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An attempt to trace ionizing particles in a gas chamber Madden, John Christopher Wyndham 1961

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AN ATTEMPT TO TRACE IONIZING PARTICLES IN A GAS CHAMBER by JOHN C. W. MADDEN B.A., The Un i v e r s i t y of B r i t i s h Columbia, 1959 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 to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1961 In presenting this thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this 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 financial gain shall not be allox^ed without my written permission. Department of P h y s i c s  The University of British Columbia, Vancouver 8, Canada. Date September, i960 ABSTRACT Experiments are described on the p o s s i b i l i t y of developing a gas luminescence chamber f o r the observation of p a r t i c l e tracks. A b r i e f review of devices c u r r e n t l y a v a i l a b l e f o r obtaining i o n i z i n g p a r t i c l e tracks, p a r t i -c u l a r l y those capable of time r e s o l u t i o n s i n the micro-second region, i s included. Attempts were made to detect s c i n t i l l a t i o n s i n several gas and vapour mixtures. For some gas mixtures the p o s s i b i l i t y of using i o n i z i n g r a d i a t i o n to i n i t i a t e a l i g h t producing chain reaction was investigated. These experiments were performed i n the temperature range from 25 to 220° centigrade. Results obtained were not of such a nature to encourage further work towards the development of a gas luminescence chamber. As a background of non-radiation induced photon emission was observed f o r several reactions, i t i s possible that the methods employed i n the experiment may be adopted to measure the onset and rate of some chemical re a c t i o n s . TABLE OF CONTENTS Page I. INTRODUCTION 1 I I . TRACK RECORDING DEVICES AVAILABLE . • 4 (a) Cloud Chamber 4 (b) Bubble Chamber 5 (c) Emulsions 7 (d) Spark Chamber 7 (e) Luminescence Chamber 15 (I) Image I n t e n s i f i e r Tubes ( i i ) The Homogeneous Chamber ( i i i ) The Filamentary Chamber (iv ) Experiments and New Developments I I I . THE RADIATION INDUCED CHEMILIMINESCENCE PROCESS 24 (a) The Gas S c i n t i l l a t i o n Chamber 24 (b) Possible Sequence f o r Radiation Induced Chemiluminescence . . 26 ( i ) Energy Transfer from P a r t i c l e to Gas Molecule ( i i ) (1) Fate of Ionized Molecules (2) De-excitation of Gas Molecules ( i i i ) Photon Capture ( i v ) Chain Reaction - The Carbon Monoxide -Oxygen Mixture i i i Page (1) Requirements f o r Chain Reaction (2) Impurity E f f e c t s (3) Light Output (4) Use of S e n s i t i z e r s (c) Previous Experiments on Radiation Induced Reactions 37 IV. EXPERIMENTAL APPARATUS 40 (a) Chamber Design 40 (b) Light Detection System 43 V. RESULTS (a) Waveshifter Vapours i n Helium, and Vapourized S c i n t i l l a t o r s 47 ( i ) Napthalene Vapour ( i i ) Helium ( i i i ) Helium Saturated with Anthracene Vapour (iv ) Helium - p-Quarterphenyl and Helium-diphenyl Stilbene Mixtures (b) Gas Mixtures 51 ( i ) 2C0 + 0 2 ( i i ) 2H 2 + 0 2 ( i i i ) H2'+ C l 2 ( i v ) C0 2 (v) Sodium Se n s i t i z e d Reactions ( v i ) Mercury Sensi t i z e d Reactions i v Page ( c ) The E f f e c t o f x - r a y s on Dark C u r r e n t . . . . 55 V I . CONCLUSIONS 56 APPENDIX - R a d i a t i o n L e n g t h 59 BIBLIOGRAPHY 60 v LIST OF FIGURES Figure f a c i n g page 1 S p a t i a l r e s o l u t i o n of the Spark Chamber . . 10 2 Diagram of Zavoisky luminescence camera . . 17 3 Light output of P 1 5 phosphor as a funct i o n of time 18 4 O p t i c a l r e l a t i o n s - homogeneous luminescent chamber 19 5a Range energy curves i n a C s l c r y s t a l . . . 20 5b Range - l i g h t y i e l d curves f o r CsI(Tl) . . 20 6 Light attenuation i n a p l a s t i c s c i n t i l l a t i n g f i b e r 22 7 Block diagram of chemiluminescent processes 27 8a Explosion peninsula f o r CO + 2O2 r e a c t i o n . 35 8b E f f e c t s of water vapour on CO + O2 r e a c t i o n 35 9 Photograph of apparatus 40 10 Diagram of chemiluminescence chamber . . . 41 11 C i r c u i t diagram - E.M.I. 9514S photomultiplier tube 44 12 Spectral response curve of E.M.I. 9514S photocathode 4 5 13 Polonium alpha p a r t i c l e spectrum i n napthalene vapour 47 14 Polonium alpha p a r t i c l e spectrum i n helium gas . 48 v i Figure f a c i n g page 15 Polonium alpha p a r t i c l e spectrum i n helium saturated with anthracene vapour 49 16 Light spectrum from carbon monoxide -oxygen mixture 50 17 Anode current vs. temperature 2C0 + 0 2 r e a c t i o n 51 18 Anode current vs. temperature 2H 2 + 0 2 r e a c t i o n 52 19 Anode current vs. temperature oxygen gas 53 20 Anode current vs. temperature H 2 + C l 2 ; H 2 + Na 54 LI3T OF TABLES Table Page 1 Properties of s c i n t i l l a t o r s 21 2 Comparison of nuclear t r a c k i n g devices, f a c i n g 23 3 Time scale of events, f a c i n g 31 v i i ACKNOWLEDGEMENTS The author wishes to thank Dr. J . B. Warren who suggested and supervised t h i s research. The many h e l p f u l suggestions supplied by Dr. B. L. White during the course of the experiment are g r a t e f u l l y acknowledged, as i s the assistance of Dr. K. L. Erdman whose help during the actual w r i t i n g of t h i s t h e s i s , when both Dr. Warren and Dr. White were absent, was invaluable. The author i s indebted to Mr. Douglas Stonebridge and others i n the Physics Department workshop f o r t h e i r f i n e workmanship and co-operation i n the construction of the apparatus. The author wishes to thank the National Research Council of Canada f o r the studentship which made t h i s research p o s s i b l e . v i i i I. INTRODUCTION In t h i s thesis a d e s c r i p t i o n of experiments con-ducted into the p o s s i b i l i t i e s of developing a gas luminescence chamber f o r the observation of p a r t i c l e tracks i s presented, along with a review of devices c u r r e n t l y a v a i l a b l e f o r obtaining i o n i z i n g p a r t i c l e tracks, with p a r t i c u l a r emphasis on those capable of time r e s o l u t i o n s i n the microsecond region. The l i g h t output from c e r t a i n vapourized s c i n t i l -l a t o r s and gas mixtures was observed over the temperature range 20 deg.-220 deg.C, and the dark current as a function of temperature was recorded. No r e s u l t s of a nature to encourage fu r t h e r work i n t h i s d i r e c t i o n were obtained. Of the track recording devices i n use today, the bubble chamber i s probably the most common. This device has several l i m i t a t i o n s . F i r s t l y , i t must be made s e n s i t i v e ( i . e . expanded) before the event of i n t e r e s t occurs, and hence cannot be triggered by the event i t s e l f . Secondly, the s e n s i t i v e time of the bubble chamber i s of the order of several m i l l i s e c o n d s . This not only makes coincidence work very d i f f i c u l t , but as i t has been found that more than twenty tracks i n each picture of a normal size device leads to confusion, there i s a severe l i m i t a t i o n on the allowable p a r t i c l e f l u x through the chamber, which i n turn d r a s t i c a l l y 2 reduces the p r o b a b i l i t y of the occurrence of rare nuclear events i n the p i c t u r e s . For these reasons, considerable e f f o r t has been expended i n an e f f o r t to develop a track reproducing device with time r e s o l u t i o n i n the microsecond rather than the mi l l i s e c o n d region. The gas luminescence chamber i s just one of the several paths followed i n t h i s endeavour. * * * * * * When i o n i z i n g r a d i a t i o n passes through c e r t a i n gases, some gas molecules are excited and subsequently emit u l t r a v i o l e t l i g h t . In a gas s c i n t i l l a t i o n counter the l i g h t emitted i n the track of an i o n i z i n g p a r t i c l e i s transformed int o an e l e c t r i c a l pulse and amplified i n a photo-multiplier tube. Such a device gives no i n d i c a t i o n of the s p a t i a l co-ordinates of the r a d i a t i o n , except that i t must have passed within the dimensions of the gas container. It was the purpose of the experiment undertaken to f i n d a mixture of gases which would y i e l d s u f f i c i e n t l i g h t along the path of an i o n i z i n g p a r t i c l e to make a picture of i t s track p o s s i b l e . I t i s u n l i k e l y that there w i l l be any s i g n i f i c a n t improvement i n photon y i e l d from gas mixtures through increased s c i n t i l l a t i o n e f f i c i e n c y alone. Most gases require the expenditure of about 29 e l e c t i o n v o l t s to ionize an atom, and no gas has yet been found which required l e s s than 20 e l e c t r o n v o l t s . While e x c i t a t i o n p o t e n t i a l s are of course lower than i o n i z a t i o n p o t e n t i a l s , the dif f e r e n c e i s not so great that s i g n i f i c a n t improvement i n s c i n t i l l a t i o n e f f i c i e n c y can he expected to be uncovered. ( I t i s worth noting that some s o l i d and l i q u i d substances are much more e f f i c i e n t than gases f o r ion p a i r production. The new s o l i d state devices require the expenditure of only 4 e.v. per ion pa i r . ) One method of increasing the photon y i e l d would be to f i n d a chemical chain r e a c t i o n which could be i n i t i a t e d by i o n i z i n g r a d i a t i o n , and which would culminate i n photon emission, the energy f o r photon m u l t i p l i c a t i o n being supplied by the chlmical r e a c t i o n . I f such a re a c t i o n were found, one would expect, f o r reasons discussed i n section I I , that time r e s o l u t i o n f o r the passage of the p a r t i c l e through the gas, of the order of lf|~k seconds could be obtained. I t i s t h i s feature which would make such a chamber, which could be c a l l e d a chemi-luminescence chamber, a useful t o o l f o r high energy physics. The devices presently a v a i l a b l e f o r t r a c i n g i o n i z i n g p a r t i c l e s w i l l be discussed below. It w i l l be seen fe that except f o r the l a s t two mentioned, i . e . the spark chamber and the luminescent chamber, the time r e s o l u t i o n i s such as to make coincidence experiments inconclusive, and to p r o h i b i t the use of high beam f l u x e s . 4-I I . TRACK' RECORDING DEVICES AVAILABLE (a) Cloud Chamber Developed by C.T.R. Wilson i n 1 9 1 2 , the cloud chamber i s the c l a s s i c way of t r a c i n g i o n i z i n g r a d i a t i o n , p a r t i c u l a r l y cosmic rays. I t works on the p r i n c i p l e that i n a super-saturated gas-vapour mixture, the vapour i s more l i k e l y to condense on charged than uncharged p a r t i c l e s , hence a vapour t r a i l forms i n the wake of an i o n i z i n g p a r t i c l e . The super saturation condition i s u s u a l l y obtained by a-d i a b a t i c expansion of the active volume. For most p r a c t i c a l cases the s e n s i t i v e time i s of the order of 1/50 s e c , the l i m i t a t i o n being the heat con-duction from the walls and the re s u l t a n t compression of the gas. A complete operational c y c l e , that i s removal of c l e a r i n g f i e l d , expansion, i l l u m i n a t i o n , admission of p a r t i c l e s , photographic recording, and the r e s e t t i n g of apparatus requires one or two seconds. The r e p e t i t i o n rate of expansions, which i s dependent on the time required to e s t a b l i s h thermal equilibrium i s u s u a l l y not more than two or three per minute. The chamber can be triggered e i t h e r by a regular clock pulse, or by the p a r t i c l e i t s e l f . In the l a t t e r method of operation the track i s more d i s t o r t e d due to the turbulence of the expansion process, which of course takes place a f t e r 5 the p a r t i c l e has passed through. Continuously s e n s i t i v e cloud chambers, c a l l e d d i f f u s i o n cloud chambers have been developed, and these have proved very usef u l i n high energy physics, p a r t i c u l a r l y before the advent of the bubble chamber, which brought with i t s i g n i f i c a n t improvements i n track stopping power a v a i l a b l e , as well as i n time and s p a t i a l r e s o l u t i o n . (b) Bubble Chamber Tracks i n bubble chambers appear as a r e s u l t of the growth of bubbles on n u c l e i excited along the paths of i o n i z i n g p a r t i c l e s i n that time during which the l i q u i d remains i n the unstable, superheated s t a t e . The i n t e r v a l of s e n s i t i v i t y i s u s u a l l y of the order of 20 m i l l i s e c o n d s . Most bubble chambers constructed to date have a r e p e t i t i o n rate of from two to twenty pulses per second. This i s comparable to the pulse rate of the beam from most high energy machines. Several d i f f e r e n t l i q u i d s have been used i n bubble chambers, some of the more common being hydrogen, deuterium, helium, propane, Preon 13 B 1 (CF^Br) and Xenon. Hydrogen chambers, so f a r , have proved most us e f u l from the standpoint of nuclear physics. Their b i g advantage i s that i n t e r p r e t a t i o n of events from a simple proton target i s r e l a t i v e l y uncomplicated, and e a s i l y r e l a t e d to theory. 6 Helium chambers are mainly of i n t e r e s t because the spin and i s o t o p i c spin of the helium nucleus are zero, hence i t i s possible to derive arguments concerning the p a r i t y , spin and i s o t o p i c spin r e l a t i o n s h i p s among p a r t i c l e s i n simple reactions (Glaser, I960). Both these l i q u i d s being l i g h t have a long r a d i a t i o n length (see Appendix) ( f o r hydrogen, 1145 cm) with the r e s u l t that there i s a r e l a t i v e l y small p r o b a b i l i t y of seeing a rare event, as high energy p a r t i c l e s pass through the chamber with a r e l a t i v e l y small energy l o s s . Herein l i e s one of the b i g advantages of the xenon chamber, f o r l i q u i d xenon has a r a d i a t i o n length of only 3*9 cm. This l i q u i d though has the drawback that momentum c a l c u l a t i o n s are subject to large e r r o r due to Coulomb s c a t t e r i n g ( f o r a r e l a t i v i s t i c p a r t i c l e i n a f i e l d of 20 kilogauss the error i s 24% compared to 1.4% f o r hydrogen). Further, because of the complexity of the xenon nucleus, i n t e r p r e t a t i o n of reactions i s very d i f f i c u l t . Propane and Freon-13 B 1 have properties i n t e r -mediate to the two extremes of hydrogen and xenon. Propane a c t u a l l y has a density of hydrogen n u c l e i about 1 .5 times that of l i q u i d hydrogen. It has the added convenience of being cheap, and operable i n the temperature range 50-60 degrees C. Xenon, at a cost of $10,000 per l i q u i d l i t r e , i s almost p r o h i b i t i v e l y expensive. The main disadvantages of bubble chambers i n general are f i r s t l y that the maximum time r e s o l u t i o n i s of the order of one millesecond, secondly that the s e n s i t i v e time i s such that i n many app l i c a t i o n s the photographs show a huge shower of p a r t i c l e s which e f f e c t i v e l y obscures i n d i v i d u a l events, and t h i r d l y that the chamber cannot be trigg e r e d by the event to be photographed, but must be sen-s i t i z e d before the event occurs. This l a t t e r shortcoming leads to the i n e v i t a b l e accumulation of large numbers of pi c t u r e s of no i n t e r e s t . (c) Emulsions nuclear r a d i a t i o n used. Discrete tracks were f i r s t observed by Reiganum i n 1911. In more recent years emulsions have proved u s e f u l f o r observing such events as pion decays, but because the emulsion contains several d i f f e r e n t r e l a t i v e l y complex n u c l e i , s c a t t e r i n g processes- are very hard to analyze. S p a t i a l r e s o l u t i o n i n emulsions i s e a s i l y the best obtainable (see Table 2 ) , but of the p a r t i c l e t r a c i n g techniques d i s -cussed, th|| time r e s o l u t i o n i s e a s i l y the worst, being of the order of seconds. As i s to be expected with a technique with such poor time r e s o l u t i o n , the emulsion i s l i a b l e to be c l u t t e r e d with a shower of tracks, of which nearly a l l are of no i n t e r e s t to the experimenter. (d) Spark Chamber Photographic emulsions were the f i r s t detectors of The spark chamber f i n d s i t s antecedent i n the spark 8 counter, which Roberts (1961) defines as any arrangement of electrodes other than that of the Geiger-Mueller counter, operated i n the discharge region. With suitable geometries, rise times of the order of 10 nanoseconds navel been obtained. This figure has since been improved to 0.15 nanoseconds (Zavoisky and Smolkin, 1957)* Keuffel (19^9) was the f i r s t to observe that the ^p-scharge between parall e l plates caused by a fast particle was located along the path of that particle, though at the time there had already been some considerable investigation of spark counters. Though some development work on the spark chamber (as distinct from the spark counter) took place from 1955 on in Germany, this largely passed unnoticed. It was Cranshaw and DeBeer (1957) who f i r s t saw the possible applications of such a chamber to high energy physics, and who introduced the idea of pulsing the high voltage, leaving only a small clearing potential on the plates to remove ions between pulses. Investigators found however that they were unable to record more than one track at a time. This c r i t i c a l barrier was hurdled by Fukui and Miyamoto (1959) who introduced noble gas f i l l i n g s and were able to show that with such f i l l i n g s , several tracks could be recorded simultaneously. They were the f i r s t to envisage the spark chamber as a track delineating, rather than a track sampling device. The spark chamber i s s t i l l i n a very e a r l y stage of (rapid) development, so that i t i s hard to describe a " t y p i c a l chamber". The chamber of Meyer and T e r w i l l i g e r (1961) at Michigan w i l l serve at le a s t as a working example. It c o n s i s t s of a set of p a r a l l e l electrodes made of 0.012 inch aluminum. These are supported and ins u l a t e d from each other by l u c i t e frames. A l l are bonded together with epoxy r e s i n to make vacuum t i g h t j o i n t s . The 3/8 inch space between electrodes was f i l l e d with neon gas. The active dimensions of the chamber were 14 x 14 x 7 inches. Alternate p l a t e s of the chamber were pulsed from a supply run at from 8-15 kv., with a pulse length of about # —8 Op sec, and a r i s e time of about 5 x 10" sec. The plates were pulsed from 0.2 to 0 .3 ju sec a f t e r the i o n i z i n g p a r t i c l e had passed through the chamber. Tri g g e r i n g was from s c i n t i l -l a t i o n counters. Stereo p i c t u r e s were taken of each event. Several gas f i l l i n g s have been t r i e d by experi-menters to date* It was not found possible to obtain more than one track at a time i n the electronegative gases t r i e d (N£» C0£, a i r ) , but successful r e s u l t s have been obtained with most of the i n e r t gases, the most common being helium, neon and argon, and with various mixtures of these gases. In some instances the mixtures were saturated with a l c o h o l . Sparking e f f i c i e n c i e s better than 99% appear easy to obtain. Rutherglen and Paterson (1961) have probably o o .25 .50 .75 8 m m .0 1.25 .50 1.75 f i g . l The percentage of sparks with deviations less than 8 plotted as a function of 8 in m.m.. The three curves represent the following angular intervals: a, 0 - 15; b, 15 - 30; c, 30 - 45. (Rutherglen and Paterson, 1961) publ i shed the most complete s t a t i s t i c s on space r e s o l u t i o n . They used a chamber with a s e n s i t i v e area 6 x 6 i n , and an e lectrode spacing of 1/4 i n c h . A gas mixture of 25% argon and 75% hel ium was used. The device was t r i g g e r e d by cosmic r a y s , a pulse voltage of 11 kv being app l i ed a f t e r a time de lay of 0.25 / i sec . There was no c l e a r i n g f i e l d . A graph of t h e i r r e s u l t s may be seen i n P i g . 1. I t w i l l be no t i ced that accuracy decreases as t rack angle r e l a t i v e to the p l a t e s increases , but even i n the range 30 degrees to 45 degrees, 80% of the sparks have a d e v i a t i o n of l e s s than 1 m.m. These r e s u l t s are roughly born out by other i n v e s t i -gators . The time r e s o l u t i o n , though not as good as s c i n t i l -l a t i o n counters , i s f a r be t t er than bubble chambers. I t appears to be a r e l a t i v e l y simple matter to s e l ec t one p a r t i c l e track from a f l u x of 10^ p a r t i c l e s per second. At higher f l u x d e n s i t i e s mul t ip l e t racks are to be expected. F i s c h e r and Zorn (1961) have done a t h e o r e t i c a l c a l c u l a t i o n of the spark formation t ime, based on a r e l a t i o n publ i shed by Dickey i n 1952. This agrees we l l with experimental r e s u l t s obta ined. The time f o r spark formation i s : -us ing the f o l l o w i n g equivalent c i r c u i t : R N A A A A A A where I . i s the current supplied by external c i r c u i t , neglecting inductance C i s the capacitance of the chamber No i s the number of free electrons i n the gap L i s the i o n i z a t i o n constant i n ion pairs/cm v i s the average d r i f t v e l o c i t y i n cm/sec In the experiment performed I _ v + . 5 amp; R=20 ohms, e x T 7 C=30 pp.f and S= 0 . 6 6 cm. The following general observations from experi-mental r e s u l t s to date were made by Fischer and Zorn:-(1) Spark formation time decreases r a p i d l y over several orders of magnitude with increasing voltages, and i s d i f -ferent f o r each gas. ( 2 ) The change i n voltage required f o r a given spark form-ation time i s roughly proportional to the gap length. ( 3 ) The a d d i t i o n of alcohol r e s u l t s i n curves which are displaced towards shorter times f o r argon and towards longer times f o r helium. As might be expected, the deadtime i s proportional to the c l e a r i n g f i e l d and to the negative ion m o b i l i t y . Hence, electronegative gases, with correspondingly low nega-t i v e ion m o b i l i t y , appear useless, as gases f o r high beam f l u x spark chambers, though they may f i n d use i n cosmic ray work. Because the negative ions i n i n e r t gases are electrons, which of course have high m o b i l i t y , very high r e p e t i t i o n rates i n i n e r t gas chambers are p o s s i b l e . 12 Roberts (1961) has stated that a deadtime, that i s , the time between the recording of one event and the time when the chamber i s ready to record the next, of the order of 10 milliseconds i s quite reasonable. This would allow about 20 pic t u r e s per pulse from an accelerator with a pulse duration of 200 m i l l i s e c o n d s . A l l experimenters have reported that the device appears to be i n s e n s i t i v e to the type or condition of the electrodes used, nor does gas p u r i t y appear to be s i g n i f i c a n t . The development of spark gaps i n which the spark follows the t r a j e c t o r y of the spark f o r large angles has been a matter of some concern. This aspect has been quite thoroughly i n v e s t i -gated i n the USSR. ( A l i k l i a n i a n and Kozodaev, I960) as well as by others i n the United States and B r i t a i n (see F i g . 1). The Russians found that i n spark gaps with electrodes spaced 30 mm apart and f i l l e d with argon at a pressure of from 0 . 5 to 1. atmosphere, the spark follows the d i r e c t i o n of r e l a t i -v i s t i c i o n i z i n g p a r t i c l e s up to an angle of 35 degrees or 40 degrees provided the high voltage pulse r i s e i s steep (0.2 u sec). Fukui and Miyamoto (1959) were able to obtain sparks f o r i o n i z i n g p a r t i c l e s t r a v e l l i n g p a r a l l e l to the electrodes, but t h i s of course only provides two co-ordinates of the p a r t i c l e l o c a t i o n . The p o s s i b i l i t y of determining the momentum of the i o n i z i n g p a r t i c l e s by p l a c i n g the chamber i n an analysing magnet has been investigated in several quarters. Aliklianian and Kozodaev (I960) make reference to a paper by Daion et a l . submitted to the Journal of Experimental Apparatus and Techniques (unavailable to author) in which measurements of particle momentums of 10" ev/c were made with "good precision". Cork (1961) reported a displacement of the sparks in the magnetic f i e l d proportional to (ExH)t where E i s the clearing f i e l d , and t i s the delay time before the high voltage i s applied, ("x" signifies a vector product.) O'Nie'll (1961) has measured 1 Bev/c tracks to 1% precision i n f i e l d s of 18 Kilogauss. He observed no spark displacement, though he makes no mention of the clearing f i e l d used. A wire chamber has been developed at Argonne (Romanowski, 1961). This digitizes the track i n two dimen-sions, as opposed to the plate chambers which digitizes i t in only one. While the d i g i t i z i n g imposes a minimum error on path track, i t i s thought the device w i l l be useful for recording curved trajectories i n a magnetic f i e l d . Attempts are at present being made by Lederman (1961) and by Pukui et a l . (I960) to confine the breakdown to the region of original ionization by using microwave, rather than D.C. voltages. So far success has been very limited. , 14 The spark chamber i s i n such an e a r l y stage of development that few experiments have been done. Lederman (1961) and a group at CERN (Cocconi, 1961) are both interested i n the chamber as a means of neutrino detection. The CERN group plan to use i r o n electrodes 20 cm t h i c k , with a t o t a l weight of i r o n of about 40 tons. In a discussion on the paper by Roberts (1961), Cork mentioned two experiments performed at Berkeley. The f i r s t was a study of the e l a s t i c s c a t t e r i n g of p o l a r i z e d protons (by the graphite e l e c t r o d e s ) , the second was a measurement of the angular d i s t r i b u t i o n of e l a s t i c s c a t t e r i n g of K mesons by protons. In the same discussion Cronin remarked that the Princeton group were using two spark chambers to measure the spin-spin c o r r e l a t i o n s i n proton-proton s c a t t e r i n g . Data handling appears to be r e l a t i v e l y much easier than f o r the cloud chamber. Cork reported that d i v i n i t y students were doing most of the scanning of the pi c t u r e s he had taken! Roberts (1961-b) ind i c a t e s that i t would be quite f e a s i b l e to t r a n s f e r the data at the rate of 8,000,000 tracks a day to magnetic tape. This rate assumes spark chamber deadtimes of 5-10 m. sec, and the use of beam pulses pre-sently a v a i l a b l e . The d i f f i c u l t y i s to f i n d a computer that can analyse with t h i s speed. Roberts points out, however, that a system capable of handling 1 0 , 0 0 0 , 50,000 or 500,000 events a day w i l l permit the design of experiments quite impossible today. One f i n a l , non-technical, but h i g h l y p r a c t i c a l comment on the spark chamber. It i s extremely cheap and easy to b u i l d . I t h a s ^ y e n been remarked that anyone could b u i l d one i n h i s own basement. (e) Luminescence Chamber Luminescence chambers can generally be broken into two main types. The f i r s t i s the homogeneous chamber which employs a s o l i d chunk of s c i n t i l l a t o r , u s u a l l y Nal (TI) or C s l (TI); the other, c a l l e d a filamentary chamber employs long t h i n rods or filaments of p l a s t i c s c i n t i l l a t o r stacked, u s u a l l y with alternate layers perpendicular to each other, to form a chamber of the desired volume. The r e s t of the equipment i s common to both types, and consists of an image i n t e n s i f i e r system — comprising one or more tuves, a camera, a high voltage pulsed supply f o r the tubes, and suitable gates and t r i g g e r i n g c i r c u i t s . In t h e i r present state of development, luminescence chambers have time and space r e s o l u t i o n very s i m i l a r to the spark chambers. The homogeneous luminescent chamber though should u l t i m a t e l y have better s p a t i a l r e s o l u t i o n as i t i s not d i g i t i z e d i n any d i r e c t i o n . 16 Fundamental to the operation of the luminescence chamber, i s the image i n t e n s i f i e r tube, which i s s t i l l very much i n the developmental stage. These tubes are fundamental to the poss i b l e development of a chemiluminescence chamber as w e l l , so they w i l l be treated i n some d e t a i l here. ( i ) Image I n t e n s i f i e r Tubes The gain requirements f o r such a tube i n order that reasonable r e s o l u t i o n be obtainable from weak photon sources i s e a s i l y c a l c u l a t e d (Jones and P e r l , 1959) . Com-me r c i a l l y a v a i l a b l e " f a s t " f i l m , such as Kodak Royal-X Pan 8 ° requires about 2 x 10 photons (near 4400 A) per square centimetre of emulsion f o r a v i s u a l l y detectable developed density increase ( 0 . 1 to 0 . 2 ) . I f a single photon i s to be resolved i n 10 ' or -4 10 square cm. on the f i l m , and the e f f i c i e n c y of the photocathode i s 20%, an image i n t e n s i f i c a t i o n of at l e a s t 4 5 10 i s required; a gain of 10^ appears to be a more p r a c t i -cable minimum. The e a r l y development of the image i n t e n s i f i e r tube was performed i n the USSR, p r i n c i p a l l y by E. K. Zavoisky (1957) . A diagram of the Russian tube i s shown i n F i g . 2 . This shows the i n i t i a l stage and several l a t e r - amplifying stages, o p t i c a l l y connected through t h i n transparent membrane s. PM PM D C S Leu /in u/u * At* r S G -AAAAAAA-PG--AAAA- -vAAA-PG -2 PG- 3 fig.2 Luminescence Camera; X - crystal s c i n t i l l a t o r ; 0 - objective lens; C - electron-opticilf! converter; D - discriminator; CS - coincidence system; Ph - photographic camera. 1 - flourescent screen; 2 - photocathode; 3 - solenoid; 4 - diaphragm; 5 - focussing electrode; 6 - deflection plates; 7,8 - electronic pulse shutters. PG-1 - Pulse generator for the shutter. PG-2 High voltage pulse generator. PG-3 Pulse generator governing the camera. SG- Scanning generator. (Zavoisky et a l . , 1957) 17 The membranes have a fluorescent screen (1) on one side and a photo cathode (2) on the other. Focusing of the el e c t r o n image i n the cascades i s achieved by the homo-geneous magnetic f i e l d of the solenoid (3)» In the entrance chamber, focusing i s per-formed e l e c t r o s t a t i c a l l y . The photo cathode (2) and the diaphragm (4), at appropriate p o t e n t i a l s , form an e l e c t r o s t a t i c lens. Electrode (5) i s used f o r f i n e r focusing of the e l e c t r o n image on screen ( 1 ) . Two p a i r s of d e f l e c t i o n plates (6), placed between the diaphragm and screen, are f o r high frequency scanning. Plates (7) and (8; form a pulsed e l e c t r o n i c shutter. (Zavoisky et a l . , 1957) This tube was capable of r e g i s t e r i n g photo-g r a p h i c a l l y single electrons emitted from the f i r s t photo-cathode, though i t has the disadvantage of having a cathode diameter of only 0.5 cm. Development of image i n t e n s i f i e r s has been somewhat slower outside Russia. Most of the e a r l y tubes used e l e c t r o -s t a t i c focusing and had r e l a t i v e l y small quantum gain, n e c e s s i t a t i n g the use of several tubes i n s e r i e s . Recently developed types, p a r t i c u l a r l y i n England (Wilcock et a l . , I960), appear to equal the Russian tubes i n performance. The En g l i s h tube employs only one photocathode, and uses potas-sium chloride f i l m s as transmitted secondary el e c t r o n m u l t i p l y i n g dynodes. The slow electrons emitted from one dynode have energies s u f f i c i e n t l y homogeneous f o r magnetic focusing. One of the l i m i t i n g f a c t o r s i n the time r e s o l u t i o n ] I I I I I I I L J I I I I I I L O I 2 3 4 5 6 7 8 9 IO II 12 13 14 15 16 17 TIME (MICROSECONOS) fig.3 Integrated light output from P15 phosphor as a function of time, with the Nai (Tl) decay folded i n . The curve is normalized to unity at i n f i n i t e time, assuming the asymptotic behaviour is exponential with a 14 microsecond time constant. 18 of the luminescent chamber i s set by the image tube i t s e l f , or, more s p e c i f i c a l l y , in the re l a t i v e l y long Jielay time of the reasonably e f f i c i e n t phosphors required on the f i r s t photocathode. A graph of the decay curve of P15, the sub-stance usually used on the f i r s t photocathode i s shown in f i g . 3 . In order to achieve reasonable time resolution, the f i r s t stage i s usually gated off after one or two micro-seconds, thus only a relatively small percentage of the total emission i s used. The photon to photo-electron conversion efficiency i s about 20%. A rel a t i v e l y new development in this f i e l d i s that of channelled image intensifiers (Burns and Neumann, i 9 6 0 ) . Such a tube^jjis made up of a large number of very small, independent channels, each acting as an individual photo-multiplier for i t s picture element. In this -system large gains can be obtained rel a t i v e l y easily by using a large number of dynodes. The ultimate limit in mesh size i s e s t i -mated to be 300 per linear inch, which corresponds to a resolution of s l i g h t l y less than |j»l millimetre. This moderate resolution would be very acceptable in applications where tubes with a large face area were required. The mesh proves useful in an unexpected fashion, for i t can be used to support glass tube faces very much thinner than could otherwise be used, thus cutting down on photon transmission loss into the tube. Work on this tube i s s t i l l proceeding. It i s not known when the f i r s t prototype w i l l be ready for use. HOMOGENEOUS C H A M B E R O P T I C S SCINTILLATING CRYSTAL n PHOTOELECTRONS PER cm. OF TRACK IN CRYSTAL SOLID A N G L E 2 SUBTENDED ft = BY L E N S d 2 FOR SPECIAL C A S E a N€ J_ 3 fig.4 Relationship between track resolution, a, depth of f i e l d , d, and track information, n, for the homogeneous luminescent chamber. (Perl, Jones and Lai, 1960) Image tubes have so f a r found a p p l i c a t i o n i n only two types of chamber, the homogeneous c r y s t a l and the f i l a -mentary p l a s t i c luminescent chambers. The advantages of the former are found mainly i n the l a r g e r number of photons per centimetre of track (about 15 times that of p l a s t i c ) , and the lack of d i s t o r t i o n problems within the chamber. The disadvantages are associated with the slow decay time of the s c i n t i l l a t o r , the l i m i t a t i o n on size of f i e l d and depth of focus imposed by the necessity of a coupling lens, and the complex nuclear structure of the s c i n t i l l a t o r . (Reynolds, I960) ( i i ) The Homogeneous Chamber The homogeneous chamber at t h i s time, i s the furthest developed, and has been the f i r s t to be used experi-mentally. P e r l and Jones have been responsible f o r most of the development i n t h i s area. They used sodium iodide c r y s t a l s i n glass encased b r i c k s 2x2x4 inches. In table I the properties of various s c i n t i l l a t o r s can be seen. Caesium iodide c r y s t a l s at a temperature of 77 degrees K. should be more e f f i c i e n t by a f a c t o r ten than sodium iodide (Jones and P e r l , 1959) but to date there has been l i t t l e work done with t h i s m a t e r i a l . Though Zavoisky et a l . make considerable mention of caesium iodide i n t h e i r 1957 paper, there i s no report a v a i l a b l e of t h e i r more recent i n v e s t i g a t i o n s . A diagram of the r e l a t i o n s h i p s between track 10 > ui 1 10' 10 >-CD cr UJ z: UJ 10" a- particle D e u t e r o n - ^ y -Proton 7r-meson 10 10' 10 10' R A N G E fig.5 (a) Range - energy curves in a Csl crystal. icr mm e o M C o o c a 10' UJ r -X 10 10 s a-particle Tr -meson—• — Proton IO" 2 10" 10' I 10 R A N G E fig.5 (b) Range - light yield curves for Csl (Tl). 10° mm 20 TABLE I PROPERTIES OF LUMINESCENT CHAMBER SCINTILLATORS (Pe r l and Jones, I960) S c i n t i l l a t o r ' Particle-producing track N, number of photons produced per cm of track i n s c i n t i l l a t o r P l a s t i c at room temperature Anthracene at room temperature Sodium Iodide at room temperature Minimum-ionizing p a r t i c l e 100-Mev proton 10-Mev proton Minimum-ionizing p a r t i c l e Minimum-ionizing p a r t i c l e 100-Mev proton 10-Mev proton 2 0 , 0 0 0 70,000 350,000 70,000 110,000 350,000 1 , 5 0 0 , 0 0 0 r e s o l u t i o n , depth of f i e l d and track information i s shown i n figure 4. P e r l and Jones have obtained r e s o l u t i o n s of 1-3 mm. i n sodium iodide c r y s t a l s up to four inches t h i c k . In f i g u r e s 5(a) and 5(b) are shown the range -energy and range - l i g h t - y i e l d curves of C s l ( T l ) as c a l -culated by Zavoisky et a l . It can be seen that the stopping power i s s u f f i c i e n t to permit the use of f a i r l y small cry-stale i n many experiments. ( i i i ) The Filamentary Chamber In the p l a s t i c materials used i n filamentary chambers, minimum i o n i z i n g p a r t i c l e s lose about 2 Mev per centimetre of path length, but the energy d i s s i p a t i o n can 2 1 e a s i l y be greatly increased by i n t e r s p e r s i n g t h i n plates of material of large stopping power with layers of filaments. A t y p i c a l chamber, as described by Lande et a l . has sides 5 cm. by 1 0 cm. and consists of alternate layers of 0 . 5 mm filaments and 0 . 5 mm t h i c k lead p l a t e s . In the filamentary chamber the l i g h t emitted by the i o n i z i n g p a r t i c l e i n a given filament i s piped to the end of that filament by i n t e r n a l r e f l e c t i o n . Stereo p i c t u r e s can be obtained by s e t t i n g alternate layers of filaments at r i g h t angles. Although a minimum i o n i z i n g p a r t i c l e can be expected to y i e l d only about 1 0 0 photons at the end of a filament 0 . 7 mm i n diameter, (Reynolds, 1 9 6 1 ) the problems of coupling to the image tube are much more e a s i l y met than f o r the homo-geneous chamber. At present much of the photon loss takes place i n the glass window of the image tube, and great e f f o r t has been expended to make t h i n windows of reasonable d i a -meter which w i l l withstand atmospheric pressure. The p l a s t i c f i b r e s at present are u s u a l l y coupled to the tube through a high speed lens or d i r e c t l y to the tube face. A tube face c o n s i s t i n g of glass l i g h t - p i p e f i b r e s i s now under develop-ment, and when completed, should b r i n g about a considerable improvement i n l i g h t gathering e f f i c i e n c y . The r e s o l u t i o n of the filamentary chambers i s l i m i t e d by the diameter of filament employed (at l e a s t down' to filament diameters of 0 . 5 mm). Because the l i g h t i s piped s t r a i g h t out, there i s no depth of f i e l d problem as 0.02 o.oi L I I I I 0 10 20 30 40 50 FIBER LENGTH (cm.) fig.6 The attenuation i n a 0.05 cm. s c i n t i l l a t i n g fiber. Internal re f l e c t i v i t y was taken to be 6.993 and the absorption constant was 0. 015 cm" . The index of refraction of the fiber was 1.58 and the surround was a i r . (Potter and Hopkins, 1960) there i s with the homogeneous chamber. The maximum depth of the chamber i s l i m i t e d by the photon at tenuat ion down the length of the f i l ament . (See f i g . 6.) A great advantage of the p l a s t i c s c i n t i l l a t o r over the Nai ( T l ) c r y s t a l i s that the former has a f a i r l y high hydrogen content , and i s composed of simple atoms (C and H) making a n a l y s i s of t racks ra ther e a s i e r than i n the l a t t e r . F u r t h e r , i t s decay t ime, being of the order of m i l l i m i c r o -seconds i s more su i ted to coincidence work. Momentum a n a l y s i s of p a r t i c l e s i n luminescent chambers i s d i f f i c u l t due to extreme s e n s i t i v i t y of the image i n t e n s i f i e r tubes to magnetic f i e l d s . A time of f l i g h t method of measuring p a r t i c l e v e l o c i t y (app l i cab le to a f i l a -mentary chamber) has been devised by Lande et a l . (I960-b) . ( i v ) Experiments and New Developments To date , P e r l et a l . (i960) have s u c c e s s f u l l y used a homogeneous chamber to perform experiments at Berkeley on e l a s t i c p ion-proton s c a t t e r i n g and on p ion product ion with slow proton r e c o i l s . Reynolds expects soon to use a f i l a -mentory chamber setup to study the r e a c t i o n 7 r + — TT° + e ++ v The rate of t h i s r e a c t i o n , compared with the more common 7r+ — - p.+ + v i s a s e n s i t i v e t e s t of theory and i s b e l i e v e d to be < I O - 8 Work has begun at the Imperia l Col lege i n London TABLE II COMPARISON OF TRACK RECORDING DEVICES Max. time S p a t i a l r e s o l u -Sensitive volume material Device r e s o l u t i o n (sec.) Counter control t i o n (mm) Pure H 2 Contain H 2 Dense high Z Luminescent chamber 10" 6-10 - 7 Yes 1. No? 1 Yes Yes Bubble chamber io-5 No 0.5 Yes Yes Yes Cloud chamber 1. Yes 0.5 Yes Yes 2 Yes^ Nuclear emulsion — No 0.001 No Yes Yes Complex s c i n t i l -l a t i o n counter array 3 x 10" 9 Yes 10 Yes Yes Yes Spark chamber 10 - 6-10 - 7 Yes 1. No Yes Yes 5 1 It may be possible to have l i q u i d hydrogen s c i n t i l l a t e by proper impurities a d d i t i o n . There are also no cl e a r data on the s c i n t i l l a t i o n properties of pure l i q u i d hydrogen. 2 Multiple plate chamber only. 3 High Z obtained by using heavy electrodes. Much of the information f o r t h i s table was obtained from P e r l and Jones, i960. 23 pn a Cerenkov chamber using freon as the r a d i a t o r . The l i g h t cone produced w i l l be r e f l e c t e d onto an image inten-s i f i e r , so that p a r t i c l e v e l o c i t y w i l l be obtainable d i r e c t l y from the radius of the r a d i a t i o n r i n g . L a s t l y i t i s worth mentioning the various complex s c i n t i l l a t i o n counter arrays which have been developed. These may be set up e i t h e r i n matrix form (Heer, I960) or as i n d i -v i d u a l s c i n t i l l a t i o n counters. The maximum s p a t i a l r e s o l u t i o n obtainable i s of the order of a centimetre, but the time r e s o l u t i o n i s of the order of 3 millimicroseconds. I t i s l i k e l y that developments i n complex s c i n t i l l a t i o n arrays and filamentary chambers w i l l eventually merge. A comparison of the properties of the track recording devices mentioned can be seen i n table I I . I I I . THE RADIATION INDUCED CHEMILUMINESCENCE PROCESS (a) The Gas S c i n t i l l a t i o n Chamber The s c i n t i l l a t i o n process i n gases has been reviewed i n some d e t a i l by G. A. Beer (1959) i n connection with gas s c i n t i l l a t i o n counters. The phenomenon can be broken into two parts. F i r s t l y , the e x c i t a t i o n and i o n i -zation produced by the passage of the charged p a r t i c l e s through the gas, and secondly the subsequent photon emission, which must compete with other forms of energy d i s s i p a t i o n . A charged p a r t i c l e t r a v e r s i n g a gas, leaves i t s energy i n the form of ions, n e u t r a l excited atoms, and atoms which are both ionized and excited. An estimate of the proportion of energy l o s t i n e x c i t a t i o n r e l a t i v e to i o n i -zation can be made i f WQ, the energy spent i n the formation of one ion p a i r , and£ , the i o n i z a t i o n p o t e n t i a l are known. The r e l a t i v e l o s s of energy by i o n i z a t i o n i s 7 7 , the remainder 0 _ comprises energy l o s s due to e x c i t a t i o n . For most gases approximately equal amounts of energy are expended i n the two processes. In the noble gases, de-excitation i s by photon emission. As neut r a l and s i n g l y ionized noble gases have a very high f i r s t excited state, of the order of two-thirds the i o n i z a t i o n p o t e n t i a l , and as the t r a n s i t i o n p r o b a b i l i t y of allowed t r a n s i t i o n s i s proportional to the energy differ e n c e between the two l e v e l s , most of the r a d i a t i o n 25 observed i s the r e s u l t of ground state t r a n s i t i o n s . This r a d i a t i o n l i e s well i n the u l t r a v i o l e t spectrum, hence most photomultipliers are i n s e n s i t i v e to i t . The usual p r a c t i c e f o r s c i n t i l l a t i o n counters, therefore i s to coat a l l walls of the container with a waveshifter which emits r a d i a t i o n i n the s e n s i t i v i t y range of the photomultiplier. p-Quarterphenyl and diphenyl s t i l b e n e are the waveshifters most commonly used (Northrup, 1 9 5 8 ) . The l i f e t i m e of the r a d i a t i o n from the noble gases i s t h e o r e t i c a l l y estimated to be of the order of a nano--9 second (10 y sec), but t h i s cannot be observed because of the phenomenon of resonance trapping. In t h i s process, the quanta are resonantly absorbed and re-emitted many times before reaching the detector. I f atoms of other molecules are present, they w i l l not, i n general, absorb the photon resonantly, and energy degradation takes place. It i s t h i s process which explains the extreme s e n s i t i v i t y of the gas counter to some contaminants. The pulse heights from the best gas counters com-pare quite favourably with s o l i d c r y s t a l s . Sayres and Wu (1957) give a r a t i o of pulse heights f o r C s l r e l a t i v e to a Helium + 10% Xenon gas mixture of 160 to 105* Further i t has been found that pulse height changes very l i t t l e over a wide range of gas pressures (up to 75 atmos) (Engelke, I960). Gas counters have a d i s t i n c t advantage over c r y s t a l 26 s c i n t i l l a t i o n counters f o r some ap p l i c a t i o n s because they are p r a c t i c a l l y i n s e n s i t i v e to gamma r a d i a t i o n . Sayres and Wu (1957) report a small e f f e c t due to y - r a y s , but experi-ments by the author have shown that t h i s could e a s i l y have been caused, at lea s t p a r t i a l l y , by gamma ray bombardment of the phototube photocathode. It i s possible that a gas s c i n t i l l a t i o n chamber could now be b u i l t using xenon at very high pressure (so as to achieve as many photons/cm as p o s s i b l e ) . At these high pressures, gas p u r i t y would become rather c r i t i c a l because of the large number of resonant absorptions and re-emissions involved. I t i s u n l i k e l y that such a chamber could make s i g n i f i c a n t improvements i n s p a t i a l r e s o l u t i o n over the s o l i d luminescent chamber, though time r e s o l u t i o n would be better, probably c l o s e r to the r e s o l u t i o n of the p l a s t i c filamentary chamber. (b) Possible Sequence f o r Radiation Induced Chemiluminescence Because i t looked very d i f f i c u l t to construct a useful noble gas s c i n t i l l a t i o n chamber at t h i s time, two d i f f e r e n t approaches were t r i e d . F i r s t l y , a b r i e f survey of the s c i n t i l l a t i o n properties of some of the conventional s o l i d s c i n t i l l a t o r s i n the vapour phase was conducted. Subsequent experiments were dire c t e d towards f i n d i n g a gas mixture from which photon m u l t i p l i c a t i o n could be obtained by a release of chemical energy. A complicated chain of E N E R G Y T R A N S F E R T O N E W M O U E C U L E A G A S M O L E C U L E S D E C O M P O S I T I O N T O F R E E R A D I C A L E X C I T E D G-AS M O L E C U L E S I O N S N E W I O N F O R M A T I O N S E N S I T I Z E R C H A I N R E A C T I O N N E U T R A L - I Z A T I O N E X C I T E D M O L E C U L E S E M I S S I O N O F E N E R G Y A S L U M I N E S C E N C E fig.7 Block diagram of processes which might lead to ionizing particle-induced chemiluminescent reactions. processes i s invo.lved i n t h i s conception, which w i l l be c a l l e d r a d i a t i o n - induced chemiluminescence (see f i g . 7). The t h e o r e t i c a l work that has been done on t h i s subject i s scanty, so that only a rather incomplete outline of the subject can be given here. The sequence of events w i l l be dealt with as fo l l o w s : -1) Energy t r a n s f e r from p a r t i c l e to gas molecule. 2) ( i ) Fate of ionized molecules, ( i i ) Fate of excited molecules. 3) Photon capture 4) Requirements f o r a chain r e a c t i o n . ( i ) Energy Transfer from P a r t i c l e to Gas Molecule It has already been mentioned that i o n i z i n g r a d i a t i o n passing through a gas, leaves not only ions, but excited molecules as w e l l . Pshezhetskii and Dmitriev (1958) state as a general p r i n c i p l e (not applicable to noble gases only), that on the average about equal amounts of energy are expended i n e x c i t a t i o n and i o n i z a t i o n . Since e x c i t a t i o n of molecules to energy l e v e l s l i k e l y to be e f f e c t i v e f o r further chemical r e a c t i o n requires from 1/2 to 1/3 the i o n i z a t i o n energy, about 2-3 times as many excited as ionized molecules are produced. Energetic secondary electrons from the i o n i z a t i o n process may cause further i o n i z a t i o n c l u s t e r s . An average of about three per secondary e l e c t r o n are produced, but more than eighteen ions per secondary e l e c t r o n have been observed. ( i i ) (1) Fate of Ionized Molecules I o n i z a t i o n i s u s u a l l y followed by n e u t r a l i z a t i o n . This process takes place so r a p i d l y i n comparison to the v i b r a t i o n a l motion, that immediately afterwards the n u c l e i s t i l l have very nearly the same p o s i t i o n and v e l o c i t y as before the jump (Franck-Condon P r i n c i p l e ) , and hence the energy of n e u t r a l i z a t i o n appears mostly as e x c i t a t i o n of the molecule. A l t e r n a t i v e l y , an ion may ionize another molecule, n e u t r a l i z i n g i t s e l f i n the process (unimolecular rearrange-ment), or i t may take part i n a bimolecular ion-molecule r e a c t i o n of the type:-AB$ + CD = ABC + + D (2) De-excitation of Gas Molecules Burton (1958) l i s t s s i x possible fates of excited molecules. These are:-( i ) Internal conversion to a lower excited state. A l l the upper excited states, no matter how pro--13 duced, i n t e r n a l l y convert within about 10 ^ seconds to the lowest excited state of the same m u l t i p l i c i t y . The energy r e l e a s e d i s p r o b a b l y absorbed as k i n e t i c energy of the atom. ( i i ) I n t e r n a l c o n v e r s i o n to the ground s t a t e . A v e r y much slower p r o c e s s than ( i ) , hence a l t e r n a t e schemes of d e - e x c i t a t i o n are p e r m i t t e d . ( i i i ) E m i s s i o n of energy as luminescence. ( i v ) Energy t r a n s f e r t o a second m o l e c u l e : -A* + B = A + B* + K.E. Aromatic compounds c o n t a i n i n g the or r i n g s f u n c t i o n e f f e c t i v e l y as 11B" i n the e q u a t i o n above. They absorb the energy and d i s s i p a t e most of i t without d e c o m p o s i t i o n . T h i s i s the p r o c e s s f o l l o w e d i n w a v e s h i f t i n g i n gas s c i n t i l l a t i o n chambers, "A" b e i n g the gas m o l e c u l e , and "B" the w a v e s h i f t e r m o l e c u l e . In t h i s case "B" sub-s e q u e n t l y d e - e x c i t e s by e m i t t i n g a photon of lower energy. (v) Decomposition r e a c t i o n - e i t h e r t o u l t i m a t e m o l e c u l e s o r to f r e e r a d i c a l s . These l a t t e r may e n t e r i n t o c h a i n r e a c t i o n s at h i g h temperatures, or be "scavenged" by the r e a c t a n t i t s e l f , or by a d e l i b e r a t e l y added i m p u r i t y . ( v i ) M e t a t h e t i c a l r e a c t i o n , t h a t i s , the i n t e r -change o f atoms between two m o l e c u l e s . For example, f o r benzene, a suggested p r o c e s s of d e c o m p o s i t i o n i s : -2C 2H 6 = 2 C 2H 5 + H 2 30 The chain of events which i s of i n t e r e s t f o r a r a d i a t i o n induced chemiluminescent r e a c t i o n i s one i n which e i t h e r the ions produced enter in t o a chain r e a c t i o n , or the excited atoms decompose and form r a d i c a l s which enter into a chain r e a c t i o n . The process must be one which t e r -minates by emission of energy as luminescence, and f u r t h e r i t i s necessary that the photons emitted are not reabsorbed by the gas mixture. l i k e l y to deactivate by c o l l i s i o n s with other molecules, while at pressures low enough so that the l i f e t i m e of the excited state i s shorter than the time between c o l l i s i o n s , l o s s of energy i s more l i k e l y to come about through r a d i a t i o n or decomposition. The reason i s e a s i l y seen from the following formula f o r the number of c o l l i s i o n s per unit time per unit volume between two molecular species (1 and 2) with respective concentrations N, and N P (Lewis and von At high pressures, an excited molecule i s most Elbe, 1951) where Z - no. of c o l l i s i o n s / ^ u n i t time/unit volume, m,, m~ - Molecular weights c, p - Average diameter of c o l l i d i n g molecules R = 8.314- . 10' ergs/ degree mole at N.T.P. TABLE III APPROXIMATE TIME SCALE FOR RADIATION CHEMISTRY (Reprinted from J . Magee, Annual Rev. of Nucl. S c i . 1 9 5 3 ) Time (sec.) Events —18 1 0 " Primary el e c t r o n ( 1 Mev) traverses molecule. -17 1 0 1 Mev alpha p a r t i c l e traverses molecule. —1 f~i 1 0 ~ Secondary el e c t r o n ( 5 ev) traverses molecule. T C Thermal e l e c t r o n ( 0 . 0 2 5 ev) traverses 1 0 ~ ^ molecule. -13 10 ' d i s s o c i a t i o n . Molecular v i b r a t i o n . Past molecular d i s s o c i a t i o n . E l e c t r o n capture i n molecular l i q u i d , - 1 2 1 0 Radical moves one jump i n d i f f u s i o n . , Q L i q u i d d i e l e c t r i c relaxes. 1 0 ~ C o l l i s i o n time f o r thermal electrons i n gas at N.T.P. C o l l i s i o n time f o r molecules i n gas at N.T.P. _o Secondary electron ( 5 ev) thermalized i n 1 0 " gas ( 1 atm.) Lifetime f o r r a d i a t i o n of excited s i n g l e t state (allowed). „ Thermal el e c t r o n captured i n gas 1 0 /capture prob. ^ IO"5") ^ c o l l i s i o n prob. ' ' Forward r e a c t i o n completed f o r alpha track i n water. Reaction time f o r r a d i c a l without solute i n molar concentration. Lifetime f o r r a d i a t i o n of t r i p l e t state 31 °~ ^ 2 ^ s °£ the order of 4 . 10"" cm., hence, at N.T.P. a molecule makes about 10"^ c o l l i s i o n s per second, so that the time between c o l l i s i o n s i s about 1 0 ~ ^ seconds. C o l l i s i o n l i f e , which i s defined to be the mean time taken f o r an excited molecule to become deactivated by c o l l i s i o n i s , fortunately, often much greater than the c o l l i s i o n frequency, due to i n e f f i c i e n t energy t r a n s f e r i n some molecular c o l l i s i o n s . F u l l y allowed r a d i a t i v e t r a n s i t i o n s have a l i f e t i m e of the order of 10~ seconds, but many of the t r a n s i t i o n s from which chemiluminescence has already been observed are f i r s t forbidden f o r i n d i v i d u a l atoms. When the atoms are joined together i n molecules, the coupling between atomic and molecular forces to some extent breaks down the "forbiddenness" of the t r a n s i t i o n , so that the l i f e t i m e of such r a d i a t i o n i s of the order of 10"~ seconds, rather than about 10~^ seconds which would be the l i f e t i m e of the i n d i v i d u a l atomic state. (See Table $:). T r a n s i t i o n s from OH * and C0 2* f a l l ' i n t o t h i s category. In 10~ seconds a molecule would undergo about ten thousand c o l l i s i o n s at normal temperature and pressure. ( i i i ) Photon Capture It frequently happens that r a d i a t i o n emitted from an excited gas molecule i s resonantly absorbed by a neigh-bouring molecule, which i n turn becomes excited. This process was r e f e r r e d to i n the d i s c u s s i o n of the noble gas counter as the means by which the r a d i a t i o n emitted eventually reached the walls of the chamber. For a p e r f e c t l y pure noble gas, the phenomenon has no e f f e c t on l i g h t output, though the time r e s o l u t i o n i s adversely a f f e c t e d . For chemiluminescence processes though, where the p r o b a b i l i t y of photon emission i s l i k e l y to be very small, due to more probable alternate methods of de-excitation, attenuation e f f e c t s due to photon capture are l i k e l y to com-p l e t e l y damp out a l l r a d i a t i o n i n a very short distance. I t i s very probable, therefore, that any r a d i a t i o n induced chemiluminescence w i l l be pressure s e n s i t i v e , f o r at low pressures the i r r a d i a t i n g p a r t i c l e s w i l l not produce very many ions or excited molecules per centimetre of path, whereas at high pressures, such e x c i t a t i o n as i s produced i s more l i k e l y to de-excite by c o l l i s i o n . Even assuming i t does de-excite by photon emission the p r o b a b i l i t y that the photon energy w i l l be completely degraded i s high. So f a r i n t h i s t r e a t i s e there has been l i t t l e mention of the chain r e a c t i o n , which i s fundamental to the whole idea of photon m u l t i p l i c a t i o n by chemical means. In order to s i m p l i f y the discussion, the carbon monoxide-oxygen mixture has been chosen as an example. While i t has not been found possible to i n i t i a t e t h i s r e a c t i o n by nuclear r a d i a t i o n , i t i s known that there i s a large photon y i e l d 33 from isothermal detonation of the same mixture i n some circumstances. ( i v ) Chain Reaction - The Carbon Monoxide - Oxygen  Mixture Lewis and von Elbe (1951) suggested the following mechanism f o r the combination of carbon monoxide and oxygen to form carbon dioxide (1) 0 + 0 2 + M = Oj + M (2) Oj + CO = C0 2 + 20 (3) Oj + CO + M = C0 2 + M + 0 2 (4) 0 + CO + M = CO* + M where M i s any other molecule. Once t h i s r e a c t i o n has been i n i t i a t e d i t i s easy to see that (2) causes a branching of the chain (chain branching r e a c t i o n ) , whereas (3) c a l l s a h a l t to any fur t h e r progress down the chain (chain breaking r e a c t i o n ) . Reaction (4) com-petes with (1) f o r the removal of oxygen atoms. The suggested processes f o r de- e x c i t a t i o n of the excited C0 2 molecule are:-(5) COJ + M = C 0 + 0 + M (6) C0| + 0 2 = C0 2 + 20 (7) COJ « C0 2 + h\> Reaction (7) i s the one from which l i g h t output should be obtained. Gordon and Knipe (1955), however, favour another scheme:-(8) CO + 0 . = CO* (9) CO* + 0 2 • C0 2 + 2 0 (chain branching rx.) (10) CO^ + M e C0 2 + M (at container wall) (11) CO + 0 + M »• C0 2 + M (chain breaking rx.) (12) 0 ) d i f f u s i o n to the wall and subsequent CO^) chain quenching. I f t h i s second scheme i s c o r r e c t , l i g h t output w i l l be dependent on the size of the container, and i s u n l i k e l y to give much i n d i c a t i o n of the p a r t i c l e track, even assuming the p a r t i c l e could i n i t i a t e the r e a c t i o n i n the f i r s t instance. (1) Requirements f o r Chain Reaction I f "n" i s the chain c a r r i e r concentration, ( [ 0 ] i n the above schemes), "n 0" the rate of the chain i n i t i a t i n g r e a c t i o n (e.g. rx. (1) or (8), and:-a = c o e f f i c i e n t of chain branching & = c o e f f i c i e n t of chain breaking, then ^§ = n - (/8-a)n (Lewis and von Elbe, 1 9 5 D cc o In general, "n" may comprise several d i f f e r e n t chain c a r r i e r s , and a several d i f f e r e n t rate c o e f f i c i e n t s . From t h i s equation i t can be seen that i f a < I £ E UJ cr D V) CO hi X a. 7 0 0 6 0 0 5 0 0 4 0 0 3 0 0 2 0 0 l O O 3 O a o > o c * o 'Z- >» O -D o w V > >» •o o o J o 2 * * o. c - 2 3i 5 5 6 0 H o m o g e n e o u s rx . a t h i g h e r t e m p , a n d p r e s s . © / _ _ — E x p l o s i o n region WaM_ _ef fects_ I I 6 0 0 6 4 0 6 8 0 T E M P E R A T U R E 7 2 0 7 6 0 C. fig.8 (a) Explosion peninsula for CO + 20 reaction (Hadman et a l . , 1932) 4 0 0 I E E UJ X 3 (/) (/) UJ cr 3 0 0 2 0 0 1 0 0 0 . 0 3 9 % H 2 O T X ^ ^ 0 . 0 0 3 9 % H 2 0 / / / 8 / i— / / o / / ^ D r y O x ° O M i x t u r e O J L _ £ l X 6 2 0 6 6 0 7 0 0 7 4 0 T E M P E R A T U R E 7 6 0 8 2 0 fig,8 (b) Effects of water vapour catalysis on the explosion limits of the CO + 2O2 reaction. (Gordon and Knipe, 1955) 35 then ^ — 0 , so that n = o . I f a=/3 , the r e a c t i o n P ~ a rate w i l l increase exponentially with time. Hence the equation a=/3 defines the explosion l i m i t . I t should also be mentioned that the r e a c t i o n rates are exponential functions of the temperature. As, i n f a c t , a and/5 are known i n only a very few cases, and these under very s p e c i a l conditions, t h i s formula i s of doubtful value, except i n so f a r as i t c l a r i f i e s the nature of the process. The e f f e c t of temperature may be seen i n f i g u r e 8(a) which shows what i s c a l l e d the "explosion peninsula". At pressures of the order of 50 mm. of Hg., explosion can occur at temperatures as low as 628 deg. C. There i s a very strong evidence to suggest that explosions at these low pressures are i n i t i a t e d at the wall of the container and so are of no i n t e r e s t i n t h i s d i s c u s s i o n . Above 200 mm. the temperature required f o r isothermal explosion increases, and at the same time a region of incomplete explosion or s e l f i n h i b i t i o n opens up. At temperatures and pressures just to the l e f t of t h i s region, i t i s possible that i o n i z i n g r a d i a t i o n could i n i t i a t e a chain r e a c t i o n by the simple mechanism of r a i s i n g the l o c a l temperature enough to s t a r t i t o f f . Further the r e a c t i o n would probably be i n h i b i t e d before i t had spread through the whole chamber. Unfortun-a t e l y , the equipment used would not withstand temperatures over 230 degrees C. so that t h i s p o s s i b i l i t y could not be explored. (2) Impurity E f f e c t s Rates of r e a c t i o n are very decidedly affected by some a d d i t i v e s . Topley (1930) observed that at 580 degrees C. the presence of water vapour (0.13 per cent upwards) i s e s s e n t i a l i f the r e a c t i o n i s to proceed at a l l , and stated that r e a c t i o n v e l o c i t y was approximately proportional to the concentration of water vapour up to several per cent. The r e s u l t s of Gordon and Knipe are s i m i l a r , and are shown i n f i g u r e 8(b), There are i n d i c a t i o n s however that the l i g h t y i e l d i s decreased i n d i r e c t proportion to the mole f r a c t i o n of water vapour. Small amounts of nitrogen peroxide increase the r e a c t i o n rate, but have l i t t l e e f f e c t on the explosion l i m i t s . Iodine, on the other hand, even i n small q u a n t i t i e s , acts as a strong i n h i b i t o r . (Lewis and von Elbe, 1951) (3) Light Output Gaydon (194-2) states that chemilumine scence from the CO + 0 2 r e a c t i o n i s the strongest known. He estimated that the l i g h t y i e l d at a pressure of 40 mm. Hg. may be as high as one quantum per one hundred and twenty f i v e C0 2 molecules formed. Most of these quanta are able to escape resonant reabsorption because of the d i f f e r e n c e i n shape of the excited and ground state C0 2 molecules. This d i f f e r e n c e i n shape leads to a changed degree of i n t e r a c t i o n between v i b r a t i o n a l and e l e c t r o n i c states, which i n turn r e s u l t s i n a d i s p a r i t y between the regions of s p e c t r a l absorpt ion and s p e c t r a l emiss ion , the l a t t e r being strongest i n the 4500-3 5 0 0 A area . (4) Use of S e n s i t i z e r s I t may be poss ib l e to make use of chain reac t ions which do not normal ly chemiluminesce, and at the same time to minimize the problem of resonant reabsorpt ion by i n t r o -ducing small q u a n t i t i e s of some other molecule . Such a molecule must be one to which energy t r a n s f e r from the exc i t ed end product of the chain i s e a s i l y achieved, and should have s trong emission l i n e i n the s e n s i t i v i t y range of common photocathodes. Sodium and mercury vapours appear to have the best chances of success as s e n s i t i z e r s . (c) Previous Experiments on Rad ia t ion Induced Reactions The chemical e f f e c t of a lpha p a r t i c l e s and e l ec t rons on var ious gas mixtures has been the subject of sporadic i n v e s t i g a t i o n f o r some t ime. Data i s u s u a l l y tabulated as a r a t i o of M/N, where M i s the t o t a l number of molecules of the new substance produced, and N i s the t o t a l number of ion p a i r s created by the r a d i a t i o n . Measurements of M are u s u a l l y made by measuring changes of pressure i n the gas mixture enclosed i n a conta iner of constant volume. The M/N r a t i o for the hydrogen-oxygen mixture has been measured by S c h i f l e t t and Lind (1934), over a wide range of temperatures. They report a r a t i o of about three f o r temperatures ranging from -180 Deg. C. up to room temperature. The r a t i o then rose s t e a d i l y up to t h i r t e e n at 400 deg. C. Above t h i s temperature r e s u l t s were not reproducible owing to the onset of the thermal r e a c t i o n . Water vapour was noted to be a strong c a t a l y s t . Experiments with other gas mixtures have y i e l d e d M/N r a t i o s which, at room temperatures at le a s t ( r e s u l t s at higher temperatures not u s u a l l y a v a i l a b l e ) were f o r the most part f r a c t i o n a l , or at l e a s t l e s s than three (Lind, 1921). The one out-standing exception to these r e s u l t s i s the hydrogen chlorine mixture, f o r which Bodenstein and Taylor (1916) obtained an M/N value of 4000. In t h i s case i t i s thought that e x c i t -a t i o n of the chlorine plays a very active r o l e . A f a i r l y complete review of the known e f f e c t s of i o n i z i n g r a d i a t i o n on simple gas mixtures i s given by Pshezhetskii and Dmitriev (1958). None of the reactions studied were tested f o r photon y i e l d . The only re a c t i o n known to be i n i t i a t e d by alpha p a r t i c l e s , and to produce l i g h t i n considerable q u a n t i t i e s , i s the oxidation of s o l i d nitrogen iodide (^H^I^). Henderson (1922) found that t h i s substance would explode a f t e r about 20 seconds i n a f l u x of about 10 p a r t i c l e s per second. In a follow up of Henderson's experiment, Poole (1922) was unable to explode any other of the more common 39 12 explosive mixtures a f t e r bombardment by about 10 alpha p a r t i c l e s . Among the mixtures t r i e d were s i l v e r azide, i dynamite, n i t r o g l y c e r i n e , fulminate of mercury and potassium p i c r a t e . It has been suggested (Bowden and Yoffe, 1958) that the explosion of nitrogen iodide was caused by the removal of ammonia from the surface. Henceforward the mechanism i s the same as f o r thermal explosion. f i g . 9 Photograph of Apparatus. 1. Thermocouple temperature meter 2. S t a i n l e s s s t e e l bellows valve 3o Molded clay i n s u l a t o r covering chamber 4. Water cooling 5. Vacuum tube voltmeter 6. Varian chart recorder 7. Anode current integrator condensers 8. Mercury test pulser 40 IV. EXPERIMENTAL APPARATUS A photograph of the apparatus can be seen i n figure 9» (a) Chamber Design The chamber i n which the gas reactions took place was made.of 2" I.D. s t a i n l e s s s t e e l tubing, s i x inches long with a wall thickness of 1/8". On one end of the tube a 1/2" t h i c k quartz disc was bolted on with 0-ring se a l s . On the opposite end was a s t a i n l e s s s t e e l face p l a t e , out of which came the gas i n l e t tubes, the evacuating tube (3/4-" I.D.), the blowout flange and the pressure gauge. (See P i g . 10). The valve connecting the chamber to the vacuum pumping system was a s t a i n l e s s s t e e l bellows type. A collimated polonium oC-source, which could be shielded from the chamber at w i l l , was mounted on one side. S t a i n l e s s s t e e l was used wherever possible on the i n t e r i o r surfaces of the chamber, because of the corrosive nature of some of the vapours and gas mixtures. The apparatus was designed to be vacuum t i g h t , and also to withstand pressures of 400 p . s . i . (blowout diaphragm excepted). A n i c k e l blowout diaphragm was inco r -porated (supplied by Black, S i v a l l s and Bryson), designed to rupture at 220 p . s . i . at 25 degrees C. and 158 p . s . i . at 417 degrees C. S I D E V I E W E N D P L A T E CHEMILUMINE S C E N C E C H A M B E R ONE H A L F F U L L S C A L E f i g . 10 1. Photomultiplier tube shield 6. Pipe to bellows valve and vacuum pump 2. Quartz disc. 7. Rupture diaphragm and holder 3. Water cooling tubes 8. Outlet to Marsh pressure gauge 4. Magnet operated source shield 9, Gas inlet tubes 5. Collimated alpha source Heater wires are not shown. They were wrapped around the cylinder 41 Heating was accomplished by the use of B & S No. 22 gauge nichrome wire which was wrapped around the outside of the chamber. Some d i f f i c u l t y was experienced i n f i n d i n g a suitable e l e c t r i c a l i n s u l a t o r f o r the heater. Asbestos was t r i e d but always broke down i n due course causing the heating c o i l to burn out. Insulation molded from modelling clay provided the ultimate s o l u t i o n . It gave excellent service once i t had been made. Correct molding of the c l a y was d i f f i c u l t however, as considerable shrinkage resu l t e d when i t was dried and hardened. I f the ins u l a t o r s were baked on metal molds they just crumbled. It was, there-fore, necessary to mold the c l a y to a larg e r diameter than was a c t u a l l y desired, and hope that i t would shrink to about the r i g h t s i z e . This was accomplished a f t e r some t r i a l and much error. The molded c l a y was f i r s t d r i e d f o r about twelve hours i n an oven at about 100 degrees C , and was then baked fo r an equal period at 600 degrees C , and cooled slowly down to room temperature. The r e s u l t i n g i n s u l a t o r s were b r i t t l e and quite hard, but could s t i l l be whittled a l i t t l e with a k n i f e . The temperature of the chamber was measured with a chromel - p-alumel thermocouple junction strapped to the outside of the s t e e l tubing. The power required to maintain the chamber at a temperature of about 220 degrees C. was 200 watts (3-5 amps. 42 into 16 ohms). Current through the heater was regulated by a Variac. Teflon O-rings were used at the seals to begin with, but were found to deform considerably at high temperatures, n e c e s s i t a t i n g continual tightening of the fastening b o l t s . Teflon has the f u r t h e r disadvantage that at high temperatures i t gives o f f h i g h l y toxic fumes. Viton A O-rings were eventually used. They proved to have more r e s i l i e n c e than Teflon rings and were safer to use. The maximum usable temperature was about the same as f o r Teflon (260 degrees C.) which set the upper l i m i t on temperature obtainable i n the chamber. The alpha source was mounted at an angle of 30 degrees to the axis of the chamber, so as to allow a reason-able path length'for the collimated p a r t i c l e s . A source s h i e l d , mounted on a swivel, could be operated by turning a small permanent magnet on the alpha source mounting plate (See F i g . 10). Alpha source strengths were about fourteen thousand counts per minute. The sources were made by depositing polonium i n hydrochloric a c i d s o l u t i o n on polished s i l v e r . In p r a c t i c e the co l l i m a t o r and source were made from the same piece of s i l v e r , which was r i v e t t e d onto the source holder before the polonium was deposited. I t was found necessary to construct several sources, as some of the 43 vapours coated the source during operation of the chamber. The inside of the chamber was coated with smoked magnesium oxide i n order to obtain maximum r e f l e c t i v i t y . In addition a small s t a i n l e s s s t e e l t r a y with several com-partments was constructed. This was used to hold any l i q u i d s or s o l i d s whose vapours were to be tested, and was placed on the bottom of the chamber. The thickness of the quartz glass disc required to stand the designed load of 400 p . s . i . was calculated from formulae supplied by the General E l e c t r i c Company, the makers of the d i s c . The thickness " t " of the disc i s given by:-«-{£}* where t-; - disc thickness - inches p - pressure d i f f e r e n t i a l ( p . s . i . ) r0 - unsupported d i s c radius (inches) S - maximum stress (7 to 1 safety factor) = 1000 p . s . i . f - f a c t o r (1.5 i f edges are clamped; 0.84 i f edges are undamped.) (b) Light Detection System A photomultiplier rather than an image i n t e n s i f i e r tube was used to c o l l e c t l i g h t from the chamber, hence no p a r t i c l e track was v i s i b l e at any time. However, i f any Anode Dynode 13, 2 5 0 pf. Dynode 12. 5 0 0 pf. H\—\ 2 7 0 K M e g . lOOpf. z j r Dynode 11_ , 5 0 pf. zjr: Dynode IO_ 2 5 pf. - j -Dynode 9 | Dynode 2_ Dynode I. Cathode f ig .11 2 7 0 K 2 2 0 K 2 2 0 K 2 2 0 K 2 2 0 K 3 3 0 K 1.21 M e g . .001 4 7 pf. IN V. II 8 8 n IO T O V T V M Min. input impedance : 10 Megohms 3 K I 5 0 K 3 3 0 K 4 9 K - 1 2 0 0 v. + 125 v. \ •§ 6 D J 8 HEAD AMPLIFIER E M I 9 5 I 4 S P H O T O M U L T I P L I E R 44 p a r t i c l e caused a r e a s o n a b l e amount of luminescence, t h i s appeared as a p u l s e e a s i l y d i s c e r n a b l e from n o i s e . The p h o t o m u l t i p l i e r tube used was a 9514S E.M.I. .2" tube, s p e c i a l l y s e l e c t e d f o r i t s low dark c u r r e n t . The n o i s e l e v e l of t h i s tube e v e n t u a l l y r o s e beyond t o l e r a b l e l i m i t s , p o s s i b l y due to i n t e r n a l breakdown, and was r e p l a c e d by a 5" 9530S E.M.I. p h o t o m u l t i p l i e r tube f o r the l a s t few run s . With t h i s tube o n l y the c e n t r a l p o r t i o n of the f a c e was used, the r e s t b e i n g b l a n k e d out. The s m a l l d i f f e r e n c e s i n the apparatus t h a t r e s u l t e d from the tube replacement w i l l be mentioned i n the course o f the g e n e r a l d e s c r i p t i o n . The p h o t o m u l t i p l i e r tube f a c e was h e l d about one i n c h b e h i n d the q u a r t z d i s c (about 2 1/2" f o r the 9530S t u b e ) , and water c o o l i n g was p r o v i d e d around the o u t s i d e of t h i s a i r space t o pr e v e n t the tube f a c e from b e i n g heated by c o n d u c t i o n from the i n s i d e of the chamber. When T e f l o n O - r i n g s were used, i t was found neces-s a r y to wrap a l l s e a l s w i t h b l a c k tape t o p r e v e n t l i g h t l e a k s . T h i s p r e c a u t i o n was not n e c e s s a r y when V i t o n - A s e a l s were used. The p h o t o m u l t i p l i e r dynode c h a i n was f e d by a n e g a t i v e 1200 v o l t s u p p l y . The anode was A.C. co u p l e d out to a cathode f o l l o w e r (see F i g . 11). Anode r e s i s t a n c e was 5.9 megohms, w i t h a tap a t 1.21 megohms from ground. From t h i s t a p an i n t e g r a t o r c i r c u i t w i t h a time c o n s t a n t o f 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 0 0 W A V E L E N G T H - A N G S T R O M U N I T S fig.12 Spectral response curve of E.M.I. 9514S photocathode. Cathode material: Sb.- Cs. (S), 25 microamps per lumen. 2 0 seconds (8 seconds for the 9530S tube) was coupled to a vacuum tube voltmeter, hence the voltage on the meter was a measure of the anode current. The output of the voltmeter (one volt f u l l scale deflection on any scale) was fed into a 1 0 m.v. f u l l scale deflection Varian chart recorder. With this system a voltage change of about 0 . 0 5 m i l l i v o l t s could be detected. Sensitivity was down a factor from 20 - 1 0 0 when the VTVM only was used. A.voltage change of 0.05 m i l l i v o l t s corresponded to a photomultiplier anode current change of about 4 . 1 x 1 0 _ ju a. At 1 2 0 0 volts the photomultiplier tube amplification 7 was about 4 . 2 x 1 0 , so that the minimum discernable change in amount of charge per second arriving at the f i r s t dynode . . 4 . 1 x 1 0 ~ 5 , r t - 1 2 n , was about 1 0 microcoulombs per second, 4 . 2 x 107 which i s roughly equivalent to six electrons per second. If i t i s assumed that the photon to photoelectron conversion efficiency of the tube i s about 7»5%» then a s c i n t i l l a t i o n rate of 80 photons/sec. in the sensitive area of the photo-tube would be detectable (See f i g . 1 2 ) . At room temperatures, the dark current of the —9 9514-S photomultiplier was about 1 0 7 amps with 1 2 0 0 volts applied across the dynode chain. Dark current increased by about a factor three when the temperature of the chamber was raised to 2 0 0 ° C . The dark current in the 9 5 3 0 S photo-46 m u l t i p l i e r was roughly double that i n the 9514-S at the same voltage. * * * * * * The cathode follower had an output f o r d i r e c t viewing i n an o s c i l l o s c o p e , and a 100 ohm output to match the cable leading to a 256 channel Nuclear Data k i c k s o r t e r . Whenever a d e f i n i t e change i n l i g h t output from the chamber was observed on the VTVM, the output spectrum was examined i n the k i c k s o r t e r . For c a l i b r a t i o n purposes, a mercury pulser, producing 60 c/s. pulses of known pulse height was coupled into the g r i d of the cathode follower. C O U N T S P E R C H A N N E L O O Q O O O O O O O O 0 20 40 60 80 0 20 40 60 80 CHANNEL NUMBER CHANNEL NUMBER POLONIUM a-PARTlCLES IN NAPTHALENE VAPOUR fig.13 1 1 1 1 1 1 1 1 1 1 ; 0 20 40 60 80 0 20 40 60 80 CHANNEL NUMBER CHANNEL NUMBER POLONIUM a-PARTlCLES IN NAPTHALENE VAPOUR fig.13 -\ L T ^ \ O) W (\) -\ 1 o o 0 l \ >^  1 1 - / X to ro o / \ 5 O O -/ \ — » 3 3 3 / \ . o 3 3 3 / y ^ X O 5 / \ ^ C O U N T S P E R C H A N N E L O O o O O O O ° O O o 0 20 40 60 80 0 20 40 60 80 CHANNEL NUMBER CHANNEL NUMBER POLONIUM a-PARTlCLES IN NAPTHALENE VAPOUR fig.13 1 1 1 1 1 1 1 i i i 0 20 40 60 80 0 20 40 60 80 CHANNEL NUMBER CHANNEL NUMBER POLONIUM a-PARTlCLES IN NAPTHALENE VAPOUR fig.13 — — ^ ^ ^ ^ ^ ^ ^ ^ 5 p> pi ^  i y ^ ^ f ? s|* O g ^ o o - • 3 3 3 I. 1 3 3 3 p M-7 V. RESULTS Experiments were conducted into the vapour phase e f f i c i e n c i e s of some organic s c i n t i l l a t o r s and waveshifters normally used i n the s o l i d phase as well as into the pos-s i b i l i t y of i n i t i a t i n g a chain r e a c t i o n i n some gas mixtures. The r e s u l t s from the vapourized s c i n t i l l a t o r s w i l l be d i s -cussed f i r s t . No s p e c i a l precautions were taken to p u r i f y the substances used. A l l the gases used were C P . grade, except f o r hydrogen which was extra dry grade (99.8% minimum p u r i t y ) and oxygen, which was welding grade only. Because the photomultiplier was set back from the quartz d i s c , and because no s p e c i a l precautions were taken to p u r i f y the gases used, i t was not expected that good r e s o l u t i o n would be obtained i n cases where s c i n t i l l a t i o n was observed. It was to be expected though that the res u l t a n t increase i n anode current would be noticeable. This proved to be the case. Experiments were not repeated unless a large l i g h t output was observed. (a) Waveshifter Vapours i n Helium, and Vapourized  S c i n t i l l a t o r s ( i ) Napthalene Vapour. The naphalene used was ob-tained from ordinary mothballs. The chamber was evacuated to 6 x 10 mm. of Hg. pressure before the valve was shut and the heating began. At the maximum temperature of O 20 40 60 80 100 CHANNEL NUMBER POLONIUM o r - P A R T I C L E S IN H E L I U M GAS f ig .14 2 0 5 ° C , the vapour pressure of the napthalene was 560 mm. As can be seen from a comparison of f i g . (13) and f i g . (14) only about 4% of the alpha p a r t i c l e s produced any noticeable pulse at a l l . The s h i f t i n g of the peak i n f i g . (13) can be explained as follows: Spectrum 1. A few alpha p a r t i c l e s impinged on the s o l i d napthalene i n the tra y at the bottom causing a small, badly smeared peak. Spectrum 2. The napthalene had melted and spread out i n the tray, allowing more alpha p a r t i c l e s to impinge on i t , and the s c i n t i l l a t i n g e f f i c i e n c y was now greater as the l i q u i d was c l e a r e r . Spectrum 3« The l i q u i d had evaporated from the tray, but was beginning to condense on the quartz d i s c which was water cooled. Because of the chamber design, more alpha p a r t i c l e s f e l l on the quartz disc than on the tr a y . Spectra 4, 5» and 6. A f a i r l y t h i c k layer of napthalene has formed on the quartz, attenuating the l i g h t output, but stopping more alpha p a r t i c l e s . It appears from t h i s e x p e r i -ment that vapourized napthalene does not s c i n t i l l a t e . ( i i ) Helium. The chamber was f i l l e d with helium to -4 25 p . s . i . a f t e r having been evacuated to about 6 x 10 mm. of Hg. I t was then heated to 200°C. and cooled down again. A summary of the pulse spectra obtained from helium i s shown i n f i g . (14). Two phenomena are immediately obvious. CHANNEL NUMBER f i g . 1 5 49 ( i ) The existence of a second smaller peak of higher energy, and ( i i ) the s h i f t of the peak towards lower pulse heights as the temperature increases, u n t i l the peak f i n a l l y merges with noise at 200 degrees C. (Spectrum J ) . When the chamber had been cooled down again, the peaks reappeared, though t h e i r amplitude was reduced by a f a c t o r of 2/3 from the f i r s t run at room temperature (Spectrum 1). The second phenomenon can be explained as being due to the poisoning of the s c i n t i l l a t o r by impurity vapours. In p a r t i c u l a r anthracene was present i n small quantities as i t s removal from the chamber a f t e r a previous run had not been complete. The existence of the second peak i s harder to explain. I t c l e a r l y i s not due to an addition of pulses obtained from two coincident p a r t i c l e emissions, as the pulse height of the second peak i s more than three times that of the f i r s t . I t i s u n l i k e l y a l s o , that i t i s caused by some p a r t i c l e s h i t t i n g a side wall due to improper source c o l -limation, as, i n that case wthe second peak would be at a lower, not a higher, energy. It i s l i k e l y that the source had become coated with a very t h i n l a y e r of organic material, which had been scraped o f f i n one place. I f t h i s was the case, then the second, smaller peak represents the true l i g h t output from polonium alpha p a r t i c l e s . O 20 40 60 80 C H A N N E L N U M B E R fig.16 50 The double peak e f f e c t showed, to a l e s s e r extent, i n the runs with anthracene and p-quarterphenyl vapour as we l l . ( i i i ) Helium Saturated with Anthracene Vapour. Thrice d i s t i l l e d anthracene was placed i n the tray on the bottom of the chamber, which was then pumped down to 5 x 1 0 - ^ mm. of Hg. The system was f i l l e d with helium to 25 p . s . i . and then heated'to 1 7 5°C, at which temperature the vapour pressure of anthracene i s about 8 mm. The s h i f t i n spectrum with increased temperature can be seen i n f i g . (15)• The curve f o r room temperature i s seen to be very s i m i l a r to that obtained f o r pure helium. The peaks gradually s h i f t e d down towards noise, u n t i l at 175 degrees C. (Spectrum 2), then were a l l but l o s t . As the chamber was cooled down the small peak only appeared at about 110 degrees C. (Spectrum 3 ) . This disappeared once more at room temperatures. (Spectrum 4). A reheating of the chamber caused the peak to reappear. ( i v ) Helium - p-Quarterphenyl and Helium-diphenyl  Stilbene Mixtures. The e f f e c t of both these vapours i n the saturated condition on the s c i n t i l l a t i o n s i n helium was t r i e d up to temperatures of 200 degrees C. The vapour pressure of the diphenyl s t i l b e n e i s about 3 mm. The vapour pressure of p-quarterphenyl at the same temperature i s considerably l e s s than 1 mm. (vapour pressure 18 mm. at 4 2 8 ° C ) . With diphenyl stilbene a poorly resolved peak with fig.17 Total pressue one p.s.i. above atmospheric at 25 degrees C. Numbers between experimental points represent elapsed time in minutes. very poor s i g n a l to noise r a t i o was observed, which improved at about 100 degrees C. but disappeared at 160 degrees C. The spectrum development with temperature f o r the p-quarterphenyl vapour was very s i m i l a r to that f o r anthra-cene (see f i g . 15), only pulse height and r e s o l u t i o n were not as good. S c i n t i l l a t i o n s from the quarterphenyl i n the tray were observed before the helium was admitted to the chamber. This peak had l e s s than 100 counts maximum and was about 30 channels wide, which would seem to support the hypothesis that the s c i n t i l l a t i o n s observed i n napthalene were from the s o l i d and l i q u i d phases only. (b) Gas Mixtures No e f f e c t which was a t t r i b u t a b l e to the alpha p a r t i c l e s was observed i n any of the gas mixtures t r i e d , there d i d however appear to be photon emission i n some gas mixtures at elevated temperatures. For most of these runs, the vacuum obtained between f i l l i n g s was of the order of 10"^  mm. ( i ) 2 CO » Oo* This was the only gas mixture studied which produced a noticeable change i n spectrum shape as the temperature changed (see f i g . 16), though the anode current change was not as great as f o r some other gases and gas mixtures. (See f i g . 17). The t o t a l pressure i n the chamber was one pound over atmospheric at room temperature. The small peak observed i n the spectrum i s probably 2000 O 40 80 120 160 200 TEMPERATURE °C. fig.18 Results when the chamber was evacuated at 175 degrees C. and r e f i l l e d with hydrogen and oxygen separately are shown. Total pressure 45 psi. The chamber was coated on the inside with 5 2 due to short chain reactions i n i t i a t e d at the quartz d i s c . The l i g h t from chain reactions i n i t i a t e d on other walls i s l i k e l y to have been absorbed or attenuated beyond recognition i n the gas mixture. A change i n dark current with time, the temperature being constant, can be seen i n f i g . (17). This was an e f f e c t common to several other gases t r i e d (see f i g u r e s 1 8 , 19, and 2 0 ) . During t h i s run p-quarterphenyl waveshifter was coated on the outside (only) of the quartz d i s c . ( i i ) 2 H Q + 0 2 » A s c a n D e seen from f i g . ( 1 8 ) , the anode current rose to a high l e v e l at 1 8 0 degrees C , (a f a c t o r ten over the dark current at that same temperature), but when the mixture was pumped out, and replaced with oxygen only, the anode current went up to over ninety times the dark current, and then dropped down again quite quickly. F i l l i n g the chamber with pure hydrogen had only a very small e f f e c t (see f i g . 1 8 ) . It appears l i k e l y that i t was the oxygen alone rather than the mixture which caused the increase i n dark current. As the insi d e of the chamber had been coated with p-quarterphenyl during t h i s run, i t was thought that oxidation of t h i s substance may have caused the l i g h t output observed. In order to check t h i s the quarterphenyl was removed from the chamber, and the run repeated using only oxygen. The r e s u l t s are shown i n f i g . (19). Though the r i s e i n dark current was not as spectacular as i n the previous experiment, Fresh filling fig.19 Oxygen pressure 5 p.s.i. above atmospheric. Numbers between experimental points represent elapsed time in minutes. the e f f e c t was s t i l l very pronounced. As the 9514-S tube had been replaced by the 9530S i n the i n t e r v a l , no d i r e c t com-parison was p o s s i b l e . I t i s very probable that the r a d i a t i o n observed i n the mixtures containing oxygen was due to the decomposition of ozone. This phenomenon has been observed before (Barbier et a l . , 194-1) at temperatures over 1 5 0°C. The oxygen used throughout these experiments was only of welding grade, hence impurities may have contributed to the observed e f f e c t . ( i i i ) + CI2. This r e a c t i o n , which took place at atmospheric pressure, might have been expected to have a large l i g h t y i e l d at higher temperatures. The anode current, however, was only about ten times the dark current at 24-0 degrees C. (see f i g . 20). ( i v ) CO2• Pure CO2 was admitted to the chamber at room temperature, but no change i n anode current was observed up to pressures of 17 p . s . i . (v) Sodium Sensit i z e d Reactions. Both sodium metal and sodium chloride were placed i n the tr a y during the sodium s e n s i t i z e d experiments. In the f i r s t run pure hydrogen was admitted to the chamber. The r e s u l t s can be seen i n f i g . (20). S e l f quenching i s very strong i n t h i s mixture. I t can be seen f i g . 20 Numbers between experimental points represent elapsed time in minutes. 54-that maximum anode current was observed at only 0.008 mm. (Hg) pressure. Luminescence from t h i s r e a c t i o n was f i r s t observed by Magee and Ri (194-1). In the second run, a one to one mixture of carbon monoxide and nitrous oxide was introduced to the chamber which was held at 24-0 degrees C. Fenimore and Kelso (1950) had reported intense chemiluminescence from t h i s r e a c t i o n at 550 - 600 degrees C. It had been hoped that alpha p a r t i c l e s might i n i t i a t e the r e a c t i o n at a lower temperature, but there was no appreciable change i n anode current throughout the run. ( v i ) Mercury S e n s i t i z e d Reactions The following reactions were tested f i r s t l y f o r alpha p a r t i c l e s e n s i t i v i t y and secondly f o r a difference i n anode current over that e x i s t i n g when the chamber i s evacuated at the same temperature. .1. Hp + NpO + Hg at 4-3 p . s . i . above atmospheric pressure 2. 2Hp + Op at 10 p . s . i . above atmospheric pressure 3. + Hg at one atmosphere The chamber was held at 24-0 degrees C. throughout, and contained mercury vapour at saturation pressure (about 50 mm.). No measureable e f f e c t s were observed. I t i s i n t e r e s t i n g to note that the mercury vapour appears to have quenched r a d i a t i o n from the decomposition of ozone which had been expected from the hydrogen - oxygen r e a c t i o n . (c) The E f f e c t of x-rays on Dark Current During the course of the experiment i t was noticed that x-rays produced by the operation of the Van de Graaf Generator (which was about f o r t y feet away) caused a very noticeable increase i n the dark current of the phototube. A r a d i a t i o n l e v e l beside the phototube of 6 m i l l e Roentgens per hour caused an increase i n dark current by a f a c t o r of . - 9 N ten (from 1.6 x 10 y amps). The x-rays were from 1.9 Mev alpha p a r t i c l e s s t r i k i n g a gold target. A s i m i l a r change i n dark current could be produced by holding a 1.1 m i l l e Curies 60 Co source about two feet from the photomultiplier photo-cathode. The e f f e c t was almost c e r t a i n l y caused by the x-rays knocking secondary electrons from the photocathode (the tube used i n t h i s experiment was a f i v e inch 9 5 3 0 S ) . Emission from the dynodes was very s l i g h t , as was shown by shorting the photocathode to the f i r s t dynode. VI. CONCLUSIONS 56 Data c o l l e c t e d through the course of the experiment often proved d i f f i c u l t to i n t e r p r e t , but i t was at le a s t c l e a r that no photon m u l t i p l i c a t i o n was obtained from chemi-luminescent chain reactions i n i t i a t e d by the passage of alpha p a r t i c l e s . I t i s u n l i k e l y that increasing the temper-ature c a p a b i l i t i e s of the chamber (to 700 degrees C. say) would much improve the chances of i n i t i a t i n g the desired chain r e a c t i o n . A 700 degree C. chamber would allow research i n two areas though which were closed to the author, namely an i n v e s t i g a t i o n of sodium s e n s i t i z a t i o n at reasonable vapour pressures (up to 100 m.m.), and an i n v e s t i g a t i o n of possible i n i t i a t i o n of chain r e a c t i o n simply by the l o c a l heating e f f e c t of the i o n i z i n g p a r t i c l e . I t i s pos s i b l e , that i f a carbon monoxide - oxygen mixture were operated i n the region of s e l f - i n h i b i t i o n (see f i g . 8a), an i o n i z i n g p a r t i c l e may r a i s e the l o c a l temperature enough to i n i t i a t e a l o c a l chain reaction, which would be i n h i b i t e d before detonation of the whole mixture occurred. The same pos-s i b i l i t y e x i s t s f o r the sodium s e n s i t i z e d r e a c t i o n of carbon monoxide with nitrous oxide, which i s reported to be very s e n s i t i v e to temperature (Fenimore and Kelso, 1950). Several other methods of obtaining photon m u l t i -p l i c a t i o n have been considered, and may merit further i n v e s t i -gation. The idea of applying high gradient e l e c t r i c f i e l d s 57 inside a chamber so that ions produced by the primary p a r t i c l e w i l l be accelerated to energies capable of pro-ducing further e x c i t a t i o n and i o n i z a t i o n i s one which has been applied, i n the extreme case, to the spark chamber. It i s possible that at lower plate voltages, and with a helium-xenon gas mixture, considerable photon m u l t i p l i c a t i o n would be obtained. Such a device, however, would have no obvious advantages over the spark chamber, and would be rather more complex and expensive. Reference has been made to attempts (so f a r with l i m i t e d success) by Lederman (1961) and by Fukui et a l . (I960) to produce a microwave spark chamber. I t may well be possible that the same techniques w i l l induce enough photon m u l t i -p l i c a t i o n to allow construction of a gas s c i n t i l l a t i o n chamber. L i t t l e i s known thus f a r of the s c i n t i l l a t i o n pro-p e r t i e s of l i q u i d hydrogen, helium and xenon. Interest i n a l i q u i d s c i n t i l l a t i o n chamber was expressed by several of those attending the Berkeley Conference on Instrumentation i n High Energy Physics (I960), but no conclusive r e s u l t s are known to have been published. It i s possible that there i s a gas with a metastable state, which could be excited by u l t r a - v i o l e t r a d i a t i o n , and induced to de-excite by i o n i z i n g r a d i a t i o n . Such a technique has not, so f a r as i s known, been t r i e d . 58 While the experiment undertaken y i e l d e d no useful r e s u l t s which might lead to a p a r t i c l e t r a c k i n g device, i t i s l i k e l y , that i f the o r i g i n of the l i g h t observed i n some gas mixtures at elevated temperatures i s c l a r i f i e d , a sen-s i t i v e device f o r measuring the onset of some chemical reactions w i l l have been uncovered. Presumably the number of photons emitted i s d i r e c t l y proportional to the re a c t i o n rate, so that the device should be useful f o r measuring rates of re a c t i o n as w e l l . APPENDIX Radiation length (X Q) i s defined by the equation -!- = 4 a - A - Z ( Z + l ) r e 2 In.(183 Z ~ i ) x o where a = r — = 1 re = 2.8176 x 10 cm, the c l a s s i c a l radius -the e l e c t r o n 23 N = 6.024- x 10 ^ - Avogadros Number A = atomic weight Z = atomic number The d e s c r i p t i o n of r a d i a t i o n phenomena i s only s l i g h t l y dependent on atomic number when thicknesses are measured i n r a d i a t i o n lengths. The dependence on atomic number becomes l e s s pronounced with increasing energy (Rossi, 1 9 5 2 ) . BIBLIOGRAPHY A. J. Alikhanian and M. S. Kozodaev, Proc. of the Inter- n a t i o n a l Conference on Instrumentation f o r High Energy  Physics, pp. 174-176, Interscience, New York and London, Sept., I960 . D. Barbier, D. Chalonge and Miguel Marriera, Comptes Rendus Acad. S c i . (Paris) 212 , 984-6, (1941). G. A. Beer, Gas S c i n t i l l a t i o n Counters, Master of Science Thesis, U n i v e r s i t y of B r i t i s h Columbia, Sept., 1959. M. Bodenstein and H. S. Taylor, J.A.C.S. 22, 24, ( 1915 ) . M. Bodenstein and H. S. Taylor, J.A.C.S. £8, 280, (1916). F. P. Bowden and A. D. Yoffe, Fast Reactions i n S o l i d s , Butterworth, London, 1958. J . Burns and M. J. Neumann, I.R.E. Trans. NS - 7.126, June -Sept., I960. M. Burton, Records of Chemical Progress 1°/, 1958. Cocconi ( i n d i s c u s s i o n ) , R.S.I. £ 2 , ^95, (1961). Cork ( i n d i s c u s s i o n ) , R.3.I. £ 2 , 497, (1961). T. E. Cranshaw and J . F. deBeer, Nuovo cimento 11 , 1107, ( 1959 ) . Frank B. Dickey J r . , Journal of Applied Physics 2J5, 1336, ( 1952 ) . Charles A. Engelke, I.R.E. Trans. NS - 7 , 3 2 , June - Sept., I960., C. Fenimore and J . Kelso, J.A.C.S. £2, 5045, ( 1950 ) . J . Fischer and G. T. Zorn, R.S.I. £ 2 , 4 9 9 , (1961). S. Fukui, S. Hayakawa, T. Tsukishima and J . Nukushina, Proc. of the International Conference on Instrumentation  f o r High Energy Physics, p. 267, Interscience, New York and London, Sept., I960. S. Fukui and S. Miyamoto, Nuovo Cimento 11, 113, ( 1 9 5 9 ) . A. G. Gaydon, Spectroscopy and - Combustion Theory, Chapman and H a l l , London, 1942. D. A. Glaser, Proc. of the International Conference on Instrumentation f o r High Energy Physics, pp. 150-153, Interscience, New York and London, Sept., I960. A. Gordon and R. Knipe, J . Phys. Chem. 5J2, 1160, ( 1 9 5 5 ) . G. Hadman, H. W. Thompson and C. N. Hinshelwood, Proceedings of the Royal Society (London) A 137, 87, 1932. Ernest Heer, Proc. of the International Conference on Instrumentation f o r High Energy Physics, pp. 284-286, Interscience, New York and London, Sept., I960. G. H. Henderson, Nature 10°,, 74-9, ( 1 9 2 2 ) . L. W. Jones, K. L a i , R. Newsome and M. L. P e r l , I.R.E. Trans. NS - 7. 145-150, June - Sept., I960. L. W. Jones and M. L. P e r l , Proc. of the International  Conference on High Energy Accelerators and  Instrumentation, pp. 561-570» Cern, Geneva, 1959. J . W. K e u f f e l , R.S.I. 20, 202, (1949). (a) K. Lande, A. K. Mann, K. Reibel and D. H. White, I.R.E. Trans. NS - 7, pp. 121-126, June - Sept., I960. (b) K. Lande, A. K. Mann, K. Reibel and D. H. 'White, Proc. of the International Conference on Instrumentation  f o r High Energy Physics, pp. 192-194, Interscience, New York and London, Sept., I960. L. M. Lederman, R.S.I. £2, 523, (1961). B. Lewis and G. von Elbe, Combustion, Flames and Explosions i n Gases, Academic Press, New York, 1951* S. C. Lind, The Chemical E f f e c t s of Alpha P a r t i c l e s and El e c t r o n s , 2nd ed. Chemical Catalogue Co., New York, J. L. Magee and R i , Journal of Chemical Physics °/» 638, (1941). D. Meyer and K. T e r w i l l i g e r , R.S.I. £2, 512, (1961). John A. Northrup, R.S.I. 2°,, 4-37, (1958). 62 G. K. O ' N i e l l , R.S.I. £2, 528, (1961). M. P e r l and L. Jones, Nucleonics 18, 9 2 - 9 7 , (I960). M. P e r l , L. Jones and K. L a i , Proc. of the International Conference on Instrumentation f o r High Energy Physics, pp. 186-191• Interscience, New York and London, Sept., I960. H. H. Poole, Proceedings of the Royal Dublin Society, 1 2 , 9 3 , ( 1 9 2 2 ) . R. J . Potter and R. E. Hopkins, I.R.E. Trans. NS - 7, 150, June - Sept., I960. S. Pshezhetskii and M. Dmitriev, The Mechanism of Some Simple Chemical Reactions Taking Place Under the Action  of Ionizing Radiations, t r a n s l . AERE Lib/Trans 800, Sept., 1958. M. Reiganum, Z e i t s c h r i f t f u r Physik 12, 1076, (1911). George T. Reynolds, I.R.E. Trans. NS - 7, 115, June -Sept., I 9 6 0 . (a) A. Roberts, R.S.I. £2, 482, (1961). (b) A. Roberts, R.S.I. £2 , 531, ( 1961) . Romanowski ( i n d i s c u s s i o n ) , R.S.I. £2 , 517, (1961). Bruno Rossi, High Energy P a r t i c l e s , pp. 5 0 - 5 4 , P r e n t i c e - H a l l , New York, 1952. J . Rutherglen and J. Paterson, R.S.I. £2 , 522, (1961). C. H. S c h i f l e t t and S. C. Lind, J . Phys. Chem. £8, 327, (193<0. H. H. Staub, Experimental Nuclear Physics, v o l . 1, (ed. E. Segr£), pp. 1-165, John Wiley and Sons, New York, 1953• B. Topley, Nature 12J?, 560, ( 1 9 3 0 ) . W. L. Wilcock, D. L. Emberson and B. Weekley, I.R.E. Trans. NS - 7, 126, June - Sept., I960. E. K. Zavoisky, M. M. Butslov, A. G. Plakhov and G. E. Smolkin, J . Nuclear Energy 4, PP« 340-344, ( 1957) . E. K. Zavoisky and G. E. Smolkin, J . Nuclear Energy 4 , 353 , (1957) . Abbreviations J.A.C.S. - Journal of the American Chemical Society J . Phys. Chem. - Journal of Physical Chemistry R.S.I. - Review of S c i e n t i f i c Instruments 

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