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A comparison of the stopping powers of hydrogen and deuterium and the angular distribution and correlation… Neilson, George Croydon 1952

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fir* fix* A COMPARISON OF THE STOPPING POWERS OF HYDROGEN AND DEUTERIUM AND THE ANGULAR DISTRIBUTION AND CORRELATION PATTERNS IN PROTON BOMBARDMENT OF F 1 9 AND N 1 5 by George Croydon N e i l s o n A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS , FOR THE DEGREE OF MASTER OF ARTS . i n the Department of PHYSICS We accept t h i s t h e s i s as conforming to the standard r e q u i r e d from candidates f o r the degree of MASTER OF ARTS Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1952 ACKNOWLEDGEMENTS This work was carried out under a research grant from the B r i t i s h Columbia Research Council. The author i s grateful f o r the continual encouragement and assistance of Dr..C. A. Barnes, under whose supervision this work was carried out.. Advice and suggestions from Drs. J . B. Warren and D. B. James were also greatly appreciated. The author wishes to acknowledge the work of Mr. A. J. Fraser i n connection with the machining of the scatter chamber. He i s indebted to Drs. H.'G. Thode and W. H. Fleming, McMaster Uni-v e r s i t y , f o r the analysis of the deuterium gas. 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 Com-pany Limited f o r the scholarship awarded him. INDEX Page CHAPTER I. A COMPARISON OF THE STOPPING POWERS OF HYDROGEN AND DEUTERIUM I. INTRODUCTION '...v. . '. 1 II. METHODS OF MEASURING THE RANGE OF ALPHA PARTICLES A. Thin Ionization Chamber •. I; B. Other Methods of Measuring Alpha P a r t i c l e Ranges 5 I I I . DESCRIPTION OF THE APPARATUS ' A. Chamber 7 B. Electronics .... 8 1. Power Supplies 8 2. Pulse Formation and Requirements of Amplifier ... 8 3. Amplifiers 10 C. Deuterium Generator 12 IV. EXPERIMENTAL PROCEDURE A. F i n a l Amplifier Adjustments 13 B. Source Preparation Ik C. Generation of Deuterium 15> D. P u r i f i c a t i o n and Handling of Gases , 16 E. Calibration of the Source Micrometer 17 V. RESULTS AND DISCUSSIONS A. Corrections to Experimental Data 19 B. Results 20 C. Possible Sources of Error i n Results 20 D. Comparison with Previous Data 23 VI. CONCLUSIONS . 2U CHAPTER II. ANGULAR DISTRIBUTION AND CORRELATION PATTERNS IN PROTON BOMBARDMENT OF F19 AND N l i ? PART A - RADIATION PATTERNS FROM EXCITED STATES OF Q-I. INTRODUCTION 26 II. DESCRIPTION OF APPARATUS A. ELECTRONIC EQUIPMENT NECESSARY -30 B. REACTION CHAMBER 30 III. FILMS FROM ORGANIC PHOSPHORS 32 IV. . RESPONSE OF THIN ORGANIC FILMS TO ALPHA PARTICLES 35 V. CONCLUSIONS ; 37 PART B - ANGULAR.DISTRIBUTIONS FOR GROUND-STATE ALPHA PARTICLES FROM N^POQCJ-2 REACTION •I. . INTRODUCTION 38 II. THEORY A. GENERAL PRINCIPLES OF TWO-STAGE PROCESSES 39 B. CALCULATED ANGULAR DISTRIBUTIONS FOR THE GROUND-STATE ALPHA PARTICLES IN THE N (P oc )C REACTION UO III. DESCRIPTION OF APPARATUS A. REACTION CHAMBER KH B. COUNTER AND AUXILIARY APPARATUS .'. HH . IV. DISCUSSION H$ APPENDIX I - SAMPLE CALCULATION ( ' .'46 APPENDIX II - NUMBER OF SCATTERED PROTONS AS A FUNCTION.OF ENERGY ' • AND ANGLE ... • k9 APPENDIX III - RATIO OF TRUE TO CHANCE COINCIDENCES FOR THE (P-^ ANGULAR CORRELATION EXPERIMENT 50 APPENDIX IV - EXCITON THEORY 52 ..• APPENDIX V - ALPHA PARTICLE RANGES FROM THE N^PO^C REACTION AS A FUNCTION OF(^ (LABORATORY COORDINATES) AND INCIDENT PROTON ENERGY 53 BIBLIOGRAPHY $K ABSTRACT 56 ILLUSTRATIONS Plate Facing Page I. The Specific Ionization f o r a Single Alpha P a r t i c l e Track, Measured from the End of the Track 2 I I . Comparison of Different Methods of Measuring Ranges 3 I I I . Shallow Ionization Chamber 7 IV. Pulse Formation i n a P a r a l l e l Plate Chamber 9 V. Pre-Amplif i e r 10 VI. Main Amplifier 11 VII. Deuterium Generator 13 V I I I . Signal-to-Noise, Argon i n Chamber I l l IX. Straggling f o r Different Source Thicknesses 15' X. Uncorrected Experimental Number-Distance Curves 20 XI. Experimental Range Number Curve with a•Photo-Multiplier and an Anthracene Crystal 2k X I I . Angular Correlation Between Alpha P a r t i c l e s and Gamma Rays i n the Reaction F 1 9(p<x ^ ) 0 1 6 27 X I I I . Scattering Chamber 30 XIV. Block Diagram of the Electronics Required f o r a High-Speed Coincidence Angular Correlation Experiment 31 XV. Response of a Thick Section of Anthracene to Alpha Par t i c l e s of Different Energy 35 XVI. Response of a Thin Section of Anthracene to Alpha Pa r t i c l e s of Different Energy 36 XVII. Response of Anthracene to Alpha P a r t i c l e s 37 XVIII. Thin Window Scattering Chamber HK XXLX. Rutherford Scattering of Protons h9 XX. Relative Response of Anthracene to Alpha P a r t i c l e s and Protons f o r Different Film Thicknesses £2 XXI. Range of Ground-State Alpha P a r t i c l e s from N1^(p<x.)C12 i n Lab. Coordinates as a Function of (ft and Proton Energy... $3 CHATTER I - A COMPARISON OF THE STOPPING POWERS OF HYDROGEN AND DEUTERIUM I. INTRODUCTION The process of energy loss by alpha p a r t i c l e s i n passing through dx • matter has been investigated by many authors and a theo r e t i c a l r e l a t i o n as deduced by Bethe"*" i s given as * "dE = UTTZ 2e UN z y i o g 2 m v 2 ) m (-—) i s the energy loss f o r c o l l i s i o n processes f o r p a r t i c l e s of charge cue eZ and speed v moving through an absorber of effective nuclear charge Z * , effective i o n i z a t i o n p otential I and atomic density N. m i s the mass of . an electron. In the derivation of th i s formula i t i s assumed that a l l energy i s l o s t i n the ioni z a t i o n and excitation of the atoms of the stopping mater-i a l . In the case of the two stable isotopes of hydrogen, the io n i z a t i o n .potential of the electron w i l l be only s l i g h t l y altered by the presence of the extra neutron i n the deuterium nucleus. 2 3 H Most previous experimental work ' ' on the stopping powers, of hydro-gen and deuterium would seem to indicate that the stopping powers are very s i m i l a r , which i s i n agreement with the theoretical predictions. In 19k9> however, i t was reported by Harrick and E i c h h o l g ^ that the stopping power of deuterium was approximately 6% greater than the stopping power of hydro-cm Z o l . ... i.... i .... t.... l.... I.... I .... I 25 2.o f.S '.o o.i o cm THE SPECIFIC IONIZATION FOR A SINGLE ALPHA-PARTICLE TRACK, MEASURED FROM WE END OF THE TRACK PLATE I 2 gen. This resu l t cast doubt on the v a l i d i t y of the the o r e t i c a l assumption that the energy loss f o r alpha p a r t i c l e s i s due only to i o n i z a t i o n and excitation of the atoms. I t was considered necessary, therefore, to make a new measurement of the stopping power of deuterium i n order to resolve the apparent discrep-ancy between theory and experiment. THEORY The mean range f o r a single alpha p a r t i c l e can be convputed from equa-t i o n ( l ) by using the r e l a t i o n R E R =/ dx - / (-dEj^dE (2) Jo I dx ; The stopping power (-i^ ?.) of an absorber f o r an alpha p a r t i c l e of a dx given energy i s related to the s p e c i f i c i o n i z a t i o n by ' " I - c p • OJ. £ i s the mean energy associated with the formation of an ion p a i r . Plate I shows 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 f o r a single alpha p a r t i c l e , of mean range (3.8U2 cm), measured from the end of the range. The increase i n s p e c i f i c i o n i z a t i o n of the alpha p a r t i c l e near the end of i t s range i s expected, since - ^ ^ i j o c p . dx v As each alpha p a r t i c l e makes only a f i n i t e number of c o l l i s i o n s and each c o l l i s i o n has a discrete energy l o s s , i t i s to be expected that the ranges of i n d i v i d u a l alpha p a r t i c l e s w i l l have a d i s t r i b u t i o n about some mean range. I f R i s the mean range of an alpha p a r t i c l e and ydR gives the f r a c t i o n of alpha p a r t i c l e s i n the beam which- comes to rest between R and R plus dR from the source, then the curve_y can be approximated by the Gaussian r e l a t i o n \ a • \ c • Vs V 3.60 3.70 3.80 3.90 4.00 cm COMPARISON OF DIFFERENT METHODS OF MEASURING RANGES PLATE H y = ( T r i e ) " W -(R-R)2/oc2 (U) c*; i s the half-width at i of the d i f f e r e n t i a l range curve (Plate I I ) . e Plate I I shows the following range relations f o r polonium alpha part-7 i c l e s i n a i r : Curve a. a number-distance curve with a number-distance extrapolated number range at 3.897 cm Curve b. a d i f f e r e n t i a l range curve with mean range 3-8ij.2 cm Curve c. a portion of a sp e c i f i c i o n i z a t i o n curve f o r an alpha p a r t i c l e beam with an io n i z a t i o n extrapolated range at 3.870 cm Curve d. a portion of a s p e c i f i c i o n i z a t i o n curve f o r a single alpha p a r t i c l e of mean range 3»8Ij.2 cm. I t can be seen that the extrapolated i o n i z a t i o n range (Rj) i s not equal to the extrapolated number-distance range. The r e l a t i o n between B^ and R i s given by Rn-R = iTT^oc (5) The stopping power of a substance can be defined i n a number of ways, but the most common d e f i n i t i o n i s the r a t i o of the mean range i n the sub-stance to that i n a standard material, usually a i r . The mean ranges are measured at 1J?°C and 760 mm mercury. I I . METHODS OF MEASURING THE RANGE OF ALPHA PARTICLES A. THIN IONIZATION CHAMBER2'8,9 In analyzing data from t h i n i o n i z a t i o n chambers, the usual procedure i s to pl o t a number-distance curve, obtain the extrapolated number range, and determine the mean range by direct substitution into equation ( 3 ) . In analyzing data from s p e c i f i c i o n i z a t i o n measurements, however, the mean range can be determined from the extrapolated i o n i z a t i o n range only from the empirical r e l a t i o n ^ - R-J--R- - (.8oC-.06)mm (6) Extrapolated number-distance range measurements suffer from chamber depth corrections, i . e . one'must know the minimum depth that the p a r t i c l e has to traverse inside the chamber to produce a pulse that w i l l be count-ed. To make th i s correction, i t i s necessary to know the va r i a t i o n of io n i z a t i o n with distance from the end of the range, as w e l l as the s i g n a l -to-noise r a t i o , amplifier response and bias v o l t s . The observed d i s t r i b u t i o n i n range always has a greater value f o r oc } the range straggling parameter, than the theoretical value of .062 f o r 8 a i r . Livingston and Holloway have been able to account f o r th i s increase by including a l l sources of straggling: 1. Range Straggling - Defined as the i width of y ( d i f f e r e n t i a l range curve, Plate II) at i of i t s maximum. e 2. • Noise Straggling - The pulse produced i n the chamber either adds to or"subtracts from the random noise pulses producing additional strag-glin g . 5 3 . Ionization Straggling - The energy required f o r io n i z a t i o n i s not constant but varies about a mean value, approximately 32 v o l t s per ion pa i r i n a i r . K* Angular Straggling - Due to angular deviations, the paths are not the same length f o r various alpha p a r t i c l e s , thus a straggling effect i s produced. 5>. F o i l Straggling - .Slight non-uniformities i n the f o i l vriiich form the chamber window can lead to an increase i n straggling. 6. Source Straggling - This and the angular straggling are the only effects mentioned so f a r that are not symmetrical about the mean range. Source straggling decreases the mean range and can be caused by source thickness, roughness or d i r t i n e s s . B. OTHER METHODS OF MEASURING ALPHA PARTICLE RANGES107*"18 1. D i f f e r e n t i a l Ionization Chamber10 The io n i z a t i o n from two shallow i o n i z a t i o n chambers with opposite potentials i s collected on a common g r i d . E s s e n t i a l l y , this method meas-ures the position of maximum change of ioni z a t i o n along the path of an alpha p a r t i c l e and leads to a direct determination of the mean range. 12 2 . S c i n t i l l a t i o n Method Alpha parti.cles may be counted on a ZnS screen placed between the source and the observer. Ranges determined by this method are usually short due to the d i f f i c u l t y of counting very small flashes v i s u a l l y . This method i s much more effective i f a photo-multiplier tube i s used i n place of v i s u a l counting. 3 . Wilson Cloud Chamber A p a i r of stereoscopic pictures enables one to measure the track , lengths and leads d i r e c t l y to a determination of the mean range. The d i s -advantages of t h i s method are (a) unknown composition of gases at time of expansion, and (b) the rapid f a l l i n g o f f of the i o n i z a t i o n , making the l a s t part of the range hard to see. h* Photographic Method Any ionizing radiation or p a r t i c l e w i l l produce activation on absorption i n the emulsion of a photographic plate. This method i s not too good f o r low energy alpha p a r t i c l e s because of the coarseness of the photographic grains, and, as a r u l e , i s used only f o r a check on the ener-gy of the alpha p a r t i c l e s . 16,17,18 5 . Magnetic Deflection The v e l o c i t y of a charged p a r t i c l e can be found from the amount that i t i s bent when passed through a magnetic f i e l d . This magnetic deflection method leads to very accurate values f o r the alpha p a r t i c l e v e l o c i t y , but, unfortunately, the range can be only approximated from the measured v e l o c i t i e s as there i s no precise range energy r e l a t i o n . 0 pen PREAMPLIFIER H '—COPPER MICA UARTZ/t ^toe/re BE>! J=iL ) ted. D QcOLUMATED SOURCE © A * M PITCH SCREW ® REGISTER SHALLOW IONIZATION CHAMBER © MICROMETER © CHAMBER ® H.T. OUTPUT Q ALUMINUM WINDOW (8) COLLECTOR (5) RUBBER GASKET PLATE HI 7 I I I . DESCRIPTION OF THE APPARATUS A. CHAMBER Plate I I I shows a cross-sectional view of the four sections of the chamber. Section I contains the movable platform upon which the collimated source i s mounted. The source position i s set by rotating a screw of one mm p i t c h . Complete revolutions are registered on a mechanical counter and p a r t i a l turns are read to the nearest 1/100 mm from a calibrated drum. Section I I shows the thin chamber i t s e l f and the method of insul a t i n g the c o l l e c t o r and the high tension face plate-. The collimated alpha part-i c l e s enter the t h i n chamber through an aluminum window which i s pasted to the copper face of the chamber with aqua-dag. The aluminum window has an air-equivalent of approximately 1.2 cm. This thickness f o r the alumi-num f o i l was chosen f o r a number of reasons: (a) the face must be r i g i d to reduce "microphonics, (b) t h i s thickness i s more uniform than thinner f o i l s , and (c) the chamber would not be deep enough f o r the f u l l range of a polonium alpha p a r t i c l e i n hydrogen or deuterium without increasing the pressure above atmospheric or interposing other absorbers. I t i s to be noted that the c o l l e c t o r must have the best i n s u l a t i o n possible i n order that none of the charge collected s h a l l be l o s t through, or soaked up by, the insul a t o r . The insulator f o r the high voltage elec-trode need not be of such high q u a l i t y , but i t i s necessary that the high voltage input lead should have a large capacity to ground i n order to by-8 pass any electromagnetic pick up before i t reaches the chamber. I t i s of the utmost importance that the capacity of the chamber be kept as low as possible so that the voltage w i l l be a maximum f o r a given charge c o l l e c t -ed. The chamber depth of 3 mm was chosen f o r t h i s experiment as th i s i s deep enough to give a good signal f o r hydrogen and i s less than 1.$% of the t o t a l path length. B. ELECTRONICS . 1. Power Supplies Two regulated power supplies were used to supply the power f o r the amplifiers. One was used as a 300 v o l t plate supply and the other as a variable negative r a i l from -100 to -15>0. In order to ensure complete i s o l a t i o n from e a r l i e r amplifier stages and from the fluctuations that are inherent i n a.c. voltage supplies, a battery supply was used f o r the plate voltage of the pre-amplifier. - A l l tubes were heated with a 6 v o l t d.c. storage battery and thus any a.c. coupling between filaments and cathodes was eliminated. The high voltage supply was a conventional regulated supply and was capable of delivering 2000 volts with very l i t t l e a.c. r i p p l e . I t was necessary to place a f i l t e r network i n the high tension l i n e just before i t entered the chamber, i n order to eliminate any spurious counts due to electromagnetic pick up. 2• Pulse Formation i n P a r a l l e l Plate Ionization Chambers1^-As a l l g a s - f i l l e d counters and chambers function by the separation and c o l l e c t i o n of the positive and negative ions formed i n the gas, i t i s important to know hovir a pulse i s formed. I f the c o l l e c t i n g electrode i s positive and has a, capacity C, i t would appear that the change of p o t e n t i a l t. t PULSE FORMATION IN A PARALLEL PLATE CHAMBER PLATE IV of the. c o l l e c t o r i s due to a charge -e being placed on a capacity C. This over-simplified picture i s wrong because i t neglects the induction effects which the two ions have been exerting since the creation of the ion pa i r . If at a time t after'the ion p a i r formation, the positive electrode has induced charges - q + ( t ) and - q _ ( t ) , the potential on that electrode w i l l be p(t) = q»(t)+g_(t) ; . . . . . ( 7 ) C where P(o) = 0 when.q+(o) = -q_(o). I t i s interesting to note that the charges induced on the other elec-trode are complementary to those considered here, and, therefore, the pulses formed on the tyro electrodes are i d e n t i c a l in' form but opposite i n sign. When the negative ion has been completely collected at time t-^, the potential of the c o l l e c t o r w i l l be The s i m p l i f i e d picture i n which the potential would be ~£ when the charge c -e has been collected i s thus incorrect, and corresponds to the case of complete c o l l e c t i o n of both negative and positive ions. Plate IV shows the actual pulse shape. As the c o l l e c t i o n time t 2 (Plate IV) of the positive ions i s more than 1000 times longer than that of free electrons, i t i s advisable to construct an amplifier with a short time constant so that electron c o l l e c -t i o n alone i s responsible f o r the pulse amplified. This i s c a l l e d a f a s t chamber as compared to a slow chamber i n which the positive ions also are collected and give an addition to the pulse height (see Plate IV). Unfort-unately, with a slow chamber, the clippin g time (shortest RG time constant of amplifier) must be made longer than the pulse r i s e time t 2 (Plate IV). This.method has the disadvantage of a slow maximum'counting rate, and PRE-AMP11F1ER ~ PLATE V 10 greatly increased microphonics. For a fa s t chamber with electron c o l l e c t i o n only {tr^? R C ^ t - ^ ) , the pulse p r o f i l e i s given by P(t) = q-(t)+Q» where Q^. = q +(o) = constant. 3» Amplifiers The output signal of an amplifier usually has superimposed on i t a variety of extraneous signal components termed "noise". The various types of noise can be c l a s s i f i e d according to t h e i r o r i g i n as follows: a. Hum introduced from power supply b. microphonic noise c. noise picked up from external sources d. noise a r i s i n g from defective components e. inherent tube and r e s i s t o r noise Theoretically, the only type of noise that can not be completely elim-inated i s that due to tube and r e s i s t o r noise. In practice, perfection i s hard to obtain, but the various types of noise can be greatly reduced with care. In constructing the amplifiers (Plates V, VI), the following points were considered. The high mutual conductance pentodes that were used are extremely microphonic as Trail as very susceptible to hum pick up from an a.c. oper-ated heater. To reduce these factors a d.c. heater supply was used and the amplifier and chamber were mounted on soft sponge rubber as a protec-tion against shock. Also, i t was found necessary to reduce the low-fre-quency response to lessen microphonic pick up. Electromagnetic pick up was greatly reduced by proper shielding of IOOK WU/ OOSuF N H — IOOK WW 120K Kyi 5K JuF~>± B /OK 10 K + 2<X> 5K 5K 50K 68K «*-— SOK . •<— A' 01 2*F ±* S470K SK B" HI* 01 0 (i 0/ 68* 4 o o/ -Hl-o 6"' J-^—• 47K - Z7K *>Z7K. '<— 47* S Z7K < 27* — ISK X /20K /SO 6AC7U -I50V 1 /0K 6,4 <? 7 MAIN AMPLIFIER PLATE E Z 11 the pre-amplifier which was completely contained i n a copper tube. Also a common ground was used i n an e f f o r t to eliminate any electromagnetic pick-up loops. For a pulse amplifier having a single RC cut-off at low and high f r e -quencies, the equivalent rms noise charge q n on the capacity of the detec-19 tor can be calculated as q n " ( g f R + 2 f R i ) T&ttJ + 2 T r k T R s ( e + C l ) 2 (10) Where R i s the input resistance between g r i d and ground, Rg i s the equiva-lent input noise resistance of the f i r s t tube due to g r i d current. R i s s the equivalent series g r i d noise resistance commonly used to express the magnitude of shot effect voltages i n the plate c i r c u i t . C i s the detector capacity, C-^  i s the input capacitance of the amplifier, K i s Boltzman's constant and T i s the temperature. The frequencies f-^ and fg' are the low-er and upper §• power frequencies. To reduce the f i r s t t e r m the g r i d of the input 6AK5 was l e f t f l o a t i n g ( R — A plot of Qn(= q n) against C (chamber capacity) shows a l i n e a r r e l a t i o n exists between the two; thus the f i r s t two terms, which are due to thermal noise and g r i d current respectively, are negligible compared to the noise from the shot e f f e c t . In order to reduce the shot e f f e c t , the input 6AK5> was triode-connected as the mean-square shot noise of a pentode i s generally three times greater than that of the same tube tr i o d e -connected"''9, due to p a r t i t i o n noise. The 6AK5 used was hand-picked f o r i t s low noise as a l l 6AK5"'s are not equally good. Elmore and Sands 1 9 give an experimentally determined curve showing the signal-to-noise r a t i o as a function of clipping time. This curve was found useful as a guide i n selecting the proper clipp i n g time. 12 Some of the advantages of the push-pull main amplifier used are: 1. Changes i n the main supply current are very small. The tubes are self-compensating as pa i r s , reaction through the supply l i n e i s v t h u s reduc-ed, and the amplifier i s more nearly independent of the impedence of the power supply, removing the necessity f o r large decoupling condensers. 2. Either sign of input may be used and either sign of output i s a v a i l -able. 3. The gain i s large only f o r out-of-phase or d i f f e r e n t i a l inputs at the grids by .virtue of the use of the cathode-coupling res i s t o r s of the l o n g - t a i l p a i r . Hence, in-phase effects such as hum on the supply l i n e cause the grids to r i s e and f a l l , but the cathodes do likewise and there i s l i t t l e amplification. U . The gain i s a function mainly of the tube current and i s more or less independent of the combination of g r i d and screen voltages used. More p a r t i c u l a r l y , i t i s independent of the cathode emission and gain i s s t a b i l i z e d against changes i n heater voltage. Gain control i s obtained by the variable low resistance joining the cathodes which gives smooth noise-free action. 6. The amplifier i s l i n e a r over a larger range of output pulses than a single tube amplifier. C. DEUTERIUM GENERATOR The deuterium generator (Plate VII) consists of an e l e c t r o l y s i s tube to generate the deuterium from heavy water, a pressure equalizer to keep the deuterium separate from the oxygen, a phosphorous pentoxide dryer, a l i q u i d a i r trap, a gas holder, and a mercury pump.to transfer the deuter-ium into the storage f l a s k . LIQUID .1 AIR TRAP RESERVOIR DEUTERIUM GENERATOR ~ PLATE M 13 IV. EXPERIMENTAL PROCEDURE A. FINAL AMPLIFIER ADJUSTMENTS The f i n a l value selected f o r the clippin g time was determined experi-mentally with hydrogen i n the chamber as the range of t h i s gas was to be measured. The maximum signal-to-noise r a t i o was realized with a clipping .time of 10 microseconds, which i s approximately the value predicted by the 19 • curves given i n Elmore and Sands . I t i s to be noted that t h i s i s not the optimum value f o r the cli p p i n g time i f the gas used forms negative ions, as, f o r example, a i r . I t i s advisable, therefore, to use a gas such as argon f o r a standard because both argon and hydrogen allow electron c o l l e c t i o n and w i l l give pulses of the same order of r i s e time. I t w i l l be noted that maximum signal-to-noise r a t i o does not imply maximum pulse height. A larger pulse w i l l be produced i f the c l i p p i n g time i s made long enough to include the r i s e due to the positive ion c o l -l e c t i o n , but when th i s i s done, microphonics increase the noise l e v e l so that there i s a net loss i n signal-to-noise r a t i o . An optimum-value for the chamber voltage was chosen i n much the same manner as for the clipping time, but, as would be expected, the signal-to-noise r a t i o does not depend on the chamber voltage as long as the voltage i s high enough for complete electron c o l l e c t i o n . The chamber i t s e l f was thoroughly washed f i r s t with concentrated n i t -r i c acid then with alcohol i n an e f f o r t to reduce the background due to alpha p a r t i c l e s from the w a l l s , and the inside was painted with pure PUTEWT SIGNAL TO NOISE. ARGON IN CHAMBER S / N - '9S S55 '495 = 8.25 4000 3000 MINUTE 2000 WNTS PER * /ooo DISCRIMINATOR VOLTS to f5 20 25 30 36 4o 45 .•nil aqua-dag. The background f i n a l l y obtained was approximately 30 counts/min. B. SOURCE PREPARATION20' 2 1> 2 2 Approximately a dozen old radon needles were, available f o r the prepar-ation of a polonium source. The procedure f o r preparing the polonium source "was as follows. The radon needles were smashed and dissolved i n concentrated n i t r i c acid, then evaporated to dryness. The residue was next dissolved i n con-centrated hydrochloric acid and again evaporated to dryness. This l a t t e r process was repeated at l e a s t three times. ( A l l evaporation was done with a water bath, not with an open flame, to avoid apattering.) F i n a l l y , the residue was dissolved i n .£N HC1. The polonium was then separated from the RaDEF solution as follows. A s i l v e r button was mounted on a source holder and polished. One drop of 'the RaDEF solution was put onto the s i l v e r button, and after one minute, the button was immersed i n a large volume of water. The f i l m of polonium thus deposited Y/as so thi n that i t was v i s i b l e only as a s l i g h t colouring to the s i l v e r when placed under, a strong l i g h t . The thi n source, prepared as outlined above, was 'compared to a much stronger source prepared by rotating the s i l v e r button i n the RaDEF solu-2'2 t i o n u n t i l a l l the polonium had been deposited by electrochemical action Plate IX shows the number-distance curves f o r the two sources. The thi n source i s much weaker as would be expected, and, therefore, i t s maximum counting rate has been normalized to that of the stronger source to allow a direct comparison. Curves A and B are the curves f o r the thick source and the thi n source, respectively, while curve C shows the range straggling expected for an i n f i n i t e l y t h i n source. I t i s to be noted that curves A PLATE LX STRUGGLING FOR DIFFERENT SOURCE THICKNESS THEORETICAL CURVE FOR RANGE STRAGGLING ONLY TUIN SOURCE THICK SOURCE 3.897 15 and B include other straggling factors besides the range straggling. C. GENERATION OF DEUTERIUM The operation of the deuterium generator can best be understood with reference to Plate VII. To commence operation the system must be complete-l y evacuated. This evacuation i s best accomplished..by pumping through taps 8 and 10 simultaneously. Taps 5 and 7 are closed after the reservoir and mercury pump are f u l l of mercury. Tap 9 i s also closed while taps 1, 2, 3 , ' h and 6 are open. When the system has been completely evacuated, taps 1, 2, 3s k, 6 , 8 and 10 are closed. Heavy.'water with potassium sulphate .(lgm/ 25>cc) as electrolyte can now be introduced into the e l e c t r o l y s i s chamber through tap .10, being sure not to admit any a i r . The electrodes can now be connected and a current of approximately .5 amps passed through the elec-t r o l y t e . Thr f i r s t stage i n the operation i s very c r i t i c a l and the gener-ator must be watched constantly; otherwise, too large a pressure w i l l be b u i l t up i n the e l e c t r o l y s i s tube. Taps 1, 2, 3 , h and 5 are opened as atmospheric pressure i s gradually b u i l t up i n each section of the apparatus. Stopcock 9 i s l e f t open after atmospheric pressure i s obtained i n the tubes'themselves. Once tap 5 i s opened the reservoj.r i s connected and the generator can be l e f t to operate by i t s e l f u n t i l the reservoir i s f i l l e d . While the storage tank i s below' atmospheric pressure, the reservoir can be emptied by simply opening tap 6 cautiously and drawing the deuterium into the storage tank. After the stor-age tank reaches atmospheric pressure the mercury pump must be used as f o l -lows. The generator i s shut off and. tap U closed. Taps 5 and 7 are then opened and by lowering.the mercury l e v e l i n the pump the reservoir i s emptied. Now tap 5 i s closed and tap 6 i s opened, and once more the mer-16 • cury l e v e l i s raised, forcing the deuterium into the storage tank. The entire process i s repeated u n t i l the storage tank contains the required amount' of deuterium. The phosphorous pentoxide and the l i q u i d a i r trap remove any water vapour that may come through the o i l pressure equalizer. (Water vapour pressure at -190°C i s less than 10 -^ mm). D. PURIFICATION AND HANDLING OF GASES As the chamber was pumped down to approximately 10 microns with a mechanical fore-pump before admitting the gases, any residual gas would amountto only about part i n 10,000, which i s c e r t a i n l y i n s i g n i f i c a n t . The heavy water used i n the deuterium generator was quoted as 99»5% pure heavy water. Upon f i l l i n g the io n i z a t i o n chamber with deuterium, the f i r s t measurements indicated that the stopping power of the deuterium was about 6% greater than that of hydrogen. The e l e c t r o l y t i c hydrogen and .argon were obtained i n lecture bottles at 15>00 Ibs/sq inch and were of high p u r i t y - better than 99.9%. Water vapour on the chamber was elimin-ated by means of a l i q u i d a i r trap. As the results f o r the stopping power of th i s deuterium were not con-s i s t e n t , i t was decided.that the generator must have o r i g i n a l l y contained some a i r i n the reservoir due to f a u l t y pumping, or to a small leak when the reservoir was below atmospheric pressure. In order to save the deuter-ium already generated, i t was decided to pump the deuterium out of the i o n i -zation chamber and p u r i f y i t by passing i t through a palladium lead. When th i s was done, the o r i g i n a l discrepancy between the stopping powers of deuterium and hydrogen was non-existent . For consistency, the hydrogen also was introduced into the chamber through a palladium leak. This process did not change the range, which was i d e n t i c a l with that obtained when the hydrogen was admitted d i r e c t l y into the chamber. 17 As a further check gas samples were taken and sent f o r analysis, (see Possible Sources of Error i n Results, Section V. -C) • E. CALIBRATION OF THE SOURCE MICROMETER Even after a l l possible precautions f o r reducing spurious noise effects had been taken, i t was found necessary to take a l l readings at night when accoustical and e l e c t r i c a l noise were at a minimum. The actual performance of the complete system can be judged by the signal-to-noise r a t i o obtained (see Plate VIII) f o r a chamber depth of 3 mm. With argon i n the chamber a signal-to-noise r a t i o of 9 was obtained and with hydrogen (or deuterium) a r a t i o of approximately 3 was obtained. The reduced signal-to.-noise r a t i o f o r hydrogen i s to be expected due to the decreased stopping power of hydrogen. • The signal-to-noise r a t i o was dependent upon the state of discharge of the filament battery. Therefore, i n order to obtain reproducible r e s u l t s , the battery was f u l l y charged before each run and the amplifiers allowed to operate f o r about two hours i n order to reach thermal equilibrium. Also, i n order to check the amplifiers performance, a signal-to-noise t e s t was taken immediately before each range measurement. The actual c a l i b r a t i o n was obtained by p l o t t i n g a number-distance curve f o r hydrogen and using the accepted value f o r the stopping power of hydrogen (.2210^ to calculate the stopping power of deuterium. Hydrogen was chosen as. a standard f o r the comparison because hydrogen and deuterium have t h e o r e t i c a l l y very s i m i l a r stopping powers, and, therefore, any error inherent, i n one measurement should be the same.in the other. Such an error might arise from the chamber penetration correction, which can be determin-ed only approximately. As an additional check on the correctness of results 18 each run with hydrogen or deuterium i n the chamber was followed by a simi-l a r run with argon i n the chamber. In th i s way, the stopping power of hydrogen was measured i n terms of the stopping power of argon, which was then compared to a i r by using the accepted stopping power of argon compared to a i r ( .929) • Two additional runs were made i n which the range of argon was compared d i r e c t l y to that of a i r to check the accepted value f o r the stopping power of argon. In order that each point on the number-distance curve should have the same s t a t i s t i c a l weight, the same number of counts were taken f o r each point on the curve. Since 2% standard deviation on each point requires approximately 2000 counts, the, source strength was chosen to given approx-imately 2000 counts per minute. 19 V. RESULTS AND DISCUSSIONS • A. CORRECTIONS TO.EXPERIMENTAL DATA The mean range i s defined as that range reached by just one-half of the p a r t i c l e s that leave the source. The mean range w i l l depend, of course, on the self-absorption of the source, but the stopping power w i l l s t i l l be given by S = fa1 a — provided Rm a i r and Rm gas are measured from the same Rm gas source. In finding the intercept f o r the extrapolated number-distance range, only the points on the l i n e a r portion of the number-distance p l o t (see Plate X) were considered. The background was subtracted from each count, and a straight l i n e was drawn through these points to give the best i n t e r -cept. The best l i n e was determined by tyro separate methods, (a) using a thread to f i t the l i n e by eye on a large scale p l o t , and (b) using least squares. Both methods gave the same r e s u l t s . ' Having obtained the intercept, the extrapolated number-distance range was found by considering: 1. The distance from the chamber face to the source. (From the c a l i -bration of the micrometer scale with range i n a standard gas.) 2. The'equivalent range of the aluminum f o i l . The stopping power of aluminum was taken as l.J?7 mg/cm2, which i s the average of the values given 1 1 by Rosenblum , and Marsden and Richardson-1-. 3. The chamber depth penetration, which was found at le a s t to a f i r s t order approximation by using the method described i n Appendix I. UNCORRECTED EXPERIMENTAL NUMBER DISTANCE CURVES ~ PLATE X 600 300 MICROMETER READING W o 3.4 3.5 3.6 ARGON JULY 25 1951 g 3.903 \1«>3 /400 k, HYDROGEN \ SEPT 25 1951 200 Vioo MICROMETER READING CM o I2.Q 20 IN The correction f o r temperature and pressure. The ranges were reduc-ed to standard conditions. Ro = RP To T O J P 0, RQ refer to. standard temperature, pressure & ranges. T D = 288°K, P 0 = 760 mm mercury. B. RESULTS Stopping Power  Relative to Hydrogen 1 1.012 TABLE I Stopping Power  Relative to A i r .929 .220 .222 Gas Argon Hydrogen Deuterium Table I shows the results obtained i n t h i s experiment. The -values quoted f o r argon and deuterium are the mean of two sets of readings, and those f o r hydrogen are the mean of four sets of readings. The stopping power i s defined from the mean range char a c t e r i s t i c of the source. Plate X shows a t y p i c a l p l o t f o r a i r , argon and hydrogen. C. POSSIBLE SOURCES OF' ERROR IN RESULTS 1. S t a t i s t i c a l The best intercepts obtained from approximately 20 points on the li n e a r section of the extrapolated number-distance curve were found i n the least squares analysis to have uncertainties of . 0 0 6 , .005 and .015 cm f o r a i r , argon and hydrogen, respectively. . 2. F o i l . Anv error caused by the f o i l would be due .to choosing the wrong a i r 21 equivalent f o r aluminum. The stopping power of aluminum used was 1.57 mg/cm equivalent to 1 cm of a i r , which i s the average of the two values, 1.62 and 1 . 5 1 1 . The uncertainty, therefore, i s plus or minus .05 mg/cm2. o Because of the thickness of the window used (I . 8 9 0 mg/cm ), there i s a pos-si b l e error of , 0 k l cm i n the range i n argon, or an error of 1.6$ i n the stopping power of hydrogen or deuterium. 3. Chamber Depth The chamber depth penetrations were 1 .25 , .90 and 2.U0 mm f o r a i r , argon and hydrogen (deuterium), respectively. (See Appendix I f o r method of ca l c u l a t i n g ) . These calculated values are probably a good f i r s t approx-imation . I4.. Temperature and Pressure The temperature was read to the closest'\ degree which corresponds to uncertainties of . 0 0 5 , .005* »020 cm f o r a i r , argon and hydrogen, respec-t i v e l y . The barometer was read to 1/10 mm, giving an uncertainty too.small to be considered. 5 . Gas Purity Argon and hydrogen were obtained i n lecture bottles and were quoted as 99.9% pure, thus any error introduced from t h i s cause was ce r t a i n l y small. As a further check on the p u r i t y of the hydrogen and the deuterium used, a mass spectrometric analysis was obtained from Drs. Thode and Fleming, McMaster University. Unfortunately, the hydrogen gas sample was contamin-ated during t r a n s i t , but the deuterium analysis i s as follows. 22 H2- D2 98.33$ D 2 0, HDO 1.29 N 2 .22 A .011 c o 2 .030 29 .008 3 1 .009 32 .019 33 .018 3U .016 35 .016 36 .018 • Atom $ Deuterium 97.5$ In explaining the analysis, Dr. Fleming stated that the values given probably represent a lower l i m i t on the deuterium content, since the most probable sources of error would resu l t i n an apparent increase i n hydrogen content. The appearance of 1.29$ water vapour i n the sample can be explained only by assuming contamination of the sample after i t l e f t the chamber, since a l l water vapour i n the chamber should have been removed by the l i q -u i d a i r trap. If.the water content i f ignored, therefore, the lower l i m i t for the deuterium content can be set as 9 8 . 7 9 $ . The hydrogen impurity, . 8 3 $ , would not introduce any appreciable error i n the stopping power, but the .38$ of other impurities, mostly nitrogen, could lead to an error of 1.6$ i n the stopping power of deuterium. I t i s quite possible that some of the impurities were introduced i n talcing the sample. Considering the above possible sources of error, the stopping powers 23 can be quoted with the following estimated errors. Gas Argon Hydrogen Deuterium TABLE I I  Stopping Power .929 .220 .222 Estimated  Probable Error ^ .007 ± .005 * .005 D. COMPARISON OF RESULTS WITH PREVIOUS DATA The following table l i s t s the values of stopping powers found i n t h i s experiment and shows the accepted values f o r hydrogen and argon as found by other workers. TABLE I I I Gas Argon Hydrogen Deuterium Shopping Power Previous Work This Work .929^ • 9 3 1 6 . 2 2 U 3 .22^ .929 ±0.007 .220 ±0.005 .222 10.005 .236 ±0.0005 The extrapolated range of polonium alpha p a r t i c l e s i n a i r used as a 8 standard was taken from Holloway and Livingston and i s quoted as being 3.897 cm at l5°C and 760 mm mercury. The stopping powers of hydrogen and argon are i n agreement with the well-established values as given by Gray 3. The stopping power obtained f o r deuterium i s i n accord with the work ) t of S c h u l t z 4 , but i s i n disagreement with the work of Eichholz and Harrick PLATE H EXPERIMENTAL RANGE NUMBER CURVE WITH A PHOTO-MULTIPLIER AND AN ANTHRACENE CRYSTAL 700 2h V. CONCLUSIONS In agreement with theoretical predictions, the stopping powers of hydrogen and deuterium appear to be the same within the accuracy of this experiment. In the theoretical derivation of (-—) the assumption i s made dx that energy loss i s due e n t i r e l y to io n i z a t i o n and excitation of the absorb-ing atoms. The results of t h i s experiment indicate that t h i s assumption i s a v a l i d one. To detect a difference i n the stopping powers of hydrogen and deuter-ium due to the difference i n io n i z a t i o n potentials would require an i n -crease i n the accuracy of. t h i s experiment by a factor of 100 or more. With present experimental techniques, t h i s accuracy would be d i f f i c u l t i f not impossible to accomplish. . . Although the object of the experiment has been met to a reasonable extent, i t i s quite l i k e l y that the thin i o n i z a t i o n chamber w i l l be replac-ed by either a d i f f e r e n t i a l chamber where chamber penetration d i f f i c u l t i e s are non-existent, or by a s c i n t i l l a t i o n counter using modern photo-multi-p l i e r tubes as pulse amplifiers. With a photo-multiplier s c i n t i l l a t i o n counter, i t should be possible to detect 10 KEV pulses as compared.to the 150 KEV required f o r the th i n i o n i z a t i o n chamber. Plate XI shows a preliminary number-range curve which was obtained from an M I photo-multiplier s c i n t i l l a t i o n counter with anthracene c r y s t a l without taking any special e f f o r t to increase the signal-to-noise r a t i o . 25 This curve compares very favourably with the ones obtained from the t h i n i o n i z a t i o n chamber after talcing a l l possible steps to increase the.signal-to-noise r a t i o . 26 CHAPTER I I - ANGULAR DISTRIBUTION AND CORRELATION PATTERNS IN PROTON BOMBARDMENT OF F19 AND N 1^ -PART A - RADIATION PATTERNS FROM EXCITED STATES OF O l 6 I . INTRODUCTION 19 When F i s bombarded by protons there are three, competing reac-t i o n s 2 3 ' 2 ^ 1. F 1 9(po< ^ ) 0 1 ^ short range ot 2. F 1 9(p<* ) 0 1 6 long range °c 3 . F 1 9(pocTT ) o 1 6 short range =v ( 7T refers to nuclear p a i r production) The f i r s t reaction i s the one of interest i n t h i s experiment. The y i e l d of alpha p a r t i c l e s changes rap i d l y with bombarding energy, and resonance, levels have been measured and found to occur at the following proton ener-gies (KEV) 3h0, k80, $90, 660, 820, 873, 890, 935, 1092. I t i s known also that the gamma radiation has three d i s t i n c t energies, approximately 6 . 1 3 , 6 . 9 , 7.1 MEV, the re l a t i v e i n t e n s i t i e s depending'on the bombarding energy of the protons. At the lowest resonance (3U0 KEV proton energy) almost a l l of the disintegrations seem to lead to the same excited state of 0±o. Gam-ma rays 6.13 MEV are then emitted i n the tr a n s i t i o n to the ground-state. No long-range alpha particles'corresponding to the direct 30' 60* 90° '2"° , 5 0° l 8 0° ANGULAR CORRELATION BETWEEN oC-PARTICLES AND Y-RAYSIMTHE REACTION F'9(paT) 0" PLATE IE 27 t r a n s i t i o n from the Ne 2 0 l e v e l to the ground-state of O1^ are observed, though they are much favoured energetically. Experimental work on alpha gamma angular correlation f o r F"^(pot )O 1^ reaction has been carried on at Cavendish laboratory f o r the 3I4O KEV reson-ance . Both alpha p a r t i c l e s and gamma rays were observed i n a plane per-pendicular to the proton beam. The alpha p a r t i c l e s were detected with a proportional counter and with a window thick enough to exclude scattered protons from the thick CaFl 2 target. The 6 .1 MEV gamma rays were detected with-a lead-walled geiger counter. Coincidences between the alpha p a r t i c l e and the gamma rays were measured as a function of the-angle between the counters. The angular d i s t r i b u t i o n of alpha p a r t i c l e s with respect to the proton 20 beam i s i s o t r o p i c , and one can take as the simplest assumption that the Ne i s formed by s-protons . This means that i t has no preferred orientation and i t i s not u n t i l the alpha p a r t i c l e i s emitted that a f i x e d d i r e c t i o n i n space i s defined. The .gamma ray then has an angular d i s t r i b u t i o n with 38 respect to th i s axis, as has been found by Barnes et a l , and the observed • 6_ l a , correlation function has a term i n cos o . (The results are shown i n Plate XII) I t was concluded from the experimental results that (a) the resonance l e v e l of neon 20 i s formed by-£ =0 (•£= o r b i t a l momentum) protons and has j= l ( j = t o t a l angular momentum) and i s probably of even p a r i t y because alpha p a r t i c l e emission to the ground state of O1^ i s not observed, (b) since neon 20 has even p a r i t y and i s formed by^ =0 protons, F 1^ also has even p a r i t y , and (c) the neon 20 emits alpha p a r t i c l e s with£ = 3 leaving an excited oxygen 16 nucleus with j=3 and odd p a r i t y . In view of the wealth of information which was obtained from the study 28 of only one resonance, i t would be very interesting to extend the study of the angular correlation to the higher resonances i n an e f f o r t to assign quantum numbers to each of the known levels of 0 " ^ and Ne2^. A very serious d i f f i c u l t y arises, however, as the proton energy i s increased. The 3U0 KEV resonance i s the only one at which the alpha range exceeds the scattered proton range. For this reason, the protons can be e a s i l y exclud-ed from the counter by interposing absorbing screens while s t i l l admitting the alpha p a r t i c l e s . Extension of the measurement of the alpha gamma angu-l a r correlation to the higher resonances, therefore, reduces to the rather d i f f i c u l t feat of designing a counter which w i l l count only alpha p a r t i c l e s i n the presence of a v a s t l y larger number of more penetrating protons. Appendix I I shows calculated curves on the number of scattered protons as a function of angle and energy. Possible Methods of Counting Alpha P a r t i c l e s i n the Presence of a Strong Flux of Protons of Longer Range 1. Magnetic Separation A magnetic analyzer could be used to separate, the two p a r t i c l e s , then a conventional proportional counter could be used f o r counting the alpha p a r t i c l e s . The disadvantages of t h i s method are (a) construction of a magnet capable of resolving the p a r t i c l e s , and (b) v a s t l y reduced count-ing rates due to a decrease i n the effective s o l i d angle of the counter. 2. Very Fast Coincidence C i r c u i t A very f a s t coincidence c i r c u i t could give a real, to random coin-cidence r a t i o of 1 i f the resolving time was less tnan 5, x 10 . Appen-dix I I I contains the calculations on r e a l to random r a t i o s . 3 . Coincidence C i r c u i t with Anthracene Film A f a s t coincidence c i r c u i t could be used i n conjunction with an 29 anthracene f i l m . The anthracene f i l m would give pulses which would depend on the rate of energy loss (-^) i n the f i l m , and as i s greater f o r dx dx alpha p a r t i c l e s than f o r protons, the pulses from the alpha p a r t i c l e s would be much .greater than the pulses from protons. I f t h i s were true, the r e s o l -ving time of the coincidence c i r c u i t would not have to be so short. Recent publications, 2''''^ however, show that the s p e c i f i c s c i n t i l l a t i o n (—) does dx not increase l i n e a r l y with ( - — ) , but instead a saturation effect i s obser-dx ved. In spite of this e f f e c t , the pulse from a 2 MEV alpha, p a r t i c l e may be made nearly twice that from a 2 MEV proton by proper choice of the f i l m thickness. (See Appendix: IV) ROT AT ABLE TARGET MOUNT INSULATED TARGET GASKET -EMI PHOTO-MULTIPLIER %<=-snpnoti SCATTERING CHAMBER—PLATE. PLUG 30 I I . DESCRIPTION OF APPARATUS A. ELECTRONIC EQUIPMENT NECESSARY Plate XIV i s a block diagram of the electronic equipment that i s required to perform an alpha gamma angular correlation experiment using a fast.coincidence c i r c u i t . An E.M.I, type photomultiplier tube i s used with a th i n anthracene f i l m as the alpha detector. A 5819 RCA photo-multiplier with a large anthracene c r y s t a l i s used, as the gamma detector. The high speed discriminator following the Chalk River design employs an EFP 60 sec-ondary emission tube and is'capable of producing a standard pulse which rises i n a few millimicroseconds. This discriminator i s being constructed by Dr. D. B. James and w i l l be available f o r the experiment. 39 The coincidence c i r c u i t i s that of Garwin 7 and has a measured resolv-8 ' " ing time of 3 x 10~ seconds. To function properly the input pulses to the mixer must be greater than 3 v o l t s . Atomic instrument amplifiers and s c a l -ers are available to complete the necessary electronics. B. REACTION.CHAMBER The scattering chamber (Plate XIII) was designed and constructed with two purposes i n mind: to study the response of thin sections of anthracene to protons and to determine the angular d i s t r i b u t i o n of the alpha p a r t i c l e s from the F 1 9(poC^ ) 0 1 ^ reaction. (This angular d i s t r i b u t i o n i s required f o r •the interpreting of the. angular correlation experiment.) . The chamber has 12 ports (30° intervals) into whicfc the magnetically MUUIPLIBR I FAST DISCRIMINATOR - I + DELAY C- > I DELAY (+) 1 COINCIDE HCE CIRCUIT I AMPLIFIER I DISCRIMIMATOH I MULT I PL l€ft FAi DISCKirvti J 1 + DELAY f-) IT COINCIDENCE CIRCUIT AM PL IFICR IT DISCRIMINATOR IT DELAY (+) JL SCALER I SCALERJL PLATE JJF BLOCK DIAGRAM OF THE ELECTRONICS REQUIRED FOR A HIGH SPEED COINCIDENCE ANGULAR CORRELATION EXPERIMENT. 31 shielded s c i n t i l l a t i o n counter can be f i t t e d . The vacuum seals fo r a l l ports are made with rubber gaskets.• . 32 I I I . FILMS FROM ORGANIC PHOSPHORS 25 - 37 Although there are many references i n l i t e r a t u r e to organic s c i n t i l l a t o r s , both l i q u i d and crystalline,•there has been l i t t l e work done on t h e i r response to heavy p a r t i c l e s , and the work that has been done i s of very recent o r i g i n . In f a c t , i t was only after most of the work reported here had been completed that anything was published on the r e l a -t i v e pulse heights produced by protons and alpha p a r t i c l e s , and nowhere i n l i t e r a t u r e has anyone published a method of producing a t h i n f i l m from org-anic s c i n t i l l a t o r s . Anthracene and terphenyl of high p u r i t y were chosen as the organic s c i n t i l l a t o r s . I t i s unfortunate that the pulse r i s e time i n inorganic s c i n t i l l a t o r s i s so long" ' , because they show a good response to alpha p a r t i c l e s . Since anthracene and terphenyl have a c r y s t a l l i n e structure, i t i s not as simple as would be supposed to produce the uniform clear layer of the pure s c i n t i l l a t o r that i s desired. . Many different methods of f i l m formation were t r i e d and the better films produced were tested f o r alpha p a r t i c l e response on an 18 channel kick sorter. Techniques Employed i n the Production of Thin Films 1. Evaporation of an anthracene solution on a water surface. The anthracene was dissolved i n a suitable organic solvent which would f l o a t on a water surface and evaporate, leaving the anthracene as a f i l m on the water surface. 33 The following table l i s t s the various solvents t r i e d and the r e s u l t s . TABLE IV Solvent (Saturated  with Anthracene) Benzene Chloroform Ether Carbon Disulphide Remarks on Film Produced  on Water Surface Too violent a surface e f f e c t Film uneven and c r y s t a l l i n e Film very t h i n Density of chloroform too great, does not form a suitable f i l m Surface tension breaks f i l m as i t i s formed Forms granular islands Other solvents such as acetone, amyl alcohol and ethyl alcohol were t r i e d also, but the s o l u b i l i t y of anthracene i n these was too small to make f i l m formation a p o s s i b i l i t y . Zapon (commercial laquer) was added to the benzene solution i n an e f f o r t to reduce surface a c t i v i t y , but i t was found that the films thus produced did not s c i n t i l l a t e . • • 2. Evaporation of Anthracene Solutions on Glass Plates • The same solutions as t r i e d above on water were painted onto glass slides and allowed to eyaporate to dryness. In a l l cases, the surface formed was very uneven due to c r y s t a l formation. The best f i l m produced i n t h i s manner came from a chloroform solution. 3. Crystals Grown from Solution Saturated solutions of anthracene i n benzene, chloroform and carbon disulphide were allowed to evaporate slowly at room temperature, producing cry s t a l s of pure anthracene. The crystals formed from a chloroform solution were, needle-shaped, and, therefore, of l i t t l e use, but the crystals formed from benzene and 3k carbon disulphide- solutions were i n a useable leaf form. The lea f c r y s t a l s , however, were either very small or i f given time to grow to a s u f f i c i e n t s i z e , too thick. This i s a good method of making f l a t crystals of anthra-cene approximately 1/10 mm thick. h- Evaporation of Anthracene onto a Glass Plate i n a Vacuum The anthracene was heated i n vacuo and allowed to condense on a glass plate. The thickness could be controlled by the amount of anthracene evaporated. The required thickness, .J? to 1, mg/cm , could be obtained e a s i l y but, unfortunately, the.surface formed was not as uniform as would be desired, because the anthracene tends to form c r y s t a l l i n e grains as i t condenses on the glass. 5 . Film Produced from an Anthracene Melt Finely powdered anthracene was placed between two glass s l i d e s which were separated by an aluminum f o i l of the f i l m thickness required (approxi-mately .001 cm). The anthracene was thus confined to the area not covered by the aluminum f o i l and was of the proper thickness. The sl i d e s were heated (on a hot plate) u n t i l the anthracene flowed, and then allowed to • cool. The top plate and the aluminum f o i l were removed when the anthracene had s o l i d i f i e d , leaving the required anthracene f i l m . This procedure r e s u l t -ed i n a sa t i s f a c t o r y f i l m . 6 . Film Produced from a P l a s t i c Phosphor The p l a s t i c phosphor consists of a small percentage of terphenyl i n a polymerized monomer of styrene. Uniform thin sections were made by d i s -solving the p l a s t i c i n benzene and painting the solution on a glass s l i d e , but the p l a s t i c f i l m did not give the large pulses that were produced by anthracene of the same thickness, (see next section) 7. Film Produced from Pure Terphenyl The above techniques were t r i e d with terphenyl as with anthracene and the results were very s i m i l a r , (see next section) 3000 PLATE XV RESPONSE OF A THICK SECTION OF AHWKACENE TO ALPHA PARTICLES OF DIFFERENT ENERGY ZOOO i /ooo PULSE HEIGHT cC ALPHA PARTICLE ENERGY ARBITRARY UNITS / ) 35 IV. RESPONSE OF THIN ORGANIC FILMS TO ALPHA PARTICLES In order to test the pulses from the films of anthracene and terphenyl, a l i g h t - t i g h t chamber was constructed within which was mounted a movable alpha source. I t was thus possible to have any.desired f r a c t i o n of the alpha range f a l l w ithin the f i l m . Plate XV shows the pulses formed by a c r y s t a l of anthracene grown from a carbon disulphide solution. The c r y s t a l was much thicker than the equiv-alent range of polonium alpha p a r t i c l e s i n anthracene. Therefore, the pulses diminished i n sige as the source was moved farther from the c r y s t a l . Plate XVI shows sim i l a r curves f o r a t h i n f i l m produced by method 5« The thickness of th i s f i l m as determined by weighing was 1.1. mg/cm . Assum-ing that I.I4. mg/cm of anthracene i s equivalent to 1 a i r cm, th i s corresponds to an equivalent range i n a i r of approximately 0.8 cm. The pulse height does not decrease as the source i s moved away from the f i l m , showing that the f i l m i s much thinner than the range of the alpha p a r t i c l e s (3«8U cm). Very similar' results were obtained using pure terphenyl i n place of anthracene, but the p l a s t i c phosphor gave a much smaller pulse. The pulses produced i n the p l a s t i c phosphor are roughly one-half the size of those pro-duced i n anthracene or terphenyl f o r the same energy of alpha p a r t i c l e s . From the pulse heights observed f o r a thick section of anthracene, Plate XV, i t i s possible to plo t the response of anthracene to alpha part-i c l e s i n the energy range 0 .5 to 5.0 MEV. Taylor et a l have published a response curve f o r alpha p a r t i c l e s of energies from 2 MEV to 20 MEV. The 3000 PLATE RESPONSE OF A THIN SECTION OF ANTHRACENE T O ALPHA PfiRTiCLeS 2000 IOOO o 36 insert i n Plate XVII shows t h i s curve. The results obtained i n this exper-iment are plotted and f i t t e d to t h i s curve at h MEV (Plate XVII), and are seen to be a smooth continuation of the results obtained by Taylor et a l 37 V. CONCLUSIONS From the preceding section, i t i s concluded that a sa t i s f a c t o r y and r e l a t i v e l y simple method'of producing thi n s c i n t i l l a t i o n films of anthra-cene has been developed. The response of these films to alpha p a r t i c l e s of varying energy agrees w e l l with the exciton theory of Birks (see Appen-dix IV). 'Assuming that t h i s theory cor r e c t l y predicts the response of these films to protons, i t would appear d i f f i c u l t to discriminate between protons and alpha p a r t i c l e s i n organic phosphors on the basis of pulse size alone. As soon as a resolved beam of protons i s available from the Van de Graaff generator, the response of these films to protons w i l l be checked. I f i t proves impossible to discriminate between protons and alpha p a r t i c l e s on pulse size alone, i t w i l l be necessary to decrease the resolv-ing time of the coincidence mixer to about 5 x 1 0 - 1 0 seconds. The Garwin c i r c u i t i s probably not capable of such a short resolving time, but a s u i t -able coincidence c i r c u i t f o r t h i s work i s being developed by Dr. D. B. James. 38 PART B - ANGULAR DISTRIBUTIONS FOR GROUND-STATE ALPHA PARTICLES FROM N^Cpoc)^ 1 2 REACTION I. INTRODUCTION In view of the power of the angular d i s t r i b u t i o n and correlation method, i t i s natural to look at the analogous set of reactions f o r the proton bom-bardment of N . These reactions are l 1-! 12 N (poCtf)C short range alpha p a r t i c l e s 15 12 N (poc)C long range alpha p a r t i c l e s The y i e l d f o r the reactions N^^(p<x ^ ) C 1 2 and N"^(p°c)C12 has been measured 16 up to a proton energy of 1.2 MEV, resonance states of the 0 compound nucleus occuring at proton energies of 0 . 9 , 1 . 0 , 1.2 MEV^. For the reson-c IP ance at 1.2 MEV, both states of C can r e s u l t , the cross-section being 0 . 6 12 and 0.2 barn f o r the reaction leading to ground and excited states of C. respectively. The gamma ray t r a n s i t i o n energy i s h»k£& MEV. Wilkinson reports an energy of h-h$ .01+'MEV and a high order of anistropy of order 14- 0 .3 cos 20 near the 900 KEV l e v e l . In order to assign the angular momentum and p a r i t y of the luU6|? MEV state of C 1 2, the angular, d i s t r i b u t i o n of the gamma rays with respect to the proton beam i s i n s u f f i c i e n t by i t s e l f . A knowledge of the p a r i t y and angular momentum of the O1^ excited state could be determined by the M 1^(poc)C 1 2 reaction, and would probably enable an unambiguous assignment of quantum numbers to a l l states involved. I f necessary further knowledge could be obtained by examining the alpha gamma angular co r r e l a t i o n . 39 ' I I . THEORY ' - ^ . GENERAL PRINCIPLES OF TWO-STAGE PROCESSES Reduced to i t s simplest terms, the two-stage process i s e s s e n t i a l l y a fluorescence problem. In fluorescence, l i g h t of one wave-length i s absorbed by a system and the capture cross section of atoms f o r radiation i s largest when the t o t a l energy ocrresponds to a stationary state of the system. Subsequently, some other l i g h t i s emitted. The dispersion of l i g h t i s described i n roughly the same terms. The essential form of the d i f f e r e n t i a l cross section i n the analogous case where a heavy p a r t i c l e (proton) i s absorbed and a heavy p a r t i c l e (ex. alpha p a r t i c l e ) i s emitted i s given as In t h i s formula gg(E) and g^ <(E*) represent slowly varying functions of energy and serve as parameters when one compares theory with experiment. They represent something l i k e i n t r i n s i c p r o b a b i l i t i e s of reaction once one has brought the nuclei together. P^  and P^ ' are ba r r i e r p e n e t r a b i l i t i e s f o r the incident and emitted p a r t i c l e s , respectively. The subscript r refers to the various resonances and the Y ^ r e f e r s to the associated Legen-dre polynomials. ^ o r b i t a l angular momentum of incident p a r t i c l e . A Z-component o f ^ , and i s zero because of the choice *-X of Z-axis j t o t a l spin angular momentum of incident p a r t i c l e and target nucleus jz Z-component of j J t o t a l angular momentum of compound nucleus ho Jz Z-component of J Jl 3Xid.Jlz are the o r b i t a l angular momentum and i t s z-component for the emitted p a r t i c l e . j'and ^ are the spin angular momentum and i t s z-component of the resultant p a r t i c l e s The transformation coef f i c i e n t s ( ^ z j j z | J J Z ) are l i s t e d i n Condon and Shortley, pages 73—78. Since conservation of angular momentum holds i n these problems I t - U + j) Jz - Jz s i n c e J^z " 0 A l l possible angular distri b u t i o n s up to J equals h f o r the compound nucleus have been worked out and are l i s t e d i n the section below. B. CALCULATED ANGULAR DISTRIBUTION FOR THE GROUND-STATE ALPHA PARTICLE IN  THE N ^ p t x Q C 1 2 REACTION The d i s t r i b u t i o n has been calculated assuming f i v e overlapping e x c i t -ed states i n 0"^ with spins of o+, 1-, 2+, 3 - , U+- where the signs indicate the p a r i t y of the state (-, odd p a r i t y , -+- even p a r i t y ) . The following assumptions following e a r l i e r experimental results were made (a) N 1^ has a spin of \ and odd p a r i t y , and 12 (b) C has a spin of 0 and even p a r i t y . 12 Therefore, C +• °C has a spin of 0 and even p a r i t y . A l l possible proton and alpha p a r t i c l e angular momenta which are con-sistent with the above assignment to the O1^ compound nuclear states have been included. In order that unknown nuclear factors, ex. pe n e t r a b i l i t y , resonance denominators, can be allowed f o r , each alpha p a r t i c l e has .assoc-iated with i t a separate amplitude and phase factor of the form Ae , where "A" i s the amplitude and "a" i s the phase angle. i l l Including t o t a l angular momentum of the compound nucleus up to k, there are nine ways i n which the reaction can-take place. These are l i s t -ed below. No Total Spin -of N 1 5+ p 1 o l 6 Total Spin-C 1 2 V Amplitude and Phase 1 1 0+" 0 0 A, a 2 1 0 1- 1 0 B, b 3 . 1 ; 2 1- 1 0 I , 7 k 1 1 ' 2 + 2 0 D , d 1 3 2 + 2 0 E, e 6 1 2 3 - 3 0 F , f 7 1 U 3 - 3 0 G, g 8 1 3 U 0 H, h 9 1 5 ii - 0 K, k Any combination of the f i v e states of the O1^ compound nucleus can be taken by c o l l e c t i n g those terms which contain the amplitude and phase f a c t -ors corresponding to them. Ex: Assuming 0 l 6 to have two overlapping l e v -els of ( 0 4-) and ( 3 - ) , the following terras must be collected,/ A 2+ F 2 H G2 4 AFcos(a-f) 4- AGcos(a-g) +• FGcos(f-g) E s s e n t i a l l y , t h i s i s a procedure whereby the amplitudes of the unwanted states are put equal to zero. The•angular di s t r i b u t i o n s of ground-state alpha p a r t i c l e s from the N 1^(poC ) C 1 2 reaction are thus as follows. The abbreviation c n = cos 1 1 i s used throughout. O1^ Spin States , • Involved Angular Dist r i b u t i o n (04-) - 2 1/6 A^ k2 0 Spin States Involved Angular Distribution (Cont'd) ( 1 - K 1 - ) (24)(24) ( 3 - K 3 - ) (h+)(k+) (o * ) ( l - ) (0-*)(24) (0+)(3-) (o+)(U+) (l-)(2+-) 3 / 2 B 2 3/20Y 2(l+-3[C 2) 3 A J27£ BYcos(b-y) ( l - 3 c 2 ) 5/12 D 2 ( l + - 3 c 2 ) 15/56 E 2 ( l - 2 c 2 + 5 c [ t ) 5/8 4 2/7 DE cos(d-e)(-l + 1 2 c 2 - l 5 c ^ ) 21 A O F 2(l - 2 c 2 - » - 5 c i ; ) 7/288 G 2 (9+li5C 2 - l65c^-«-175c 6 ) 7/16 FGcos(f-g)(3-69c 24- 225c^-175c 6 ) 9/22U.H 2 (9-»A5c 2 -l6Sc [ U- 1 7 5 c 6 ) U5AU08 K 2 ( 9 9 3 6 c 2 + 2 9 U c l t - 6[^c 6 - + - i t U l c 8 ) 9 M J 5/77 HKcos(h-k)(-9-»-360c 2 -2130cV3920c 6 -2205c 8 ) I ABcos(a-b)(-c) 1/JlO AYcos(a-y)(-Hc) 1/6 J ^ A ADcos(a-d)(l-3c 2 ) l A J577 AEcos(a-0,)(-l4-3c2) 1 A . 4 ^ 7 5 AFco.s (a-f) (3c - 5 c 3 ) £ 47/27 AGcos(a-g)(-3c-»-5c 3) 3/8 J 5 7 2 1 AHcos(a-h)(-3 +-'30c2-3£ck) 3/16 J5/33 AKcos(a-k)(3-30c 2 ^ 3 5 ^ ) I 4l0"BDcos(b-d)( c) u-)(a+) (2+)(3-) O 1 6 Spin States Involved Angular Dist r i b u t i o n •. (Cont'd) ( l - ) O - ) _ . f >)63/5 BFcos(b-f )(-l - h 3 c ) 1/2U J l l BGcos(b-g)(-3H-30c 2-35c I t) 3/UO J l i r Y F c o s ( y - f ) ( - H - 1 2 c 2 - l 5 c l i ) 1/16 YGcos(y-g ) ( -3-6c 2 -H25c I i ) .3/2 4 3 A BHcos(b-h)(-3c4-£c3) 3/16 > | l 5 / l l BKcos(b-k)(- l5c 4 - 7 0 c 3 - 6 3 c ^ ) 3/16 >|6/35 YHcos(y -h)(-21c+110c 3 -105c^) 3/16 J6/11 YKcos(y-k)(-3c - 1 0 c 34 - 2 1 c ^ ) • | 4 l i r D F c o s(d-f ) ( c 3 ) 1/U8 470/3 DGcos(d-g)(-21c +110c 3 -105c^) 3/8 EFcos(e-f ) ( - 5 c 4-26c3-25'c^) l / 8 j V 3 EGcos(e-g)(3c-10c 3 4- l$c^) 1/16J30/7 DHcos(d-h)(-3-6c 2-4- 25c^) 5/32^6/11 DKcos(d-k)(3-75c 2 4- 2 U 5 c ^ - l 8 9 c 6 ) 3 / l l 2 j l ? EHcos(€-h)( 3 - 6 9 c 2 4 - 225c^-175c 6 ) 15/32J3/77 EKcos(e-k)(-3 4-l5c 2 -U5c J 4 -+- h9c6) 3/8jp/5 FHcos(f-h)(3c-10c 3-4-30c 5 ) ' 3/32J21/II FK cos(f-k ) ( 2 1 c - 2 0 5 c 3 + U 5 3 c ^ - 3 l 5 c 7 1/32 GHcos(g-h)(8lc-795c 3 + 1775c^-1225c 7 ) ': 1/16 J3 5/11 GKcos(g-k ) ( 3 0 c 3 - 8 U c 54 - 7 0 c 7 ) (3-)(U+) BALL BEARING STOP ROTATING ARM — COUNTER'ADJUSTMENT THIN WINDOW. SCATTERING CHAMBER PIATElUIl hk I I I . DESCRIPTION OF APPARATUS A. REACTION CHAMBER •Calculations (see Appendix V) shovf that the range of the alpha part-i c l e s at 180° to the beam fdirection i n the laboratory system of coordin-ates would have a range of only 2.3 cm i n a i r . The chamber shown i n Plate XVIII was constructed with 21 windows a l l of which have an equiva-lent a i r range of less than 1.56 cm. The windows on one side of the chamber are 10° apart, while the three on the other f o r monitoring pur-poses are placed at 1|0 , 90° and 1 3 0 ° , respectively. The proportional counter can be accurately moved from window to window by a calibrated arm with a ball-bearing stop every 1 0 ° . The counter can be moved towards or away from the target by means of a screw. B. COUNTER AND AUXILIARY APPARATUS A proportional counter i s used to distinguish between alpha pulses and any other pulses from other sources. The counter i s shown i n Plate XVIII. A very t h i n window i s the main feature of the proportional counter ( 0 . 5 cm a i r equivalent). At 180° the alpha p a r t i c l e ' w i l l thus expend O.I4.MEV (2.30-2.06=0.2l|. cm) i n the counter, which i s s u f f i c i e n t to give a good-sized pulse. The necessary electronic c i r c u i t s have been constructed and tested and have been found to be satis f a c t o r y . An a u x i l i a r y reaction chamber has been constructed f o r the measurement of the gamma ray angular d i s t r i -bution. IV. DISCUSSION A l l tests of equipment indicate that the experiment i s feasible and completion of 'the experiment awaits a stable resolved beam from the Van de Graaff generator. he APPENDIX I SAMPLE CALCULATION The experimental extrapolated range f o r a i r corresponded to a read-ing of 3 « 7 1 2 on the micrometer. To f i n d the actual distance from the o r i g i n , the known value of the extrapolated range f o r a i r at l£°C and 760 mm, i . e . 3 . 8 9 7 cm, was used. Now, not a l l of the 3 . 8 9 7 cm i s i n a i r . The aluminum window has an a i r equivalent of . 9 6 8 cm, therefore, the actual range i n a i r i s 3 . 8 9 7 - . 9 6 8 = 2 . 9 2 9 cm from the o r i g i n , i f the experimental range was at l£°C and 760 mm mercury. But since the exper-imental temperature and pressure d i f f e r e d from these values, a correction i s necessary to make the two i d e n t i c a l . R 1 = RQPQT PT 0 Therefore, a reading of 3 . 7 1 2 cm on the micrometer corresponds to an actual distance of 3 . 0 2 6 . The micrometer correction is. then •3.712 - 3 . 0 2 6 = . 6 8 6 cm For argon, which was found to have an uncorrected extrapolated range corresponding to a distance of U .003 om on the micrometer, the following corrections are applied. • 1 . Micrometer Correction R 1 - U .003 - . 6 8 6 = 3 . 3 1 7 cm hi 2. Temperature and Pressure Correction Ro ° R ' ^ £ ~ 3-317 x .968 = 3.210 P 0T 3. Chamber Depth Correction Since the signal-to-noise r a t i o i s greater f o r argon than f o r a i r , the alpha p a r t i c l e m i l not have to penetrate the chamber as f a r to pro-duce the same size pulse i n argon as i n a i r . This means that the o r i g i n from which we measured the a i r range w i l l have sh i f t e d toward the front of the chamber f o r argon. In order to have the o r i g i n the same, we must sub-tract the difference i n chamber penetration from' the observed argon range. Experimentally, the signal-to-noise r a t i o f o r a i r at the maximum of the Bragg curve was 6 (see Plate V;1U). Since the maximum sp e c i f i c i o n i z a -t i o n i s approximately 6000 ion pairs per mm- , a chamber depth of 3 mm would correspond to.a t o t a l of 18,000 ion pairs formed. I f the signal-to-noise r a t i o i s 6 , then 1/6 x 18,000 or. 3000 ion pairs are. required before a pulse i s recorded. By numerical integration of the s p e c i f i c i o n i z a t i o n D curve f o r a single alpha p a r t i c l e - , the penetration required to produce 3000 ion pairs was found to be 1.25 mm. For argon, the signal-to-noise r a t i o was found to be approximately 8 1/3. Therefore, 25/18 times as many ions were formed by argon i n the 3 mm as were formed from a i r . I f the r e l a t i v e i o n i z a t i o n i s constant over a l l energies f o r a i r to argon, one would assume that the penetration would be given by 18/25 :o.f the value found f o r a i r . 18/25 x 1.25 - .90 mm The correction i s , therefore, 1.25 - .90 = .35 mm, which i s subtracted from 3.210 to give a corrected extrapolated range of 3 . 175 . The mean ranges are now e a s i l y found from the graph by subtracting the observed straggling parameter, S = polated range. Mean Range i n A i r = Mean Range i n Argon = Therefore, the stopping power of argon Rm a i r = 2.8l>2 Rm argon 3 .067 • • - U8 R e x - Rm, from the corrected extra-2.929 - .087 - 2.8U2 3.175 - .108 = 3.067 i s given by' » .927 RUTHERFORD SCATTERING OF PROTONS—— PLATE. h9 APPENDIX I I Number of Scattered Protons as a Function of Energy and Angle Plate XIX shows the family of curves obtained when the number of pro-tons scattered at various angles i s plotted against the energy of the incident protons. The curves were calculated from the Rutherford scattering-formula. N Q = number of incident p a r t i c l e s per second N = number of atoms/cc i n target t = target thickness cm e = electronic charge Z *» charge on incident p a r t i c l e Z' = atomic number of target •§mv •- energy of the incident p a r t i c l e The curves, were calculated f o r an aluminum target of 2 mm a i r equiv-alent thickness. Clearly the number of scattered protons could be further reduced by using a thinner target of low atomic number. Thin films of collodion or formar would reduce the- number of scattered protons. 50 APPENDIX I I I Ratio of True to Chance Coincidences f o r the F 1 9 ( p o c y ) 0 l 6 Angular Correlation Experiment The number of true coincidences i s given by Np = ^0^^\^7^ where k = p r o b a b i l i t y that the incident p a r t i c l e s h a l l produce the requir-ed reaction, and N D - number of incident p a r t i c l e s per second ^ (^= the e f f i c i e n c i e s of two counters "»fl7kss the s o l i d angle subtended by the counters The number of chance counts i s given by N c = 2N 1N 2C T i s the resolving time i n seconds, N-j_ and N 2 are the single channel count-ing rates. If. N]_ refers to the alpha counter, then the scattered protons w i l l determine the counting rate i n that channel because t h e i r number f a r exceeds the number of alpha p a r t i c l e s i n the reaction. N]_ - N0X, where X i s the Rutherford scattering, p r o b a b i l i t y then the r a t i o of true coincidences to chance coincidences i s 2N]_N2 f 2N0X C For the experiment to be f e a s i b l e , R should not be less than unity. If we substitute the following numerical values 51 k = 10 , W0 = 6 x K r V s e c = 1 microamp of beam £ ; - 1, ^.= 0 . 2 5 , y[t = ^ 2 = 1 0 ~ 2 , and K = lofysec, we obtain 2* = £ x 1 0 ~ 1 0 sec. The value of N]_ above was calculated f o r 2 mm a i r equivalent thickness of aluminum . By substituting a thinner hydrocarbon f i l m , N]_ can be reduced by a factor of 100 without too much d i f f i c u l t y . Then the required resolv-ing- time i s f - 5 x 10~ which i s e a s i l y obtainable. UJ a b_ o o !< oc: Equivalent Air Range of Film 0.2 cm 0.5 cm I.O cm P R O T O N E N E R G Y H > M E V PLATtXST RELATIVE RESPONSE OF ANTHRACENE TO ALPHA PARTICLES AND PROTONS FOR DIFFERENT FILM THICKNESSES 52 APPENDIX IV  Exciton Theory The v a r i a t i o n of — (spe c i f i c fluorescence) with (-—) (s p e c i f i c ener-dx dx gy loss) may be explained using the exciton theory 2^. On th i s theory, the electronic energy excited by the i o n i z i n g p a r t i c l e (the exciton) i s transfer-red from molecule to molecule within the c r y s t a l , u n t i l i t i s either emit-ted as radiation or quenched by a damaged molecule. I f the number of excitons produced per unit path length i s A^. and the dr dE l o c a l concentration of damaged molecules i s B-?~ molecules ner undamaged dx molecule and the exciton capture p r o b a b i l i t y of a damage molecule r e l a t i v e to an undamaged molecule i s k, then the sp e c i f i c fluorescence i s dE dL = Adx~ dx 1 + kBdE dx _ The values f o r anthracene are A = 82.5 and kB = 7 . 15 , as calculated from the experimental data on alpha p a r t i c l e s . In Plate XX> the r a t i o of the s p e c i f i c fluorescence of alpha p a r t i c l e s to the sp e c i f i c fluorescence of protons i s plotted against proton energy for films of different thickness. The incident alpha p a r t i c l e energy i s assumed to be 2 MEV. PLATE MI RANGE OF GROUND-STATE ALPHA PARTICLES FROM N'5(p *)c'2 IN LAB. COORDINATES AS A EttMTlON OF 0 AND PROTON ENERGY _ N " ALPHA PARTICLE RANGE SCATTERED PROTON RANGE 53 APPENDIX V Alpha p a r t i c l e ranges from the (p o<.)C reaction as a function of (ft (lab coordinates) and proton energy have been calculated from the gen-e r a l formula ME: A L i 2' — \ = ( M ^ ) 2 ^ Cos(^+(MM3Q +• MQM^ E-^  - M-jMgE-^sin (f)* The subscript 0 refers to the target nucleus, 1 to the incident p a r t i c l e , 2 to the emitted p a r t i c l e , and 3 to the residual nucleus. MQ...M^ are the masses of the four n u c l e i , and M the mass of the compound nucleus. EQ...E are the k i n e t i c energies of the p a r t i c l e s . Plate XXI shows the scattered proton range at = 0 as a function of energy. • 9x BIBLIOGRAPHY 1. Livingston and Bethe 2. Rutgers and Milatz 3• Gray U. Schultz 5 . N. J. Harrick 6 . Harrick, Eichholz 7. Halliday 8 . Holloway and Livingston 9 . Milatz and Rutgers , 1 0 . Lewis and Wynn-Williams 11 . Lewis 12. Hoag and Korff 1 3 . Hoag l U . Wilkinson 15 . Curran and Craggs 16. Mano 17. Chang 18. Wadey 1 9 . Elmore and Sands 2 0 . Rutherford, Chadwick and E l l i s Review of Modern Physics - 9 , 2 6 3 , 1937 Physica 7 , 5 0 8 , I9U0 Proc. Cambridge Phil.Soc. 72, 19UU Phys i c a l Review 53,62 2, 193 8 M.A. Thesis, University of B.C. A p r i l , I9I+9 Physical Review 7 6 , 5 8 9 , 19U9 Introductory Nuclear Physics Wiley, 1950 Physical Review 5 U , l 8 , I938 Physica VI 529, 1939 Proc.Royal Society 13>6,3k9, 1932 E l e c t r i c a l Counting Cambridge University Press 19U2 Electron and Nuclear Physics Van Nostrand, 19U9 Electron and Nuclear Counters Van Nostrand, 1938 Ionization Chambers and Counters Cambridge University Press 1950 Counting Tubes Academic Press Inc. 19U9 Annales de Physique • 1 ,72 , 193U Physical Review 6 9 , 6 0 , 193H Physical Review 7 U , l 8 U 6 , 19U8 •Electronics McGraw H i l l , 19^9 Radiations from Radioactive Substances Cambridge University Press 1930 2 1 . Hevesy and Paneth Radioactivity . Oxford University Press 1938 55 22. Bjorksted, M i t c h e l l ' Physical Review U 6 , 6 2 9 , 193U 23. Chao, F o l l s t r u p , Fowler and Lauritsen Physical Revie?f. 79,108, 1950 2U. Hornyak, Lauritsen, Morrison and Fowler Review of Modern Physics 22,357, 1950 25. S. B. Birks Physical Review 8U,36ii, 1951 26. Taylor, Remley, Jentschke and Kruger Physical. Reviexv 83,169, 1951 27. Frey, Grim, Preston and Gray Physical Review 82,170, 1951 28. Robinson, Cook and Jefferson Journal Chem. Physics l 8 , l U 8 , 1950 29. Jordan and B e l l Nucleonics 30, Oct . l9U9 30. Scharr and Farmer Physical Review 81,891, 1951 31. . Scharr and Franklin Physical Review 80, U7UA950 32. Birks Proc.Physical Soc. London A 63,129U, 1950 33. Reynolds Nucleonics 5, May-1950 3U. Hartmut Kallman and Furst Nucleonics 1, U9;JUiy-i950y 35. Koski Physical Review 76,308,19^9 36. Gettings Physical Review 75,205, 19U9 37. B e l l Physical Review 77,li| .05, 19U8 38. Barnes, French and Devons Nature I 6 6 , l i i 5 , 1950 39. Garwin R.S.I. 21,569, 1950 Uo. Schardt, Fowler and Lauritsen Physical Review 80,136, 1950 i l l . W. R. Arnold Physical Review 8o,3U, 1950 U2. French Mimeographed Lecture Notes Cambridge, 1950 •ABSTRACT The stopping powers of hydrogen and deuterium have been compared, using a t h i n i o n i z a t i o n chamber. The results were found to be consistent with the preseht-day theory on the method of energy l o s s . A method f o r the preparation of thin films of organic phosphors has been devised and the response of these films to alpha p a r t i c l e s has been tested. • • A l l necessary apparatus f o r the study of angular correlation and d i s t r i b u t i o n patterns f o r F l ! ?(p<*.t)0^ and N l 5(pac)C 1 2 reactions has been constructed. The theoretical angular d i s t r i b u t i o n patterns f o r the U ?(p«)C reaction have been calculated. 

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