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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 Gilbert James P h i l l i p s A thesis submitted i n p a r t i a l fulfilment of the requirements for the degree of . . MASTER OF ARTS i n the Department ' of PHYSICS We accept this thesis 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 0.28$, Ne 9.21%, has been bombarded with 2.68 Mev. neutrons from the reaction H (dn)He . A gridded ion chamber, with 1 l i t r e sensitive volume, f i l l e d with neon to 6^ atmospheres pressure, was used to detect disintegrations. Pulses from the ion chamber were amplified, arid recorded on an 18-channel kicksorter. The ground-state t r a n s i t i o n of Ne (n«i)0 was observed, with 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 with previously reported r e s u l t s . Careful search f a i l e d to reveal any evidence . 17 of excited states i n 0 . A l e v e l at 0.87 Mev. i s known,. and one at 1.6 Mev. i s suspected, but calculation of b a r r i e r penetrability f o r alpha p a r t i c l e s corresponding to these levels indicates that they are not l i k e l y to be observed at the neutron energy used. A second reaction, with disintegration energy corresponding to a Q,-value of +0.48*0.10 Mev. was also observed. I t was not clear whether t h i s reaction was N ^ n p j C ^ o r Ne^U^Oo'* . This point i s to be c l a r i f i e d by further investigation. ACKNOWLED&EMEHTS The author i s pleased to express his thanks to Dr. J. B. Warren for 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 this thesis i s deeply appreciated. Thanks i s also due to Dr. S. B. Woods f o r the construction of portions of the apparatus, and f o r helpfu l suggestions. INDEX I. INTRODUCTION A. Knowledge of Nuclei 1 33. (n ) Reactions 2 C. Choice of Experiment 3 I I . EXPERIMENTAL METHOD A. The Experimental Problem 4 33. Production of Past Neutrons 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 Radiation from Neutron Source 9 C. Measurement of Alpha-Particle Energies 10 1. Choice of Detectors 10 2. The Gridded Ion Chamber 11 3 . Description 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 Amplification and Measurement 18 8. Energy Calibration 20 9 . Experimental Procedure 21 I I I . RESULTS AND DISCUSSION 23 A. Experimental Results 23 33. Discussion 2^ 1. Threshold Energies for Fast Eeutron Disintegration of Neon 24i= 2. Interpretation of the Main Peak 25 3 . Interpretation of the Minor Peak 28 l+. Summary of Results 30 C. 3Turther Investigations 31 IV. CONCLUSIONS 33 V. BIBLIOGRAPHY 3k ILLUSTRATIONS Plate 4 3 Facing Page I. H (dn)He Cross Section vs. Deuteron Energy 5 I I . H* (dn)He* Thick Target Yield Function 6 I I I . 4 3 H (dn)He Neutron Energy vs. Angle of Emission 7 IV. Heavy Ice Target 8 V. Pulse Formation i n Ungridded Ion Chamber 12 VI. Gridded Ion Chamber 13 VII. Electrode System f o r Gridded Ion Chamber 14 VIII. Wide-33and Low Noise Head Amplifier IS IX. 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: Details 24 1. THE PI SI MSG-RAT I ON OF NEON BY FAST NEUTBOMS I. INTRODUCTION A. KNOWLEDGE OF NUCLEI It is much easier to speak with optimism regarding the future of Nuclear Physics than to discuss with satisfaction i t s present status. While the general principles and methods of quantum mechanics appear.to he satis-factory for dealing with nuclear phenomena, yet the most f r u i t f u l approach, particularly in dealing with nuclear reactions and energy levels, i s essen-t i a l l y a phenomenological one. The main d i f f i c u l t i e s in attempting to formulate a theory of nuclei, comparable in 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 mathemat-i c a l d i f f i c u l t y of dealing with such a many body problem accurately is s t i l l formidable. .-'In this case even approximation methods make l i t t l e headway—in the words.of Gamow., "the theory of complex nuclei is d i f f i c u l t mathematically because everything is of the same order of magnitude, and nothing can be neg-lected. " (GAMOW, 1937) The problem has then been approached experimentally by collecting as much data as possible on a l l aspects of nuclear structure: excitation functions, energy levels, spin values, and phenomenological theory, such as the dispersion formula, deduced to f i t the observed results. Attempts to progress further have resulted in the proposal of various nuclear models. One example is the "alpha particle model", which pictures the nucleons as being grouped within the nucleus as alpha particles, 2. the alpha i n turn forming shells, with extra nucleons i n outer orbits. Such a picture suggests exceptional stability for closed, shells of alpha particles, as i n C and 0 . It was hoped that some of this model's pre-dictions for nuclei of the 4N + n type might "be tested in the present exper-iment as one such nucleus, namely 0 (4<*+n ) was a known disintegration; product in the reaction T$e*° (xi* ) 0 1 7 The alpha particle model is "by no means the only model postulated, and recently the "orbit model" has been extensively and successfully developed by MAYER, (1948). However, with light nuclei the alpha particle model has had some success i n correlating experimental data. It is 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 particles. For neutron bombardment, capture adds about 8 Mev. to the nucleus, this energy being distributed among the nucleons. The compound nucleus so formed remains in this excited state u n t i l random fluctuations concentrate sufficient energy i n one or more nucleons for their emission. Photon emission may also occur, but the levels for this process are sharper than for particle emission, and the latter process is much more probable, i f energetically possible. The most favoured transition i s that which leaves the residual nucleus in the most stable configuration. Generally, neutron emission requires somewhat less excitation energy than proton or alpha emission, because of the Coulomb barrier for the charged particles. Alpha emission, however, commonly occurs for the light nuclei, in which the Coulomb barrier is relatively low, while the 3 . average energy per nucleon in the compound nucleus is high, due to the sharing of excitation energy between relatively few particles. C. CHOICE OF EXPERIMENT Much work has been done with fast neutrons, and the disintegration of certain nuclei, in particular those of carbon and nitrogen by such particles is well known. Neon however, has received l i t t l e attention. Previous experiments, using neutrons of less than 3 Mev. energy, showed with rather poor statistics only one single-energy group of emitted alphas. This group was identified with the ground state transition of Ie*°(n<t) o ' r . No evidence of excited states has been reported, but since a level i n o'at 0 . 8 7 Mev. is known, and another at 1 . 6 Mev. suspected, further investigation seemed worthwhile. Neon is readily obtainable in pure form, and like the other noble gases is suited for use in an ion chamber (WILKINSON, 1 9 5 0 ) . With an 18 channel pulse amplitude analyser available in this laboratory, i t was expected to obtain data; more rapidly•and hence more precisely than heretofore. It was decided therefore to bombard neon with fast neutrons, using an ion chamber as a detector. The Devalue of the Ne (n«) 0 reaction was to be carefully measured, and a thorough search made for other groups of particles, particularly any associated with excited states of o ' ' . The remainder of this thesis discusses the details of this experiment,- and the results obtained. 4. II. EXPERIMENTAL METHOD A. THE EXPERIMENTAL PROBLEM Since neon like the other noble gases has excellent electron collection properties, i t seemed obvious to detect the reaction directly by ionization .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 linearity between pulse-size and energy. Three sources of fast neutrons were available for the experiment: a 5 0 millicurie Radium-Beryllium source, a 50 kev. ion 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 yield of neutrons, but the straggling of alphas within the source produces a very broad spectrum of neutron energies. Neutrons can be generated in both the 5 0 kev. accelerator and the Van de G-raaff by means of a nuclear reaction, such as H*(dn) He 3 or H 3 (dn ) He * . Since an intense, monoenergetic source was required, i t was decided to start by using the H (dn) He reaction with the 50 kev. ion accelerator and a thick target of heavy ice. B. PRODUCTION 03? EAST NEUTRONS 1. Fast Neutron Sources Many nuclear reactions may be used as neutron sources. In particular, bombardment of light nuclei with protons or deuterons produces neutrons of a wide range of energies. The choice of a reaction depends on the yield, on the number of neutron energy groups produced, and on the energies required. A few of the better known fast neutron sources are: (HANSON et a l 1949; HORNYAK et a l , 1 9 5 0 ) . Plate I 10 /o 10 or z cms. JO D(dn)f He ; Cros s Section {cy) vs. D euteron Er lergy (Ed; • \ T • > ' /o •It -J9 /O o.S AO Ed ; Mev. JO 5 . Reaction quvalue Threshold Ranee of Neutron Energies # H* (dn) He3 + 3.256 Mev. < 0 . 0 1 5 Mev. 1 .8 -> 7 . 2 Mev. H 3 (pn) He 3 - 0.764 » 0 . 9 8 " 0.001-^4. " E3 (dn) He * +17.60 " ,< 0 . 0 1 5 " 12. - > 2 0 . » li'(pn) Be" - 1.647 " 1.882 « 0.001 3 . " . V*'(pn) C r 4 7 - 1 . 5 0 1.53 " 0.002 -> 0.020 Mev. # for bombarding energies up to about 4 Mev. As mentioned, H (dn) He neutrons generated "by the 5 0 Kev. ion accelerator were chosen for the i n i t i a l investigation. It was hoped to extend the neutron energy up and down from the 2.68 Mev. value by using this reaction with faster deuterons, the H (dn ) He reaction, and slower neutrons from the V (pn) Cr reaction. 2. The Ha(dn) He' Reaction Since i t was f i r s t reported by OLIPHANT,'HARTECK and ' RUTHERFORD, ( 1 9 3 4 ) , this reaction has received the attention of many workers. The literature contains extensive information on the yield function (ROBERTS, 1937; AMALDL et a l , 1937) the cross section (LADENBURG et a l 1937) and the angular distribution (BONIER, 1 9 3 7 ) , for 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 probability: U* (dn) He 3 Q* * 3 . 2 5 6 ± 0.018 Mev. H* (dp) H 3 + 4 . 0 3 6 ± 0 . 0 2 2 Mev. These 0-values are magnetic spectrometer determinations (TOLLESTRUP et a l , 1 9 4 9 ) . Exact relative yields of protons and neutrons have s t i l l not been established, but are known to be approximately equal. Plate II D2(dn)He3-, Thick Target Yield Function Total Yield (Y) vs. Deuteron Energy (Ed) oS to /.s 20 Ed ; Mev. 6. Curves of cross section and total yield for the reaction show-smooth increases with the energy of the incident deuterons. These are i l l -ustrated i n Plates I and II respectively, for deuteron energies up to 2 Mev. The former gives the integrated cross section, and the latter the total yield, over the whole solid angle, for a thick heavy ice target. These curves are "based on several investigations, as listed "by HORNYAK et a l , (1950) The angular distribution of neutrons i s anisotropic. For deuteron energies of less than 500 kev., i t is adequately described by a relation of the form N (?) • B ( 1*A cos*^ ) i n the centre of mass system. (MANNING et a l , 1 9 4 2 ) . At higher deuteron energies, more neutrons are observed in the forward direction than this relation predicts, and terms in higher powers of cos*^ must be added. The more extensive investigations of this aspect of the reaction include those of BENNETT et a l , (1946) for deuterons of up to 1.8 Mev., HUNTER and RICHARDS, (1949), 0.5 to 3.7 Mev. deuterons, and ERICKSON et a l (1949) for 10 Mev. deuterons. In the present experiment, the angular distribution was calculated using the value of A = 0.45 for 50 kev. deuterons. (HUNT00N, 1940) The neutron energy varies with the angle of emission measured relative to the deuteron beam, and is also a function of the deuteron energy. This dependence may be calculated from the masses and energies of the particles involved. HANSON et a l (1949) have carried out these calculations for several of the reactions used as neutron sources, for a wide range of deuteron energies. Plate III shows the results of their calculations for deuterons up to 2 Mev. It is 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 truly homogeneous depends on several factors. Variation in energy of the incident deuterons, and % note especially BRETSCHER et al(l948)for low energies. Plate III En Mev. . J E n M e v frf? 0° * 3 D(dn)He Neutron Energy (En) vs. Angle of Emiss ion {^) straggling of deuterons in thick targets w i l l produce a spread i n neutron energies. Unless the solid angle subtended by the detector is very small, a range of neutron energies w i l l be observed, due to the angular dependence of the reaction. Variations in deuteron energy may be controlled by magnetic analysis of the ion 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 is not well known. In the present experiment, the effect of this mechanism on the homogeneity of the neutron energy w i l l be small. In the region of deuteron energies used (50 kev.), the yield function of the reaction rises very rapidly. Therefore, for deuterons of even slightly less than average energy, the neutron yield drops very sharply. Thus relatively few low energy neutrons w i l l result from straggled deutrons. 3. The 50 kev. Ion Generator The accelerating voltage of the 50 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. Ripple voltage was stated to be much less than 1 Construction of the unit i s described elsewhere (KIRKALDY, 1 9 5 1 ) . The ion beam was produced by.a radio-frequency ion source, of a type developed i n this department. (KIRKALDY, 1951; WOODS, 1 9 5 2 ) . Energy spread in the discharge is less than 100 volts. Provision i s included for magnetic analysis of the ion beam. Deuterium gas was obtained from heavy Water in this laboratory. The heavy water ( 9 9 * $) was electrolysed i n a closed system, which included a cold trap and phosphorous pentoxide drier. The gas was passed into"the ion i P L A T E IV H E A V Y ICE T A R G E T 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. The target was biased to + 3 0 0 volts to prevent back electron current. Beams of 100 to 20Cyfc-amp. resolved H ions were obtained. h. Heavy Ice Target The f i r s t targets used consisted of heavy water adsorbed on Phosphorous Pentoxide but the neutron yields obtained were low. A heavy ice target, as shown in Plate IV was then constructed. The target was attached to the ion generator by a two inch'Pyrex pipe and flare f i t t i n g s . After/ evacuation, the inner chamber of the target was f i l l e d with liquid a i r . Heavy water vapour was then admitted to the body of the target via a needle valve and the copper tube, indicated in Plate IV. The vapour froze to the copper target-leaf, forming a layer of heavy ice. The 0-ring seal on the copper tube allowed i t to be moved out of the way of the beam after the target was formed. Formation of the target was f i r s t observed by looking into the target body through the Pyrex pipe. This view proved to be rather restricted, and the target was later modified by cutting a circular 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 effectively insulated the target from the ion generator, and allowed the beam current to be metered. It was found i n i t i a l l y that the interior of the glass pipe became charged, causing serious defoc-ussing of the beam and so a great reduction in the useful beam current. A sleeve of wire gauze on the interior of the pipe, connected to the target, overcame the d i f f i c u l t y . 9. The inner chamber of the target accomodated about 200 cc. of liquid a i r . With a normal beam of 150y(f.amp. on the target, i.e. 7«5 watts beam-power, this was sufficient for 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 ; f 1 0 deuterons per second, and from the yield curve for the reaction, (Plate II), the yield for 50 kev. deuteron energy is approximately 2/10 'neutrons per deuteron, so that the total yield was about 2 * 10 ' neutrons per second. \\ v, 5. Background Radiation from Neutron Source It was necessary to consider the possibility of other penetrating- radiations originating in the heavy ice target, i.e. other (dn) reactions. The light nuclei exposed to the deuteron beam were B' , E* , o " , 0/f , o'*from the heavy water, and traces of c'^and C / J f rom pump o i l contamination. The only reactions to be considered were: o'**(dn) J?"- Q . - - 1 . 6 Mev. (NEWS0N, 1935; REYDENBURG-, 1948) o"(dn) F " ft « +3.5 Mev. (WELLES, 1946) (isotopic abundance 0.04$) C M(dn) N / J Q» -0 .26 Mev. . (BONNER et a l 1949) (threshold 0.328 Mev.) »x ft The high thresholds eliminate the C - + 0 reactions, while both /a 17 C and 0 have too few atoms present to be significant. 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 steel walls of the ionization chamber. Additional protection was given by a l/4 " sheet of lead-wrapped around the chamber. There is expected to be a background of scattered neutrons from the concrete walls of the laboratory, and from such massive objects in the vicinity as the analysing magnet. Tests with a cadmium absorber would indicate whether thermal neutrons were producing any effects, such as (n* ) 10. reactions in 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 distributions. 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 in windows can be avoided by detecting the ionization of the products of the reaction i n the target gas i t s e l f . The Wilson Cloud Chamber has often been used for -such observations—the f i r s t records of Neon disintegrations were so obtained, (HARZINS, 1 9 3 3 ) . However, there are several drawbacks in using i t for accurate measurement of particle energies. Unless special high-S pressure chambers are used, there w i l l be relatively fev; target atoms, and correspondingly few disintegrations w i l l be observed. Fairly acc-urate energy determinations from cloud chamber photographs are poss-ible, but both the collection and analysis of the data are tedious. Proportional counters are capable of fast and accurate energy determinations. Most d i f f i c u l t i e s arise i n maintaining s t a b i l i t y of the device. For detecting energetic particles, the gas pressure must be high in order to confine the particle range to the gas volume. High voltages are then required to obtain gas-amplification. For consistant results, the gas-amplification must remain constant, and this requires well-stabilized high voltage. The amplification is also very sensitive to slight changes in 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 is f a i r l y high i n proportional 11. counters, because of their sensitivity to stray radiations, such as soft x-rays. Such an increase i n "noise" w i l l raise the useful lower limit of the device, making the detection of low energy events d i f f i c u l t . Finally, the pure noble gases are unsuited for use i n proportional counters, chiefly because of their tendency to form metastable states. Addition of a quenching gas is necessary to de-excite these states, and so prevent spurious pulses. (WILKINSON, 1950) Ionization chambers, and particularly gridded ion chambers, are well suited to these investigations.' They require very stable amplifiers, but modern circuits meet these demands. Ion chambers are less sensitive to variations i n collecting voltage than proportional counters, are much less sensitive to background radiations, and are relatively unaffected by slight changes in the composition of the f i l l i n g gas. The speed with which data may be collected depends on the recording equipment available. Frequently the voltage pulses are displayed on an oscilloscope and photographed. This mejbhod yields accurate results, but; the rate of collecting data i s usually limited by the photographic equipment, and analysis of the data is a bit tedious. Greatest speed is attained with a pulse-amplitude analyser, or "kicksorter". Such a device was available for the present experiment. 2. . The Gridded Ion Chamber Measurement of particle energies with an ion chamber requires that the ionization produced be proportional to the particle-energy, and that the voltage pulses from the chamber be pro-portional to this ionization. Events of a single energy may in practice P L A T E V P U L S E FORMATION IN UN6RIDDED ION CHAMBER P U L S E HEIGHT P U L S E D U R A T I O N , S E C O N D S 12. produce a distribution of pulse-heights. The spread of this dis-tribution depends on several factors—source thickness, i n the case of solid sources placed in the chamber, straggling of ionization, non-ionizing, excitation of the f i l l i n g gas, noise .in the associated electronic circuits, and the influence of positive ions on pulse-form-ation. In a gridded ion chamber, the effect of the positive, ions is eliminated. How this is achieved may be understood by considering pulse-formation in an ungridded chamber. We assume that electron-attachment does not occur i n the f i l l i n g gas, and that the collecting f i e l d is sufficient to prevent recombination of the ions. An ionizing event occurs between the electrodes, and the electrons are attracted to the positive collector. Electron collection -S requires 10 seconds or less, and results in a sharply rising voltage pulse on the collector (see Plate V). The positive ions, whose mobility is about 1000 times less than that of the electrons, do not move appreciably before electron collection is completed. While the positive ions are being collected, the potential of the collector continues to rise, and reaches-i t s maximum when a l l positive ions have been collected (at the negative electrode). The pulse then decays as the charge leaks away through the resistance and capacity of the electrode system. Hot only is such pulse-formation slow, but the height and duration of the portion resulting from the collection of positive ions depends on the orientation of the ions at the time of formation. In many experiments this w i l l be a f a i r l y random matter, and the resulting spread of the pulse-height distribution w i l l be correspondingly widened. An amplifier to reproduce such slow pulses requires a low low-frequency cut-off, thus introducing the additional problem of microphonics. L A T E VI GRIDDED ION C H A M B E R 1 8 2 0 - 2 0 8 H O L E S 8 HOLES SIDE S E C T I O N S C A L E - INCHES O I 2 13. The d i f f i c u l t y i s completely overcome "by placing a grid between the electrodes, near the collector. The grid shields the collector electrostatically from the f i e l d of the positive ions. When an ionizing event occurs, the collector potential remains con-stant u n t i l the electrons have diffused past the grid, then rises rapidly to i t s maximum value as electron collection occurs. The pulse then decays as the charge leaks from the electrode system. Much faster amplifiers may be used for such pulses, improving the signal-to-noise ratio. The faster pulses also allow shorter resolving time and much higher counting rates. Suitable choice of the grid voltage prevents electron collection by the grid. Some fra.ction of the ionizing events w i l l occur in the portion of the sensitive volume between the grid and the collector. This region w i l l behave as an ungridded chamber, with collection of positive ions at the grid. Such pulses w i l l have random heights, and w i l l cause some spreading of the distribution. For some events, only a portion of the ionization w i l l occur in the sensitive volume, the particles either striking 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 distribution of pulses from single energy ionizing events. The degree of asymmetry w i l l depend upon the range of the ionizing particles in the chamber. 3- Description of the Ion Chamber Plate VI shows the gridded ion chamber used in the experiment. The body of the chamber was steel tubing, with welded flanges. 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. P L A T E VII E L E C T R O D E S Y S T E M FOR GRIDDED ION C H A M B E R S C A L E : INCHES O I 2 M A T E R I A L \ B R A S S , E X C E P T WHERE NOTED C O L L E C T O R AND G U A R D RING G Rl 1^. C O L L E C T O R AND GUARD RING GRID, '////////////////////;//////\ MYCALEX ^ SIDE E L E V A T I O N END ELEVATION 14. The volume of the chamber was 4.64 litres.. A l l interior surfaces were coated with "aquadag" colloidal graphite, to reduce the background from natural alpha-activity i n the steel. One end plate carried the gas purifier, constructed of brass and copper tubing. An electric heating c o i l of about 30 ohms (cold) resis-tance was wound about the centre of the purifier, and insulated with glass wool and asbestos paper. 75 volts across the heater produced a O temperature of about 300 C. at the interior. A copper cooling c o i l at one end aided i n convection of the gas, and protected the 0-ring seal on the f i l l i n g cap. The most common impurities are oxygen, nitrogen, and water vapour. Calcium metal i s known to be effective in removing a l l of these, and so was used as the purifying agent. (REIMANN, 1934; WILKINSON, 1950); The opposite end-plate carried a needle-valve for f i l l i n g the chamber, and three kovar terminals. The electrode system was also mounted on this 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, 1952) 4. Electrode System The design of grids for such chambers has been carefully investigated, both i n theory and experiment by BUNEMANN, CHANSBAW, and HARVEY,1 1 9 4 9 . The electrode system of the present chamber was designed largely according to their recommendations. The electrodes were constructed of brass, with Mycalex and Lucite insulation. Plate 7;II shows the arrangement. The grid was wound from 36 Coppel wire, spaced 1 mm. centre-to-centre. Grid-collector spacing was 1-52 cms. and grid to plate 7 . 6 cms. For such 15. an arrangement, the "grid inefficiency", i.e. the extent to which the number of lines of force ending on the collector is dependent upon the f i e l d due to positive ions, is 0 . 0 1 , indicating very efficient shielding of the collector. The curvature of the high-voltage plate is a slight modif-ication to the design of Bunemann et a l . Field plots have shown that the curvature increases the number of lines of force ending on the collector, and insures that the sensitive volume approximates to the actual volume above the collector. The sensitive volume was estimated to be about 1 l i t r e . About 15$ of this volume constitutes the unshielded region between the grid and the collector. To a f i r s t approximation, we may assume that the ionizing events are distributed uniformly throughout the sensitive volume, and so about 15$ of the events w i l l contribute to the spreading of the pulse-distribution. 5- F i l l i n g Gas Like a l l noble gases, neon is well suited to use in an ionization chamber . Electron attachment does not occur, and electron collection can be achieved, even at high pressures, with reasonable collecting voltages. Although the purest obtainable gas was used, the purifier on the chamber was s t i l l necessary, because of occluded gases on the chamber walls. Outgassing of such a chamber is 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, prior to f i l l i n g . The manufacturers ^ of the neon used stated the limits of # The Matheson Co., East Rutherford New Jersey; private communication 1 6 . impurity to be Helium 0 . 2 $ Nitrogen 0 . 0 2 $ Other less than 0 . 0 2 $ The three stable neon isotopes were present in the normal ratios: Ne 2 0 9 0 . 5 1 $ Ne1' 0 . 2 8 $ Ne a* 9 . 2 1 $ (MATTAUCH and FLAMMERSFELD, 1949) 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 ionizing particles, so increasing the number of events which are completely confined to the sensitive volume, relative to those which are only partially in this region. The pressure, may be limited i f the electrostatic f i e l d i s not sufficient to achieve electron collection, but this consideration did not affect the present experiment. After 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 liquid 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 particles prodticed in the Ne (n«;0 reaction with 2 . 6 8 Mev. neutrons'would be 0 . 2 cms., a relatively small dis-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 volts. This was more than adequate to achieve saturation, i.e. complete electron collection, without recombination. In operation, a plate voltage of «800 volts and grid voltage of - 2 0 0 volts was used. The plate voltage was obtained from a Dynatron Hadio Type 200 regulated supply, and the grid voltage was supplied from the plate by a resistor network. The chamber was stationed with i t s centre-line about 5 1/2 inches from the heavy ice target. Jn_this position the sensitive volume received about l / 3 5 of the total yield of neutrons, i.e. about 5X 10 * neutrons per second for a 150^-amp. beam. The range of neutron energies received by the sensitive volume was determined by the half-angle subtended at the target by the sensitive volume, i n this case about 45 degrees. A value for the average neutron energy was obtained as follows. The solid angle was treated in three regions: 0 - 1 5 , 1 5 - 3 0 , and 3 0 - 4 5 degrees. A value of neutron energy for each region was calculated from the general formula for the angular energy dependence in c o l l i s i o n processes (HANSON et a l , 1 9 4 9 ) . Substituting appropriate masses and energies for 50 kev. deuterons bombarding deuterium, the exp-ression for neutron energy i s : E^r ( 0 . 0 1 2 5 ) | ^ c o s ^ - H 9 6 + cos ^ / c o s V+ 3 9 2 Neutron energies for the three regions were obtained from this formula w i t h * s 7 . 5 , 2 2 . 5 , and 3 7 . 5 degrees. The relative number of neutrons in each region was then P L A T E V I I I 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 distribution function N(^) = B(l>A cos"*/), which is valid for 50 kev. deuterons, using A = 0 . 4 5 (HUHT00N, 1 9 4 0 ) . In this relation, f& is measured i n centre-of-mass co-ordinates. The angle •& i n the laboratory system is related to^by 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 is 2 degrees, which was considered negligible. Finally, adjustment was made for the number of target atoms exposed to each of the three neutron groups, i.e. for the fraction of the sensitive volume included i n each of the angular regions. The f i n a l value obtained for the average neutron energy was 2.68 t 0 . 0 7 Mev. The error on this figure makes allowance for slight variations in deuteron energy, chiefly due to straggling in the heavy ice target. 7. Pulse Amplification and Measurement A wide band, low noise head amplifier with an approximate gain of 100 was mounted directly on the chamber. Plate VIII gives the circuit diagram. 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 collector and f i r s t grid was measured by observing the reduction i n height of standard pulses when ten a known capacity was connected in para l l e l and was found to beyyt.farads. The plate voltage was obtained from a Lambda Model 28 regulated supply, 19. and stabilized DC was used for 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 Electric Type 1444 Linear Amplifier. This amplifier has a gain of about 10 , and gain stability better than 1$ over long operating periods. Provision is included for up to 33 db. attenuation, variable top and bottom cuts, and variable duration of output pulses. Eor optimum signal-to-noise ratio, 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 pulse-amplitude analyser, or "kicksorter", (CHALK EIVEE design). Amplitude stability of the kicksorter discriminators i s stated as approaching 0 . 0 2 volts under the existing operating conditions. The kicksorter has a maximum input voltage of 40 volts, and incorporates a stabilized amplifier with a gain of 5 , so that maximum input to the discriminator is 200 volts. The kicksorter channels were set up using pulses from a Standard Pulse Generator. The amplitude of these pulses was stable to about 0 . 0 1 volts over long periods. These pulses were fed through the kicksorter amplifier when adjusting the channels. The minimum practical 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. disintegration energy, which provided adequate resolution for the present experiment. P L A T E I X l O O O l P U L S E - H E I G H T D I S T R I B U T I O N F R O M P O L O N I U M A L P H A S NO. O F C O U N T S S O O 5.25 DIS INTEGRATION E N E R G Y , MEV. 20. 8. Energy Calibration A series of ionizing events of the same energy in the chamber should, within s t a t i s t i c a l limits, produce voltage pulses of the same amplitude. Moreover, the pulse amplitude is expected to depend linearly on the disintegration energy. The energy scale was JL/o calibrated by a Po source in the chamber. This source was attached to the inner surface of the high voltage plate, and covered with a thin aluminum collimator. The alphas from Po have 5«3 Mev. energy, and an air-range of 3.842 cms. This corresponds to a range of about 1 cm. in 6 l / 4 atmospheres of neon. Since i t requires 29*3 ev. per ion pair in neon (STETTER, 1943), such particles should produce about 290/*volt pulses on the lOO^ifarads capacity of the collector and f i r s t grid.. The signal-to-noise ratio for those polonium alphas was about 20:1. A l l polonium alphas are stopped in the sensitive volume, and none reach the unshielded region between the grid and the collector. Plate IX shows a typical distribution of pulses from the polonium alphas. The half-width of this distribution is 68 kev., or 1.3$ of the total energy. Ionization produced by disintegrations in the f i l l i n g gas differs somewhat from that produced by alpha particles from radio-active sources in the chamber. In the case of a neon disintegration, the reacion energy is shared between an alpha particle and a recoiling oxygen ion, both of which w i l l produce ionization i n the f i l l i n g gas. For the alpha particles, the relation between ionization and energy is linear down to low energies (region of 100 kev.) after which the proportionality may break down, due to electron attachment and inelastic non-ionizing collisions. The variations in total ionization produced w i l l however be rather small, so that the determination of alpha particle energies is quite accurate. 2 1 . In the case of the larger fragments, the mechanism of ionization i s not clearly understood, though some investigations have "been made on recoil particles in cloud chambers; (see WBENSHALL, 1940) and discussion and references given by WILKINSON ( 1 9 5 0 ) ) . Two points must be considered with regard to energy determination for such particles. F i r s t , the energy loss may be less uniform than for alpha particles, so that the amount of ionization released by single energy events is somewhat more random than for alphas. In this case, the result would be a spreading of the pulse-height distribution. Secondly, the energy calibration i s based on the energy loss of Po alphas i n neon, i.e. 29.3 ev. per ion pair. If the energy loss per ion pair for oxygen ions has some other value, the apparent disint-egration energy w i l l be in error. The linearity 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 grid-collector capacity. After amplification, they were displayed on the kicksorter, where they appeared in not more than two channels. Pulses of the same amplitude, without attenuation, were then fed directly into the kick-sorter amplifier, with the same discriminator settings. This procedure was carried out for a range of pulse amplitudes. Comparison of the two sets of kicksorter readings showed the system to be linear to within 1 $ . 9 . Experimental Procedure Except when shut-downs of several days duration were anticipated, a l l electronic equipment, with the exception of the 22. collecting voltage on the chamber, ran continually, thus maintaining, the greatest s t a b i l i t y . The polonium alphas were observed for 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. At the end of the run a second polonium alpha-peak was taken to ensure that the gain of the amplifiers, and conditions within the chamber were unchanged. The calcium purifier was heated for 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. 2 0 0 0 - , NO. O F C O U N T S I O O O P L A T E X C O M P L E T E E N E R G Y S P E C T R U M 2.0 2.5 3.0 3.5 DISINTEGRATION E N E R G Y , M E V . 4.0 4.5 23. > III. RESULTS AMD DISCUSSION A. EXPERIMENTAL RESULTS Plate X shows the complete energy spectrum. The spectrum was examined i n sections, and in moving the kicksorter from one section to another, an overlap region of six channels was allowed. The data shown in 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 flux, but a neutron monitor was not available at the time this data was collected. The position of the large peak corresponds to a disinteg-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 electronic noise at about 450 kev. The background over this region is due mainly to scattered neutrons from the walls of the laboratory and massive objects in the v i c i n i t y of the ion chamber. Neon recoils from neutrons scattered in the chamber have a maximum energy of about 490 kev. Some contri-bution may also come from disintegrations i n the unshielded region between the grid and collector of the chamber. Investigation of the high energy side of the main peak disclosed a second peak of about 100 times less intensity. This regio of the spectrum is shown in greater detail i n Plate XI. Position of this peak corresponds to a disintegration 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 for 3Jast Neutron Disintegration of Neon The gridded ion chamber might be expected to detect any (noc) and (n p) reactions arising from the disintegration of neon nuclei. The following disintegrations must then be considered: Isotope Reactions Calculated Qr-Yalue (n ot ) - 0.54 Mev. (n p ) - 6.14 » Ne Ne*' Ne (n «. ) + 0 . 7 1 ( u p ) (n * ) - 5 . 0 6 (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 in the masses, but are useful in est-imating the thresholds of reactions. 25-2. Interpretation of the Main Peak There i s no question "but that the main peak is due to the reaction Ne'^Cn*) 0 "\ From the measured energy release, Q ; - 0 . 7 ? ± 0 . 0 8 Mev. 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 railli-barns. No information v?as given on the number of events recorded. Their Q-value was later revised to -0 .8 to - 0 . 8 5 Mev. in a private communication to JOHNSON et a l (see below). (b) SIKKEMA (1950) used a large proportional counter f i l l e d 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 (1951) used a pro-portional counter containing 30 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 millibarns. The published curves show peak about 100 counts high. It is of interest to recall the considerations of WILK-INSON (1950) mentioned previously, regarding the use of pure noble gases in proportional counters. The formation of metastable states in 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 in the ion chamber. In the present experiment, the 0„-value' was obtained with 1% counting st 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/r= 17.004,507*0.000,015 Ne' = 19.998,771*0.000,012 17 The 0 mass i s f a i r l y well 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'7 at 0.876 * 0.009 Mev. The gamma ray from this level; to ground has been reported by ALB0EGER (1949). POLLARD and ^ DAVIDSON (1947) have observed the same level through the reaction N («p)0 , and in addition a second particle-groug, which may indicate a second ,; level at 1.6 Mev. However, i t i s possible that the particles observed were deuterons from N (<*d)0 . It i s generally assumed that the grounds-state of Ne has zero spin and even parity, (HORNYAK , 1950). BUTLER, (1950) i n an analysis of the work of BURROWS et al (1950) on the angular distribution H . . 17 / 7 of protons from 0 (dp}0 concludes that the ground-state of 0 i s 5/2 or 3/2., even, and that the f i r s t excited level i s 1/2 , even. The spin value of 5/2 for 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 transition of Ne (n<* }0 may be viewed as follows: 27. Spin: Parity: JO Ne 0 even Angular momentum: n i g 1 =-0 1 = 1 Ne 0 + 0 5/2 even ^ (even) 1 = 2 1 = 3> 0 1 = 2. ' | (odd) .3/2 (odd) 13/2 (even). 5/2 (even). f5/2 (odd) [7/2 (odd) The arrows indicate the possible transitions for neutrons up to 1 = 3 and alpha particles up to 1 = 4 . In view of the cross section of the reaction, i t seems unlikely that larger 1-values need be considered. 17 The transition to the excited level in 0 , which was not observed with the 2.68 Mev. neutrons, has the po s s i b i l i t i e s : 17* JO Spin: Parity: Angular momentum: Ne 0 even n i z 1-0 1=1 1=2 1 = 3 Ne ii* 0 3 even * -a (even) i - (odd) 1 * 1 0 1 (.3/2 (odd) ^ (3/2 (even) 1 - 2 [5/2 (even) '5/2 (odd) -^1 = 3 (7/2 (odd)-Thus, assuring up to F-wave neutrons, (1 = 3) , leads to SI* seven possible states for the compound nucleus, Ne . O f these the the ground state transition i s forbidden for the even and -J- odd states. 28. •It was hoped that the lack of excited levels of 0 i n the present experiment would give some indication as to which state of Ne appeared. However, the penetrability of the potential barrier for the alpha particles must also be taken into account. Calculation of the penetrability § indicates that the intensity of the excited-state transition would be approximately 10 times that of the ground-state transition. A reaction of such low intensity would not have been detected. Therefore, the non-observance of excited levels of 0 yields no information regarding the state of the compound nucleus, Ne 3. . Interpretation of the Minor Peak No such subsidiary peak has been reported by other workers. This i s not surprizing i n view of i t s low intensity, 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 for this 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 f i l l i n g gas. The possibility of higher energy neutrons has been previously # see for example BETHE, H. A. R. M. P. 9.-164--1937 £ 9 -discussed, and shown to "be most unlikely. The only energetically possible reaction in We or Ne */. , /% for the neutrons used is Ne (n * ) 0 . For this reaction, the observed energy release gives Q, - *0.48 * 0.10. EWALD (1951) 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 - 2 1 . 0 0 0 , 3 3 9 * 0 . 0 0 0 , 0 6 8 Using the cross section of JOHNS Oil et a l (1951) of 2 0 millibarns for Ne (n«) 0 , the intensity of this peak would indicate a cross section of about 65 millibarns for Ne (n«0 0 The limits of inrpurity for the neon were stated as 0.2$ helium and 0.02$ nitrogen, while nitrogen, oxygen and water vapour may have been present in' the chamber prior to f i l l i n g . The energetically possible reactions for these elements are / ' ( n p ) 0 "* Q r f O . 6 3 Mev. l/"(n « ) B " q - - 0.28 " 0" (n « ) C/S Qr - 2.31 • 1 1 o " (n « ) c " + 1 . 7 3 " The f i r s t of these reactions is well known, and i t s Q,-value has been carefully established as + 0 . 6 3 0 * 0 .006 Mev. (FRANZEN et a l , 1950; H0RNYAK et a l 1950) . T h i s value l i e s close to that of the minor peak, although outside the probable error, which is believed to be generous. Por 2 . 8 Mev. neutrons, the cross section for N (n p )C is 40 millibarns (BALDINGER and RUBER, 1 9 3 9 ) . T h i s would indicate more than 0 . 4 $ nitrogen in the chamber, though such an estimate is very approximate, as this cross section may not hold for 2 . 6 8 Mev. neutrons, ^his amount of nitrogen should be accompanied by about 0 . 1 $ oxygen. WILSON et a l (1950) have stated that 5 parts per million of oxygen in 6 atmospheres of deuterium w i l l prevent electron collection entirely. It would be expected therefore that 0 . 1 $ oxygen would have noticeable effects in 6-£ atmospheres of neon. The calcium pur i f i e r is expected to remove both nitrogen and oxygen. If nitrogen were present in the chamber, the reaction N (n «)B should also be observed. The cross section is 160 millibarns for 2 . 8 Mev. neutron, and so a second small peak should be observed at a dis-integration energy corresponding to Q, - - 0 . 2 8 Mev. Some irregularities have been noted i n this region of the spectrum, but at the time of writing, sufficient data was not available to determine whether such a peak exists. 4 . Summary of Results This table shows the Q-values for Ne ( n * ) 0 from various experimental investigations, and from calculations based on mass spectrograph data. Q,-values GRAVES and COON (1946) - 0 . 8 0 to. - 0 . 8 5 . Mev. SIKKEMA (1950) - 0 . 6 " JOHNSON et a l (1951) - 0 . 7 5 i 0 . Q 5 " Present experiment - 0 . 7 7 0 . 0 8 " Mass Spectrograph MATTAUCH and ELAMMERSPELD (1949) - 0 . 5 4 ± 0 . 1 2 " EWALD ( 1 9 5 D - 0 . 6 4 ± 0 . 0 5 " 31. C. FURTHER INVESTIGATIONS The extension of t h i s investigation i s now being a c t i v e l y pursued i n the following ways: (a) Reaction energies are to be determined more exactly . by reducing the error on the neutron energies. This w i l l be done by moving the ion chamber away from, the target, thus reducing the s o l i d angle subtended by the sensitive volume, and so decreasing the angular va r i a t i o n of neutron energy. (b) The region of the spectrum corresponding to a reaction 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 established, i t would seem to indicate that both small peaks are due to nitrogen i n the ion chamber. This would however raise two inte r e s t i n g points. F i r s t , how could t h i s amount of nitrogen appear i n the ion chamber, and remain i n spite of the calcium p u r i f i e r ? Second, i f the minor peak i s due to N (np)C , why i s the reaction energy not closer to the accepted value? ' (c) Investigations are to be carried out with thermal neutrons, •t 3 by moderating the H (dn)He neutrons wifch p a r a f f i n . This 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 int e n s i t y of t h i s peak would be expected, i f the reaction cross section follows the l / v law. The s h i f t i n the position of t h i s peak w i l l give an additional check on the l i n e a r i t y of the amplifier system. (dj 1 Some i n i t i a l attempts to thermalize the neutron f l u x were not completely successful i n that the main peak s t i l l appeared, though with 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 reaction, which are 3 0 / 7 below the threshold f o r Ne (n<*)0 • (e) The reactions w i l l be examined at higher neutron energies, using neutrons from H (dn)He and H (dn)He , with 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 interest 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 desirable. 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 has been measured as -0^77*0.08 Mev. The 1% counting st a t i s t i c s obtained are considerably better than those reported in previous investigations of this reaction. Careful search of the energy spectrum gave no evidence n 'J-for the appearance of excited states of 0 , using 2.68 Mev. neutrons. At least one such state,is known, (0.87 Mev.) .' Calculation of the penetrability of the potential barrier for the alpha particles corresponding to this level indicates an intensity approximately 10 times that of the ground-state transition, at this neutron energy. A reaction of such low intensity 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. There i s some -2/. . IS evidence that the reaction is' Ne (n^jO , but the reaction N (np)C may also be responsible, i f nitrogen is" present in the ion chamber. Further investigation i s necessary to c l a r i f y this point. 34. V. BIBLIOGRAPHY Alburger, D. S. P. R. 75-51—1949 Alder, F. ; Yu, F. C. P. R. 81-1067—1951 Amaldi, E. ; Hafstad, L. R. ; Tuve, M. A. P. R. 51-896—1937 Baldinger, E. ; Huber, P. Helv. P. Acta. 12-330—1939 Bennett, W. E. ;• Mandeville, C. E. ; Richards, H. T. P. R. 69-418—1946 BOnner, T. W. P. R. 52-685—1937 Bonner, T. W. :'Evans, J. E. : Harris, J-. C. ; P h i l l i p s , G. C. P. R. 75-1401—1949 Bretscher, E. ; French, A. P. ; S e i d l , F. G. P. P. R. 73-815—1948 Buechner, W. W. ; S t r a i t , E. N. ; Sperduto, A. ; Malm, R. P. R. 76-1543—1949 Bunemann, 0. ; Cranshaw, T. E. ; Harvey, J. A. Can. J. Res* 27-191—1949 Burrows, H. B. ; Gibson, W. M. ; Rotblat, J. - P. R. 80-1095—1950 Butler, S. T. P. R. 80-1095—1950 Devons, S. Excited States of Nuclei; Cambridge U. Press, 1949 Erickson, K. W. ; Fowler, J. L. ; Stoval, E. J. P. R. 76-1141—1949 Ewald, H. Z e i t s . f u r Naturforschung 6A-293—1951 Franzen, W. ; Halpern, J. ; Stephens, ¥. E.' P. R. 77-641—1950 Gamow, G. Atomic Nuclei and'Nuclear Transformations, Oxford U. Press,1937 Geschwind,' S. ; Gunther-Mohr, G. R. ; Silvey,' G. P. R. 85-474—1952 Goldsmith, H. H. ; Ibser, H. ; Feld, B. T. R. M. P. 19-259—1947 Graves, E. R. ; Coon, J. H. P. R. 70-101—1946 ' Hanson, A.TO. ; McKibbon, J. L.' ' ' P. R. 72-675—1947 Hanson, A. 0. ; Taschek, R. F. ; Williams, J. H. R. M. P. 21-655—1949 Harkins, W. D. ; Gans, D. M. ; Newson, H. W. ' P. R. 44-529—1955 Heydenburg, N. P. ; I n g l i s , D. R.. P. R. 75-250—1948 Hornyak, W. : Lauritsen, 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. ; Barschall, H. H. P. R. 82-117—1951 Kirkaldy, J. S. M. A. Sc. Thesis, U. B. C., 1951 Ladenburg, R. ; Kanner, M. H. P. R. 52-911—1937 'L e i t e r , H. A. ; Rodgers, F. A. ; Kruger, P. G. P. R.' 78-663—1950 35. Manning, H. P. ; Huntoon, R. D. ; Myers, F. ; Young, V. P. R. 61-371-Mattauch, J. ; Flammersfeld, A. Isotopic'Report, Tubingen, Mayer, M. G. P. R. 74-235— Newson, H. W. P. R. 48-790-Oliphant, M. L. E. ; Rutherford, E. Proc. Roy. Soc. A141-259— Oli pliant-, M. L. E. ; Harteck, L. ; Rutherford, E. Proc. Roy. Soc. A144-692— Pepper, T. P. Pollard,E. ; Davidson, P. W. Reimann, A. L. Roberts, R. B. Schiff, L. I. Sikkema, C. P. Stetter, G. Tollestrup, A. V. Can. J. Res'. 27-143-P. R. 72-756-P h i l . Mag. (7) 18-1117-P. R. 51-810-Quantum Mechanics, McGraw-Hill, '" Nature 165-1016-Zeits. Phys. 120-659— 1942 1949 •1948 •1935 •1933 •1934 •1949 •1947 -1934 •1937 1949 1950 1943 Welles, S. B. Wilkinson, D. H. 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. : Collie, C. H. ; Halban, H. : Bishop, G. R. R. S. I". 21-699-Woods, S. B. Ph. D. Thesis, U. B. C., 1950 1952 

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