{"Affiliation":[{"label":"Affiliation","value":"Science, Faculty of","attrs":{"lang":"en","ns":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","classmap":"vivo:EducationalProcess","property":"vivo:departmentOrSchool"},"iri":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","explain":"VIVO-ISF Ontology V1.6 Property; The department or school name within institution; Not intended to be an institution name."},{"label":"Affiliation","value":"Physics and Astronomy, Department of","attrs":{"lang":"en","ns":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","classmap":"vivo:EducationalProcess","property":"vivo:departmentOrSchool"},"iri":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","explain":"VIVO-ISF Ontology V1.6 Property; The department or school name within institution; Not intended to be an institution name."}],"AggregatedSourceRepository":[{"label":"Aggregated Source Repository","value":"DSpace","attrs":{"lang":"en","ns":"http:\/\/www.europeana.eu\/schemas\/edm\/dataProvider","classmap":"ore:Aggregation","property":"edm:dataProvider"},"iri":"http:\/\/www.europeana.eu\/schemas\/edm\/dataProvider","explain":"A Europeana Data Model Property; The name or identifier of the organization who contributes data indirectly to an aggregation service (e.g. Europeana)"}],"Campus":[{"label":"Campus","value":"UBCV","attrs":{"lang":"en","ns":"https:\/\/open.library.ubc.ca\/terms#degreeCampus","classmap":"oc:ThesisDescription","property":"oc:degreeCampus"},"iri":"https:\/\/open.library.ubc.ca\/terms#degreeCampus","explain":"UBC Open Collections Metadata Components; Local Field; Identifies the name of the campus from which the graduate completed their degree."}],"Creator":[{"label":"Creator","value":"MacDonald, Jack Robert","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/creator","classmap":"dpla:SourceResource","property":"dcterms:creator"},"iri":"http:\/\/purl.org\/dc\/terms\/creator","explain":"A Dublin Core Terms Property; An entity primarily responsible for making the resource.; Examples of a Contributor include a person, an organization, or a service."}],"DateAvailable":[{"label":"Date Available","value":"2011-10-20T18:05:18Z","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/issued","classmap":"edm:WebResource","property":"dcterms:issued"},"iri":"http:\/\/purl.org\/dc\/terms\/issued","explain":"A Dublin Core Terms Property; Date of formal issuance (e.g., publication) of the resource."}],"DateIssued":[{"label":"Date Issued","value":"1964","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/issued","classmap":"oc:SourceResource","property":"dcterms:issued"},"iri":"http:\/\/purl.org\/dc\/terms\/issued","explain":"A Dublin Core Terms Property; Date of formal issuance (e.g., publication) of the resource."}],"Degree":[{"label":"Degree (Theses)","value":"Doctor of Philosophy - PhD","attrs":{"lang":"en","ns":"http:\/\/vivoweb.org\/ontology\/core#relatedDegree","classmap":"vivo:ThesisDegree","property":"vivo:relatedDegree"},"iri":"http:\/\/vivoweb.org\/ontology\/core#relatedDegree","explain":"VIVO-ISF Ontology V1.6 Property; The thesis degree; Extended Property specified by UBC, as per https:\/\/wiki.duraspace.org\/display\/VIVO\/Ontology+Editor%27s+Guide"}],"DegreeGrantor":[{"label":"Degree Grantor","value":"University of British Columbia","attrs":{"lang":"en","ns":"https:\/\/open.library.ubc.ca\/terms#degreeGrantor","classmap":"oc:ThesisDescription","property":"oc:degreeGrantor"},"iri":"https:\/\/open.library.ubc.ca\/terms#degreeGrantor","explain":"UBC Open Collections Metadata Components; Local Field; Indicates the institution where thesis was granted."}],"Description":[{"label":"Description","value":"The cross section for the photodisintegration of helium-3 has been measured at gamma ray energies of 8.06 and 9.17 mev. The He\u00b3 (\u0263,p)D reaction cross section at 8.06 and 9.17 mev was found to be 0.493\u00b1 0.066 and 0.723\u00b1 0.087 millibarns respectively. The He\u00b3 (\u0263,n)2p reaction cross section at 9.17 mev was found to be 0.25\u00b1 0.13 millibarns. These results are compared with other experimental work on the photodisintegration of helium-3 and tritium.\r\nThe photodisintegration reaction was observed in a cylindrical gridded ionization chamber using a helium-3, methane, and argon gas mixture. The C\u00b9\u00b3 (p,\u0263) N\u00b9\u2074 reactions at proton bombarding energies of 0.554 and 1.75 mev were used as the source of gamma rays of well defined energy. The preparation of carbon-13 targets is discussed in detail.\r\nTheoretical calculations on the photodisintegration of mass 3 nuclei are summarized. Photodisintegration and electron scattering measurements are compared as methods of determining the nature of the ground state wave function of the mass 3 system.","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/description","classmap":"dpla:SourceResource","property":"dcterms:description"},"iri":"http:\/\/purl.org\/dc\/terms\/description","explain":"A Dublin Core Terms Property; An account of the resource.; Description may include but is not limited to: an abstract, a table of contents, a graphical representation, or a free-text account of the resource."}],"DigitalResourceOriginalRecord":[{"label":"Digital Resource Original Record","value":"https:\/\/circle.library.ubc.ca\/rest\/handle\/2429\/38126?expand=metadata","attrs":{"lang":"en","ns":"http:\/\/www.europeana.eu\/schemas\/edm\/aggregatedCHO","classmap":"ore:Aggregation","property":"edm:aggregatedCHO"},"iri":"http:\/\/www.europeana.eu\/schemas\/edm\/aggregatedCHO","explain":"A Europeana Data Model Property; The identifier of the source object, e.g. the Mona Lisa itself. This could be a full linked open date URI or an internal identifier"}],"FullText":[{"label":"Full Text","value":"THE PHOTODISINTEGRATION OF HELIUM-3 AT PHOTON ENERGIES OF 8.06 AND 9.17 MEV JACK ROBERT MACDONALD B.A.Sc\u2022, The U n i v e r s i t y of B r i t i s h Columbia, 1960 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of PHYSICS We accept th i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1964 In p r e s e n t i n g t h i s t h e s i s 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 an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study, I f u r t h e r agree that per-m i s s i o n f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t , c o p y i n g or p u b l i -c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission.-, Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8 f Canada Date O C T ^ g ^ ~7 , (^4 The University of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOB, THE DEGREE OF DOCTOR OF PHILOSOPHY B \u201e A \u201e S c c j The Univ e r s i t y of B r i t i s h Columbia* 1960 TUESDAY, OCTOBER 6, 1964, AT 3;-00 P.M. ROOM 12, HEBB BUILDING, (PHYSICS) COMMITTEE IN CHARGE Chairman; I. McT. Cowan of JACK ROBERT MACDONALD W . M o Armstrong K. L\u00bb Erdman C. Froese. D,\u201eL..Livesey J o Mo McMillan J . Bo Warren External Examiner: Leon Katz Department of Physics University of Saskatchewan THE PHOTODISINTEGBATION OF HELIUM-3 AT PHOTON ENERGIES OF 8.06 AM) 9,17 MEV. ABSTRACT The cross section f o r the photodisintegration of helium-3 has been measured at. gamma ray energies of 8,06 and 9.17 mev. The He 3 ( F 20 25 3o P H O T O N E M E R G Y ( M E V ) FIGURE 3 'Theoretical Estimates of the Gross Section f o r the Reaction He 3(V , n) 2p - 8 -plane wave and the deuteron wave function was of the form These r e s u l t s are also given i n Figure 2. Eichmann (1963) has Improved the 2-body calculations by-considering quadrupole contributions to the cross section, by including a non-symmetrical component S' i n the ground state wave function, and by Including f i n a l state i n t e r a c t i o n s between the outgoing p a r t i c l e and the deuteron, Eichmann takes the symmetric part of the ground state wave function as a sum of two terms of the form exp \\\\-\/Az { r\\ +\u2022 r 4| + r 2|)] and chooses the parameters yj^ from a v a r i a t i o n a l c a l c u l a t i o n of the binding and coulomb energies of t r i t i u m and helium-3. The mixed symmetry state S' i s added with an amplitude r a t i o of % - O, \\ , consistent with the neutron absorption by deuterium at low energies (Austern, 1.951, 1952). The f i n a l state wave function i s taken as the product of wave functions f o r the deuteron ground state (the sum of two Gaussians) and the outgoing nucleon. The nucleon wave function Is assumed to be the Jl- 1 part of a plane wave i n one c a l c u l a t i o n . The e f f e c t of f i n a l state i n t e r a c t i o n i s calculated f o r the t r i t o n i n a second c a l c u l a t i o n , where the int e r a c t i o n i s calculated using a Serber p o t e n t i a l with a Gaussian shape. The e f f e c t s of including the mixed symmetry component, the quadrupole contribution and the f i n a l state i n t e r a c t i o n are shown i n Figure 4. The mixed symmetry component contributes to 0.9-o.e-0.7-0.6-(rr,b) 0-5-0.4-03-0.2-0.1-I.O-i 0.9-0.9-W 06-0.5-0.4-0.3-0.2-0.1-' \/ V % \/ \/ \/ \/ M e B l * , p ) D \" ^ ^ 0 \/ \/ E l (S-^P) \" ^ O ^ \/ \/ E l CS-*-P) + E 2 (S-*-D) \"^r^ \/\/ E l (.S+S'-^P) + E2 (S-t-S'-^D) 5.5 10 15 v io 25 30 PHOTON ENERGY (rAE\\l) \/ \/ I ^ _ \\ 1 S ^ N . \\ If \/ ; T ( t , n ) D N N ^ 1 El ( S - * - P ) jl \u00a3| (S -* - P) + FINAL S T A T E INTERACTIONS (o'.zto \\'o ife 20 2'S 3b PHOTOM ENERGY (MEV) FIGURE 4 He 3()T , p)D and ?(t > n)D Cross Sections from Eichmann (1963) -9-the e l e c t r i c dipole t r a n s i t i o n s (S* \u2014>\u2022 P) and lowers the energy of the cross section maximum by 10$, increasing the magnitude of the cross section by 10%. The e l e c t r i c quadrupole contribution has a dramatic e f f e c t on the > t o t a l . cross section as i s shown i n Figure 4, even for channel energies of 15 mev. The f i n a l state i n t e r a c t i o n increases the maximum cross section f o r the photodisintegration of t r i t i u m by 25% and causes the cross section curve to f a l l more r a p i d l y above the maximum. Delves (1960, 1962) has calculated the 2-body cross section f o r helium-3 at low energies and the 3-body cross section f o r t r i t i u m including the effects of f i n a l state i n t e r -actions i n the l a t t e r case. These calc u l a t i o n s are not ap p l i c -able to the present work and w i l l not be discussed here. The t h e o r e t i c a l c a l c u l a t i o n s and experimental results for the photodisintegration of helium-3 w i l l be compared i n Chapter VI. -10-CBAPTER I I I Photodisintegration of 5-body Nuclei Experimental Results Apart from the U.B.C. r e s u l t s , there have been four experimental measurements reported on the photo-effect i n helium-3 and one on t r i t i u m . Each of these w i l l be discussed i n some d e t a i l with p a r t i c u l a r emphasis on the experimental technique. Berman et a l (1963, 1964) measured the 90\u00b0 d i f f e r e n -t i a l cross section f o r the 2-body breakup at photon energies from 8.5 to 22 mev. The experiment was done with the brems-strahlung photon beam of a 22 mev betatron incident on a helium-3 gas target. Protons and deuterons from the 2-body breakup were counted i n coincidence using two CsI(Tl) s c i n t i l -l a t o r s as detectors. Because the photon momentum i s small at these energies, the proton and deuteron are emitted at approximately 180\u00b0 to each other, with nearly equal momenta. Consequently, the energy of the proton, i s twice that of the deuteron. This constitutes a unique signature for a 2-body event and also determines the energy of the photon which caused the event. 2-body events are thus e a s i l y separated from 3-body events f o r which no such angular c o r r e l a t i o n or kinematical r e s t r i c t i o n s apply. The photon f l u x per unit energy i n t e r v a l was determined by monitoring the t o t a l energy i n the photon beam and assuming a brem^strahlung spectrum shape. -11-The r e s u l t s obtained by Berman et a l are shown i n Figure 5 and tabulated i n Table I . TABLE I 90\u00b0 D i f f e r e n t i a l Cross Section for H e 3 ( ^ ,p)D from Berman, Koester and Smith (1964) Gamma Ray Energy (mev) ( 0 ^ ) 9 0 \u00b0 steradian 9 80.0 5.0 10 90.8 5.7 11 91.6 5.9 12 88.8 6.0 13 84.5 6.1 14 78.2 5.9 15 77.1 6.5 16 6,4.8 + 6.0 17 59.9 \u00b1 5.8 18 59.7 \u00b1 6.0 19 57.6 \u00b1 6.2 20 57.5 \u00b1 6.7 21 42.7 + 6.6 .09 STeroduqn \u202206 \u202205-\\ \u2022o4-\u2022cs-\u202202.-\u202201H S T E W A R T E T A L , Y A L E B E R M A N E T A L . ILLIWOIS FlMCKH ET AL, HEIDELBERG -1\u2014\u20141 1 1 1 1 1 1 r -10 15 ~ i 1 r \u2014 r ~ 20 -< 1\u2014 25 30 G A M M A R A Y E N E R G Y (MEV) FIGURE 5 He3()T , p)D Differential Cross Section at 90\u00b0 in the Laboratory System -12-The errors quoted are c e r t a i n l y most optimistic i n view of the continuous photon energy spectrum produced by the betatron. In addition to the 2-body data, Berman et a l observed 3-body breakup i n the cases where the two protons were emitted at 90\u00b0 and 180\u00b0 to each other. Although an attempt i s made to extract a cross section for 3-body breakup from the data, the assumptions made are most l i k e l y i n c o r r e c t , and we s h a l l not reproduce the data here. Finckh et a l (1963) also measured the 90\u00b0 d i f f e r e n t i a l cross section for the 2-body breakup. Their technique was almost i d e n t i c a l to that of Berman et a l . A 31 mev bremsstrahlung beam was used to i r r a d i a t e a helium-3 gas target. The reaction products were detected by CsI(Tl) s c i n t i l l a t i o n counters. The absolute value of the cross section was determined by comparing the He ( )f ,p)D y i e l d to the known y i e l d from the reaction C 1 2 ( ) f j n J C 1 1 . In th i s manner, uncertainties i n the bremsstrahlung spectrum shape were eliminated. The re s u l t s are shown i n Figure 5. The estimated uncertainty i n the absolute value of the cross section amounts to about \u2022 1: 12%. Stewart et a l (1964) have also measured the d i f f e r -e n t i a l cross section at 90\u00b0 i n the laboratory system f o r the 2-body, breakup of helium-3. Helium-3 contained i n a c e l l was i r r a d i a t e d with, a 40 mev bremsstrahlung beam produced by an electron l i n e a r accelerator. The reaction products emitted at 90\u00b0 to the photon beam passed through a thi n window i n the c e l l and were momentum selected by a quadrupole magnet. The magnet -13-focussed the p a r t i c l e s onto two s o l i d state transmission counters arranged to stop the deuterons i n the f i r s t counter and protons i n the second counter. The energy spectrum of the photons was determined by replacing the helium-3 with deuterium and normalizing the He (^ J P)!* y i e l d to the known D()f ,p)n y i e l d . The r e s u l t s are given i n Figure 5 and tabulated i n Table I I . TABLE I I 90\u00b0 D i f f e r e n t i a l Cross Section f o r He 3(tf ,p)D from Stewart, Morrison and O'Connell (1964) Gramma Ray Energy (mev) VdJl\/90\u00b0 \/steradian 8.9 \u00b1 0.4 83 13 10.3 \u00b1 0.5 90 11 12.6 \u00b1 0.8 87 \u00b1 13 13.6 \u00b1 1.0 90 \u00b1 11 14.4 \u00b1 1.2 85 \u00b1 10 15.6 \u00b1 1.3 74 \u00b1 8 16.8 \u00b1 1.5 70 \u00b1 9 18.8 \u00b1 1.8 64 \u00b1 8 20.0 \u00b1 2.0 59 \u00b1 7 23.0 2.5 50 \u00b1 6 27.7 \u00b1 3.3 32 + 4 34.3 \u00b1 4.3 19 \u00b1 4 41.0 5.5 18 \u00b1 8 46.1 \u00b1 6.1 10 7 -14-Due to momentum considerations, the number of deuterons counted by the s i l i c o n detector should be equal to the number of protons produced i n the 2-body breakup of hellum-3. Thus any \"excess\" protons could be attributed to 3-body breakup. Stewart et a l measured the number of 3-body events as a function of proton energy and compared the i r r e s u l t s with a t h e o r e t i c a l c a l c u l a t i o n using the Irving wave function. Becchi et a l (1964) have used a 30 mev bremsstrahlung photon beam and s o l i d state p a r t i c l e detectors to measure the 90\u00b0 d i f f e r e n t i a l cross section f o r the 2-body breakup of helium-3. Their r e s u l t s suffer from poor s t a t i s t i c s and are reported i n terms of a t o t a l cross section, i n spite of the f a c t that t h i s i s not what was measured. No information i s tendered as to the method of determining the bremsstrahlung spectrum shape. Certainly, more information about the experimental technique i s required before the measurements can be taken s e r i o u s l y . The work of Gorbunov and Varfolomeev (1963, 1964) together with the U.B.C. r e s u l t s represent the only measurements of the t o t a l cross section f o r the 2 and 3-body breakup of helium-3. Gorbunov and Varfolomeev used a cloud chamber contain-ing helium-3 and a bremsstrahlung beam with a maximum energy of 170 mev. Each reaction i n the chamber was photographed and i d e n t i f i e d by i t s klnematical features. The t o t a l cross sections, 0\"(2-body) and F R O M S . 5 T O \\ T O M G V . F-Rot-A G O R ^ U M O V A M D V A R F O L O M E E V (V9\u20ac>4-) C A L C U L A T E D A N G U L A R D \\ S T R \\ F C U T I O N A T A P H O T O N E N E R S Y O C 1 9 . 5 M \u00a3 V \/ F O R C L E C T R V C D I P O L E . A N D E L E C T R I C Q U A D R U P O L E A ^ S O R R T I O N . F R O M E ICr tMA^IN (V9631) 0 C M FIGURE 19 Angular Distribution of the Reaction Products for the Reaction He (ft , p)D -40-a s t h e r e s u l t s of Gorbunov and Varfolomeev. The assumption can be made that the angular d i s t r i b u -t i o n at photon energies riear 20 mev i s approximated by curve A of Figure 19. This assumption Is p a r t i a l l y j u s t i f i e d by the calculated e f f e c t of the e l e c t r i c quadrupole contribution on the d i f f e r e n t i a l cross section. In t h i s case the t o t a l cross section, C h a m b e r A c J i v e V o l u m e In terms of these functions, equation A - l becomes Yie l d = f \u2022 p H \u20ac ' ^ ' H ( d ) Eq A-2 where H(d) - a Q H 0(d) -f eL\u00b1 H-^d) 4- a 2 H 2(d)4 .... Robertson has evaluated H n(d) by numerical Integration for n = 0 , 2 , 4 - and BlacJtmore (private communication) has repeated the c a l c u l a t i o n with greater accuracy. The r e s u l t s f o r n = 0,2 are shown i n Figure 21.for the chamber at 90 to the d i r e c t i o n of the proton beam. I t should be noted that the graph given i n Robertson's thesis corresponding to Figure 21 i s Incorrect. B. Atom Density of Helium-3 In the course of the three years taken to investigate the cross section f o r the photodisintegration of helium-3, the gas mixture of argon, methane, and helium-3 was cycled i n and out of the chamber several times. During one period, the chamber developed a leak and part of the gas was l o s t . In a l l cases, when the gas was transferred, accurate pressure readings were taken using a March \"Master Test\", Model 210 pressure gauge so that the atom density of helium-3 was always known to within 1%, In c e r t a i n cases, the argon and methane were separated from the hellum-5 by freezing out these gases at a temperature of 65\u00b0K i n a trap cooled by s o l i d nitrogen. The vapour pressure of argon at 65\u00b0K i s 0.44 psia and t h i s r e s i d u a l pressure of argon was accounted for i n the pressure corrections. At the time of the 8.06 mev runs, the chamber contained 2,24 \u00b1 0.03 atmospheres of helium-3 at 20\u00b0C. This i s equivalent to an atom density of (6.02 \u00b1 0.08)(10) 1 9 atoms per cm 3. The pressure i n the chamber during the 9.17 mev 2-body di s i n t e g r a -t i o n runs was 1.515 \u00b1. 0.02 atmospheres and the atom density of helium-3 was (4.07 \u00b1 0 . 0 7 ) ( l O ) 1 9 atoms per cm3. Both of these figures were checked by repeating the work of Warren et a l \" T 0.1 0.2 0.3 0.4- 0.5 0.6 d\/v''3 FIGURE ?IQ CHAMBER AT 90 TO PROTON BEA.M DIRECTION FIGURE 21 b FIGURE 21 H Q(d) vs. d and H 2(d)\/H 0(d) vs. d for Chamber of Volume V and Diameter to Length Ratio 0.783 -50-(Appendix E) at 7.12 mev and using the known cross section at this energy to determine p H g . The agreement i s w e l l within the \u00b1 10% experimental error of the comparison measurement. During the 9.17 mev 3-body run the atom density of helium-3 at 20\u00b00 was (3.79 \u00b1 .04)(10) . G. Wall E f f e c t At a given gamma ray energy Ey. and f o r a fi x e d t o t a l density of gas i n the chamber, the photodisintegration products have a d e f i n i t e range, R. A f r a c t i o n of these produots w i l l c o l l i d e with the c y l i n d r i c a l wall or pass out of the active volume defined by the guard rings at the end of the chamber. The e f f e c t of th i s \"wall l o s s \" i s to remove counts from the f u l l energy peak at E Q = (E^ - Qj mev to a lower energy i n the spectrum. On the other hand, a f r a c t i o n of i o n i z a t i o n tracks which originate outside the sensitive volume w i l l extend into the active region. These events w i l l a l s o appear i n the low energy part of the spectrum and are termed \"end e f f e c t \" . The combination of the processes which add counts to the low energy section of the spectrum i s termed wall e f f e c t . 1. Wall Loss For an accurate measurement of the number of reactions occurring i n the active region, i t Is necessary to know the magnitude of the w a l l l o s s and i t s influence on the shape of the spectrum. Let the f r a c t i o n of tracks of length R cm o r i g i n a t i n g i n the a c t i v e volume which eith e r s t r i k e the wall or extend into the end region of the chamber be P(R). -51-For a cylinder of radius b cm and length L cm, Robertson (1963) has calculated P(R) to be He approximates the cylinder by a plane and shows thi s approximation to be v a l i d f o r R E 0\/3 and a p a r t i a l w a l l l o s s . This has the further advantage of removing the end e f f e c t from the c a l c u l a t i o n . \" 3. 2-body Photodisintegration Speotrum Analysis The method of spectrum analysis i s best explained with reference to the hypothetical spectrum shown i n Figure 23. K(E) i s the number of counts per u n i t energy so that the t o t a l number of counts N Q i s given by Due to the f i n i t e energy r e s o l u t i o n , the f u l l energy peak has a width 2 S E which buries a part of the w a l l loss spectrum i n the peak. We assume that the f i n i t e energy resolution does not a f f e c t the shape of the wall loss spee-trum to an appreciable degree. The number of counts i n the peak Kp, excluding the w a l l loss contribution, i s given by Eo+SE Eq A-4 E o \/ 3 -54-Eq A-5 where the wall loss removes N QP(R) counts from the peak. The actual number of counts i n the peak i s denoted by Np'. Np' i s the sum of Np and the wal l loss contribution to the peak. The wall l o s s contribution i s that f r a c t i o n of the t o t a l number of wal l loss counts f o r which the energy i s greater than E 0 - BE . This f r a c t i o n i s e s s e n t i a l l y the r a t i o of the area under the wal l loss spectrum f o r energy greater than E 0~ SE to the t o t a l area and i s given by 0 + \u00a3 0 \/ 3 Since the t o t a l number of wall l o s s events i s N QP(R), Np' i s given by Combining equations A-5 and A-6 and solving f o r N Q y i e l d s Equation A-7 gives the t o t a l y i e l d N Q i n terms of the measured number of counts i n the peak and the calculated -55-wall l o s s . In order to reduce the dependence of the t o t a l y i e l d on the calculated w a l l loss and spectrum shape, we calculate the t o t a l y i e l d i n terms of a y i e l d f o r E >2\"^'5\/3 and a p a r t i a l w a l l loss P*(R). P* (R) i s the f r a c t i o n of the t o t a l number of counts which appear i n the spectrum at an energy less than ^^\/^\u00bb P'(R) i s given by Hence the t o t a l y i e l d N 0 Is given by N 0 - - ^ N(E)^E + ( * \u00ae \u2022 N p Eq A-8 where Np i s given by equation A - 6 . The difference between the t o t a l y i e l d as calculated using equation A-7 and the t o t a l y i e l d calculated from equation A-8 was always l e s s than 2%. I t i s assumed that equation A-8 gives a more accurate value of the y i e l d . -56-APPENDIX B The Preparation of Carbon-15 Targets Commercially available C^3 targets are produced by separating the C-1-3 isotope i n a mass spectrometer and allowing 15 the C beam to h i t a target backing. This technique has certain inherent weaknesses. The C i s only loosely attached to the target backing and i s not able to withstand bombardment by proton beams greater than a few microamps. The targets so produced are non-uniform and consequently have a low y i e l d r e l a t i v e to the calculated y i e l d based on the measured target thickness. Further-more, the thickness of such targets i s l i m i t e d to a few kev f o r 'l mev protons. For these reasons i t was necessary to produce C 1 3 targets free of these shortcomings. P h i l l i p s and Richardson (1950) have reported a technique whereby benzene\"is thermally \"cracked\" on s i l v e r f o i l s using a high temperature oven. In an alternative procedure they used an induction heater to crack methyl iodide, CH 3I, on n i c k e l disks. Seagrave (1952) prepared carbon targets by passing current through a tantalum s t r i p i n a chamber containing CH^ or CHgl. Holmgren et a l (1954) and McCormick et a l (1961) report success with similar, techniques. The method used i n the present work i s simple, e f f i c i e n t , and reproducible. Methyl iodide, enriched to 59.5% 13 C was obtained from Merck, Sharp and Dohme of Canada Limited. The CHgl was purchased i n a sealed glass container (11) which was -57-f i t t e d with a flanged pyrex top and attached to the apparatus shown i n Figure 24. The apparatus consisted of a valve assembly (8); (9), (10) and a pyrex tube (7) i n which a 0.002 i n thick platinum s t r i p was suspended (6). The platinum s t r i p was heated by ac current from a 5 v o l t i s o l a t i n g transformer fed by a 110 volt v a r i a c . The output of the transformer was connected to the Kovar seals (1) and the brass plate (15). The apparatus was made vacuum tight by means of a neoprene 0-ring(2) and a flange assembly (5),(4). The procedure used for making targets i s outlined below. 1. A clean platinum s t r i p 1.4 cm wide x 7.5 cm long was spot welded to 0.040 i n . n i c k e l wire (5) and to a n i c k e l plate (12) and suspended i n the pyrex tube. 2. The tube assembly was evacuated to a pressure of 2(10) mm, of Hg. through a pumping port (13) and the platinum was outgassed at 800\u00b0C (red heat) by passing 40 amperes ac current through i t . 3. The tube was closed to the pump and the valve to the CHgl was also closed. For the f i r s t target, the f r a g i l e glass seal on the v i a l containing the CHgl was broken using an i r o n rod held within the tube with a permanent magnet. This allowed the volume of tube up to the Viton A valve diaphragm to become saturated with CH I vapour at a pressure of 30 cm of Hg. FIGURE 24 Target Preparation Apparatus. Scale - Full Size -58-4. With the platinum heated to 600\u00b0C ( d u l l red heat), the valve to the CH 3I was opened and the vapour diffused into the craoking chamber. Thermal decomposition of the CHgl proceeded s a t i s f a c t o r i l y and the carbon deposited on the platinum. The copious quantities of gaseous iodine released i n the process did not condense on the hot platinum but deposited on the sides of'the pyrex tube which was cooled with an a i r blower. Aft e r 5 minutes of cracking, the valve to the CHgl was closed and the iodine was pumped o f f . This procedure could be repeated many times, depending upon the thickness of target desired. Each f i v e minute run deposited about 100 micrograms\/cm 2 of carbon on each side of the platinum i n the form of an extremely tough layer. I f the platinum was cooled quickly, the carbon layer b l i s t e r e d and could be removed from the platinum i n t a c t . I f the platinum was cooled slowly, the carbon adhered to the platinum which then served as a target backing. Approximately 45% of the carbon i n the CHgl was deposited on the platinum. The \"one run\" targets were 10 to 15 kev thick to 1.8 mev protons and withstood beam currents of 35 micro amps f o r hundreds of hours when suitably cooled. The exc i t a t i o n function f o r such a target i s shown i n Figure B5. The maximum y i e l d i s 7.4 gemma rays of energy 9,17 mev per \/ * 9 (10) protons* This compares favourably with the t h e o r e t i c a l 1.740 1.750 1.7(60 1.770 P R O T O N E N E R G Y ( M E V ) FIGURE 25 Yield of 9.17 mev Gamma Rays as a Function of Proton Energy -59-g y i e l d of 7.5 gamma rays per (10) protons as calculated using the measured cross section and resonance width (Appendix C). A C 1 5 target 600 kev thick to 1 mev protons was prepared by peeling the carbon o f f the platinum and placing several layers i n a gold target holder. This target produced a maximum y i e l d of 5.8 photons of energy 8,06 mev per (10) 9 protons of energy 730 kev. The y i e l d calculated from the measured cross section and resonance width i s 6.0 photons per 9 (10) protons. -60-APPENDIX C The. Reaction C 1 3 ( p , ) N 1 4 and the Measurement of the Gamma Flux A. Gamma Ray Counter E f f i c i e n c y The measurement of the e f f i c i e n c y of the 2-3\/4 i n . diameter by 4-| i n . long Nal(Tl) s c i n t i l l a t i o n counter i s discussed by Singh (1959), G r i f f i t h s et a l (1962), Larson (1957) and Robertson (1963). The gamma counter e f f i c i e n c y f o r gamma rays of energy Ey> i s given i n terms of the number of counts i n the gamma spectrum above an energy bias E^\/2 r e l a t i v e to the t o t a l number of gamma rays impinging on the counter. The \u2022effic i e n c y i s w r i t t e n \u00a3(E Y ; E^ > \/2) and i s shown i n Figure 26 as a function of Ey \u2022 Robertson (1963) derives expressions for the e f f i c i e n c y i n terms of an energy bias other than Ey. \/2, and th i s straightforward c a l c u l a t i o n w i l l not be repeated here. There i s one ess e n t i a l difference between the work of Robertson (1963) and the present work. The absolute e f f i c i e n c y of the s c i n t i l l a t i o n counter had been measured at a photon energy of 6.14 mev and could be extrapolated to 6.92 and 7.12 mev with l i t t l e e r r o r . Such extrapolation i s not s u f f i c i e n t l y r e l i a b l e for the 8.06 and 9.17 mev gamma rays used i n the present experiment, and the factor which l i m i t s the accuracy of the photodisintegration cross sections at these energies i s knowledge of the gamma counter e f f i c i e n c y . I t would be worth-while therefore, to measure the e f f i c i e n c y of the gamma counter FIGURE 26 Gamma Counter Efficiency for Half Energy Bias - 2 3\/4 in diam. by 4 1\/2 i n Long Nal(Tl) Crystal -61-at an energy greater than 6.14 mev. This could be done in the following way. Two steps are required. F i r s t l y , the absolute efficiency of the counter could be measured at 4.433 mev by counting the gamma rays to the ground state of C 1 2 from the f i r s t excited state at 4,433 1 5 1 2 * mev. This level can be populated by the reaction N (p,\u00b0<)C \u2022 The gamma yield from the consequent de-excitation can be determined exactly by counting the associated alpha particles with a sol i d state counter, providing the alpha and gamma angular distributions and the alpha-gamma angular-correlation are taken into account, l ? Secondly, one could populate the 16.11 mev level in C by the proton bombardment of at the well-known 160 kev resonance. This level decays 90% hrough the 4.433 mev level mentioned above, and gamma rays of energy 11.7 and 4.43 mev can be observed in coincidence. Hence the determined efficiency at 4,43 mev would lead directly to an experimental measurement of the efficiency at 11,7 mev. B, The Reaction C 1 5(p, T )H 1 4 as a Source of Gamma Rays 1\u00ab Introduction In order to obtain an accurate measurement of the cross section for the photodisintegration of helium-3 at gamma ray energies of 8.06 and 9,17 mev, we must know the angular distributions and relative intensities of the gamma rays produced by the proton bombardment of C ^ 5 at the 0,554 -62-and 1,75 mev resonances. As i s shown i n Appendix A, the angular d i s t r i b u t i o n has a large e f f e c t on the fact o r H n(d) which enters d i r e c t l y i n t o the cross section c a l c u l a -t i o n . The r e l a t i v e i n t e n s i t i e s of cascade gamma rays must be known In order to calculate the 8,06 or 9,17 mev gamma flux from the recorded gamma spectra. 2, 0,554 mev Resonance The reaction C^Cp,^ )N\"*\"4 was f i r s t studied, by Curran et a l (1939), They observed a resonance i n the gamma y i e l d at a proton energy of 0,57 mev and determined the maximum energy of gamma ray to be 8.5 mev. Subsequent work by Lauritzen and Fowler (1940), Fowler et a l (1948), and Seagrave (1952) fixed the energy of the resonance at 0,554 mev and the energy of the gamma ray to the ground state of at 8.06 mev, Seagrave also measured the resonance width PR and obtained a value of 32,5 kev. Devons and Hine (1949) measured the angular d i s t r i b u t i o n of the 8.06 mev gamma rays from the 8,06 (.554)- mev l e v e l and found i t to be i s o t r o p i c . This i s consistent with the l e v e l being formed by S-wave protons as expected. Subse-quent workers have Investigated the de-excitation of the 8,06 (.554)- mev l e v e l i n d e t a i l (Woodbury et a l , 1953; Hird et a l , 1954; Lehman et al,1956; Wilkinson and Bloom, 1957; Broude et a l , 1957). The accumulated data has been compended by Ajzenberg-Selove and Lauritzen (1959) and i s -63-shown i n Figure 27. 3. 1,75 mev Resonance Day and Perry (1951) found a sharp resonance i n the reaction C 1 3(p,}f ) N 1 4 at a proton energy of 1.75 mev, y i e l d -ing 9.17 mev gamma rays. Several workers (Table XII) have since measured the angular d i s t r i b u t i o n s or the anisotropy A f o r the 9.17 mev to ground state t r a n s i t i o n . The r e s u l t s of Rose et a l (1960) are used i n t h i s present work. TABLE XII Measured Angular D i s t r i b u t i o n of the 9.17 mev Radiation at the 1.75 mev resonance Angular D i s t r i b u t i o n Anisotropy = Yield(0\u00b0) - i Yield(90\u00b0) Day and Perry (1951) Woodbury et a l (1953^ Rose et a l (1960) Segal et a l (1961) 1 - (0.55\u00b1 0.02)cos 2 e \\Q+\\ <0.02 1 - (0.59 \u00b10.03)oosf 6 + (0,03 \u00b1 0.03)cos*G - 0.399\u00b1 0.013 - 0.48 \u00b10.03 - 0.55+0.02* - 0.56\u00b1 0.04* ^Calculated from the. measured angular d i s t r i b u t i o n . Several people have measured the r e l a t i v e i n t e n s i t i e s of the cascade gamma rays produced In the de-excitation of the 8.06-5.69-4-.9I -3.95-2.31-CM 00 t \u2014 r in m vO X 8.0k M E V L E V E L D E C A Y (O) ! + 9 . 1 7 -4-7.03-6.44-5.Q3. 5.69-5.10-3.95-231 SQI CM VI +\u00bb ort CM +\u2022 0D m o m N 1 * 9.17 M E V L E V E L D E C A Y 3 FIGURE 27 Decay Schemes f o r the 8.06 and 9.17 mev Levels of N x . Only those le v e l s involved i n the decay of the 8.06 and 9.17 mev states are shown. Broken arrows denote uncertain t r a n s i t i o n s and parentheses denote uncertain assignments. -64-9,17(1*75)-mev resonanoe l e v e l i n N 1 4 (Seagrave, 1952; Wood-bury et a l , 1953; Rose et a l , 1959; Rose, 1960), The re s u l t s of Rose (I960) are reproduced i n Figure 27 and are used i n the ca l c u l a t i o n of the gamma flux i n the present work, 4. Doppler S h i f t When a nucleus i n an excited state of energy E Q , moving with v e l o c i t y v, decays by emission of a gamma ray to the ground state, the energy of the gamma ray i s given by Eq C I where 14 i s the mass of the nucleus and 0 i s the angle between v and the dir e o t i o n of emission of the gamma ray. The f i r s t term i n equation C - l i s the Doppler s h i f t , the second term i s the r e c o i l s h i f t , For the decay of the 9,17 14 mev excited state i n N , the r e c o i l s h i f t i s 3,2 kev. Since the i o n i z a t i o n chamber was placed at 90\u00b0 to the proton beam and hence to ^ , the Doppler s h i f t i n the present case i s zero. The maximum Doppler s h i f t occurs at 8 = 0 \u00b0 or 180\u00b0 and when v has the maximum value allowed by the kinematics of the reaction; that i s , when decay takes place before the -65-nueleus slows down i n the target. The magnitude of t h i s maximum s h i f t for the decay of the 9.17 mev state i s \u00a3 40 kev. A knowledge of the stopping time X and the stopping oross section \u00a3 f o r the nucleus leads to a direct measure-ment of the l i f e t i m e of the excited state. Such a s h i f t i n gamma ray energy would r e s u l t i n a corresponding s h i f t i n the 2-body photodisintegration peak i n the i o n i z a t i o n chamber. However, the expected s h i f t f o r the 9.17 mev gamma rays i s too small to be observed and an estimate of the l i f e t i m e of the state i s not possible by this technique. C. Analysis of the Gamma Spectra 0*554 mev Resonanoe The gamma spectrum obtained by bombarding the thick target described i n Appendix B with 0.730 mev protons i s shown i n Figure 28. The de-excitation of the 8.06(.554)-mev 14 l e v e l i n N y i e l d s 8.06, 5.7 and 4.11 mev gamma rays i n the r a t i o 82:11:4. The 5.7 mev ra d i a t i o n i s due to the sum of the 5.69 -*-0 and 8.06->-2.31 tran s i t i o n s which cannot be separated i n the s c i n t i l l a t i o n counter spectra. Any aniso-tropy i n the 5.7 mev component can be neglected at the angle of observation used i n the present work. The t o t a l number of counts above 4.03 mev In the spectrum shown i n Figure 28 i s given by M * [o.82\u00a3(8.O6;4:0B) + 0.O4r\u00a3(5j;4-.03)-\u00bb-OJl\u00a3(4.l\\;4:O5)]N' NUMBER OF COUNTS -66-where ^ ( E ^ ; E b ) i s the s c i n t i l l a t i o n counter e f f i c i e n c y f o r gamma rays of energy E ^ at an energy bias E ^ and N ' i s the t o t a l number of 8.06, 5.7 and 4.11 mev gamma rays impinging on the counter. \u00a3(5.7;4.03) was determined using the e f f i c i e n c y to a ha l f energy bias, \u00a3(5.7; 2.85) 3and an assumed spectrum shape. The gamma ray spectrum from the 340 kev resonance of the reaction F 1 9(p , \u00a9 0 n o 1 6 which y i e l d s 98% 6.14 mev radi a t i o n (Dosso, 1957) was assumed to give the shape of the spectrum f o r the 5.7 mev rad i a t i o n and i s shown i n Figure 29. The r a t i o of the number of counts above 4.03 mev to the number above 2.85 mev was 0.82 \u00b1 0.01. Hence \u00a3(5.7; 4.03) i s given i n terms of the e f f i c i e n c y to a half energy bias of 2.85 mev by E(5.7;4-.03)= 0.82 \u00a3(5.7; 2.85) = 0.6OB \u00b1 0.008 where \u00a3,(5.7; 2.85) i s given by Figure 26. The uncertainty i n \u00a3(5.7; 4.03) does not include the systematic error i n \u00a3 (5.7; 2.85). As seen In Figure 28, the 4.11 mev rad i a t i o n i s e a s i l y accounted f o r by extrapolation of the 8.06 and 5.7 mev t a i l , again using an assumed spectrum shape. The r a t i o of the number of counts above 4.03 mev contributed by the 4.11 mev ra d i a t i o n to the t o t a l number above 4.03 mev was found to be 0.023 \u00b1 0.003. Hence the r a t i o of the number of counts above 4.03 mev due to the 8.06 mev ra d i a t i o n to the t o t a l I IOO -, E N E R G Y ( M E V ) FIGURE 29 6.14 mev Gamma Spectrum from the Reaction F 1 9 ( p,=*Y)016 at the 340 kev Resonance -67-number above 4.03 mev i s 1 - 0.04 \u00a3(5.7}4.03) - 0.023 - 0,953 \u00b1 0,011 The number of 8,06 mev gamma rays incident on the counter i s given i n terms of the number of counts N above 4,03 mev in the spectrum by ^ c o u ^ T e r _ 0 . 9 5 3 N 8 - 0 G E(8.06;*h03) = \\.2B Kl 2, 1.75 mev Resonance 13 The gamma spectrum from the proton bombardment of C at 1.75 mev i s shown i n Figure 30. As well as the primary component at 9,17 mev, the speotrum contains contributions from 7,03, 6,44, 5,7 and 5,1 mev gamma rays. In order to determine the number of 9,17 mev gamma rays from the spec-trum the contribution from the other gamma rays must be subtracted. Two methods of subtraction were used. In the f i r s t case, the 9,17 mev contribution was assumed to have the same shape as the pure 8.06 mev spectrum obtained previously. Using this shape and the known efficiency \u00a3,(9.17; 4,585), the number of 9,17 mev gamma rays incident on the counter is given in terms of the number of counts N above 4,585 mev by Sooo 4ooo Y-z 3ooo-o (J 2ooo I ooo B I A S I 1 1 1 1 1 1 1 1 1 90 l O O U O I20 <3o 14-0 150 1 6 0 170 \\QO KlCKSORTER C H A N N E U i 1 1 1 1 1 1 1 1 1 1 1 5 6 7 8 9 10 E N E R G Y ( M E V ) FIGURE 30 Gamma Spectrum from the Reaction C 1 3 ( p , Y ) N 1 4 at the 1.75 mev Resonance -68-counter N = 1.156N 9.17 In the second method, the e f f i c i e n c i e s \u00a3(E\u00a3;4\u00ab585) f o r each gamma ray were calculated using the known e f f i c i e n c i e s EfE^E^y^) as given i n Figure 26 and assuming the spectrum shape to be as given i n Figure 29. In terms of the r e l a t i v e i n t e n s i t i e s R^O) of the gamma rays a t an angle \u00a9 to the proton beam, the number of 9.17 mev gammas incident on the counter i s given by 2 R t(e) \u00a3 ( E i ; 4-.SS5) R 9. l 7(e) \u00a3(ai7;+.505) Table XIII shows the values of E^tG) and (E i;4.585) as calculated. Using these values one obtains, counter N - 1.162N 9.17 The two methods are i n good agreement and a value counter N = 1.16N was used i n c a l c u l a t i n g the 9.17 mev 9.17 gamma f l u x . N counter 9.17 TABLE XIII Angular Distributions, Relative Intensities and Scintillation Counter Efficiencies for the Gamma Rays from the 9.17(1.75)-mev Resonance Level in N-Gamma Ray Energy, Ej. Angular Distribution Relative Intensity integrated over 4 TT steradians R i ( G ) 9=152.3\u00b0 \u00a3(Ei ; f ) \u00a3(E i ; 4.585) 9.17 1 - 0.55cos28 100 70 0.782 0.782 7.03 isotropic 3 3 0.767 0.662 6.44 1 + 1.6cos 2\u00a9 - l.lcos^\u00a9 7 8.5 0.76 0.606 5.7 1 - 0.55cos2\u00a9 3 2.1 0.744 0.518 5.1 1 - 0.32cos 2e 3 2.5 0.728 0.223 -69-APPENDIX D Neutron Induced Background and Neutron Shielding One of the main experimental d i f f i c u l t i e s associated with the measurement of the cross section f o r the photodisin-tegration of helium-3 was the problem of neutron-Induced back-ground. The reaction He (n,p)T has a po s i t i v e Q-value of 0.765 mev. The cross section as a function of neutron energy i s shown i n Figure 31. Because of the high cross section and beoause of the unique energy release f o r t h i s reaction, several workers have used helium-3 f i l l e d Ionization chambers and proportional counters as neutron spectrometers (Batchelor et a l , 1955; M i l l s et a l , 1962; Freeman and West, 1962; Brown, 1964; Sayres and Coppola, 1964; Friedas and Chrien, 1964). As discussed i n Chapter IV, the thermal neutron capture by helium-3 served as an energy c a l i b r a t i o n f o r the io n i z a t i o n chamber. The neutron disintegrations were dis t r i b u t e d through-out the chamber and produced counting conditions s i m i l a r to those met i n the photodisintegration measurement. In a l l other respects, the advantages of the chamber as a neutron detector became disadvantages f o r the photodisintegration work. For the 2-body photodisintegration at 8.06 and 9.17 mev, the thermal neutron capture peak at 0,765 mev d i d not i n t e r -fere with the photodisintegration spectrum. Hence f a s t neutron d i s i n t e g r a t i o n r e s u l t i n g i n an energy release i n the chamber of greater than 1.5 mev was the only source of neutron Induced FIGURE 31 Cross Section for the Reaction He ( n, p)T. From Hughes and Schwartz (1958) -70-background whioh proved troublesome. Elaborate p a r a f f i n and cadmium shielding eliminated t h i s background. The neutrons were moderated by the p a r a f f i n and absorbed by the cadmium which has a high capture cross section f o r thermal and epithermal neutrons. For the 3-body photodisintegration at 9.17 mev, the thermal neutron peak f e l l i n the middle of the 3-body spectrum (Figure 1, Chapter I I ) , and a l l neutrons, thermal or f a s t , i n t e r f e r e d with the measurement. The main source of neutrons, and that most e a s i l y eliminated, was i n the e x i t seotion of the Yan de Graaff column and i n the 90\u00b0 analyzing magnet vacuum box where the neutrons were produced by the deutron bombardment of deuterium and carbon. The deuterons a r i s e from the 0.0156% natural deuterium contamina-tion of the hydrogen used i n the Yan de Graaff ion source whioh res u l t s i n deuterium ions being accelerated along with the proton beam. The 90\u00b0 analyzing magnet selects the mass one proton beam; the mass two and mass three beams (Hg, D and HD ) s t r i k e a tantalum s t r i p i n the magnet box (Figure 32). The number of neutrons produced i n the column and magnet box was kept to a minimim by p e r i o d i c a l l y cleaning the tantalum collimator i n the accelerating column and the tantalum beam catcher i n the magnet box. The neutrons which were produced were prevented from entering the chamber by wax shielding walls and a wax and cadmium castle around the chamber and target. For the 8.06 mev runs at a proton energy of 0,730 mev, the proton beam i t s e l f produced no neutrons and the precautions MASS 2 BEAt-1 J IONIZATION CHAMBER V A N O E . G R A A F F C O L U M N T A N T A L U M . C O L L I M A T O R 90 A N A L Y Z I N G M A & M E T | T A N T A L U M C O L L I M A T O R S ' W A X A N D C A D M I U M C A S T L . E WAX AMD CAOMIUM WALL 5 r PLATlWut TUBE LlNf E L & C T R O S T A T I F O C U S S I N G P R O T O N B E A M T A N T A L U M B E A M C A T C H E R PYRE.H. T U B E \" C O L D T R A P C A R B O N - 13 ON PLATINUM T A R G E T R A C K I N G FIGURE 32 Beam Collimators and Neutron Shielding -71-outlined above were adequate. However, f o r the 9.17 mev runs, the proton beam energy was 1.75 mev, we l l above the (p,n) threshold f o r most materials, as shown i n Table XIV. By introduc-ing collimators i n the ho r i z o n t a l pyrex beam tube, the proton beam was prevented from s t r i k i n g the glass, a major source of neutrons from (p,n) reactions. In a s i m i l a r manner, the proton beam scattered o f f the platinum target backing was shielded from the beam tube w a l l by a 0.001 inch thick tantalum l i n e r . Such extra precautions e f f e c t i v e l y eliminated a l l neutrons which resulted i n an energy release i n the chamber greater than 1.5 mev and thus were adequate f o r the S-body photodisintegration at a photon energy of 9.17 mev. Such was not the case f o r the 3-body runs at 9.17 mev. A l l attempts to completely eliminate the thermal neutron peak f a i l e d . Table XIV shows that there are very few materials suitable f o r use as a target backing, as beam collimators, or as a beam tube l i n e r f o r which the (p,n) threshold i s less than -1.75 mev. Carbon and n i c k e l are obvious choices. However, impurities i n both materials produced a neutron f l u x which resulted i n 20 thermal neutrons counted by the chamber per 15 (6)(10) protons incident on the material. The concentration of impurities need not be greater than one part i n (10) to account for t h i s neutron count r a t e . The i o n i z a t i o n chamber i s \"black\" to thermal neutrons and the observed neutron count rate represents a thermal neutron f l u x of only 0.1 neutrons per cm per minute through the chamber for the proton beam currents used. TABLE XIV (p,n) Q-values. Calculated from the Mass Values of Everling et al (i960) Isotope % Natural Abundance (p,n) Q-value Isotope % Natural Abundance (p,n) Q-value Isotope X Natural Abundance (p,n) Q-value L i 7 92.48 -1.646 Mg 2 6 11.3 -4.808 C a 4 2 0.64 -6.660 Be9 100 -1.854 Al 2 ? 100 -5.607 C a 4 4 2.13 -4.432 B10 18.83 -4.56 S i 2 8 92.28 -14.545 Sc45 100 -2.827 BH 81.17 -2.764 Si 2? 4.67 -5.745 1^ 46 7.95 -8.082 C12 98.89 -18.453 p31 100 -6.219 T i 47 7.75 -3.695 C13 1.11 -3.003 S32 95.06 -13.812 T i 4 8 73.45 -4.805 N i 4 99.6 -5.934 S33 0.74 -6.231 Ti49 5.51 -1.383 016 99.8 -16.360 S34 4.18 -5.774 T i 50 5.34 -2.991 F19 100 -4.038 C I 3 5 75.4 -6.039 V51 99.75 -1.534 Ne 2 0 90.5 -16.121 C I 3 7 24.6 -1.598 C r 5 0 4.41 -8.433 N e22 9.2 -3.622 A 4 0 99.6 -2.274 C r 5 2 83.46 -5.485 Na 2 3 100 -4.875 K39 93.1 -7.638 C r 5 3 9.54 -1.370 Mg 2 4 78.6 -14.80 K41 6.9 -1.220 Cr5 4 2.61 -2.15 Mg 2 5 10.1 -5.074 C a 4 0 96.92 -14.768 Mn 5 5 100 -1.014 TABLE XIV (continued) Isotope % Natural Abundance (p,n) Q-value Isotope % Natural Abundance (p,n) Q-value Isotope % Natural Abundance (p,n) Q-value F e 5 4 5.90 -9.622 Ga 6 9 60.0 -3.02 Z r 9 6 2.8 -0.500 F e56 91.52 -5.383 Ga 7 1 40.0 -1.015 Nb 9 3 100 -1.265 F e 5 7 2.245 -1.649 Ge 7 0 20.45 -7.32 Mo 9 2 15.05 -7.200 Cc59 100 -1.857 Ge 7 2 27.41 -5.143 Mo 9 4 9.35 -5.102 N i 5 8 67.8 -9.317 Ge?3 7.77 -1.155 Mo 9 5 15.78 -2.439 N i 6 0 26.2 -6.912 Ge?4 \u2022 36.58 -3.347 Mo96 16.56 -3.760 N i 6 1 1.2 -3.076 Ge 7 6 7.79 -1.760 Mo9? 9.60 N.C. N i 6 2 3.7 -4.62 A s 7 5 100 -1.648 Mo 9 8 24.60 -2.500 N i 6 4 1.1 -2.46 B r 7 9 50.57 -2.404 M o 1 0 0 9.68 N.G. C u 6 3 69.1 -4.149 B r 8 1 49.43 -1.030 R h 1 0 3 100 -1.342 C u 6 5 30.9 -2.131 Rb85 72.15 -1.890 Pd102 0.8 N.C. Z n 6 4 48.87 -7.84 Rb8? 27.85 -0.510 p d104 9.3 -5.062 Z n 6 6 27.62 -5.96 Z r90 51.46 -6.900 p d105 22.6 -2.800 Z n 6 7 4.12 -1.781 Z r 9 l 11.23 -2.380 P d106 27.1 -3.753 Z n 6 8 18.71 -3.70 Z r 9 2 17.11 -2.860 P d108 26.7 -2.619 Zn70 0.69 -1.435 Z r 9 4 17.40 ^1.550 P d110 13.5 -2.230 TABLE XIV (continued) Isotope % Natural Abundance (p,n) Q-value Isotope % Natural Abundance (p,n) Q-value Isotope % Natural Abundance (p,n) Q-value Agl07 51.35 -2.223 SnU? 7.67 -2.602 W184 30.64 N.C. A g 1 0 9 48.65 -0.940 S n 1 1 8 23.84 N.C. W186 28.64 -3.300 Gdl\u00b0 6 1.22 N.C. S n 1 1 9 8.68 -1.361 Pt-192 0.78 -4.020 C d 1 0 8 0.89 -5.880 S n 1 2 0 32.75 -3.505 P t 1 9 4 32.8 -3.348 C d 1 * 0 12.43 -4.742 S n 1 2 2 4.74 -2.37 P t 1 9 5 33.7 -1.053 CdUl 12.86 -2.020 S n 1 2 4 6.01 -1.380 P t 1 9 6 25.4 -2.570 C d 1 1 2 23.79 -3.398 Sb*2* 57.25 -0.399 P t 1 9 8 7.23 -1.440 CdH3 12.34 -0.473 S b 1 2 3 42.75 -0.820 A u 1 9 7 100 N.C. CdH4 28.81 -2.203 jl27 100 -1.480 P b204 1.37 -5.068 G d 1 1 6 7.66 -1.370 C s l 3 3 100 -1.273 p b206 26.26 -4.383 I n 1 1 3 4.16 -1.466 C e 1 4 0 88.45 -4.042 P b207 20.82 -3.183 I n 1 1 5 95.84 -0.282 C e 1 4 2 11.1 -1.480 p b208 51.55 -3.659 Sn l l 2 1.01 N.C. P r 1 4 1 100 -2.582 B i 2 0 9 100 -2.68 S n 1 1 4 0.68 N.C. T a 1 8 1 100 -0.982 y238 99.28 -0.633 S n 1 1 5 0.35 N.C. W182 26.31 N.C. S n 1 1 6 14.28 -5.48 W183 14.28 N.C. -72-Of a l l the materials used as collimators and target backings, gold, platinum and reactor grade graphite produced the lowest neutron count rate i n the chamber (20 thermal neutrons per milliooulomb of proton beam on target) . Stopping the proton beam outside the castle reduced the count rate to less than 100 neutrons per coulomb of stopped beam. The number of neutrons produced by (\u00a3 ,n) reactions i n the material surrounding the target and chamber was at least an order of magnitude lower than those produced by (p,n) reactions. That i s , the presence of a 9.17 mev gamma f l u x did not increase the neutron f l u x to a measurable extent over the neutron f l u x observed with no gamma rays. The 3-body spectrum shown i n Chapter V was obtained using a platinum target backing and a platinum beam tube l i n e r . A self-supporting carbon-13 target would allow the proton beam to pass through the target and be stopped outside the c a s t l e . Preliminary runs with no target but with the beam passing through the castle indicate that neutron count rates of less than 200 neutrons per coulomb of proton beam can be attained. APPENDIX E Reprinted from T H E P H Y S I C A L REVIEW, Vol. 132, No. 4, 1691-1692, 15 November 1963 Printed in U. S. A. Photod i s in teg ra t ion of H e 3 nea r the T h r e s h o l d * J . B . W A R R E N , K . L . E R D M A N , L . P . R O B E R T S O N , ! D . A . A X E N . J AND J . R . M A C D O N A L D J Physics Department, University of British Columbia, Vancouver, British Columbia, Canada (Received 1 July 1963) The total cross section for the reaction He3 (y,p)T> has been measured at gamma-ray energies of 6.14, 6.97, and 7.08 MeV. The cross section was found to be 0.102, 0.298, and 0.307 mb at the three energies. The experimental cross-section values are compared with those of the inverse reaction D(p,y)Hc?, as an accurate check on the principle of detailed balance. TH E photodisintegration of He 3 has been observed by Cranberg1 and Berman et al? This letter describes the measurement of the total cross section of the reaction He3(Y,\/>)D (Q=\u2014 5.493 MeV) at gamma-ray energies of 6.14, 6.97, and 7.08 MeV. The experi-mental cross-section values are compared with those of the inverse reaction as an accurate check on the principle of detailed balance. The reaction was observed in a cylindrical, gridded ionization chamber of active volume 1.485 liters. The chamber contained 1.05 atm of He 3 , 0.0187 atm of methane, and 1.36 atm of argon. The methane was added to reduce the resolving time of the chamber to less than 2 usee, whereas the argon served as a stopping gas for the photodisintegration products. It was necessary to reduce the tritium contamination in the He 3 to eliminate the electron background due to the \/3 decay of tritium. The tritium contamination was reduced from one part tritium per 106 parts He 3 to five parts tritium per 1010 parts He 3 by freezing the tritium at 4 .2\u00b0K. The charac-teristics of the ionization chamber and the purifica-tion technique will be published in a separate communication.3 Two identical chambers were used in the experiment and were placed symmetrically on either side of the gamma-ray source. One chamber contained He 3 , methane, and argon, the other contained He 4 , methane, and argon. The outputs of the two chambers were fed through separate amplifying systems into separate halves of the memory of a model N D 103 Nuclear Data pulse-height analyzer. In this way, all effects not specific to the He 3 could be monitored. Elaborate wax and cadmium shielding was required to attenuate the thermal neutron background and thus reduce the cap-ture of thermal neutrons by the He 3. The gamma rays were produced by bombarding CaFs targets with protons accelerated by a 3 MeV Van de Graaff generator. Table I shows the relative yields and F I G . 1. Photodisinte-gration spectra. 1 3 0 1 120 IIO IOO 9 0 SO-TO 60 80 40 SO 20 IO-I 0.5 6 7 3 . 5 KEV RESONANCE 995 KEV RESONANCE He3+ Y '7.12 M \u00ab V I 1.0 I 1.5 '\u2022>.\u00ab\u2022'\u20ac N ERG Y IN M E V * Research supported by a grant from Atomic Energy ,of panada, Ltd. t Holder of a National Research Council Studentship 1960-:<>2f.Present'address: Rutherford High,Energy Laboratory, Oxford, England. ' \u2022''\u2022*'\u00a3\u2022'\u2022;'. \u2022 \u2022 ' \"\"; \" '\u2022' J's'iSi?.\/ i Holder of a National Research Council Studentship 1961-63. 1 L. Cranberg, Bull. Am. Phys. Soc. 3, 173 (1938).\" \u00abi. \u2022-.\u2022..->:,\u2022>-- ...... 8 B. L. Berman, L. J. Koester, Jr., and J. H. Smith, Phys. Rev. Letters 10, 527 (1963). ' K. L. Erdman, L. P. Robertson, D. A. Axen, and J. R. MacDonald, Can. J. Phys. (to be published). 1691 1692 W A R R E N , E R D M A N , R O B E R T S O N , A X E N , A N D M A C D O N A L D 0.5 l.o I.S ^ R E A C T I O N * [ E Y \" 5 . 4 9 3 ] M E V F I G . 2. Comparison of cross sections for the inverse reactions Ke?(y,p)T) and D(p,y)Ke'. angular distributions of the gamma rays from the reac-tion F 1 9 ( ^ , a 7 ) 0 1 6 at the two proton bombarding energies used i n the exper iment . 4 - 6 T A B L E I. Relative yields and angular distribution of gamma rays. - ^ p r o t o n (keV) Relative gamma-ray yields 6.14 6.92 7.12 MeV MeV MeV p\u2014y angular distribution 873.5 73% 20% 7% 6.14 MeV 1-0.1 cos^fl 6.92 MeV 1+0.51 cos=0-O.22 cos'0 7.12 MeV 1+0.622 cosV 935 75% 3% 22% Isotropic at all energies \u00abJ. M. Freeman, Phil. Mag. 41, 1225 (1950). 6 H. J. Martin, \\V. A. Fowler, C. C. Lauritsen, and T. Lauritsen, Phys. Rev. 106, 1260 (1957). 6 R. W. Peterson, W. A. Fowler, and C. C. Lauritsen, Phys. Rev. 96, 1250 (1954). T A B L E II. Cross section for the reaction He?{y,p)D. Total probable error including Experi- uncertainties in cr mental relative gamma-(MeV) (mb) error ray yields 6.14 (873-keV resonance) 0.109 9% 14% (935-keV resonance) 0.102 6% 7% 6.97 0.298 5% 16% 7.08 0.307 5% 8% The gamma flux was measured with an accurately calibrated N a l ( T h ) scinti l lat ion counter. The efficiency for this counter had been previously measured by Grif-fiths, Larson, and Rober tson 7 at a gamma-ray energy of 6.14 M e V . _ The photodisintegration spectra at the two fluorine resonances are shown in F i g . 1. Below 1.2 M e V , the spectra from the two resonances are essentially identical. I n calculating the cross sections at 6.92- and 7.12- M e V gamma-ray energies, no attempt was made to separate the peaks. Instead, the cross section is given in terms of a mean gamma-ray energy assuming the ratios of 6 . 9 2 -and 7.12-MeV gamma rays as given in Table I , and taking into account the rate of change of cross section wi th change i n gamma-ray energy. The experimental cross section values are shown in Table I I . The inverse reaction, D(\/>,7)He 3, has been studied by several worke r s . 7 - 1 1 Figure 2 shows the results of the application of the principle of detailed balance to the experimental measurements of Griffi ths. 1 1 The photo-disintegration cross-section values are shown for com-parison. The agreement is well wi th in experimental error. 7 G. M. Griffiths, E. A. Larson, and L. P. Robertson, Can. J. Phys. 40, 402 (1962). 8 W. A. Fowler, C. C. Lauritsen, and A. V. Tollerstrup, Phys. Rev. 76, 1767 (1949). 8 D. H. Wilkinson, Phil. Mag. 43, 659 (1952). 10 G. M. Griffiths and J. B. Warren, Proc. Phys. Soc. (London) A68, 781 (1955). 11 G. M. Griffiths (private communication). lM'l-'WWIA, if Reprinted from THE REVIEW OF SCIENTIFIC INSTRUMENTS, Vol . 33, No. 10, 1111-1112, October, 1962 Printed in U. S. A. Gas Flow Regulator for an rf Ion Source* B . L . WHITE, L . 1'. ROHERTSON,| K . L . ERDMAN, AND J . R. MACDONALDI Physics Department, University of British Columbia, Vancouver S, B. C, Canada (Received June 28, 1962) THE beam extracted from an ion source for injection into a Van de Graaff accelerator must be stable in intensity and focus for long periods of time. The intensity and focus are both functions of two externally controllable OSCILLATOR PLATE SUPPLY RF OSCILLATOR TO ACCELERATOR TUBE 1 A ION SOURCE OSCILLATOR COIL OAS FLOW REGULATOR i DIFFERENCE VR AMPLIFIER TO OAS BOTTLE FIG. 1. Block diagram of regulator system. ion source parameters, the plasma density and the ex-tracting field. Geometrical factors affecting the beam usually remain fixed while the ion source is operated, and the extracting field is easily regulated by conventional means. This leaves the plasma density as the factor whose stability will ultimately determine the beam stability. The plasma density is in turn a function of the power coupled into the discharge from the rf oscillator, and of the gas pressure within the discharge tube. However, > o o o e \u00a9 o o o o o\/o o o \u00a9\/o \u00a9 FIG. 2. Thermal leak. Body made of brass. The leak is mounted in an evacuated case for thermal stability and minimum power con-sumption. (1) Set screw, (2) soft solder vacuum seal, (3) Nichrome heater, 22.5 il, (4) stainless steel rod, (5) asbestos, (6) steel ball, (7) ball seat, taper formed with drill tip, ball forms tight shut-off seat by pressure deformation of brass. when the power supply voltages for the oscillator are set at fixed values, the power coupled into the plasma is directly proportional to the gas pressure in the discharge tube. This note describes an apparatus which stabilizes the plasma density by maintaining the oscillator power output constant at its optimum level by means of a servo system controlling the pressure in the discharge tube (Fig. 1). A signal proportional to the oscillator power regulates the flow of gas into the discharge tube. The gas flow rate is controlled by the power dissipation in the thermal leak heater (Fig. 2 ) . As the heater power is increased from zero, the leak temperature rises, and the differential expansion of the brass case and the stainless steel rod allows a gap to form between the ball and its seat, the size of the gap determining the leak rate from the high pressure reservoir into the discharge. The set screw is adjusted so that at a heater power input of 15 W the leak just begins to open; it will then shut off tight at OSCILLATOR PLATE SUPPLY FIG. 3. Servo circuit. a power input of about 6 W. In the present system, how-ever, the leak is not required to act as a tight shutoff, since the various gas supply bottles are shut off separately with solenoid valves. The servo circuit (Fig. 3) is driven by an error signal derived by comparing the voltage Vi .developed across Ri (by passing through Ri a fraction of the direct current in the rf oscillator plate circuit) to the reference voltage VJI developed across the Zener diodes D 3 and Di. The comparison is made in the difference amplifier Ti, after the dc level of V i has been shifted by means of the Zener diodes Di and D-.. The emitter-followers T\u00bb and T 3 provide successive stages of power gain. The gain feedback product around the loop (oscillator power level \u2014> difference am-plifier \u2014> power amplifier \u2014> thermal leak\u2014> gas pressure \u2014> oscillator power level) is about \u201410, which is sufficient to stabilize the discharge conditions so that the beam current onto the target of the accelerator is maintained constant to within a few percent over periods of many hours. * Research supported by a grant from Atomic Energy of Canada, Ltd. f Recipient of a National Research Council of Canada Studentship. APPENDIX G Rojirintcd from Tut: REVIEW OF SCIENTIFIC I N S T R U M E N T S , Vol. 34, No. 11, 1280-1281, November. 1963 Printed in U. S. A. Removal of Tritium from He 3* K. L. E R D M A N , \u2022 L. P. R O B E R T S O N , ! D . AxEN, f AND J. R . MACDONAI-Df Physics Department, University of British Columbia, Vancouver 8, B.C., Canada (Received 22 July 1963) CO M M E R C I A L L Y available H e 3 has a t r i t ium con-centration of approximately 2 parts in 10 s. As t r i t ium is beta active the concentration must be reduced to less than 1 part in 10 1 0 before H e 3 can be used in ionization chambers or proportional counters. Hanson et al.1 report concentrations of 7 parts in 10\" by dist i l l ing the H e 3 at 1\u00b0K. E l l i o t t 2 reports concentrations of 1 part in 10\" by absorbing the t r i t ium in pyrophoric uranium. We have found that similar concentrations are obtainable b y adding approximately 1% hydrogen to act as a carrier gas and then freezing the hydrogen and t r i t ium from the mixture at 4 .2\u00b0K. A schematic diagram of the purification system is shown i n F i g . 1. The helium cryostat was of standard design. The H e 3 was precooled in a l iquid nitrogen bath above the l iquid H e 4 . T w o German silver tubes 0.278 cm in diameter and 30 cm long, containing spiral inserts to increase the effective tube, length, were soldered together to serve as a heat exchanger. The trap in the cryostat had a volume of 6 cm 3 . The transfer pump was constructed of commercially available micro-finished Shelby steel tubing. The piston was fitted wi th a double O-ring seal and the port ion of the upper cylinder below the piston was connected to a reser-voir filled wi th argon such that the argon pressure always exceeded the H e 3 pressure above the piston. This pre-caution insured that no nitrogen or oxygen would con-taminate the H e 3 and destroy its qual i ty as a counter gas. The piston was driven by compressed air admitted to the lower of the two cylinders. The purification procedure was as follows. The system TO GEIOER COUNTER 6 SYSTEM F I G . 1. Schematic of He* pur-ification system. TO AIR CONTROL FOR TRANSFER PUMP DRIVE HELIUM CRYOSTAT 2 N OTES \u2022was evacuated and tested for leaks. The cryostat was cooled to liquid helium temperature with the vacuum system connected to ensure that no leaks developed during the cooling process. Carrier hydrogen was then added to the impure He 3. With the valve to the transfer pump closed, the gas from the bottle was leaked slowly into the trap in the cryostat until the pressure in the system came to equilibrium. This usually took 2 to 3 min. The initial pressure in the He 3 bottle was slightly below atmospheric pressure. The valve to the bottle was closed and the gas in the trap was transferred to the clean storage bottle using the transfer pump. The process was repeated until the He 3 bottle was evacuated. During the purification, samples of the purified gas were let into a Geiger counter of 25-cm3 active volume and the activity.measured. With 1900 V applied to the central electrode of the counter, a Geiger plateau of 150 V was achieved with a filling of 2.5 cm Hg of helium and 10 cm Hg of methane. The counting was done with a Berkeley scaler model 2001. The background activity was measured with a He4-methane mixture. The counter had to be etched after a few runs to remove the background due to tritium occluded at the walls. 6.5 liters of He 3 were purchased from the Monsanto Corporation with a quoted tritium concentration of 1.7 parts in 108. With the addition of 1% hydrogen to the He 3, the tritium concentration was reduced to 6 \u00b1 2 parts in 10\". Approximately 2.5 liters of liquid helium were evaporated in the process. * Research supported by a grant from Atomic Energy of Canada Ltd. t Recipient of a National Research Council of Canada Studentship, 1961-63. 1 E. R. Hanson, H. H. Otuski, L. Passell, W. H. Lien, and N. E. Phillips, Rev. Sci. Instr. 30, 591 (1959). 2 M. J. W. Elliott, Rev. Sci. Instr. 31, 1218 (1960). APPENDIX H Reprinted from T H E REVIEW OE SCIENTIFIC I N S T R U M E N T S , Vol. 35, No. 2, 241, February 1964 Printed in U. S. A. Simple Gas Circulation Pump* K . L . E R D M A N , J. R . M A C D O N A L D , ! G . A . BEER,*; 'AND D . A . A X E N | Physics Department, University of British Columbia, Vancouver 8, British Columbia, Canada (Received 10 October 1963) ' | V H E standard method of purifying gases in ionization chambers, proportional counters, and gas scintilla-tion counters is to pass the gas over hot calcium-magnesium eutectic mixtures.1'2 One usually depends on convection currents to circulate the gas through the purifier. For efficient circulation of the gas, the tubes leading to and from the purifier must be large, and thus the volume of purifier and associated connections may represent a large fraction of the total volume of the chamber. This note describes a pump with a small volume and relatively high rate of gas circulation. The details of construction of the pump are shown in Fig. 1. The cycle is as follows: (1) -The piston is shown in the position with the solenoid energized. The solenoid is energized by a pulse of 0.3-sec duration that drives the piston upwards. During this part of the cycle the upper valve is closed, pushing the gas out of the pump cylinder, and the lower valve opens to admit gas to the lower part of the pump cylinder. (2) When the voltage pulse is removed from the solenoid, the piston falls under gravity and is stopped by the spring. The lower valve closes and the upper valve opens, allowing the gas to leak into the space above the piston. The piston clearance in the cylinder is 0.002 in. and negligible leakage occurs during the stroke. The cycle is repeated once per second. The total dis-placement of gas per cycle is 4 cm 3 for the size pump shown. One can thus pump 14 liters\/h. The pump may be operated continuously for several months without excessive wear or loss of efficiency and has been tested at pressures up to 11 atm. * Research supported by a grant .from Atomic Energy of Canada, Ltd. t Recipient of a National Research Council of Canada Studentship (1961-1963). X Present Address: University of Saskatchewan, Saskatoon, Saskatchewan, Canada. ' O. Ruff and H. Hartmann, Z. Anorg. Allgem. Chem. 121, 167 (1922). 2 N. Colli and U . Facchini, Rev. Sci. Instr. 23, 39 (1952). SCALE - INCHES FIG. 1. Gas circulation pump. All joims are silver soldered. (1) copper tube; (2) adjustable clamp to hold solenoid; (3) soft iron cap, slotted for removal; (4) mild steel valve; (5) soft iron piston; (6) ac or dc solenoid; (7) stainless steel cylinder; (8) piano wire spring; (9) screw cap, slotted; (10) 3 equally spaced ports; (11) Neoprene O-ring; (12) 6 equally spaced screws; (13) mild steel valve; and (14) copper tube. APPENDIX I Reprinted from T H K RKVIKW OK SCIENTIFIC INSTRUMENTS, Vol. 35, No. 1, 122-123, January 1964 rrintcd in U. S. A. Simple Electron Bombardment Apparatus for Evaporating Boron* K . L . E R O M A N , D . A X E N,! J. R. M A C D O N A L D , ! AND L . P . R O B E R T S O N ! Physics Department, University of British Columbia, Vancouver S, B. C, Canada (Received 22 July 1963; and in final form, September 16, 1963) ' | % HIS note describes an electron bombardment appara-tus which has proven to be successful in preparing evaporated films of high resistivity materials. Tempera-tures in excess of 2500\u00b0C were obtained. The evaporator (Fig. 1) was enclosed in a commercially available1 2-in. Pyrex tee connection (1). This design has the advantages that no high voltage vacuum feedthrough and no water-cooling connections are required. The filament power was supplied by a 115-A 5-V trans-former. The high voltage was supplied by a Scientific Electric model P3-238 dc power supply rated to deliver FIG. 1. Schematic vertical section of electron bombardment apparatus. 20 mA current at 20 kV. The glass tee was cooled with a Dayton model 2C 610 air blower. The evaporator was attached to a mercury diffusion pump which produced a residual pressure of approximately 2 X IO - 5 mm Hg. Two threaded brass rods served as filament terminals. One of the rods (2) was attached to the brass plate (3) at the top of the glass tee. The other (4) was admitted through a Kovar seal (5). The upper brass plate was at ground potential. Two 0.010-in.-thick tantalum plates \\ in. apart, (6), served as focusing electrodes for the electron beam. The hole through the center of these plates was XTT in. in diameter. The filament (7) consisted of a single turn of O.OlO-in.-diam tungsten wire 2 in. long. The material to be evaporated was placed in a well at the top of a carbon rod (8) \\ in. in diameter and \\ \\ in. long. For the reasons dis-cussed by H i l l , 2 the carbon rod was attached to a brass Sylphon bellows (9) movable in all three directions by the adjusting screw (10). The metal backing (11), onto which the film was deposited, was clamped to the grounded filament electrode approximately l^ - in. above the upper tantalum plate. The high voltage lead was connected directly onto the brass plate (12) at the lower end of the glass tec. Initially, the carbon rod was outgassed by applying 5 k V to the lower plate and enough filament current to produce approximately 1 mA of electron current and then pumping the system for \\ h. A typical set of operating conditions during the evaporation of boron, which requires tempera-tures above 2500\u00b0C, was filament current, 6A; accelerating voltage, 5 k V ; electron current, .20-30 m A ; and pressure, 4 X l f r s nun Hg. The apparatus was operated under these conditions for periods up to 3 h. Under these conditions red diffraction rings appeared on the film at a rate of one per minute. This rate of deposition was extremely sensitive to the position of the boron. The best results were obtained when the boron was situated approximately ^ in.'below the lower tantalum plate. < * Research supported by a grant from Atomic Energy of Canada Ltd. ! Recipient of a National Research Council of Canada Studentship 1961-63. 1 Pyrex tec available from Corning Glass Works, Corning, New \u2022\u2022'-Sifork.' 2 H. A. Hill, Rev. Sci. fnstr. 27, 10S6 (1956). APPENDIX J Charged Photoparticles from Argon M.A. Reiraann, J.R. MacDonald and J.B. Warren Abstract 4 0 j. Charged photoparticles from A have been observed i n a gridded i o n i z a t i o n chamber i r r a d i a t e d with 9.17 MeV and 17.71 MeV gamma rays. P a r t i a l and t o t a l cross sections are given for the reactions 4 0 A ( r , p ) 3 9 C l and 4 oA ( j<,a ) 3 6 S . I. Introduction Because i t i s r e l a t i v e l y well understood, the electromagnetic i n t e r a c t i o n suggests i t s e l f as a powerful t o o l for the investigation of nuclear structure. Thus quanta of r a d i a t i o n have the advantage over p a r t i c l e s as participants i n nuclear reactions, i n that the i n t e r a c t i o n Hamiltonian i s known. Over the past two decades t h i s know-ledge has been u t i l i z e d i n the construction of nuclear models enjoying considerable success i n accounting f o r the giant dipole absorption resonance observed i n nuclear reactions . The advance of our understanding of the mechanism involved i n photonuclear d i s i n t e g r a t i o n following the large amount of experimental work which has been done^ , has been hampered by the lack of a suitable source of gamma rad i a t i o n , and shortcomings i n instrumentation f o r monitoring the a v a i l a b l e sources. Bremsstrahlen have been used e f f e c t i v e l y to e s t a b l i s h the shape of the dipole resonance, and to reveal the gross features of the energy d i s t r i b u t i o n of the photo-di s i n t e g r a t i o n products. The r e s u l t s have been used to estimate l e v e l densities i n heavier n u c l e i , and have lead to descriptions of the d i s i n t e g r a t i o n process i n terms of both s t a t i s t i c a l 6 ^ and d i r e c t photoeffect 7^ models. In order to assess the pertinence of these models, and i n order to examine the f i n e structure, both of the dipole absorption cross section as a function of energy and of the energy spectrum of emitted photoparticles, the photodisintegration must be i n i t i a t e d by a monochromatic source. To t h i s end the photodisintegration of argon at 17.6 MeV e x c i t a t i o n was investigated by Wilkinson and 8) Carver 9 ] Our present knowledge of the reaction Q-values ', as pointed out by S p i c e r * 0 ^ , does hot admit the i n t e r -pretation placed by Wilkinson and Carver upon t h e i r r e s u l t s . For t h i s reason the reactions 4 0 A ( ^ , p ) 3 9 C l and 4 o A ( ^ , a ) 3 6 S have been re-examined at a photon energy Of 17.71 MeV and the cross sections determined. In addition the photoalpha cross section was measured at a photon energy of 9.17 MeV. 2 . E x p e r i m e n t a l The e n e r g y s p e c t r a o f c h a r g e d p h o t o p a r t i c l e s were o b t a i n e d u s i n g a g r i d d e d i o n i z a t i o n c h a m b e r . The chamber i s d e s c r i b e d by Hay a n d W a r r e n 1 1 \\ b u t i n t h e p r e s e n t c a s e t h e i n t e r i o r was l i n e d w i t h a 35 um t h i c k n e s s o f M y l a r w i t h s u f f i c i e n t g o l d e v a p o r a t e d o n t o i t t o r e d u c e t h e r e s i s t i v i t y o f t h e i n n e r s u r f a c e t o a p p r o x i m a t e l y a t h o u s a n d ohms p e r cm. T h i s t e c h n i q u e was f o u n d more e f f e c t i v e t h a n a g r a p h i t e c o a t i n g f o r r e d u c i n g b a c k g r o u n d due t o a c t i v i t y i n t h e chamber w a l l s . The chamber was f i l l e d w i t h h i g h g r a d e w e l d i n g a r g o n c o n t a i n i n g l e s s t h a n 0 . 0 2 % o f i m p u r i t i e s , t h e s e b e i n g o x y g e n , n i t r o g e n , h y d r o g e n and c a r b o n d i o x i d e . A f t e r f i l l i n g , t h e i m p u r i t y c o n t e n t was f u r t h e r r e d u c e d by c o n v e c t i o n c i r c u l a t i o n o f t h e gas t h r o u g h a h e a t e d s i d e a r m c o n t a i n i n g a c a l c i u m - m a g n e s i u m ' 12) \"\" '\" e u t e c t i c m i x t u r e W i t h an a r g o n p r e s s u r e o f 6 . 8 0 atm a few h o u r s o f p u r i f i c a t i o n i n t h i s manner r e s u l t e d i n an e n e r g y r e s o l u t i o n o f 4% f o r 5 . 1 5 MeV a l p h a p a r t i c l e s f r o m a Pu239 c a l i b r a t i n g s o u r c e . The c a l c i u m - m a g n e s i u m e u t e c t i c c o n t a i n e d r a d i o a c t i v e 222 i m p u r i t i e s w h i c h e m i t t e d t h e i s o t o p e Rn . T h i s i s o t o p e was i d e n t i f i e d f r o m t h e ; p e a k s p r o d u c e d i n t h e chamber b a c k -g r o u n d c o r r e s p o n d i n g t o t h e e n e r g i e s p r e d i c t e d by t h e d e c a y scheme f o r t h i s n u c l e i d e . W h i l e t h e s e p e a k s i n t u r n p r o v i d e d a u s e f u l c h e c k on t h e e n e r g y s c a l e s e t by t h e c a l i b r a t i n g s o u r c e , t h e b a c k g r o u n d a c t i v i t y c o u l d be r e d u c e d t o n e g l i g i b l e p r o p o r t i o n s f o r t h e d u r a t i o n o f t h e e x p e r i m e n t a l work by b e g i n n i n g t h e r u n s w i t h i n a few h o u r s o f f i l l i n g t h e chamber w i t h f r e s h g a s . V o l t a g e p u l s e s f r o m t h e chamber were a m p l i f i e d by a D y n a t r o n P r e a m p l i f i e r U n i t m o d i f i e d t o i n c o r p o r a t e a t y p e 7586 N u v i s t o r a s a f i r s t s t a g e , f o l l o w e d by a D y n a t r o n M a i n A m p l i f i e r 1430 A . D i f f e r e n t i a t i n g a n d i n t e g r a t i n g t i m e c o n s t a n t s o f 8 u s were u s e d f o r p u l s e s h a p i n g , a n d p u l s e s were a n a l y s e d i n 128 c h a n n e l s o f a N u c l e a r D a t a M o d e l ND 103 256 c h a n n e l p u l s e h e i g h t a n a l y s e r . Gamma f l u x measurements were made w i t h a 6 . 9 8 cm d i a m e t e r by 1 1 . 4 cm l o n g N a l ( T l ) c r y s t a l . The e f f i c i e n c y o f t h i s s c i n t i l l a t i o n c o u n t e r has been d e t e r m i n e d t o w i t h i n 15% a t 1 7 . 7 MeV, a n d i s known t o w i t h i n 10% a t 9 . 1 7 M e V 1 3 - 1 5 ) . The gamma s p e c t r a were a l s o m o n i t o r e d b y a N u c l e a r D a t a M o d e l ND 120 256 c h a n n e l p u l s e h e i g h t a n a l y s e r . The 9 . 1 7 MeV gamma r a y s were o b t a i n e d by b o m b a r d i n g a 1 3 C t a r g e t 3 0 keV t h i c k w i t h 1 .75 MeV p r o t o n s f r o m t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 3 MeV Van de G r a a f f a c c e l e r a t o r . The t a r g e t was p r e p a r e d b y c r a c k i n g i s o t o p i c a l l y e n r i c h e d m e t h y l i o d i d e o n t o a p l a t i n u m b a c k i n g 1 6 K The y i e l d o f 9 . 1 7 MeV gamma r a y s was d e t e r m i n e d by s p e c t r u m s t r i p p i n g t o s e p a r a t e o u t t h e 6 . 4 4 MeV c o m p o n e n t 1 6 ) . F o r t h e 9 . 1 7 MeV w o r k , t h e i o n i z a t i o n chamber was s i t u a t e d a t 90\u00b0 t o t h e p r o t o n beam. Gamma r a y s o f e n e r g y 1 7 . 6 4 MeV were o b t a i n e d f r o m t h e r e a c t i o n ^ L i C p , ? ' ) ^ e a t t h e 441 keV r e s o n a n c e . A d i f f e r e n t t a r g e t was u s e d f o r e a c h r u n , t h e s e b e i n g p r o d u c e d by e v a p o r a t i n g m e t a l l i c l i t h i u m o n t o a c o p p e r b a c k i n g . The t a r g e t t h i c k n e s s v a r i e d f r o m 3 0 KeV t o 60 k e V . The i o n i z a t i o n chamber was a t 0\u00b0 t o t h e p r o t o n beam. A t t h i s p o s i t i o n t h e D o p p l e r s h i f t i s + 67 k e V , g i v i n g p h o t o n s o f e n e r g y 1 7 . 7 1 MeV. x The r a t i o o f 1 7 . 7 MeV t o 1 4 . 8 MeV gamma r a y s was o b t a i n e d by s p e c t r u m s t r i p p i n g . F o r t h i s p u r p o s e t h e s p e c t r a l s h a p e o f t h e 1 7 . 7 MeV component was assumed t o be s i m i l a r t o t h a t o f t h e 1 7 . 1 MeV component f r o m t h e r e a c t i o n ^B(p,y)^C a t a b o m b a r d i n g e n e r g y o f 1 .2 MeV. A t t h e s c i n t i l l a t i o n c o u n t e r , p o s i t i o n e d a t 138\u00b0 t o t h e p r o t o n beam, t h e r a t i o o f 1 7 . 6 t o 1 4 . 8 MeV gamma y i e l d s was f o u n d t o v a r y f r o m 1 , 8 7 +_ 0 . 1 f o r a 3 0 keV t a r g e t t o 1 . 7 8 + 0 . 1 f o r a 60 keV t a r g e t . T h e s e r e s u l t s a r e c o n s i s t a n t w i t h t h o s e o b t a i n e d by M a i n s b r i d g e i 7 ) . The r a t i o o f y i e l d s i n c r e a s e s by a p p r o x i m a t e l y 4% when o b s e r v e d a t 0\u00b0 t o t h e p r o t o n beam, due t o t h e a n i s o t r o p i e s i n v o l v e d 1 \u00ae ) . The gamma f l u x t h r o u g h t h e chamber was c a l c u l a t e d a c c o r d i n g l y . 3 . R e s u l t s A c a l c u l a t i o n o f t h e c r o s s s e c t i o n f o r t h e e v e n t s o b s e r v e d r e q u i r e s k n o w l e d g e o f t h e e f f i c i e n c y o f t h e i o n i z a t i o n c h a m b e r . T h i s i s a g e o m e t r i c c a l c u l a t i o n c o m p r i s i n g i n t e g r a t i o n over the s e n s i t i v e volume of the chamber to e s t a b l i s h the mean gamma f l u x - p a t h l e n g t h product. This c a l c u l a t i o n was 19) c a r r i e d out by Axen and Robertson ' a s a f u n c t i o n of chamber dimensions and t a r g e t p o s i t i o n . The number of events a s s o c i a t e d w i t h each peak i d e n t i f i e d i n the p h o t o p a r t i c l e energy d i s t r i b u t i o n s was obtained by c o r r e c t i n g the number of counts i n the peak f o r time dependent background, w a l l e f f e c t and, i n the 17.7 MeV work, estimated c o n t r i b u t i o n due to 14.8 MeV r a d i a t i o n . Wall e f f e c t i s due t o two causes, and r e s u l t s i n the accumulation of counts i n the s p e c t r a a t energies lower than a c t u a l l y a s s o c i a t e d w i t h the charged p a r t i c l e s causing them. The most important c o n t r i b u t i o n to w a l l e f f e c t i s the w a l l l o s s . T h i s r e s u l t s i f the charged p a r t i c l e s o r i g i n a t i n g from a p h o t o d i s i n t e g r a t i o n event do not l o s e a l l t h e i r energy by i o n i z a t i o n of the gas i n the s e n s i t i v e volume of the chamber, e i t h e r because of c o l l i s i o n w i t h the chamber w a l l or because they leave the s e n s i t i v e volume and proceed i n t o the dead spaces at the ends of the chamber. The other c o n t r i b u t i o n t o w a l l e f f e c t i s the end e f f e c t . This r e f e r s t o events t a k i n g place i n the dead spaces g i v i n g r i s e t o charged p a r t i c l e s which enter the s e n s i t i v e volume a f t e r l o s i n g some of t h e i r energy ou t s i d e i t . The c a l c u l a t i o n of the w a l l e f f e c t i s a l s o a geometric one, i n v o l v i n g the chamber dimensions and the p a r t i c l e ranges, and has been treated i n d e t a i l for c y l i n d r i c a l chambers by Robertson 1 9). The necessary corrections to the cross sections varied from 41% for the most energetic protons to 6% for the lea s t energetic alphas. The predominating source of uncertainty i n our values for the cross sections i s the gamma spectrometer e f f i c i e n c y . The experimental uncertainties i f the gamma counter e f f i c i e n c y were known exactly, are also indicated with the experimental r e s u l t s i n table 1. A considerable uncertainty, +_ 15% for the 9.17 MeV r e s u l t s , i s due to the r e l a t i v e l y large background subtraction involved. This uncertainty i s smaller for the 17.71 MeV work. Other sources of uncertainty, such as knowledge of the angular d i s t r i b u t i o n s of the gamma rays, have been considered i n estimating the possible error of our r e s u l t s , but these are much smaller than the two already mentioned and w i l l not be discussed further. 3.1 Results at 9.17 MeV The energy d i s t r i b u t i o n of charged photoparticles from argon i r r a d i a t e d with 9.17 MeV gamma rays i s shown i n f i g . 1. We i d e n t i f y the peak at 2.29 MeV with the reaction 40 *3fi A(y,a) S leaving the re s i d u a l nucleus i n i t s ground state. We obtain a Q-value of =6.88 + 0.1 MeV for t h i s reaction, i n 9) agreement with Everling ejt aJL , who obtain -6.81 +_ 0.01 MeV. The t o t a l c r o s s s e c t i o n f o r t h e r e a c t i o n i s 31 +_ 6 u b . 3 . 2 R e s u l t s a t 1 7 . 7 1 MeV The i r r a d i a t i o n w i t h l i t h i u m gamma r a y s gave t h e c h a r g e d p h o t o p a r t i c l e s p e c t r a shown i n f i g . 2 . F i g . 2 ( a ) c o m p r i s e s s p e c t r a o b t a i n e d w i t h 6 . 8 0 atm a n d 3 . 4 0 atm o f a r g o n p r e s s u r e r e s p e c t i v e l y , a n d f i g . 2 ( b ) shows r e s u l t s o b t a i n e d a t 7 . 0 1 atm w i t h i m p r o v e d e n e r g y r e s o l u t i o n . The i d e n t i f i c a t i o n o f p e a k s B a n d C a s p h o t o a l p h a p e a k s i s c o n f i r m e d by c o m p a r i s o n o f t h e s p e c t r a i n f i g . 2 ( a ) . B o t h g i v e t h e same c r o s s s e c t i o n w i t h i n e x p e r i m e n t a l e r r o r when w a l l e f f e c t c o r r e c t i o n s a p p r o p r i a t e t o a l p h a p a r t i c l e s a r e made f o r p e a k s B a n d C , a n d a p p r o p r i a t e t o p r o t o n s a r e made f o r peak D . D i f f e r e n t a s s i g n m e n t s w o u l d n o t g i v e s u c h a g r e e m e n t . B e c a u s e o f t h e w i d t h o f t h e 1 4 . 8 MeV component o f t h e gamma f l u x , i t s c o n t r i b u t i o n t o t h e p h o t o p a r t i c l e s p e c t r u m i s d i s t r i b u t e d o v e r a l a r g e e n e r g y i n t e r v a l a n d hence d o e s n o t i n f l u e n c e t h e s t r u c t u r e s i g n i f i c a n t l y . 2 0 ) F u r t h e r m o r e , t h e b r e m s s t r a h l u n g work o f M c P h e r s o n e t a l shows t h e 4 0 A ( y , p ) 3 9 C l c r o s s s e c t i o n a t 15 MeV t o be down b y a f a c t o r o f s i x f r o m t h a t a t 18 MeV. We assume t h a t t h i s r e d u c t i o n i n t h e c r o s s s e c t i o n i s e n t i r e l y due t o t h e shape o f t h e d i p o l e r e s o n a n c e , s o we u s e t h e same f a c t o r i n c o m p u t i n g t h e y i e l d o f p h o t o a l p h a s due t o t h e 1 4 . 8 MeV r a d i a t i o n from that measured at 17.71 MeV. Thus the number of counts due to the 14.8 MeV gammas was estimated for each possible reaction by taking the number of counts due to the 17.71 MeV gammas for the same reaction corrected for background but not wall e f f e c t , reducing i t by the McPherson et a l factor of s i x , and further reducing i t i n proportion to the r a t i o of 14.8 to 17.71 MeV gamma rays. For the purpose of subtraction from the charged p a r t i c l e spectrum, the approximation was made that these counts were evenly d i s t r i b u t e d over a 2 MeV i n t e r v a l centred on the appropriate energy. In no case did t h i s correction exceed 5%. The r e s u l t s at 17.71 MeV are summarized i n table 1. 4. Discussion If the photonuclear process i n argon proceeds l a r g e l y through the formation of a compound nucleus, the r a t i o of p a r t i a l cross sections f o r decay by charged p a r t i c l e emission through various channels should simply be that of the respective p e n e t r a b i l i t i e s . It would there-fore be expected that the p a r t i a l cross section for 40 39 A(JSP) c l t o t h e ground state of \u00b09C1 would be larger than that to i n d i v i d u a l excited states. We therefore suppose that the peak \u00a3 of f i g . 2 represents t r a n s i t i o n s t o more t h a n one e x c i t e d s t a t e o f C l n o t r e s o l v e d i n our e n e r g y d i s t r i b u t i o n . T h i s p o s s i b i l i t y i s s u p p o r t e d by E n d t a n d v a n d e r L e u n 2 * ^ who show l e v e l s i n ^ 9 C 1 a t 0 . 3 6 and 0 . 8 MeV. On t h e o t h e r h a n d , t h e s u p p o s i t i o n t h a t peak E r e p r e s e n t s two o r more l e v e l s i n ^ C l makes t h e p a r t i a l c r o s s s e c t i o n f o r t h e g r o u n d s t a t e t r a n s i t i o n a p p e a r l a r g e r t h a n p r e d i c t e d f r o m s t a t i s t i c a l c o n s i d e r a t i o n s ; l a r g e r by an e x t e n t d e p e n d e n t u p o n t h e s p i n and p a r i t y a s s i g n m e n t s f o r t h e e x c i t e d s t a t e s , a n d d e p e n d e n t upon how much l o w e r t h a n c l a s s i c a l t h e Coulomb b a r r i e r h e i g h t i s due t o f u z z i n e s s o f t h e n u c l e a r s u r f a c e 2 2 S i n c e t h e p r e d i c t i o n s o f t h e r a t i o o f p e n e t r a b i l i t i e s c a n o n l y be c o n s i d e r e d as a p p r o x i m a t e , i t i s n o t p o s s i b l e t o e s t i m a t e t o what e x t e n t t h i s p a r t i a l c r o s s s e c t i o n r a t i o h a s been i n f l u e n c e d b y c o n t r i b u t i o n s f r o m a d i r e c t p h o t o -e f f e c t . The r a t i o o f t h e p a r t i a l c r o s s s e c t i o n t o t h e g r o u n d 36 s t a t e o f S by a l p h a e m i s s i o n a n d t o t h e g r o u n d s t a t e o f 39 C l b y p r o t o n e m i s s i o n a t 17.71 MeV e x c i t a t i o n was o b s e r v e d t o be a n o r d e r o f m a g n i t u d e s m a l l e r t h a n p r e d i c t e d f r o m t h e p e n e t r a b i l i t i e s . T h i s m i g h t i n p a r t be due t o a s m a l l f o r m a t i o n f a c t o r 2 3 ) f o r a l p h a p a r t i c l e s i n t h e compound n u c l e u s , b u t t h i s c o u l d n o t a c c o u n t f o r t h e e n t i r e d i s -c r e p a n c y . I n t h e a b s e n c e o f known s e l e c t i o n r u l e s o p e r a t i n g 36 a g a i n s t t h e a l p h a t r a n s i t i o n t o t h e g r o u n d s t a t e o f S , t h e e x i s t e n c e o f a n o t h e r l o w l y i n g l e v e l i n ^Cl c o n t r i b u t i n g t o peak D o f f i g . 2 may be i n d i c a t e d . A l t e r n a t i v e l y , t h e c r o s s s e c t i o n f o r a l p h a e m i s s i o n t o t h e g r o u n d s t a t e o f 3 6 S may have b e e n o b s e r v e d a t an e x c u r s i o n b e l o w i t s a v e r a g e v a l u e , d e m o n s t r a t i n g t h e e x i s t a n c e o f e x c i t a t i o n e n e r g y d e p e n d e n t c r o s s s e c t i o n f l u c t u a t i o n s a s d e s c r i b e d b y E r i c s o n 2 4 * 2 5 ) . We a r e i n d e b t e d t o t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada f o r f i n a n c i a l s u p p o r t a n d f o r a w a r d i n g s t u d e n t -s h i p s t o two o f u s ( M . A . R . a n d J . R . M . ) . We w i s h t o t h a n k D r . E . V o g t f o r h i s most h e l p f u l comments , a n d M r . E . W . B l a c k m o r e f o r r e p r o g r a m m i n g t h e gamma f l u x - p a t h l e n g t h c a l c u l a t i o n s f o r t h e i o n i z a t i o n c h a m b e r . Figure 1 Figure 2(a) Figure 2(b) Table 1 Experimental r e s u l t s at 17.71 MeV Spectrum peak Reaction A 4 0 A ( r , a ) 3 6 S Remarks To ground state of 36, Q-value MeV - 6.6+0.4 Cross section mb 0.08+0.05 B 4 0 A ( r , a ) 3 6 S To excited state(s) of 36s at e& 3.5 MeV -10.1+0.1 0.35+0.06 4\u00b0A( r,\u00ab)36 S To excited state(s) of ,36s at 4.7 MeV -11.3+0.1 0.40+0.08 40., ,36c A(^,a) S Total cross section 0.83+0.16 (+ 0\".09) D 4 0 A ( r , p ) 3 9 C l oq To ground state of C l -12.46+0.1 1.15+0.2 E 4 0 A ( r , p ) 3 9 C 1 39 To excited states of C l at 0.5 MeV -12.99+0.1 1.26+0.2 F,G 4 0 A ( r , p ) 3 9 C l To excited states of 3 9 C 1 4.75+1.0 4 0 A ( r , P ) 3 9 C l Total cross section 7.16+1.26 (+0.5*6) The uncertainties i n brackets are estimated on the assumption that the e f f i c i e n c y of the gamma counter i s known exactly. FIGURE CAPTIONS F i g . 1: The photodisintegration of 4 0 A at 9.17 MeV showing the photoalpha peak. 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Barnes, CA,, Carver, J.H,, and Stafford, G.H. (1952), Phys. Rev, 86, 359. Wilkinson, D.H, and Bloom, S.D. (1957), P h i l . Mag. 2, 63. Woodbury, H.H., Day, R.B., and Tollestrup, A.V. (1953), Phys. Rev. 92, 1199. ","attrs":{"lang":"en","ns":"http:\/\/www.w3.org\/2009\/08\/skos-reference\/skos.html#note","classmap":"oc:AnnotationContainer"},"iri":"http:\/\/www.w3.org\/2009\/08\/skos-reference\/skos.html#note","explain":"Simple Knowledge Organisation System; Notes are used to provide information relating to SKOS concepts. 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