnetic F i e l d . Production of a magnetic f i e l d without the use of i r o n eliminates the need for measuring small f i e l d s since the f i e l d must vary l i n e a r l y with the current i n the magnet TO V A c U U M J y s r £ M TO COUNTER FILL ING SYSTEM f / 6 V £ f 3 I « T £ H\ H A L CONSTRUCT/ON OF S P E C T R_ 0 M £ T £ 1^ 8. 6tF£Ltl(oi,ff,N*l) <* BtFFLi ( 9 t St * It t l) h CIVN Ti i s * LH> Sttttli C S 8 u C € ft a L 0 £ ^ 20 c o i l . The magnetic f i e l d required by t h i s spectrometer should be uniform over a c y l i n d r i c a l region, 12 cm. i n dia> meter and about 2 cm. deep, with the magnetic vector p a r a l l e l to the axis of the cylinder. A set of c o i l s has been de-signed and b u i l t by Brown25 as i l l u s t r a t e d i n F i g . 4 i n order to produce such a f i e l d . The c o i l system consists of three coplanar c o l l s , one of 1400 turns of radius 17.5 cm., a second of 270 turns of radius 11 cm., and a t h i r d of 12 turns of radius 8.5 cm. Passing 10 amperes of current through these c o i l s , with the current In the second c o i l flowing i n the opposite sense to the current i n the other two c o i l s , was found to produce a magnet f i e l d of approx-imately 360 gauss with a maximum inhomogeneity of less than 1% over the desired region. The uniformity of the magnetic f i e l d was tested by means of two inverse connected matched search c o i l s and a b a l l i s t i c galvanometer. The results of t h i s test are shown i n F i g . 5. The inner c o i l has been eliminated because t h i s test showed the f i e l d to be more uniform without i t . D. Mathematical Treatment of a Spectrometer. G e o f f r i o n 2 ^ has deduced mathematically the cha r a c t e r i s t i c s of a semi-circular focussing spectrometer. In the spectrometer shown i n F i g . 6 electrons of a given momentum w i l l form an image of the source at the e x i t s l i t . 26. G. Geoffrion, R.S.I., 20,638(1949). It tjo 14-0 o T a A. N i T (I UN ! T tt S f I Q If d £ 4 C O N l T l i U C T I O N OF THE M A 6 N £ T 0.1<f /• o/o MIS 1 I I I I I I O / 1 3 4 5 x(cm) 6 F I G U JL E 5 0 B S E A. V E 0 r A JF / A T / 0 W OF M A G N E T I C F I E I P (H SPECTt^OM E T E R E G I O N A IN N £ 4. COll CONNECTED 8 INN £ A. COIL PIS CONNECTED The width of t h i s image i s given by Q1 — Q, + 2 r ( l - cos A 0 cosB 0) (1) where Q 1 i s the width of the image, Q i s the width of the source, r i s the radius of the electron trojectory, A Q i s one-half the angle subtended by the ba f f l e s at the source, and B 0 i s the maximum angle with the median plane of the spectrometer at which an electron may enter the e x i t s l i t . The r i g h t hand edge of t h i s image i s located a distance 2r from the r i g h t hand edge of the source. Since r depends on the momentum of the elctrons i n a constant f i e l d H, a number of images w i l l be formed with d i f f e r e n t values of r f o r d i f f e r e n t values of the momenta. I f a detector i s placed behind the e x i t s l i t i t w i l l receive electrons with a certain range of values of momenta. The width of the e x i t s l i t adds to the e f f e c t i v e width of the image of the source so that i t now becomes equal to F •+ Q, + 2 r ( l - Cos A Q Cos B Q) (2) where F i s the width of the e x i t s l i t and (2) i s the uncertainty i n the measurement of 2r for a constant f i e l d H. The l i m i t of resolution of a spectrometer i s defined as the r a t i o A p / p where A p i s the range of the momenta of those electrons passing through the e x i t s l i t . F I 6 U H E 6 0 / A G Z A M OF E L E C T I O N P A T H S I N F H E S P S C T l o M E T E H 22 Since p = cHr/e (3) 4 p « ( H A r + rAH)e/e (4) and since H i s constant 4 P a cH4r/e *cH/2e(F+ Q, -f 2 r ( l « Gos A 0 Cos B 0)) (5) and therefore 4-E - F Q •+ 2 r ( l - Cos A 0 Cos Bft) (6) p " 2r and since A© and B 0 are small t h i s can be s i m p l i f i e d to 4_E „ F + 0, A Q2 + B Q2 (7) p 2r 2 The spectrometer described i n t h i s report has a trajectory radius of curvature of 3.05 cm. and A 0 was chosen as 0.1. The minimum value of Q obtainable i s about 0.07 cm. since t h i s i s the projection of a 0.10 cm. wide l i n e . a t 45° to the plane of the source backing. The width of the e x i t s l i t (width of the counter window) i s about 0.025 cm. Sub-s t i t u t i n g the actual values of Q, F, A 0 and B 0, the above formula yi e l d s a reso l u t i o n of about 1.5%. The luminosity of a spectrometer i s defined as the product of the transmission times the source area. For t h i s Instrument the luminosity i s 0.58 x 10-3 cm2, which i s larger than most comparable spectrometers. Geoffrion26 shows a curve of the i n t e n s i t y to be expected under optimum conditions using an e x i t s l i t of i n f i n i t e s i m a l width. The t h e o r e t i c a l l i n e p r o f i l e f or an e x i t s l i t of a given width F can be obtained by p a r t i a l integration of Geoffrion's curve for F - 0.025 cm. and F « 0.06 cm. These p r o f i l e s are shown i n F i g . 7. I t i s evident from these curves that the resolution f or the wider e x i t s l i t i s appreciably the same as f o r the narrower one (the one we are using). Attempts to use the wider e x i t s l i t with i t s resultant increase i n transmission were unsuccess-f u l because the wider zapon windows were unable to withstand the counter f i l l i n g pressure. T H E O R E T I C A L A E X I T SLIT B £ X J T S I I T F f G 1/ Al £ 7 l I hf E P R O F I L E S 0- 0 75 CM. fy I D E 0 06 CM. N I 0 E I l l RESULTS A. Preliminary Considerations. Before attempting the analysis of the low energy beta spectrum of Eul52-4 ^ ± a necessary to c l a r i f y two important points. F i r s t l y , some attempt should be made to estimate the effe c t of source absorption on the broadening and s h i f t i n g of spectral peaks, and secondly, some proof should be shown for the v a l i d i t y of the spectrometer c a l i -b ration. Although i t i s d i f f i c u l t , i f not impossible, to determine the effect of source absorption from a purely t h e o r e t i c a l argument, nevertheless i t i s possible to estimate these effects i n an empirical way. White and Millington27 have derived from experimental result s the momentum d i s t r i b u t i o n of an I n i t i a l l y monochromatic beam of electrons a f t e r passing through t h i n f o i l s . A f t e r taking into account the f i n i t e resolving power of t h e i r equipment they plot the natural l i n e p r o f i l e s f or electrons af t e r being straggled by various thicknesses of f o i l . From these r e s u l t s they are able to plot a fundamental l i n e p r o f i l e curve from which a l l other straggling curves may be obtained from a simple r e l a t i o n s h i p . These calculations show that the displacement i n momentum of the maximum of any straggled curve from the true value of the momentum of the incident 27. F. White and G. M i l l i n g t o n , Proc. Roy. S o c , A12C,701(1928). 25 monochromatic electrons i s a constant multiple (1.3) of the half-width of the straggled curve, where the h a l f -wldth i s defined as the percentage width of the peak at h a l f - i n t e n s i t y on a momentum scale. Although electrons emitted by a t h i n radio-active source do not a l l t r a v e l through the same thickness of straggling material, the rel a t i o n s h i p regarding peak displacement may be used since I t i s independent of the thickness of the straggling material. By taking into account the natural l i n e width due to the f i n i t e resolving power of the spectrometer employed i n t h i s research we obtain the corrected value of the momentum corresponding to a spectral peak: H^ = (H^) 0(1+ 0.013(P - 1.5)) where H i s the corrected value of the momentum, (Hj>)0 i s the momentum value corresponding to the maximum of the experimental peak, and P i s the percentage half-width of the experimental curve, as defined above. The o r i g i n a l c a l i b r a t i o n of the spectrometer made by Brown i s considered by t h i s w r i t e r to be some-what i n error. Brown's determination of the energy of the dominant i n t e r n a l conversion l i n e i n Ra D (600 gauss-cm), although i n substantial agreement with the work of Rutherford, Ohadwick and E l l i s 2 ^ , Richardson 28. S i r E. Rutherford, J . Chadwick and C. D. E l l i s , Radiations from Radio-active Substances, 1930. and Leigh-Smith 29, Tslen San-Tsiang30 and others, does not take into account the peak s h i f t phenomenum discussed above. The c a l i b r a t i o n used i n the present work has been established by accepting the experimental work of Brown reinterpreted i n the l i g h t of the previous paragraph, together with the w e l l substantiated value f o r the domin-ant L conversion l i n e i n Ra D. This c a l i b r a t i o n i n terms of the control potentiometer s e t t i n g i s 980 1 15 gauss-em. per v o l t . Using t h i s c a l i b r a t i o n , together with the method of analysis described above, none of the peaks i n Ra D reported by Brown i s appreciably s h i f t e d from the values he has assigned to them. B. E u r o p l u m 1 ^ 2 - ^ When the stable isotopes of Eu (mass numbers 151 and 153) are i r r a d i a t e d by slow neutrons, active isotopes of atomic weights 152 and 154 are formed by (n,y) processes. A mass speetrographic analysis by Ingham and Hayden3l showed that a c t i v i t i e s could be assigned to these isotopes as follows: TABLE I Atomic wt. 152 H a l f - l i f e 9.2 h 152 5-8 y 154 5-8 y 29. H. 0. W. Richardson and A. Leigh-Smith, Proc. Roy. S o c , 160,454(1937) 30. Tsien San-Tsiang, Phys. Rev., 69,38(1946). Cork, S h r e f f l e r and Fowler^ 2 studied the long-l i v e d a c t i v i t i e s i n a photographic 180° spectrometer, and reported i n t e r n a l conversion l i n e s corresponding to t r a n s i t i o n s of 122, 247, 286, 34-3 a n d 400 Kev, as wel l as a continuous beta spectrum of energy 0 .93 Mev. Wiedenbeck and Chu^, using a coincidence count ing technique, reported the continuous beta spectrum of long-lived Eu to be complex with upper l i m i t s 0.62 and 1.0 Mev. S h u l l ^ , using a magnetic double-focussing beta ray spectrometer, reported i n t e r n a l conversion l i n e s corresponding to t r a n s i t i o n s of 123, 124, 247, 286, 344 and 412 Kev, as w e l l as additional external conversion l i n e s corresponding to t r a n s i t i o n s of 442, 772, 959, 1082 and 1402 Kev. Cork et al^5 summarize t h e i r investigation of Eu by proposing the decay schemes i l l u s t r a t e d i n F i g . 8. The same workers report Auger l i n e s at 32.0 and 37.9 Kev corresponding to an atomic t r a n s i t i o n i n Sm of 39.1 Kev. For the present investigation the source was made from europium oxide (see Appendix I I ) , the active material having been l e f t standing several months to 32. J . M. Cork, R. G. S h r e f f l e r and G. M. Fowler, Phys. Rev., 72, 1209(1947); 73,78L(1948). 33. M. L. Wiedenbeck and K. Y. Chu, Phys. Rev., 72, 1164(1947). 34. F. R. S h u l l , Phys. Rev., 74,917(1948). 35. J . M. Cork et a l , Phys. Rev., 77,848(1950). H - 62 S5y CKJL IO86 336.4 7 7 3 K ? 1 243.6 • < F I G U R E S PfLOPOJED DECAY fCHEMES F O R Eu,sz'4' eliminate the sho r t - l i v e d isomer of Eu 1^ 2. Table I I shows the binding energies of the o r b i t a l electrons i n Sm and Gd. TABLE I I S h e l l 62 Sm 64 Gd •- Kev Kev K 46.7 50.1 L 7.3 7.9 M 1.4 1.6 The four highest energy peaks of the spectrum - (Fi g . 9) correspond to the four lowest energy peaks reported by Cork et al35 (Table IV). The seven peaks found In the present in v e s t i g a t i o n have t e n t a t i v e l y been assigned as follows: TABLE I I I Peak No. Electron energy Kev Interpre-.tation Energy Sum Z = 62 Kev Z - 64 Kev 1 8.0 ± 1.0 M(Aup;er(62) 9.4 ± 1.0 2 15.0 ± 1.0 M(62)(64) 16.4 ± 1.0 16.6 * 1.0 3 26.4 ± 1.2 K(62)(64) L(62)(64) 73.1 * 1.2 33.7 ± 1.2 76.5 ± I - 2 34.3 ± 1.2 4 33.2 ± 1.4 L(Au«er)(62) 40.5 ±1.4 5 38.3 ± 1.5 M(Au*er)(62) 39.7 ± 1.5 6 73.1 ± 2.2 K(64) 123.2 - 2.2 7 74.8 i 2.2 K(62) 121.5 ± 2.2 (Third column numbers i n brackets r e f e r to mass numbers of assigned daughter nucleus). TABLE IV Electron energy Kev Interpre-t a t i o n Energy Sum Z= 62 Kev z= 64 Kev 3 2 . 0 L(Auger) (62) • 3 9 . 1 37.9 M(Auger)(62) 39.1 72 .9 K(64) 123.2 7 5 . 0 K(62) 121.8 Peaks (1) and (2) l i e at approximately the correct energies f o r L and M conversion of a 16 Kev gamma-ray t r a n s i t i o n In Gd or Sm. Some support i s given for t h i s assignment by the work of Mr. R. Azuma (pri v a t e l y communi-cated) using a sodium iodide proportional s c i n t i l l a t i o n spectrometer. Azuma finds d i s t i n c t evidence of a weak gamma-ray at t h i s energy. Peak (1) i s very broad. This may be p a r t l y caused by source-absorption but i t should be noted that peak ( l ) corresponds roughly to the energy at which one would expect to f i n d the Auger electrons due to atomic t r a n s i t i o n s which f i l l the L s h e l l . Peak (2) cannot be ascribed to any Auger electron group i n Gd or Sm because of energy considerations. Peak (3) may be either a K conversion l i n e corresponding to a gamma t r a n s i t i o n of about 73 Kev or an L conversion l i n e corresponding to a gamma t r a n s i t i o n of about 34 Kev. I f i t i s the former, an L conversion l i n e would be expected at about 66 Kev. This was not detected i I f peak (3) Is an L conversion l i n e an M conversion l i n e would be expected at about 32 Kev. This l i n e , i f i t e x i s t s , i s possibly hidden by Auger peak (4). There i s i n s u f f i c i e n t Information available to determine i n which daughter the t r a n s i t i o n occurs or which Isotope of Eu i t i s due to. Because of energy considerations peak (3) cannot be ascribed to Auger groups i n G-d or Sm. Peaks (4) and (5) have been reported by Cork et al32 f and peaks (6) and((7) have been reported by these same workers as w e l l as by Shull34. The present work i s i n good agreement with these experiments. Peak (6), although not f u l l y resolved, i s j u s t i f i e d since the best smooth curve through the experimental points misses two consecutive experimental points by more than four times the probable error of these points. The present work has extended the low-energy beta spectrum of Eul52-4 to energies lower than any previously reported and although I n s u f f i c i e n t information exists to f i t the newly found t r a n s i t i o n s into an energy l e v e l scheme i t i s clear from t h i s work that the decay schemes proposed by Cork et a l ^ 2 are Incomplete, i f not i n error. I t might prove f r u i t f u l to repeat t h i s experiment using, i f possible, a thinner source of higher s p e c i f i c a c t i v i t y together with a more accurate c a l i b r a t i o n . Such an experiment could possibly resolve the two components suggested i n peak ( l ) and thus provide conclusive evidence fo r the 16 Kev t r a n s i t i o n . The fact that the source contains two a c t i v i t i e s , and E u 1 ^ of almost i d e n t i c a l h a l f - l i v e s , plus the added d i f f i c u l t y of ascribing the gamma-ray tr a n s i t i o n s to the proper daughter product, i . e . , 0 * 1 5 2 , G d 1 5 \ 3 m 1 5 2 ) or makes the task of assigning a reasonable decay scheme to any of these elements impossible. I t would appear that many gamma-rays are s t i l l undetected and the p o s s i b i l i t y s t i l l e xists that the beta-spectra of both parent nuclei may contain more groups than the two already reported. U n t i l more information i s available over the entire energy region, the l e v e l sequences must remain unknown. 3 2 IV RECOMMENDATIONS The modifications incorporated into the spectro-meter have s a t i s f a c t o r i l y f a c i l i t a t e d i t s operation, nevertheless, more work must be done i f t h i s equipment i s to produce accurate and r e l i a b l e r e s u l t s quickly and e f f i c i e n t l y . The magnet c o i l , because i t seriously overheats, cannot be operated continuously with currents i n excess of 3 amperes. The necessity for intermittent operation reduces the e f f i c i e n c y of the spectrometer so that i t i s necessary to spend several weeks obtaining a single spectrum. Counter windows were found to l a s t two or three days on the average. I f continuous operation of the spectrometer were possible i t i s l i k e l y that a complete spectrum could be run without rupture of a counter window and Its consequent laborious and time-consuming repair. To t h i s end the magnet c o i l should be redesigned to permit continuous operation carry-ing the current necessary to produce a magnetic f i e l d of about 400 gauss. Several improvements can be suggested for the spectrometer chamber. F i r s t l y , a reduction i n r e f l e c t i o n of electrons from the walls could be effected by l i n i n g the chamber with l u c i t e . For t h i s reason the base plate as w e l l should be made of l u c i t e . Secondly, the leads from the 33 anodes of the counters should be brought out of the chamber through kovar seals to a shielded preamplifier mounted d i r e c t l y beneath the chamber. This would minimize pick-up as we l l as guard against sparkover inside the chamber and at the same time provide an adequate vacuum seal. Pro-v i s i o n against source charging could be made by mounting a small filament above the source holder. Such a filament could l i k e l y provide s u f f i c i e n t thermal electrons near the source to discharge i t without resorting to the use of an accelerating electrode. The chamber l i d could be more quickly and r e l i a b l y sealed to the chamber by using an O-ring rather than a f l a t rubber washer. The counting rate of the Geiger counters was noted to deteriorate s l i g h t l y with time. This effect i s probably due to impurities i n the heptane, e.g., dissolved oxygen could ef f e c t the counter c h a r a c t e r i s t i c s . In any case an experiment should be performed to determine the cause of th i s defect so that i t may be r e c t i f i e d . A j i g could be constructed to f a c i l i t a t e the application of the counter windows. This would consist of a hinged support for the f i l m holder and would permit the wet f i l m to be swung uniformly into contact with the counter face where i t could be l e f t u n t i l dry. In thisv.way r e l i a b l e windows could be applied to the counters without taxing the dexterity- of the operator. Attempts should be made to devise new techniques for making thinner and more uniform sources, f o r without question t h i s i s the crux of the problem of producing accurate r e s u l t s at energies below 20 Kev. An accurate c a l i b r a t i o n of the instrument should be made either by a dir e c t measurement of the magnetic f i e l d or by use of an accurately measured i n t e r n a l conversion l i n e . 35 APPENDIX I COUNTER WINDOWS The windows are produced by the method f i r s t published by Backus 2^ About 20 m i c r o l l t r e s of a solution of one part zapon lacquer i n two parts amyl acetate Is dropped on the clean surface of d i s t i l l e d water. The solution spreads out over the surface of the water and the solvent evaporates leaving a t h i n f i l m of zapon on the surface. This f i l m i s picked up on a wire frame so that the f i l m f a l l s on both sides of i t making a double layer. The f i l m was found to become b r i t t l e when l e f t on the water surface for more than 20 seconds. In the process employed by Brown these windows were permitted to dry before applying them to the counters. The counter face was prepared by cleaning i t with amyl acetate and applying to i t a t h i n f i l m of v l n y l i t e r e s i n prepared i n the same manner as the zapon f i l m s . This f i l m was applied immediately a f t e r i t was stripped from the water. The entrance s l i t was then cleaned out with the sharp corner of a piece of paper and a dry zapon f i l m was placed on the counter face, the v i n y l i t e acting as an adhesive. Fortunately i t has been found that the wet zapon films w i l l adhere to the counter face quite adequately without the use of any bonding material. This i s an advantage not only because i t i s a s i m p l i f i c a t i o n i n the window application technique, but also because i t had been found d i f f i c u l t to clean out the counter window s l o t without disrupting the; whole v i n y l l t e f i l m , thus rendering i t useless. Brown has shown that zapon films prepared i n t h i s manner are less than 10 micrograms/cm2 t h i c k and that t h e i r threshold f o r beta-particle transmission i s below 2 Kev. For operation i n the energy range above 10 Kev laminated wind-ows of two or three zapon films have been u t i l i z e d to good e f f e c t . The windows, once applied, are tested by reducing the pressure inside the counters by about 1 cm of mercury. A small mercury manometer i s provided i n the system for measuring t h i s pressure. Aft e r 5 or 10 minutes any leakage through the windows i s r e a d i l y detected by a change i n reading of t h i s manometer. 37 APPENDIX I I PREPARATION OF SOURCES The source holder i s made from a piece of 3/l6 Inch l u c i t e 2 inches square. A rectangular s l o t 2 cm. long "by 0.5 cm. wide i s m i l l e d through the l u c i t e with the back cut away at an angle of 45°, leaving a narrow edge around the s l o t . A hole i s d r i l l e d i n the l u c i t e f or mounting the source holder on the base plate of the spectrometer and a l i n e i s scribed on the l u c i t e to indicate the pos i t i o n of the source. The source backing i s made of zapon i n the same manner as the counter windows. The f i l m i s stripped from the surface of the water on a frame made of l / l 6 inch l u c i t e and l a i d on the source holder while s t i l l wet. The f i l m i s then allowed to dry before further work i s done i n preparing the source. Attempts to use LC600 films f a i l e d because t h i s lacquer did not form a homogeneous solution i n amyl acetate. The thinnest films that could be made with nylon were found to be about ten times as thick as those made with zapon. A solution of one part zinc insulin36 to eight parts d i s t i l l e d water i s made up and about 0.25 m i c r o l i t r e s of i t i s deposited with a pipette at each end of the f i l m i n l i n e with the mark on the holder. The pipette i s made 36. L. M. Langer, R.S.I., 20,216(1949). 38 from tubing pulled down to 0 . 5 mm. outside diameter and 0 . 2 mm. inside diameter. The solution i s brought up into the pipette and the end wiped clean with a paper towel. By car e f u l l y blowing into.the pipette, the l i q u i d can be made to bulge from the end without forming a drop. The end of the pipette when touched to the film.leaves a droplet about 0 . 5 mm. i n diameter. The source i s now placed i n a j i g constructed so as to permit the droplets to be run along the f i l m i n a t h i n l i n e . The j i g i s made of brass and comprises a milled slideway beneath an adjustable b a l l - p o i n t pen which may be raised or lowered by turning a screw. The b a l l - p o i n t i s ca r e f u l l y lowered to contact the f i l m at the i n s u l i n droplet and the source holder i s then moved i n the slideway of the j i g so that the b a l l - p o i n t i s drawn along the source f i l m . With care the droplet may be drawn back and f o r t h along the f i l m to form.a l i n e of i n s u l i n about 1 mm. wide. A pipette s i m i l a r to that used for depositing the i n s u l i n s o l u t i o n but made i n the form of an eye-dropper i s then used to deposit the active material. The active solution was made by di s s o l v i n g about 0 .5 mg. of europium oxide i n a few drops of concentrated HCl. The europium oxide had been activated i n the Chalk River p i l e and was subsequently l e f t standing f o r about s i x months, by which time the short-l i v e d isomer of Eu^52 had been reduced to a n e g l i g i b l e amount. The solution was concentrated by evaporation under a heat lamp u n t i l about 10 m i c r o l i t r e s remained. The t o t a l a c t i v i t y of the europium oxide was about 500 microcurles, so that p i p e t t i n g off 0.25 m i c r o l l t r e s of solution onto the source f i l m would produce a source of t o t a l a c t i v i t y between 10 and 15 microcuries. By c a r e f u l l y manipulating the pipette one can make the active material run along the i n s u l i n l i n e without flowing to the remaining f i l m . After drying for a short time the source i s ready f o r use. 40 APPENDIX I I I  THE COUNTER FILLING SYSTEM The counters are f i l l e d with heptane (CyH^g) maintained at a pressure of 1.1 cm, of mercury by keeping i t i n contact with l i q u i d heptane i n an ice bath. Commercial heptane was found to contain a small amount of water which required removal i n order to permit proper operation of the counters. This was done by connecting a f l a s k containing a few grams of phosphorous pentoxide to the neck of the f l a s k which contains the l i q u i d heptane. Because heptane i s a good solvent for stopcock grease i t was necessary to connect the heptane reservoir to the rest of the counter f i l l i n g system by means of a graphite lubricated mercury seal stopcock. The l i q u i d heptane i s introduced into the system by f i l l i n g a small f l a s k with heptane, freezing i t i n l i q u i d a i r , and then sealing i t to the spectrometer glasswork by means of a torch. The heptane f l a s k i s then c a r e f u l l y pumped while s t i l l frozen to remove a l l the a i r from i t , the pumping being continued u n t i l evaporating heptane has completely flushed the system. The c h a r a c t e r i s t i c s of the heptane f i l l e d Geiger counters have been determined by Brown25. Although they do not exhibit a long plateau, they may be operated s a t i s f a c t o r i l y at about 1250 v o l t s giving uniform pulses of about 4 v o l t s peak amplitude with a r i s e time of less than one micro-second and a length of about 50 microseconds. 41 1 -APPENDIX IV AUXILIARY ELECTRONIC APPARATUS A. Current Regulator. In order to set and maintain a constant magnetic f i e l d to focus the beta-particles In the spectrometer i t i s necessary to be able to regulate and measure the current i n the f i e l d c o i l . Since there i s no ir o n i n the system the magnetic f i e l d i s l i n e a r with current. I t i s therefore possible to make a secondary c a l i b r a t i o n of the spectrometer at a single check point using an i n t e r n a l conversion l i n e of accurately known energy or a l t e r n a t i v e l y a primary c a l i b r a t i o n may be made by measuring the magnetic f i e l d produced by a measured current. Since the resolution of the spectrometer i s of the order of 1%, the magnetic f i e l d should be regulated to at least 0.1$. The c o i l current 1B regulated by passing i t through a standard r e s i s t o r and comparing the voltage developed across t h i s r e s i s t o r with the control voltage which i s generated i n a Student Potentiometer. The difference voltage i s then amplified and fed back to control and grids of the current regulating tubes. The standard r e s i s t o r has a value of about 0.1 ohm and I s made of 10 feet of 1 inch manganin s t r i p . This r e s i s t o r can carry the largest currents used with a tempera-ture increase of less than 1°C, consequently i t s resistance 42 w i l l stay constant to about 1 part i n 105. The amplifier comprises two channels. One channel amplifies the frequency range 0-10 cps. This channel i s ca l l e d the D.C. amp l i f i e r . The A. C. channel amplifies the frequency range 5-1000 cps. Amplific a t i o n at higher frequen-cies i s unimportant since the load i s highly inductive. The signals from the two channels are mixed i n a 6SL7 double triode which i s used to drive the grids of a bank of eight 6AS7's i n series with the magnet and the standard r e s i s t o r . The input to the D.C. amplifier i s 'chopped1 i n a Brown Converter, fed into an A.C. amplifier and subsequently de-tected In a phase sensitive detector. The o v e r a l l voltage gain from the input of the D.C. amplifier to the output of the mixer i s 5x10^. The eight 6AS7Ts w i l l conduct currents up to 2 amp-eres. For current greater than t h i s i t i s necessary to shunt the 6AS7's with an external shunt resistance. An additional variable resistance i n series with the 6AS7's permits opera-t i o n from 0 to 10 amperes from the 250 v o l t D.C. mains. A block diagram of the regulating system i s shown In F i g . 10 and a c i r c u i t diagram i n F i g . 11. The potentiometer output voltage i s accurage to 5 parts i n lO^-. The loop gain for the servo-mechanism i s about 1.5x10^". The voltage generated across the standard r e s i s t o r i s V = V 0 (1 + 1/G) Galvanometer Calibration Batte ry AC Amplifier 6 S L 7 DC Driver Amplifier Student Potentiometer Standard Ceil F t $ U H S 10 S L 0 c X D I A G R A M Of C (/ £ $ £ N T » ^ £ a u L A T i N $ s y f T £ M f / G 1/ % £ 11 £ i A\ C O i r P / A G < A M OF C U < < £ N T /^£GU LAVING f V f V f flf 43 where V Q i s the potentiometer voltage, and G i s the loop gain. Therefore the current i s regulated to better than 1 part i n 104 of the value corresponding to the s e t t i n g on the control potentiometer. The A. C. r i p p l e current i n the magnet c o i l was measured to be less than 1 part i n 103. B. H. T. Regulation. The voltage on the counter anodes must be well s t a b i l i z e d , since these counters do not exhibit a w e l l de-fined plateau. A voltage regulator has been b u i l t by Brown25 f o r t h i s purpose. Tests made on t h i s regulator show a change i n output voltage of less than 5 v o l t s f o r a change of 20% i n A.C. input. The A.C. r i p p l e i n the output i s less than 0.1%. C. Pulse Amplifier and Scaler. A simple 2 tube pulse amplifier with high input impedance and low output impedance was designed u t i l i z i n g a double triode to serve h a l f as an input stage and half as an output stage. I t was necessary to use a shielded lead from the counters to the amplifier to reduce pickup. The output of the amplifier was fed into a scale of 1000, composed of 3 Berkeley scales of 10 followed by a mechanical r e g i s t e r . 

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