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The angular correlation of annihilation radiation and a study of high energy nuclear reactions in neon Erdman, Karl Lembit 1953

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THE ANGULAR CORRELATION OP ANNIHILATION RADIATION AND A STUDY OF HIGH ENERGY NUCLEAR REACTIONS IN NEON. by KARL LEMBIT ERDMAN A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN PHYSICS We accept this thesis as conforming to the standard required from candidates for the degree of DOCTPR OF PHILOSOPHY IN PHYSICS Members of the Department of Physics THE UNIVERSITY OF BRITISH COLUMBIA May, 1953. A B S T R A C T In the f i r s t section of this thesis the measurement of the angular correlation of annihilation radiation in various materials is described. It is found that the angular depen-dence of the coincidence counting rate leads to results which are not in agreement with the present theories of the mech-anism of positron decay processesin crystal and metal latt i c e s . In particular, positrons do.not appear always to reach ther-mal energies before annihilation with electrons takes place. A search for the photodisintegration of Ne2^ using the gamma rays from the Li^(p , V )Be^ reaction is described. An uausually low cross-section for the photo-alpha process to the ground state of O1^ is found, 8 x 1 0 ~ 3 0 cm2 for the 17.6 Mev. gamma ray and 3 x 10~ 2 9 em2 for the 14.8 Mev. gamma ray. Cross-sections of the order of lO" 2^ cm2 are obtained for alpha particle transitions to the 6 and 7 Mev. excited state of 0XO. These cross-sections are of the usual order of magnitude for photodisintegration reactions. 20 Irradiation of Ne with high energy neutrons gives rise 21 to alpha particle transitions from the Ne compound nuclear levels so formed to several excited states of 9^7. The ex-citation energies of these states are in good agreement with those measured in other nuclear reactions. THE UNIVERSITY OF BRITISH COLUMBIA F a c u l t y o f Graduate S t u d i e s PROGRAMME OF T H E F I N A L ORAL E X A M I N A T I O N FOR T H E D E G R E E OF DOCTOR OF P H I L O S O P H Y of K A RL L E M B I T ERDMAN B.Sc. (University of Alberta) 1948 M.Sc. (University of Alberta) 1949 FRIDAY, MAY 8 t h , 1953, at 10:00 A.M. IN ROOM 300 PHYSICS BUILDING COMMITTEE IN CHARGE: H. F. Angus, Chairman C. A. Barnes W. A. Bryce K. C. Mann H. C. Gunning G. M. Shrum S. A. Jennings J. B. Warren B. Savery LIST OF PUBLICATIONS Positive P a r t i c l e s Associated with Beta-ray Emitters, K. L. Erdman, G. Kokotailo, D.B. Scott, Physical Review 1262, 1949. T H E S I S THE ANGULAR CORRELATION OF ANNIHILATION RADIATION AND A STUDY OF HIGH ENERGY NUCLEAR REACTIONS IN NEON In the f i r s t s e c t i o n of t h i s t h e s i s the measurement of the angular c o r r e l a t i o n of a n n i h i l a t i o n r a d i a t i o n i n various materials i s described. I t i s found that the angular depen-dence of the coincidence counting rate leads to r e s u l t s which are not in agreement with the present theories of the mechanism of positron decay process i n c r y s t a l and metal l a t t i c e s . In p a r t i c u l a r , positrons do not appear always to reach thermal e n e r g i e s before a n n i h i l a t i o n with e l e c t r o n s takes p l a c e . 90 A search for the photodisintegration of Ne using the gamma rays from the L i (p.V)Be^ r e a c t i o n i s described. An unusually low cross-section for the photo-alpha process to the ground state of 0 1 6 i s found, 8 x 10~ 3 0 cm2 for the 17.6 Mev. 9Q 9 gamma ray and 3 x 10 cm z f o r the 14.8 Mev. gamma ray. Cross-sections of the order of 10~ 2^ cm2, are obtained f o r alpha p a r t i c l e t r a n s i t i o n s to the 6 and 7 Mev. excited states of 0^6. These cross-sections are of the usual order of magni-tude for photodisintegration reactions. 90 I r r a d i a t i o n of Ne with high energy neutrons gives r i s e to alpha p a r t i c l e t r a n s i t i o n s from the Ne 2^ compound nuclear l e v e l s so formed to several excited states of 0 . The e x c i ; t a t i o n energies of these states are i n good agreement with those measured i n other nuclear reactions. GRADUATE STUDIES F i e l d o f Study: P h y s i c s Spectroscopy -- K. B. Newbound Electromagnetic Theory -- W. Opechowski Nuclear Physics -- K. C. Mann Quantum Mechanics -- G. M. Volkoff Chemical Physics -- A. J . Dekker Radiation Theory -- F. A. Kaempffer Theoretical Nuclear Physics -- G. M. Volkoff Other S t u d i e s : Probability and S t a t i s t i c s -- E. S. Keeping Advanced D i f f e r e n t i a l Equations -- T. E. Hull Radiochemistry -- M. Kirsc h and K. Starke TABLE OF CONTENTS Page PART I - THE ANGULAR CORRELATION OF ANNIHILATION RADIATION I. INTRODUCTION i l II. EXPERIMENTAL ARRANGEMENT (a) Source . . . . . . . . . . . . . . k (b) Detectors and Geometry 5 (c) Coincidence Counting Circuits 6 (d) Measurement of Resolving Time 7 (e) Experimental Procedure 8 III. CORRECTIONS TO OBSERVED DATA (a) Counting Corrections 9 (b) Geometrical Corrections 9 (c) Scattering Corrections (i) Compton Scattering . 10 ( i i ) Rayleigh Scattering 10 IV. RESULTS (a) Annihilations i n Copper . 13 (b) Other Substances . . . . . 13 V. DISCUSSION . 16 I II PART II - THE PHOTODISINTEGRATION OF NEON . Page I. INTRODUCTION (a) Photonuclear Reactions . . . . . 17 (b) Photo-Alpha Processes 17 (c) Methods of- Investigation • 18 (d) Theoretical Estimate of the Cross Section... 19 (e) Purpose of Present Experiment 21 II. THE EXPERIMENTAL TECHNIQUE (a) General Considerationsin Cross Section Measurements 22 (b) The Ionization Chamber 23 (c) Source of Gamma Rays 2k III. THE EXPERIMENTAL ARRANGEMENT (a) Gridded Ion Chamber . . '-.26 (b) Electronics and Pulse Analysis .' 28 (c) Lithium Target Arrangement 30 (d) Gamma Ray Flux Determination 30 IV. EXPERIMENTAL PROCEDURE (a) Energy Calibration . . . . . . . 33 (b) Photodisintegration Measurements 3U V. RESULTS (a) Discussion of Pulse Spectrum . 35 (b) Cross Section Calculations'. . . . . . . . . . ' 3 8 VI. DISCUSSION OF RESULTS h-0 I l l PART III - DISINTEGRATION OF NEON BY FAST NEUTRONS "I. INTRODUCTION . . . . . . . II. ' EXPERIMENTAL TECHNIQUE . III. EXPERIMENTAL PROCEDURE IV. RESULTS APPENDIX I . . Rayleigh Scattering . . . . .• .. . . •. .-. APPENDIX II Direct Flux Measurement by NaI (T|) Crystal • BIBLIOGRAPHY . . . . . . . . . / TABLE OF ILLUSTRATIONS To Face Page Figure I Experimental Arrangement ' 5 II Source Mount III Photomultiplier Circuit 6 IV Fast Coincidence Mixer * " • ' ' • 6 V Side Channel Amplifier and Discriminator 7 VI Measurement of Resolving Time by Pulse Delay Method 7 VII Dead Time of Circuits 9 VIII Scattering Amplitude as a Function of Angle 11 IX Effect of Correction for Rayl.eiglr Scattering 11 X Angular Correlation for Annihilation i n Copper and Teflon 13 XI Angular Correlation for Annihilation i n Ag, Cu, Cd, PB lit XII Angular Correlation for Annihilation i n C, CCl^, S, Cs,Cl, L i , L i C l , . L i I lU XIII Angular Correlation for Annihilation in L i and Be lU XIV Average Momenta of Mass Centre 15 XV Electrode System of Ion Chamber 26 XVI Outside Casing of Ionization Chamber 26 XVII Lithium Metal Target Evaporating Chamber 30 XVIII Ll(p)f) Excitation Function 30 - IV -- V -To Face Page Figure XIX Experimental Arrangement for Photodisinteg-ration Measurement 31 XX Po210 Alpha Peak 33 XXI Li(p,vO Gamma Ray Spectrum 3I4. XXII Background in Ionization Chamber with Accelerator off 3I4. XXIII Background i n Ionization Chamber Bombarded with Fluorine Y -rays 3k XXIV Spectrum of Pulses i n Chamber above 8 Mev. 35 XXV Spectrum of Pulses in Chamber below 8 Mev. 35 XXVI Experimental Arrangement for Neutron Bombardment of Neon I4J4. XXVII Pulse Spectrum of (D-D) Neutrons on Neon I46 XXVIII Pulse Spectrum below 6.5 Mev. 1*6 XXIX Pulse Spectrum between 6.5 Mev. and 9 Mev. • 1|6 XXX Pulse Spectrum above 9 Mev. lj.6 XXXI Observed Alpha Groups to Levels i n 0 1 7 kl ACKNOWLEDGEMENTS The author is indebted to Dr. J. B. Warren for his kind supervision of the work which was done in this thesis and, in particular, for the experiment on positron annihilation which was f i r s t suggested by him. The experiment on the angular correlation of annihilation quanta was made possible by the oppor-tunity of working at the Chalk River pile and the suggestions and help given by Dr. L. G. E l l i o t t and Mr. E. P. Hincks. It i s a pleasure to acknowledge the aid and suggestions of the many members and staff of the Physics Department in the experiments conducted with neon. In particular, thanks are given to Dr. C. A. Barnes for his discussions on the photo-:disintegration of the neon nucleus and to Dr. D. B. James for his valuable aid in making the measurements. The author's part in this work was made possible by scholarships awarded by the National Research Council and by the British Columbia Telephone Company. PART I THE ANGULAR CORRELATION OF ANNIHILATION RADIATION .1. INTRODUCTION The radiation from the annihilation of positron electron pairs was f i r s t shown to be made up of pairs of coincident quanta emitted i n opposite directions by Klemperer^. Calculations based on the Dirac hole theory1- show that a l l but a few percent of the positrons reach the end of their path of ionization before anni-hilating. If the energy of the positron i s small, a relationship can be developed between the momentum of the annihilating pair and the angle between the two annihilation quanta. By the laws of the conservation of energy and momentum i t can be seen that: and where 2ht = mc2 ( 1 + 1 B p = 2 ™ sin -0/2 c B = v/c - 1 -- 2 -by eliminating hv we have p = mc(l 1 ) sin 6/2. If the momentum i s small then v i s small and by neglecting Bj p/mc £4 2 sin & /2, and since 6 w i l l also be small then mc An attempt to measure the coincidence rate as a function of counters as detectors. The negative result together with the increased detection efficiency of s c i n t i l l a t i o n crystals and photomultipliers led to a further investigation by Argyle and Warren-^ and De Benedetti, Cowan, Konneker and Primakoff . Calcul-ations made by De Benedetti and his co-workers seemed to indicate that the slowed-down positrons lost their remaining kinetic energy by collisions with the crystal l a t t i c e i n a time of the order of 3 x 10""^ seconds, and the thermalized positrons proceeded to wander about i n spaces of the crystal or metal l a t t i c e not pre-cluded by the positive ions u n t i l they found a suitable mating electron, talcing a time of the order of 10"? seconds. The momen-r turn distribution of the annihilating pairs could therefore be expected to be the momentum distribution of the combining elec-trons i n the l a t t i c e , so i n the case of a metal these would be expected to be the conduction electrons. The observed exponential dependence of the coincidence rate with the angle 0 i n copper, of the form Nc = No e - k ®/®<>, gave an average momentum for the electrons i n copper which was close to the theoretically predicted average momentum for the conduction electrons. It was noted by Warren and Griffiths^- that a l l of the this angle was made by Beringer and Montgome using Geiger - 3 -measurements did not give a simple exponential result, and further-more that the average momentum, although reasonable, did not vary-i n a too systematic way among the materials tested. A further measurement of this last effect was carried out by H. Maier Leibnitz who noticed that i n some cases the change i n momenta was entirely in the wrong direction from what would be expected on the basis of electronic shell structure. To gain more information about the way in which the average momentum varied among various substances, i t was f e l t that measure-ments should be made with increased angular resolution, geometry which involved few corrections, and with the angular range extend-ed as far as possible. I I . EXPERIMENTAL ARRANGEMENT (a) Source The source of positrons was a .0005" f o i l of copper of size 1 cm. x 3 cm. The thickness was chosen so that 90 percent of the positrons escaped from the f o i l before annihilation. This was done by assuming the momentum distribution was that predicted by the beta decay theory of Fermi with an end point corresponding to 0.66 Mev. Calculations by Reitler^, mentioned earlier, on the small amount of high energy annihilation also allowed the assump-tion that the Feather range energy relationship for beta particles would apply for positrons, i n which case the maximum range of the fastest positron should be equal to the thickness of absorber corresponding to a mass of 0.21) gms. per cm2. The ele c t r o l y t i c a l l y pure copper was irradiated i n a flux of 2 x 10^3 neutrons/sec. i n the N.R.X. pi l e at Chalk River for periods of about 21). hours. At this time the acti v i t y due to annihil-ation radiation corresponded to 300 millicuries. The f o i l s were placed between two sheets of absorber of thickness corresponding to the maximum range of the positrons and the assembly was placed in a slot i n a 1" round rod of styrofoam ( a i r - f i l l e d polystyrene i 6 145 Cms. Movable Counter Source 145 Cms. Q Fixed Counter Absorber .0005 Cu Foil 10* T~ . - \ — — Fig. X Experimental Arrangement Cu Foil Absorber^ J ^Absorber (Styrofoam (D 'I «-Styrofoam Fig. II Source Mount of density 0.02 gm./cm.-3) which was fastened to a larger block of the same material for/rigidity (Figure II). The source was placed midway between two detectors at a height of 3' above the floor. (b) Detectors and Geometry The gamma-ray detectors consisted of two E.M.I, photo-multipliers, run at 2800 volts supplied from Chalk River Mk V i l a H.T. units, with 1" x l j " x ^" slices of transrrstilbene mounted on the photocathodes. Trans-stilbene was used due to i t s high speed as a phosphor. The detection efficiency per gamma quantum of .51 Mev. radiation i s 0.27 per inch of crystal thickness. Dow Corning DC 200 silicone o i l of 20,000 centistokes viscosity was used as the ligh t coupling medium between the crystals and the multipliers. The crystals were covered with 0.001" aluminum f o i l and the assembly was made light tight by wrapping with black Scotch brand cellulose electrician's tape. The counters were positioned 290 cm. apart with the source midway between them (Figure I ) . One counter was r i g i d l y fastened to the end of a table; the other was fastened-to the carriage of a travelling microscope. The vernier scale of this instrument thus acted as the azimuthal angle indicator. The l8o° counter-source-counter line was established roughly with a tightly stretched'?threadrfast'ened to the. face of the fixed counter, passing through the source and over the face of the mov-able counter. The central line was established more exactly i n each individual run by counting over this position. 2800 Volts HT. EMI. 5311 ^ | Photocathode i 6 8 M 6AK5 ^ 220 4=01 « 4- 150 Volts 0 Fast Mixer Variable Screen Supply + 120 Volts -Q Side Channel Amp. Fig. I l l Photomultiplier Circuit Sig. IV Fast Coincidence Mixer - 6 -As shown i n Figure I the source was turned through a. > small angle. This served two purposes: (i) The gamma-ray absorption path through the source was decreased. ( i i ) The "width" of the source contributing to the geometrical overlap of the counters and the source was constant for absorbers of varying density since i t now was a function of the width of the f o i l and the angle of the source and to a lesser extent of the thickness of the absorber used. The source subtended an angle of 0.08° i n the horizontal plane at the detector. The detectors subtended angles of 0.25° i n this plane at the source. With this geometry, the counting rates obtained per detector were of the order of 80,000 counts per min-ute for a 200 millicurie positron source. (c) Coincidence Counting Circuits The pulses from the photomultipliers were fed into a coin-cidence c i r c u i t of the Bell-Petch type . Pulses were taken from both the collector and f i n a l dynode of the photomultipliers (Figure III). A \ volt rise oh the collector was sufficient to cut off. the 6 AK5 pulse shaper. The standing current i n this tube was controlled by the screen voltage. The shaped pulses travelled down 100 ohm co-axial lines to a 5>0 ohm shorted stub and diode (Figure IV). The level of the diode was set by vary-ing the bias with counts i n one channel only. Pulses from this point were passed to an Atomic Instruments amplifier (Model 20lj.-C) with a rise time of 0.1 microseconds and a gain of 1000. The discriminated output was fed to a triple coincidence mixer with a resolving time of 1 microsecond. .01 T J 6J6 6AL5 40 47K •91 Input G 1|-I00K 6J6 IK IOOK IOK f O O f r s j l K 330 -o + 300 V. .01 —1| 9 To Scaler IOK -9 To Triple Mixer Fig. Y Side Channel Amplifier and Discriminator Fig. VI Measurement of Resolving Time by Pulse Delay Method - 7 -The side channel amplifiers (Figure V) were driven by the cathode followers on the f i n a l dynodes. Discrimination was car-ried out at a level high enough to remove a large fraction of the noise pulses and the discriminated output was monitored and fed to the triple coincidence mixer. Coincidences oocuring i n the mixer represented the desired coincidence count. Monitoring was carried out with N.R.C. scales of 128.. (d) Measurement of Resolving Time The resolving time of the ci r c u i t was measured i n two ways. (i) The counters were placed closely together and a source of positrons was placed between them. The length of 100 ohm (RG7U) cable between one pulse shaper.and the fast coincidence mixer was then varied. This gave a curve of coincidence rate as plotted against length of delay cable used. By the use of the velocity of propogation of a wave i n a cable of this type the time delay in the pulses was known and the resolving time could be read d i r -ectly from the curve (Figure VI). (i.i) A random coincidence rate was taken with the counters at a large angle from co-linearity. The resolving time was calcul-ated from the 'known single channel rates and the equation T • N random  2 N1 N2 _-.-yThe resolving time measured, by •both^ .-pf these methods"wr.? the same and was equal to ,2.0 + ,.;1 x 10 7 sec. Yi/hen a fast phosphor such as trans-stilbene i s used, the resolving time of a c i r c u i t of the above type i s largely deter-mined by the s t a t i s t i c a l variations i n the time required for the - 8 -the f i r s t electron from the photocathode to reach the f i r s t dynode of the photomultiplier. Due to the long d r i f t space in the 5311 E.M.I, photomultiplier, i t i s d i f f i c u l t to achieve resolving times as short as can be attained by the use of a tube with a short photocathode to dynode distance such as the 1P21. (e) Experimental Procedure Sources obtained from the pi l e were immediately mounted as previously described. The central position or position at 180° geometry of the annihilation gamma rays was established by count-ing over the central region. A short check on the resolving time was made by placing the counters i n the l80° position and varying the cable length of one counter and comparing the curve of counting rate versus pulse delay with the curve used for obtaining the resolving time. The coincidence rate v.s. angle curve was taken beginning at large values of Q and ending at the position of co-linearity. Theoretical Curve = ne" t - 87 /isec. Rate xlO Counts per Minute Dead Time of Circuits III. CORRECTIONS TO OBSERVED DATA (a) Counting Corrections The observed coincidence rate was f i r s t corrected for dead times i n the circuitry. This dead time correction was absolutely determined by the use of several of the 0.0005" copper f o i l s , having a low activity, with an absorber of copper to stop a l l of the positrons. The counting rate for each f o i l alone i n the absorber was measured. The counting rate for several f o i l s was* taken ;and plotted against the counting rate from the addition of the rates of single f o i l s (Figure VII). Since the dead time losses for the single f o i l s were negligible due to the low counting rates, the 87; microsecond dead time of the ci r c u i t may be considered to be a good approximation of the true dead time. The decay correction for a half l i f e of 12.88 hours? was applied to the counting rates. (b) Geometrical Corrections As i s pointed out by Griffiths and Warren^ a f i n i t e source and counter width has an effect of decreasing the slope of the log coincidence rate v.s. angle curve, i f this curve i s a pure exponential. Since the experiment was carried out to determine - 9 -- 10 -a dependence of this curve on some property of the absorber and no absolute measurement was to be made, no correction for the f i n i t e widths and lengths of the source and counters was applied. As was outlined before, by suitable arrangement these were kept the same for a l l the absorbers. (c) Scattering Corrections (i) Compton Scattering The "good" geometry of the experiment reduces the Compton scattered quantum counting rate to a neg-l i g i b l e value. Since the s o l i d angle subtended at the source by the counter i s only krr x 10"^ steradians and since the Compton cross section for this energy i s not far from isotropic i t i s easy to see that the counter would only receive _ i _ part of the total 10** scattered radiation. Since the source was positioned as far as possible from any significant mass of surrounding material, there i s likewise l i t t l e effect from external scattering. ( i i ) Rayleigh Scattering Coherent scattering of gamma ray by o atoms has been demonstrated by Moon to be up to 100 times as great as Compton scattering at small angles. Measurements made by Storruste^ have agreed with the cross section values as c a l -culated by Franz"*"0 for this process. The differential cross e UA 2 section for Rayliegh scattering may be written d(r = jr-*r where Onoc) o 11 A i s obtained from the x-ray scattering considerations of Debye A 2 (See Appendix I) who has given — \ as a function of u. Z u = 2.U3 x 10 2Z" 1/^ sin.0/2 I u q C 2 Fig. VIII Scattering Amplitude aa a Function of Angle from P. Debye, Zeits. fur Phys. 1935 - 11 -A = d i f f e r e n t i a l scattering amplitude at angle e E = energy of the quanta being scattered 0 = angle of scattering m0c = rest energy of the electron Z = atomic number of the scatterer. The curve of Debye was extended to large and small values of u (Figure V I I I ) . Using these results an abso-lute correction f o r single Rayleigh' scattering was made to the experimental data. The magnitude of t h i s correc-t i o n i s shown i n (Figure IX) where the dashed curve repre-sents a plot of the uncorrected coincidence rate. The large Z dependence which can be seen by the absence of an appreciable correction f o r Lithium i s more obvious i f 2 one considers the region where A u i s almost a constant. z2 Here Moon° has written the cross section as The Rayleigh cross section f o r various angles was calculated from Franz's equations. (A private communica-t i o n from Professor P.B. Moon confirmed that t h e i r measure-ments agreed with the absolute magnitude of the number of scattered quanta as calculated from the formulae.) The assumption was made that i n any one a n n i h i l a t i o n only one ^  scattering event occurred. The thickness of scatterer was therefore taken as the thickness of source .'crossed by ^ l i n e between the counters .when they were i n the p o s i t i o n of - 12 -c o l i n e a r i t y . Since the experimental Nc v.s. 9 curve drops so r a p i d l y with angle i t i s easy to see that only the scattering from the central region to the sides w i l l have a ^ i g n i f i c a n t effect on the counting rate. A numeri-c a l integration was carried out over the experimentally obtained Nc v.s. 0 curve using the calculated cross section f o r scattering into the s o l i d angle subtended by the counter at various angles. The curve so obtained f o r scattered quanta was then subtracted from the experimental curve. Thus fo r gold the cross section f o r scattering i n -26 2 to the detector varied from 4 x 10" cm. at 0.4° to 1 x 10"*^ cm. at 2 . 0 ° . The large Z dependence reduced these values by a f a c t o r of 1000 i n the case of L i . The largest correction was obtained f o r Uranium and was 0 . $ counts per minute at an angle of 1 . 2 ° when the coincidence peak height was 1600 counts.per minute. l*ig. IX Effect of Correction for Rayleigh Scattering 1 2 3 4 5 6 COUNTER D I S P L A C E M E N T C M S . Fig. X -Angular Correlation for Annihilation in Copper and Teflon IV. RESULTS (a) Annihilation i n Copper A comparison of the results obtained. (Figure X) with those of previous workers shows good agreement. Q0 uncorrected for fi n i t e source and counter width (3.80 x 10"^ radians) agrees with the value given by Griffiths and Warren*1 ( 3 . 8 l x 10"^ radians). The thickness of absorber was increased by a factor of 6 to determine the effect of scattering and was found to make no s i g -nificant difference to the shape or the slope of the logarithmic plot. An exponential type of dependence of the form N c = N Q e" where N 0 and Q0 are constants f i t s well, i n which case the aver-age value for the momentum calculated from p ay. = 2 mc0o i s 7.2 x 10~3 mc and E a y > = 22 ev. Of the remaining absorbers only teflon gave a dependence of this simple exponential type so i t i s displayed i n the same figure. Q0 for teflon was different from that of copper, giving an average p of £.8 x 10 - ^ mc. and an E a v > = 13 ev. (b) Other Substances '•••' • , *" A series of 18 metals, salts and co-valent compounds were used as absorbers i n an attempt to establish a dependence of the - 13 -COUNTER DISPLACEMENT CMS. Fig. XI -Angular Correlation for Annihilation in Ag, Au, Cd, Pb Fig. XII Angular Correlation for Annihilation in C, CC1A, S, CsCl, Li,. L i C l , L i I Fig. XIII Angular Correlation for Annihilation in L i and Be . Absorber Z A P 0o Av. p Av. E Li 3 , 7 53qgfer3 2.6I0"3 5.2IO"3mc 10 e.v. Be 4 8 1.8 2.3 4.6 8 C 6 12 2.3 2.5 5.0 9 Mg 12 24 1.7 3.0 6.0 14 Al 13 27 2.7 2.4 4.8 9 S 16 32 2.0 2.4 4.8 9 Fe 26 56 7.9 3.4 6.8 ! 18 Ni 28 58 8.9 3.4 6.8 1 18 Cu 29 63 8.9 3.8 7.6 22 Ag 47 107 10.5 3.3 6.6 17 Cd 48 1 12 8.7 3.3 6.6 17 Sn 50 118 7.3 3.3 6.6 17 Au 79 197 19.3 3.3 3.2 6.6 17 Pb 82 208 11.4 6.4 15 U 92 238 18.6 2.7 5.4 II Li CI 2.0 2.7 5.4 II Li 1 3.5 2.7 5.4 II CsCI 3.9 2.7 5.4 II CCI4 1.6 2.7 5.4 11 Teflon 2.0 2.9 5.8 13 Fig. XIV Average. Momenta of Mass Centre (from f i r s t three half lives J -Ik-^c/& curve on some property of the absorber. The thickness of the absorber i n each case was the mass thickness required to stop the positrons completely within i t . Since the source assemb-l y was turned through a small angle as previously mentioned, the source width was constant for a l l the materials except for absorb-ers of very low density such as lithium. The geometrical regions of overlap of source and counters are marked on a l l curves. The results are displayed i n Figures Xt, XII, and XIII. Statistics varied from 2% on the peaks of the curves to 6% at the t a i l s . The counting rates were not normalized to source strength as their display would then have been very confused. A normalization to source strength plus a correction due to photo-electric absorption of 0.5>1 Mev. quanta, which i s as great as 50% i n the heavier elements for the thickness of absorber used, yields a superposition of the peaks of the curves. It i s immediately obvious that a "good" exponential type of dependence of the logarithm of the coincidence rate with angle i s not a general rule. Groups of materials do have the same gen-eral shape. A group of metals of different electronic shell structure but ha.vj.ng a similar Mc /Q dependence is displayed i n Figure XI. Due to the shape of the curve the average momentum for a depend-ence of this type was based on an average exponential slope over the f i r s t three half l i v e s . Curves for a number of metals, salts and co-valently bound compounds;'are displayed i n Figure XII. There is no pronounced dependence on chemical binding or la t t i c e structure. Certainly the proposed theory of variation with negative ion change as i n going through a series of salts such as L i C l to L i l i s not borne '•A out. An anomalous curve (Figure XIII) was obtained for beryllium. There appears to be an inflection i n the NQ/Q curve for a beryl-lium positron absorber at an angle of 1.1°. The energy of pairs giving this inflection would be 130 ev. Thus, to summarize: (i) No curves with the exception of copper and teflon appear to be simple exponentials, ( i i ) The table (Figure XIV) of values for p a v # and E a v < from the slopes of the curves immediately outside of the geometrical overlap region shows no correlation with the density or atomic number. The slopes, as mentioned before, are average slopes over the f i r s t three half lives of the curves, ( i i i ) Most curves are markedly similar, particularly i n the t a i l s • V. DISCUSSION It i s considered that these measurements are sufficiently accurate to have shown variations i n average electron momenta i n various compounds i f the current ideas of such momenta are correct and i f positrons do reach "thermal velocities" before annihilation. In view of the short lifetime of positrons i n solids as measured 12 13' by both Deutsch • and Bell , i t seems unlikely that such a com-plete thermalization process as postulated by De Benedetti et al. does occur. It seems more l i k e l y that the distribution of mom-enta observed i s that of the positrons rtfiich have slowed down in the l a t t i c e . An attempt to explain these results might be aided by a measurement of the Nc/^ dependence through a change in state (liquid to crystalline s o l i d ) . The mean energy of the electrons should be different i n these two cases and possible diffraction effects may also be present. It would also be of interest to see i f there i s a variation i n materials such as amorphous and crystal-line quartz that exhibit a change in the lifetime of the positron. -16-PART II THE PHOTODISINTEGRATION OF NEON I. INTRODUCTION (a) Photonuclear Reactions The photodisintegration of a nucleus was f i r s t achieved by Chadwick and Goldhaber i n 193U. They succeeded i n sp l i t t i n g the deuteron into a proton and a neutron by irradiation with sufficiently-energetic gamma rays. Other examples of this type of p r o c e s s 1 ? ' 1 8 ' 1 ? ' 2 0 ' 2 1 ' 2 2 ' 2 3 have been discovered since that time. The threshold energy or minimum energy required to pro-duce the reaction i s simply the energy equivalent to the binding energy of the particle which i s ejected. The cross section or probability of occurrence of such a reaction i s . . . impor-tant i n the study of nuclear forces. (b) Photo-Alpha Processes In the region of low Z i n the periodic table,models have been postulated for certain nuclei (Be 8, C 1 2, O1^, Ne 2 0) which consist of groups of alpha particles bound together i n a geom-et r i c a l way1''. This hypothesis i s based on the fact that the binding energy per nucleon i n an alpha particle i s about 7 Mev., a large proportion of the mean binding energy per nucleon for - 17 -- 18 -) nuclei of low mass, and i f the nucleons are grouped i n alpha particle configurations the alpha particles w i l l be bound rather loosely to each other. Thus evidence for a tetrahedron type of configuration for 0 ^ has been obtained i n angular correlation measurements made by Barnes, French and Devons"^ on the F 3-9(p o<y)0 1 6 reaction. The corresponding model for the Ne 2 0 nucleus i s a b i -pyramid of alpha particles, and i n this case one would expect that excitation could result i n the splitting off of one of the alpha groups. Another prediction of the alpha particle model i s that there are few low-energy excited states i n the nuclei investigated. A on photo-alpha reaction i n Neon might also give rise to several groups of alpha particles of different energies corresponding to transitions to excited levels i n 0 . This should i n principle i £ allow relatively low excitation levels i n Cr- to be found, i f such exist. (c) Methods of Investigation Photodisintegration processes w i l l i n general have low cross —71 2 sections of 10 'cm. or less due to the weak electromagnetic interaction existing between photons and nuclei. Methods involv-ing high sensitivity of detection and large fluxes of gamma rays must therefore be used. ( Y, °< ) reactions i n C 1 2 and O1^ have been observed i n photographic emulsions. Unfortunately, i t would 20 be extremely d i f f i c u l t to introduce Ne nuclei into an emulsion. Photo processes involving unstable products have been investigated by betatron irradiations, but direct measurements are made with - 19 -d i f f i c u l t y due to the high background from these machines. More-over, the gamma spectrum from them is"white" and interpretation i s therefore correspondingly confused. Ionization chamber methods seem suited because neon i s a gas having good electron collection properties, and so the photo-alpha disintegration process has been investigated using a gridded ion chamber. (d) Theoretical Estimate of the Cross Section Preston has published a summary of a theoretical analysis "I C, TO of the 0 (Y,oO.C reaction i n which an alpha particle model has been used. The results are sufficiently general that an extension can be made to the ( )ff °C) reaction i n Ne . By a study of unpublished results of Millar and Cameron for the 01^(y,o()C12 reaction and the results of Goward, Telgedi and Wilkins Preston concludes that since the cross section peaks sharply at an energy well above the Coulomb barrier, the alpha particle may be treated as a plane wave leaving the nucleus and a Coulomb penetrability factor may be applied at lower energies. The wave function of the i n i t i a l nucleus i s written as (^(A-)!^ J where >^0(-^ ) i s a function of the co-ordinates of the ejected alpha particle and ~tyQ is the wave function of the remaining group. Preston also assumes that the gamma interaction must be quadrupole, due to symmetry, and so obtains matrix elements of the form file*** OLrfbMVoclZ for the transition probability of the reaction, where Q * "X-ty l s o n e °£ the t e r m s f ° r the quadrupole interaction - 20 -of the photon with the ejected alpha particle, and T^ p i s the wave-function of the residual nucleus. He assumes that consider-able overlap occurs between the wave-function of C^2 and the residual wave-function i n 0^ i n which case the approximation can be made Tfo df % 1 . A specific reference to 0 i s lost at this point and only an upper li m i t can be set on the cross section by evaluating the remaining matrix elements. For ^>0CJV) Preston uses: _ An exponential function Q~ A Gaussian function Q 6% A modified Wheeler function e ~ >* fC^i a and b are parameters i n the function which give a maximum cross section at the gamma ray energy at which i t i s experimentally observed, i.e. 17 Mev. However, the results are not markedly different, i n the case of the Wheeler function, from the Gaussian. This last assumption, together with the estimate that the overlap of ljff and \ for the Ne 2 0(y, o()0 l 6 should be about the same as i n the case, immediately leads to the conclusion that the two cross sections should be of about the same order of magni-tude. The use of the modified Vftieeler formula would tend, however, to change the value of the gamma ray energy for which the maximum cross section occurs. 20 The threshold energy for the Ne ()f, <*) process i s 1*.6 Mev., as calculated from mass values; that of the 0"^(y, eC ) i s 7.2 Mev. -13 If a nuclear radius of $ x 10 cms. i s used the Coulomb barrier - 21 -height i s 6.1 Mev. for the Ne 2 0 reaction but only £.0 Mev. in the latt e r case. The threshold plus Coulomb barrier height corres-ponds to 10.7 Mev. for neon. Any gamma ray of lower energy would therefore have a decreased probability of interaction due to the penetrability of the Coulomb barrier. It is interesting to note that the total energy required i n the case of O1^ is 12.2 Mev. which would jfead to the speculation that, with the gamma ray On energies above this barrier plus the threshold level, the Ne ( Yt °0 cross section should be of about the same size as the O1^ thresh-old or perhaps a l i t t l e ; larger i f the previous argument about overlapping wave-functions i s reasonable. (0~, «0 reaction cross sections have been measured i n the 16 26 case of 0 by Waffler and Younis who report a cross section of (1.8 ±.0.6) x 10~ 2 8 cm.2 for 0 l 6(V } <*)C12 with gamma rays of 17.6 Mev. energy and in the case of C by Goward, Telgedi and Wilkins 2^ who report a cross section of 10~ 2 8 cm.2 f o r C ^ C ^ * ^ 6 4 with gamma rays of 18 Mev. energy at the maximum cross section. The cross section for the reaction Ne 2 0( JT, *<)016 would therefore v be expected to be of the same order of magnitude. (e) Purpose of Present Experiment Since there i s a considerable controversy as to. the mechan-ism involved i n a photodisintegration process and a previous 27 experiment by Tfoods 1 seemed to indicate an unexpectedly low cross section for the { f9 o{) process i n Ne 2 0 (less than 10 - 2? cm.2) i t was f e l t worthwhile to examine this reaction again with refine-ments in technique which would allow a cross section 'of 1 x 1 0 - 3 0 cm2, to be detected with certainty. II. THE EXPERIMENTAL TECHNIQUE (a) General Considerations i n Cross Section Measurements A reaction cross section may be defined by the equation Y = cTnN. Y - y i e l d of the reaction per unit time & - cross section of the reaction occuring N = number of nuclei in the" target capable of taking part i n a reaction which can be detected n * number of bombarding nuclei per unit area per unit time Thus the determination of a cross section consists of two parts: one the measurement of the yield of the reaction, and the other the measurement of the flux of the bombarding particles, or quanta, as the case may be. The number of effective target nuclei i s usually easily found. For the Ne 2 0( If, o<)<9-^ reaction a gridded ionization chamber was chosen as a detector and a sodium iodide crystal mounted on a photomultiplier was used to measure the absolute magnitude of the gamma ray flux which was obtained by the bombardment of a thick lithium metal target with protons from the University of B.C. Van de Graaff generator. - 22 -- 23 -(b) The Ionization Chamber The passage of a proton, an alpha particle, or heavier charged particle through a pure, inert, rare gas produces' electron and positive ion pairs i n number closely proportional to the kinetic energy of the particle. Hence by arranging a suitable electrostatic f i e l d , the whole of the electrons may be collected on a positive electrode, giving rise to a voltage pulse at the electrode, the amplitude of which i s proportional to the total energy released by the ionizing event. The details as to the shape and formation of this pulse have been discussed by Wilkin-son 2^ and Tfoods2?. In normal two-electrode chambers the pulse has a slow rise time due to the space charge of positive ions which have a low mobility compared to the electrons. To remove the slow rise due to this positive ion effect and hence allow the use of an amplifier with a much narrower band pass, Frisch 2^ has suggested that a third electrode or grid be placed i n front of the collec-tor. The pulse rise time i s then dependent only on the electron collection time from the moment the f i r s t electron passes the grid. The position of the grid for most effective shielding has been investigated by Bunemann, Cranshaw ,and Harvey^. A series of experiments-^-*-^,33»3k ^ n v a r i o u s types of ion chambers containing argon gas has shown that W(the energy loss per ion pair on the average) isva^onstant for various alpha particle energies. This allows quantitative energy measurements to be made because a natural alpha source of known energy can be used to give the energy calibration of a chamber. - 2h -op Wilkinson has also determined that a high purity of gas is necessary (as l i t t l e as 1 part in 10-* of oxygen, an electro-negative gas, i n argon at atmospheric pressure w i l l capture one percent of the liberated electrons i n 6 cm. of motion and this effect increases rapidly with pressure). Because of the release of occluded gases a continuous purification i s necessary. (c) Source of Gamma Rays A monochromatic source of gamma radiation of energy above 10.7 Mev. in energy was required for the experiment i n order to 20 raise the Ne to a high-enough excitation that alpha particles could be emitted with an energy greater than that of the poten-t i a l barrier. Of the gamma rays produced i n nuclear reactions, two suggested themselves: (1) H 3 + p He1** -± Eeh -hY (20 Mev.) (2) L i 7 + p —> Be 8* — ^ Be 8 + 1T (17.6 Mev.) —>- Be8*-*-* (m.8 Mev.) The tritium reaction has not yet been used since the gamma-ray yield from the zirconium-impregnated targets available appears to be much lower than that from the lithium reaction. Two groups of gamma rays are emitted i n the Li(p, /.a) reac-tion, of which only the 17.6 Mev. gamma ray has a narrow, line energy spread (about 12 Kev. ). The II4..8 Mev. gamma ray is actually a spectrum with an energy width of 2 Mev. which arises from the transition from the excited state at 17.6 Mev. to the broad excited state of Be 8 at 2.9 Mev. The reaction has been - 25 -studied i n detail by Walker and McDaniel and cross sections for the production of both the II4..8 and 17.6 Mev. gamma rays using a thick lithium metal target with various energies of bom-barding protons have been given by them. There i s also a slowly rising non-resonant yield i n this reaction which allows the gamma-ray energy (at bombarding energies of protons greater than the khO Kev. resonance) to be varied by changing the bombarding energy. The cross section for this non-resonant yield i s much lower than the resonant cross section and the gamma ray contribution from i t i s small. ELECTRODE SYSTEM FOR GRIDDED ION CHAMBER SCALE : INCHES O I 2 MATERIAL: BRASS,EXCEPT WHERE NOTED COLLECTOR AND GUARD RING GRID. COLLECTOR AND GUARD RING. \ SIDE ELEVATION MYCALEX END ELEVATION Fig. XV Electrode System of Ion Chamber 6RIDDED ION CHAMBER GASKET 8 HOLES 8 HOLES EQUALLY SPACED EQUALLY SPACED Y7TCT7\ S \r_I~J NEEDLE VALVE S E A ^ L S 1 " " ^ ^ IN THIS P L A T E [ Y/X BRACKET FOR J r 1 E L E C T R O D E S L E A D - ^ 5 \ w E L D E D S T E E L ^ WELDED ^  g g ^ G A S K E T SIDE SECTION S C A L E : INCHES O I 2 Fig.. XVI Outside Casing of Ionization Chamber I III. THE EXPERIMENTAL ARRANGEMENT (a) ;.Gridded Ion Chamber (Figure XV) The chamber was constructed according to the formulae given by Bunemann et a l 3 0 . The electrode structure was contained i n a seamless steel tube (Figure XVI), 6 inches in diameter and 10 inches i n length with a ^ -inch wall thickness. ^-inch thick end plates were bolted to welded flanges and the seal was accom-plished with lead gaskets to prevent exposure of the f i l l i n g gas to rubber. The chamber was painted with colloidal graphite on a l l the internal surfaces to reduce the background of natural radioactivity from the walls and to reduce the effect of ( processes. The gas purifier was carried on one end plate. It was of 37 the type used by Jentschke and Prankl and described by Wilkin-28 son y. and was connected to the chamber by two copper tubes. It consisted of a brass tube 6 inches long and one inch i n diameter around which a heater c o i l was wound which was capable of raising the temperature to 300°C. The inside was f i l l e d with turnings of calcium metal which readily forms solid compounds with gases such as oxygen and nitrogen above 220°C. and should take out water when cold. The opposite end plate supported the electrode struc-- 26 -- .27 -tore, the kovar seals by which e l e c t r i c a l connections were made to the H.T. voltage and amplifier, and the valve by which the chamber was f i l l e d . The electrodes were of brass with mycalex and lucite insul-ation, the grid being of #36 Copel wires spaced 1 m.m. apart on a brass frame. The high voltage electrode was curved, to increase the number of lines of force arriving at the collector, i n a fashion determined by measurements made in an electrolytic tank. The collector, 6 inches by 3 inches, was surrounded by a guard ring 1-g- inches i n width. The grid was 0.6 inches from the co l -lector and 3 inches from the high voltage electrode. With these dimensions the grid inefficiency or the extent to which the number of lines of force ending on the collector i s dependent on the f i e l d of the positive ions was 1%. -30 The voltages recommended by Bunemann et al were not quite suitable for the chamber with neon gas. From observation of the pulse height as the grid voltage was changed,^it was decided to work with Vg = 0.U£ Va. The high voltage electrode was run at 800 volts. The pulse height increased i n size u n t i l -700 volts was reached and then remained constant u n t i l -900 volts at which time breakdowns occurred i n the neon due to i t s known poor elec-t r i c a l quality. The chamber contained pure neon gas at a pressure of 1|73»5 cm. of mercury or 6.25> atmospheres. The choice of gas pressure was made so that the range-of the alpha particles" should be kept small (3 cm.) to keep the wall effect low for the high energy groups expected (10 and 13 Mev.). The high-energy incident gamma quanta are well above the threshold for the gamma- p processes and - 28 -at this pressure the maximum energy which may be dissipated by a proton i n the sensitive volume i s 7*8 Mev. Since no proton of higher energy can liberate more than 7.8 Mev. and the chamber was f i l l e d with neon gas Containing Ne 2 0, Ne2-1- and Ne 2 2 i n abun-dances of 90.$1%, 0.28$, and 9.21% 6 lJ 0 n l y (/,«.) reactions i n the neon can produce pulses above this l e v e l . F i l l i n g was accomplished by passing the compressed gas of a high purity (He 0.02%; N 0.02%; others less than 0.02%) through a l i q u i d nitrogen cold trap and directly into the chamber. The resulting pressure was obtained from the equalization of the pres-sures i n the chamber and f i l l i n g bottle. Electron collection was obtained with no-difficulty but the heater was run for"some time and the pulse size increased slightly. (b) Electronics and Pulse Analysis The head amplifier vras the low-noise model 500 type of Elmore and Sands*10. A cathode follower fed the pulses from this amplifier, which had a gain of 100, down a 100 ohm cable properly terminated at the main amplifier. The main amplifier was a Northern Electric Type lkhk ampli-f i e r with, a maximum gain of lO** and a s t a b i l i t y of better than 1% over long periods. The differentiation and integration time were both set by the controls available on this instrument, to 5 microseconds. At 6 db attenuation the 5.29 Mev. alpha peak from P o 2 1 0 was 26.2 volts in amplitude. The voltage distribution of the amplified pulses was measured by feeding them into an 18 channel "pulse amplitude analyzer" or "kicksorter", designed by Westcott and Hanna*4^ and built accord-- 29 -ing to specifications of the National Research Council of Canada, Chalk River Laboratory, by Canadian Marconi Ltd... The discriminators determining the voltage intervals were set from a precision pulse generator designed by Bowers**2. Its o output was stable to 0.01 volts over long ^periods of time. The kicksorter unit has a stated amplitude s t a b i l i t y of less than 0.02 volts. This was found to be the case by a 2U-hour check with the pulse generator. The l i n e a r i t y of the equipment was determined by feeding pulses of calibrated amplitude through a potentiometer into the grid of the ion chamber and measuring the output voltage level for various input pulse amplitudes. The lin e a r i t y of the elec-tronic system was found to be better than 1%. The energy scale was determined by a Po2-1-0 alpha source which was fastened to the inside of the high voltage electrode and gave about 3 alpha particles per minute. The range of 5-3 Mev. alpha particles i n 6.2£ atmospheres of neon i s about 1 cm., so a l l of the alphas were stopped i n the sensitive volume. A typi-cal alpha particle pulse distribution i s shown i n Figure XVI. The width of the pulse spectrum i s largely due to amplifier noise as IV' was > seen by the comparison with a standard pulse spectrum, which was produced by feeding pulses from a pulse generator into the grid of the head amplifier. The small extra width i s due to straggling i n ionization and source thickness. A l l the amplifiers, power supplies and the H.T. voltage were supplied from regulated 110 volts A.C. obtained from constant voltage Sola transformers loaded to at least 8$% of their rated load. The Solas were isolated from the mains with f i l t e r s to • LT • Brass-Lucite O-ring Seals —> Position of Furnace while Evaporating i 1 Water Cooling - Coil Copper Target Plate Steel Wire Lithium Heater Bakelite Insulator Fig. XVII Lithium Metal Target Evaporating Chamber - 30 -X prevent . spurious line pulses from entering the equipment. (2) Lithium Target Arrangement A target chamber was constructed for evaporation of lithium metal under evacuation. This system is shovm in Figure XVII. The lithium metal was placed i n a heater of spiral spring steel wire. The. system was evacuated and the shutter was closed to the main system. The heater was turned on and a lithium metal layer was evaporated on the l / 8-inch thick copper end plate. The water cooling c o i l of copper tubing was hard soldered to the periphery of the plate to give a minimum mass thickness of absorb-er between the source of the"v gamma rays and the ionization chamber. The thickness of the targets used was about 700 Kev., as deter-mined from the gamma ray excitation function (e.g. Figure XVIII). (d) Gamma Ray Flux Determination : The flux of gamma rays was determined i n two ways to give a check on the integrated number of gamma rays passing through the gas i n the sensitive volume of the ionization chamber. In the f i r s t place, the integrated proton current to the target was measured and the flux calculated from cross section 7 data. The L i (p , Jf ) cross section has been accurately measured by Walker and McDaniel 3^ with a pair spectrometer, and has been given for various bombarding energies of protons on thick l i t h -ium targets. The copper plate on which the target was fastened was held at a potential of +300 volts with respect to the evapor-ating chamber and so the integrated proton current to the"target could be expected to be very closely the true current of protons, To Accelerator Column. il To Cooling Water M Lead Shield J vr\ i \ t Beam Stabilizing vSHts Magnet Valve -60 cmsr Gamma Ray Monitor t. Beam Shutter Ionization Chamber : —68 cms.— Fig. XIX Experimental Arrangement for Photodisintegration Measurement - 31 -since the chamber acted as a Faradfycage preventing electrons from being boiled off the target by secondary emission effects. ratios The Walker and McDaniel cross section ^ were extrapolated to the bombarding energy of 6^0 kilovolts used i n the experiment, since the shape of the thick target excitation function was known (Fig-ure XVIII). The flux of gamma rays at the target was calculated using the integrated current and the extrapolated cross sections. Secondly, absolute measurements of the flux were made with a phot omul tiplie.r and large NaI (T -0 crystal. The crystal, i;.l|6 cm. in diameter and-5>.08 cm. i n length, which was'mounted on the end of an E.M.I. 6262ji ll^-stage photomultiplier, according to the method of Swank, and Moenich^3, was used as the gamma ray detector. JThe mounting method has been further developed and modified by Griffiths 5 and, Azuma w . The total gamma ray flux was calculated* (Appendix II) from the shape of the observed gamma ray spectrum and the counting rate obtained from the crystal with the discrim-ination level set at a pulse height corresponding t'o 10.5 Mev. The pair production cross section as well as the cross section for Compton effect given by Heitl e r 1 were used i n the flux calcul-ation. Pair production i n the f i e l d of the electrons and the effect of the thallium, activating impurity has been neglected but would only contribute a very 'small correction of the order of 1% or 2%. The target' chamber and gamma ray monitor were mounted i n a direct line (Figure XIX). The chamber was removed to measure the flux from the target alone and then was inserted to give an absorption measurement for the chamber walls. Repeated readings * I am indebted to G.M. Griffiths for his aid i n making the flux determination. i - 32 -of t h i s absorption correction agreed to within 2%. The gamma-ray f l u x calculated by the s c i n t i l -l a t i o n counter method and the f l u x calculated by the cross section and integrated current method agreed to within 10% i n a two-hour bombardment of the target at the beginning of the experiment. The agreement became worse during the course of the ex-periment due to the deterioration of the target, but at no time was i t worse than 20%. 5.3 Mev. Fig. XX P o C i U Alpha Peak IV. EXPERIMENTAL PROCEDURE (a) Energy Calibration / The ionization chamber was consected via the Northern Electric amplifier to the kicksorter and a run was taken on the Po2-*-0 alpha peak (Figure XX) to calibrate the voltage scale from the known alpha energy (5*29 Mev.) and to check the s t a b i l i t y of the amplifiers. At the attenuation of 12 db on the Northern Electric amplifier this peak appeared at 13 volts. The kick-sorter was set up to cover the energy range from 5 Mev. to II4 Mev. in 530 Kev. energy steps (530 Kev. was equivalent to 1.30 volts \vidth per channel). To give a quick check on the s t a b i l i t y dur-ing the course of the experiment, pulses from a standard pulse generator xrere fed on to the collector of the ionization chamber. This was necessary as the specific activity of the Po i U calibrat-ing source was low. The gamma ray monitor counter output was fed via an Atomic Instruments Company 20I4.C amplifier and discriminator to a scaler. In order- to set the discriminator bias to an energy level corres-ponding to pulses produced i n the crystal due to a 10.5 Mev. energy loss, the energy at which the bias was set i n the c a l i -bration run for the gamma ray monitor, an energy scale was set PULSE HEIGHT IN VOLTS Fig. XXI' Li(p,iT) Gamma Ray Spectrum 2500 2000 I 1500 o u | 1000 3 Z 500 -i t = V 5 6 7 Fig. XXII 210 Po w alpha Peak Counting time 86400 seconds I Count • i-10 II 12 J Energy in Mev. Background in Ionization Chamber with Accelerator off 200 -• P o2 1 0 alpho peak with 6 Mev 8 7 Mev gamma flux. 3 O o E z 100 I Count 4 8 10 II 12 Energy in Mev 13 F i g i XXIII Background i n Ionization Chamber Bombarded with Fluoriney-rays - 3k -up using the 2.62 Mev. Th C" gamma rays. The gain was reduced by a factor of h and the bias was re-adjusted to correspond to a discriminator level of 10.5 Mev. The two spectra are shown i n Figure XXI. The bias level was checked at intervals throughout the course of the experiment to ensure the constancy of the mon-itoring. (b) Photodisintegration Measurements A- resolved proton beam of 25 microamperes was focussed on the target and the electric timer, current integrator, gamma-ray monitor, and kicksorter were switched on simultaneously from a master relay unit. Readings were recorded every 15 minutes and once every two hours a complete gain check was made for both the ionization chamber and the gamma-ray monitor. Measurements on background were made i n two ways: (i) A long count (6 hours) was taken with the chamber i n the standard position and no proton beam on the target (Figure XXII). ( i i ) A run was made with the proton beam on a calcium fluoride target. This i s a p r o l i f i c source of 6 and 7 Mev. gamma rays and the large gamma flux should produce noise due to "build ups" in the ionization chamber. The results are shown i n Figure XXIII. Finally having established the existence of the reaction the energy spectrum of the disintegrations i n the chamber was examined with finer resolution by spreading the spectrum out over the kicksorter so that each channel covered 200Kev. of energy. In this way an investigation was made of the spectrum i n three separate sections i n an attempt to ascertain i f any fine structure was present. I 4 0 0 300 h UJ z z <t X o cr Ui 0. (0 z 200| 3 O O 586J Photodisintegration ot Neon. High Energy Spectrum. 100 8 9 10 II 12 ENERGY IN MEV. 13 Fig. SOT Spectrum of Pulses i n Chamber above 8 Mev. I4000h - l UJ < 5 iboooh K 111 a. <o t-z § o 60001-2000r Photodisintegration of Neon. Low Energy Spectrum. S3 Mev. II _, 1 iT'm 3 4 ENERGY IN Mev. Fig. XXV' Spectrum of Pulses in Chamber below 8 Mev. V. RESULTS (a) Discussion of Pulse Spectrum The pulse height distribution on an energy scale i s dis-played i n Figures XXIV and XXV. It i s divided into two regions, one below and one above the pulse height corresponding to an energy release of 7 Mev. in the chamber. Hie two regions are normalized to the same integrated flux of gamma rays. Peaks may be seen to occur i n the distribution at 3"i-l» II?> 5.9, 6 . 8 , 8, 10.5 and 12.9 Mev. The background of pulses occur-ring above the Po x u alpha peak was negligible, as, shown in Figure XXIII, even when the proton beam was incident on a calcium fluoride target which gave a very large flux of gamma radi-ation. Since a resolved beam was used i n bombarding the lithium target only those neutrons arising from secondary reactions i n the target i t s e l f could give neutron-induced reactions i n the chamber gas or the walls. The maximum neutron'energy..which can be produced ihihe-"secondary L i ( oC, /A)' reaction, from the low percentage of ground state alpha particles from the excited state Pi ' \ of the Be° compound nucleus produced by the bombardment of the target with protons, i s 5.2 Mev. This could only produce 5.1 Mev. - 35 -36 -20 alpha particles i n the chamber via Ne (n,e() processes, which would give pulses below the alpha peak. A possible N"^(n, o() reaction i s energetically too low as well, since the Q of this on reaction, like that of the Ne'lu(n, o<) reaction, is negative. The poss i b i l i t y of this reaction must be considered due to i t s high cross section. Furthermore, a measurement made with a ZnS detec-tor (ZnS on the photocathode of a 5819 photomultiplier ) -with a hydrogenous radiator for detecting high energy neutrons did not give a counting rate which was significantly above i t s back-ground rate. Measurements made with the same detector i n a bom-bardment of D with protons easily detected the neutrons pro-duced by the "knock-on" deuterons i n this experiment. It i s con-cluded, therefore, that no significant flux of high energy neut-rons was present,and the pulses i n the ionization chamber corres-ponding to energy release greater than 7.8 Mev. (Figure XXIV) could be ju s t i f i a b l y attributed to: (i) Ne 2 0(y, o()0 1 6 ground state from both 17.6 and 1U.8 Mev. gamma'radiation which give rise to energy releases 12.9 and 10.1 Mev. respectively. 22 18 ( i i ) Ne (iff o( )0 from the 17.6 Mev. gamma ray with an energy release of 8.0 Mev. The cross sections for the Ne20(X'_>oC)01^ reactions can be calculated from the yield figures and are 8.0 i 2 x 10~^° cm.2 i f or the 17.6 Mev. gamma ray and 3.0 £ 0.6 x 10~ 29 c m. 2 f o r the 111.8 Mev. gamma ray. Although the cross sections are stated to 20$ accuracy the ratio of the two cross sections can be defined much more exactly from the ratio of the number of pulses i n the - 3-7 -two peaks, v i z . 117/VJ.. By taking the ratios of l[i . 8 to 17.6 Mev. gamma radiation according to Walker and McDaniel-^, and extrapol-ating to the 6£o Kev. bombarding energy used i n the experiment, a value of .6/1 is obtained for the ratio of these gamma rays. From these two ratios the ratio of the cross sections becomes Iris = ™ * ™-Although the amount of Ne^ present in the chaxnber:Lis known op "l ft (9%) the cross section for the Ne (tf, «()0 reaction canjiot be calculated to better than an order of magnitude because the disintegration group i s on the edge of a steeply-rising curve due —28 2 to the (Y > p) background. This cross section i s about 10 cm. . a value which i s not inconsistent with other photodisintegration cross sections. 210 The energies of the two groups above the Po alpha peak at 5.9 and 6.8 Mev. (Figure XXV) agree so well with the expected energies of alpha groups to the 7.1* 6.9 and 6.13 Mev. excited states of 0 1 6 for the reaction Ne 2 0( Y, oQO1^''' (17.6 Mev. gamma radiation) that they may be ascribed to i t . The cross section i ft for the reaction to the 6.05 Mev. level'of 0 , which has the same spin and parity as the ground state, would be expected to be only of the same order of magnitude as the ground state. The energy of the higher alpha group i s certainly much closer to 6.7 Mev. than to 6.9 Mev. (bearing out this assumption). By using the mass difference between 0 ^ and Ne 2 0 as given by Ajzenberg and Lauritsen^ 8 of 1^75 Mev., the reaction to the 6.05 Mev. level should have a Q of 6.80 Mev. and to the 6.13 Mev. level a Q of 6.72 Mev. It seems l i k e l y that the reaction i s proceeding through the 6.13 Mev. state of 0 1 6. The mean Q of the reactions proceeding through the 6.9 and 7.1 Mev. levels of O1^ i s 5.85 Mev. The centre of gravity of this peak is close to this value. With this interpretation the cross section for the reaction Ne 2 0(r, 0 O 0 1 6 " * i s U.O + 1.6 x 10~ 2 8 cm. to the 6.9 and 7.1 Mev. pQ excited states and 3.0 + 1.2 x 10 cm. to the 6.13 Mev. excited state. An unambiguous assignment can not be made to the peaks i n 210 the pulse distribution curve below the Po alpha group due to the large number of possible ( Yy p) processes (Ne^O^^,);?-1-?). It i s interesting to note that the energy of the peak at 3 Mev. does correspond to the reaction Me20(Y,e><)0^ '"' (IJ4..8 Mev. gamma radiation) to the excited states of 0 ^ a t 6.9 and 7.1 Mev. and the shape of the curve i s such that there is also an indication of this same reaction to the 6 Mev. excited states. The cross section for the reaction corresponding to this peak i s 1.0 + 0.5 x 10~27cm.2. (b) Cross Section Calculations The cross sections for the reaction's as calculated by the two methods previously described agreed very well. As an example, the cross section calculated for the Ne 2 0( 3\o^O1^"" (17.6 Mev. gamma radiation) to the 6.9 and 7.1 Mev. excited states of nl6 i s treated below. The yield of the reaction can be obtained from a sum of the counts under the peak at 6 Mev. which l i e above the background in Figure XXV. This background was drawn in from the general shape - 39 -of the curve and the knowledge that no (y;p) process i n the chamber could give rise to pulses greater than 7.8 Mev. The unknown shape of the background introduces an error into the cross section, but this error can not be greater than 30%. The yield in this case was 290 events. During the run the current integrator recorded a total flux t of 2.17 x K r microcoulombs of charge and the gamma ray flux through the chamber as measured by the s c i n t i l l a t i o n counter was U.7 x 10° gammas per cm The total number of neon atoms i n the chamber as calculated from the s i z e of the sensitive volume (15.2 x 7.6 x 7.6 cm.3) and the pressure (lj.73.5 cm. of mercury) was 1.5 x 102-^ atoms. The cross section from the measured gamma ray flux follows "Y" 28 ? immediately from <f = —• and i s U.O x 10~ cm. . nN The gamma ray flux from the integrated current was based on the thick target yield figures of Fowler and Lauritsen^ of —Pi 1.90 x 10~ gammas per proton from the resonance and yiel d (includ-—ft ing the resonance yield), of 2.6 x 10 gammas per proton at 850 Kev. for a thick lithium target. The thick target yield for the o 650 Kev. bombarding energy used was estimated to be 2.k x 10 gammas per proton. The ratio of 17.6 to lU.8 Mev. gamma rays calculated from the figures of Walker and McDardel-^ (1.6/1) -8 was used to give a yield of 1.5 x 10 gammas per proton of Q 17.6 Mev. radiation and 0.9 x 10~ gammas per proton of lU.8 Mev. ' .ir-radiation. The distance of the sensitive volume (mean distance) was estimated to be 12 cm. Using these figures the cross section was found to be I4.8 x 10~2^cm.2, a value in f a i r agreement with that obtained from the gamma flux measured in the direct way. VI. DISCUSSION OF RESULTS The low cross section for the photo-alpha reaction i n Ne to the ground state of 0 ^ and the relatively much larger cross sections to the excited states may be due to the operation of selection rules leading to a decreased probability of a reaction proceeding to the ground state. The relatively larger <S" at llj.,8 than 17.6 Mev. may perhaps be an indication that the photodis-sharply integration process is venergy dependent, the wide gamma-ray energy spread at llj.8 Mev. encompassing regions of large d' whereas the sharp 17.6 Mev. does not. It i s d i f f i c u l t to account for the relatively large ratio on the basis of a slov/-,' smooth change i n cT' for probability of disintegration with hv. If so, this would indicate that the whole nucleus i s excited by the interaction of thephoton. Bj using the non-resonant portion of the cross section for ' 1 8 gamma ray production in the Li(p,jr )Be reaction, an experiment is feasible in which the energy of the gamma rays used for the irradiation of the neon i s varied. If there i s a definite depend-ence of the cross section for the (y, oC) reaction on energy, fur-ther information may be obtained for establishing the mechanism of the interaction between a gamma ray and a nucleus. -ko-PART III THE DISINTEGRATION OF NEON BY FAST NEUTRONS I. INTRODUCTION The study of nuclear energy levels may be carried out by particle excitation as well as by excitation by radiation as was discussed in Part II. In general, low-lying levels of a compound nucleus can_not be excited by particles due to the binding energy of the orderyof 8 Mev. associated with a nucleon. Levels close to the binding energy'require slow bombarding particles and i f the particles are charged, such as protons, d i f f i c u l t i e s are experienced i n overcoming the Coulomb barrier. Uncharged particles (neutrons) do not experience this d i f f i c u l t y and much useful work has been done using them. ' A compound nucleus with an excitation of 8 Mev. or more is most l i k e l y to decay by remission of a neutron due to the Coulomb barrier for charged particles. Light nuclei may decay by alpha emission, however,'as the numbercof particles i s low and the excit-ation per particle i s therefore high. In this way, fast neutron - Ul -Ik l i 5 0 ) disintegrations have been investigated for nitrogen N On,e()B 10 7 ( 5 1 ) and boron B (n^c()Li' . 20 17 The reaction Ne (n,o()0 ' has been investigated by several workers^ 2 j 53j5k,55 # I t i s o f i n t e r e s t because both the Ne 2 1 compound nucleus and the 0"^ residual nucleus are of the kn + 1 type where an extra neutron has been added to the k alpha-particle model of 0"^ and to the 5 alpha-particle model of Ne2*"1, both of which were discussed.in Part II. The reaction was established i n cloud chambers ' . using "natural" neutron sources. Thawork of Johnson-^ and of Sikkema-^ i s the best to date on the energy levels of Ne for neutron energies up to 3 Mev. They found transitions to the ground state of 017 from several 21 levels i n the Ne compound nucleus. A further investigation of the same reaction was carried out by Flack who obtained resonances corresponding to l£ levels i n Ne2^-'from 10 Mev. to 12 Mev. excitation and alpha particle transitions to the .87 excited state of 0 1 ? as well as to the ground state. The neu-trons were produced by the well known D(d,n)H^ reaction-' 8. Neutrons of 5 Mev. energy are produced i n this reaction with a deuteron bombarding energy of 2 Mev. and the neutrons into a small, solid angle i n the forward direction are closely mono-energetic. It was f e l t that the use of neutrons of s t i l l higher energy 21 would enable higher levels of Ne to be measured from reaction resonances and would also give evidence for higher energy levels in the CI? nucleus. Since tritium targets became available to the laboratory from the N.R.C. pile at Chalk River, the high - U3 -energy neutron source H^(d,n)He^ could be used for producing the neutron flux. This reaction has a Q of 17.6 Mev. giving a neutron energy of l l ; . ! Mev. at zero bombarding energy. To Ion Source r Q . Differential Pumping Aperture—»j M £ To Pump Liquid Air Traps J I r ir i 1 Tritium Target Mercury Diffusion Pump Original High Speed Jet Modified Jet Glass Bead Insulated Shield Fig. XXVT Experimental Arrangement for Neutron Bombardment of Neon II. EXPERIMENTAL TECHNIQUE The neon gas was contained i n the same gridded ionization chamber as discussed i n Part II. The tritium target used i n the experiment was absorbed on a tantalum layer melted on to a tungsten backing. The exact thickness of the target was not known, but was perhaps as much as one hundred kilovolts. The plate carrying the target was mounted on a copper back which was water-cooled (Figure XXVI). Considerable modifications were made to the vacuum system of the SO kv accelerating set built by Kirkaldy^ to prevent cracked o i l products,and.backstreaming o i l vapours from the pumps from getting into the system. The baffle system for the main1 diffusion pump was modified, a second diffusion pump (Hg vapour) was added and with suitable cold traps throughout the system, this effect was reduced to a large extent; 2000 microampere hours of bombardment resulted i n a decrease i n neutron flux of a factor of 3 and only a very slight darkening of the surface of the t r i -tium target. • At the end .of ,.thi.s' timef..,the- target contained a ' ' large amount of absorbed D from the beam, which caused the product tion of (D-D) neutrons. - kh -- kB -The neutron flux was monitored by a long boron counter i n paraffin*of the type discussed by Hanson and McKibben^, with an efficiency of l/2£0 per neutron impinging on the sensitive area. 1 Mr Heiberg constructed the neutron Monitor. 2 0 0 0 < C O M P L E T E E N E R G Y S P E C T R U M NO. OF COUNTS IOOO- 1 c D 1 '10^ >0 >^0o2<toOoooOOQ<KjO Of r 2IO Po 1 I 1 1 O.S I.O 1 1 1.5 2.0 2.5 3.0 DISINTEGRATION ENERGY 3.5 4.0 , MEV. 4.5 1 f — 1 — I N < S.O 5.5 1 6.0 Fig. XOTI Pulse Spectrum of (D-D) Neutrons on Neon III. EXPERIMENTAL PROCEDURE The neon gas contained i n the ionization chamber was bombard-ed with neutrons from the dcon d reaction using a heavy-ice target. The spectrum of pulses so obtained is displayed i n Figure XXVII. The peak i n the distribution i s due to the reaction Ne 2 0(n ground state which has a Q of - .70 Mev. as measured by both P h i l -l i p s 3 8 and F l a c k 3 9 . The tritium target was placed i n position and bombarded with a flux of about 100 microamps of resolved protons at 5>0 k i l o -volts bombarding energy. The neutron flux as measured by the 7 monitor was 2 x 10 neutrons per second at the target. The kick-sorter was. set up to record the pulse height spectrum i n 75 k i l o -voit steps beginning at 1 Mev. When sufficient :.iQ>Xt\ts.;,.c.:, had been obtained, the unit was set up to cover the next energy range in the same channel widths and the procedure was repeated u n t i l the whole spectrum had been covered to lk Mev. The distribution obtained i n this way is plotted i n three sections which are given in Figures XXVIII, XXIX and XXX. The region below k Mev. i s not plotted as the' distribution was'a monotonically-rising function in this region and had no interesting features. - 1*6 -0.87 0.70 Ne20+n-Fig. XXXI Observed; Alpha Groups to Levels i n IV. RESULTS The pulse height distribution shows a large number of peaks superimposed on a large background at energies below 7.8 Mev., which is the maximum range of protons i n the chamber as outlined in Section II. The background may be attributed to (n, p) pro-cesses in the walls and i n the gas i t s e l f . The peaks correspond to energy release i n the chamber of 13.3, 12.3, 10.25, 8.70, 8.20, 7.85, 7.30, 6.95, 6.50, 5.93 and 5.72 Mev. From the energy-level diagram as given by Ajzenberg and Lauritsen^ 8, i t i s seen that these values are i n very good agreement with the energy levels of 0-L7 at 0 , .87, 3 .06, lx.56, 5.08, 5.31, 5.9I4, 6.2U, 6.87, 7.37, and 7.51 Mev. The transitions involved i n the reaction are shown on the diagram in Figure XXXI. Since (n,p) processes i n neon arise from the reaction Ne 2 0(n,p)F 2 0 and this reaction has a Q of -6.25 Mev.^8, only the lowest three peaks in the pulse height distribution curve may 20 be said to be i n doubt due to (n,p) processes to levels i n F . The agreement of the energy values i s so good, however, that i t i s doubtful that the 'peaks 'do-; represent (n,p): processes. Additional verification of these levels in 017 are given by the measurements. In particular, the level at 8.07 Mev., which was doubtful in the summary of Ajzenberg and Lauritsen, has received additional support. - 1x7 -APPENDIX I Rayliegh Scattering Coherent scattering of gamma radiation from electrons which are bound to atoms is known as Rayliegh scattering. This i s distinguished from nuclear scattering of gamma radiation which has been called Thomson scattering. Coherent scattering from the bound electrons i s essentially an interference effect. The amplitudes of the gamma ray scat-tered by the electrons, with their various phases, must be summed over a l l the electrons i n an'^atom. The electron distribution must therefore be known to some degree of approximation. The best approximation i s given by the Hartree model, but for large Z the Fermi-Thomas gas clouj model i s equally valid. The method of evaluating the cross section i s given in 10 Compton and Allison and has been worked i n detail by Franz , who has evaluated the cross section for hard radiation. The Fermi-Thomas distribution for the electrons may be expressed by: g(a)da = 2 . l y / 3 f a ^ l l a , where Z i s the atomic number and o< is a radius characteristic -1/3 of the atomic number Z, defined by .U7 Z angstroms. The electronic structure factor, f, is given by the equation The angle Q i s the direction of scattering. * Compton and Allison - X rays i n Theory & Experiment Van Nostrand 1935 - hfi -- U9 -The scattering factor i s then Z f 2 . Debye has evaluated this function for large Z and has plotted the scattering amplitude as a function of the scattering angle 1 1. This function i s reproduced in Figure VIII, where the differential scattering amplitude i s plotted against u where u = We( sin 9/2. For .51 Mev. radi-ation u becomes 2.k3 x 1 0 2,Z~^ sin 9/2. APPENDIX II Direct Flux Measurement by Nal(Tl) Crystal The intrinsic efficiency for pair production and Compton effect of the Nal crystal can be calculated from the theoretical cross sections for both of these effects for the 17.6 and the II4..8 Mev. gamma rays of the Li(p,y)Be reaction. The wall effect i n the crystal w i l l reduce the energy dissipated by the secondary electrons in some cases, and further, some secondaries w i l l produce bremstrahlung which may escape from the crystal and so reduce the energy released therein. The fraction of the maximum efficiency which i s u t i l i z e d by biasing the counter at a level corresponding to an energy dis-sipation of 10.5 Mev. in the crystal may be deduced from the shape of the pulse spectrum obtained (Figure XXI). This approxi-mation is helped by comparison with the spectrum obtained from the single gamma ray i n the p +• d —> He3 + f reaction^. Since $0% of the spectrum l i e s above the 10.5 Mev. energy level, the fractional efficiency i s most certainly accurate within 20% limits. The Nal crystal was cylindrical in shape of a diameter of lt.lj.6 cm. and a length of 5.08 cm. The cross sections for absorp-tion of the 17.6 and lit.8 Mev. gamma radiation are 8.77 and 7.92 x 10~2^cm.2 per molecule of Nal, neglecting the effect of the (TJ2). - 50 -The intrinsic efficiency of the crystal as calculated from these values i s .557 for the 17.6 Mev. gamma rays and .538 for the lU.8 Mev. radiation. The fraction of this maximum efficiency which i s u t i l i z e d i s 0.70 +. .15 and 0.5? ± .Irrespectively. The absorption of the chamber walls was taken into consider-ation by measuring i t s magnitude by inserting the chamber between the target and the gamma ray monitor. The fraction.v.absorbed in this way was O.I4.O + 0 .05. The gamma ray flux was monitored dur-ing the course of the experiment with the chamber^, this position. By defining:' • w = solid angle subtended by the monitor & = intrinsic efficiency f - fraction of maximum efficiency u t i l i z e d •5 r fraction absorbed by ionization chamber. This fraction i s assumed equal for IJ4..8 and 17.6 Mev. gammas. - No = number of disintegrations in target giving rise to gamma rays Nc = number of counts i n counter due to gamma rays with-out chamber absorption Np = number of counts in counter with chamber absorption and i f these same quantities for II4..8 and 17.6 Mev. gamma rays are defined by superscripts LU and 17, then The ratio of . may be obtained from the curves of Walker and No" McDaniel as previously described. Since, the flux into the ioni-zation chamber i s only reduced by b/2,(there i s only one wall between the valume of the chamber and the target) the flux/cm. in the chamber i s given by; |\J •=• 0 ~ ^ SL>) Klo'1 d •= mean distance of the sensitive volume of the chamber. BIBLIOGRAPHY ' 1. Heitler, The Quantum Theory of Radiation Oxford, P231 , (l9Ut) 2. De Benedetti, Cowan, Kenneker and Primakoff, Phys. Rev. , 77, 205 (l95o) 3 . Argyle and Warren, Can.J .Phys. , 29:32 (1953) h. Griffiths and V/arren, Can.J.Phys,, 29:325 (1951) 5. Maier - Leibnitz, H . , Zeits. fur Nat., 11 665 6-95l) 6. B e l l , Graham and Petch, Can.. J. Phys., 30:35 (l950) 7. Silver, Miss L.M., Can.J .Phys.f 29:59 (l95l) 8. Moon, Proc. Phys.Soc. London ., A63, H 8 9 (l95o) 9. Storruste , ii A63,1197 (l950) 10. Franz, Zeits.fur Phys., 98,3lU (l935) 11. Debye , u 317, 98 (1935) 12. Deutsch , Phys. Rev., 82,16$ (l95l) 13. Bell , Private Communication. 1U." Chadwick & Goldhaber^ Nature, 137,123U (19314) 15. fi/heeler , Phys. Rev.., 52,1083 (1937) 16. Barnes, French & Devons? Nature , 166, Ili5(l950) 17. Goldhaber & Telfer, Phys. Rev., 71^10U5 (l9)-i8) 18. Levinger and Bethe, Phys. Rev., 78,115 d95o) 19- Courant ^  II 82,703 (l95l) 20. Baldwin and Koch^ II 67, 1 (19 U5) 21. McEhinney, Hanson, Becker^ Duffield & Diven, II 75,5U2 (X9h9) - 52 -- 53 -22. Johns, Katz, Douglas & Haslam, Phys. Rev„ , 80:1062 (i95o) 23. Halpern & Mann, 83, 370 C1951> 2k. Preston, 1! 80,307 (19 5o) 25. Goward, Telgedi & Wilkins f Proc. Roy.Soc., A63, U02 (1950) 26. Waffler & Younis, Eelv„ Phys. Acta., 22,6lU 0Sk9) 27. ?foods, Unpublished thesis, U.B.C. 28. Wilkinson, Ionization Chamber & Counters, Cambridge University Press. 0-919) 29- Frisch,* - British A.: E.R.E. Report BR-1+9. 30. Bunemann, Cranshaw & Harvey, Can. J.Research, A27,191 Ll9k9) 31. Gray 9 Proc. Camb. Phil. Soc. , ho , 95 (!9kD 32. Jesse, Forstat and Sadauskis, Phys. Rev. , 77, 782 (1950) 33. Cranshaw and Harvey, Can.J.Research, A26 7U3 (19U6) 3U. Hanna , Phys. Rev. , 80 530 (1950) 35. Tangen 1 Det. Kgl. Norske vid. Sels. Skrifter ; NRI, d°U6) 36. Walker and McDaniel, Phys. Rev. 7 7h ,3l5 (19U8) 37. Jentsche and Prankl, Phys<Zeits. ^  1x1,52k (1939) 38. Phillips, M.A. Thesis U.B.C, {19^ 39. 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Rev. , hi , 52 (1935) 57. Zagor and Valente, Phys. Rev. , 67,133 (19U5) 58. Oliphant, Harteck and Rutherford^ Proc. Roy. Soc.?Allj.U?692 (193U) 59. Kirkaldy, M.A. Thesis, U.B.C. 1^951) 6o. Hanson & McKibben, Phys. Rev., 72 ? 673 (19U7) 61. Mattauch Flajmnersfeld^ Isotopic Report; Tubingen, (I9k9) 62. Sample. Unpublished reoort on Particle Detector U.B.C, (1953) 

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