Science, Faculty of
Physics and Astronomy, Department of
DSpace
UBCV
Harrick, Nicolas James
Master of Arts - MA
University of British Columbia
[no abstract submitted]
6f i MEASUREMENT OP RANGES OP ALPHA PARTICLES IN GASES by N i c o l a s James H a r r i c k A t h e s i s submitted i n p a r t i a l f u l f i l m e n t o f the requirements f o r the degree o f MASTER OP ARTS i n the Department o f PHYSICS. The U n i v e r s i t y o f B r i t i s h Columbia A p r i l , 1949 AC KNOWLED GEMENT S T h i s work was c a r r i e d out under a r e s e a r c h grant from the N a t i o n a l Research C o u n c i l o f Canada. The author i s p l e a s e d to express h i s g r a t i t u d e t o Dr. G. G. E i c h h o l z under whose s u p e r v i s i o n t h i s work was c a r r i e d out. He a l s o wishes t o thank ot h e r members o f the s t a f f , e s p e c i a l l y Dr. J . B. Warren, f o r h e l p f u l s u g g e s t i o n s . I n a d d i t i o n he wishes t o acknowledge the work o f Mr. A. J . F r a s e r i n c o n n e c t i o n w i t h the machining o f the shallow i o n i z a t i o n chamber and the work o f Mr. A. W. Pye i n c o n n e c t i o n w i t h the g l a s s blowing. F i n a l l y he would l i k e to thank the B r i t i s h Columbia Telephone Company L i m i t e d f o r the s c h o l a r s h i p awarded him. INDEX Page I. INTRODUCTION A. Object 1 B. Theory, concepts and definitions 1 C. Methods of doing experiment 8 D. Previous work 12 II. JffiPARATUS AND DETAILS OP CONSTRUCTION A. Source 15 B. Ionization chamber 16 C. Power supplies 21 D. Pulse generator... 22 E. Amplifiers 23 P. Deuterium generator 27 G. Auxiliary equipment 28 III. EXPERIMENTAL PROCEDURE A. Adjustments and assembly 30 B. Calibration 32 C. Production of deuterium 32 D. Method of taking readings 34 IV. RESULTS AND DISCUSSIONS A. Method of treating data 36 B. Results ' 39 C. Possible error in results 40 D. Comparison of results with previous data.. 43 V. CONCLUSIONS 46 VI. BIBLIOGRAPHY 48 ILLUSTRATIONS P l a t e P a c i n g page I . Number-distance curve and Bragg curve 2 I I . C r o s s - s e c t i o n a l s k e t c h o f shallow i o n i z a t i o n chamber ." 16 I I I . Photograph o f shallow i o n i z a t i o n chamber.. 17 IV. B l o c k diagram o f experimental arrangement 18 V. C i r c u i t diagram o f v o l t a g e r e g u l a t e d power supply 21 VT. C i r c u i t diagram o f h i g h t e n s i o n r e g u l a t e d power supply 22 V I I . C i r c u i t diagram o f p u l s e g e n e r a t o r 23 V I I I . C i r c u i t diagram o f p r e a m p l i f i e r 24 IX. C i r c u i t diagram o f main a m p l i f i e r 26 X. General view o f apparatus 27 X I . Photograph o f deuterium g e n e r a t o r — 28 X I I . Number-distance curves f o r a i r , argon and methane 39 MEASUREMENT OF RANGES OF ALPHA PARTICLES IN GASES I. INTRODUCTION A. OBJECT The o b j e c t o f t h i s r e s e a r c h i s t o develop the apparatus and the techniques o f the shallow i o n i z a t i o n chamber and to use t h i s method i n measuring the ranges o f polonium a l p h a p a r t i c l e s i n argon, methane and deuterium and therefrom t o de-termine the s t o p p i n g powers o f these gases r e l a t i v e t o a i r . B. THEORY. CONCEPTS AND DEFINITIONS There o c c u r i n nature many n a t u r a l l y r a d i o a c t i v e sub-stances, t h a t i s substances which d i s i n t e g r a t e spontaneously e m i t t i n g gamma r a y s , b e t a p a r t i c l e s and a l p h a p a r t i c l e s . The gamma r a y s are photons o r l i g h t quanta. The wave l e n g t h o f the emitt e d gamma r a y i s w e l l - d e f i n e d and i s char-a c t e r i s t i c o f the e m i t t i n g substance. B e t a p a r t i c l e s are e l e c t r o n s whose e n e r g i e s are d i s t r i b -u t e d c o n t i n u o u s l y from zero t o s e v e r a l M.E.V., the upper l i m i t b e i n g p o o r l y d e f i n e d and c h a r a c t e r i s t i c o f the e m i t t i n g sub-stance. Beta p a r t i c l e s form crooked and s p a r s e l y i o n i z e d t r a c k s . PLATE I 2 Alph a p a r t i c l e s are doubly charged helium i o n s w i t h a w e l l d e f i n e d i n i t i a l energy and range c h a r a c t e r i s t i c o f the e m i t t i n g substance. Due to the charge and l a r g e mass the a l p h a p a r t i c l e s form s h o r t , s t r a i g h t and h e a v i l y i o n i z e d t r a c k s . As a l p h a p a r t i c l e s p e n e t r a t e the surrounding medium they g r a d u a l l y slow down due t o c o l l i s i o n s p r i m a r i l y w i t h e l e c t r o n s o f the medium. The mean range i s d e f i n e d as the most pr o b a b l e d i s t a n c e which an a l p h a p a r t i c l e o f the group t r a v e l s i n a g i v e n medium before i t l o s s e s s u f f i c i e n t energy so t h a t i t can no l o n g e r i o n i z e atoms o f t h a t medium. I t has been i m p l i e d t h a t the a l p h a p a r t i c l e s emerge from the r a d i o a c t i v e atom without b e i n g a f f e c t e d by n e i g h b o r i n g atoms--this, however, i s not always s a t i s f i e d . The number o f encounters f o r a s i n g l e a l p h a p a r t i c l e i n a group may d i f f e r from the average f o r a l l p a r t i c l e s and the p a t h l e n g t h s o f the p a r t i c l e s i n a homogenous group are d i s -t r i b u t e d about a mean value i n a manner r e p r e s e n t e d v e r y c l o s e l y by the Gaussian e r r o r f u n c t i o n . T h i s e f f e c t i s c i l l e d range s t r a g g l i n g . I n g e n e r a l the number o f p a r t i c l e s w i t h range between x and x + dx i s g i v e n by f ( x ) d x Z ( l / 7 r i ( C ) ( e - ( % ^ - ) 2 ) d x (1) where R 0 - mean range < = range s t r a g g l i n g parameter The range s t r a g g l i n g parameter i s d e f i n e d as the h a l f w i d t h o f the range d i s t r i b u t i o n curve ( P l a t e I , curve A) a t l / e o f the maximum and f o r polonium has a v a l u e o f 0.062 cms. 3 I n t e g r a t i o n o f t h i s f u n c t i o n g i v e s number-distance curve ( P l a t e I , curve B) where the mean range (a) i s the d i s t a n c e at h a l f -maximum ( p o i n t o f i n f l e c t i o n o f c u r v e ) . Thus the mean range f o r a group o f c o l l i m a t e d a l p h a p a r t i c l e s i s t h a t range f o r which h a l f o f the p a r t i c l e s have a g r e a t e r and h a l f have a s m a l l e r range. The e x t r a p o l a t e d number-distance range (b) i s d e f i n e d as the i n t e r c e p t o f the s t r a i g h t l i n e p o r t i o n o f the number-d i s t a n c e curve w i t h the d i s t a n c e a x i s . T h i s i s the most read-i l y determined f e a t u r e o f the number-distance c u r v e s . I t may be shown 1 t h a t the d i f f e r e n c e between the mean and the ex t r a p -o l a t e d ranges, s, i s a l i n e a r f u n c t i o n o f the s t r a g g l i n g par-ameter and i s g i v e n by s z ....(2) An a l p h a p a r t i c l e moving t h r u a gas l o s s e s energy by i o n i z a t i o n and e x c i t a t i o n o f the gas atoms. The i o n i z a t i o n produced p e r cm l e n g t h o f p a t h i s c a l l e d the s p e c i f i c i o n i z -a t i o n . S p e c i f i c i o n i z a t i o n i s a f u n c t i o n o f d i s t a n c e from the end o f the range. The Bragg curve ( P l a t e I) i s a p l o t o f s p e c i f i c i o n i z a t i o n v e r s u s d i s t a n c e from the end o f the range f o r a c o l l i m a t e d group o f alpha p a r t i c l e s . The curve f o r a i r r i s e s a s y m p t o t i c a l l y from zero a t the end o f the range t o a maximum 5 o r 6 mm from the end o f the range and then decreases s l o w l y t o 0.5 o f the maximum f o r p a r t i c l e s o f about 2.5 cm r e s i d u a l range. The Bragg curve shows a pronounced t a i l due t o s t r a g g l i n g . A number o f ranges may be d e f i n e d from the Bragg curve a — - p e a k range b — - c h a r a c t e r i s t i c range ( h a l f way between maximum and zero i o n i z a t i o n ) c — ^ e x t r a p o l a t e d i o n i z a t i o n range d maximum range The u s u a l method o f determining ranges i s to p l o t number-d i s t a n c e curves and to o b t a i n the e x t r a p o l a t e d number-distance range therefrom. The main advantage o f t h i s method i s t h a t i t l e a d s d i r e c t l y t o the mean range u s i n g ( 2 ) . T h i s g i v e s a v a l u e o f the range independent o f the shape o f the Bragg curve. The s t r a g g l i n g parameter can be determined from the observed s t r a g g l i n g o f the f r o n t o f the number-distance curve. The mean range may be determined from the e x t r a p o l a t e d i o n i z a t i o n range o n l y t h r u an e m p i r i c a l r e l a t i o n , x z 0.8< - 0.06mm, as deduced by Lewis and Wynn-Williams3, where x i s the d i f f e r e n c e between the mean range and the e x t r a p o l a t e d i o n i z a t i o n range. A l l p r e v i o u s experimental d e t e r m i n a t i o n s have l e d t o l a r g -e r v a l u e s o f s t r a g g l i n g than any t h e o r e t i c a l v a l u e s . The t h e o r e t i c a l v a l u e s may be too low because energy l o s s by e x c i t a t i o n <Jf the i n n e r e l e c t r o n s has been n e g l e c t e d . Observed d i s t r i b u t i o n s a l s o i n c l u d e o t h e r sources o f s t r a g g l i n g . Some o f the more important ones a r e — 1. Range S t r a g g l i n g As i s i n d i c a t e d by the d i f f e r e n t i a l number-distance curve ( P l a t e I , curve A) there i s a Gaussian d i s t r i b u t i o n o f the range about the mean. Experimental v a l u e s o f polonium are 5 given by Lewis and Wynn-Williams^ I. Curie 7 Rayton and Wilk ins 8 S e h u l z e 9 ct z at z O.64 cms << z 0.067 cms o< z 0.052 cms 0.062 cms 2. Noise Straggling Noise straggling i s due to fluctuations i n the amplifiers. 3. Ionization Straggling determine the number of ionizat ion co l l i s ions which vary rapidly at the end of the range. Further, there i s a s ta t i s -t i c a l variat ion of energy loss per c o l l i s i o n about the mean (32 vol t s / ion pair) which results i n additional straggling. 4. Angular Straggling rection due to the fact that the average alpha par t ic le de-tected may mafte a s l ight angle with the normal. The variat ion i n angle about this average angle within collimation l imi t s introduces a straggling nearly symmetrical about the mean angle. Good geometry has been defined by Oliphant 1 when c i rcu la r opening radius _ a , 1 distance between ends b" ~ T2 5. Chamber Depth Straggling The method of determining the chamber depth used by Holloway and Livingston 2 essential ly reduces the chamber depth to zero. Since the counts versus bias curves are not l inear and the result can only be obtained by extrapolating, the The exact depth of the chamber must be measured to The geometry of the collimation s l i t leads to a cor-6 e l e c t r i c a l f a c e o f the chamber as determined i s a*^average value and w i l l v a r y f o r i n d i v i d u a l p a r t i c l e s e n t e r i n g the chamber. 6. F o i l S t r a g g l i n g The r a t i o o f the s t o p p i n g power o f aluminum to air 1® i s 1.7X1G-3 thus any s l i g h t n o n - u n i f o r m i t y o f the f o i l w i l l r e s u l t i n a v a r i a t i o n i n the range o f the i n d i v i d u a l p a r t i c l e s . 7 . Source S t r a g g l i n g A l l the s t r a g g l i n g e f f e c t s mentioned so f a r are symmetrical about a mean range and as such have no e f f e c t on the value o f the mean range. Source s t r a g g l i n g i s due to t h i c k n e s s , roughness and d i r t i n e s s o f the source. I t i s not symmetrical. I t i n c r e a s e s the t o t a l s t r a g g l i n g and r e s u l t s i n a s m a l l e r v a l u e o f the mean range. A good estimate o f the source s t r a g g l i n g may be o b t a i n e d from observed d i f f e r e n c e s i n range o f p a r t i c l e s from a r e l a t i v e l y t h i c k source and a f r e s h l y p r e pared source i n which s p e c i a l p r e c a u t i o n s are taken to keep the source t h i n (monomolecular l a y e r ) , smooth and c l e a n . F o r low e n e r g i e s (below 1 M.V.) the alpha p a r t i c l e l o s e s and c a p t u r e s e l e c t r o n s thus the exact charge o f the a l p h a p a r t i c l e i s not known and i t i s d i f f i c u l t t o o b t a i n a range-energy r e l a t i o n . T h i s process o f capture and l o s s uses up energy t o o . B e t h e 1 gave a for m u l a f o r energy l o s s by i n -e l a s t i c c o l l i s i o n s and t h i s i s v a l i d o n l y i f the v e l o c i t y o f the i n c i d e n t p a r t i c l e i s l a r g e compared t o the v e l o c i t i e s o f the e l e c t r o n s i n the atom ( i . e . E ^ ( M / m ) E c i ) . , 7 dE _ 47Te4-z2 dx ~ mv 2 NB (3) where E i n c i d e n t energy o f p a r t i c l e E c i - i o n i z a t i o n p o t e n t i a l o f e l e c t r o n s B l Z l o g ( 2 M v 2 / l ) M mass o f i n c i d e n t p a r t i c l e m — m a s s o f e l e c t r o n v v e l o c i t y o f i n c i d e n t p a r t i c l e e z — c h a r g e o f i n c i d e n t p a r t i c l e W n u c l e a r charge I average e x c i t a t i o n p o t e n t i a l o f the atom Mano^ i n t e g r a t e d the eq u a t i o n o f Bethe t o o b t a i n the t h e o r e t i c a l ranges o f a l p h a p a r t i c l e s . The range o f an a l p h a p a r t i c l e i s g i v e n by y0 t' Range i m p l i e s mean range a t 1 5 ° C and 7 6 0 mm Hg. E x t e n s i o n o f the range-energy r e l a t i o n t o v e l o c i t i e s g r e a t e r than those o f al p h a p a r t i c l e s i s r e l i a b l e except r e l a t i v i s t i c c o r r e c t i o n s must be a p p l i e d . B e t h e 1 has g i v e n a s t o p p i n g formula f o r a l p h a p a r t i c l e s the o n l y d i f f i c u l t y b e i n g the accurate d e t e r m i n a t i o n o f I . I f I i s known, the s t o p p i n g power may be determined f o r a l l e n e r g i e s except below 1 M.V. where the capture and l o s s o f e l e c t r o n s becomes important. Since no th e o r y i s a v a i l a b l e f o r v - . dy . J M I ' dy' " 327rz^e 4HZ J l o g y + 128irz*e4ric*NZ J l o g y ' where y : 4m 2y4 f 1* ..(4) 8 e n e r g i e s below 1 M.V. one must depend upon experimental r e -s u l t s . Bethe d e f i n e s the s t o p p i n g power, s, r e l a t i v e t o a i r as s - B / B a i r (5) Due to the u n c e r t a i n i t y o f the th e o r y o f s t o p p i n g f o r low e n e r g i e s s t r a g g l i n g c a l c u l a t i o n s are not r e l i a b l e . Gibson and E y r i n g ^ have p r e s e n t e d some d i s c u s s i o n o f a t h e o r e t i c a l e x p r e s s i o n f o r the i o n i z a t i o n o f high-energy p a r t i c l e s i n gases. The i o n i z a t i o n p e r cm p e r second (dJ/dR) has been gi v e n as dJ/dR : (NS/2)[R ( 1 * I 3 1 ^ 2 ) t 2 e 9 R 2 / 4 3 H ^ . .(6) where the con s t a n t NS/2 may be ob t a i n e d by the c o n d i t i o n t h a t the i n t e g r a l t o the end o f the range must give the t o t a l i o n i z a t i o n . C. METHODS OF DOING EXPERIMENT The most important methods f o r measurement o f range o r i o n i z a t i o n o f alpha p a r t i c l e s are--1. Shallow I o n i z a t i o n Chamber 2 The shallow i o n i z a t i o n chamber may be used i n two waya a. I t may be used to count the number o f p a r t i c l e s a r r i v i n g at a c e r t a i n p o i n t , i . e . a l l p u l s e s are counted r e -g a r d l e s s o f s i z e . By v a r y i n g the d i s t a n c e from the source one may p l o t number-distance curVes and then the most e a s i l y determined q u a n t i t y i s the e x t r a p o l a t e d number-distance range. The d e t e r m i n a t i o n o f the exact mean range i s handicapped by the f i n i t e r e s o l u t i o n o f the chamber due to i t s depth. 6 n l y 9 p u l s e s are counted which exceed a c e r t a i n amplitude, determined by the n o i s e l e v e l , thus p u l s e s due to p a r t i c l e s j u s t e n t e r i n g the chamber are n o t counted. The minimum depth a p a r t i c l e must p e n e t r a t e the chamber to be d e t e c t e d must be added t o the d i s t a n c e to the fa c e o f the chamber i n determining the range. I n o r d e r t o determine the chamber depth p e n e t r a t i o n i t i s ne c e s s a r y t o know the v a r i a t i o n o f i o n i z a t i o n w i t h d i s t a n c e from the end o f the range. b. The shallow i o n i z a t i o n chamber may a l s o be used i n measuring the s i z e o f the p u l s e s . Prom a measurement o f the p u l s e s i z e a l o n g the t r a c k o f an alpha p a r t i c l e the Bragg curve may be determined. The s p e c i f i c i o n i z a t i o n o f a s i n g l e a l p h a p a r t i c l e t o the end o f i t s range may a l s o be determined from a c o n s i d e r a t i o n o f the p u l s e s i z e . 2. D i f f e r e n t i a l I o n i z a t i o n Chamber 2 1 A v a l u a b l e m o d i f i c a t i o n came from the development o f the d i f f e r e n t i a l i o n i z a t i o n chamber. Here the i o n i z a t i o n from two shallow i o n i z a t i o n chambers w i t h o p p o s i t e a p p l i e d poten-t i a l s i s c o l l e c t e d upon a common g r i d . Por p a r t i c l e s c r o s s i n g b o t h chambers the i o n i z a t i o n e s s e n t i a l l y c a n c e l s g i v i n g no p u l s e — o n l y those p a r t i c l e s are r e c o r d e d which stop w i t h i n the chamber. T h i s method e s s e n t i a l l y measures the p o s i t i o n o f maximum change o f i o n i z a t i o n a l o n g the p a t h o f an a l p h a p a r t -i c l e . T h i s maximum response determines a c h a r a c t e r i s t i c range d i f f e r i n g from the mean range o n l y by a con s t a n t d i f f e r e n c e which depends on the measuring apparatus and not upon the s t r a g g l i n g o f the alpha-group. 10 3. D i s c r i m i n a t i n g Chamber 2 1 Having o b t a i n e d a rough t r a c e o f the d i s t r i b u t i o n o f the i o n i z a t i o n a l o n g the t r a c k i n an i o n i z a t i o n chamber one may d i s c r i m i n a t e between alpha p a r t i c l e s and protons and o t h e r p a r t i c l e s . The d i s c r i m i n a t i n g chamber i s e s s e n t i a l l y a g r i d e d shallow i o n i z a t i o n chamber. The purpose o f the g r i d b e i n g t o a v o i d charge induced on the c o l l e c t i n g e l e c t r o d e b e f o r e the a c t u a l c o l l e c t i o n o f i o n s . The p o t e n t i a l s are arranged so t h a t i n the i o n i z a t i o n chamber p r o p e r the d r i f t v e l o c i t y i s s m a l l e r than i n the chamber behind the g r i d . The r a t e o f c o l -l e c t i o n o f i o n s i s p r o p o r t i o n a l t o the r a t e o f a r r i v a l a t the g r i d which i n t u r n i s p r o p o r t i o n a l to the i o n i z a t i o n d e n s i t y of t h a t p o r t i o n o f the t r a c k . 4. S c i n t i l l a t i o n M e t h o d 2 2 I n d i v i d u a l appha p a r t i c l e s w i l l , on s t r i k i n g a s p e c i a l l y p r e p a r e d s c r e e n , give r i s e t o momentarily i l l u m i n a t e d spots and these s c i n t i l l a t i o n s may be counted i f n o t too num-erous . Alpha p a r t i c l e s may be counted on a z i n c s u l p h i d e , barium p l a t i n o - c y a n i d e o r c a l c i u m t n n g s t a t e screen when p l a c e d a t v a r i o u s p o s i t i o n s between the souree and the o b s e r v e r . The p l o t s are number-distance d i s t r i b u t i o n curves. I t has been found t h a t ranges determined i n t h i s way are u s u a l l y s h o r t compared to those o b t a i n e d by o t h e r methods. T h i s may be due to the i n a b i l i t y o f the o b s e r v e r to count f a i n t f l a s h e s i n the presence o f b r i g h t ones. A l s o , t o o b t a i n a f l a s h , i t i s r e q u i r e d to p l a c e the source a t a d i s t a n c e l e s s than t h a t a t which the a l p h a p a r t i c l e stops i o n i z i n g and t h i s d i s t a n c e 1 1 depends on the energy r e q u i r e d t o produce d e t e c t a b l e r a d i o -lumeniscence. 5. Wilson C l o u d Chamber The W i l s o n c l o u d chamber measures the most prob a b l e d i s t a n c e a p a r t i c l e w i l l t r a v e l b e f o r e i t stops p r o d u c i n g c l o u d f o r m i n g i o n s . A p a i r o f s t e r e o s c o p i c p i c t u r e s are taken s i m u l t a n e o u s l y o f the c l o u d chamber t r a c k s a t a d e f i n i t e i n -s t a n t d u r i n g the c y c l e o f the chamber. Prom the n e g a t i v e s p r o j e c t e d on a sc r e e n one may measure the l e n g t h o f the t r a c k images. The most probable v a l u e o f the range i s i n t e r p r e t e d here as the mean range o f the group. T h i s method g i v e s a good estimate o f s t r a g g l i n g i f e r r o r s o f measurement are con-s i d e r e d . The c l o u d chamber method has a few disadvantages however; the g r e a t e s t disadvantage b e i n g perhaps the d i f f i c u l t y i n de t e r m i n a t i o n o f the gas d e n s i t y a t the time o f f o r m a t i o n o f the t r a c k s . There i s a f u r t h e r disadvantage i n e s t i m a t i n g the exa c t range from c l o u d chamber t r a c k s f o r the i o n i z a t i o n o f an alpha p a r t i c l e decreases r a p i d l y towards the end of i t s range. 6 . Photographic Method Any i o n i z i n g r a d i a t i o n o r p a r t i c l e w i l l produce a c t i v a t i o n on a b s o r p t i o n i n the emulsion o f a photographie p l a t e . Thus i n development the a c t u a l t r a c k i s r e v e a l e d . F o r r e l i a b l e r e s u l t s the emulsion must be f i n e g r a i n e d . T h i s method i s a means o f g e t t i n g v a l u e s o f a mean range but the e r r o r s i n measuring t r a c k g r a i n s are l a r g e enough t o mask s t r a g g l i n g e f f e c t s . The photographic method i s u s e f u l o n l y as 12 a check. 7. Magnetic D e f l e c t i o n The magnetic d e f l e c t i o n method i s used t o measure v e l o c i t i e s o f a l p h a p a r t i c l e s and v e l o c i t y s t r a g g l i n g coef-f i c i e n t s i n c o n j u n c t i o n w i t h c l o u d chambers o r i o n i z a t i o n cham-b e r s . T h i s method i s v e r y u s e f u l f o r s t r a g g l i n g i n v e s t i g a t i o n s but s i n c e there does not e x i s t a t r u e r e l a t i o n between range and v e l o c i t y the range d e t e r m i n a t i o n s thus o b t a i n e d are i n e x a c t . 8. E l e c t r o s c o p e Method I o n i z a t i o n v e r s u s d i s t a n c e curves are o b t a i n e d by c o v e r i n g the source w i t h f o i l s o f v a r i o u s t h i c k n e s s ( o f which the a i r e q u i v a l e n t s are known) and measuring the r a t e o f d i s -charge o f the e l e c t r o s c o p e . The r e s u l t i n g curves show two or more d i s t i n c t s l o p e s from which an a l p h a p a r t i c l e range may be deduced. Bragg curves may a l s o be p l o t t e d from a c o n s i d e r -a t i o n of the r a t e o f discharge o f the e l e c t r o s c o p e . D. PREVIOUS WORK 1. Holloway and L i v i n g s t o n 2 A shallow i o n i z a t i o n chamber o p e r a t i n g a l i n e a r p u l s e a m p l i f i e r and counter was used f o r the study o f the extreme range o f polonium a l p h a p a r t i c l e s . The output p u l s e s were measured by a v a r i a b l e t h y r a t r o n b i a s and were s t r i c t l y pro-p o r t i o n a l t o the i o n i z a t i o n i n the chamber. U s i n g two cham-bers o f 1 mm and 2 mm depth, the i o n i z a t i o n n e a r the end o f the range was determined. Por p a r t i c l e s which stopped i n the chamber the i o n i z a t i o n was d i r e c t l y p r o p o r t i o n a l t o the r e s i d u a l energy a t the f a c e . The d i f f e r e n c e i n the two cham-bers gave d i r e c t l y the range-energy r e l a t i o n t o w i t h i n the l a s t mm. The Bragg curve was o b t a i n e d w i t h the 1 mm chamber throughout the l a s t 2.8 cm o f range. C o n s i d e r a t i o n o f d a t a l e d t o range-energy r e l a t i o n t o w i t h i n 0.005 cm o f the end o f the range. The e s t i m a t e d average e r r o r was l e s s than 0.5$ which gave energy measurements to w i t h i n t5K.V. (assuming ave-rage energy l o s s p e r i o n formed remained c o n s t a n t ) . 2. 1. D i c k ; P. P a l k - V a i r a n t and J . R o s s e l 1 1 U s i n g a shallow i o n i z a t i o n chamber w i t h a s i x stage a m p l i f i e r , they measured energy o f i o n i z a t i o n , r e combination e f f e c t s and s t o p p i n g power f o r a l p h a p a r t i c l e s i n v a r i o u s gases. Work had been done p r e v i o u s l y f o r p r e s s u r e s below atmospheric and a t atmospheric p r e s s u r e so they chose to work a t p r e s s u r e s above atmospheric. R e s u l t s were o b t a i n e d f o r the f o l l o w i n g gases (99% pure) N 2 , O 2 , 0 1 2 , CO,,, BP^, GH^ and C F 4 . They found t h a t recombination i s v e r y marked a t p r e s s u r e s o f s e v e r a l atmospheres. 3. Gurney 1 2 Gurney measured the i o n i z a t i o n i n monatomic and d i -atomic gases as w e l l as the s t o p p i n g power o f gases f o r a l p h a p a r t i c l e s o f d i f f e r e n t v e l o c i t i e s by means o f a shallow i o n i z -a t i o n chamber w i t h a g o l d l e a f e l e c t r o s c o p e as deleting d e v i c e . 4. R. N a i d u 1 3 Naidu made a c a r e f u l study o f the Bragg curve by measuring i o n i z a t i o n c u r r e n t s . H i s apparatus c o n s i s t e d o f a p a r a l l e l p l a t e i o n i z a t i o n chamber and a Wulf e l e c t r o m e t e r as 1: d e t e c t i n g d e v i c e . He deduced ranges and s t o p p i n g powers ( d e f i n e d as s l R Q / R ; R O— r a n g e i n a i r ) f o r helium, neon and argon from a study o f t h e i r normal Bragg curves. 5 . l e w i s and Wynn-Williams^ Groups o f a l p h a p a r t i c l e s were analysed by means o f the d i f f e r e n t i a l chamber connected t o a pul s e a m p l i f i e r ; Re-s u l t s were o b t a i n e d i n the form o f s t r a g g l i n g curves (G-aussian e r r o r c u r v e s ) , by the shape o f which the homogeneity o f each alpha p a r t i c l e group was determined and-from the p o s i t i o n o f which the mean range was o b t a i n e d d i r e c t l y . Mean ranges i n a i r were o b t a i n e d f o r alpha p a r t i c l e s from radium A , Thoron, thorium A, a c t i n o n and a c t i n i u m A; the range f o r a l p h a p a r t -i c l e s from polonium was a l s o measured. 6 . Mano 1 4 Mano used the magnetic d e f l e c t i o n method t o study the a b s o r p t i o n o f alpha p a r t i c l e s i n a i r , hydrogen, helium, neon and o t h e r gases. 7. C h a n g 1 5 Chang s t u d i e d the alpha p a r t i c l e s from polonium w i t h as a l p h a - r a y spectrograph. He found t h a t the main group o f alpha p a r t i c l e s from polonium have an energy o f 5 . 3 0 3 M.e.V. There was no i n d i c a t i o n o f a d i s c r e t e group i n the energy r e g i o n above 5 . 3 0 3 M.e.V., however he found twelve much weaker groups i n the lower energy r e g i o n r a n g i n g down t o 3 . 6 8 5 M.e.V. These r e s u l t s have s i n c e been d i s c u s s e d by Wadey 2 8 and i t seems l i k e l y t h a t the weaker l i n e s were due t o Chang's method o f p r e p a r i n g s ources. 15 I I . APPARATUS AND DETAILS OF 0OBSTRUCTION A. SOURCE^ > 20 An alpha p a r t i c l e source was r e q u i r e d f o r t h i s experiment. 3/10 o f a m i l l i c u r i e o f RaD was a v a i l a b l e a t the time from which the polonium ( R a F — a n alph a e m i t t o r ) was e x t r a c t e d f o r t h i s experiment and a t the same time a RaE source ( a b e t a e m i t t o r ) was p r e p a r e d f o r another experiment. The method o f p r e p a r a t i o n was e l e c t r o - c h e m i c a l d e p o s i t i o n ; the d i r e c t i o n o f the r e a c t i o n s b e i n g determined by the e l e c t r o -motive s e r i e s and the metals used were chosen a c c o r d i n g l y . A 5/8 i n c h b r a s s cap was made i n the c e n t e r o f which was s o l d e r e d a s i l v e r b u t t o n f o r the RaF source and a s i m i l a r cap w i t h a n i c k e l b u t t o n was made f o r the RaE source. The RaD was d i s s o l v e d i n 6 c. c. o f 1/2 normal HC1. The s i l v e r b u t t o n was tho r o u g h l y c l e a n e d i n n i t r i c a c i d and then r o t a t e d i n the RaD-HCl s o l u t i o n a t 1000 r.p.m. f o r one hour. A c c o r d i n g to B j o r k s t e d and M i t c h e l l 2 0 the r o t a t i o n ensures a t h i n uniform d e p o s i t i o n and i n one hour 98% o f the polonium i s removed. The r e s i d u a l s o l u t i o n then c o n t a i n e d o n l y RaD and RaE. The r e s i d u a l s o l u t i o n was c a r e f u l l y evaporated t o dryness and r e d i s s o l v e d i n 0.6 normal HC1. The n i c k e l d i s c was r o t a t e d i n a s i m i l a r f a s h i o n to remove the RaE. H H W <! P4 I 16 The edge o f the source h o l d e r ( b r a s s c a p ) , which s h o u l d be c l e a n , was p a i n t e d w i t h g l y p t a l b e f o r e p r e p a r a t i o n o f the source and c a r e f u l l y washed a f t e r p r e p a r a t i o n o f the source and then r e p a i n t e d w i t h g l y p t a l . I n t h i s way the a c t i v e a r e a was w e l l d e f i n e d and the source c o u l d be handled s a f e l y . The source was c o l l i m a t e d w i t h a 'pepper pot' which con-s i s t s o f a p e r f o r a t e d b r a s s cap and f i t s t i g h t l y over the source h o l d e r . The h o l e s were made o f diameter and l e n g t h so t h a t d i a m e t e r / l e n g t h = 1/15 which i s i n l i n e w i t h the recom-mended r a t i o o f O l i p h a n t who used d i a m e t e r / l e n g t h z 1/12 and then the angular s t r a g g l i n g i s s m a l l . The number o f h o l e s i n the pepper p o t was chosen so as t o gi v e a maximum c o u n t i n g r a t e of 2000 p e r minute ( a p p r o x i m a t e l y ) . B. IONIZATION CHAMBER 1. C o n s t r u c t i o n P l a t e I I shows a c r o s s - s e c t i o n a l view o f the shallow i o n i z a t i o n chamber and P l a t e I I I i s a photograph o f the shallow i o n i z a t i o n chamber i n p a r t s . The c y l i n d r i c a l s h e l l o f the chamber and f l a n g e s are made o f 3/16 i n c h copper. Each sec-t i o n i s at t a c h e d t o the next by means o f s i x #10 b o l t s . The chamber as a whole i s made up o f f o u r s e c t i o n s . Prom l e f t t o r i g h t -S e c t i o n I c o n t a i n s the p r e a m p l i f i e r and i s separated from the i o n i z a t i o n chamber p r o p e r by a p a r t i t i o n which has been i n s e r t e d so t h a t the chamber may be evacuated. The l e a d t o the f i r s t tube o f the p r e a m p l i f i e r i s supported by mica a t t h i s PLATE I I I SHALLOW IONIZATION CHAMBER 17 p o i n t . S e c t i o n I I p r o v i d e s the space r e q u i r e d between the pre -a m p l i f i e r and the source o f p u l s e s . The l e a d t o the f i r s t tube which c o n s i s t s o f a b r a s s r o d runs down the c e n t e r o f t h i s s e c t i o n . S e c t i o n I I I i s the i o n i z a t i o n chamber p r o p e r and c o n t a i n s the c o l l e c t i n g e l e c t r o d e , guard r i n g and h i g h p o t e n t i a l p l a t e . The c o l l e c t i n g e l e c t r o d e and guard r i n g are made o f copper and are p o l i s h e d . The c o l l e c t i n g e l e c t r o d e i s supported by quartz c y l i n d e r s as shown. The h i g h p o t e n t i a l p l a t e i s made of t h i n sheet copper. There i s a h o l e i n the c e n t e r and s m a l l e r h o l e s around the c e n t e r h o l e . The hole i n the c e n t e r i s covered by aluminum f o i l o f a p p r o p r i a t e t h i c k n e s s . The h i g h p o t e n t i a l p l a t e i s supported and i n s u l a t e d from the chamber s h e l l by a l u c i t e c y l i n d e r . The h i g h p o t e n t i a l l e a d runs t h r u the c o a x i a l socket which i s mounted on the chamber s h e l l . S e c t i o n I T c o n t a i n s the moveable c a r r i a g e on which i s mounted the c o l l i m a t e d source. The c a r r i a g e i s mounted on a screw and i s supported by two p a r a l l e l r o d s . The screw i s o f one mm p i t c h and i s p r o v i d e d w i t h a drum which i s graduated i n hundreds so t h a t , t h e o r e t i c a l l y , p o s i t i o n may be r e a d accur-a t e l y t o 1/100 mm. P a c k i n g has been p u t around the screw where i t emerges from the chamber t o make i t a i r - t i g h t . The p o s i t i o n o f the source may be rea d from an i n d i c a t i n g device which i s l o c k e d t o the screw by means o f a c h a i n d r i v e and re c o r d s the number o f complete t u r n s o f the screw. Two tubes are p r o v i d e d so t h a t the chamber may be PLATE IV Jl oni z, FiT/orv PRE- > < M- POT Supply Pu RTE SUPPLY 5 c S>uPPLy 7 P £ £CCPJ>£/? BLOCK DIAGRAM OF EXPERIMENTAL ARRANGEMENT 18 evacuated. 2. Theory o f O p e r a t i o n The p r i n c i p l e o f o p e r a t i o n i s d e s c r i b e d w i t h r e f e r -ence to P l a t e I I . A c o l l i m a t e d source o f a l p h a p a r t i c l e s i s mounted on the c a r r i a g e and i f the c a r r i a g e i s c l o s e enough to the c o l l e c t i n g e l e c t r o d e the a l p h a p a r t i c l e s w i l l p e n e t r a t e the aluminum f o i l and i o n i z e the gas i n the r e g i o n between the h i g h p o t e n t i a l p l a t e and the c o l l e c t i n g e l e c t r o d e . The p o s i t i v e and n e g a t i v e i o n s formed between the f o i l and the c o l l e c t i n g e l e c t r o d e w i l l move i n opposite d i r e c t i o n s due t o the p o t e n t i a l g r a d i e n t . The charge c o l l e c t e d on the e l e c t r o d e w i l l produce a p u l s e , V z Q/C ( C — c a p a c i t y o f chamber), on the g r i d o f the f i r s t tube o f the p r e a m p l i f i e r where the p u l s e i s a m p l i f i e d . The p r e -a m p l i f i e r i s designed t o give a l a r g e s i g n a l to n o i s e r a t i o . The p u l s e i s then a m p l i f i e d to such an amplitude i n the main a m p l i f i e r t h a t i t i s able to operate the s c a l e r which i n t u r n operates a mechanical r e c o r d e r . An o s c i l l o s c o p e i s a l s o a t t a c h e d t o the output o f the main a m p l i f i e r so t h a t the p u l s e s may be examined. A b l o c k diagram o f the experimental arrangement i s shown i n P l a t e IV. 3. Requirements and S p e c i a l C h a r a c t e r i s t i c s The c a p a c i t y o f the i o n i z a t i o n chamber ( c o l l e c t i n g e l e c t r o d e t o ground) must be kept as s m a l l as p o s s i b l e . The charge c o l l e c t e d due t o a l p h a i o n i z a t i o n f o r a chamber depth o f t h r e e mm i s o n l y o f the o r d e r o f 1 0 " 1 5 coulombs and the 19 v o l t a g e , V - 0/0, produced by t h i s charge must be o f such a magnitude t h a t the s i g n a l , V, r i s e w e l l above the n o i s e l e v e l o f the a m p l i f i e r . F o r t h i s reason the c a p a c i t y must be kept at a low v a l u e . The c o l l e c t i n g e l e c t r o d e s h o u l d be made of copper and should have a guard r i n g . Copper makes a good conductor o f e l e c t r i c i t y and i s the ' c l e a n e s t * (radi©actively) o f the more common metals. The guard r i n g maintains a uniform f i e l d be-tween the c o l l e c t i n g e l e c t r o d e and the h i g h poteri&al p l a t e which i n s u r e s a more u n i f o r m c o l l e c t i o n o f charges. The i n s u l a t o r s must be chosen c a r e f u l l y . E s p e c i a l l y imp-o r t a n t i s the i n s u l a t i o n o f the c o l l e c t i n g e l e c t r o d e where su r f a c e leakage o r a b s o r p t i o n o f charges ( s o a k i n g up) i s un-d e s i r a b l e s i n c e i t i s important t o keep the i n p u t p u l s e s as l a r g e as p o s s i b l e . F o r t h i s reason the s u r f a c e s o f the i n s u l -a t o r s must be kept c l e a n . Quartz, amber and s u l f u r are some of the b e s t i n s u l a t o r s . The i n s u l a t i o n f o r the c o l l e c t i n g e l e c t r o d e used here was q u a r t z . I n s u l a t i o n o f the h i g h p o t e n t i a l e l e c t r o d e i s n o t so important. Ebonite rubber o r l u c i t e w i l l do. l u c i t e has been used here. Complete e l e c t r i c s h i e l d i n g i s i m p e r a t i v e . Any r a d i a t i o n p i c k - u p would r e s u l t i n extraneous p u l s e s which would i n c r e a s e the u n c e r t a i n ! t y o f the f i n a l r e s u l t s . The t h i c k copper s h e l l was meant t o p r o v i d e s h i e l d i n g among o t h e r t h i n g s . A l s o , t o minimize s t r a y pick-up o f e l e c t r o m a g n e t i c d i s t u r b a n c e s the h i g h p o t e n t i a l e l e c t r o d e should have a l a r g e c a p a c i t y to e a r t h and the h i g h v o l t a g e l e a d should be w e l l s h i e l d e d and t h i s 20 s h i e l d must be grounded. The window may be made o f mica coated w i t h g o l d f o i l o r s p u t t e r e d w i t h s i l v e r o r a t h i n m e t a l l i c f o i l may be used. The a i r e q u i v a l e n t o f the f o i l s h ould be as sm a l l as p o s s i b l e ye t c o n s i s t e n t w i t h r i g i d i t y and u n i f o r m i t y . F o r a f o i l where uni f o r m t h i c k n e s s i s d e s i r e d and y e t small a i r e q u i v a l e n t and knock-on p r o t o n s are u n d e s i r a b l e aluminum f o i l i s s u i t a b l e . The f o i l used here was r o l l e d aluminum o f a i r e q u i v a l e n t 1.020 ems. The a i r e q u i v a l e n t o f the f o i l was determined by weigh-i n g a sheet o f known a r e a and u s i n g the average o f Marsden & Richar d s o n and Rosenblum 1 value f o r s t o p p i n g power o f aluminum compared t o a i r . 1.62 » 1.51 = 1.57 m s/cm2 The f o i l must make good e l e c t r i c a l c o n t a c t t o the h i g h poten-t i a l e l e c t r o d e . I t was found t h a t Aqua-dag which i s a c o l l o i d o f g r a p h i t e served w e l l as an adhesive f o r a t t a c h i n g the aluminum f o i l t o the h i g h p o t e n t i a l p l a t e . A l l p a r t s o f the chamber must be kept c l e a n . Sometimes the i n s i d e o f the chamber i s p a i n t e d w i t h carbon b l a c k o r ot h e r very pure o r g a n i c chemical such as c o l l o d i a n t o keep a l p h a p a r t i c l e c ontamination t o a low v a l u e . A chamber depth o f Qt.2 to 1.5 cms may be u s e d — t h e c h o i c e depending on the s i g n a l t o n o i s e r a t i o o f the a m p l i f i e r s . A c t u a l l y , the more shallow the chamber the more r e l i a b l e are the r e s u l t s s i n c e t h e r e i s l e s s s t r a g g l i n g due to chamber depth. A depth o f 0.3 cm has been chosen here which was p o s s i b l e due to the h i g h s i g n a l t o n o i s e r a t i o o f the a m p l i f i e r s . 21 The p o t e n t i a l g r a d i e n t a c r o s s the chamber may be 100 t o 10,000 vo l t s / c m . The upper l i m i t i s governed by the breakdown o f the gas and the lower l i m i t by the c o l l e c t i n g time o r p u l s e l e n g t h r e q u i r e d . At a h i g h p o t e n t i a l there i s a corona d i s -charge between the h i g h p o t e n t i a l p l a t e and the i n s u l a t o r which may gi v e extraneous p u l s e s ; f o r t h i s reason i t i s a d v i s -able t o operate below maximum p o t e n t i a l . The p o t e n t i a l grad-i e n t used here was about 5000 v o l t s / c m ( i . e . 1500 v o l t s f o r 3 mm). Such a g r a d i e n t g i v e s a p u l s e l e n g t h f o r n e g a t i v e i o n s o f about 20 microseconds. I f p a r t s o$ the chamber move r e l a t i v e l y tooeach o t h e r the c a p a c i t y o f the system changes and v a r y i n g p o t e n t i a l s w i l l be produced which r e s u l t i n sp u r i o u s p u l s e s . Attempts have been made t o keep the raicropheny t o a low v a l u e by p e r f o r a t i n g the h i g h p o t e n t i a l p l a t e w i t h l a r g e h o l e s f o r low f r e q u e n c i e s and sm a l l h o l e s f o r h i g h f r e q u e n c i e s (see P l a t e I I I ) and mounting the i o n i z a t i o n chamber and a m p l i f i e r s on sponge rubber. The chamber has been p l a c e d i n s i d e a box to reduce a c o u s t i c a l v i b r a t i o n s ; The p r e a m p l i f i e r was mounted i n s i d e the c y l i n d r i c a l s h e l l o f the i o n i z a t i o n chamber so t h a t i t was w e l l s h i e l d e d and the l e a d t o the f i r s t tube was kept s h o r t . C. POWER SUPPLIES 1. P l a t e and Cathode S u p p l i e s Two v o l t a g e r e g u l a t e d power s u p p l i e s have been b u i l t the c i r c i i t o f which i s shown i n P l a t e V. They are used f o r the plate and cathode supplies of the amplifiers. The power supplies were s tabi l ized against fluctuations i n the input voltage and fluctuations i n the output current. These power supplies regulate sa t i s fac tor i ly i n the 200 to 300 vo l t range. The posit ive side of the cathode supply was grounded so that i t s output voltage was negative. A dropping res is tor was inserted i n series with the output since the voltage required for the cathode bias was i n the -50 to -150 vol t range. 2. High Tension Power Supply This i s the power supply (Plate VT) for the high potential on the ionizat ion chamber. The only requirement* was s tab i l iza t ion against input voltage fluctuations and a ripple free output. After assembly, oscillatory-discharge type pulses appeared i n the output which produced spurious counts even though the pulses were only of the order of m i l l i -vo l t s . I t was found that insert ion of capacitance between output and ground smoothed the output. This power supply delivers up to 2500 vol ts and regulates well for a var iat ion of input voltage between 80 and 135 volts with zero load current. D. PULSE GENERATOR The pulse generator (Plate VII) consists of a cathode coupled multivibrator, amplifier and attenuator output. I t was bu i l t for cal ibrat ing and adjusting the amplifier and for cal ibrat ing the soaler discriminator. The frequency of pulses may be varied continuously from 23 0.41 to 3*1 KC/S. The p u l s e l e n g t h may be v a r i e d i n steps from 2.5 to 63 microseconds. The a t t e n u a t o r was b u i l t so t h a t output p u l s e s w i t h the same o r d e r o f amplitude as those from the i o n i z a t i o n chamber were o b t a i n e d . The p u l s e amplitude output was v a r i a b l e from 28.9*10*"^ to 0.21 v o l t s peak. E. AMPLIFIERS 1. General Since the i n p u t p u l s e s are o n l y o f the order o f 6.1K 10"*^ v o l t s and 5 to 100 v o l t s are r e q u i r e d to actuate the s c a l e r , an over a l l v o l t a g e g a i n o f 10^ to 10^ i s r e q u i r e d . The g a i n o f the a m p l i f i e r used was about 10^. The i n p u t p u l s e s are about 20 microseconds l o n g and have a f a i r l y sharp r i s e time. I n o r d e r t o accept the p r o p e r frequency components a f a i r l y wide frequency band i s r e q u i r e d . I t i s important to choose the band width c a r e f u l l y s i n c e too wide a band would o n l y i n c r e a s e the n o i s e l e v e l and too narrow a band would attenuate and d i s t o r t the p u l s e s . T h i s a m p l i f i e r has a frequency response a t 3db down from 2000/S t o 100 K D / S . Since each r u n i s taken over a p e r i o d o f hours the ampli-f i e r s must be v e r y s t a b l e . To i n s u r e s t a b i l i t y b a t t e r i e s and r e g u l a t e d power s u p p l i e s have been used as p l a t e and cathode s u p p l i e s . The a m p l i f i e r must be l i n e a r over the range o f p u l s e s i z e s which may be o b t a i n e d from the i o n i z a t i o n chamber. T h i s a m p l i f i e r was found t o be l i n e a r over the r e q u i r e d range. A storage b a t t e r y has been used f o r h e a t e r supply so t h a t PLATE V I I I PRE-AMPLIFIER 24 'hum' may be kept down. The amplifier and a l l connecting leads must be completely shielded to reduce electro-magnetic pick-up. The preamplifier was mounted inside the s h e l l of the i o n i z a t i o n chamber thus completely shielded even from the main amplifier. The main amplifier was mounted by i t s e l f inside a metal chassis and was thus shielded. Common ground points were used through-out to avoid c i r c u l a t i n g currents i n the chassis. Microphonic response to acoustical noise or v i b r a t i o n has been minimized by mounting the chamber and tubes on sponge rubber supports. The amplifiers have been provided with balanced cathode follower outputs. Cathode followers provide maximum power gain rather than voltage gain. 2. Preamplifier The preamplifier (Plate VIII) consists of one stage of amplification and a cathode follower output. The tubes chosen are minirtrure 6AK5 tubes because the space a l l o t t e d f o r the pre-amplifier i s very small and the 6AK5 has a high mu and a com-parat i v e l y low noise factor. Amplification was s a c r i f i c e d i n the f i r s t stage i n order to obtain a high signal to noise r a t i o . Pair)titbn noise often amounts to one-third of the noise. I t i s customary to connect t h i s tube as a triode to keep p a r t i t i o n noise down. Since the suppressor and cathode are i n t e r n a l l y connected i n the 6AK5, only the screen was t i e d to the plate. low plate and screen 25 v o l t a g e (20 t o 30 v o l t s ) on the f i r s t tube reduce the gas n o i s e . Noise due to cathode emission v a r i e s e x p o n e n t i a l l y as f i l a m e n t v o l t a g e thus n o i s e may be reduced by u n d e r h e a t i n g the f i r s t tube. The f i l a m e n t v o l t a g e used here was 5.2 v o l t s i n -stead o f 6.3 which gave some improvement. The time co n s t a n t o f the i n p u t c i r c u i t s hould be as l a r g e as p o s s i b l e to o b t a i n a good s i g n a l to n o i s e r a t i o . F o r i f I s i g n a l c u r r e n t 1 ^ — i o n c u r r e n t o r c u r r e n t due t o g r i d e m i s s i o n . I t i s f i x e d and p r a c t i c a l l y independent o f g r i d v o l t s . I 2 — c u r r e n t due to e l e c t r o n s a r r i v i n g a t the g r i d from cathode and decreases w i t h i n c r e a s i n g R. R - — i n p u t g r i d l e a k . 0 frequency k Boltzmann con s t a n t T - — a b s o l u t e temperature Mean square n o i s e c u r r e n t l 2eI]A* ¥ 2 e l 2 ^ E q u i v a l e n t n o i s e c u r r e n t due to r e s i s t a n c e R Z 4kT R ( S i g n a l ^ 2 _ I 2 x N o i s e ' " 4kTa v » 2 e I l A J + 2el2*>> R i n c r e a s i n g R, a l l terms i n the denominator decrease except the second which remains co n s t a n t . The above i n d i c a t e s t h a t i n c r e a s i n g the i n p u t g r i d l e a k i n c r e a s e s the s i g n a l t o n o i s e r a t i o . A number o f e x t e r n a l g r i d l e a k s were t r i e d f o r the i n p u t c i r c u i t but the b e s t s i g n a l to n o i s e r a t i o was o b t a i n e d w i t h f l o a t i n g g r i d . The e s t i m a t e d g r i d l e a k due to s u r f a c e 26 leakage of the insulators and leakage thru the atmosphere was about IO 9 ohmg. ^n electrometer triode with a f l o a t i n g g r i d i s much more sensitive than an electrometer triode with ex-t e r n a l g r i d leak as has been shown i n the above c a l c u l a t i o n . The s e n s i t i v i t y l i m i t , however, i s set by the input time-con-stant. To reduce the r e s i s t o r noise which arises due to the granular nature of carbon, wire-wound r e s i s t o r s were used i n the early stages of the amplifier. A separate battery was used f o r the plate supply of the preamplifier. In t h i s way the plate of the f i r s t tube of the preamplifier had no common impedances with the plates of the main amplifier and thus the p o s s i b i l i t y of spurious counts due to feedback was reduced. The input capacity i s mainly due to the i o n i z a t i o n cham-ber but s t i l l may be kept at a low value by choosing a miniature tube f o r the f i r s t tube as was done here. Reducing the input capacity increases the signal to noise r a t i o since the noise i s independent of the input capacity. 3. Main Amplifier The main amp l i f i e r ( P l a t e l l ) consists of two p a r a l l e l channels of three tubes (6AC7) with a single cathode follower output (6AG7)• I t i s extremely d i f f i c u l t to make a high gain a m p l i f i e r stable. Various'tricks'are employed to ensure s t a b i l i t y . In t h i s amplifier the t r i c k i s the p a r a l l e l construction. The p a r a l l e l construction reduces the necessity of very large de-PLATE X GENERAL VIEW OP APPARATUS 27 c o u p l i n g condensers. As P l a t e IX i n d i c a t e s , the g r i d o f the f i r s t tube o f the lower channel i s grounded and thus any s i g -n a l on the g r i d o f the upper channel w i l l induce a s i g n a l o f equal amplitude but 180° out o f phase i n the lower channel. Any hum f e d back t o the p l a t e o f a tube w i l l be i n phase i n both channels. I n t h i s way the s i g n a l s impress the anodes o f the p a r a l l e l c o n s t r u c t i o n i n op p o s i t e d i r e c t i o n s and r i p p l e impresses the anodes i n the same d i r e c t i o n . T h i s i n e f f e c t tends t o c a n c e l the r i p p l e and propagate the s i g n a l . I n o r d e r t o i n c r e a s e the steepness o f the back o f the p u l s e and to prevent m u l t i p l e 'back-kicks' o r ' r i n g ' i n the a m p l i f i e r i t i s u s u a l t o have the time c o n s t a n t o f one (and o n l y one) o f the i n t e r s t a g e c o u p l i n g s (so c a l l e d f a s t stage) a p p r e c i a b l y s h o r t e r than the remaining s t a g e s . T h i s means t h a t the lower frequency response o f the a m p l i f i e r i s almost wholly dependent upon t h i s time c o n s t a n t . T h i s time c o n s t a n t w i l l attenuate the s i g n a l i f i t i s o f the same o r d e r as the chamber c o l l e c t i n g time so i t must be chosen a c c o r d i n g l y . I t i s p r e f e r a b l e t o have the f a s t stage i n one o f the e a r l y stages. I n the main a m p l i f i e r the f i r s t stage i s the f a s t stage. The remaining stages have much l a r g e r time c o n s t a n t s . P. DEUTERIUM GENERATOR The deuterium g e n e r a t o r i s d e s c r i b e d w i t h r e f e r e n c e t o P l a t e X I . The e l e c t r o l y s i s system has p l a t i n u m e l e c t r o d e s and a water c o o l i n g j a c k e t . An o i l manometer i s p r o v i d e d which a c t s as a p r e s s u r e e q u a l i z e r and keeps the gases separate. PLATE XI 28 There are two phosphorus pentoxide drying chambers and a glass spi ra l which when immersed i n l i q u i d a i r further reduces the water vapor pressure. The gas i s collected i n a gasholder which floats on mercury i n a stainless steel container. A mercury pump i s provided whereby the deuterium may be trans-ferred to the two l i t e r flask to any desired pressure. A l l this i s mounted on the two sides of an asbestos board. G. AUXILIARY EQUIPMENT 1. Scaler The scaler used i s a commercial scale of 64 type made by the Atomic Instrument Company. I ts discriminator i s so constructed that the scaler w i l l respond only to posit ive or negative pulses of amplitude exceeding the discriminator setting which i s useful i n eliminating background. The res-olution of the scaler i s such that short pulses (of order of 1/2 microsecond) w i l l be counted i f they are not closer to-gether than five microseconds. A mechanical counter i s used to record every 64th count of the scaler. 2. Oscillograph The cathode ray oscilloscope used i s a Cossor, double beam one. I t was very useful i n examining and comparing s ig-nals from the various stages of the amplifier and i n analysing the pulses from the ionizat ion chamber. 3. Pump and f i l l i n g System The pump used was a Cenco Megavac pump manufactured by Central Scient i f ic Company. I t was good down to pressures of the order of t h i r t y microns. Ho d i f f u s i o n pump was used. A glass system was b u i l t to connect the pump to the io n i z a t i o n chamber, mercury manometer and p i r a n i gauge. Copper tubing provided with a needle valve connected the io n i z a t i o n chamber to the lecture bottles of argon or methane. A connection was made d i r e c t l y from the i o n i z a t i o n cham-ber to the deuterium generator f o r the deuterium f i l l i n g . 30 I I I . EXPERIMENTAL PROCEDURE A. ADJUSTMENTS AND ASSEMBLY 1. Amplifier After design of the amplifier with special character-i s t i c s as indicated the signal to noise r a t i o was 16 to 1 when c o l l e c t i n g a i r ions from 3 mm chamber depth. With further precautions i t would be possible to increase the signal to noise r a t i o . For t h i s work, however, such a high signal to noise r a t i o i s not required so by shortening the time constant of the f i r s t stage of the main amplifier the system was made le s s sensitive to vibrations and acoustical noise with a s a c r i f i c e of signal to noise r a t i o . The f i n a l signal to noise r a t i o was 10 to 1 when c o l l e c t i n g a i r ions from a 3 mm chamber depth. 2. Ionization Chamber A l l parts of the i o n i z a t i o n chamber were thoroughly washed i n alcohol before f i n a l assembly to keep the background as low as possible. There i s a l i m i t to the reduction of background since a metallic surface as free as possible from contamination of radioactive substances s t i l l emits from one to f i v e alpha p a r t i c l e s per hour per cm2. Cosmic rays and gamma rays produce together from two to ten ion p a i r s per second per cm3 of a i r at normal pressure. 31 Lucite rings of accurately known widths were made which were used as spacers for the high potential plate and thus the chamber depth was accurately determined. The amplifiers had a good signal to noise ratio thus i t was possible to work with small chamber depth. The depth chosen was 3™a. All inside measurements were made with a micrometer so that the exact position of the source was known—this, however, was not essential since the accepted range of polonium alphas in air was used as standard. The source was well collimated and the source strength was adjusted by the number of holes in the collimator. The maximum counts was approximately 1800 per minute and chosen thus to keep^ flf/N small. A number of compounds were tried for sealing the chamber. Apieson *W' sealing wax was found suitable for the permanent flange seals and stopcock grease for the less permanent seals. Graphite grease was used for the packing around the shaft which moves the carriage and source. 3. Pilling the Chamber The argon and methane were obtained in lecture bottles at 1000 lbs pressure. The deuterium was stored at two atmos-pheres pressure in the two liter flask of the deuterium generator. Before the actual filling the system was pumped out to thirty microns and then flushed with the gas in question and then repumped after which the gas was slowly admitted into the chamber and allowed to come to equilibrium before readings 32 were taken. B. CALIBRATION The f i n a l cal ibrat ion was obtained with a p lo t of the number-distance curve i n a i r . This curve was checked to 2.6 cms from the source. C. PRODUCTION OF DEUTERIUM In choosing the proper electrolyte the mobility of the ions and the relat ive s t a b i l i t y and the puri ty of the compounds were considered. The compounds KC1, NaNO ,^ Na202 were t r i ed and rejected because of the above considerations. K 2 S O 4 was found to meet a l l the requirements. One gram of K2SO4 per 25 grams D2O was used. The concen-trat ion was chosen so that the deuterium could be produced at a reasonable rate, say, one l i t e r per five hours at a potential of 20 to 25 vo l t s . The potential was chosen thus to avoid over heating, bumping and decomposition. The two l i t e r flask was f i l l e d with deuterium to two atmospheres so that there was sufficient deuterium to f i l l the ionizat ion chamber (capacity approximately two l i t e r s ) to one atmosphere pressure. Details of operation of the deuterium generator are given with reference to Plate X I . To start the operation the system must be pumped out. To pump out the system apply a pump beyond tap 10 and open taps 2, 3, 4, 5, 6, 7, 8, 9, and 10 and close taps 1 and 11. When the mercury leve l r ises to the tops of the 33 c o n t a i n e r s c l o s e taps 5 and 9 and then continue t o pump to the d e s i r e d p r e s s u r e . When the d e s i r e d p r e s s u r e i s reached c l o s e taps 2, 3, 7, 8, 10. Now t u r n on the water supply f o r the c o o l i n g j a c k e t and make sure there i s a s m a l l continuous f l o w o f c o l d water d u r i n g the e l e c t r o l y s i s . Switch on the e l e c -t r i c a l supply and a d j u s t the c u r r e n t t o the d e s i r e d v a l u e (between 0.3 and 1 ampere). The gases are s l o w l y developed by e l e c t r o l y s i s and pass over the water t r a p s and on through the pr e s s u r e e q u a l i z e r ( o i l manometer). The p r e s s u r e b u i l d s up a t d i f f e r e n t r a t e s atoove the p r e s s u r e e q u a l i z e r . Keep the p r e s -sures equal by opening and c l o s i n g taps 1 and 2. .When the pr e s s u r e has reached atmospheric p r e s s u r e on both s i d e s above the o i l open tap 1 and l e a v e i t open so t h a t the oxygen passes on to the atmosphere ( o r may be c o l l e c t e d ) . Allow the deuter-ium to pass through tap 2 i n such a manner as to keep the o i l i n the mamometer a t the same l e v e l on both s i d e s . A f t e r p a s s i n g tap 2 the deuterium passes t h r u the d r y i n g chambers which c o n t a i n phosphorus p e n t o x i d e . The phosphorus pentoxide reduces the water vapor p r e s s u r e t o 10"5 mm. The deuterium then passes on through the s p i r a l which i s dipped i n l i q u i d a i r and which reduces the water vapor p r e s s u r e t o below 10"^ mm and f r e e s the deuterium o f a l l vapors which condense above -180° 0. A f t e r the p r e s s u r e has b u i l t up t o atmospheric i n t h i s s e c t i o n admit the deuterium s l o w l y through taps 7 and 5 i n t o the gasholder. As more deuterium i s developed the g l a s s dome r i s e s on the mercury. When the gasholder i s f u l l stop the e l e c t r o l y s i s , c l o s e tap 7, open tap 9 and by a d j u s t i n g the 34 mercury head admit the deuterium into the container below tap 9, at the same time immerse the gas holder into the mercury and then close tap 5. How open tap 8 and force the gas into the large container with the mercury pump. F i n a l l y close taps 8 and 9, open taps 5 and 7 and switch on the current. C o l l e c t deuterium i n the gasholder again and transfer i t to the large f l a s k as before. This process may be repeated u n t i l the desired pressure i s attained i n the two l i t e r f l a s k . I f through any cause the deuterium should be contaminated the system must be repumped and the procedure as outlined above must be followed f o r r e s t a r t i n g the generator. D. METHOD OF TAKING READINGS Spurious counts due to acoustical noise could not be e n t i r e l y avoided)for t h i s reason most of the readings were taken at night. The discriminator of the scaler was set so that i t would just eliminate the background i . e . a s e t t i n g at about three v o l t s as compared to the maximum signal of about t h i r t y v o l t s . I t was found necessary to l e t the apparatus warm up from four to f i v e hours before taking readings to ensure consistent r e s u l t s . Since the r e s u l t s must be traated s t a t i s t i c a l l y consider-ation must be given tofff where N i s the number of counts and to the weight of the i n d i v i d u a l readings. For t h i s reason the source strength was chosen so that within ten minutes at the most, one would be able to obtain, say, 5000 counts even on 35 the lower portion of the number-distance curve. In 5000 counts the expected error due to random distr ibut ion (Poisson d i s t r ib -ution) is\J5000/5000* 100 z 1.4$. To f i t a proper curve to the results i s ve ry-d i f f i cu l t unless the readings are of equal weight. Approximately 5000 counts were taken at each point on the l inear portion of the curve to keep the weights of the individual readings equal. The f i n a l individual readings were indicated as counts per minute with equal weight and a possible error of 1.4^ The readings were always taken with the source moving i n the same direction to avoid "back-lash' of the screw. Along the l inear portion of the curve the readings were taken 0.1 mm apart. A number of readings were taken to check the background and the maximum counts. The accepted range of polonium alphas i n a i r was used as standard thus i n a l l cases two number-distance curVes were plotted one for a i r and one for the gas i n question. 36 IV. RESULTS AND DISCUSSIONS A. METHOD OF TREATING DATA The mean range ( R Q ) i s t h a t range which i s reached by j u s t one h a l f o f the p a r t i c l e s . T h i s i s r e s t r i c t e d t o a beam o f p a r t i c l e s p e r f e c t l y homogeneous i n i t i a l l y . The m a t e r i a l emit-t i n g the p a r t i c l e s must be p e r f e c t l y smooth. F o r an inhomogeneous beam the p o i n t a t which the i n t e n s i t y i s reduced t o one h a l f has no p a r t i c u l a r s i g n i f i c a n c e except t h a t i t may be d e f i n e d as a mean range c h a r a c t e r i s t i c o f t h a t p a r t i c u l a r source. I f the inhomogeneity a r i s e s from f i n i t e t h i c k n e s s o f the source the p a r t i c l e s o f l a r g e r range come from the top l a y e r s o f the source so t h a t the e x t r a p o l a t e d range i s always w e l l d e f i n e d . Since the mean range o b t a i n e d from such a source (Rm) i s c h a r a c t e r i s t i c o f the source, i t may be used t o d e f i n e the s t o p p i n g power (s) o f gases r e l a t i v e t o a i r . g _ Rmalr Rmgas The shallow i o n i z a t i o n chamber method as o u t l i n e d i n t h i s experiment c o n s i s t s o f a study o f the number-distance curve. The b e s t d e f i n e d f e a t u r e o f t h i s curve i s the e x t r a p o l a t e d number-distance range (E.N.R.). Many o f the range r e s u l t s o b t a i n e d up to date have been o b t a i n e d from a study o f the Bragg, ( i o n i z a t i o n ) curve and the range r e s u l t s are quoted as 37 extrapolated ionizat ion range (E.I .R.) which i s not the same as the extrapolated number-distance range. The number-distance curves were plotted and analysed as outlined here. Only the points on the l inear portion of the curve (Plate XII) were considered. Prom each count the back-ground was subtracted and the best l ine through these points f i t t ed by means of least squares determined the best value of the intercept of th is curve with the distance axis . The f o l -lowing considerations were applied to th is intercept to obtain the extrapolated number-distance range. 1. Distance of the face of the chamber from the source. 2. The stopping power of aluminum used was -the average of the values as given by Rosenblum and Marsden & Richardsonl which i s 1.57 mg/cm2 of aluminum i s equivalent to one air-cm at 15° C and 760 mm. The a i r equivalent of the f o i l was deter-mined by weighing a large area and found to be 1.020 air-cm. 3. The chamber depth penetration for a i r was determined as follows. The discriminator of the scaler was calibrated with the pulse generator. The amplification of the amplifier was obtained by measuring the pulse size at the peak of the Bragg curve and using the known value of the specific ionizat ion of an alpha par t ic le at this point 2 ^. The pulse size required to t r i p the scaler when the discriminator was set just above the noise l eve l (as used throughout the experiment) was then measured from which the number of ion pairs required to actuate the scaler at this discriminator setting was determined. The chamber depth penetration was then determined by integrating 38 graphically the curve as given by Holloway and Livingston2 f o r the s p e c i f i c i o n i z a t i o n of a single alpha p a r t i c l e to the end of i t s range. For the other gases i t was assumed that the alpha part-i c l e i o n i z a t i o n i s proportionally the same from the peak of the Bragg curve to the end of i t s range compared to a i r . The chamber depth penetration fov each of the gases was then obtained by int e g r a t i n g the Holloway and Livingston curve with a d i f f e r e n t s p e c i f i c i o n i z a t i o n scale. The r e l a t i v e s p e c i f i c i o n i z a t i o n was obtained by comparing to a i r the pulse sizes at the peaks of the Bragg curves. 4. Ranges are us u a l l y quoted at 15° C and 760 mm. The ranges were reduced to standard conditions d i r e c t l y by apply-ing 5. The correction f o r r e l a t i v e humidity was made using the generally accepted stopping power of water vapor 1 r e l a t i v e to that of a i r at the same temperature and pressure as 0.74. This correction was applied d i r e e t l y knowing the water vapor pressure at various temperatures. There are other corrections which may be considered although they are small. 1. Correction f o r the actual thickness of the f o i l . 2. Correction f o r the secant e f f e c t . For good geometry (Page 5) t h i s correction i s n e g l i g i b l e . For a i r the lin G a r portion of the number-distance curve extends over a range of 39 2 mm so the c o r r e c t i o n i s c e r t a i n l y n e g l i g i b l e . Even i n deuterium where the l i n e a r p o r t i o n o f the curve extends o v e r a range o f one cm the c o r r e c t i o n i s s m a l l . 3. C o r r e c t i o n t o sea l e v e l at l a t i t u d e 45°. 4. C o r r e c t i o n f o r C02 content. I n a c l o s e d room the CO2 content i s s l i g h t l y l a r g e r than i n open a i r . B. RESULTS P l a t e X I I shows a p l o t o f the curves o b t a i n e d f o r a i r , argon and methane. The d o t t e d curve i s an i d e a l curve f o r a i r d e f i n e d by the accepted v a l u e s o f the mean range and the e x t r a p o l a t e d number-distance range. T h i s i d e a l curve i s one which would be o b t a i n e d i f the source were i n f i n i t e l y t h i n and o t h e r sources o f s t r a g g l i n g were e l i m i n a t e d . The graph i n d i c -a t e s t h a t the s t r a g g l i n g w i t h the source used i s somewhat g r e a t e r than t h a t o f an i n f i n i t e l y t h i n source. The mean range o b t a i n e d from the experimental curve i s a range c h a r a c t e r i s t i c of the source (R m) and i s somewhat l e s s than the accepted mean range. T h i s c h a r a c t e r i s t i c mean range has been used to deter-mine the s t o p p i n g powers o f the gases r e l a t i v e t o a i r . One may o b t a i n an estimate o f the t o t a l s t r a g g l i n g f o r the v a r i o u s gases from a c o n s i d e r a t i o n o f the d i f f e r e n c e be-tween E.N.R. and R m. The s t r a g g l i n g i n deuterium as compared to the s t r a g g l i n g i n a i r i s 0.385 t o 0.091. The r a t i o o f these i s 0.237 which i s j u s t the s t o p p i n g power o f deuterium r e l a t i v e to a i r s i n c e the t o t a l s t r a g g l i n g i s p r o p o r t i o n a l t o the range. 40 The re s u l t s obtained i n t h i s experiment are tabulated i n Table I . The values quoted f o r argon and methane are the mean of two sets of readings and those f o r deuterium are the mean of f i v e sets of readings. TABLE I Gas Ejj^cms) E.N.R.(cms) s Accuracy In Range (cms) A i r 3.806 3.897 1.000 ±.005 Argon 4.090 4.185 .931 ±.009 Methane 4.462 4.556 .853 ±.010 Deuterium 16.124 16.509 .236 + .029 where Ti^—range c h a r a c t e r i s t i c of the source. s= gMaIr Rmg a g C. POSSIBLE ERROR IN RESULTS 1. S t a t i s t i c a l In determining the best intercept of the l i n e a r por-t i o n of the curve with the distance axis the uncertain!ties found were .004, .007, .008 and .013 f o r a i r , argon, methane and deuterium respectively. The error f o r deuterium i s some-what larger which i s due to the large range and thus increased range straggling. 2. F o i l Non-uniformity of the f o i l would tend to increase the straggling of the alpha p a r t i c l e s . I t i s d i f f i c u l t to obtain 41 a good estimate o f the n o n - u n i f o r m i t y , however, i t would o n l y i n c r e a s e the s t r a g g l i n g and to the same ex t e n t i n a l l the gases so t h a t the E.N.R.s o f the gases r e l a t i v e t o a i r are s t i l l w e l l d e f i n e d . Any e r r o r i n t r o d u c e d due to the f o i l would be mainly i n weighing and c o n v e r t i n g to air-cms. The p o s s i b l e e r r o r thus i n t r o d u c e d would not be more than 0.1 mg i n 60 mg. T h i s g i v e s a p o s s i b l e percentage e r r o r o f 0.16$ of g a s - e q u i v a l e n t o f the f o i l and t h i s i s s m a l l except perhaps i n deuterium where i t i n t r o d u c e s a p o s s i b l e e r r o r i n range o f not more than .0048cms. 3. Chamber Depth The r e l a t i v e i o n i z a t i o n compared to a i r a t the peak o f the Bragg curve was found to be 2.4, 1.17, and .97 f o r argon, methane and deuterium r e s p e c t i v e l y . The method used i n o b t a i n i n g the peak i o n i z a t i o n w i t h the s c a l e r d i s c r i m i n a t o r , as d e s c r i b e d on page 38, i s n o t i d e a l so the r e s u l t s may n o t be as accurate as may be o b t a i n e d w i t h , say, a p u l s e h e i g h t s e l e c t o r (P.H.S.). The chamber depth p e n e t r a t i o n s are o n l y 0.205, 0.122, 0.186 and 0.209 cms f o r a i r , argon, methane and deuterium r e s p e c t i v e l y thus any e r r o r i n t r o d u c e d by i n a c c u r a t e measurement o f the r e l a t i v e peak i o n i z a t i o n i s p r o b a b l y s m a l l . As d e s c r i b e d on page 37, these chamber depths were o b t a i n e d from i n t e g r a t i o n o f the Holloway and L i v i n g s t o n curve f o r the s p e c i f i c i o n i z a t i o n o f a s i n g l e a l p h a p a r t i c l e t o the end o f i t s range i n a i r . One i s c e r t a i n l y w e l l j u s t i f i e d i n u s i n g t h i s method f o r the d e t e r m i n a t i o n o f the chamber depth f o r argon s i n c e the 42 Bragg curve f o r argon has been s t u d i e d by Gibson and Gardiner^ and o t h e r s and i * i s f a i r l y w e l l e s t a b l i s h e d t h a t the r e l a t i v e i o n i z a t i o n i n argon to a i r i s c o n s t a n t throughout the range o f the alpha p a r t i c l e . The Bragg curve has not been so w e l l s t u d i e d f o r methane and not a l a l l f o r deuterium thus there may be an e r r o r i n t r o d u c e d i n assuming such a curve f o r these two gases. Since the chamber depth p e n e t r a t i o n i s s m a l l and i n any case, the s p e c i f i c i o n i z a t i o n curve f o r alpha p a r t i c l e s o f 3 o r 4 mm r e s i d u a l range i n gases o t h e r than a i r would n o t show a wide divergence compared to a i r , the e r r o r i n t r o d u c e d by assuming the a i r - c u r v e f o r the o t h e r gases i s p r o b a b l y s m a l l . A t r u e estimate o f the e r r o r ( i f any) c o u l d o n l y be o b t a i n e d by p l o t t i n g t h i s s p e c i f i c i o n i z a t i o n curve f o r these gases. 4. Temperature and P r e s s u r e Temperature was r e a d t o the c l o s e s t h a l f degree. An u n c e r t a i n i t y i n temperature o f 1/2 degree would i n t r o d u c e e r r o r s o f .0049, .0052, .0057 and .0207 cms f o r a i r , argon, methane and deuterium r e s p e c t i v e l y . The temperature should be r e a d to the c l o s e s t 1/10 degree and attempt should be made to keep the temperature c o n s t a n t throughout the experiment. The barometer was r e a d t o the c l o s e s t 1/10 mm. Any e r r o r would o n l y be due t o f l u c t u a t i o n o f p r e s s u r e throughout the experiment and these were s m a l l . 5. P u r i t y o f Gases Argon and methane were o b t a i n e d i n l e c t u r e b o t t l e s a t 1000 l b s p r e s s u r e and were quoted as 99$ C P . I n the case I 43 of argon any impurity was most l i k e l y a i r so that any error introduced was small. For methane, however, i t i s quite possible that the impurity was hydrogen and then the error would be of the order of 0.12 cm. The heavy water was quoted as 99175$ pure. After trans-ferring the heavy water a number of times and after electrolyz-ing the purity should s t i l l be, say, 99.5$. Any impurity was most l i k e l y ordinary water and i n the case of deuterium the impurity would be hydrogen. The error introduced due to such an impurity would be slight since, theoretically at least, the stopping power of hydrogen i s of the same order of magnitude as the stopping power of deuterium. From the above considerations the possible errors on the range results for the various gases are eargon - V(.007)2 + (.0052)2 I .009 ^methane = V(.008) 2 * (.0057)2 Z .010 ^deuterium =V( &3)Z • (.0207)2 •f.004efc .029 where only s t a t i s t i c a l errors, temperature uncertain!ties and air-equivalent uncertainities of the f o i l have been considered. Any discrepancies between these results and those of other workers especially for methane and deuterium may be explained by considering the uncertain!ties due to chamber depth correction and purity of gases. D. COMPARISON OF EXPERIMENTAL RESULTS WITH PREVIOUS DATA The results of other workers along with the results of this experiment for ranges and stopping powers, of polonium 44 alpha particles i n argon, methane and deuterium gjse tabulated in Table II. TABLE II Gas E.I.R. E.N.R. s Previous This Previous This Work Work Work Work Air 3.872 3.8972 3.897 1 1 (Standard) Argon 4.17 1 8 4.185 .93-.95127 .931 4.174 .936 4.177 Methane 4-556 .86-.9127 .853 .9211 Deuterium 16.509 .22 2 4 .236 The values for a i r as quoted in Table II from Holloway and Livingston 2 are generally accepted; these have been obtained by many experimenters. The results for argon have been f a i r l y well established too. The E.I.R.s quoted i n Table II were obtained from a study of the Bragg curve. Gibson and Gardiner^ studied the ionization i n argon by the electroscope method. The results quoted from the International C r i t i c a l Tables 2 7 were obtained by the ionization method and the s c i n t i l l a t i o n method. The stopping power for methane as obtained i n this experiment does not agree so well with the previously obtained values as does the stopping power for argon. This discrepancy, i f i t i s a discrepancy, may well by explained, as incdiated, 45 by the purity of the gas and the chamber depth correction. The values for methane as quoted from the International Crit-i c a l Tables were obtained by the ionization method and the track condensation method. The value obtained by Dick, Falk-Variant and Rossel1-*- i s somewhat high. Their experiments were carried out at pressures above atmospheric as described on page 13 which may explain their high value for the stopping power. Bethe-*- has given a calculation for the stopping power of methane knowing the stopping power of carbon and hydrogen for a given range. For protons of 55 cms range, Bethe calculated a stopping power i n methane of 0.83. A similar calculation has been made by the present writer for alpha particles of mean range (in air) of 3-842 cms. From the stopping power curves given by Bethe, the stopping powers of carbon and hydrogen for 3.842 cms range are 0.928 and 0.218 respectively. From this the stopping power of methane i s SCH 4 " ^(0.928) + 2(0.218) Z 0 . 9 0 The only publication mentioning stopping power of deuterium i s the paper by Schultz 2 4. I t i s stated that the approximate stopping power of deuterium relative to air i s as 17 cms to 76 cms pressure which i s equal to 6.22. There i s no mention as to how the stopping power i s defined. Presumably i t i s defined as every where else--from the mean range. This value only provides a check since i t i s approximate. Gibson and Gardiner^ studied the Bragg curve for hydrogen and found that the stopping power for hydrogen varied from .214 to .293 along 46 the p a t h o f a polonium a l p h a p a r t i c l e . I t i s reasonable t o assume t h a t the s t o p p i n g power i n deuterium v a r i e s i n the same manner so t h a t the va l u e o b t a i n e d i n t h i s experiment as d e f i n e d from the mean range i s an average. V. CONCLUSIONS The o b j e c t o f t h i s experiment has been met to a reason-able e x t e n t . The apparatus and a technique o f the shallow i o n i z a t i o n chamber have been developed and the ranges o f polonium a l p h a p a r t i c l e s i n argon, methane and deuterium have been measured from which the s t o p p i n g powers o f these gases r e l a t i v e t o a i r have been computed. The r e s u l t s o b t a i n e d i n t h i s experiment f o r argonand methane are i n agreement w i t h the r e s u l t s o f p r e v i o u s workers. The deuterium r e s u l t s are o f the r i g h t o r d e r o f magnitude si n c e from formula (4) one would expect approximately the same value f o r the range o f a l p h a p a r t i c l e s i n deuterium as i n hydrogen and t h i s was shown t o be the case. The s t o p p i n g power of deuterium r e l a t i v e t o a i r as o b t a i n e d i n t h i s experiment checks w i t h *he rough v a l u e quoted by S c h u l t z 2 4 . The shallow i o n i z a t i o n chamber i s v a l u a b l e i n o b t a i n i n g ranges and s t o p p i n g powers f o r alpha p a r t i c l e s i n gases. I t has advantages o v e r many o f the p r e v i o u s l y used methods (Page 8) f o r accurate v a l u e s o f ranges o f a l p h a p a r t i c l e s i n gases may be o b t a i n e d d i r e c t l y and q u i c k l y . The e s t i m a t e d u n c e r t -47 a i n i t y i n the v a l u e s f o r ranges and s t o p p i n g powers o b t a i n e d i n t h i s experiment i s not more than 0.2$ which i s b e t t e r than may be ob t a i n e d w i t h many o f the p r e v i o u s l y used methods. The shallow i o n i z a t i o n chamber, however, s u f f e r s from l i m i t a t i o n s t o o . As i n d i c a t e d on page 37, the d e t e r m i n a t i o n of the exact mean range i s handicapped by the f i n i t e r e s o l -u t i o n o f the chamber depth. F o r t h i s reason an e s s e n t i a l p a r t o f the apparatus should be a pu l s e h e i g h t s e l e c t o r (P..H.S.). One c o u l d then study the i o n i z a t i o n o f a s i n g l e a l p h a p a r t i c l e t o the end o f i t s range i n much the same way as d i d Holloway and L i v i n g s t o n 2 o r perhaps by some o t h e r i n g e n i o u s method. With a P.H.S. the ranges c o u l d then be determined from a c o n s i d e r a t i o n o f the Bragg curve o r the number-distance curve. Without too much d i f f i c u l t y the shallow i o n i z a t i o n cham-be r c o u l d be transformed i n t o a d i f f e r e n t i a l chamber where the chamber depth i s reduced t o zero so t h a t 'chamber depth d i f f i c u l t i e s ' do n o t a r i s e . S e v e r a l attempts have been made t o observe the non-cap-t u r e d i s i n t e g r a t i o n o f the deuteron a c c o r d i n g t o i H 2 + 2He 4 > 3H 1 • o n 1 + 2He4 Attempts w i t h i o n i z a t i o n chamber o r Wilson c l o u d chamber f a i l -ed. S c h u l t z 2 f however, o b t a i n e d p o s i t i v e r e s u l t s w i t h a mod-i f i e d i o n i z a t i o n chamber. Since o n l y S c h u l t z has had p o s i t i v e r e s u l t s i t would be i n t e r e s t i n g t o attempt t o r e p e a t h i s experiment. The shallow i o n i z a t i o n chamber c o u l d e a s i l y be m o d i f i e d t o perform t h i s experiment. 48 VI. BIBLIOGRAPHY 1 Bethe and L i v i n g s t o n 2 Holloway and L i v i n g s t o n 3 Lewis and Wynn-Williams 4 Mano 5 Rosenblum 6 Gibson and E r y i n g Gibson and Gar d i n e r 7 I . C u r i e 8 Rayton and W i l k l n s 9 Sehulze 10 R a s e t t i 11 L. Dick, P. P a l k - V a r i a n t and J . R o s s e l 12 Gurney 13 R. Naidu 14 G. Mano 15 W. Y. Chang Review Of Modem P h y s i c s 9, 1936 & 1937 P h y s i c a l Review 54, 18, 1938 Proceedings o f the Royal S o c i e t y , 136, 349, 1932 Annales de Physique 1, 407, 1934 Annales de Physique 10, 408, 1928 P h y s i c a l Review 30, 299, 1925 Annales de Physique 3, 299, 1925 P h y s i c a l Review 51, 818, 1937 Z e i t s c h r i f t f u r P h y s i k 94, 104, 1935 Elements o f N u c l e a r P h y s i c s H e l v e t i c a P h y s i c a A c t a 20, 357, 1947 P r o c e e d i n g s o f the Royal S o c i e t y , 107, 332, 1925 Annales de Physique 1, 72, 1934 Annales de Physique 1, 407, 1934 P h y s i c a l Review 69, 60, 1934 49 16 Rutherford, Wynn-Williams, lewis and Bowden 17 Briggs 18 Harper and Salaman 19 Hevesy and Paneth Proceedings of the Royal Society, 139, 617, 1933 Proceedings of the Royal Society, 139, 638, 1933 143, 604, 1934 157, 183, 1936 Proceedings of the Royal Society, 127, 175, 1930 Radioactivity 20 Bjorksted and M i t c h e l l 21 Lewis Physical Review 46, 629, 1934 E l e c t r i c a l Counting 22 Hoag-Korff Electron and Nuclear Physics 23 Korff Electron and Nuclear Counters 24 Schultz 25 King and Rayton 26 Rutherford, Chadwick And E l l i s P h y s i c a l Review 53, 622, 1938 Physical Review 51, 826, 1937 Radiations from Radioactive Substances 27 International C r i t i c a l Tables 28 Wadey Physi c a l Review 74, 1848, 1948 ABSTRACT A shallow i o n i z a t i o n chamber and a u x i l i a r y equipment was designed and c o n s t r u c t e d . A technique i n u s i n g the shallow i o n i z a t i o n chamber to measure the ranges o f a l p h a p a r t i c l e s i n gases was then developed and used to measure the ranges o f polonium a l p h a p a r t i c l e s i n argon, methane and deuterium. The s t o p p i n g powers o f these gases r e l a t i v e t o a i r was then com-puted from the range v a l u e s . The r e s u l t s are c o n s i d e r e d t o be more accurate than p r e v i o u s l y p u b l i s h e d r e s u l t s and f o r argon and methane are i n agreement w i t h them. No d e f i n i t e r e s u l t s have p r e v i o u s l y been quoted f o r deuterium. The deuterium r e s u l t s obtained, however, check w i t h the rough value quoted by S c h u l t z and w i t h the t h e o r e t i c a l l y expected v a l u e .
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Physics
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Measurement of ranges of Alpha particles in gases
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