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Efficiency calibration of a neutron long counter. MacFarlane, Tracy Lynn 1968

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EFFICIENCY CALIBRATION OF A NEUTRON LONG COUNTER by TRACY LYNN MacFARLANE B . S c , The Univers i ty of New Brunswick, 19&5 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of PHYSICS We accept th is thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1968 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 o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t 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 , I a g r e e t h a t t h e 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 a nd S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d b y t h e Head o f my D e p a r t m e n t o r b y hits r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . The 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 V a n c o u v e r 8 , C a n a d a D e p a r t m e n t ABSTRACT The U.B.C. Hanson-McKibben type Long Counter has been modified to permit inser t ion of a larger diameter BF^ proportional counter. An invest igat ion was made to determine the ef fects of the modif icat ion on the neutron detect ion cha rac te r i s t i cs of the counter. 2.60 MeV and •3 3 .09 MeV neutrons were obtained from the react ion D(d,n) He and 4.5 MeV 241 neutrons from a standard AmBe source. The e f f i c iency of the Long Counter was determined in terms of counting rate per unit neutron f lux occurr ing at the e f fec t i ve centre of the counter. The count rate used was that due to the ^ B ( n , a ) ^ L i pulses from the BF^ counter located in the centre of the Long Counter. The e f fec t i ve centre was defined as that point inside the counter such that the counting rate varied as the inverse square of the distance of the source from the e f fec t i ve centre. The fol lowing table l i s t s the neutron energy, e f f i c iency and distance from the front reference face of the Long Counter to the e f fec t i ve centre. Neutron Energy Ef f i c iency Distance to MeV counts / neutron Ef fec t ive Centre per cm2 at the cm e f fec t i ve centre 2.580 ( ± . 0 6 % ) 26.6 ( ± 2 0 % ) . 12.6 {±3%) 3 .004 (±.3%) 29.4 ( ± 1 9 % ) 10 .3 ( ± 7 * ) 4.5 ( ± 1 0 % ) 14.1 (±2.2%) 10.8 (±6%) The e f f i c iency for 4.5 MeV neutrons was found to have increased i i i by at least an order of magnitude from that of the o r ig ina l Long Counter. However, the dependence of the e f f i c iency on energy has been increased, varying by 50% over the range of energies measured. The sh ie ld ing on the sides of the Long Counter was found to reduce the i n t r i n s i c e f f i c iency for neutrons incident from the side to O.A of that for neutrons incident on the front face. TABLE OF CONTENTS ABSTRACT . . . . . . . . i i LIST OF FIGURES • • • • • • o » « * o * « 0 o » o o * o « * « o o o * « * « « » o o o « « « « « * o » « * o o v ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v i CHAPTER I INTRODUCTION 1.1. General Introduction „ 1 1.2. The Long Counter A CHAPTER II MEASUREMENT OF THE LONG COUNTER EFFICIENCY USING AN 2 l t l AmBe NEUTRON SOURCE 2.1. The Standard Neutron Source . . . . . . . . . . 6 2.2. Measurements 7 2.3. Ca lcu la t ion of E f f i c iency . . . . . . . . . . . . . . . . . 10 CHAPTER III MEASUREMENT OF LONG COUNTER EFFICIENCY USING THE REACTION D(d,n) 3He 3.1. The Reaction D(d,n)^He . . . 13 3.2. The Gas Target 14 3.3. Current Integration . . . . . . . . . . . . . . . . . . . . . . . 16 3.4. Measurements 19 3.6. Gas Target Thickness . . . . . . . . . . . . . . . . . . . . . . 22 3.7. Beam Energy in the Gas Cel l . . . . . . . . . . . . . . . 23 3.8. Neutron Flux from the Gas Target . . . . . . . . . . 23 3.9. Calcu la t ion of E f f i c iency . . . . . . . . . . . . . . . . . 26 Iv CHAPTER IV CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 APPENDIX LONG COUNTER ELECTRONICS A.1 . Proport iona 1 Counter • o « # » o o » » « o » o o » o « 3 ^  241 A . 2 . E lect ron ics for AmBe Ca l ib ra t ion . . . . . . . 31 A.3- E lect ron ics for D(d,n) He Ca l ib ra t ion . . . . . 32 LIST OF FIGURES Figure Follows Page 1. U.B.C. Long Counter . . . . . . . . . . . . . . . . 4 2. Comparison of RaBe and AmBe spectra . . . . . . . . . . . . 5 3. Arrangement of Long Counter and neutron source for e f f i c i ency measurement . in text p. 7 4. Long Counter spectrum for AmBe neutrons 7 5. Arrangement of Long Counter and neutron source for s ide sh ie ld ing invest igat ion in text p. 9 6 . Inverse square plot for AmBe source at end of counter . 9 7. Inverse square plot for AmBe source at side of counter. 10 8. Var ia t ion of energy with angle for neutrons from the react ion D(d,n) 3He . . . . . . . 12 3 9 . Var ia t ion of D(d,n) He cross sect ion with incident deuteron energy 12 10. Gas target and beam co l l imat ion system 13 11. Current measurement system for study of current d i s t r i bu t i on in target 16 12. Results of current d i s t r i bu t i on study . . . . . . . . . . . . . . . . . 17 13. E lec t ron ics for D(d,n) He ca l i b ra t i on 18 1h. Beam dependent background increase with running time . . 20 • 3 15. Inverse square plot for D(d,n) He runs 25 16. E f f i c iency var ia t ion with neutron energy . . . . . . . . . o . . . . 27 17. E f fec t i ve centre va r ia t ion with neutron energy 27 ACKNOWLEDGEMENTS I wish to express my sincere grat i tude to Dr. G„M. G r i f f i t h s for his supervision and assistance throughout the course of th is work. I am also grateful to the students, s ta f f and facu l ty of the Van de Graaff Group for the i r generous help in the laboratory, espec ia l l y Duncan Hepburn, Dr. Grahame Ba i ley , Richard Helmer and Miguel 01ivo. CHAPTER I INTRODUCTION 1 . 1 . General Introduction The Hanson-McKibben (19^7) Long Counter developed at Los Alamos is probably the most widely used of a l l neutron detectors. It consis ts of a long BF^ proportional counter placed in a c y l i n d r i c a l moderator of pa ra f f i n . The moderator is surrounded a l l but on one end by a thermal neutron absorber, usual ly a th in sheet of cadmium, and a second fast neutron moderating layer surrounds the absorber. Neutrons incident on the open end are thermalized in the center moderator and have a high probab i l i t y of entering the BF^ counter, where they produce ^ B ( n , a ) ^ L i reactions in the counter gas. In general the gas is enriched to about 90% in ^ B , compared to the 19-8% of found in natural boron. Ions produced by the charged par t i c les from the reactions are co l lec ted and produce an e l e c t r i c a l pulse which is ampl i f ied and counted e l e c t r o n i c a l l y . Neutrons incident from the side are largely thermalized in the outer moderator and e i ther escape or are captured in the absorber. Therefore they have a small p robab i l i t y of entering the center region and do not contr ibute s ign f i can t l y to the counting rate. Thus, the counter has the highest s e n s i t i v i t y only to those neutrons that enter the center region at one end. Pangher and Nichols (1966) at the P a c i f i c Northwest Laboratory in Richland, Washington have made an improvement by replacing the paraf f in 2 with polyethylene, supported by aluminum rather than steel used by McKibben. The resul t was a highly reproducible detector with considerably better resolut ion on the neutron peak than the McKibben model. The Long Counter is much used in the neutron range from 25 keV to 14 MeV. The absolute ca l i b ra t i on is usual ly based on observing the counting rate for a neutron source with a known neutron output. Standard neutron sources are made of a natural or a r t i f i c i a l l y produced alpha-emitter int imately mixed with beryl l ium in which the neutron y ie ld ar ises 9 12 241 from the react ion Be(a,n) C. Typical sources such as Ra-Be or Am-Be produce a broad spectrum of neutrons from a few MeV to 10 or 11 MeV, with a tendency to peaking in the 3 to 6 MeV range. This provides a reasonable ca l i b ra t i on over the energy range in which the counters are used because the neutron detect ion e f f i c iency of the counter with the usual dimensions is r e l a t i ve l y independent of neutron energy. (Hanson and McKibben, 1947; Pangher and Nicho ls , 1966) The o r ig ina l U.B.C. Long Counter was constructed by Heiberg (1954) using a Chalk River one inch diameter BF^ counter. In 1966, when the counter began to f a i l , a decis ion was made to introduce gas counters of larger diameter and higher pressure containing e i ther BF^ or He. The aim of the present work was to invest igate the propert ies of these counters and in par t i cu la r to make a more accurate measurement of the absolute e f f i c i ency for neutron detect ion using both accurately ca l ibra ted neutron sources and monoenergetic neutrons produced by the Van de Graaff generator. Using monoenergetic neutrons from reactions of known cross sect ion in has been possible to invest igate the e f f i c iency of the detector as a funct ion of neutron energy. A major disadvantage with the Long Counter is i t s slow response to incoming neutrons, making i t of l i t t l e use in time of f l i g h t or concidence measurements. In these cases a s c i n t i l l a t i o n detector is often used. This type of detector consists of a s c i n t i l l a t i n g material mounted on a photomul t ip l ier . Some mater ials used are ^L i - I (T l ) c r y s t a l , ^Li loaded g lass , a compound of 15% Zn-S act ivated with s i l v e r and dispersed in l uc i t e (Hornyak 1952), and various l i qu id and p l as t i c s c i n t i l l a t o r s . These however require elaborate pulse shape analysis to separate the pulses o r ig ina t ing from neutrons and those or ig ina t ing from gamma ray f l ux . The major advantege of the Long Counter, i t s r e l a t i ve l y constant e f f i c i ency with varying energy, is not always a desirable feature. Such a case is health monitoring of neutrons. The re la t i ve health hazard for a given f lux of neutrons increases with energy. It was found by Bramlett, Ewing and Bonner (1960) that a ^L i - l (Eu) c rys ta l placed in the center of a 12 inch polyethylene sphere exhibi ted an e f f i c iency var ia t ion with energy of the incident neutrons which c lose ly approximated the energy va r ia t ion of the health hazard of neutrons. 4 1.2. The Long Counter The Long Counter depends on the detect ion.of the alpha par t i c les emitted by the react ion ^ B ( n , a ) ^ L i produced in boron t r i f l u o r i d e gas in a c y l i n d r i c a l proport ional counter. The energy release in the react ion ^ B ( n , a ) ^ L i is 2.79 MeV. This is considerably greater than the energy lost by electrons t raversing the counter volume. Proport ional operation then ensures that processes leading to a s ing le electron e jec t i on , such as gamma in te rac t ions , produce pulses considerably smaller than the react ion pulses. This is important for a neutron detector since a neutron f l ux is often accompanied by a large gamma ray f l u x . Table 1 shows the cross sect ion of the react ion ^ B ( n , a ) ^ L i as taken from Hughes and Schwartz (1958). TABLE 1 ^B(n,cx)^Li Cross Sections Neutron Energy Cross Section 0.0235 eV 3820 barns 6.0 eV 85 barns 10.0 keV 7 barns C lear ly the e f f i c iency of the proportional counter depends strongly on the energy of the incident neutrons. The counter developed by Hanson and McKibben (19^7) had a paraf f in moderator arrangement to give a counting ra te , as nearly as poss ib le , proportional to the incident neutron f lux and independent Of the neutron energy. By making the moderator appreciably longer than the mean free path of fast neutrons, neutrons k ' 0 >J ft «J > <L<fc -mi OJ <L CL t o 'J. ft ct d S \ N.. 0 (J (0 'L 0 C9 entering the moderator are slowed by successive c o l l i s i o n s in the moderator so that the f rac t ion that d r i f t s into the counting tube at thermal energies var ies l i t t l e with energy. The U.B.C. Long Counter, shown in F i g . 1, is based on the Hanson - McKibben design, however the present version has a larger BF^ counter( 2 inches in diameter) f i l l e d to 70 cm of Hg with 96% ^ B enriched BF^. Further de ta i l s concerning the counter and operating condit ions are given in the Appendix. CHAPTER I I MEASUREMENT OF THE LONG COUNTER EFFICIENCY USING AN 2 i* 1Am-Be NEUTRON SOURCE 2 .1 . The Standard Neutron Source The e f f i c i ency of the long counter shown in F i g . 1 was measured using an accurately ca l ib ra ted neutron source placed at various distances from the counter ranging from 10 cm to 150 cm. The neutron source (type AMN-18) obtained from the Radio-chemical Center, Amersham, England consisted of the alpha-emitter Americium-241 fused with beryl l ium metal. It produces neutrons by the 9 12 react ion Be(a,n) C. The absolute neutron output was measured by K.W. Geiger at the National Research Council of Canada by a d i rec t intercomparison with the Canadian Standard source #118 using a Prec is ion Long Counter (Pangher and Nichols 1966). The Canadian standard had been absolutely ca l ib ra ted by the manganese bath method (Geiger and Baerg, 1965), and intercompared with the standards of other countr ies. The neutron output for the present source was 7.62 x 10^  (±2%) neutrons per second. (Cal ib ra t ion Report APXNR-2095, January 1968) 2h1 Some data on the alpha par t i c les emitted by Am and on the 237 Y~rays from the daughter product Np are given in Table 2 with the strongest l ines underl ined. The neutron energy spectrum is compared with that from a Ra-Be source in F i g / 2 . 7 TABLE 2 241 *\m ->- a + " Np \ i 'Am •  2 3 7 N p (T, = 458 years) a-Energy % Y~ E n e r9y (%) MeV MeV a 0 5-545 0.25 0 a 3 3 5.513 0.12 0.0332 (100) a 6 0 5.486 85.5 0.0594,(94) 0.026 (6) a i o 3 5.443 12.7 0.1029 (22) 0.043 (78) a i 5 9 5.389 1.3 0.125 (11) 0.099 (50) 0.055 (39) plus 18 other alpha groups, a l l less than 0.02%. The complex alpha spectrum, combined with the fact that alpha pa r t i c l es of a l l energies slowing down in the source can produce neutrons 1 2 which leave C ei ther in the ground state or in the f i r s t exci ted state at 4.43 MeV, gives r i se to a broad neutron energy spectrum. 2.2. Measurements d tfiBICHT- SOURCE Figure 3- Arrangement of Long Counter and Neutron Source. The Long Counter and source were arranged as shown in Figure 3, such that wall scat ter ing of the neutrons was much less than scat ter ing from the f l oo r . The pulses from the Long Counter were analyzed in an ND-101 256 channel k icksor te r , using the ampl i f i ca t ion system described 0 \ 8 in Appendix A. 1 . A t yp ica l neutron spectrum is shown in Figure 4 . This spectrum resul ted from a 3 0 minute exposure of the Long Counter to the Am-Be source at a distance d = 6 0 cm. The lop-sided appearance of the peak is explained by Pangher and Nichols ( 1 9 6 6 ) - i n the i r work on an accurately reproducable long counter using polyethylene rather than pa ra f f i n . Their Prec is ion Long Counter had better resolut ion than the Hanson-McKibben type; as a resul t two d i s t i nc t peaks, a main peak and a smaller one at s l i g h t l y higher energy, could be c l ea r l y resolved. The main peak was at t r ibuted to the case when the alpha p a r t i c l e from the react ion 1 °B(n ,a)^L i (Q. = 2 i 3 1 MeV) leaves the \ i nucleus in i t s f i r s t exci ted state ( 4 7 8 keV above the ground s ta te ) , and the second small peak, which produces the hump on the high energy side of Figure 4 , to the case when the ^L i nucleus is l e f t in the ground state ( Q = 2 . 7 9 MeV ) ( about 5% of the t ime-) . To determine the number of counts in the spectrum due to neutrons, the background was subtracted and the spectrum integrated from a bias level to the point where the spectrum returned to background l e v e l . So as to make the operation independent of the par t i cu la r neutron source being used, a bias level was chosen to cut of f a l l pulses resu l t ing from gamma rays. This gives a bias unnecessari ly high for such sources as Am-Be which produce l i t t l e accompanying gamma f l u x , however i t is appropriate for the many high gamma f lux s i tuat ions in which the counter may be used in fu ture. The bias was determined by running a spectrum s imi la r to the Am Be one of F i g . 4 . but using a Ra Be source with neutron f lux of 5 7 . 5 x 10 neutrons per second accompanied by a high gamma f l u x . The spectrum was s im i la r to that for Am Be but with a very high low energy t a i l , as dotted in on F i g . 4 . The ef fect iveness of the shie ld ing layers around the sides of the Long Counter was investigated by determining the e f f i c iency of the Long Counter for neutrons from the Am Be neutron source placed at various distances along a l i ne at r ight angles to the axis of the Long Counter. The arrangement is shown in F i g . 5 . Figure 5. Arrangement of Long Counter and neutron source for shie ld ing ef fect iveness inves t iga t ion . 10 2.3. Ca lcu la t ion of E f f i c iency In order to define an e f f i c iency independent of the distance from the source to the Long Counter, a plot of C 2 against the distance from the counter face to the source was made as shown in F i g . 6, where C is the count rate from the Long Counter. In the range of distances d $ 100 cm th is produced a reasonable approximation to a s t ra ight l i ne . Beyond th is point the room scattered neutron background became s ign i f i can t as indicated by the deviat ion of the data from the inverse square law. This l i ne was extrapolated back to cross the distance ax is at a point " e " cm from the o r i g i n , ca l led the e f fec t i ve centre. The Long Counter count rate can be related to i t s i n t r i n s i c e f f i c iency and the neutron source strength as fo l lows: C = n A 4TT (d+e)2 Eff where: C = counts per unit time from Long Counter. n = neutrons per unit time emitted into 4TT sterradians by the neutron source. A = area presented by the counter to the incident neutrons, d = distance from the counter face to the source, e = distance from the counter face to the e f fec t i ve centre of the Long Counter. D = d + e e = the probab i l i t y of obtaining a count when a neutron enters the so l i d angle of the counter. Eff = Ae-= e f f i c iency of. the Long Counter in terms of counts per unit neutron f lux at the e f fec t i ve centre. This can be rearranged to give the e f f i c iency e x p l i c i t l y Eff = c n or in terms of the slope S of the inverse square p lo t , where S - - 1 = 4 IT the e f f i c i ency can be expressed as Eff = The slope S of F?g„ 6 for d £ 100 cm was found to be 2„549 x 105 ( ± 0.1% ). This combined with the neutron source strength of 7-62 x 105 ( ±2% ) neutrons per second, gives an effeciency Eff = 14.1 ( ±2.2% ) counts per neutron per at the e f fec t i ve centre cm2 Using the same s lope, the e f fec t i ve centre was found to be at e = 10.8 ( ±5.5% ) cm behind the front reference face of the counter shown in F i g . 3. The e f f i c i ency can also be stated in terms of the f rac t ion of neutrons emitted by a source which produce counts in the counter. This c l ea r l y depends on the distance of the source from the counter. The present counter can be compared with the counter in i t s o r ig ina l form in terms of the relateve number of counts obtained when the source was placed at 150 cm from the face. The o r ig ina l U.B.C. Long Counter contained a 1 inch diameter BF^ counter as described on page 2. Using a RaBe neutron source at 150 cm th is counter gave 4.6 x 10 ^ counts per neutron emitted by the source (Heiberg, 1954). For an AmBe source of approximately the same strength, at the same distance, the new counter gave 4.74 x 10 counts per neutron emitted by the source, more than an order of magnitude improvement. Results for the source at the side of the counter are shown in F i g . 7 where the inverse square root of the counting rate is plotted against the distance from the counter centre l ine to the source. 12 _ c The slope of th is plot for d s $100 cm was found to be 7.74 x 10 ± 1.6%„ Using th is slope and the method described previously ( page 10 ), the Long Counter e f f i c iency for neutrons incident from the side was calcu lated to be 15.3 ± 5-2% counts per unit neutron f lux at the e f fec t i ve centre. The e f fec t i ve centre in th is case was obtained by extrapolat ion of the plot in F i g . 7 to the distance ax i s , and found to be 1.6 ( ±2% )cm behind the centre l i ne of the counter. The e f f i c iency per unit f lux for neutrons incident on the side is thus s l i g h t l y larger than that previously calculated for the same f lux incident on the end. However, since the cross sect ional area of the sens i t i ve region from the s ide , A , is 2.7 times that from the end,A , S 3 th is implies a lower i n t r i n s i c e f f i c iency e for neutrons incident on the s ide . From the data we have for the end and for the side (Eff) = A e = 14.1 a a a (Eff) = A e = 15-3 s s s Therefore the ra t io of the i n t r i n s i c e f f i c i enc i es is — S = 15.3 A 3 = x — = 0.402 £ a 14.1 A° 14.1 2.7 This indicates that the sh ie ld ing on the Long Counter reduces the i n t r i n s i c e f f i c iency for neutrons incident from the side compared to the e f f i c i ency for those entering the front end. This reduces the ef fect of background and room scattered neutrons. I . 1 , . _ l I I 1_ :_t I t O -do 80 ISO /60 o 90 /so C EN 77/?£T 0/= rt&SS / V ' £ O C T e w RNCLS Ft CURE p. JDCJj n) ffe o//=/=e R e^TS^u c / f o s s SH/FT£-E> DCJhJ/V £3.y • • S~ fnti/sti firs & GFtCht- of: T H E CURVES £zj J>> 0.99 SYe V HRS r3G£M ^Hi>=-r{^D UP Gy S /sZ s=-f?ot-t CHAPTER III MEASUREMENT OF LONG COUNTER EFFICIENCY USING THE REACTION D(d,n) 3He 3.1• The Reaction D(d,n) He At low energies, c o l l i s i o n s between two deuterons resul t in the two reactions D(d,n) He and D(d,p)T with approximately equal probab i l i t y and Q-values of 3.268 MeV and 4.033 MeV respect ive ly . If the input energy of the react ion is increased, a te r t i a ry process, involving the breakup of the deuteron, occurrs with a Q-value of -2.225 MeV. F i g . 8 shows the var ia t ion in neutron energy with the neutron angle and incident deuteron energy for the react ion D(d,n) He (Fowler and Bro l l ey , 1956). F i g . 9 shows the d i f f e ren t i a l cross sect ion for the reaction D(d,n) He for various incident deuteron energies as a funct ion of the centre of mass angle. The data shown were obtained by Hunter and Richards (19^9) using a ca l ib ra ted Hanson-McKibben Long Counter to detect the neutrons produced in a gas target 2 cm in length. The neutrons were 3 observed rather than the He in order that measurements could include a l l angles in centre of mass from 0° to nearly 180°. Background neutrons from the f o i l and co l l imat ing apertures amounted to only 2% of the D(d,n) He y ie l d at a deuteron energy of 0.7 MeV. The data shown in F i g . 9 have been corrected for the var ia t ion of Long Counter e f f i c iency with neutron energy using the curve given in McKibben (19^7) and converted to 14 centre of mass. The appearance in the data of the symmetry of the d i f f e r e n t i a l cross sect ion about 90° in the centre of mass system as required by the ident i ty of the i n i t i a l p a r t i c l e s , v e r i f i e s the a p p l i c a b i l i t y of McKibben's curve to the Long Counter used. The s t a t i s t i c a l errors in the data were ^ ±5% and the Long Counter ca l i b ra t i on was accurate to ±5% g iv ing a total error on the cross sect ions of 10%. 3•2. The Gas Target The gas target system used is i l l u s t ra ted in F i g . 10. The Nickel Fo i l Entrance Window Manufacturer: Chromium Corporation of America, Waterbury, Conn.,U.S.A. Spec i f i ca t i ons : Grade C, Dimensions x i " x 50yinches The f o i l thickness was determined for =0.9 MeV protons by invest igat ing the sh i f t in the 0.873 MeV gamma resonance from proton bombardment of f l u o r i n e . Su f f i c ien t f luor ine contamination was present on the gas target backstop and on the metal backing of a beamstop in the beamline before the co l l imators to make actual deposit ion of a f luor ine target unnecessary. The f luo r ine gamma peak before the window was centered about 876 ±2 keV with a hal f -width of 7 keV, and that af ter the window , at 1015 ±4 keV with half width 12 keV, indicat ing a window thickness of 139 ±6 keV for =0.9 MeV protons. The f o i l was soldered over the end of the brass support tube using " S t a y - B r i t e " so lder . The^nd of the tube was dipped in a puddle of molten solder and f lux on an aluminum plate so as to reta in a small 15 amount of solder on the end. The tube was then clamped solder end up, a n ickel f o i l l a id across the end and a small aluminum block set on the f o i l . The block was then heated, melting the so lder . Fo i l s attached in th is manner were found to withstand pressure di f ferences of =740 mm Hg, the lower pressure being inside the support tube, before ruptur ing. Using an e l e c t r i c a l contact system, a pressure di f ference of 300 mm Hg was found to cause a depression in the f o i l of 0.012 cm from the posi t ion of the f o i l with equal pressures on both s ides. This represents a 1,4% change in the 0.833 cm length of the beam path in the gas eel 1 . Target Thickness The purpose in using a gas target was the ease of determination of the number of target n u c l e i , which can be calculated using the path length in the gas from the entrance f o i l to the backstop, the gas pressure in the target , and the temperature in the beam region of the gas. The pressure of the target gas was measured using a 0 to 800 mmHg Wallace and Tiernan absolute pressure meter, and the target region length was measured with a t r ave l l i ng telescope. Because of the power d iss ipated by the incident deuteron beam, the temperature in the beam region is general ly s l i g h t l y greater than that in the bulk of the gas in the target. The extent of th is ef fect has been measured and plotted for various beam currents by Robertson et al (1961) using the same gas target being used in the present experiment. Due to the large copper backstop' i t was assumed that the temperatures of the bulk of the gas in the target and that of the backstop would be 16 essen t i a l l y the same. This temperature was measured using a copper-constantan thermocouple af f ixed to the backstop. The e f fec t i ve temperature in the beam region is described in terms of the temperature T of the bulk of the g'as as T effV ( P i / P o > T where ( P./Pq) is given by Robertson's plot as Beam Current ( P./P ) i o .02 yamp 1 .001 .1 1.005 .2 1.012 3 - 3 . Current Integration Even at very low beam currents the target gas w i l l experience considerable ion za t ion . This was confirmed by the fact that a d i s t i nc t blue glow was seen in the beam region in the gas, which in fact was used to check the centering of the beam in the f o i l window support tube. The s l i gh tes t potent ia l d i f ference between the window and backstop portions of the target would cause charge separat ion, making current measurements from the backstop completely unre l iab le . The only way to el iminate errors in the current measurement resu l t ing from charge separation in the gas, or from preferent ia l c o l l e c t i o n of secondary electron emission, was to connect the window and backstop port ions e l e c t r i c a l l y and measure the total current entering and stopped in the gas c e l l . This method, however, presents the problem that i f some of the deuteron beam is lost on the sides of the tube holding the window, pos i t i ve current is measured which does not ac tua l ly / SfiCKS-roP FOIL. \~>IN-DOVJ U L 'OO A--Ti-9° V- . go /. )0 v. 300 V. 17 pass through the target gas. To prevent any current f a l l i n g on the tube supporting the f o i l at the entrance to the gas c e l l , the beam co l l imat ion system was designed such that a p a r t i c l e passing through the col l imators at even the most extreme angle would pass through the window into the gas without h i t t ing the wal ls of the entrance tube. A lso , the focussing was arranged to give as nearly as poss ib le a pa ra l l e l beam and rule out any chance of a focus inside the co l l imat ion system. These precautions were shown to be e f fec t i ve in a current d i s t r i bu t i on check using the apparatus described in F i g . 11. The f i na l col 1imator.nwas held at +300 v in order to reta in any electrons knocked out of i t by the incident beam. The window port ion was held at +90 v so that the potent ia l d i f ference between i t and the backstop could be var ied from +90 v to -90 v continuously. Using th is arrangement an invest igat ion was made, with no gas in the target , of the currents on d i f fe ren t parts of the target chamber and co l l imat ion system, resul t ing from both a proton beam d i r ec t l y and the secondary electron emission. The to ta l current was measured, with a +300 v b ias , on a removable metal beam stop jus t in fromt of the f i na l co l l ima to r . Then, with the beam stop removed, simultaneous currents were measured on the f i n a l co l l ima to r , the f o i l window and i t s supporting tube, and on the backstop, as shown in F i g . 11, for various re la t i ve potent ia ls on the f o i l window and backstop. To 10% the net current ' agreed with the tota l current measured on the removable stop, indicat ing that no beam was being lost other than 18 on the three parts considered. This agreement at the point where the window current is zero further indicates that a l l current that goes through the co l l ima to r , passes through the f o i l window and the gas c e l l and h i t s the backstop with none being lost on the sides of the entrance tube. The high saturat ion currents at negative backstop b ias , indicate a higher rate of secondary electron emission from the backstop than from the foi1 window. The to ta l charge entering and stopped in the gas c e l l was measured on the connected window and backstop using an Eldorado Elect ron ics model CI-110 current integrator ( Eldorado E lec t ron ics , Berkeley, C a l i f . ) . The tr ip-meter pointer was set to f u l l scale before the runs with the Long Counter were begun and was l e f t unchanged unt i l a l l runs and the current integrator ca l i b ra t i on check had been completed. The current integrator ca l i b ra t i on on the range used during the runs was checked using a 1.01902 (±0.01%) vol t standard c e l l , and an accurately measured 5.0116 (±0.005%) ohm res i s to r . These gave an input current of 0.20303 ya which was measured by the current integrator for 52'' V , or 14 cycles of kS ycoulombs each. Comparison of the input charge and the charge integrated indicated the current integrator to read wi th in 1.25%. X si •A ft. u k U ,1 O HI Q: 0 • o o o o k 0 it 0 \ i 1 Q ft Jl s 0 ^ 0. •HI o a: 3 J '41 31 19 3.4. Measurements The experimental arrangement was that shown in F i g . 3 (page 7), the neutron source in th is case being the gas target. The pulses from the BF^ tube were ampl i f ied and analyzed as shown in F i g . 13: The l i n e a r i t y of the k icksor ter was checked, and the window l im i t s of the s ing le channel analyzer were set using a pulse generator which was able to produce pulses very s imi lar to those from the BF^ tube. These test pulses were injected with a 51 fl termination at the preampl i f ier test input. Using a 776 keV deuteron beam of 250±20 nanoamps at the gas c e l l from the U.B.C. Van de Graaff generator, a ser ies of 44 runs were made, each having a tota l co l lec ted charge of 45 ucoul . or 90 y c o u l . , an even one or two cycles of the current integrator . For each run the Long Counter was placed at a distance between 30 cm and 200 cm, measured from the front face of the centre paraf f in region to the centre of the gas c e l l , and along one of two l ines drawn at 60° and 90° to the d i rec t ion of the incident deuteron beam in the gas c e l l . The gas target was f i l l e d with 200 torr of e i ther deuterium or hydrogen, the la t te r for background runs. Table 3 gives an out l ine of the run schedule. 20 TABLE 3 Run Schedule Run Long Counter Di rect ion Target Gas 9 16 23 26 33 38 42 1 8 15 22 25 32 37 41 4 4 The resu l t ing spectra were punched on paper tape d i r ec t l y from the kick sorter memory and la ter converted to cards by the U.B.C. computing centre for use with a neutron peak integrat ing program. A l l spectra had the same cha rac te r i s t i c shape shown in F i g . 4 . A short computer program was used to integrate a l l the spectra, with the bias set as shown in F i g . 4, and the accumulated running time at the beginning of each run. A beam-off run was made to determine the time dependent room background. A l l counts were corrected for k icksor ter dead time, which only twice exceeded k%, and for time dependent background, which never exceeded 3% of the total count rate and was almost always considerably less than 3%. 3-5. Background The accumulated time was used as a c lock to study the background increase with time due to neutrons from deuterium build-up on the co l l imators and backstop. The neutron counts remaining in the H, runs 21 af ter subtract ion of the time dependent background represented the background due to deuterium contamination in the target chamber and co l l imat ion system. These count rates were plotted against the ' c lock ' time of the midway point of the run, to produce a family of curves showing the background buildup with beam-on time at Long Counter distances of 30 cm, 50 cm, 75 cm, 100 cm, 130 cm, 150 cm, and 200 cm at 90°; and 35 cm, 75 cm, and 130 cm at 60°. Two of these curves were normalized to produce F i g . 14. It can be seen that even a f ter almost 3 hours with actual beam on target , or 2385 ucoulombs of deuterium stopped in the gas c e l l , the neutron y ie ld due to bui ld-up of deuterium on the surfaces shows no tendency to level- of f as would resul t from saturat ion of the surfaces. The greatest port ion of th is background w i l l be from the backstop where most of the beam is stopped. The f a i l u r e to saturate is in agreement with a ca lcu la t ion based on a discussion by J . H . Coon in Marion and Fowler (i960), which indicates that for the beam current and conf igurat ion, and the gold backstop used in th is experiment, a total charge of about 4200 ucoulombs would be co l lec ted by the backstop before saturat ion became s i g n i f i c a n t . The o r ig ina l curves for individual distances were used to read of f the background seen by the Long Counter at a par t i cu la r d is tance, angle and time. These backgrounds were then subtracted from the runs to determine the count rate due to the target gas alone. To determine the beam dependent background for runs at Long Counter distances where no background data was taken, suth as 100 cm at 90°, p lots were made 22 of B(d,t) 2 , where B(d,t) is the background count rate at distance d and at t, the mid-time of the run in quest ion, versus the distance d. Interpolat ion then gave the background at the desired d is tances. 3.6„ Gas Target Thickness The thickness of gas traversed by the deuteron beam is defined by the pressure and temperature of the gas in the beam region, and the geometrical distance between the entrance window and the backstop. The gas c e l l length was measured, using a t rave l l i ng telescope, to be 0.833 ( ±0.3% ) cm. Thus, including the f o i l depression, the beam path length was 0.845 ( ±0.5% ) cm. The gas pressure for a l l runs was 200 torr (±2%). Any di f ference between the meter pressure and the gas pressure in the beam region was corrected for by the e f fec t i ve temperature cor rec t ion . The temperatures measured on the backstop cool ing f i n varied by ± 0.24% about an average of 295-2°K. For the current range 250 ± 20 nanoamps used in the runs, the temperature correct ion factor for gas heating in the beam region is 1.001 ( ±0.1% ) ( Robertson et a l , 1961 ) . This gives an e f fec t i ve temperature in the beam region of 295-5 (±0.34%) ° K . Using these parameters, N^, the number of target nuclei per square centimeter in the gas c e l l was: N = ~ ° x L x £ x i> = 1.11 x 10 1 9 ( ±2.8% ) t v 0 K c 1 target nuclei / cm2 23 where: T P L Avagadro's number. Volume of one mole of gas at STP. Length of beam path in gas c e l l . Gas pressure. 760 t o r r . 273°K Ef fec t i ve temperature in beam region. 3.7. Beam Energy in the Gas Cel l A check of the Van de Graaff energy ca l i b ra t i on indicated that the meter read 3 keV high, so that the meter energy reading of 776 keV used during the runs indicated a beam energy of 773 ( ±0.4% ) keV entering the f o i l window. The f o i l window thickness was 138 ( ±2.3% ) keV for 900 keV protons, which corresponds to a thickness of 228 ( ±3.6% ) keV for 773 keV deuterons ( page 683, Marion and Fowler, 1960 ). Thus the deuteron beam entered the gas c e l l with an energy of 5^ 5 ( ±2.0% ) keV. Deuterons of th is energy are slowed in deuteriom gas at the rate of -15 2 3.1 x 10 ev per gas atom per cm (page 681, Marion and Fowler, i960 ). Thus the approximate energy of the deuteron beam midway across the gas c e l l would be 528 ( ±2.1% ) keV. Reca lcu la t ing, using th is approximate energy in the gas to determine a better average energy loss rate, the beam energy midway through the gas was found to be 527 ( ±2.1% ) keV. 3.8. Neutron Flux from the Gas Target The number of neutrons emitted from the target in the d i rec t ion of the Long Counter can be calculated using the react ion cross sect ion for the appropriate angle and deuteron beam energy, the thickness of gas in the gas c e l l , and the number of deuterons incident on the target 2h gas. The average beam energy in the gas c e l l was found in sect ion 3.7 t o be 527 ( ±2.1% ) keV. Interpolat ing to th is energy using the curves o f F i g . 9, the d i f f e ren t i a l cross sections for the react ion D(d,n) He a t 60° and 90° in the laboratory system were found to be 3.18 ( ±10% ) mi l l i ba rns per sterradian and 3.03 ( ±10% ) m i l l i ba rns per s ter radian, respect ive ly . The gas target thickness was found in sect ion 3.6 to be 19 2 1.11 x 10 ( ±2.8% ) target nuclei per cm . The charge co l lec ted during each cyc le of the current integrator was found, on page 18, to be 45 ( ±1.25% ) ucoulombs. Thus, during each cyc le , the number of deuterons entering the gas eel 1 was .1 45-ycoui  N d = Zyi : 1.6 x 10 y c o u l . / deuteron = 2.81 x 10 ( ±1.25% ) deuterons The d i f f e r e n t i a l cross sect ion at angle 8 is defined as N N , x N d t where: N = Number of neutrons per sterradian emitted per integrator cyc le at angle 0. = Number of deuterons incident orj the target per cyc le . N^ = Number of target nuclei per cm . Thus N = a_ N . N . In order to use the method described in sect ion n 6 d t 2.3 to ca lcu la te the e f f i c i ency , the number of neutrons emitted by the react ion must be expressed as the number of neutrons emitted into 4TT sterradians by an isot rop ic source. In sp i te of the f i n i t e so l i d angle subtended at the gas c e l l by the Long Counter, i t was found that the neutron energy and cross sect ion 'var ied l i t t l e across the sens i t i ve 25 region, so that no correct ion was necessary for the spread in angles due to the f i n i t e s ize of the detector at 60° and 90°. The react ion was therefore s u f f i c i e n t l y iso t rop ic over the range of angles included by the counter at e i ther 60° or 90° that n in the effeciency ca lcu la t ion on page 10 could be expressed as n. = ATT N = 4TT a. N , N 0 n 0 d t " 8 Therefore n ^ = 1.25 x 10 ( ±14% ) neutrons per integrator cyc le emitted in a l l d i rec t ions g and n q Q = 1.19 * 10 ( ±14% ) neutrons per integrator cyc le emitted in a l l d irect ions,, Due to the geometry of the gas c e l l , at angles d i f ferent from 90° the counter face w i l l be p a r t i a l l y shadowed by the backstop. When the Long Counter is at 60° to the d i rec t ion of the deuteron beam in the gas c e l l , 32.7% of the beam path length is shadowed by the backstop. The neutrons emitted in the d i rec t ion of the Long Counter by th is shadowed port ion of the beam pass through an average of 0.276 cm of copper in the backstop. The total cross sect ion for neutrons in copper is .4 barns ( Hughes and Schwartz, 1958 ), so that 9.2% of the neutrons entering the backstop w i l l be l os t . Combining these f r ac t i ons , i t is estimated that at 60°, 3.01% of a l l the neutrons emitted into the so l i d angle of the Long Counter w i l l be lost in the backstop. Applying th is correct ion to n ^ gives n ^ = 1.21 x 10 ( ±14% ) neutrons per integrator c y c l e , emitted in a l l d i r ec t i ons . 26 3.9. Ca lcu la t ion of E f f i c iency Following the procedure used in sect ion 2.3 for the AmBe c a l i b r a t i o n , a plot was made of C 2 against the distance from the counter face to the centre of the gas c e l l , where C is the number of counts per integrator cyc le from the Long Counter. The slope of th is p lo t , shown in F i g . 15, was found to be S^Q = 5-94 x 10~5 ( ±2.4% ) for the Long Counter at 60°, and S = 6.3 x 10~5 ( ±2.8% ) for the counter at 90°, and the corresponding e f fec t i ve centres were found" to be at -10.3 ( ±7% ) cm and -12.6 ( ±9% ) cm, respect ive ly . Both slopes represent the region d^100 cm, as the room scattered neutron background becomes s ign i f i can t beyond that point , as indicated by the deviat ion of the data from the inverse square law. The e f f i c iency was then calculated as • E f f . = k « 6 "71 • where n is the number of neutrons emitted by the react ion in a l l 6 direct ions", assuming isotropy. It was found in sect ion 3.8 that g n^Q = 1.21 x 10 ( ±14% ) neutrons per cycle into 4TT assuming isotropy and n q f ) - 1.19 x 10 ( ±14% ) neutrons per cycle into 4TT assuming isotropy so that E f f 6 Q = l o 2 1 x 1 0« x (V.?-* Yl x 10-!° = 29-4 ( ±19% ) counts per neutron per cm2 at the e f fec t i ve centre a n d E f f90 = 1.19 x 10y x U.30' ) 2 x 10-^ = 26.6 ( ±20% ) counts per neutron per cm at the e f fec t i ve centre. 27 The uncertainty on these e f f i c i enc i es would be considerably reduced by more accurate measurement of the d i f f e ren t i a l cross sect ions, which in th is ca lcu la t ion accounted for 10% of the 19% or 20% uncerta inty. _ J , ! 1 1 1 ll —inj VJ Oc 3 A /-LTD S3-VOi/j' „ 3> ^  ^ fV & S / C7 CHAPTER IV CONCLUSIONS In F i g . 16 the Long Counter e f f i c i enc i es are plotted against neutron energy. The energy of the AmBe neutrons was determined from F i g . 2 to be approximately 4.5 MeV. The neutron energies produced at 60° and 90° by the react ion D(d,n) 3He were interpolated from the data of Fowler and Bro l ley ( 1956 ) shown in F i g . 8, to be F i g . 16 indicates considerable va r ia t ion of the e f f i c iency of the modified Long Counter with energy. This is due to the fact that the larger diameter BF^ counter displaced a considerable amount of pa ra f f i n , thus reducing the amount of moderating material and leading to the loss of a higher proportion of high energy neutrons than with the smaller BF^ counter. This reduction of paraf f in would, on the other hand, al low a higher proportion of those neutrons that have been thermalized to reach the BF^ tube and be counted. F i g . 17 shows the e f fec t i ve centre pos i t ion plotted against neutron energy. Thus for a gain of an order of magnitude in e f f i c iency for high energy neutrons, and even more for low energy neutrons, one has sac r i f i ced energy independence of the Long Counter. This is acceptable, however, as the neutron evergy spectrum is general ly known in an experiment. 29 By modifying the o r ig ina l counter, the old ca l i b ra t i on was destroyed, but a factor of ten increase in e f f i c iency was achieved and the modified counter has been ca l ib ra ted to better accuracy than the o r i g i n a l „ BIBLIOGRAPHY Bramlett , R .L . , R . l c Ewing, and T.W. Bonner, Nucl . Instr . Methods 9, 1 (1960). de Pangher, J . , and L .L . Nichols , BNWL-260, P a c i f i c Northwest Laboratory, Richland, Washington, 1966. Fowler, J . L . , and J . E . Bro l ley J r . , Rev. Mod. Phys. 28,112 (1956). Geiger, K.W., and A . P . Baerg, Can. J . Phys. 43,373 (1965). Hanson, A . O . , and J . L . McKibben, Phys. Rev. 72,673 (1947) Heiberg, S . A . , Ph D Thesis, Univers i ty of B r i t i s h Columbia, 1954. Hornyak, W.F., Rev. S c i . Instr . 23,264 (1952). Hughes, J . D . , and R.B. Schwartz, Neutron Cross Sect ions, Brookhaven National Laboratory, Upton, New York, 1958. Hunter, G.T. , and H.T. Richards, Phys. Rev. 7_6, 1445 (1949). Marion, J . B . , and J . L . Fowler, Fast Neutron Physics Part I, Interscience Publ ishers Inc., New York, i960. Robertson, L . P . , B.L. White, and K.L. Erdman, Rev. S c i . Inst r . 32., 1405 (1961). APPENDIX LONG COUNTER ELECTRON ICS A . 1 . Proport ional Counter Reuter Stokes, Model # RSN 44A F i l l i n g Gas: BF^ (96% 1 °B) F i l l i n g Pressure: 70 cm Hg Outside Diameter: 2 1/32 " Sensi t ive Length: 12" Operating Voltage: +2200 v Maximum Voltage: +3500 v 13 2 o Max. Neutron Flux:2.5x10 n/cm sec Maximum Temperature: 150 C 12 Res stance: £ 10 ft Capacitance: 8 pf A . 2 . E lec t ron ics for AmBe Ca l ib ra t ion D.C. Power Supply for Proport ional Counter: Hewlett-Packard, Harrison 6I56A (Hewlett-Packard Co . , Palo A l t o , Ca l i f o rn ia ) Preampl i f ier Input: PRO PORT/0£SRL_ CO UMT£~R •+•'2 2.0 0 vr--Ih /oo -o - V W - O U T P U T -170 H -O-T 3 2 Preamplif i e r : Atomic Instruments, model 205 B (Atomic Instruments Co . , Cambridge, Mass.) Gain: 24 Limi t ing occurrs for input £ 0.2 v Linear Amp l i f i e r : Atomic Instruments, model 204 C (Atomic Instruments Co. , Cambridge, Mass.) Input Time Constant: 7 Feedback: 1 Gain: 16 (x700 ampl i f icat ion) 32 (x1200 ampl i f ica t ion) Output: High Level K icksor te r : Nuclear Data, model ND 101, (Nuclear Data Inc., Pa la t ine , Ca l i f o rn ia ) 3 A . 3 o E lec t ron ics for D(d,n) He Ca l ib ra t ion See F i g . 13. Pulse Generator: B.N.C. model PB-2, (Berkeley Nucleonics Co . , Berkeley, C a l i f . ) Frequency: 10-100 cps Fine Frequency: 10 Width: 1-10 usee Fine Width: 0.0 P o l a r i t y : negative Amplitude: 10-560 Attenuator: 100 Rise Time: 0.5 ysec 33 Linear Amp l i f i e r : C . I . Linear Ampl i f ie r , model # 1410, (Canberra Industr ies, Middleton, Conn.) Input Mode: neg unterm Integrat ion: 1 Coarse Gain: 28 Fine Gain: 7 F i r s t D i f f e ren t i a t i on : 1 Second D i f f e ren t i a t i on : of f Output: pos. , un ipolar , prompt K icksor ter : ND-160 (Nuclear Data Inc., Pa la t ine , Ca l i fo rn ia ) Input: b ipolar 10 v Conversion Gain: 1024 Zero Leve l : 8.0 Threshold: 1.0 Coarse Gain: external amp Fine Gain: 0.0 Experimental Conf igurat ion: IQ24 X LOW Single Channel Analyzer: C . I . Timing Single Channel Analyzer, model # 1435 (Canberra Industr ies, Middleton, Conn.) Window Width: 0.94 Basel ine: 0.31 Var iab le Delay: 0.0 Input Mode: uni Analyzer Mode: window Output: fast Preampl i f ier and Preamplidier Input: See A .2 . 

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