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Gas scintillation counters Beer, George Atherley 1959

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GAS SCINTILLATION COUNTERS by GEORGE ATHERLEI BEER A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Physics We accept t h i s t h e s i s as conforming t o the standard required from candidates f o r the degree of MASTER OF SCIENCE Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA September, 1959 ABSTRACT Construction d e t a i l s of a 5 " diameter 1 . 7 l i t r e gas s c i n t i l l a t i o n counter are given. The behaviour of the counter when containing He^ gas and a mixture of He^ " and 1 0 $ Xe at various t o t a l pressures has been i n -vestigated. Two 5 " photomultiplier tubes and a v a r i e t y of d i f f e r e n t gas p u r i f i e r s were tested i n an attempt to obtain the best po s s i b l e r e s o l u t i o n . The pulse height was found t o depend s e n s i t i v e l y on the p u r i f i c a t i o n procedure adopted. A r e s o l u t i o n of 10$ f o r the P o 2 ^ oc-peak has been a t -tained i n two d i f f e r e n t chambers containing He^ plus 10% Xe. The voltage pulses rose i n ^300 nsec. Intense i r r a d i a t i o n of a counter with 6 Mev Jf-rays caused only a s l i g h t increase i n the maximum noise l e v e l already present from the photomultiplier tube. The behaviour of the counter as a f a s t neutron detector has been i n -vestigated using an uncollimated beam of l+.l Mev neutrons. Pulses from r e -c o i l i n g protons and C^ 2 n u c l e i were observed as w e l l as the He^ r e c o i l s . In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of this thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia, Vancouver 8, Canada. ACKNOWLEDGEMENTS The author wishes t o thank Dr. D.L. Livesey and Dr. K.L. Erdman who suggested and supervised t h i s research. Dr. Erdman's patience i n reading much of t h e f i n a l d r a f t while i l l i s g r a t e f u l l y acknowledged. Thanks are due t o Mr. G. Jones and Dr. B.L. White f o r h e l p f u l suggestions concerning e l e c t r o n i c s problems, and u s e f u l discussions on the observed behaviour of the counter. The excellent workmanship and the cooperation of the Physics Department Workshop s t a f f has made the construction of these gas counters possible. The author would l i k e to thank h i s wife f o r assistance i n the preparation of the t h e s i s and patience during the research. The author g r a t e f u l l y acknowledges the r e c e i p t of a scholarship from the B r i t i s h Columbia Telephone Company. T A B L E O F CONTENTS Page I. INTRODUCTION 1. The advantage of noble gas s c i n t i l l a t i o n counters 1 2. Relative e f f i c i e n c i e s o f counter gases 2 I I . A SURVEY OF PREVIOUS WORK 3 I I I . THE THEORY OF SCINTILLATIONS PRODUCED BY CHARGED PARTICLES IN A GAS 1. E x c i t a t i o n and i o n i z a t i o n produced by the slowing of charged p a r t i c l e s 7 2. Photon emission 8 IV. CONSTRUCTION DETAILS OF GAS SCINTILLATOR COUNTERS 1. Mechanical s p e c i f i c a t i o n s f o r H e 3 - f i l l e d s c i n t i l l a t i o n counters 13 2. The gas chamber 13 3. C i r c u l a t i o n pumps 17 4. Valves and couplings 19 5. P u r i f i e r s 20 6. C a l i b r a t i o n sources and source holders 22 7. Mounting of the components 22 8. E l e c t r o n i c s 23 A Components 23 R Observed behaviour of the components 24 V. THE BEHAVIOUR OF HE 4 GAS SCINTILLATION COUNTERS 1. Preliminary measurements with 5" diameter chambers ....... 28 2. The e f f e c t cf co o l i n g the s c i n t i l l a t i o n counter 28 3. Gas s c i n t i l l a t i o n pulses from helium plus xenon 29 4. Further factors a f f e c t i n g the r e s o l u t i o n of He^ and He^ • Xe counters 31 a. F i l l i n g procedure 31 b. Waveshif t e r s 31 c Windows 32 d. Impurity content i n helium counters 32 e. E l e c t r o n i c s voltage l e v e l s 33 f. Xenon concentration 33 5. The s t a t i s t i c s of photon observation i n gas s c i n t i l l a t i o n counters 33 TABLE OF CONTENTS CONTD. Page 6. The counting e f f i c i e n c y f o r x p a r t i c l e s 34 7. Photon c o l l e c t i o n e f f i c i e n c y 35 8 . The e f f e c t of x - r a d i a t i o n on gas s c i n t i l l a t i o n counters. 35 VI. A HELIUM GAS SCINTILLATOR USED AS A DETECTOR OF FAST NEUTRONS 1. Types of fa s t neutron counters 36 a. Ionization detectors 36 b. S c i n t i l l a t o r s 37 c. Time-of-flight s e l e c t i o n of neutron energy 37 2. A He^ - plus Xe gas s c i n t i l l a t i o n counter used as a neut ron spectrometer 38 a. Discussion 38 b'. Neutron spectra obtainedwwith a 5" diameter He plus Xe f i l l e d gas s c i n t i l l a t i o n counter 40 c. Counter e f f i c i e n c y 41 d. Conclusion 41 APPENDIX 43 BIBLIOGRAPHY 46 I. INTRODUCTION 1 . The advantages of noble gas s c i n t i l l a t i o n counters Pulses from a gas counter are f a s t e r than those from i o n chambers and some s o l i d s c i n t i l l a t o r s . Rise and f a l l times of the order of 1 0 " ^ sec t o 1 0 " sec have been reported but are dependent on the type and p u r i t y of gas used. The pulse height i s a l i n e a r function of energy and i s independent of both the charge and mass of the e x c i t i n g p a r t i c l e . Continuous p u r i f i c a -t i o n of the gas maintains a constant pulse height i n d e f i n i t e l y . By varying the pressure the stopping power of the counter gas may be varied over a wide range without serious a l t e r a t i o n of the pulse c h a r a c t e r i s t i c s . S i g n a l t o noise r a t i o s of 1 5 0 : 3 and a corresponding r e s o l u t i o n of 4 . 8 $ have been attained using a 2 " quartz window photomultiplier (Sayres and Wu 1 9 5 7 ) , however pulse height and r e s o l u t i o n are dependent on the type of gas i n the counter. Almost complete i n s e n s i t i v i t y to gamma r a d i a t i o n makes these counters extremely u s e f u l for photodisintegration experiments. Because of the s i m p l i c i t y of i t s construction and operation, the s i z e and shape of the gas chamber may be adjusted over a wide range t o meet the require-ments of a p a r t i c u l a r experiment. A l l counters reported i n the l i t e r a t u r e have been 2 " diameter and the 5 " diameter chamber reported i n t h i s t h e s i s i s at present the l a r g e s t . The cost of a counter i s determined p r i m a r i l y by the photomultiplier tube and e l e c t r o n i c s and i n some cases by the counter gas. A number of a p p l i c a t i o n s f or gas s c i n t i l l a t i o n counters have been suggested. a) The fast pulses could be c l i p p e d t o reduce «. p i l e u p during f i s s i o n counting. b) Gas s c i n t i l l a t o r s provide f a s t pulses f o r counter telescopes. c) Using a mixture of 1 0 $ Xe i n He^ a gas s c i n t i l l a t i o n neutron spectro-meter of high e f f i c i e n c y using the He^ (n,p) H 3 r e a c t i o n could be b u i l t . - 2 -d) The counters are ddeal f o r observing the photodisintegration of the noble gases. e) Gamma ray promptness measurements could be made using the gas counter as a f a s t detector of the r e c o i l s . f) A He^ " f i l l e d counter may be used as a p o l a r i z i n g analyser f o r f a s t neutrons. 2 . Relative e f f i c i e n c i e s of counter gases The e f f i c i e n c y cf a counter i s dependent on chamber geometry, r e f l e c t i -v i t y , type and thickness of waveshifter and many other factors so i n t e r -comparison of r e s u l t s from d i f f e r e n t counters i s d i f f i c u l t . The following i s a l i s t of s u c c e s s f u l counter gases with an estimate of t h e i r r e l a t i v e e f f i c i e n c i e s . Xe (6 psia) - Pulse height 105, increased t o 145 with quartz tube. Kr (8 psia) - 50, increased t o 83 with quartz. A (10 psia) - 15 , increased to 28 with quartz. He (45 psia) - 3 8 , increased t o 40 with quartz. He • Xe - 10% Xe i n He increases pulse height t o 125. Other mixtures of noble gases are reported by Northrop and Gursky ( 1 9 5 8 ) . He t - Ng i n small q u a n t i t i e s increases the pulse height. - 3 -I I . A SURVEY OF PREVIOUS WORK Most of the photons emitted i n a noble gas following the passage of a charged p a r t i c l e were found t o have wavelengths i i i the u l t r a v i o l e t (Muehlhause, 1953, Eggler and Huddleston 1956, Northrop and Nobles 1956 and Sayres and Wu 1957). These photons have been detected e i t h e r by using an u l t r a v i o l e t - s e n s i t i v e photomultiplier (Nobles 1956, and Sayres and Wu 1957) or by u s i n g a waveshifter which absorbs u l t r a v i o l e t r a d i a t i o n and re-emits photons i n a s p e c t r a l region more nearly matching the s e n s i t i v i t y of conventional photomultipliers, Northrop (1958) l i s t s the r e l a t i v e e f f i c i e n c i e s of a representative group of organic-phosphor Waveshifters which have been used by various experimenters. He found diphenyl s t i l b e n e most e f f i c i e n t whereas Sangster (1952) found that quaterphenyl combines the highest e f f i c i e n c y with the shortest decay time. (~8 x IXT^ s e c ) Northrop's r e s u l t s on quaterphenyl and diphenyl s t i l b e n e used with He + Xe are i n d i s -agreement with fur t h e r work published by Northrop and Gursky. I t i s po s s i b l e that t h i s discrepancy could be resolved by using MgO and waveshifter instead of aquadag on the chamber.walls. Forte (1956) used sodium s a l i c i l a t e success-f u l l y . The phosphors were applied e i t h e r by vacuum evaporation (Northrop and Nobles 1956, Sayres and Wu 1957) or by p a i n t i n g with a s o l u t i o n of phosphor-p l a s t i c mixture (Eggler and Huddleston 1956). The counters were found t o be susceptible t o quenching of the l i g h t pulses by contaminants present i n the gas or a r i s i n g i n the counter walls and gaskets. Out-gassing from the counter system has been c o n t r o l l e d by using a clean, out-gassed metal system and T e f l o n gaskets (Sayres and Wu 1957). For low-priced counter gases f r e s h gas was allowed t o flow through the counter at a rate which was s u f f i c i e n t t o overcome the rate of evolution from the System* (Boicourt and B r o l l e y 1954). The t h i r d method which a l s o removed contaminants present i n the gas used to f i l l the counter was t o c i r c u l a t e the gas i n a closed system which included a p u r i f i e r containing a r e a c t i v e material such as hot calcium (Sayres and Wu 1957), uranium (Northrop and Nobles 1956), calcium magnesium a l l o y (Forte 1956), or barium (Bennett 1957). These p u r i f i e r s e f f e c t i v e l y remove H 0, 0_, N and CH,, the contami-<c 2 <c 4 nants suggested by Northrop and Gursky (1958). Forte (1956) observed that extremely small q u a n t i t i e s of N^, 0^  and COg poisoned the pulses from argon. To obtain maximum energy r e s o l u t i o n , the i n t e r i o r walls of the chamber were coated with a l i g h t r e f l e c t o r . The most commonly used material was smoked Magnesium oxide (Northrop and Nobles 1956, Sayres and Wu 1957, Pasma 1957, Rubbia and T o l l e r 1958 and Ba l d i n et a l . 1957). Eggler and Huddleston (1956) used s i l v e r e d and aluminized surfaces and Forte (1956) used white enamel. In a l l cases the r e s o l u t i o n was reasonable. Because the r e f l e c t i v i t y of these materials f a l l s o f f i n the vacuum u l t r a v i o l e t , more e f f i c i e n t opera-t i o n r e s u l t e d i f the r e f l e c t o r was coated with a l a y e r of waveshifter. Northrop and Nobles (1956) found that the optimum thickness f o r tetraphenyl-butadiene and quaterphenyl was 30 /ig/cm 2. This i s i n agreement with t h e o r e t i -c a l p r e d i c t i o n s . Quantitative measurements of the properties of gas s c i n t i l l a t i o n counters by various i n v e s t i g a t o r s are not i n agreement. The decay of the s c i n t i l l a t i o n l i g h t has been reported as being f a s t (10~^ sec) by Nobles (1956), having more than one decay period and having a decay period i n v e r s e l y p r o p o r t i o n a l t o pressure (Northrop and Nobles, 1956, A v i v i and Cohen 1957) and i n the case of "nitrogen as a waveshifter", being a function of the f r a c t i o n a l amount of nitrogen present (Eggler and Huddleston 1957). Nobles (1956) reported a -9 voltage r i s e time of 3-5 x 10 7 sec f o r oc-particles i n Xenon. Forte (1956) reported a r i s e time o f 1.5 x 10""''' sec f o r a-particles i n pure argon and 5 x 10"^  sec i n "tank" argon. - 5 -Unlike s o l i d s c i n t i l l a t o r s , noble-gas pulse heights are a l i n e a r func-t i o n of energy and independent of the charge and mass of the e x c i t i n g p a r t i -c l e s . Nobles (1956) reported measurements of pulse height i n xenon gas versus energy f o r protons, dueterons and ot-particles from 2 t o 5 Mev f o r which a s t r a i g h t l i n e could be drawn through a l l the points. An energy i n t e r -cept of 0.5 Mev was unexplained. Boicourt and B r o l l e y (1954) measured f i s s i o n and ot-particle spectra i n Krypton gas and found the proper r a t i o between the energies of the l i g h t and heavy f i s s i o n fragment mass groups, but each was too large with respect t o the 4.76 Mev oc group. Few measurements of l i g h t output have been made. Northrop and Nobles (1956) l i s t r e l a t i v e l i g h t outputs f o r oc-particles on Nal (TI) and a number of noble gasses, and Northrop (1958) found Xe gas and diphenyl s t i l b e n e waveshifter as e f f i c i e n t f o r oc-particle d e t ection as Nal ( T I ) . Sayres and Wu (1957) found «-particle; pulse heights i n C s l , Anthracene, and.He + 10$ Xe to be i n the r a t i o of 160:22:105. A l l r e s u l t s show that even the most e f f i c i e n t combination of gas and waveshifter r e s u l t s i n a l e s s e f f i c i e n t p a r t i c l e , d e t e c t o r than s o l i d Nal ( T I ) . Resolution v a r i a t i o n s with gas pressure have been observed, however t h i s e f f e c t was a t t r i b u t e d t o "concentration of « t r a c k s " at higher gas pressure (Sayres and Wu 1957) and no extrapolation t o large-volume counters or t o d i s t r i b u t e d source conditions can be made. Ba l d i n et a l . (1957) report an almost l i n e a r increase i n pulse height with pressure i n the range from ID t o 80 atmospheres for.pure He and for He + 3$ We. In the range from 1 t o 10 atmospheres the pulse height was constant. Binary gas mixtures have been extensively investigated by Northrop and Gursky (1958). A l l 10 mixtures studied have the same general features. There i s a large drop i n l i g h t output f o r a small amount of the heavy gas i n a large f r a c t i o n of the l i g h t e r one. For example, the minimum of the He • Xe - 6 -curve extends o v e r . f r a c t i o n a l Xe concentrations from 10"^  t o 10"3. A peak occurs at 1Q% Xe 90$ He i n which the l i g h t output was found t o be s l i g h t l y l a r g e r than for pure Xenon. I t has been suggested that t h i s mixture, because of i t s large l i g h t output and r e l a t i v e l y high stopping power, would be of use i n constructing a f a s t He 3 counter f o r neutron detection (Batchelor, Aves and Skyrme, 1955). For a l l binary mixtures a large f r a c t i o n of l i g h t e r noble gas i n a heavier one d i d not s e r i o u s l y decrease the l i g h t output of the heavier gas. Baldin et al.(1957) and Eggler and Huddleston (195°) have both found an i n t e n s i t y peak i n the l i g h t output from He when l O " ^ N 2 i s added. The pulse height drops q u i c k l y as the concentration i s increased t o 1$ N 2. B a l d i n et a l . (1957) report a s i m i l a r peak at K f 2 ^ 0 2 i n He with very r a p i d quenching of the pulses as 0 2 percent i s increased. I I I . THE THEORY OF SCINTILLATIONS PRODUCED BY.CHARGED PARTICLES IM A GAS 1. E x c i t a t i o n and I o n i z a t i o n Produced by the Slowing of Charged P a r t i c l e s The slowing of a f a s t charged p a r t i c l e i n a gas leaves energy i n the form of ions, n e u t r a l excited atoms, and ionized and excited atoms. For 1 Mev of oc-particle energy expended i n Xe, i n He^ and Argon the Allowing d i v i s i o n of energy was c a l c u l a t e d . i ) Xe Wo = 21.4 ev spent i n the formation of one i o n p a i r . (Rossi & Staub) 6 Nxe = 10 - Number of i o n pai r s formed per Mev expended. •Wo Exe = 12.13 ev i o n i z a t i o n p o t e n t i a l . (Richtmyer, Kennard & Lauritson) N x e - 46,800 i o n p a i r s T o t a l energy expended i n i o n i z a t i o n - N x e E x e = 570 Kev T o t a l energy used i n e x c i t a t i o n - 1000 - 570 - 430 Kev i i ) He Wo = 31.0 ev Ejfe = 24.58 ev N H e = 32,200 i o n p a i r s T o t a l energy expended i n i o n i z a t i o n = 790 Kev T o t a l energy l o s t i n e x c i t a t i o n = 210 Kev i i i ) Argon r e s u l t s i n 40,000 i o n p a i r s with 630 Kev i o n p a i r energy and 370 Kev e x c i t a t i o n energy. These r e s u l t s cannot be d i r e c t l y correlated with the observed pulse h e i ^ i t from each gas, thus they i n d i c a t e that the photon emission and absorp-t i o n processes are s e n s i t i v e to more than j u s t the r e l a t i v e energy of i o n p a i r s to e x c i t a t i o n energy, or t o the number of i o n p a i r s created. 2. Photon emission The energy l e v e l diagrams of the n e u t r a l and s i n g l y ionized noble gases are characterized by a very high f i r s t excited state, t y p i c a l l y of the order of two-thirds of the i o n i z a t i o n energy. Since the t r a n s i t i o n p r o b a b i l i t y f o r allowed t r a n s i t i o n s between any two l e v e l s i s p r o p o r t i o n a l t o s'(^) ,.-(Hfeitle"r-) the - 8 -majority of the r a d i a t i o n w i H be from ground state t r a n s i t i o n s . From the r e l a t i o n >(A) = 12,380 and using E = 2 E i o n i z a t i o n . E (ev) 3 "XH = 750 A M e = 1530 A Both these primary emission l i n e s l i e i n the vacuum u l t r a v i o l e t and cannot be observed d i r e c t l y with glass envelope photomultipliers. The speed of the l i g h t pulses from a gas s c i n t i l l a t o r w i l l , along with other f a c t o r s , depend on the speed of these ground state t r a n s i t i o n s . A t h e o r e t i c a l estimate of the l i f e t i m e of the lowest resonance s t a t e . i n He, f o r example, gives a value of 5 x 10-1° sec (Wheeler 1933). Measurements made on allowed l i n e s i n the v i s i b l e region vary from 10 x 10"^ sec t o s e v e r a l jut sec, and i t i s known that u l t r a v i o l e t t r a n s i t i o n s are f a s t e r . Unfortunately the speed of t h i s primary process i s not observed i n the s c i n t i l l a t i o n l i g h t because any allowed t r a n s i t i o n ending on the ground s t a t e of the n e u t r a l atom produces resonance r a d i a t i o n . ( M i t c h e l l and Zemansky) There i s a very large p r o b a b i l i t y f o r subsequent re-absorption of such a quantum before i t can escape from the gas and be detected. This phenomenon, c a l l e d resonance trapping, requires such quanta to be absorbed and re-emitted many times before escaping from the gas or being l o s t due t o c o l l i s i o n proces-ses. Radiation from allowed jumps between the higher l e v e l s as w e l l as a l l r a d i a t i o n from excited ions and atoms has a much lower p r o b a b i l i t y of being re-absorbed due to the low density of states involved i n the t r a n s i t i o n . Competing c o l l i s i o n processes could provide important modes of de-excita-t i o n . A c o l l i s i o n with an impurity atom may quench the photon or, i n the case of a nitrogen atom, may s h i f t the wavelength of the primary r a d i a t i o n as discussed i n a f o l l o w i n g s e c t i o n . The e f f e c t s of c o l l i s i o n a l t r a n s f e r of charge i n b i n a r y gas mixtures i s of major importance i n understanding t h e i r - 9 -l i g h t emission processes. To obtain a grasp on the r e l a t i v e importance of c o l l i s i o n s t h e c o l l i s i o n frequency and c o l l i s i o n period were c a l c u l a t e d f o r He and Xe at standard temperature and pressure. The c o l l i s i o n frequency f o r a gas atom i s given by the equation Z = 4d RT (Glasstone) Z = C o l l i s i o n frequency (sec~^) d - Gas d e n s i t y (g/cvar) Yf - Gas v i s c o s i t y (Poises) R = 8.31 x IX) 7 (erg/deg C Mole) T = Absolute Temperature M = Atomic weight of the gas Values f o r d and^obtained from the Handbook of Physics and Chemistry. 29th e d i t i o n y i e l d : Z x e = 2.05 x 10' c o l l i s i o n s / s e c Z j j e a 2.3 x 1C-9 c o l l i s i o n s / s e c The corresponding c o l l i s i o n periods, T , are: ? xe = 5 * ° x 2 D ~ 1 0 s e c  H e = 4.3 x 1 0 " 1 0 sec The v i s c o s i t y of a gas i s independent of pressure except f o r very high or very low pressures, hence the c o l l i s i o n frequency varies d i r e c t l y with pressure and the c o l l i s i o n period v a r i e s i n v e r s e l y . Because of t h i s short c o l l i s i o n period, energy t r a n s f e r between excited and unexcited atoms i n i n e l a s t i c c o l l i s i o n s competes strongly with d i r e c t r a d i a t i v e t r a n s i t i o n s . This.high c o l l i s i o n . f r e q u e n c y combined with the l a r g e number of times that a resonance photon i s emitted and re-absorbed i n d i c a t e s the "way i n which minute quantities of contaminants can act as poisons i n gas s c i n t i l l a t i o n s . - 10 -A contaminant molecule absorbs energy e i t h e r i n an i n e l a s t i c c o l l i s i o n with an excited noble gas atom or by d i r e c t photon absorption. The energy i s then degraded i n t o thermal energy or i n t o the i n f r a r e d region where de-t e c t i o n i s not po s s i b l e . P a r e n t h e t i c a l l y i t may be noted that a waveshifter i s e s s e n t i a l l y a poison except that re-emission i s i n the v i s i b l e region rather than the i n f r a r e d . Bennett and Wu (1957) have investigated the u l t r a v i o l e t spectra due t o the passage of ae-particles through helium and through argon. I t was found - 5 that even with a nitrogen content of less than 10 , the most intense l i n e s observed were due t o nitrogen. This e f f e c t has been a t t r i b u t e d to c o l l i s i o n e x c i t a t i o n of N 2 molecular band heads. In helium they observed the f i r s t negative system o f the molecule with the strongest band head at 3914.4 °A and a l l nitrogen emission between 3900*A and 5300 "A. In argon, the second p o s i t i v e system of the n e u t r a l N 2 molecule was excited with the strongest band head at 3371.3 *A. In a s i m i l a r study, they found that the o p t i c a l spectrum from xenon gas was a continuous d i s t r i b u t i o n from 2300 A t o 6000 A. In a d d i t i o n to the l e v e l s i n excited atoms that combine with the ground state i n allowed t r a n s i t i o n s , there are al s o many metastable l e v e l s excited f o r which d i p o l e r a d i a t i o n t o a lower l e v e l i s forbidden (Phelps and Molnar 1953). E x c i t a t i o n of these l e v e l s by the primary charged p a r t i c l e i s not a forbidden process and a s i g n i f i c a n t amount of energy may be deposited i n these states which have long r a d i a t i o n l i f e times. Fortunately there i s frequently a close resonance l e v e l . With the c o l l i s i o n frequency which was c a l c u l a t e d for the gas there i s a rapid exchange of energy between allowed and metastable l e v e l s , and the decay of the metastable l e v e l i s c o n t r o l l e d by the l i f e t i m e s of these nearby resonance l e v e l s and not by i t s r a d i o a c t i v e l i f e t i m e . Sayres and Wu (1957) have explained N 2 e x c i t a t i o n i n helium as being c o l l i s i o n s with the 2*So0 ( 19.8 ev) metastable state of helium. This c o l l i s i o n process - 11 -with the metastable states may explain the observed f a c t that the r i s e time of a noble elenent s c i n t i l l a t o r containing nitrogen i s dependent on the nitrogen concentration. No consideration has yet been made of the l i g h t coming from the primary p a r t i c l e energy expended i n i o n formation. This l i g h t a r i s e s when the ions recombine with thermal electrons. A study of t h i s e f f e c t was made by Biondi. et a l . (1951-1957). Using microwave techniques, the rate of disap-pearance of electrons following an electrodeless discharge at sever a l mm Hg pressure was observed. Recombination rates f a r higher than those t h e o r e t i -c a l l y estimated f o r the d i r e c t r a d i a t i v e capture process were observed. A subsequent i n v e s t i g a t i o n of mixtures of noble gases showed that the usual recombination process i n a pure noble gas was as follows. F i r s t a stable molecular i o n was formed i n a radia t i o n l e s s . c o l l i s i o n , then the r a d i a t i o n l e s s capture of an e l e c t r o n l e f t the molecule unstable and i t s p l i t i n t o two ex-c i t e d atoms. Biondi estimated that the atomic i o n recombination rate i s at 3 l e a s t a f a c t o r of 10 smaller than the molecular i o n c o e f f i c i e n t , thus the majority of the l i g h t emitted by ions r e s u l t s from the formation and de-ex c i t a t i o n of molecular ions. In the mixture of He + 0.1$ A studied, another process predominates i n which c o l l i s i o n s between excited or ionized helium atoms and argon atoms r e -s u l t s i n a nearly complete t r a n s f e r of the charge i n the gas to the argon atoms because a heavier noble gas always has a lower i o n i z a t i o n p o t e n t i a l . Since the low density of argon makes argon-argon c o l l i s i o n s r a r e , there i s e s s e n t i a l l y no molecular i o n formation. Subsequent recombination with e l e c -trons takes place only i n a r a d i a t i v e c o l l i s i o n . a n d hence has a much smaller pr o b a b i l i t y . t h a n the r a d i a t i o n l e s s s p l i t t i n g of unstable molecules. This e f f e c t was used by Northrop t o q u a l i t a t i v e l y i n t e r p r e t the dip i n l i g h t - 12 -i n t e n s i t y observed i n binary gas mixtures. He concluded that the l i g h t was s t i l l emitted, however the decay time was much longer than the e l e c t r o n i c r e s o l v i n g time and hence was not observed. No measurements with r e s o l v i n g time greater than 0.5 ju sec were made. The stopping time of the primary p a r t i c l e places an inherent l i m i t a t i o n on the r i s e time of l i g h t pulses i n a noble gas. Using the ranges l i s t e d i n Figure 1 and assuming constant d e c e l e r a t i o n , the stopping times f o r 5.1 Mev o c-particles i n He and i n Xe at various pressures were c a l c u l a t e d . a) He (45 psia) - 7.5 x 10~9 sec He (60 psia) - 6.0 x 10""9 sec b) Xe (5 psia) - 6.2 x 10~9 sec Figure 2. Component Layout of a T y p i c a l Counter To Vacuum Pump Through Cold Trap To Vacuum Pump Gas Inlet x-- X — X Cold Pinger for Xe Storage > -X X-Gas Purifier Pressure Gauge -x-f 1 I fWindow Gas Chamber Photomultiplier / X Circulation Pump Harwell Valves - 13 -IV. CONSTRUCTION DETAILS OF GAS SCINTILLATION COUNTERS 1. Mechanical s p e c i f i c a t i o n s f o r H e ^ - f i l l e d s c i n t i l l a t i o n counters a) S i z e 3 To obtain an adequate count rate three l i t r e s of He w i l l be used i n the gas chamber. Assuming that the maximum length of the c y l i n d r i c a l chamber i s twice i t s diameter, the following He pressures were c a l c u l a t e d . Table 1 Chamber djameter Pressure (0°C) Volume 2 inches 220 p s i a 210 cn? 3 64 700 5 14 3200 An i n t e r n a l pressure of 220 p s i a was considered excessive. Also a g" diameter source tube must be inserted through one end of the chamber and t h i s would use up a large f r a c t i o n of the i n t e r i o r volume i n a 2" or even 3" chamber. Three 5 inch photomultipliers were a v a i l a b l e but 3 inch tubes were not, thus a 5 inch chamber diameter was chosen. b) The chamber and p u r i f i e r must be vacuum and pressure t i g h t . c) The walls of the s e n s i t i v e volume should be t h i n t o allow if-ray penetration. d) A l l metal parts must withstand 150 p s i a i n t e r n a l pressure. e) Materials used must have low vapour pressures to reduce out-gassing, and consequent quenching of the l i g h t pulses. 2. The Gas Chamber A c r o s s - s e c t i o n a l view of a t y p i c a l chamber i s given i n Figure 3. The walls and end are a t h i n w a l l s t a i n l e s s s t e e l beaker manufactured by V o l l r a t h . F i g u r e 3. Cross Section of a 5" Gas S c i n t i l l a t i o n Counter Window -Gas -Source chamber MgO + Waveshifter I T Photomultiplier Mu-Metol Shield scale • * * * 0 l" 2" - 14 -The top f l a r e on the beaker was removed and a flange containing an o-ring was fastened on. Designs incorporating V a l l r a t h beakers were l i m i t e d by an inherent stress i n the beaker w a l l near the top. A l l holes d r i l l e d i n the w a l l cracked the beaker when heat was applied t o s i l v e r - s o l d e r the j o i n t s , but holes very near the bottom of the beaker caused no t r o u b l e . Two gas chambers were b u i l t , one with a brass flange and the other with a s t e e l one. Thermal stress was not serious i n e i t h e r chamber when heated to 200°C. The brass flange was fastened t o the beaker with 1200°F s i l v e r solder flowed around the outside j o i n t . This construction allowed the i n l e t holes to be d r i l l e d through the flange without serious cracking of the beaker but i t also l e f t a gas pocket i n s i d e the chamber. The s t e e l flange was welded as shown i n Figure 3 . Because t h i s construction gave no support t o the beaker w a l l s , the i n l e t holes had t o be d r i l l e d near the bottom . A window- r e t a i n e r r i n g was threaded onto the brass flange whereas the ri n g f or the s t e e l flange was bolted on. Both ri n g s sealed the window but the threaded r i n g r e s u l t e d i n a smaller flange assembly. The front face of the r e t a i n e r rings was machined smooth t o insure a l i g h t t i g h t s e a l with the photomultiplier s h i e l d . A compound retarded bourdon gauge with a 2g" diameter d i a l read i n -t e r i o r pressure to - 3% i n the range from 0 p s i g t o 50 p s i g and a l s o i n -dicated approximate vacuum. The gauge was sealed to a 3" length of 3/16" diameter copper pipe with an annealed copper washer and the copper tubing was s i l v e r soldered t o the chamber. This length of copper tubing allowed the gauge to be moved during vacuum evaporation. The chamber i s operated at both vacuum and pressure, thus o-ring grooves were cut to the exact outside and i n s i d e diameters of the o-rings and the - 15 -depth was chosen t o allow the o-ring t o just f i l l the groove when pressed very hard. Great care was exercised i n p o l i s h i n g the o-ring grooves. T e f l o n o-rings were used f o r the coupling j o i n t s and the end plate s e a l on A A A A the chamber but a more e l a s t i c material such as Kel-F or Viton-A was used f o r the f r o n t window s e a l . Teflon has a low vapour pressure and withstands a temperature of 540°F, however the o-rings r e q u i r e a greater sealing pressure than neoprene o-rings and permanently take the shape of the groove. Viton-A and Kel-F are chemically s i m i l a r t o Teflon, have low vapour pressures and withstand high temperatures (Viton-A 500°Fj Kel-F 300°F). Their greater e l a s t i c i t y allows them to be re-used and reduces the se a l i n g pressure to that of neoprene. To eliminate contamination of the counter gas no grease was used on any of the o-rings. The i n t e r i o r of the chamber was cleaned by swabbing i t for 15 minutes with each o f the solutions l i s t e d t o remove scale from the s t a i n l e s s s t e e l . a) Scale loosener Sulphuric a c i d (cone.) 1 part Hydrochloric a c i d (cone.) 1 part Water (65°G) 8 parts b) Scale Remover Hydrochloric a c i d (cone.) 5 parts N i t r i c a c i d (cone.) 1 part Water ("hot") 14. parts A f t e r a chamber had been thoroughly out-gassed by heating i t to 200°G while the i n t e r i o r was evacuated, a t h i c k l a y e r of MgO was smoked onto the i n t e r i o r metal surface by burning a t o t a l of about 6 feet of Magnesium ribbon near the chamber mouth. The chamber i n t e r i o r was then coated with a reasonably uniform l a y e r of waveshifter by two vacuum evaporations from a tantalum boat. To obtain a uniform l a y e r of the correct thickness two boat k Obtained from Anchor Packing Company, Vancouver, B.C. itk Obtained from V i n y l l o y d Company, 720 N. Broadway, Los Angeles 12, C a l i f o r n i a . - I m -positions were used, one c o a x i a l with the chamber and at the mouth, and the second about 4 inches outside the mouth. The tantalum boat was covered with fine-mesh s t a i n l e s s s t e e l screen when quaterphenyl was evaporated t o stop i t from "jumping" from the boat. Diphenylstilbene on the other hand di d not "jump" at a l l and was very easy t o apply. Three types of window have been tested on the chamber. Destructive pressure t e s t s were not made thus comparative strengths are unknown, but each type was run to 75 p s i a . The three types with t h e i r d i s t i n g u i s h i n g properties are l i s t e d below. P i l k i n g t o n p late glass - available i n Vancouver i n £", 3/8" and thicknesses. Both faces are f l a t but the edge i s chipped and the glass appears greenish. The strength of these windows i s unknown. Hobbs Herculite glass discs - a v a i l a b l e i n any s i z e but 1 month d e l i v e r y time. The faces are f l a t and the edge i s rough ground t o the ex-act diameter. This glass i s s l i g h t l y green. The company supplied a g" t h i c k p late f o r use at 150 psig with no guarantee. Both faces have two clamp marks from the edge which makes i n s t a l l a t i o n on the p - r i n g d i f f i c u l t . Quartz d i s c s - available i n a l l sizes but the cost i s high and d e l i v e r y i s at least one month. The faces are f l a t and the edge i s accurately ground. Quartz i s known t o be transparent to v i s i b l e and u l t r a v i o l e t l i g h t and i t s strength i s greater than that of glass. Windows were coated with a t h i n layer of waveshifter by mounting them on a 5 inch in s i d e diameter brass r i n g held 30 cm from a tantalum boat i n s i d e an evacuated b e l l j a r . The thickness of the l a y e r was c o n t r o l l e d by the quantity of waveshifter i n the boat. A measurement of the thickness was 2 made by mounting 2 cm of aluminum f o i l near the window and noting the i n -crease i n weight. This method was very inaccurate but i t indicated two - 17 -2 f i l l e d boats c o n s i s t e n t l y deposited l e s s than 100 /igm/cm at the distance used. I f windows are mounted c l o s e r to the evaporation boat a non-uniform l a y e r r e s u l t s . With c a r e f u l a t t e n t i o n to the o-ring seals, both types of chamber were vacuum and pressure t i g h t and have withstood an i n t e r i o r pressure of 75 p s i a . The MgO and waveshifter l a y e r i s mechanically stable but i s destroyed by moisture. During out-gassing, o i l ( p o s s i b l y from the spinning of the beaker) was given o f f from the walls so a week of hard pumping at 200°C was necessary before MgO was ap p l i e d . The end of t h e chamber snaps when changing from vacuum to pressure thereby l i m i t i n g the type of apparatus which may be mounted on the end but no chipping of the MgO la y e r has been observed. In one of the chambers the i n l e t pipe from the c i r c u l a t o r points toward the front window, and a f t e r continued operation a small mark shows on the waveshifter l a y e r however no other d e t e r i o r a t i o n of the waveshifter has been noted. 3. C i r c u l a t i o n Pumps The c i r c u l a t i o n pump c o n t i n u a l l y forces chamber gas through the gas p u r i f i e r , or through a cold t r a p t o separate the Xenon from the Helium (see Figure 2). The gas flow rate should be v a r i a b l e t o give f l e x i b i l i t y i n ex-perimental runs, and the i n t e r n a l volume must be small. a) Mylar diaphragm pulser with check valves A 2 inch diameter 7 m i l Mylar diaphragm as described by Sayres and Wu (1957) was mounted approximately as shown i n Figure 1 of t h e i r paper. Two u n i - d i r e c t i o n a l check valves and a 3 way solenoid valve were connected as shown i n t h i s Figure. The 110 v o l t ac used to d r i v e the 3 way valve was pulsed by a cam-activated microswitch and a Variac was used to c o n t r o l the speed of the ac-dc motor which drove the cam. High pressure a i r used t o pulse the diaphragm was taken from the laboratory a i r l i n e . scale i i i i i 0 1/4" 1/2" Figure 4. Gross Section of a Magnetic C i r c u l a t i o n Pump - 18 -This pump was much too large and was very d i f f i c u l t t o mount i n the p u r i f i c a t i o n c i r c u i t . The 3-way valve made by General Controls needed frequent cleaning and at times chips from the valve scored the Mylar diaphragm. The i n t e r i o r volume of the pump was large r e l a t i v e t o i t s pumping speed and i t was d i f f i c u l t t o increase the speed above 15 cc per minute. A number of the diaphragms ruptured allowing gas to leak out. This type of pump was used during preliminary work with the counters but was rejected i n favour of the solenoid type described i n the next section. b) Magnetic solenoid pump The construction of t h i s type of pump i s shown i n Figure 4. S t a i n -les s s t e e l and brass were used f o r a l l parts with the exception of the pump slug which was turned from magnet i r o n . Valve seats which were ground with emery dust were almost 100$ e f f i c i e n t when operated v e r t i c a l l y . A commercial-l y manufactured solenoid of in s i d e diameter and l£ M long was mounted as shown. With a 30 v o l t dc supply i t was capable of l i f t i n g the i r o n slug v e r t i c a l l y 1^ ", but t o insure r e l i a b l e operation the pump stroke was kept to 5". When tested at atmospheric pressure the pump deli v e r e d 1.3 cc of a i r per stroke, and i t could be pulsed at 150 cycles per minute. This pump was found to be superior i n the following ways. i ) i t may be operated with any i n t e r i o r gas pressure. i i ) i t i s small and e a s i l y mounted i n the c i r c u l a t i o n l i n e . (Volume l e s s than 3 cm3) i i i ) i t may be serviced e a s i l y . i v ) the pumping speed can be greater than 130 cc per minute, v) i t i s r e l i a b l y sealed and may be operated s a f e l y at high i n t e r n a l pressures. One such pump has been c i r c u l a t i n g Helium at 30 to 60 p s i g f o r s e v e r a l months. The metal slug stuck i n the s t a i n l e s s s t e e l tube several times but t h i s defect has been eliminated by p o l i s h i n g the slu g with f i n e emery. Figure 5. Construction D e t a i l s of a Harwell Valve - 19 -c) E l e c t r i c a l pulsers f o r the pumps The cam-activated microswitch driven with an ac-dc s t i r r i n g motor was found t o be noisy and u n r e l i a b l e , p r i m a r i l y because the motor was o l d . A number of cams was made to allow greater v a r i a t i o n of the pulse rate but without an unreasonable number of them i t would not be possible t o adjust the "on" and " o f f " times. To d r i v e the solenoid pump at about IOO cycles per minute, a m u l t i -vibrator was b u i l t with a 10,000 ohm telephone r e l a y as one p l a t e r e s i s t o r . A 6J6 tube i n a free-running m u l t i v i b r a t o r c i r c u i t with a 300 v o l t p l a t e supply switched s u f f i c i e n t current to a c t i v a t e the r e l a y . Two 1 megohm potentiometers controlled the pulse r a t e and the "on" - " o f f " time. A full-wave bridge using Westinghouse TP25L r e c t i f i e r s supplied 110 v o l t s dc and a 500 ohm r e s i s t o r i n s e r i e s with the pump solenoid l i m i t e d the c o i l voltage t o 30 v o l t s dc. An 8yuf capacitor was placed across the contacts. 4 . Valves and Couplings A l a r g e number of valves were required f o r use i n conjunction with the 1/8" i n s i d e diameter (I.D.) copper tubing which connects the components i n the p u r i f i c a t i o n c i r c u i t . These valves must be r e l i a b l e when operated at a pressure of 150 psig and must have a gas flow rate equivalent t o 1/8" I.D. copper tubing. To eliminate vapours i n the counter they must e i t h e r be 3 packless or have a T e f l o n seat. The dead volume must be < 1 cm . A number of commercial valves meeting these s p e c i f i c a t i o n s were pr i c e d and found too c o s t l y . A form of "Harwell" valve shown i n Figure 5 was b u i l t . A 1/16" t h i c k T e f l o n diaphragm e f f i c i e n t l y sealed these valves. To overcome creep of the Teflon diaphragm, the modified valve shown i n Figure 5 replaced t h i s f i r s t design. The double seats "trap" a small bead of Teflon and thus reduce* creep. This valve has been completely r e l i a b l e u n t i l d i r t accumulates on the brass seat. - 20 -Any number of holes may be d r i l l e d to the centre post of the valve thereby increasing i t s f l e x i b i l i t y . The flow r e s i s t a n c e from the centre hole t o the outside hole i s low and a s e r i e s of these valves w i l l not decrease gas flow noticeably. To permit removal of components from the p u r i f i c a t i o n c i r c u i t and t o connect valves to components, a small o-ring coupling was designed. D e t a i l s are s i m i l a r t o a standard coupling used i n t h i s laboratory but were scaled t o a number 5 o-ring. E i t h e r T e f l o n or Viton-A o-rings s e a l the couplings, but T e f l o n i s badly d i s t o r t e d and should not be re-used t o insure a vacuum s e a l . The o-ring seats must be c a r e f u l l y polished. 5. P u r i f i e r s Many d i f f e r e n t metals have been used t o p u r i f y the noble gases i n s c i n t i l l a t i o n counters. Of these, hot calcium and calcium-magnesium a l l o y (Ca~Mg.) were chosen. Both metals must be maintained at about 400°C while the gas flows through the p u r i f i e r tube, and f o r e f f i c i e n t operation both must be out-gassed at 500°C or higher. A g" outside diameter s t a i n l e s s s t e e l tube 2^" long contained the p u r i -f i e r metal. A bottom p l a t e sealed with an o-ring allowed access to the i n -t e r i o r , and gas flowed i n from the top and out through the bottom p l a t e . A l l j o i n t s were s i l v e r soldered with 1200°F solder. Two 3/16" O.D. copper coolant c o i l s were soldered t o the top and the bottom of the s t a i n l e s s s t e e l tube. Cold water flowed through the two c o i l s i n ser i e s t o keep the o u t l e t gas temperature low and t o protect the o-ring and the soldered j o i n t s from high temperatures. A Chromel-alumel thermocouple j u n c t i o n was strapped i n contact with the s t a i n l e s s s t e e l tube by wrapping the tube with four turns of 2" wide asbestos paper which was t i e d at the top and bottom with one t u r n of :n'i~«hrome wire. - 21 -A close-wrapped c o i l of #28 nichrome wire was wrapped over the i n s u l a t i n g layer of asbestos and an average c o i l resistance of 34 ohms at 20°C r e s u l t s . Four or f i v e turns of asbestos paper over t h i s c o i l thermally i n s u l a t e d the unit and the assembly i s capable of reaching 700°C (surface temperature) with 2 amps ac flowing. An accurate measurement of the i n t e r i o r temperature i s impossible, however with the thermocouple placed on the surface an out-gassing temperature at least 100°C higher than the maximum operating temperature i s assured. Allowance should be made f o r the change from vacuum during out-gas-sing to pressure during p u r i f i c a t i o n . A 3 inch long p u r i f i e r b a r r e l was found t o be much more r e l i a b l e than a 2" b a r r e l and i f the heating c o i l i s at l e s s than 70 v o l t s with respect t o the b a r r e l four layers of asbestos provide adequate e l e c t r i c a l i n s u l a t i o n . A charge of calcium consisted of four t o f i v e grams of clean calcium metal turnings cut into small pieces. A degreased s t a i n l e s s s t e e l screen and s t e e l wool b a f f l e was in s e r t e d followed by the calcium chunks. The bottom was blocked with a second b a f f l e held i n place by the cover p l a t e . The calcium magnesium a l l o y used i n the p u r i f i e r was made as follows. Seventy percent calcium and t h i r t y percent magnesium by weight were placed i n an oxidized s t e e l container, covered and placed on the bottom of a c y l i n d r i c a l s t e e l vacuum chamber 3 inches i n diameter. The top of the chamber was sealed with a cooled neoprene o-ring and when the chamber had been thoroughly evacu-ated with a ro t a r y pump, heat was , ;applied u n t i l the outside turned b r i g h t orange. This temperature was. maintained f o r | hour. The resultant calcium-magnesium has a b r i l l i a n t m e t a l l i c l u s t r e when cracked with a hammer. I t i s pyrophoric so must be handled cautiously, and storage under argon stops surface oxidation. 700 600 500 Count per Channel 400 300-200-100 Source 5A Uncollimated Diff'n a Integ'n 3.2 t^sec 500volts Count x 1/2 Source 6A Collimated Diff'n 8 Integ'n 3.2 fisec 500 volts Diff'n a Integ'n 1.6 fisec. 500 volts Diff'n a Integ'n 3.2 ftsec 1000volts 20 40 60 Kicksorter Channel 100 Figure 6. Ion Chamber Spectra of Two Po <x Sources - 22 -6. C a l i b r a t i o n sources and source holders To determine the r e s o l u t i o n of the counter and t o provide one point f o r energy c a l i b r a t i o n of the counter an i n t e r n a l ccnsource was mounted on an i r o n h o l i e r which may be activated from the outside t o blank o f f the oc-p a r t i c l e s . The holder was inse r t e d through the diameter hole i n the end of the chamber and the source l a y f l u s h with the end surface. A t y p i c a l source holder i s shown i n Plate 2. Two sources were used f o r c a l i b r a t i o n . A Pu239 source manufactured at Chalk River was used f o r a l l the preliminary runs with pure He 4. The t h i c k -ness of t h i s source was measured as 1.4 Mev with the i o n chamber described i n Appendix A, thus r e s o l u t i o n s measured with the source were i n e r r o r . To replace t h i s Pu^39 source a number of t h i n sources were made by dipping 1/8" diameter polished s i l v e r buttons i n t o a 0.5N HC1 s o l u t i o n which contained a d i s s o l v e d radium needle. Extremely high count rates were observed with a t h i n window geiger tube run as a proportional counter even with a dip time of les s than 1 second. The r a t i o of ^ c - p a r t i c l e s to^J-particles was almost 1:1. The radium s o l u t i o n was d i l u t e d 10:1 with d i s t i l l e d water, and co n t r o l l e d dip times followed by vigorous washing immediately a f t e r dipping reduced the /S to oc r a t i o and allowed the count rate t o be chosen. F u l l width at h a l f height of t h e 5.3 Mev cc-particle peak was better than k% when measured with an i o n chamber. Representative source spectra are shown i n Figure 6. 7. Mounting of the components The most e f f i c i e n t plan f o r connecting the basic components of a gas s c i n t i l l a t i o n counter i s shown i n Figure 2. With these connections a number of advantages are apparent. a) the chamber may be sealed from the p u r i f i e r c i r c u i t . b) the gas p u r i f i e r may be i s o l a t e d and evacuated to out-gas the p u r i f i e r metal. Figure 7. C i r c u i t Diagram of a Photomultiplier Head A m p l i f i e r High Tension Rl 5 0 0 Anode 47 pf 47pf 300 Dynode 10 47pf Dynode 9 Test 47pf R3 220 47pf Dynode 2 > 3 0 o DynodeI Focus R2 47 500 ZO Cww) 30 O volts O.I8 I Lo -ve 4 7 0 |47pf Out All Resistances in Kohms - 23 -c) when f i l l i n g the gas chamber, incoming gas i s passed through the gas p u r i f i e r d) the xenon i n the gas chamber may be removed by c i r c u l a t i n g the chamber gas through a cold fingar with the c i r c u l a t i o n pump. Mechanical p o s i t i o n i n g of the components i s shown i n Plate 3. The gas s c i n t i l l a t i o n counter was mounted 41" from the f l o o r on a dexion rack. A l l components inc l u d i n g pumps and power supplies were mounted on lower shelves. The u n i t can e a s i l y be moved in t o a p o s i t i o n with the gas chamber very close to a ta r g e t on the Van de Graaff. 8. E l e c t r o n i c s A. Components The head cathode follower shown i n Figure 7 i s used t o match the output voltage s i g n a l from the photomultiplier t o a 100-chm c o a x i a l cable v/hich carries-, the pulses t o the cathode follower input of a Dynatron wide band a m p l i f i e r . The cable i s terminated with a 100 ohm r e s i s t o r at the cathode follower. Both p o s i t i v e and negative pulses are amplified by the head am p l i f i e r but a l l experimental r e s u l t s were taken using a negative output s i g n a l . With negative input pulses t o the Dynatron a m p l i f i e r , the p o s i t i v e output pulses must be inverted before a c t i v a t i n g the 100 channel C.D.C. k i c k s o r t e r . This i s done with a standard i n v e r t e r c i r c u i t using a 403B tube. A Northeast S c i e n t i f i c Supply Company regulated voltage supply, s e r i a l 82, provided high voltage f o r b i a s s i n g the photomultiplier dynodes. Both a DuMont #6364 and an E.M.I. #6099B f i v e - i n c h end-window photomultiplier tube were used t o observe s c i n t i l l a t i o n s i n the gas. The bias r e s i s t o r c i r c u i t f o r the DuMont tube i s shown i n Figure 7. The c i r c u i t f o r the E.M.I, tube i s s i m i l a r (with the exception of the 20 K«W.W. focus c o n t r o l which i s un-necessary) but was constructed with g Watt, 10$ r e s i s t o r s . - 24 -The s t a b i l i t y and l i n e a r i t y of the a m p l i f i e r s and the k i c k s o r t e r were measured by i n j e c t i n g , p r e c i s e pulses at the negative t e s t input of the head cathode follower. The pulses were formed by discharging a 0.1 jifd capacitor through a 34 ohm r e s i s t o r . A Westinghouse 276D mercury switch driven by 60 cycle l i n e current was used to charge and discharge the capacitor, and the precise charging voltage was supplied from the wiper of a 100 KjLHelipot with 15 v o l t s dc across i t (Robertson, 1955) • B. Observed behaviour of the components a) Photomultipliers The photomultiplier tube was expected t o l i m i t the r e s o l u t i o n a t t a i n a b l e with the s c i n t i l l a t i o n counter and also t o be the major source of e l e c t r o n i c noise, consequently an i n t e n s i v e i n v e s t i g a t i o n of the properties of the photomultiplier tubes was c a r r i e d out under the following headings, i ) Photomultiplier noise DuMont Two #6364 tubes were te s t e d . Both were darkened f o r two days with 1700 v o l t s on the dynode bia s chain. The maximum noise pulses from the tube which was discarded were twice the height of those from the tube which was used. A Pye Scalamp galvanometer placed i n s e r i e s with the anode load measured the dark current. This current increased as the voltage was r a i s e d and at 1700 v o l t s the dark current was 0.18 ^iamp. A more u s e f u l measure of the noise f o r s c i n t i l l a t i o n counter a p p l i c a t i o n s i s the pulse height spectrum. T y p i c a l noise spectra are included i n the c a l i b r a t i o n spectrum p l o t s '^Figure 9). Numerous noise spectra taken during the i n v e s t i g a t i o n showed that: The tube was very noisy immediately a f t e r i t had been exposed to room l i g h t . A f t e r two or three hours of operation on the gas chamber i t returned t o normal. Even when the tube had been darkened with the voltage o f f , i t was more noisy immediately a f t e r the voltage was turned on. - 25 -Two t r i v i a l observations were that the maximum tube noise decreased t o about three-quarters of its value at room temperature when the tube and the gas chamber were cooled with l i q u i d a i r , and that tube noise increased with in c r e a s i n g bias voltage. E.M.I. The current noise i n the #6099B tube was 0.037/amp at 1700 v o l t s . This i s i n agreement with published data and in d i c a t e s that the tube has not been roughly handled. The noise behaviour was s i m i l a r to that of the DuMont tube and t y p i c a l pulse height spectra are shown, i i ) Photomultiplier gain A two inch diameter Nal (TI) c r y s t a l was o p t i c a l l y coupled t o the centre of the photocathode of both photomultiplier tubes. The voltage was varied from 800 v o l t s upward f o r both tubes and the .peak height of Co^° gamma pulses was p l o t t e d against voltage.' DuMont The pulse height varied as V^* 0 0 , 2 where V = voltage on the dynode bias chain E.M.I. The pulse height varied as v 8 , 0 ± 0* 2 Intercomparison of gas s c i n t i l l a t i o n pulse heights at 1500 v o l t s and 1700 v o l t s showed that E.M.I. Pulse Height = 3.2 DuMont Pulse Height Both tubes exhibit a gain s h i f t during the f i r s t hour of operation. The apparent pulse height changes as much as 10$ wraith time. No s t a b i l i t y measurements have been made on the high voltage supply f o r the photomultiplier. A gradual s h i f t i n t h i s voltage would a l t e r the pulse h e i ^ i t and cause an increase i n apparent r e s o l u t i o n . Count times ranging from two minutes t o three hours taken with clean gas resulted i n constant r e s o l u t i o n thus voltage d r i f t s , i f they occur, must be over a longer period than 6 hours. DuMont Photocathode E.M.I. Photocathode Figure 8. Photocathode S e n s i t i v i t y of the Two 5" Photomultipliers Used - 26 -i i i ) Photoinultiplier r e s o l u t i o n A two-inch diameter Nal (TI) c r y s t a l was o p t i c a l l y coupled t o the 6 0 centre of the photocathode of the E.M.I, tube and the Go gamma peaks were counted using the e l e c t r o n i c s described i n t h i s chapter. A r e s o l u t i o n of 8.5$ was obtained far the most energetic gamma peak at both 1000 v o l t s and 1200 v o l t s . A l a r g e r c r y s t a l was not a v a i l a b l e , however Sharpe (1957) r e -ports 8$ r e s o l u t i o n with a 4~J inch Nal (TI) c r y s t a l coupled t o a 5 inch #6099B tube. i v ) Photocathode uniformity The uniformity of both photocathodes was measured by moving a 137 diameter anthracene c r y s t a l and Gs beta source over the photocathode and observing the pulse height with a Tektronix #541 os c i l l o s c o p e connected to the high output of the head cathode fol l o w e r . Two one-gallon paint cans were used f o r t h e l i g h t - t i g h t container and the photomultiplier tube neck was l i g h t - s e a l e d through the centre of the bottom can thereby allowing the associated head cathode follower t o be mounted outside the dark volume. A mu— metal s h i e l d placed around the photomultiplier reduced s t r a y magnetic f i e l d s . The tube was mounted v e r t i c a l l y i n the bottom can with the photocathode f a c i n g up. The top can was sealed t o the bottom one with an o-ring which allowed i t to rotate, and the source and c r y s t a l hung downward from a 3/16" rod which passed h o r i z o n t a l l y through the centre of i t at a height which allowed the c r y s t a l to r e s t on the photocathode when the c r y s t a l and source were hanging downward. By s l i d i n g the rod and rotating the top can the c r y s t a l could be moved accurately over the whole photocathode. Results of a number of t e s t s are shown i n Figure 8. The e f f e c t of vary-ing the focus voltage of the DuMont tube i s c l e a r l y seen, b) Head cathode follower Rise time The input time constant of the load r e s i s t o r and anode s t r a y capacity - 27 -i s very much longer than the f a l l time of the current pulse on the anode, thus the output voltage pulse w i l l have a r i s e time equivalent to the f a l l -9 time of the exponential current pulse, the order of 10 second. The cathcde-to-ground stray capacity of the p a r a l l e l e d 6J6 tube i s the major l i m i t a t i o n of pulse r i s e time. Assuming 25 pf s t r a y capacity, the r a t e of change of cathode voltage f o r a negative step input on the g r i d i s dv - i s 18 x 10~-3 amp = 0 . 7 v o l t s / n sec dt C 25 x 1 0 " ^ farad For a 2 v o l t output pulse the r i s e time i s ~ 3 n sec. F a l l time The d i f f e r e n t i a t i o n time constant of t h i s c i r c u i t may be adjusted by varying either the anode load r e s i s t o r (Rl) or R2 ( i n t h i s case R3 must also be adjusted to maintain the bias voltage on the g r i d ) . The f a l l time was set at 1 .5 /t- seconds. This choice eliminates n o n - l i n e a r i t y i n the C.D.C. ki c k s o r t e r caused by pulses which are l e s s than —1 p. sec wide. The wideband a m p l i f i e r settings used throughout the i n v e s t i g a t i o n were 0.16 ji sec. in t e g r a t i o n time and 8)x sec d i f f e r e n t i a t i o n time, c) L i n e a r i t y and s t a b i l i t y of the components With a 100 ohm cable connected t o the Lo output jack, the head a m p l i f i e r was l i n e a r f o r input pulses up to at l e a s t 18 v o l t s . The s t a b i l i t y of the head cathode follower, Dynatron a m p l i f i e r , i n v e r t e r and k i c k s o r t e r was - 1 .5$ i n absolute pulse height over a period of 4 days. - 28 -V. THE BEHAVIOUR OF HE 4 GAS SCINTILLATION COUNTERS 1. Preliminary measurements with 5" diameter chambers The f i r s t counter system t e s t e d consisted of a Plutonium ot-source mounted on the end of the brass-flanged chamber. The chamber i n t e r i o r was coated with MgO and quaterphenyl and the t h i c k glass window was excessively and unevenly coated with quaterphenyl ( p o s s i b l y as much as 1,000 .ugm/cm at the centre). The chamber and p u r i f i e r c i r c u i t were f i r s t evacuated with a rotary pump which was connected t o the chamber with a rubber hose. A f i l l i n g of He 4 from a Helium c y l i n d e r gave pulses s l i g h t l y above the noise l e v e l of a 5" E.M.I, photomultiplier tube run at 1650 v o l t s . The r i s e time of these pulses was^40 x 10"^ sec but the r e s o l u t i o n was not measurable. P u r i f i c a t i o n with calcium heated to 400°C resulted i n a recognizable cc group which could be switched o f f , but the r e s o l u t i o n was never better than 50$. V a r i a t i o n of the p u r i f i e r temperature produced a r a d i c a l change i n pulse height (and r e s o l u t i o n ) . The pressure was varied from 0 p s i g t o 40 p s i g and w a l l e f f e c t occurred at 15 p s i g , however no s i g n i f i c a n t change i n r e s o l u t i o n with pres-sure was observed beyond the w a l l e f f e c t region. The i n c l u s i o n of an out-gassed charcoal trap i n the f i l l i n g l i n e resulted i n 50$ r e s o l u t i o n immediately a f t e r f i l l i n g , but the calcium p u r i f i e r caused a decrease i n pulse height when heated to le s s than 400°C. A s u b s t a n t i a l increase i n r e s o l u t i o n was obtained by r e p l a c i n g the calcium metal i n the p u r i f i e r with clean out gassed calcium chips. The ultimate r e s o l u t i o n a t t a i n a b l e with such a counter was 38$ using the 5" E.M.I, photomultiplier tube. The oc peak was a f a c t o r of 3 higher than the average height of the photomultiplier noise. 2. The e f f e c t of c o o l i n g the s c i n t i l l a t i o n counter The chamber and mu-metal photomultiplier s h i e l d were cooled t o l i q u i d Figure 9. The E f f e c t of Cooling the Gas Chamber Pulse Height - 2 9 -nitrogen temperature by covering the metal surface with a s o f t c l o t h and c a r e f u l l y pouring on l i q u i d nitrogen. The chamber gas pressure dropped due to leaks i n the o:-ring s e a l s , but t h i s was counteracted by flowing Helium i n d i r e c t l y from the c ^ i n d e r . The e f f e c t on pulse height and r e s o l u t i o n i s shown i n Figure 9 for both a 5 " DuMont photomultiplier tube and the E.M.I, tube. The pulse heights were adjusted so that both tubes gave peaks i n approximately the same k i c k s o r t e r channel. A s i m i l a r run was made i n which only the gas chamber was cooled and the pulse height increased without a decrease i n photomultiplier noise, thus the e f f e c t i s not caused by a gain s h i f t i n the photomultiplier. The pulse height i n a chamber whose gas has been more completely p u r i f i e d shows l i t t l e increase on c o o l i n g . The increase i n pulse height has been a t t r i b u t e d t o " f r e e z i n g out" of impurities, however an attempt t o duplicate these r e s u l t s using a cold f i n g e r attached t o the chamber was unsuccessful. Gas c i r c u l a t i o n through a cold f i n g e r was not t r i e d . I t i s conceivable that the increased pulse height was p a r t i a l l y caused by a decrease i n c o l l i s i o n frequency at the lower gas temperature thereby reducing the p o s s i b i l i t y of quenching c o l l i s i o n s . Un-f o r t u n a t e l y t h i s c o o l i n g procedure may not be used with He^ " t Xe i n the counter because the Xe would l i q u i f y , but i t may be of use i n producing Ee* counters without external p u r i f i e r s . 3. Gas s c i n t i l l a t i o n pulses from Helium plus Xenon The a d d i t i o n of 10$ by pressure of Xe t o the He^ " increased the pulse height by a f a c t o r of three. The corresponding increase i n r e s o l u t i o n r e -vealed a t h i c k source t a i l on the « peak. The counter r e s o l u t i o n f o r the Pu ac-particles as measured from the high energy edge of the peak decreased t o 16$ and the s i g n a l t o noise r a t i o increased t o 38:3. Following p u r i f i c a t i o n with hot calcium the gas chamber was sealed o f f and the pulse height decreased 0.6$ per hour during the f i r s t 24 hours and F i g u r e 10. The E f f e c t of Calcium P u r i f i c a t i o n on He • 10$ Xe 800 700 Relative Pulse Height Cold Calcium 200 Calcium ot 400°C 8 Time (hours) 12 16 Fresh Calcium in / Purifier / 400°C(for 9hr) to20°C t o 4 2 0 ° C to4gO!C to430°| to 400° to 360° - 3 0 -0.2$ per hour during the second 2 4 hour period. This decrease was not caused by wet gas because cold calcium p u r i f i c a t i o n caused no increase i n the pulse height. Thin Po a sources with a r e s o l u t i o n of l e s s than 4 $ were used, f o r sub-sequent i n v e s t i g a t i o n s because t h i n Pu sources of the s i z e required were not a v a i l a b l e . One unfortunate consequence of using Po sources i s contamination of the chamber and source holder caused by the w e l l known phenomenon of "creeping" of Po-alpha sources. To counteract t h i s e f f e c t as much as p o s s i b l e a l l Po-coated s i l v e r d i s c s were mounted w i t h i n a 0.125 i n c h i n s i d e diameter brass cup. With a source holder i n the o f f p o s i t i o n the shutter covered the top o f the brass cup thereby stopping a l l oc p a r t i c l e s . With Po source #5A i n the chamber a preliminary f i l l i n g of 30 p s i g He^ which had been passed through the charcoal p u r i f i e r resulted i n a r e s o l u t i o n of 3 2 $ and a s i g n a l to noise r a t i o s ((X peak height: average photomultiplier noise height) of 2 6 : 3 . The ultimate r e s o l u t i o n obtained with He^ plus 10$ Xe at a t o t a l pressure of 2 2 psig was 12$ and the s i g n a l t o noise r a t i o with the tube operating at 1600 v o l t s was 9 0 : 3 . Recent work with a source mounted midway onaside w a l l of the chamber indicated that a r e s o l u t i o n of l e s s than 10$ may be obtained. The source mount caused some broadening of the peak and an exact r e s o l u t i o n could not be c a l c u l a t e d . The effectiveness of the calcium p u r i f e r and solenoid pump was i n v e s t i -gated using source #5A with 3 0 psig He plus 10$ Xe. The resultant r e l a t i v e pulse height graphj Figure 10, shows the importance of using f r e s h w e l l out-gassed calcium operated at the correct temperature. The sharp r i s e i n pulse height when the calcium was cooled t o room temperature was a r e s u l t of moisture removal from the gas by cold calcium. When the calcium has become i n e f f e c t i v e f o r removing impurities from the gas, the surface l a y e r of the - 31 -i n i t i a l l y m e t a l l i c calcium chunks i s black and may be crumbled. Removal from the p u r i f i e r b a r r e l showed that only the calcium near the c e n t r a l region of the c o i l has deteriorated, thus a more e f f e c t i v e heater could be developed i n order that a l a r g e r percentage of the calcium would react t o remove N£ from the gas more quickly. An a l l o y of 70$ calcium -30$ mag-nesium was used i n the p u r i f i e r . No comparison, with calcium was made, however Ca^Mg^ + Ca. (the resultant mixture when 70$ Ca and 30$ Mg are mixed) has given equally good r e s o l u t i o n i n He^- and Xe gas mixtures. Cold c a l c i u m -magnesium a l l o y maintained the s c i n t i l l a t i o n pulse height f o r 10 days following p u r i f i c a t i o n of the gas with hot Ca^Mg^. Before re-using the p u r i f i e r at a high- temperature i t must be out-gassed. 4. Further f a c t o r s a f f e c t i n g the r e s o l u t i o n of He^ and He^ + Xe counters a:.) F i l l i n g procedure Following a chamber evacuation using a rotary pump, two flushes of helium to a vacuum of -~15 inches Hg before the f i n a l f i l l i n g of helium g r e a t l y increased the i n i t i a l pulse height. This procedure was found unnecessary when the chamber was pumped t o 10~^ m Hg through a §" diameter tube. The decay time of t h e chamber was a l s o lengthened by higher vacuum before f i l l i n g . There was no measurable improvement when a cold t r a p was placed i n the pumping c i r c u i t of the rotary pump which seems to i n d i c a t e that the impurities are occluded on the chamber walls and are not due t o back streaming from the forepump. b">) Waveshifters One unsuccessful attempt was made t o observe v i s i b l e l i g h t pulses from He^- without waveshifters on the i n t e r i o r of the chamber and the window. Following t h i s , both quaterphenyl and diphenyl s t i l b e n e were t r i e d as wave-s h i f t e r s . No apparent d i f f e r e n c e was found, s o t o standardize the i n v e s t i g a t i o n quaterphenyl has been used e x c l u s i v e l y . The thickness of waveshifter l a y e r s - 32 - .. on windows must be greater than 20 ;ugm/cm and l e s s than a few hundred /igra/cm^ t o obtain maximum pulse height. Two f u l l tantalum boats deposit about 40 jigm/cm^ at a distance of one foot (with an allowance f o r l o s s due to jumping). Such a la y e r when deposited on a glass window shows in t e r f e r e n c e rings g" wide. c) Windows A He • Xe f i l l e d counter w i t h a quartz window has attained a r e s o l u t i o n of 610$ with the DuMont tube whereas the ultimate r e s o l u t i o n with a P i l k i n g t o n glass window counter i s ""10$ with the DuMont tube and a s i d e -mounted source. Because of the large number of f a c t o r s a f f e c t i n g r e s o l u t i o n , i t i s not c e r t a i n that t h i s increased r e s o l u t i o n r e s u l t e d from the quartz. There i s no measurable d i f f e r e n c e i n r e s o l u t i o n between the two types of glass window. d) Impurity content i n Helium counters Immediately a f t e r f i l l i n g a counter with p u r i f i e d helium the pulse height i s unsteady. A f t e r one f i l l i n g i t rose 19$ above i t s i n i t i a l value ( r e s o l u t i o n 15g$) over a period of 4 hours then f e l l t o 10$ below and remained constant during p u r i f i c a t i o n . The pulse r e s o l u t i o n changed ac-cordingly, thus the increase was not caused by a photomultiplier gain s h i f t . I f , on the other hand, the i n i t i a l f i l l i n g i s d i r t y and the r e s o l u t i o n i s poor, the pulse height increased with p u r i f i c a t i o n but no maximum has been observed or i f the chamber was sealed o f f the pulse height dropped. The e f f e c t i s most probably caused by small v a r i a t i o n s i n the nitrogen content of the gas. I f , for example, the i n i t i a l f i l l i n g of helium had a nitrogen content of 5 x 10~J$ than f u r t h e r nitrogen removal would cause the pulse height to r i s e and then f a l l t o Isss than the i n i t i a l height (Eggler and Huddleston 1956 - Figure 2 ) . Figure 11. Light Output from He Plus Xe as Observed by Northrop and Gursky (1958) - 33 -e) E l e c t r o n i c s voltage l e v e l s The h i $ i tension voltage on the DuMont tube was v a r i e d from 1300 v o l t s to 2000 v o l t s . The r e s o l u t i o n and s i g n a l t o noise are shown i n Table 2 . Table 2 High Tension Resolution S i g n a l t o Noise Comments 1300 v o l t s 1500 v o l t s 1700 v o l t s 1800 v o l t s 1900 v o l t s 2000 v o l t s 17$ 13.5$ 13.5$ 13.5$ 13.5$ 13.5$ 29:3 30:3 39:3 37:3 39:3 peak i n low k i c k s o r t e r channel observed at Bias 5 V a r i a t i o n of the focus v o l t s on the DuMont tube over the range shown i n Figure 8 had no e f f e c t on the oc peak shape or height, hence photocathode s e n s i t i v i t y i s not a serious l i m i t a t i o n on chamber r e s o l u t i o n , f ) Xenon concentration As Xe i s added to the He, the pulse height r i s e s l i n e a r l y when observed with the Tektronix o s c i l l o s c o p e , however a w e l l defined maximum pulse height i s not apparent because of the r e l a t i v e l y slow mixing rate of the Xe i n He. No decrease i n pulse h e i ^ i t was observed when Xe concentration was r a i s e d from 13$ to 16$. This i s i n agreement with Northrop and Gursky who found a maximum i n pulse h e i ^ i t f o r a 10$ Xe i n He mixture followed by a small decline with increasing Xe concentration (Figure 11). 5. The s t a t i s t i c s of photon observation i n gas s c i n t i l l a t i o n counters About 10^  photons/Mev expended i n a gas should be given o f f , however the - 34 -e f f i c i e n c y of the photocathode, waveshifter, r e f l e c t o r and window w i l l reduce t h i s by a f a c t o r of about 10-^ , thus a 5 Mev oc-particle r e s u l t s i n roughly 500 photoelectrons being emitted from the photocathode. One expects a Poisson d i s t r i b u t i o n of these photoelectrons about the mean number, m, hence the standard deviation, ff-, of the d i s t r i b u t i o n i s V"nT and the f u l l width at h a l f maximum i s 2 V 2 1 n 2.3540" (Evans). As-suming that the peak of the photomultiplier voltage pulse i s a l i n e a r function of the number of photoelectrons emitted by the photocathode, the re s o l u t i o n , R, of a pulse height d i s t r i b u t i o n would be: R - width at h a l f maximum - 2 .35 Vm"* pulse height m R = 2 - 3 5 or m s 5.52 This treatment has neglected the s t a t i s t i c a l nature of the secondary emis-sion process which Morton (1949) estimated would increase the width l e s s than 15$. Maximum noise pulses i n a photomultiplier tube correspond t o ~ & photoelectrons emitted from the photocathode. Using the r e l a t i o n s h i p be-tween m and R, m was ca l c u l a t e d f o r a representative number of ae-particle peaks. Assuming a l i n e a r r e l a t i o n between m and the channel number, the channel corresponding t o 8 photoelectrons was calc u l a t e d . There was close agreement with a reso l u t i o n of 40$ but with a r e s o l u t i o n of ^  20$ the channel corresponding t o 8 photoelectrons was a f a c t o r of 2 below the maximum of the photomultiplier noise spectrum f o r the E.M.I, tube and 4 below f o r the DuMont tube. 6. The counting e f f i c i e n c y f o r <x-particles The areas of twenty-five representative o c-particle spectra were measured - 35 -and converted t o count r a t e s . The count rates f o r a v a r i e t y of gas con-d i t i o n s i n c l u d i n g pure and impure He^ " and He^ " + Xe were found to be w i t h i n * % of the value obtained by i n t e g r a t i n g the source peak obtained with an i o n chamber. Err o r s i n p l o t t i n g and measuring the curves explain t h i s d e v i ation, thus the gas s c i n t i l l a t i o n counter has been c o n s i s t e n t l y 100$ e f f i c i e n t . 7. Photon c o l l e c t i o n e f f i c i e n c y The ultimate r e s o l u t i o n a t t a i n a b l e with a gas s c i n t i l l a t i o n counter i s strongly dependent on the photon c o l l e c t i o n e f f i c i e n c y of the photo-m u l t i p l i e r and chamber. The «c-pulse height was increased by a f a c t o r of as much as 1.5 when the photocathode was o p t i c a l l y coupled t o the chamber window with a t h i n layer of glycerine and Russian o i l . The r a t i o of centre t o average photocathode e f f i c i e n c y f o r the DuMont tube i s 2.0 thus some v a r i a t i o n i n voltage pulses out f o r a constant l i g h t pulse near the photocathode i s expected, however the path of photons l e f t by charged p a r t i c l e s and the d i f f u s i n g by the walls w i l l average the l i g h t over a larger region of the photocathode. 8. The e f f e c t of ^ - r a d i a t i o n on gas s c i n t i l l a t i o n counters Both a t h i n w a l l chamber and a r e c e n t l y b u i l t t h i c k w a l l chamber were inte n s e l y i r r a d i a t e d with 2 Mev and 6 Mev 5f-rays ('-lO7 photons). The resultant low energy pulses increased the count i n the noise channels but had no e f f e c t on channels higher than 500 Kev on the energy scale. The shape of the ¥-ray spectrum was s i m i l a r t o that of the noise pulses. - 36 -VI. A. HELIUM GAS SCINTILLATOR. USED AS A DETECTOR OF, FAST NEUTRONS 1. Types of fast neutron counters Neutrons produce very l i t t l e d i r e c t i o n i z a t i o n i n t h e i r passage through a gas, and so they cannot be detected d i r e c t l y by Geiger counters, i o n chambers or cloud chambers whose operation depends on ions produced by the entry of a p a r t i c l e . Nevertheless, instruments of t h i s kind have been adapted to detect and count neutrons by using secondary e f f e c t s of these n e u t r a l p a r t i c l e s . The following types of detectors are s u c c e s s f u l l y used f o r f a s t neutron detection. These counters have a wide range of c h a r a c t e r i s t i c s and no one type can be chosen as superior f o r a l l f a s t neutron a p p l i c a t i o n s , a) I o n i z a t i o n detectors i ) Boron t r i f l u o r i d e p r oportional counter ("long counter"), with moderator: - This counter e f f i c i e n t l y detects slow, intermediate and f a s t neutrons, but there i s a considerable time delay due t o moderation and d i f -f u s i o n e f f e c t s as w e l l as the pulse r i s e time of the proportional counter ('-1 jasec). There i s very good d i s c r i m i n a t i o n against Jf-rays but neutron energy i s not measured. i i ) He^ f i l l e d p r o p o r t i o n a l counter: - This counter has been used by Batchelor et a l . as a neutron spectrometer by the a n a l y s i s of the pulse height d i s t r i b u t i o n from the r e a c t i o n He 3 (n,p) H^. Fundamentally there i s no l i m i t a t i o n on the neutron energy or the number of neutron groups which 3 may be detected, however above a neutron energy of 1 Mev the He r e c o i l spectrum tends t o o b l i t e r a t e the spectrum of groups from the (n,p) r e a c t i o n . The quantity measured by such a counter corresponds unambiguously t o one neutron energy. The r i s e time of the pulses obtained i s l i m i t e d to ~1 /isec by the e l e c t r o n c o l l e c t i o n time. - 37 -i i i ) Proton r e c o i l counter: - The primary neutron target i n such a counter may be either gas or a t h i n p l a s t i c l a y e r . This counter may be used as a f a s t neutron spectrometer by the analysis of the pulse height spectrum of the knock-on protons. Unfortunately the range of energetic protons even i n heavy gas mixtures makes w a l l e f f e c t s a serious l i m i t a t i o n , and c o l l e c t i o n time i s increased i f the gas pressure i s r a i s e d , h) S c i n t i l l a t o r s i ) ZnS i n p l a s t i c (Hornyak Button or Sandwich Counter): - This f a s t neutron counter detects proton r e c o i l s from the p l a s t i c by observing the r e -sultant ZnS s c i n t i l l a t i o n s with a photomultiplier. The ZnS l i m i t s the speed' of the pulses to ~.3 /isec. The height of the pulses i s not l i n e a r with neutron energy and the o v e r a l l e f f i c i e n c y i s low (<-'10"3). With the correct pulse d i s c r i m i n a t i o n l e v e l the counter i s i n s e n s i t i v e t o 2?-rays. i i ) Organic s c i n t i l l a t o r s : - Small stilbene c r y s t a l s have been used as f a s t neutron detectors. The pulses are very f a s t (~6 nsec) and the ef-f i c i e n c y of a | cm t h i c k c r y s t a l f o r 1 Mev neutrons i s ~5$. P l a s t i c s c i n t i l l a t o r s give s l i g h t l y f a s t e r pulses than s t i l b e n e and have the added advantage that a v a r i e t y of s i z e s and shapes i s a v a i l a b l e . A combination of p l a s t i c i n a l i q u i d s c i n t i l l a t o r provides even f a s t e r pulses (2 nsec), however mounting of the s c i n t i l l a t o r i s more d i f f i c u l t . A l l of these organic s c i n t i l l a t o r s have a high jf-ray s e n s i t i v i t y , c) Time-of-flight s e l e c t i o n of neutron energy The determination and s e l e c t i o n of f a s t neutron energy by v e l o c i t y measurement i s at present the most accurate method. Measurements of neutron f l u x can be c a r r i e d out simultaneously provided the o v e r a l l e f f i c i e n c y of the detecting system i s accurately known. A method frequently used i s to - 33 -t r i g g e r a timing device with the 3f-ray emitted i n a (djn, r e a c t i o n and to observe the neutron r e a c t i o n a measured time l a t e r . I f the neutron path length i s known, the neutron energy may be s p e c i f i e d within l i m i t s set by the timing device and the i£-ray counter. In cases where no Jf-ray i s emitted at the same time as the neutrons from the group under i n v e s t i g a t i o n , i t i s possible to achieve the same r e s u l t by using a f a s t "chopper" i n the path of the neutron beam or by e l e c t r o s t a t i c a l l y p u l s i n g the deuteron beam r a p i d l y on the t a r g e t . Mechanical "choppers" produce monoenergetic neutron beams of up t o 1 Mev energy from intense neutron sources such as reac t o r s . A precise path length i s defined by the apparatus and only neutrons which traverse t h i s distance i n a s p e c i f i e d time emerge from the "chopper". In general the t i m e - o f - f l i g h t methods are extremely powerful and t h e i r a p p l i c a t i o n i s l i m i t e d only by the d i f f i c u l t y of obtaining a precise e s t i -mate of the o v e r a l l e f f i c i e n c y . 2. A He 4 plus Xe gas s c i n t i l l a t i o n Counter used as a neutron Spectrometer a) Discussion The absence of complex e l e c t r o n energy bands i n a gas makes the analysis of pulse spectra from gas counters more simple i n p r i n c i p l e than those from s o l i d s c i n t i l l a t o r s . The gas counter pulses should be more ac-curately p r o p o r t i o n a l to energy expended than pulses from the s o l i d s c i n t i l l a t o r s but evidence on t h i s point i s not yet c l e a r . Northrop (1956) found that the pulse height i n a Xe gas counter varied l i n e a r l y with energy but the graph published shows an energy inte r c e p t of 0.5 Mev. Further i n -v e s t i g a t i o n of the pulse height i n He 4 or He 4 plus Xe under many d i f f e r e n t conditions could be c a r r i e d out by exposure of the gas counter to neutrons of various energies. - 39 -At the same time, the properties of the gas s c i n t i l l a t o r as a f a s t neutron detector are worthy of examination. The choice of a gas t o provide suitable r e c o i l s i s important. Hydrogen would be preferable f o r purposes of pulse analysis because the n-p s c a t t e r i n g i s i s o t r o p i c i n the centre-of-mass system at moderate neutron energies. This r e s u l t s i n an id e a l spectrum from a s i n g l e neutron group which i s rectangular with a sharp upper l i m i t at the incident neutron energy. On the other hand, the long range of proton r e c o i l s causes pronounced w a l l e f f e c t s . A f u r t h e r l i m i t a t i o n i s set by the f a c t that hydrogen even i n small qu a n t i t i e s ( < 5 $ ) reduces the l i g h t output of gas s c i n t i l l a t o r s . He^ r e c o i l s are much shorter f o r the same neutron energy, but i n the 1 Mev - 5 Mev region the sc a t t e r i n g i s markedly a n i s o t r o p i c (Adair 1 9 5 2 ) , with the e f f e c t that the i d e a l pulse spectrum i s peaked both at zero energy and at the maximum r e -c o i l energy ( 1 6 / 2 5 En as shown below). The maximum t r a n s f e r of neutron energy i n a s i n g l e e l a s t i c c o l l i s i o n may be derived most e a s i l y by considering the c o l l i s i o n i n the centre of mass coordinates. The r e s u l t a n t laboratory expression i s : Emax = 4 MnMx En ( M n t Mx)^ where E ^ ^ i s the maximum energy of the r e c o i l p a r t i c l e . Mx i s the mass of the r e c o i l p a r t i c l e Mn'is the mass of the incident neutron En i s the i n i t i a l energy of the neutron A gas s c i n t i l l a t i o n counter f i l l e d with He plus Xe i s a sui t a b l e counter f o r observing the r e c o i l spectrum produced by neutrons. The pulses approach the speed of those from the organic s c i n t i l l a t o r s while the He r e c o i l spectrum maintains the r e l a t i v e s i m p l i c i t y of a gas counter. 2000 39.5psia He 58.7psia He 15001 Count per Channel lOOOr 500 High Pressure Curve — Low Pressure Curve 10 20 30 40 Kicksorter Channel 1 . j Figure 12. The Spectra of 4.1 Mev Uncollimated Neutrons at Two Helium Pressures - 40 -Almost complete i n s e n s i t i v i t y t o ^ "-rays and the a b i l i t y t o vary the gas pressure without a f f e c t i n g the pulse r i s e time are further advantages. The basic d i f f i c u l t y i n operating such a counter l i e s i n the presence 12 of r e c o i l protons and C n u c l e i from the waveshifter. In p r i n c i p l e these can be eliminated by operating the counter at two He pressures then subtracting the low pressure spectrum from the high pressure one. The background pulses plus the photomultiplier noise w i l l be equal f o r both spectra and thus w i l l cancel. The subtraction process can succeed only i f a l l w a l l e f f e c t s and s p a t i a l d i s t r i b u t i o n e f f e c t s are absent from the r e c o i l pulses obtained. b) Neutron spectra obtained with a 5" diameter He plus Xe f i l l e d gas s c i n t i l l a t i o n counter The spectra shown i n Figure 12 were obtained by i r r a d i a t i n g a gas s c i n t i l l a t i o n counter with uncollimated 4.1 Mev neutrons obtained by bom-barding a 30 Kev t h i c k heavy-ice target with a t o t a l of 1 ,060 p. coulombs of 1 Mev deuterons accelerated by the Van de Graaff generator. I n i t i a l l y the counter was f i l l e d t o 39.5 psia He^ and 6 p s i a Xe (15$). Immediately a f t e r obtaining a number of low pressure spectra the Helium pressure was increased to 58.7 p s i a (10$ Xe). A preliminary c a l i b r a t i o n with the 5 .3 210 Mev Po oc-source "on" established one point (Channel 86=5*3 Mev) on the energy scale. Assuming p r o p o r t i o n a l i t y , the points on an energy scale corresponding to the maximum energy of r e c o i l protons, Helium atoms, Carbon atoms and Oxygen atoms were found to be: H^ " max =4.1 Mev a Channel 66 He 4 max = 2 . 6 Mev = Channel 42 C 1 2 max = 1 # 2 Mev s Channel 18 O 1^ max =0.91 Mev a Channel 14 - 41 -I t can be seen from Figure 12 that the high energy t a i l caused by protons from the waveshifter l a y e r produce maximum pulses i n Channel 60 f o r both the high and the. low helium pressure. A s i m i l a r curve obtained with collimated neutrons showed the same maximum proton energy. The range of protons i n the Xe plus He mixture was calculated t o be~13g cm whereas the chamber length i s I65 cm, thus protons up t o Channel 66 should be observed. This discrepancy has not been resolved. The peak e f f e c t of the He^ " r e c o i l s i s expected i n Channel 42 whereas the sharp upper edge (knee) of the observed He^ " r e c o i l s i s i n Channel 37. I t i s of i n t e r e s t to note that i f a 0 .5 Mev energy intercept i s assumed as i n Northrop's paper, the knee of the He^ " r e c o i l s should occur i n Channel 38. 12 The expected r i s e of the curve at low energy from C r e c o i l s occurs i n a higher channel than predicted. This e f f e c t may be caused by a large number of low energy He^ r e c o i l s produced by w a l l e f f e c t s . The subtracted curve r e s u l t i n g from the two spectra i s also shown i n Figure 12. Neither proton subtraction nor carbon subtraction i s s a t i s -factory. A small s h i f t i n the observed Po*""^ pulse height, the lack of neutron c o l l i m a t i o n , probable wall e f f e c t s and the unexplained poor agree-12 ment i n the G. r e c o i l channels gave inconclusive r e s u l t s . c) Counter e f f i c i e n c y No measure of the neutron e f f i c i e n c y could be extracted from these preliminary r e s u l t s , however, considering a 20 cm long gas chamber and using a s c a t t e r i n g cross section of 3 barns f o r 4.1 Mev neutrons, an e f f i c i e n c y of 0.15$ per atmosphere of He^ w a s c a l c u l a t e d . d) Conclusion An attempt has been made to analyze the pulse spectrum produced i n the He plus Xe gas s c i n t i l l a t o r by an incident neutron beam of 4.1Mev energy. - 42 -12 The r e s u l t s show c l e a r l y . t h e presence of r e c o i l protons and C n u c e l i as w e l l as r e c o i l He^ " nuciki. but the maximum energy of the l a t t e r i s c l e a r l y v i s i b l e i n the pulse spectrum. Further work i s required to t e s t the p r o p o r t i o n a l i t y of pulse height with energy i n the counter before ac-curate neutron energy determinations are p o s s i b l e . Also gas mixtures of higher stopping power are needed t o t e s t the idea of subtracting a low He pressure spectrum from a high pressure spectramto f i n d the He^ " r e -c o i l spectrum alone. I f these d i f f i c u l t i e s can be overcome, the He gas s c i n t i l l a t o r should prove a valuable type of f a s t neutron detector. - 43 -CONSTRUCTION AND OPERATION OF AN-UNGRIDDED ELECTRON CHAMBER To obtain an accurate measurement of the thickness of c a l i b r a t i o n alpha-sources used with the gas s c i n t i l l a t i o n chambers, an a r g o n - f i l l e d ungridded el e c t r o n chamber was constructed. The chamber was designed to be r i g i d l y connected to a Dynatron type 1430-A pre-amplifier, s e r i a l 1208. a) The size of the chamber To f a c i l i t a t e construction, the chamber was c y l i n d r i c a l with the source c e n t r a l on one end and the c o l l e c t o r at the other. Five t h i n e q u i p o t e n t i a l rings were mounted c o a x i a l l y near the side walls t o insure a uniform f i e l d gradient throughout the s e n s i t i v e volume (see Pla t e 3:i). At least 20 p s i a argon i s used i n the chamber t o eliminate the inflow of contaminant r^gases, hence the required s i z e of the s e n s i t i v e volume i s determined by the range of *~5 Mev a l p h a - p a r t i c l e s i n Argon at 20 p s i a . From Figure 1 t h i s range i s 2.9 cm. The i n s i d e radius of the equ i p o t e n t i a l d i s c s was chosen large enough to allow a small decrease i n argon pressure without introducing w a l l e f f e c t s . An a d d i t i o n a l ^" was added to the O C -p a r t i c l e range f o r the distance from the end pl a t e t o the c o l l e c t o r , because the source-to-collector distance i s dependent on the thickness of the source mount. b) The mechanical construction of the chamber Four-inch diameter t h i n w a l l brass tubing was used f o r e x t e r i o r side walls, and i n t e r i o r dimensions were adjusted to allow a press f i t of the e q u i p o t e n t i a l r i n g assembly. E l e c t r i c a l contact with the wire from the Kovar s e a l was ob-tained by t i n n i n g both surfaces and heating the metal p l a t e from above when the un i t was i n p o s i t i o n . A s i m i l a r e l e c t r i c a l connection was made on the c o l l e c -t o r . Before placing the e q u i p o t e n t i a l assembly i n p o s i t i o n , a resistor-and-bypass-capacitor chain was soldered between the electrodes near t h e i r outer edge so that the f i e l d l i n e s i n the se n s i t i v e volume were not disturbed. - 44 -The ex t e r i o r part of the Kovar seals passed through holes cut i n the end of the Dynatron pre - a m p l i f i e r chassis and the chamber vas r i g i d l y -screwed onto the a m p l i f i e r chassis. In t h i s way the lead from the c o l l e c t o r t o the input was kept very short, c) The e l e c t r i c a l behaviour of the chamber The c o l l e c t o r was connected to the a m p l i f i e r input terminal and the equipotential lead to the terminal of the f i r s t capacitor of the dc l i n e f i l t e r . This procedure i s o l a t e d the c o l l e c t o r from the e q u i p o t e n t i a l p l a t e s , and also placed a 10 Megohm r e s i s t o r i n se r i e s with the bleeder r e s i s t o r s i n s i d e the chamber. Ten-Megohm bleeder r e s i s t o r s were chosen t o bias the equipotential d i s c s i n order t h a t only a small f r a c t i o n of the high tension voltage would be l o s t across the 10 Megohm r e s i s t o r i n the pre-amplifier. I t was found necessary to bypass the r e s i s t o r s with 0.02 /lfarad capacitors t o eliminate bleeder noise which was fed to the plate by capacity coupling within the chamber. With a t y p i c a l collimated alpha source mounted i n the chamber the c h a r a c t e r i s t i c s of the yoltage pulses from the Dynatron type 1430-A a m p l i f i e r are shown i n Table 3. ;;: Table 3 Pulse C h a r a c t e r i s t i c s Argon High Am p l i f i e r Pulse P u r i t y f ension Integration D i f f e r e n t i a t i o n Rise Time F a l l Time Gas flushed through chamber 4 times 500 v o l t s 0.08 usee 250 /zsec 2.0 jxsec 100 J&sec Gas flushed twice chamber d i r t y 500 v o l t s 0.08 jusec 250 jusec >50 jusec The a m p l i f i e r r i s e and f a l l times should be equal, and close t o the r i s e time of the input pulse from the ion chamber ( G i l l e s p i e 1953). The optimum - 45 -a v a i l a b l e a m p l i f i e r settings for use with t h i s chamber are thus: 3.2 jisec i n t e g r a t i o n time constant and £3.2 Jtisec d i f f e r e n t i a t i o n time constant. A m p l i f i e r pulses are fed through the i n v e r t e r t o the 100 channel C.D.C. ki c k s o r t e r to obtain the energy r e s o l u t i o n of the source. To properly a c t i v a t e the k i c k s o r t e r the pulses must r i s e i n l e s s t h a n 4/isec. This r e s t r i c t i o n i s met with the above a m p l i f i e r settings provided the argon i s not d i r t y . Increasing the high tension from 500 t o 1000 v o l t s caused a d i s t o r t i o n i n the pulse height d i s t r i b u t i o n probably due t o f r i n g i n g f i e l d s near the c o l l e c t o r . This e f f e c t i s shown i n Figure 6 . Because the chamber i s un-gridded, there w i l l be a low energy t a i l on a l l curves. An exact measure of t h i s e f f e c t has not been made, however i t adds l e s s t han one channel i n three t o the peak width at h a l f maximum. A more accurate r e s o l u t i o n may be obtained by drawing a curve symmetrical with respect t o the peak and the high energy t a i l as shown dotted i n Figure 6 . d> d) Summary The ungridded e l e c t r o n chamber described above may be used to measure ~ 5 Mev alpha source r e s o l u t i o n s to w i t h i n + 20$, -5$. Allowance must be made f o r the low energy t a i l due to the lack of a g r i d . The chamber i s f i l l e d by f l u s h i n g i t several times with tank argon, pumping i t down with a small rotary vacuum pump, and then f i l l i n g t o 7?20 p s i a argon. The pulse height should remain constant f o r sev e r a l hours, and the pulse r i s e time at the a m p l i f i e r output should be l e s s than 3.5 /isec with Dynatron a m p l i f i e r settings of 3.2 _usec and high t e n s i o n 500 v o l t s . Pulses may be displayed on the 100 channel C.D.C. k i c k s o r t e r t o obtain accurate r e s o l u t i o n measurements. BIBLIOGRAPHY P. A v i v i and S.G. Cohen, Phys. Rev. 108, 972 (1957). S.A. Baldin, V.V. G a b r i l o v s k i and F.E. Chukreev, Atomnaya Energiya 3_ 331 (1957). Batchelor, Aves, and Skyrme, Rev. S c i . I n s t r . 26, 1037 (1955). W.R. Bennett and C.S. Wu, Rev. S c i . I n s t r . 28, 1092-1099 (1957). M.A. Biondi and S.C. Brown, Phys. Rev. ^ 6, 1697 (1949) J M.A. Biondi and T. Holstein, Phys. Rev. 82, 962 (1951); M.A. Biondi, Phys. Rev. 83_, 1078 (1951). G.P. Boiccurt and J.E. B r o l l e y , J r . , Rev. S c i . I n s t r . 2J5, 1218 (1954). C. Eggler and C.M. Huddleston, Nucleonics 3A_, No. 4, 34 (1956). R.D. Evans, The Atomic Nucleus (McGraw H i l l Book Company, Inc. New York 1955) Ch. 26. M. Forte, Nuovo Cimento (Ser. 10), 3, 1443 (1956). A.B. G i l l e s p i e , S i g n a l , Noise and Resolution i n Nuclear Counter A m p l i f i e r s (Pergamon Press 1953) p. 16. W. H e i t l e r , The Quantum Theory of Radiation, Second E d i t i o n (Oxford U n i v e r s i t y Press, 1944) p. 107. A.C.G. M i t c h e l l and M.W. Zemansky, Resonance Radiation and Excited Atoms, (The MacMillan Company, New York, 1934) Ch. IV; T. H o l s t e i n , Phys. Rev. 22, 1212 (1947); T. Hol s t e i n , Phys. Rev. 83_, 1159 (1951). Morton, R.C.A. Review, 10. No. 4 (1949). C. Muehlhause, Phys. Rev. 31, 495 (1953) R.A. Nobles, Rev. S c i . I n s t r . 2J, 280 (1956). J.A. Northrop, Rev. S c i . I n s t r . 29_, 437 (1958). J.A. Northrop and Jud i t h Gursky, Nuclear Instruments 2, 207-212 (1958). J.A. Northrop and R. Nobles, Nucleonics 2k» N o» 4, 36 (1956). P.J. Pasma, Nuclear Physics 6 141-150 (1958). A.V. Phelps and J.P. Molnar, Phys. Rev. 89_, 1202 (1953). Richtmyer, Kennard and Lauritson, Introduction to Modern Physics (McGraw-H i l l Book Co. New York, 1955) p. 648. - 47 -B. Rossi and H.H. Staub, I o n i z a t i o n Chambers and Counters (McGraw-Hill Book Company, Inc., New York, 1949) p. 227. C. Rubbia and M. T o l l e r , Nuovo Cimento, 10, N2. 413-414 (1958). R.C. Sangster, Mass. Inst. Technol. Tech. Rpt. No. 55, (Jan 1, 1952). A. Sayres and C.S. Wu, Rev. S c i . I n s t r . 28, 758 (1957). J . Sharpe, Communications and E l e c t r o n i c s , Aug. 1957. J.A. Wheeler, Phys. Rev. L£, 258 (1933). 

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