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The disintegration of neon by fast neutrons Phillips, Gilbert James 1952

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THE DISINTEGRATION OF NEON BY FAST NEUTRONS by G i l b e r t James P h i l l i p s  A t h e s i s submitted i n p a r t i a l f u l f i l m e n t of the requirements f o r 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 required from candidates f o r the degree of MASTER OF ARTS  Members of the Department o PHYSICS THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1952  ABSTRACT A normal mixture of the stable neon isotopes, Ne  90.51%, Ne  w i t h 2.68 Mev.  0.28$, Ne  9.21%, has been bombarded  neutrons from the r e a c t i o n H (dn)He  .  A gridded ion chamber, w i t h 1 l i t r e s e n s i t i v e volume, f i l l e d w i t h neon to 6^ atmospheres pressure, was  used  to detect d i s i n t e g r a t i o n s . Pulses from the i o n chamber were a m p l i f i e d , arid recorded on an 18-channel k i c k s o r t e r . The  ground-state t r a n s i t i o n of Ne (n«i)0  was  observed, w i t h 1% counting s t a t i s t i c s . The Q,-value was measured as  -0.77*0.08 Mev.,  i n f a i r agreement w i t h  previously reported r e s u l t s . Careful search f a i l e d to r e v e a l any  evidence  17  .  of excited states i n 0 . A l e v e l at 0.87 and one at 1.6 Mev.  Mev.  i s known,.  i s suspected, but c a l c u l a t i o n of  b a r r i e r p e n e t r a b i l i t y f o r alpha p a r t i c l e s corresponding to these l e v e l s i n d i c a t e s that they are not l i k e l y to be observed at the neutron energy used. A second r e a c t i o n , w i t h d i s i n t e g r a t i o n energy corresponding to a Q,-value of +0.48*0.10 Mev. observed. I t was  was  not c l e a r whether t h i s r e a c t i o n  N ^ n p j C ^ o r N e ^ U ^ O o ' * . This point i s to be by f u r t h e r i n v e s t i g a t i o n .  also was  clarified  ACKNOWLED&EMEHTS The author i s pleased to express h i s thanks to Dr. J . B. Warren f o r the suggestion and d i r e c t i o n of t h i s research. The assistance of 'Mr. F. C. Flack throughout the course of the experiments,  and i n the preparation  of t h i s thesis i s deeply appreciated. Thanks i s also due to Dr. S. B. Woods f o r the c o n s t r u c t i o n of p o r t i o n s of the apparatus, and f o r h e l p f u l suggestions.  INDEX I.  II.  INTRODUCTION A.  Knowledge of N u c l e i  1  33.  (n  2  C.  Choice of Experiment  ) Reactions  3  EXPERIMENTAL METHOD A.  The Experimental Problem  33.  Production of Past Neutrons  4  1.  East Neutron Sources  k  2.  The H* (dn)He* Reaction  5  3.  The 5 0 kev. Ion Generator  7  4.  Heavy Ice Target  8  5.  33ackground R a d i a t i o n from 9  Neutron Source C.  Measurement of A l p h a - P a r t i c l e Energies  10  1.  Choice of Detectors  10  2.  The Gridded Ion Chamber  11  3.  D e s c r i p t i o n of the Ion Cham"ber 13  4.  Electrode System  14  5.  P i l l i n g Gas  15  6.  Operating Conditions  17  7.  Pulse  Measurement  Amplification  and 18  8.  Energy C a l i b r a t i o n  20  9.  Experimental Procedure  21  III.  RESULTS AND DISCUSSION  23  A.  Experimental Results  23  33.  Discussion  2^  1. Threshold Energies f o r Fast Eeutron D i s i n t e g r a t i o n of Neon 2.  24i=  I n t e r p r e t a t i o n of the  Main Peak  25 3.  I n t e r p r e t a t i o n of the  Minor Peak  28 l+. C.  Summary of Results  3Turther Investigations  30 31  IV.  CONCLUSIONS  33  V.  BIBLIOGRAPHY  3k  ILLUSTRATIONS Plate 4  3  I.  H  II.  H* (dn)He* Thick Target Y i e l d Function  III. IV.  H  (dn)He  4  Cross Section vs. Deuteron Energy  Facing Page 5  3  (dn)He  Neutron Energy v s . Angle of Emission  Heavy Ice Target  6 7 8  Pulse Formation i n Ungridded Ion Chamber  12  Gridded Ion Chamber  13  Electrode System f o r Gridded Ion Chamber  14  Wide-33and Low Noise Head A m p l i f i e r  IS  Pulse-Height D i s t r i b u t i o n from Polonium Alphas  20  X.  Complete Energy Spectrum  23-:  XT.  Energy Spectrum: D e t a i l s  24  V. VI. VII. VIII. IX.  1.  THE PI SI MSG-RAT I ON OF NEON BY FAST NEUTBOMS I. A.  INTRODUCTION  KNOWLEDGE OF NUCLEI It i s much easier to speak with optimism regarding the future of  Nuclear Physics than to discuss with s a t i s f a c t i o n i t s present status.  While  the general p r i n c i p l e s and methods of quantum mechanics appear.to he s a t i s factory f o r dealing with nuclear phenomena, yet the most f r u i t f u l approach, p a r t i c u l a r l y i n dealing with nuclear reactions and energy l e v e l s , i s essent i a l l y a phenomenological  one.  The main d i f f i c u l t i e s i n attempting to formulate a theory of n u c l e i , comparable i n scope to that of atomic structure, are twofold.  F i r s t l y , the  nature and proper description of the short-range forces between the protons and neutrons of which the nuclei are composed are not understood, and secondly, even with a comprehensive knowledge of the short-range forcesj the mathemati c a l d i f f i c u l t y of dealing with such a many body problem accurately i s s t i l l formidable. .-'In t h i s case even approximation methods make l i t t l e headway—in the words.of Gamow., "the theory of complex nuclei i s d i f f i c u l t mathematically because everything i s of the same order of magnitude, and nothing can be neglected. "  (GAMOW, 1937) The problem has then been approached experimentally by c o l l e c t i n g  as much data as possible on a l l aspects of nuclear structure: functions, energy l e v e l s , spin values, and phenomenological the dispersion formula, deduced to f i t the observed  excitation  theory, such as  results.  Attempts to progress further have resulted i n the proposal of various nuclear models.  One example i s the "alpha p a r t i c l e model", which  pictures the nucleons as being grouped within the nucleus as alpha p a r t i c l e s ,  2.  the alpha i n turn forming s h e l l s , with extra nucleons i n outer o r b i t s . Such a picture suggests exceptional s t a b i l i t y f o r closed, shells of alpha p a r t i c l e s , as i n C  and 0  .  It was hoped that some of this model's pre-  dictions f o r nuclei of the 4N + n type might "be tested i n the present experiment as one such nucleus, namely 0 product i n the reaction T$e*° (xi* )  (4<*+n ) was a known d i s i n t e g r a t i o n ; 0  1 7  The alpha p a r t i c l e model i s "by no means the only model postulated, and recently the "orbit model" has been extensively and successfully developed by MAYER,  (1948).  However, with l i g h t nuclei the alpha p a r t i c l e model has  had some success i n correlating experimental data.  It i s hoped that the  present experiment w i l l contribute another small fact to the ever increasing information about nuclear reactions. B. (n oc ) REACTIONS Nuclear energy levels are usually excited by bombardment with nucleons, or other heavy p a r t i c l e s .  For neutron bombardment, capture adds about 8  to the nucleus, t h i s energy being d i s t r i b u t e d among the nucleons.  Mev.  The compound  nucleus so formed remains i n this excited state u n t i l random fluctuations concentrate s u f f i c i e n t energy i n one or more nucleons f o r t h e i r emission.  Photon  emission may also occur, but the l e v e l s f o r this process are sharper than f o r p a r t i c l e emission, and the l a t t e r process i s much more probable, i f energetically possible. The most favoured t r a n s i t i o n i s that which leaves the r e s i d u a l nucleus i n the most stable configuration. Generally, neutron emission requires somewhat less e x c i t a t i o n energy than proton or alpha emission, because of the Coulomb b a r r i e r f o r the charged p a r t i c l e s .  Alpha emission, however, commonly occurs f o r  the l i g h t nuclei, i n which the Coulomb b a r r i e r i s r e l a t i v e l y low, while the  3.  average energy per nucleon i n the compound nucleus i s high, due to the sharing of e x c i t a t i o n energy between r e l a t i v e l y few p a r t i c l e s . C.  CHOICE OF EXPERIMENT Much work has been done with f a s t neutrons, and the d i s i n t e g r a t i o n  of c e r t a i n nuclei, i n p a r t i c u l a r those of carbon and nitrogen by such p a r t i c l e s i s well known.  Neon however, has received l i t t l e attention.  using neutrons of less than 3 Mev.  energy, showed with rather poor s t a t i s t i c s  only one single-energy group of emitted alphas. the ground state t r a n s i t i o n of Ie*°(n<t) o ' . r  This group was No evidence  been reported, but since a l e v e l i n o'at 0 . 8 7 Mev. 1 . 6 Mev.  Previous experiments,  i d e n t i f i e d with  of excited states has  i s known, and another at  suspected, further i n v e s t i g a t i o n seemed worthwhile. Neon i s readily obtainable i n pure form, and l i k e the other noble  gases i s suited f o r use i n an ion chamber (WILKINSON,  With an 18 channel  1950).  pulse amplitude analyser a v a i l a b l e i n this laboratory, i t was expected to obtain data; more rapidly•and hence more p r e c i s e l y than heretofore. It was decided therefore to bombard neon with fast neutrons, an ion chamber as a detector.  The Devalue of the Ne  (n«)  0  using  reaction was  to be c a r e f u l l y measured, and a thorough search made f o r other groups of p a r t i c l e s , p a r t i c u l a r l y any associated with excited states of o ' '  .  The  remainder of this thesis discusses the d e t a i l s of this experiment,- and results obtained.  the  4.  II. A.  EXPERIMENTAL METHOD  THE EXPERIMENTAL PROBLEM Since neon l i k e the other noble gases has excellent e l e c t r o n  c o l l e c t i o n properties, i t seemed obvious to detect the reaction d i r e c t l y by i o n i z a t i o n .produced i n a gas target.  Among the various possible detectors,  cloud chamber, proportional counter, etc., a gridded ion chamber was chosen as being a fast operating detector with excellent l i n e a r i t y between pulse-size and energy. Three sources of fast neutrons were a v a i l a b l e f o r the experiment: a 5 0 m i l l i c u r i e Radium-Beryllium source, a 50 kev. i o n accelerator, and a Van de G-raaff generator, at present producing protons with energies up to 2 Mev. The Ra-Be source provides a reasonable y i e l d of neutrons, but the straggling of alphas within the source produces a very broad spectrum of neutron energies. Neutrons can be generated i n both the 5 0 kev. accelerator and the Van de G-raaff by means of a nuclear reaction, such as H*(dn) He Since an intense, monoenergetic by using the H  (dn) He  3  or H  3  (dn ) He * .  source was required, i t was decided to start  reaction with the 5 0 kev. i o n accelerator and a thick  target of heavy i c e . B.  PRODUCTION 03? EAST NEUTRONS 1.  Fast Neutron Sources Many nuclear reactions may be used as neutron sources.  In p a r t i c u l a r , bombardment of l i g h t n u c l e i with protons or deuterons produces neutrons of a wide range of energies.  The choice of a reaction depends on the  y i e l d , on the number of neutron energy groups produced, and on the energies required. A few of the better known fast neutron sources are: HORNYAK et a l , 1 9 5 0 ) .  (HANSON et a l 1 9 4 9 ;  Plate 10  D(dn)f He ;  I  Cros s Section {cy) vs. D euteron Erlergy (Ed;  /o  10  or z  cms.  JO •  •It  /o  \  -J9  /O T  •  > '  o.S  AO  E d ; Mev.  JO  5.  Reaction  H* (dn) He  3  H  3  3  (pn) He  quvalue  Threshold  + 3.256 Mev. - 0.764  "  l i ' ( p n ) Be"  - 1.647  "  V*'(pn) C r  - 1.50  3  #  (dn) He *  4 7  < 0 . 0 1 5 Mev. 0.98  »  +17.60  E  (dn)  He  "  ,< 0 . 0 1 5  "  1 . 8 -> 7 . 2  Mev.  0.001-^4. 12.  ->20.  3.  " »  1.882  «  0.001  1.53  "  0.002 -> 0.020 Mev.  f o r bombarding energies up to about 4  As mentioned, H  Ranee of Neutron Energies #  "  .  Mev.  neutrons generated "by the 5 0 Kev. ion accelerator  were chosen f o r the i n i t i a l i n v e s t i g a t i o n .  It was hoped to extend the neutron  energy up and down from the 2.68 Mev. value by using t h i s reaction with f a s t e r deuterons, the H  (dn ) He  reaction, and slower neutrons from the V  (pn) Cr  reaction. 2.  The H (dn) He' Reaction a  Since i t was f i r s t reported by OLIPHANT,'HARTECK and ' RUTHERFORD, ( 1 9 3 4 ) , t h i s reaction has received the a t t e n t i o n of many workers. The l i t e r a t u r e contains extensive information on the y i e l d function (ROBERTS, 1937;  AMALDL et a l , 1937) the cross section (LADENBURG et a l 1937) and the  angular d i s t r i b u t i o n (BONIER, 1 9 3 7 ) , f o r deuteron energies from 15 kev. (BESTSCHERT;, et a l , 1948) to 10 Mev.  (LEITERS et a l , 1950)  The reaction proceeds i n two ways, with about equal p r o b a b i l i t y : U*  (dn)  He  H*  (dp)  H  3  3  Q*  * 3 . 2 5 6 ± 0.018 +4.036± 0.022  Mev. Mev.  These 0-values are magnetic spectrometer determinations (TOLLESTRUP et a l , 1 9 4 9 ) . Exact r e l a t i v e y i e l d s of protons and neutrons have s t i l l not been established, but are  known to be approximately equal.  Plate D (dn)He -, Thick T a r g e t 2  Total  3  Yield (Y)  oS  II Yield  vs. D e u t e r o n  to E d ; Mev.  Function  Energy  (Ed)  /.s  20  6. Curves of cross section and t o t a l y i e l d f o r the reaction showsmooth increases with the energy of the incident deuterons.  These are i l l -  ustrated i n Plates I and II respectively, f o r deuteron energies up to 2 Mev.  The former gives the integrated cross section, and the l a t t e r the t o t a l  y i e l d , over the whole s o l i d angle, f o r a thick heavy i c e target.  These curves  are "based on several investigations, as l i s t e d "by HORNYAK et a l , The angular d i s t r i b u t i o n of neutrons i s anisotropic.  (1950)  For deuteron  energies of less than 5 0 0 kev., i t i s adequately described by a r e l a t i o n of the form N (?)  •B (  1*A c o s * ^ ) i n the centre of mass system. (MANNING et a l ,  1942).  At higher deuteron energies, more neutrons are observed i n the forward d i r e c t i o n than this r e l a t i o n predicts, and terms i n higher powers of cos*^  must be added.  The more extensive investigations of t h i s aspect of the reaction include those of BENNETT et a l , (1946) f o r deuterons of up to 1.8 Mev., HUNTER and RICHARDS, (1949), 0.5 to 3.7 Mev. deuterons, and ERICKSON et a l (1949) f o r 10 Mev. deuterons.  In the present experiment, the angular d i s t r i b u t i o n was calculated  using the value of A = 0.45 f o r 50 kev. deuterons. (HUNT00N, 1940) The neutron energy varies with the angle of emission measured r e l a t i v e to the deuteron beam, and i s also a function of the deuteron energy.  This  dependence may be calculated from the masses and energies of the p a r t i c l e s involved. HANSON et a l (1949) have carried out these calculations f o r several of the reactions used as neutron sources, f o r a wide range of deuteron energies. Plate III shows the results of t h e i r calculations f o r deuterons up to 2 Mev. It i s evident that i f f a i r l y high energy deuterons are available, as with a Van de Graaff generator, the reaction provides a wide and continuously variable range of neutron energies. The extent to which the neutrons obtained are t r u l y homogeneous depends on several f a c t o r s . %  Variation i n energy of the incident deuterons, and  note e s p e c i a l l y BRETSCHER et a l ( l 9 4 8 ) f o r low energies.  Plate  En *  Mev.  D(dn)He  3  .  Neutron  III  J  Energy  E  (En)  vs.  n  Angle  M  e  v  of  frf?  0°  Emission  {^)  straggling of deuterons i n thick targets w i l l produce a spread i n neutron energies.  Unless the s o l i d angle subtended by the detector i s very small,  a range of neutron energies w i l l be observed, due to the angular dependence of the reaction. Variations i n deuteron energy may be controlled by magnetic analysis of the i o n beam.  Deuteron straggling i n thick targets i s d i f f i c u l t to determine,  as the rate of energy loss, (yj  i s not well known.  In the present experiment,  the effect of t h i s mechanism on the homogeneity of the neutron energy w i l l be small.  In the region of deuteron energies used (50 kev.), the y i e l d function  of the reaction r i s e s very rapidly.  Therefore, f o r deuterons of even s l i g h t l y  less than average energy, the neutron y i e l d drops very sharply.  Thus r e l a t i v e l y  few low energy neutrons w i l l result from straggled deutrons. 3.  The 50 kev. Ion Generator The accelerating voltage of the 5 0 kev. generator was  supplied by a commercial half-wave transformer-rectifier set, (FEREANTI X-RAY) with condenser f i l t e r i n g .  Ion voltage was monitored by observing the current  through a .calibrated o i l immersed resistance stack. to be much less than 1 (KIRKALDY,  Ripple voltage was stated  Construction of the unit i s described elsewhere  1951).  The i o n beam was produced by.a radio-frequency i o n source, of a type developed i n this department. (KIRKALDY,  1951;  i n the discharge i s less than 100 v o l t s .  WOODS,  1952).  Energy spread  Provision i s included f o r magnetic  analysis o f the i o n beam. Deuterium gas was obtained from heavy Water i n t h i s laboratory.  The  heavy water ( 9 9 * $) was electrolysed i n a closed system, which included a cold trap and phosphorous pentoxide d r i e r .  The gas was passed into"the i o n  i  PLATE  IV  HEAVY  ICE  TARGET  source through a Palladium  leak, thereby eliminating most contamination, with  the exception of hydrogen. Beam currents were measured by- metering the target current to ground. biased to + 3 0 0 v o l t s to prevent back electron current.  The target was  of 100 to 20Cyfc-amp. resolved H h.  Beams  ions were obtained.  Heavy Ice Target The f i r s t targets used consisted of heavy water adsorbed  on Phosphorous Pentoxide but the neutron y i e l d s obtained were low. target, as shown i n Plate IV was  A heavy i c e  then constructed.  The target was attached to the ion generator by a two inch'Pyrex pipe flare fittings.  After/ evacuation,  f i l l e d with l i q u i d a i r .  the inner chamber of the target  Heavy water vapour was  was  then admitted to the body  of the target v i a a needle valve and the copper tube, indicated i n Plate  IV.  The vapour froze to the copper target-leaf, forming a layer of heavy i c e . The 0-ring seal on the copper tube allowed i t to be moved out of the way the beam a f t e r the target was  formed.  Formation of the target was target body through the Pyrex pipe. and the target was  of  f i r s t observed by looking into the This view proved to be rather r e s t r i c t e d ,  l a t e r modified by cutting a c i r c u l a r part i n the side  of the body, (position indicated i n Plate IV), and attaching a glass window by means of an 0-ring seal. The Pyrex pipe e f f e c t i v e l y insulated the target from the i o n generator, and allowed the beam current to be metered.  It was  found  that the i n t e r i o r of the glass pipe became charged, causing serious  initially defoc-  ussing of the beam and so a great reduction i n the u s e f u l beam current.  A  sleeve of wire gauze on the i n t e r i o r of the pipe, connected to the target, overcame the d i f f i c u l t y .  and  9. The inner chamber of the target accomodated about 200 cc. of With a normal beam of 150y(f.amp. on the target, i . e . 7«5  liquid air.  watts beam-power, this was s u f f i c i e n t f o r a maximum of one hour's operation. Normally i t was topped up every half hour. A beam of 150  amp.  corresponds to 9 . 3 o 1 0 ; f  deuterons per  second, and from the y i e l d curve f o r the reaction, (Plate I I ) , the y i e l d f o r 50 kev. deuteron energy i s approximately 2 / 1 0  'neutrons per deuteron,  so that the t o t a l y i e l d was about 2 * 10 ' neutrons per second.  \\ v, 5.  Background Radiation from Neutron Source It was necessary to consider the p o s s i b i l i t y of other  penetrating- radiations o r i g i n a t i n g i n the heavy ice target, i . e . other (dn) reactions. E* , o " ,  0  The l i g h t n u c l e i exposed to the deuteron beam were B'  , o'*from the heavy water, and traces of c'^and C  /f  o i l contamination.  o"(dn) F " M  / J  ft  Q.--1.6  Mev.  (NEWS0N, 1935;  « +3.5  Mev.  (WELLES, 1946) (isotopic abundance 0.04$) (BONNER et a l 1949) (threshold 0.328 Mev.)  Q» -0.26  Mev. . »x  C  REYDENBURG-, 1948)  ft  The high thresholds eliminate the C - + 0 /a  f rom pump  The only reactions to be considered were:  o'**(dn) J ? " -  C (dn) N  / J  ,  reactions, while both  17  and 0  have too few atoms present to be s i g n i f i c a n t . Soft (50 kev.) jtf-rays w i l l be present from the ion generator, but  would not be expected to penetrate the l/4 inch s t e e l walls of the i o n i z a t i o n chamber.  Additional protection was given by a l/4 "  sheet of lead-wrapped  around the chamber. There i s expected to be a background of scattered neutrons from the concrete walls of the laboratory, and from such massive objects i n the v i c i n i t y as the analysing magnet.  Tests with a cadmium absorber would  indicate whether thermal neutrons were producing any e f f e c t s , such as ( n * )  10.  reactions i n the walls of the ion chamber.  Scattered neutrons with  somewhat less energy than those from the heavy ice target would be expected to cause some spreading of pulse d i s t r i b u t i o n s .  C.  MEASUREMENT 03? ALPHA PARTICLE ENERGIES !•  Choice of Detectors As mentioned previously,' such d i f f i c u l t i e s as  straggling i n windows can be avoided by detecting the i o n i z a t i o n of the products of the reaction i n the target gas  itself.  The Wilson Cloud Chamber has often been used f o r -such observations—the f i r s t records of Neon disintegrations were so obtained, (HARZINS,  1933).  However, there are several drawbacks i n using i t  for accurate measurement of p a r t i c l e energies.  Unless s p e c i a l high-S  pressure chambers are used, there w i l l be r e l a t i v e l y fev; target atoms, and correspondingly few disintegrations w i l l be observed.  F a i r l y acc-  urate energy determinations from cloud chamber photographs are possi b l e , but both the c o l l e c t i o n and analysis of the data are tedious. Proportional counters are capable of f a s t and accurate energy determinations. the device.  Most d i f f i c u l t i e s a r i s e i n maintaining s t a b i l i t y of  For detecting energetic p a r t i c l e s , the gas pressure must  be high i n order to confine the p a r t i c l e range to the gas volume. voltages are then required to obtain gas-amplification.  High  For consistant  r e s u l t s , the gas-amplification must remain constant, and this requires w e l l - s t a b i l i z e d high voltage.  The a m p l i f i c a t i o n i s also very sensitive  to s l i g h t changes i n the constitution of the f i l l i n g gas, and the release of even small amounts of occluded gases from the walls of the chamber can be very troublesome.  33ackground i s f a i r l y high i n proportional  11.  counters, because of t h e i r s e n s i t i v i t y to stray radiations, such as soft x-rays.  Such an increase  i n "noise" w i l l raise the useful lower l i m i t  of the device, making the detection of low energy events d i f f i c u l t . F i n a l l y , the pure noble gases are unsuited f o r use i n proportional counters, c h i e f l y because of t h e i r tendency to form metastable states. Addition of a quenching gas i s necessary to de-excite so prevent spurious pulses.  these states,  and  (WILKINSON, 1950)  Ionization chambers, and p a r t i c u l a r l y gridded ion chambers, are well suited to these investigations.' They require very stable amplifiers, but modern c i r c u i t s meet these demands. l e s s sensitive to v a r i a t i o n s i n c o l l e c t i n g voltage  Ion chambers are than proportional  counters, are much less sensitive to background radiations, and r e l a t i v e l y unaffected filling  are  by s l i g h t changes i n the composition of the  gas. The  speed with which data may  recording equipment a v a i l a b l e .  be collected depends on  the  Frequently the voltage pulses are  on an oscilloscope and photographed.  displayed  This mejbhod y i e l d s accurate r e s u l t s ,  but; the rate of c o l l e c t i n g data i s usually l i m i t e d by the photographic equipment, and analysis of the data i s a b i t tedious. attained with a pulse-amplitude analyser, was  Greatest speed i s  or "kicksorter".  Such a device  available f o r the present experiment.  2. . The Gridded Ion Chamber Measurement of p a r t i c l e energies with an ion chamber requires that the i o n i z a t i o n produced be proportional to the particle-energy,  and that the voltage pulses from the chamber be pro-  p o r t i o n a l to t h i s i o n i z a t i o n .  Events of a single energy may  i n practice  PLATE  V  PULSE  FORMATION  IN  UN6RIDDED  PULSE HEIGHT  PULSE  DURATION  ,  SECONDS  ION  CHAMBER  12.  produce a d i s t r i b u t i o n of pulse-heights.  The spread of t h i s d i s -  t r i b u t i o n depends on several f a c t o r s — s o u r c e thickness, i n the case of s o l i d sources placed i n the chamber, straggling of i o n i z a t i o n , nonionizing, e x c i t a t i o n of the f i l l i n g gas, noise .in the associated e l e c t r o n i c c i r c u i t s , and the influence of p o s i t i v e ions on pulse-formation. In a gridded ion chamber, the effect of the positive, ions i s eliminated.  How  t h i s i s achieved may  pulse-formation  i n an ungridded chamber.  be understood by considering We assume that electron-  attachment does not occur i n the f i l l i n g gas, and that the c o l l e c t i n g f i e l d i s s u f f i c i e n t to prevent recombination of the ions. An i o n i z i n g event occurs between the electrodes, and electrons are attracted to the p o s i t i v e c o l l e c t o r .  the  Electron collection  -S requires 10  seconds or l e s s , and r e s u l t s i n a sharply r i s i n g voltage  pulse on the c o l l e c t o r (see Plate V). i s about 1000  The p o s i t i v e ions, whose mobility  times less than that of the electrons, do not move appreciably  before electron c o l l e c t i o n i s completed.  While the p o s i t i v e ions are being  c o l l e c t e d , the p o t e n t i a l of the c o l l e c t o r continues  to r i s e , and  i t s maximum when a l l p o s i t i v e ions have been c o l l e c t e d (at the electrode).  reaches-  negative  The pulse then decays as the charge leaks away through the  resistance and capacity of the electrode system. Hot only i s such pulse-formation  slow, but the height and  duration  of the portion r e s u l t i n g from the c o l l e c t i o n of p o s i t i v e ions depends on the o r i e n t a t i o n of the ions at the time of formation.  In many experiments t h i s  w i l l be a f a i r l y random matter, and the r e s u l t i n g spread of the pulseheight d i s t r i b u t i o n w i l l be correspondingly widened.  An a m p l i f i e r to  reproduce such slow pulses requires a low low-frequency cut-off, thus introducing the a d d i t i o n a l problem of microphonics.  LATE  1820-20  VI  GRIDDED  8  H  O  L  E  ION  CHAMBER  8  S  SIDE  SECTION  HOLES  SCALEO  INCHES I 2  13.  The  d i f f i c u l t y i s completely overcome "by placing a g r i d  between the electrodes, near the c o l l e c t o r .  The g r i d shields the  c o l l e c t o r e l e c t r o s t a t i c a l l y from the f i e l d of the p o s i t i v e  ions.  When an i o n i z i n g event occurs, the c o l l e c t o r potential remains constant u n t i l the electrons have diffused past the g r i d , then r i s e s to i t s maximum value as electron c o l l e c t i o n occurs. as the charge leaks from the electrode system.  rapidly  The pulse then decays  Much f a s t e r  amplifiers  may be used f o r such pulses, improving the signal-to-noise r a t i o . f a s t e r pulses also allow shorter resolving rates.  The  time and much higher counting  Suitable choice of the g r i d voltage prevents electron  collection  by the g r i d . Some fra.ction of the i o n i z i n g events w i l l occur i n the p o r t i o n of the sensitive volume between the g r i d and the c o l l e c t o r .  This region  w i l l behave as an ungridded chamber, with c o l l e c t i o n of p o s i t i v e at the g r i d .  ions  Such pulses w i l l have random heights, and w i l l cause some  spreading of the d i s t r i b u t i o n . For some events, only a portion of the i o n i z a t i o n w i l l occur i n the sensitive volume, the p a r t i c l e s either s t r i k i n g an electrode, or passing out of the sensitive  region.  This effect w i l l cause an asymmetry  of the low-energy side of the d i s t r i b u t i o n of pulses from single energy i o n i z i n g events.  The degree of asymmetry w i l l depend upon the range of  the ionizing p a r t i c l e s i n the chamber. 3-  Description of the Ion Chamber Plate VI shows the gridded ion chamber used i n the  experiment. flanges.  The body of the chamber was s t e e l tubing, with welded  The end-plates were sealed with lead gaskets, thus avoiding  the exposure of the f i l l i n g gas to rubber, a common source of contamination.  PLATE ELECTRODE  VII SYSTEM  FOR GRIDDED  ION  S C A L E : INCHES O I 2  COLLECTOR  AND  GUARD  GUARD  BRASS,EXCEPT WHERE NOTED  GRID,  1  AND  MATERIAL \  RING  G Rl ^. COLLECTOR  CHAMBER  RING  '////////////////////;//////\ MYCALEX ^ SIDE  ELEVATION  END  ELEVATION  14.  The volume of the chamber was 4.64 l i t r e s . .  A l l i n t e r i o r surfaces were  coated with "aquadag" c o l l o i d a l graphite, to reduce the background from natural alpha-activity i n the s t e e l . One end plate carried the gas p u r i f i e r , constructed of brass and copper tubing.  An e l e c t r i c heating c o i l of about 3 0 ohms (cold) r e s i s -  tance was wound about the centre of the p u r i f i e r , and insulated with glass wool and asbestos paper.  75 v o l t s across the heater produced a  O  temperature of about 3 0 0 C. at the i n t e r i o r . at  A copper cooling c o i l  one end aided i n convection of the gas, and protected the 0 - r i n g  s e a l on the f i l l i n g cap. and water vapour.  The most common impurities are oxygen, nitrogen,  Calcium metal i s known to be e f f e c t i v e i n removing  a l l of these, and so was used as the p u r i f y i n g agent. (REIMANN, 1 9 3 4 ; WILKINSON, 1950); The opposite end-plate carried a needle-valve f o r f i l l i n g the chamber, and three kovar terminals.  The electrode system was also  mounted on t h i s plate, and the electrodes connected  to the kovars.  The chamber was designed by Mr. 3P. Flack, and the electrode system constructed by Dr. S. B. Woods.  Its use i n other experiments  has been described elsewhere (WOODS, 1 9 5 2 ) 4.  Electrode System The design of grids f o r such chambers has been  c a r e f u l l y investigated, both i n theory and experiment by BUNEMANN, CHANSBAW, and HARVEY, 1 9 4 9 . 1  The electrode system of the present  chamber was designed largely according to t h e i r recommendations. The electrodes were constructed of brass, with Mycalex and Lucite i n s u l a t i o n . was wound from  Plate 7;II shows the arrangement.  The g r i d  3 6 Coppel wire, spaced 1 mm. centre-to-centre.  c o l l e c t o r spacing was 1 - 5 2 cms. and g r i d to plate 7 . 6 cms.  Grid-  For such  15.  an arrangement, the "grid i n e f f i c i e n c y " , i . e . the extent to which the number of l i n e s of force ending on the c o l l e c t o r i s dependent upon the f i e l d due to p o s i t i v e ions, i s 0 . 0 1 , i n d i c a t i n g very e f f i c i e n t shielding of the c o l l e c t o r . The curvature of the high-voltage plate i s a s l i g h t modifi c a t i o n to the design of Bunemann et a l . F i e l d p l o t s have shown that the curvature increases the number of l i n e s of force ending on the c o l l e c t o r , and insures that the sensitive volume approximates to the a c t u a l volume above the c o l l e c t o r . The sensitive volume was estimated to be about 1 l i t r e . About 15$  of t h i s volume constitutes the unshielded region between the  g r i d and the c o l l e c t o r .  To a f i r s t approximation, we may  the i o n i z i n g events are d i s t r i b u t e d uniformly throughout  assume that the s e n s i t i v e  volume, and so about 15$ of the events w i l l contribute to the spreading of the p u l s e - d i s t r i b u t i o n . 5-  Filling  Gas  Like a l l noble gases, neon i s w e l l suited to use i n an i o n i z a t i o n chamber .  E l e c t r o n attachment does not occur, and electron  c o l l e c t i o n can be achieved, even at high pressures, with reasonable c o l l e c t i n g voltages. Although the purest obtainable gas was used, the p u r i f i e r on the chamber was s t i l l necessary, because of occluded gases on the chamber walls.  Outgassing of such a chamber i s very d i f f i c u l t ,  though  some attempt was made by evacuating the chamber while baking i t under a heat lamp, p r i o r to f i l l i n g . The manufacturers ^ of the neon used stated the l i m i t s of  # The Matheson Co., East Rutherford New  Jersey; private communication  16.  impurity to be 0.2$  Helium Nitrogen  0.02$  Other less than  0.02$  The three stable neon isotopes were present i n the normal r a t i o s : Ne  2  0  Ne ' 1  Ne * a  90.51$ 0.28$  9.21$ (MATTAUCH and FLAMMERSFELD, 1 9 4 9 )  It was desirable to have as high a pressure of neon as possible, for  two reasons.  F i r s t , high pressure increased the number of  target atoms, making disintegrations more probable.  Second, the  higher pressure shortened, the tracks of the i o n i z i n g p a r t i c l e s , so increasing the number of events which are completely confined to  the sensitive volume, r e l a t i v e to those which are only p a r t i a l l y  i n this region.  The pressure, may be limited i f the e l e c t r o s t a t i c  f i e l d i s not s u f f i c i e n t to achieve electron c o l l e c t i o n , but this consideration d i d not a f f e c t the present experiment. A f t e r evacuation, the chamber was f i l l e d by equalizing pressures between the chamber and the neon storage cylinder.  The  gas was passed through a l i q u i d nitrogen cold-trap during f i l l i n g . The f i n a l pressure was 4 ? 3 - 5 cms. mercury (at 22*C) and hence the to,  .  n  range of the alpha p a r t i c l e s prodticed i n the Ne (n«;0 reaction with 2 . 6 8 Mev. neutrons'would be 0 . 2 cms., a r e l a t i v e l y small d i s tance compared to the dimensions of the sensitive volume, (roughly ?.5  *9.*  15.  cms.)  17.  6.  Operating Conditions In the present chamber, the voltage was  limited  by sparking at the high-voltage kovar seal inside the chamber, which occurred at about - 9 5 0 v o l t s .  This was more than adequate to achieve  saturation, i . e . complete electron c o l l e c t i o n , without recombination. In operation, a plate voltage of «800 v o l t s and g r i d voltage of - 2 0 0 v o l t s was used.  The plate voltage was obtained from a Dynatron  Hadio Type 200 regulated supply, and the g r i d voltage was  supplied  from the plate by a r e s i s t o r network. The chamber was stationed with i t s centre-line about 5 1/2 inches from the heavy i c e target.  Jn_this p o s i t i o n the s e n s i t i v e  volume received about l / 3 5 of the t o t a l y i e l d of neutrons, i . e . about 5X 10 * neutrons per second f o r a 150^-amp. beam. The range of neutron energies received by the s e n s i t i v e volume was determined by the half-angle subtended at the target by the sensitive volume, i n t h i s case about 45 degrees.  A value f o r the average neutron  energy was obtained as follows. The s o l i d angle was treated i n three regions: 0 - 1 5 , and 3 0 - 4 5 degrees.  15-30,  A value of neutron energy f o r each region was  calculated from the general formula f o r the angular energy dependence i n c o l l i s i o n processes (HANSON et a l , 1 9 4 9 ) .  Substituting appropriate  masses and energies f o r 5 0 kev. deuterons bombarding deuterium, the expression f o r neutron energy i s : E^r ( 0 . 0 1 2 5 ) | ^ c o s ^ - H 9 6  +  cos ^  /cosV+392  Neutron energies f o r the three regions were obtained from this formula with*s7.5,  2 2 . 5 , and 3 7 . 5 degrees.  The r e l a t i v e number of neutrons i n each region was  then  P L A T E  VIII  W I D E - B A N D  L O W  N O I S E  H E A D  A M P L I F I E R  18.  calculated fa?om the angular d i s t r i b u t i o n function N(^) = B(l>A cos"*/), which i s v a l i d f o r 5 0 kev. deuterons, using A = 0 . 4 5 (HUHT00N, 1 9 4 0 ) . In this r e l a t i o n , f& i s measured i n centre-of-mass co-ordinates.  The  angle •& i n the laboratory system i s related t o ^ b y the expression sin  $  tan •tf.cos 0 4- ST  (SCRTFF, / 9¥  9)  For the present reaction, V = 0 . 0 5 0 4 , and at 45 degrees the correction i s 2 degrees, which was considered n e g l i g i b l e . F i n a l l y , adjustment was made f o r the number of target atoms exposed to each of the three neutron groups, i . e . f o r the f r a c t i o n of the sensitive volume included i n each of the angular regions. The f i n a l value obtained f o r the average neutron energy was 2.68 t 0 . 0 7 Mev.  The error on t h i s figure makes allowance f o r  s l i g h t variations i n deuteron energy, c h i e f l y due to straggling i n the heavy i c e target.  7.  Pulse Amplification and Measurement A wide band, low noise head amplifier with an  approximate gain of 100 was mounted d i r e c t l y on the chamber. VIII gives the c i r c u i t diagram.  Plate  Gain of the f i r s t tube was about 1 0 .  The tube i t s e l f was selected as having the lowest noise of those available.  The input capacity of the c o l l e c t o r and f i r s t g r i d was  measured by observing the reduction i n height of standard pulses when  ten a known capacity was connected i n p a r a l l e l and was found to b e y y t . f a r a d s . The plate voltage was obtained from a Lambda Model 28 regulated supply,  19. and s t a b i l i z e d DC was used f o r the filaments of the f i r s t  tube and  the ring-of-three. The output pulses were fed from the cathode follower through 120 feet of co-axial cable to a Northern E l e c t r i c Type 1444 Linear Amplifier.  This a m p l i f i e r has a gain of about 10 , and gain  s t a b i l i t y better than 1$ over long operating periods.  Provision i s  included f o r up to 33 db. attenuation, variable top and bottom cuts, and variable duration of output pulses.  Eor optimum signal-to-noise  r a t i o , the amplifier was operated with both top and bottom cuts set at 5/**-seconds. The amplified pulses were fed to an 18 channel Marconi Type  155-935  design).  pulse-amplitude analyser, or "kicksorter", (CHALK EIVEE  Amplitude s t a b i l i t y of the kicksorter discriminators i s stated  as approaching 0 . 0 2 v o l t s under the e x i s t i n g operating conditions.  The  k i c k s o r t e r has a maximum input voltage of 40 v o l t s , and incorporates a s t a b i l i z e d a m p l i f i e r with a gain of 5 , so that maximum input to the discriminator i s 200 v o l t s . The k i c k s o r t e r channels were set up using pulses from a Standard Pulse Generator.  The amplitude of these pulses was stable to  about 0 . 0 1 v o l t s over long periods.  These pulses were fed through the  k i c k s o r t e r amplifier when adjusting the channels. The minimum p r a c t i c a l channel-width was found to be 0 . 2 V i n terms of the Standard Pulse Generator.amplitude.  This channel width  corresponded to about 0 . 0 4 5 Mev. d i s i n t e g r a t i o n energy, which provided adequate  resolution f o r the present experiment.  P L A T E  P U L S E -  H E I G H T  I X  D I S T R I B U T I O N  lOOOl F R O M  P O L O N I U M  A L P H A S  NO. O F COUNTS  SOO  5.25 DISINTEGRATION  ENERGY ,  MEV.  20.  8.  Energy Calibration A series of i o n i z i n g events of the same energy i n  the chamber should, within s t a t i s t i c a l l i m i t s , produce voltage pulses of  the same amplitude.  Moreover, the pulse amplitude  depend l i n e a r l y on the d i s i n t e g r a t i o n energy.  i s expected to  The energy scale was  JL/o calibrated by a Po to  source i n the chamber.  This source was attached  the inner surface of the high voltage plate, and covered with a  t h i n aluminum collimator. The alphas from Po and an air-range of 3.842 cms.  energy,  This corresponds to a range of about  1 cm. i n 6 l / 4 atmospheres of neon. ion  have 5«3 Mev.  Since i t requires 29*3 ev. per  p a i r i n neon (STETTER, 1943), such p a r t i c l e s should produce about  290 *volt pulses on the l O O ^ i f a r a d s capacity of the c o l l e c t o r and  first  /  grid.. The signal-to-noise ratio f o r those polonium alphas was about 20:1.  A l l polonium alphas are stopped i n the sensitive volume, and  none reach the unshielded region between the grid and the c o l l e c t o r . Plate IX shows a t y p i c a l d i s t r i b u t i o n of pulses from the polonium alphas. 1.3$  The half-width of this d i s t r i b u t i o n i s 68 kev., or  of the t o t a l energy. Ionization produced by disintegrations i n the f i l l i n g  gas  d i f f e r s somewhat from that produced by alpha p a r t i c l e s from radioactive sources i n the chamber.  In the case of a neon d i s i n t e g r a t i o n ,  the reacion energy i s shared between an alpha p a r t i c l e and a r e c o i l i n g oxygen ion, both of which w i l l produce i o n i z a t i o n i n the f i l l i n g  gas.  For the alpha p a r t i c l e s , the r e l a t i o n between i o n i z a t i o n and energy i s l i n e a r down to low energies (region of 100 kev.) a f t e r which the proportionality may break down, due to electron attachment and i n e l a s t i c non-ionizing c o l l i s i o n s .  The variations i n t o t a l i o n i z a t i o n  produced w i l l however be rather small, so that the determination of alpha p a r t i c l e energies i s quite accurate.  21.  In the case of the larger fragments, the mechanism of i o n i z a t i o n i s not c l e a r l y understood, though some investigations have "been made on r e c o i l p a r t i c l e s i n cloud chambers; (see WBENSHALL, 1940)  and discussion and references given by WILKINSON  (1950)).  Two  points must be considered with regard to energy determination f o r such particles.  F i r s t , the energy loss may be less uniform than f o r alpha  p a r t i c l e s , so that the amount of i o n i z a t i o n released by single energy events i s somewhat more random than f o r alphas.  In this case, the r e s u l t  would be a spreading of the pulse-height d i s t r i b u t i o n . Secondly, the energy c a l i b r a t i o n i s based on the energy loss of Po  alphas i n neon, i . e . 2 9 . 3 ev. per i o n p a i r .  If the energy loss  per ion p a i r f o r oxygen ions has some other value, the apparent d i s i n t egration energy w i l l be i n error. The l i n e a r i t y of the detecting system was tested by the following method.  Pulses from the Standard Pulse Generator were  attenuated and fed into the head amplifier through the g r i d - c o l l e c t o r capacity.  A f t e r amplification, they were displayed on the kicksorter,  where they appeared i n not more than two channels.  Pulses of the same  amplitude, without attenuation, were then fed d i r e c t l y into the kicksorter amplifier, with the same discriminator settings. was carried out f o r a range of pulse amplitudes.  This procedure  Comparison of the  two sets of kicksorter readings showed the system to be l i n e a r to within 1 $ .  9.  Experimental Procedure Except when shut-downs of several days duration  were anticipated, a l l e l e c t r o n i c equipment, with the exception of the  22. c o l l e c t i n g voltage on the chamber, ran continually, thus maintaining, the greatest s t a b i l i t y . The polonium alphas were observed f o r at least an hour at the beginning of each run.  A fresh target of heavy ice was then  prepared, and the kicksorters set to cover the desired part of the energy spectrum. peak was  At the end of the run a second polonium alpha-  taken to ensure that the gain of the amplifiers, and conditions  within the chamber were unchanged. The calcium p u r i f i e r was heated f o r several hours at intervals of a few weeks.  The f i l l i n g gas was thus kept free of contamination, such  as occluded gases from the walls of the chamber.  P L A T E  X  2000-, C O M P L E T E  E N E R G Y  S P E C T R U M  NO. O F COUNTS  IOOO  2.0  2.5  DISINTEGRATION  3.0 ENERGY,  3.5 MEV.  4.0  4.5  23. >  III. A.  RESULTS AMD DISCUSSION  EXPERIMENTAL RESULTS Plate X shows the complete energy spectrum.  The spectrum  was examined i n sections, and i n moving the kicksorter from one section to another, an overlap region of s i x channels was allowed. The data shown i n Plate X was taken from a number of runs, normalized to approximately the same-average-number of counts per-channel i n the overlap regions.  It would have been preferable to normalize  on the basis of integrated neutron f l u x , but a neutron monitor was not available at the time this data was  collected.  The p o s i t i o n of the large peak corresponds to a d i s i n t e g ration energy of 1.91 * 0.01 Mev.  Examination of the low energy  side of this peak showed no evidence of further structure, down to the onset of e l e c t r o n i c noise at about 450 kev. The background over t h i s region i s due mainly to scattered neutrons from the walls of the laboratory and massive objects i n the v i c i n i t y of the ion chamber.  Neon r e c o i l s from neutrons scattered  i n the chamber have a maximum energy of about 490 kev.  Some c o n t r i -  bution may also come from disintegrations i n the unshielded region between the g r i d and c o l l e c t o r of the chamber. Investigation of the high energy side of the main peak disclosed a second peak of about 100 times less i n t e n s i t y .  This regio  of the spectrum i s shown in greater d e t a i l i n Plate XI.  P o s i t i o n of  this peak corresponds to a d i s i n t e g r a t i o n energy of 3.16  ± 0.03  Mev.  24.  No other structure was observed at energies below those of the polonium alphas, and no pulses of energies greater than these were observed. 33.  DISCUSSION 1.  Threshold Energies f o r 3Jast Neutron D i s i n t e g r a t i o n  of Neon The gridded ion chamber might be expected to detect any (noc) and (n p) reactions a r i s i n g from the d i s i n t e g r a t i o n of neon nuclei.  The following disintegrations must then be considered: Isotope  Reactions  Calculated Qr-Yalue  (n ot )  - 0.54  Mev.  (n p )  - 6.14  »  (n «. )  + 0.71  Ne  Ne*' (up) (n * )  - 5.06  Ne (n p ) These Q-values have been calculated from the mass tables of MATTAUCH and 3TMMMERS3TELD (1949).  Such values are only approx-  imate, due to uncertainties i n the masses, but are u s e f u l i n estimating the thresholds of reactions.  25-  2.  Interpretation of the Main Peak There i s no question "but that the main peak i s  due to the reaction Ne'^Cn*) 0 " \ Q ; - 0 . 7 ? ± 0 . 0 8 Mev.  From the measured energy release,  This reaction has previously been reported as'  follows: (a)  GRAVES and COON ( 1 9 4 6 ) , using an ion chamber, and ,  photographing pulses displayed on an oscilloscope, obtained a value of Q,= - 0 . 6 Mev., and estimated the cross section to be about 5 barns.  No information v?as given on the number of events  railli-  recorded.  Their Q-value was l a t e r revised to - 0 . 8 to - 0 . 8 5 Mev. i n a p r i v a t e communication to JOHNSON et a l (see below). (b)  SIKKEMA ( 1 9 5 0 ) used a large proportional counter  filled  with 2 atmospheres of neon, and recorded pulses on a photographic strip.  The reaction energy was observed as - 0 . 6 Mev.  The published  curves show peaks 200 counts or less high, with a background as high as 75 counts. (c)  JOHNSON, B0CKELMAN and BARSCHALL ( 1 9 5 1 ) used a pro-  p o r t i o n a l counter containing 3 0 atmospheres of neon, and operating at a gas amplification of about 5 -  They obtained the values Q=-0.75  t 0 . 0 5 Mev. and cross section about 20 m i l l i b a r n s .  The published  curves show peak about 100 counts high. It i s of interest to r e c a l l the considerations of WILKINSON ( 1 9 5 0 ) mentioned previously, regarding the use of pure noble gases i n proportional counters.  The formation of metastable states  i n these gases, and the emission of photo-electrons from the cathode of the counter may produce an objectionably high background.  Such an  26. effect was observed by SIKKEMA, though he attributed i t to hydrogen or helium contamination i n the ion chamber. In the present experiment, the 0„-value' was obtained with 1% counting s t a t i s t i c s , much better than previously reported. The mass spectrograph data of EWALD (1951)  gives the  isotopic masses: 0 = 17.004,507*0.000,015 /r  Ne' = 19.998,771*0.000,012 17  The 0 mass i s f a i r l y w e l l agreed upon, and from the above value, and the observed Q,-value i n the present experiment, the.mass of Ne " i s calculated-to be 19.998,628 ± 0.000V845. -  BUECBHER et a l (1949) have observed the protons from 0 (dp)0  i n a magneticspectrometer and measured an excited state  of o' at 0.876 * 0.009 Mev. 7  The gamma ray from t h i s l e v e l ; to ground  has been reported by ALB0EGER (1949).  POLLARD and ^DAVIDSON (1947)  have observed the same l e v e l through the reaction N («p)0  , and  i n addition a second particle-groug, which may indicate a second l e v e l at 1.6 Mev.  ,;  However, i t i s possible that the p a r t i c l e s observed  were deuterons from N (<*d)0  .  It i s generally assumed that the grounds-state of Ne has zero spin and even p a r i t y , (HORNYAK , 1950).  BUTLER, (1950) i n an  analysis of the work of BURROWS et a l (1950) on the angular d i s t r i b u t i o n H .  .  17  of protons from 0 (dp}0 5/2  /7  concludes that the ground-state of 0 i s  or 3/2., even, and that the f i r s t excited l e v e l i s 1/2 , even.  The spin value of 5/2 f o r the ground state has been confirmed by the nuclear resonance experiments of ALDER and YJJ, (1951) , and by GESCHWTND et a l , (1952) from microwave spectroscopy. Assigning these values of spin and parity, the ground-state t r a n s i t i o n of Ne (n<* }0  may be viewed as follows:  27. JO  Spin: Parity:  i  0  +  Ne  n  Ne  5/2  0  g  even  even  Angular momentum:  0  1 =-0 1= 1  0  ^ (even)  1  ' | (odd) .3/2 (odd)  1 =2  1  3/2 (even).  = 2.  5/2 (even).  1 = 3>  f5/2 (odd) [7/2 (odd)  The arrows indicate the possible t r a n s i t i o n s f o r neutrons up to 1 = 3  and alpha p a r t i c l e s up to 1 = 4 . In view of the cross  section of the reaction, i t seems u n l i k e l y that l a r g e r 1-values need be considered. 17  The t r a n s i t i o n to the excited l e v e l i n 0  , which was not  observed with the 2.68 Mev. neutrons, has the p o s s i b i l i t i e s : 17* JO Ne ii* Ne n Spin: Parity:  i  0  0  z  3  even  even  Angular momentum:  1-0 1=1  * -a (even)  i - (odd) (.3/2 (odd)  1=2  (3/2 (even)  1  0  * 1  1  ^ 1 -2  [5/2 (even) 1 =3  '5/2 (odd)  -^1 = 3  (7/2 (odd)Thus, assuring up to F-wave neutrons,  (1 = 3) , leads to SI*  seven possible states f o r the compound nucleus, Ne the ground state t r a n s i t i o n i s forbidden f o r the states.  . O f these the even and -J- odd  28. •It was  hoped that the lack of excited l e v e l s of 0  in  the present experiment would give some i n d i c a t i o n as to which state of Ne  appeared.  However, the p e n e t r a b i l i t y of the  p o t e n t i a l b a r r i e r f o r the alpha p a r t i c l e s must also be taken into account. Calculation of the p e n e t r a b i l i t y §  indicates that  the  i n t e n s i t y of the excited-state t r a n s i t i o n would be approximately 10  times that of the ground-state t r a n s i t i o n . A reaction of  such low i n t e n s i t y would not have been detected. Therefore, the non-observance of excited l e v e l s of 0 regarding  y i e l d s no  information  the state of the compound nucleus, Ne  3. . Interpretation of the Minor Peak No workers.  such subsidiary peak has been reported by other  This i s not surprizing i n view of i t s low i n t e n s i t y ,  and the s t a t i s t i c s which they obtained on the  Ne  ln« )0  reaction. There are three possible causes f o r t h i s peak: (a) A group of neutrons of higher energy than the main group from ,  H (dn)He  . a/,  ,  '  lb) A charged-particle  reaction i n Ne  or Ne  (c) A charged-particle  reaction i n some impurity i n the  filling  gas. The p o s s i b i l i t y of higher energy neutrons has been previously  # see f o r example BETHE, H. A.  R. M. P.  9.-164--1937  £9-  discussed, and  shown to "be most u n l i k e l y .  The only energetically possible reaction i n We */. for  the neutrons used i s Ne  ,  (n * ) 0  or Ne  /%  .  For this reaction, the  observed energy release gives Q, - *0.48 * 0.10.  EWALD ( 1 9 5 1 )  gives  the mass values: o r  18.004,875 * 0.000,013  He's 21.000,393* 0.000,022 ' The above Q,-value corresponds to a mass of Ne  - 21.000,339 * 0.000,068  Using the cross section of JOHNS Oil et a l ( 1 9 5 1 ) of 2 0 m i l l i b a r n s for  Ne  (n«)  0  , the i n t e n s i t y of t h i s peak would indicate a cross  section of about 65 millibarns f o r Ne The  (n«0  0  l i m i t s of inrpurity f o r the neon were stated as 0.2$  and 0.02$ nitrogen, while nitrogen, oxygen and water vapour may been present in' the chamber p r i o r to f i l l i n g .  helium  have  The e n e r g e t i c a l l y  possible reactions f o r these elements are /'(np  l/"(n 0"  «  (n «  ) 0 "*  QrfO.63  Mev.  ) B "  q - - 0.28  "  ) C  Qr - 2.31  /S  o " (n « ) c " The f i r s t has 1950;  •  +1.73  11  "  of these reactions i s well known, and  been c a r e f u l l y established as + 0 . 6 3 0 * 0 . 0 0 6 Mev. H0RNYAK et a l 1950) .  T  i t s Q,-value  (FRANZEN et a l ,  h i s value l i e s close to that of the  minor peak, although outside the  probable  error, which i s believed to  be generous.  Por 2 . 8 Mev. neutrons, the cross section f o r N  i s 4 0 millibarns (BALDINGER and RUBER, 1 9 3 9 ) .  (n p )C  h i s would indicate  T  more than 0 . 4 $ nitrogen i n the chamber, though such an estimate i s very approximate, as this cross section may not hold f o r 2 . 6 8 Mev. neutrons, 0.1$  ^his amount of nitrogen should be accompanied  oxygen.  WILSON et a l ( 1 9 5 0 ) have stated that 5 parts per m i l l i o n  of oxygen i n 6 atmospheres entirely.  by about  of deuterium w i l l prevent electron c o l l e c t i o n  It would be expected therefore that 0 . 1 $ oxygen would have  noticeable effects i n 6-£ atmospheres  of neon.  The calcium p u r i f i e r  i s expected to remove both nitrogen and oxygen. If nitrogen were present i n the chamber, the reaction N should also be observed.  (n «)B  The cross section i s 160 millibarns f o r 2 . 8  Mev. neutron, and so a second small peak should be observed at a d i s integration energy corresponding to Q, -  - 0 . 2 8 Mev.  Some i r r e g u l a r i t i e s  have been noted i n this region of the spectrum, but at the time of writing, s u f f i c i e n t data was not available to determine whether such a peak e x i s t s . 4.  Summary of Results This table shows the Q-values f o r Ne ( n * ) 0  from various experimental investigations, and from calculations based on mass spectrograph data. Q,-values GRAVES and COON ( 1 9 4 6 )  - 0 . 8 0 to.-0.85.  SIKKEMA ( 1 9 5 0 )  -0.6  Mev. "  JOHNSON et a l ( 1 9 5 1 )  -0.75  Present experiment  -0.77  0.08 "  MATTAUCH and ELAMMERSPELD ( 1 9 4 9 )  -0.54 ±  0.12 "  EWALD ( 1 9 5 D  -0.64±  0.05 "  i  0.Q5 "  Mass Spectrograph  31. C. FURTHER INVESTIGATIONS The extension of t h i s i n v e s t i g a t i o n i s now being a c t i v e l y pursued i n the f o l l o w i n g ways: (a) Reaction energies are t o be determined more e x a c t l y . by reducing the error on the neutron energies.  This w i l l be done  by moving the i o n chamber away from, the t a r g e t , thus reducing the s o l i d angle subtended by the s e n s i t i v e volume, and so decreasing the  angular v a r i a t i o n of neutron energy. (b) The region of the spectrum corresponding t o a r e a c t i o n  energy of -0.28 Mev. i s to be c a r e f u l l y re-examined. I f the presence of a second small peak i s e s t a b l i s h e d , i t would seem to i n d i c a t e that both small peaks are due to nitrogen i n the i o n chamber.  T h i s would  however r a i s e two i n t e r e s t i n g p o i n t s . F i r s t , how could t h i s amount of nitrogen appear i n the i o n chamber, and remain i n s p i t e of the calcium purifier?  Second, i f the minor peak i s due to N (np)C , why i s the  r e a c t i o n energy not c l o s e r to the accepted value?  '  (c) I n v e s t i g a t i o n s are to be c a r r i e d out w i t h thermal neutrons, •t  3  by moderating the H (dn)He  neutrons wifch p a r a f f i n .  T h i s should  eliminate the main peak, and move the minor peak to the low-energy end of the spectrum, just above the noise.  Some increase i n the  i n t e n s i t y of t h i s peak would be expected, i f the r e a c t i o n cross s e c t i o n f o l l o w s the l / v law.  The s h i f t i n the p o s i t i o n of t h i s peak  w i l l give an a d d i t i o n a l check on the l i n e a r i t y of the a m p l i f i e r system. (dj Some i n i t i a l attempts to thermalize the neutron f l u x 1  were not completely successful i n that the main peak s t i l l appeared, though w i t h much lower i n t e n s i t y . I f the moderation cannot be improved,  si  //  i t i s proposed to use neutrons from the V (pn)Cr r e a c t i o n , which are 3 0 / 7 below the threshold f o r Ne (n<*)0 • (e) The r e a c t i o n s w i l l be examined at higher neutron energies, u s i n g neutrons from H (dn)He  and H (dn)He  , w i t h deutrons accelerated  by the UBC Van de G-raaff Generator, which at present i s operating at energies up to 2 Mev.  32..  (f) I t would be of i n t e r e s t to introduce some 2% of nitrogen i n the chamber and repeat the observations. For a l l these experiments, a neutron monitor i s d e s i r a b l e . A neutron counter, of the type designed by HANSON and McKIBBEN (1947) i s at present under construction i n t h i s laboratory.  33.  IV.  CONCLUSIONS  The Q-value of the reaction Ne (n°<-)0 as -0^77*0.08 Mev.  has been measured  The 1% counting s t a t i s t i c s obtained are  considerably better than those reported i n previous investigations of t h i s reaction. Careful search of the energy spectrum gave no evidence n 'Jf o r the appearance of excited states of 0 , using 2.68 Mev. neutrons. At least one such s t a t e , i s known, (0.87 Mev.) .' Calculation of the p e n e t r a b i l i t y of the p o t e n t i a l b a r r i e r f o r the alpha p a r t i c l e s corresponding to t h i s l e v e l indicates an i n t e n s i t y approximately 10 times that of the ground-state t r a n s i t i o n , at t h i s neutron energy. A reaction of such low i n t e n s i t y would not have been detected i n the present experiment. A subsidiary peak was observed at a reaction energy corresponding to a Q,-value of +0.48*0.10 Mev. -2/.  .  There i s some  IS  evidence that the reaction is' Ne ( n ^ j O , but the reaction N (np)C may also be responsible, i f nitrogen is" present i n the ion chamber. Further i n v e s t i g a t i o n i s necessary to c l a r i f y t h i s point.  34. V.  BIBLIOGRAPHY  Alburger, D. S.  P. R. 75-51—1949  Alder, F. ; Yu, F. C.  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R. 77-641—1950  Atomic N u c l e i and'Nuclear Transformations, Oxford U. Press,1937  Geschwind,' S. ; Gunther-Mohr, G. R. ; Silvey,' G. Goldsmith, H. H. ; Ibser, H.  ; F e l d , B. T.  P. R. 85-474—1952 R. M. P. 19-259—1947  Graves, E. R. ; Coon, J . H.  P. R. 70-101—1946 '  Hanson, A.TO. ; McKibbon, J . L.' '  '  Hanson, A. 0. ; Taschek, R. F. ; W i l l i a m s , J . H. Harkins, W. D. ; Gans, D. M. ; Newson, H. W. Heydenburg, N. P. ; I n g l i s , D. R..  P. R. 72-675—1947 R. M. P. 21-655—1949 '  P. R. 44-529—1955 P. R. 75-250—1948  Hornyak, W. : L a u r i t s e n , T. ; Morrison, P. ; Fowler, W. R. M. P. 22-291—1950 Hunter, G. T. ; Richards, H. T. P. R. 76-1445—1949 Huntoon, R. D. : E l l e t t , A. : Bayley, D. S. ; Yan A l l e n , J . A. P. R. -58-97—1940 Johnson, C. H. ; Bockelman, C. K. ; B a r s c h a l l , H. H. K i r k a l d y , J . S.  P. R. 82-117—1951  M. A. Sc. Thesis, U. B. C., 1951  Ladenburg, R. ; Kanner, M. H. ' L e i t e r , H. A. ; Rodgers, F. A. ; Kruger, P. G.  P. R. 52-911—1937 P. R.' 78-663—1950  35. Manning, H. P. ; Huntoon, R. D. ; Myers, F. ; Young, V. Mattauch, J . ; Flammersfeld, A.  P. R. 61-371- 1942  Isotopic'Report, Tubingen, 1949  Mayer, M. G.  P. R. 74-235— •1948  Newson, H. W.  P. R. 48-790- •1935  Oliphant, M. L. E. ; Rutherford, E.  Proc. Roy. Soc. A141-259— •1933  O l i pliant-, M. L. E. ; Harteck, L. ; Rutherford, E. Proc. Roy. Soc. A144-692— •1934 Can. J . Res'. 27-143- •1949  Pepper, T. P. Pollard,E. ; Davidson, P. W. Reimann, A. L. Roberts, R. B. S c h i f f , L. I. Sikkema, C. P. Stetter, G. Tollestrup, A. V. Welles, S. B. Wilkinson, D. H.  P. R. 72-756- •1947 P h i l . Mag. (7) 18-1117- -1934 P. R. 51-810- •1937 Quantum Mechanics, McGraw-Hill, 1949 '" Nature 165-1016--1950 Z e i t s . Phys. 120-659— 1943  Jenkins, F. A. ; Fowler, W. A. ; Lauritsen, C. C. P. R. 76-181--1949 P. R. 69-586—1946  Ionization Chambers and Counters, Cambridge TJ. Press, 1950 Wilson, R. ; Beghian, L. : C o l l i e , C. H. ; Halban, H. : Bishop, G. R. R. S. I". 21-699- 1950 Woods, S. B.  Ph. D. Thesis, U. B. C., 1952  

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