photoneutron cross section measurement on 9b by means of inverse compton scattering of laser

7/28/2019 Photoneutron Cross Section Measurement on 9B by Means of Inverse Compton Scattering of Laser

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Hadron and Nuc lear Physics w ith Electromagnet ic ProbesK. M aruyama and H. Okuno (Editors)9 2 0 0 0 Elsev ier Sc ience B .V . A l l r ig hts reserv ed . 87

P h o t o n e u t r o n c ro ss s e c ti o n m e a s u r e m e n t o n 9B e b y m e a n s o f i nv e rs eC o m p t o n s c a t t e r i n g o f l as e r p h o t o n sH . U t s u n o m iy a , Y . Y o n e z a w a , H . A k im u n e , T . Y a m a g a ta a n d M . O h ta aM. Fu j i sh i ro bH . T o y o k a w a , a n d H . O h g a k i c

D e p a r tm e n t o f P h y s i c s, K o n a n U n iv er s it y , O k a m o to 8 -9 -1 , H ig a s h in a d a , K o b e658-8501 , JapanbOsa ka Pre fec tu re Un iver s i ty , Gaku ench o 1 -2 , Saka i , Osak a 599-8570 , Ja pa nC Q u a n tu m R a d ia t i o n D iv . , E l e c t r o t e c h n ic a l L a b o r a to r y , U m e z o n o 1 - 1 - 4 , T s u k u b a ,I b a r a k i 3 0 5 - 8 5 6 8 , J a p a n

P h o to n e u t r o n c r o s s s e c t i o n s w e r e m e a s u r e d f o r 9 B e in t h e e n e r g y r a n g e f r o m 1 . 8 t o4 .0 MeV us ing a lase r - e lec t ron MeV pho ton sou rce . These c ross sec t ions a re r e levan t tothe r eac t ion r a te o f the f i rs t s tep o f c~-p rocess , c~ (an , ~ /)gBe, in a neu t r ino -d r iven win d o fco re -co l lapse supernovae . R s lts f or t h e 1 /2 + , 5 / 2 - , 1 / 2 - , a n d 5 /2 + s ta t e s a r e p r e s e n t e di n c o m p a r i s o n w i t h t h e d a t a t a k e n w i t h o t h e r p h o t o n s o u r c e s .1 . I N T R O D U C T I O N

T h e n e u t r i n o - d r i v e n wind fo rmed in co re -co l lapse supernovae may be an idea l s i te fo rthe r -p ro cess [1 ] . In th i s r ap id ly e xpa nd in g env i ronm en t , n - and c~-r ich m ate r ia l i s f i r stp rocesse d in to 9Be th rou gh c~(c~ n , -y )gBe by b r idg ing ma ss gap s A = 5 and 8 , wh ich i sfo llowed by the 9Be(c~ , n )12C. T h is sequence o f I2C p ro duc t ion p roceeds m ore e f f ic ien t lyth a n t h e t r i p l e c~ p r o ce s s . T h e s u b s e q u e n t r e a c t i o n n e tw o r k is c o n s id e r ed t o p r o d u c e s e e dnuc le i be yon d 56Fe fo r r -p rocess .9Be i s a loos e ly -b oun d nuc lear sy s tem cons is t ing o f two c~ 's and a neu t ron . Non o f anytw o c o n s t i t u e n t s a lo n e c a n f o r m a b o u n d s y s t e m . T h e l i fe t im e s o f 5 He a n d S B e a r e 1 .1x 10 -91 sec an d 0 .97 x 10 -16 sec, respect ively . Th ree adj ace nt thre sho lds e xis t a t 1 .573M eV for c~ + a + n, at 1.665 MeV for aBe + n and at 2.467 MeV for 5He + c~. In view ofth e l if e tim e s , t h e s y n th e s i s o f 9 Be is c o n sid e r e d t o p r o c e e d m o r e d o m in a n t ly t h r o u g h S B eth a n t h r o u g h 5 H e in tw o s u c c es s iv e b in a r y p r o ce s s es . T h e t e r n a r y p r o c e s s is l ik e ly t o p l a ya m in o r r o le . R e s o n a n c e s n e a r t h e s e t h r e s h o ld s d e t e r m in e t h e r a t e o f t h e 9 B e s y n th e s is .R e s o n a n c e p a r a m e t e r s o f t h e s e s t a t e s c a n b e o b t a i n e d b y p h o t o d i s i n t e g r a t i n g 9 Be .

P r e v io u sly , p h o to n e u t r o n c r os s s e c ti o n s f or 9 Be w e re m e a s u r e d n e a r t h r e s h o ld s u s in g tw ok in d s o f r e a l p h o to n s o u r c es , B r e m s s t r a h lu n g a n d r a d io a c t iv e i so to p e s. B r e m s s t r a h lu n gm e a s u r e m e n t s w i th a p h o to n d i f f e r e n c e m e th o d l a c k e d g r e a t a c c u r a c y d u e t o p o o r e n e r g yreso lu t ion [2 ]. M eas u rem en ts wi t h r ad ioac t ive i so topes (24Na, 2SA1 , asC1, 5SCo , 65Ni , s sy ,l~ 124Sb, 142 '144pr an d 2~ were l im ited below 2 .75 M eV [3-8] . Lo w-ly in g s ta tes in


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9 B e w e r e a l s o i n v e s t i g a t e d i n e l e c t r o n s c a t t e r i n g [ 9 , 1 0 ] . H o w e v e r , v i r t u a l p h o t o n s p o p u -l a t e d t h e f ir s t e x c i t e d s t a t e ( 1 / 2 + ) o n l y v e r y w e ak l y. N o s in g l e m e a s u r e m e n t s p r o v i d e dd a t a f or a ll r e s o n a n c e s t a t e s n e a r t h r e s h o l d s ( 1 / 2 + , 5 / 2 - , 1 / 2 - , 5 / 2 + ) .2 . E X P E R I M E N T A L M E T H O D2 . 1 . v - r a y p r o d u c t i o n a n d n e u t r o n d e t e c t i o n

I n v e r s e C o m p t o n s c a t t e r i n g o f l a s e r p h o t o n s i n c i d e n t o n r e l a t i v is t i c e l e c t r o n s i n t h e s t o r -a g e r in g T E R A S o f t h e E l e c tr o te c h n ic a l L a b o r a t o r y ( E T L ) p r o d u c e s q u a s i -m o n o c h r o m a t i c7 r a y s i n t h e e n e r g y r a n g e f r o m 1 t o 4 0 M e V [ 11 ]. T h i s n e w M e V p h o t o n s o u r c e , L E M P S( l a s e r - e l e c t r o n M e V p h o t o n s o u r c e ) , is a v a i la b l e a s a p e n c il -l ik e b e a m t y p i c a l l y w i t h a fl u xo f 104 p h o t o n s / s e c / m m 2 a n d e n e r g y r e s o l u ti o n o f a fe w % . T h e b e a m p r o p e r t i e s w e r ep r e v i o u s l y i n v e s t i g a t e d w i t h a n a n t i - C o m p t o n s p e c t r o m e t e r [ 1 2 ] .

A N d : Y L F l a s e r (A - 1 05 3 n m ) w a s u s e d . T h e l a se r s y s t e m w a s o p e r a t e d a t 1 k H z a n dp r o d u c e d 1 0 0 % l i n e a r l y p o l a r i z e d l i g h t . U n p o l a r i z e d l i g h t w a s a l s o p r o d u c e d b y p a s s i n gp o l a r i z e d l i g h t t h r o u g h a n o p t i c a l e l e m e n t c a l l ed d e p o la r i z e r . E l e c t r o n e n e r g y w a s v a r i e df r o m 3 1 4 M e V t o 4 75 M e V t o p r o d u c e V r a y s in t h e e n e r g y r a n g e o f 1 .8 M e V - 4 .0 M e V . 3 'r a y s w e r e c o l l i m a t e d i n t o 2 m m i n d i a m e t e r w i t h a 2 0 c m t h i c k P b b l o c k a t 5 . 5 m f r o m ah e a d - o n c o ll is io n p o i n t b e t w e e n e l e c t r o n s a n d l a s er p h o t o n s . 7 - r a y e n e r g y w a s m e a s u r e dw i t h a 1 5 5 c m 3 p u r e - G e d e t e c t o r o f c o a x i a l ty p e . A f ul l e n e r g y p e a k w a s e n e r g y - c a l i b r a t e dw i t h n a t u r a l s o u r c e s o f 4 ~ a n d 2~ a s w e ll a s a s t a n d a r d s o u r c e o f 6 ~

A 4 c m t h i c k 9 B e r o d o f 9 9 .5 % e n r i c h m e n t ( 2 .5 c m in d i a m e t e r ) w a s i r r a d i a t e d . N e u -t r o n s w e re m e a s u r e d w i t h f o ur 60 c m H g B F a c o u n t er s ( M I T S U B I S H I N D 8 5 3 4 - 6 0 : 2 . 5c m i n d i a m e t e r x 2 4 . 1 c m i n e f f e c t i v e l e n g t h ) e m b e d d e d i n a 3 0 c m p o l y e t h y l e n e c u b e .T h e B F a c o u n t e r s w e r e l o c a t e d , t w o e a c h v e r t i c a l l y a n d h o r i z o n t a l l y , i n a c o n c e n t r i c r i n ga t 7 .5 c m f ro m t h e b e a m a x is . N e u t r o n m o d e r a t i o n t i m e in t h e p o l y e t h y l e n e w a s m e a -s u r e d i n 1 m s t i m e r a n g e w i t h a n O R T E C 5 66 T A C m o d u l e . F i g. 1 s h o w s a m o d e r a t i o nt i m e d i s tr i b u t i o n . B a c k g r o u n d n e u t r o n s , a t i m e - i n d e p e n d e n t c o m p o n e n t , m a d e a m i n o rc o n t r i b u t i o n t o t h e n e u t r o n m e a s u r e m e n t .

I n c i d e n t p h o t o n s w e r e d e t e c t e d d i r e c t ly w i t h a B G O d e t e c t o r (2 " in d i a m e t e r x 6 " inl e n g t h ) p l a c e d a t t h e e n d o f t h e b e a m l i ne . P i l e - u p s p e c t r a w e r e o b t a i n e d f o r t h e p h o t o nb e a m p u l s e d a t 1 k H z . T h e a v e r a ge n u m b e r o f p h o t o n s p e r b e a m p u ls e w a s d e t e r m i n e df r o m t h e p i l e - up s p e c t r a w i t h u s e o f a s in g le p h o t o n s p e c t r u m t h a t w a s s e p a r a t e l y t a k e nw i t h a D C b e a m . T h e t o t a l n u m b e r o f p h o t o n s w a s o b t a i n e d f r o m th e f r e q u e n c y ( l k H z )a n d t h e d a t a a c q u i s i t io n ti m e . I n d e p e n d e n t m e a s u r e m e n t s w e re c a r r ie d o u t w i t h a D Cb e a m , i n w h i c h foregroundn e u t r o n s a n d backgroundn e u t r o n s w e r e m e a s u r e d b e f o r e a n da f t e r e a c h m e a s u r e m e n t . T h e r e s u l t s o f t h e pu l s ed - a n d D C - b e a m m e a s u r e m e n t s a g r e e dt o e a c h o t h e r w i t h i n t h e a c c u r a c y o f 5 % .2 .2 . N e u t r o n d e t e c t i o n e f f i c ie n c y

A 2 . 1 6 m C i 2 4 N a (h a lf -l if e 1 5 h o u r s ) w a s m a d e b y i r r a d i a t i n g a 5 0 m g s a m p l e o f N a O Hf or 15 m i n u t e s w i t h t h e r m a l n e u t r o n s a t t h e R e s e a r c h R e a c t o r I n s t i t u t e o f K y o t o U n i -v e r s it y . I t w a s m i x e d w i t h 2 c m 3 D 2 0 i n a n a l u m i n u m c o n t a i n e r ( 2 c m i n d i a m e t e r 4c m i n l e n g t h ) . T h e 2 4N a b e t a - d e c a y t o t h e 4 + s t a t e i n 24 M g w i t h a 9 9 .9 4 % b r a n c h i n gr a t i o , p r o d u c i n g 2 7 5 4 k e V 7 r a y s in t r a n s i t i o n t o t h e 2 + t r a n s i t i o n . T h e s e 7 r a y s d i s i n t e -g r a t e d d e u t e r o n s a n d p r o d u c e d 2 65 k e V n e u t ro n s . C o n t r i b u t i o n s o f o t h e r s t a t e s i n 2 4M g


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89

6O

4

200 400 600 800 1000C h a n n e l N u m b e r

Figure 1. Moderation time distribution for neutrons in a polyethy-lene cube. Full scale is 1 ms. The time con stan t for mod era tio n isabo ut 140 # sec. Background neutrons tha t arrived at BFa counterstime-independently are clearly distinguished from photoneutrons ofinterest.

to neutron production were neglected because of small branching ratios.The absolute intensity of the 265 keV neutron source was determined with the water

ba th me th od [13]. The source was placed at the center of a 400 liter water bath. Neu tro nflux was measured from 2.5 cm to 17.5 cm in 2.5 cm steps with a 1.5 cm a BFa coun ter(MITSUBIS HI ND8522-60). The efficiency calibra tion of the small counter for the rma lneutrons had separately been done with the standard graphite pile at ETL [14]. Back-ground n eutro ns were measured with the same counter covered by a 5 mm thick cadmium.Time elapsed during the measurement was corrected for. The source intensity at the timeof pro duc tion was dete rmined to be 2.83 10 a counts/ sec/4r r.

The n eut ron source was placed at the center of the neutr on detector. A neutr on countingwas performed five times, each for 300 seconds, with two bare BFa counters and two Cd-covered counters. The cont ributio n of background neutr ons was very small. The detec tionefficiency thu s de te rmined for 265 keV neu trons was 1.67 -+- 0.09 % per BFa counter . Theuncertainty stemmed from 1% systematic error in neutron counting, 5 % in calibrationof the small BFa counter and 1.7 % in determination of the source intensity.

Fig. 2 shows the tot al emciency of the ne utron det ecto r as a function of neu tro n energy.The efficiency was also measure d wi th a 252Cf source whose average ne ut ron energy is 2.35MeV. The energy dependence of the emciency was calculated with a Monte Carlo codeMCNP [15].3 . R E S U L T S

Cross sections were obtained with a formulaO m_ NnN . r e n ( E n ) N e f ' ( 1 )


4/6

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1210

M C N P2 4N a OH + D 0 M o n t e C a r l o C a l .2

2 5 2 C f

5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0E (keY)n

F i g u r e 2 . N e u t r o n d e t e c t i o n e f fi ci en c y a s a f u n c t i o n o f n e u t r o ne n e r g y . T h e e f f i c i e n c y w a s e x p e r i m e n t a l l y d e t e r m i n e d w i t h s o u r c e so f 2 4 N aO H + D 2 0 a n d 2 52 C f. T h e n e u t r o n e n e r g y d e p e n d e n c e o ft h e e f f i c i e n c y w a s c a l c u l a t e d w i t h t h e M C N P M o n t e C a r l o c o d e .

w h e r e N n is t h e n u m b e r o f n e u t r o n s d e t e c t e d , N ~ i s t h e n u m b e r o f V r a y s i n c i d e n t o nt h e B G O d e t e c t o r , N t is t h e n u m b e r o f t a r g e t n u c l e i p e r c m 2, a n d C n ( E n) i s t h e n e u t r o nd e t e c t i o n e ff ic ie n cy . A c o r r e c t i o n f a c to r , f , w a s n e e d e d f or t h e m e a s u r e m e n t w i t h a t h i c kt a r g e t .

f = e t , t (1 - e - u t ) (2 )# t '

w h e r e # i s t h e t o t a l v - a t t e n u a t i o n c o e ff ic i en t in 9B e , a n d t is t h e l e n g t h o f t h e 9 B e t a r g e tm a t e r i a l . T h e a t t e n u a t i o n c o e ff ic i en t w a s t a k e n f r o m [1 6].

F i g . 3 s h o w s m e a s u r e d p h o t o n e u t r o n c r o s s s e c t i o n s a s a f u n c t i o n o f 7 e n er g y . N o t et h a t r e s u l t s f o r u n p o l a r i z e d -~ r a y s ( o p e n c i rc le s ) a g r e e w i t h t h o s e f o r p o l a r i z e d o n e s( s o li d c i rc l e s) w i t h i n s t a t i s t i c a l u n c e r t a i n t i e s . T h e p r e s e n t m e a s u r e m e n t f u l ly c o v e r e d t h ee x c i t a t i o n - e n e r g y r a n g e o f t h r e e r e s o n a n c e s ( 5 / 2 - , 1 / 2 - , 5 / 2 + ) , w h i le it c o v e re d o n ly ah i g h - e n e r g y t a i l o f t h e f i rs t e x c i t e d s t a t e ( 1 / 2 + ) . T h i s i s b e c a u s e a s a p p r o a c h i n g t o t h e1 . 6 6 5 - M e V t h r e s h o l d , i t b e c a m e d i ff ic u lt t o a c c u r a t e l y e v a l u a t e w h a t f r a c t i o n o f t h e v - r a yf lu x la y a b o v e t h e n e u t r o n t h r e s h o ld . A M o n t e C a r l o c a lc u l a t io n o f t h e a s y m m e t r i c e n e r g ys p r e a d o f L E M P S m a y h e l p d e d u c e t h e n e c e s s a r y V f lu x . T h i s w i ll b e d o n e i n t h e f u t u r e .C u r r e n t l y , a l e a s t - s q u a r e f i t t o th e d a t a is i n p r o g r e s s t o d e d u c e r e s o n a n c e p a r a m e t e r s( v - d e c a y w i d t h s e t c . ) t o w a r d s a n e v a l u a t i o n o f t h e r e l e v a n t r e a c t i o n r a t e .


5/6

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.. QEv

2 . 0 . . . . ~ . . . . , . . . . i . . . . ~ . . . . J . . . .

9 P r e s e n t r e s u l t s ( p o l a r i z e d )o P r e s e n t r e s u l t s (u n p o l a r i z e d )n Fu j ish i r o e t a l . ( 1982) R I+ B . H a m e r m e s h e t a 1 . (1 9 5 3 ) R Io B . R u s s e l l e t a l . ( 1 9 4 8 ) R I1 . . 5 A J . H . G i b b o n s e t a 1 . (1 9 5 9 ) R I

W . J o h n e t a 1 . (1 9 6 2 ) R I. . . . . J a k o b s o n e t a l . ( 1 9 6 1 ) B r e m s s .

i

N'

0 5 .

1.5 2.0 2.5 3.0 3.5 4.0 4.5E (MeV)T

Figure 3 . Ph oto neu tron cross sect ions for 9B e. For com parison,resu l t s o f the p reced ing measurem ents wi th Brem ss t rah lung andr a d i o a c t i v e i s o t o p e s a r e a l s o s h o w n .

4 . A C K N O W L E D G M E N TWe are indeb ted to Mr . K . Okam oto (Research Reac to r Ins t i tu te o f Kyoto Univers ity )

for h is technical ass is tance in the i r radia t ion of NaO H w ith therm al neutron s , to M r. Y .Ogawa (Kinki Univers i ty) for h is help in running the MCNP code, and to Dr. K. Kudo(Elect rotechnical Laboratory) for h is cooperat ion in the detector cal ibrat ion wi th thes tand ard g raph i t e p il e. Th i s work was suppor ted by the Jap an Pr iva te Schoo l Prom ot ionFoundat ion .R E F E R E N C E S1. S.E. W oosley and R .D. Hoffma n, Astro phy s. J . 395 (1992) 202.2. M .K. Jakobso n, Phys . Rev. 123 (1961) 229.3. B. Ru ssel e t a l . , Phys. Rev. 73 (1948) 545.4. B. H am erm esh and C. Kim ball , Phy s. Rev. 90 (1953) 1063.5 . J .H. Gibb ons e t a l . , Phy s. R ev. 114 (1959) 1319.6. W . John an d J.M. Prosser, Phys. Rev. 163 (1967) 958.7. M. Fu jishiro e t a l . , Can . J . P hys. 60 (198 2) 1672.8. M. Fujish iro e t a l . , Can . J . Ph ys. 61 (1983) 1579.9. H.-G. Clerc e t a l . , Nucl. Ph ys. A120 (1968) 441.10. G. Kuechler e t a l . , Z. Phys. A326 (1987) 447.


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11. H. Toyokawa e t a l . , Nucl . Ins t r . and Methods in Phys . Res . A422 (1999) 95 .12 . H . Ohgak i e t a l . , IEEE Tran. Nucl . Sci . 38 (1991) 386.13. E xp er im en ta l n uc l ear phys i cs , V o l . 2 , ed i t ed by E . Segre ( John . W i ley & Son , Inc . ,

New York, NY 1978) p . 455.14. Y. Inoue e t a l . , Bul l . Elect ro tech. Lab. 31 (1967) 60.1 5 . J .F . B r i e s me i s t e r , M C N P , A G en e r a l M o n t e C a r l o N - P a r t i c l e T r an s p o r t C o d e , V e r s i o n

4 B , Las A l amo s N a t i o n a l Lab o r a t o r y , 1 9 9 7 .1 6. En g i n ee r i n g C o m p en d i u m o n R ad i a t i o n S ch ie l di n g, V o l . 1 , ed i t ed b y R .G . J aeg e r , E .P .

B l i zard , A .B . Ch i l t on , M. Gro tenhu i s , A . HSnig , Th . A. Jaeger , and H.H. Ei sen lohr(Spr inger -V er l ag N ew York Inc . 1968) p . 173 .

photoneutron cross section measurement on 9b by means of inverse compton scattering of laser

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