Д 33_ Г(/ 34. Ж
KFKI-71-9
CENTRAL RESEARCH
INSTITUTE FOR PHYSICS
BUDAPEST
L Jóki G y. K luge
A. Lajtai SURVEY O F MEASUREMENTS O F THE
FISSION NEUTRON SPECTRUM O F
252Cf
K PK I-71-9
SURYEY OP MEASUREMENTS OP THE PISSION NEUTRON SPECTRUM OP 252Cf L . J e k i , G y .K lu g e , A . L a j t a i
C e n t r a l R e s e a r c h I n s t i t u t e f o r P h y s i c s , B u d a p e st, Hungary N u c le a r P h y s i c s Department
1 . INTRODUCTIOH
An a c c u r a t e knowledge o f the sh a p e o f th e n e u tr o n s p e c tr u m i s o f prim ary i n t e r e s t in r e a c t o r p h y s i c s c a l c u l a t i o n s . This i s tru e f o r t h e f i s s i o n s p e c t r a o f i s o t o p e s o t h e r than the e x t e n s i v e l y i n v e s t i g a t e d 2 ’ * U . Such d a ta can a l s o lend i n s i g h t by co m p a riso n s w i t h s im i
l a r measurements on 2^ U . Most o f the measured n e u t r o n s p e c t r a a r e de
s c r i b e d v e r y w e l l by the M a x w ellia n d i s t r i b u t i o n . E x p e r im e n ta l n e u tr o n s p ectru m d ata on 2 ^2Cf s p o n ta n e o u s f i s s i o n a re c o m p ile d in T a b le I and are d i s p l a y e d P i g . 1 , which g i v e s the M a x w ellia n tem p era tu re ( T ) a s a c h a r a c t e r i s t i c p a ra m eter o f th e s p e c t r a l s h a p e . When s t u d y i n g t h e t a b le one i s s t r u c k by th e o b s e r v a t i o n t h a t i n s p i t e o f th e r e l a t i v e l y sm a ll e r r o r s quoted t h e r e are r a t h e r la r g e d i v e r g e n c e s b e tw e e n the v a l u e s . T his i n d i c a t e s l a r g e s y s t e m a t i c u n c e r t a i n t i e s .
Prom t h e t h e o r e t i c a l p o i n t o f v i e w , T e r r e l ’ s r e l a t i o n [ 2 l ] b etw een T and v /a v e r a g e number o f n e u t r o n s from f i s s i o n / r e p r e s e n t s a g u id e f o r an e x p e c t e d t r e n d . However, i t g i v e s a l a r g e s c a t t e r i n g be
tw een d a ta p o i n t s and a t b e s t one can o n l y sa y t h a t th e T-v r e l a t i o n d e m o n s t r a t e s some d eg ree o f t r e n d . I t i s c l e a r , t h e r e f o r e , t h a t th e r e
i s a need f o r new a c c u r a t e measurements and f o r a s e r i o u s e v a l u a t i o n o f a l l im p o rta n t s o u r c e s o f e r r o r .
The i n c r e a s i n g a v a i l a b i l i t y o f 252Cf has made i t a c o n v e n i e n t n e u t r o n source and thus i t i s v e r y d e s i r a b l e to have an a c c u r a t e know
l e d g e o f i t s n e u t r o n en ergy d i s t r i b u t i o n . We have a tte m p te d i n t h i s paper to e x p l a i n the l a r g e spread i n the ^ Cf f i s s i o n n e u t r o n s p ectru m data and t o e s t im a t e th e b e s t v a l u e o f T from a d e t a i l e d a n a l y s i s o f the e x p e r i m e n t a l c i r c u m s t a n c e s and the n e c e s s a r y c o r r e c t i o n s .
2 . BACKGROUND CORRECTIONS IN TOP MEASUREMENTS
Most o f the measurements r e f e r r e d to were c a r r i e d o u t u s in g t i m e - o f - f l i g h t (TOP) t e c h n i q u e s . B e fo r e d e a l i n g w i t h th e m easurem ents we s h a l l d is c u B s th e p o s s i b l e s o u r c e s o f background and the c o r r e c t i o n s
t h a t t h e y e n t a i l i n TOP measurements on f i s s i o n n e u t r o n s p e c t r a in g e n e r a l . The d i s c u s s i o n i s a p p l i c a b l e to e x p e r im e n t s w here:
t
a / th e s t a r t s i g n a l t o the t i m e - t o - p u l s e h e i g h t c o n v e r t e r i s p r o v id e d by a n e u t r o n d e t e c t o r s e n s i t i v e to both n e u t r o n s and gamma rays:.
2
b / th e a t o p s i g n a l i s p r o v id e d by a f i s s i o n d e t e c t o r s e n s i t i v e t ó f i s s i o n f r a g m e n t s o n l y . The f i s s i o n d e t e c t o r r e c o r d s a l l t h e f i s s i o n e v e n t s , so t h a t th e t o t a l n e u tr o n sp e c tr u m a v e r a g e d o v e r a l l a n g l e s can be m easured.
A. Random b a c k g r o u n d .
The random c o i n c i d e n c e background can be measured s i m u l t a n e o u s l y i n a g iv e n r a n g e o f the sp ectru m by s u p p r e s s i n g s y s t e m a t i c e v e n t s i n t h i s r e g io n w i t h a p p r o p r i a t e d e l a y s . The d i s t r i b u t i o n o f background c a u s e d by the random c o i n c i d e n c e s i s f l a t .
A s p e c i a l type o f random c o i n c i d e n c e s , t h e '’s y ste m a tic -r a n d o m "
c o i n c i d e n c e s , em erges i n t h e f i s s i o n n e u tr o n sp e c tr u m m ea su rem en ts, b u t th e s e are n o t m ea su ra b le d i r e c t l y . The number o f a c c i d e n t a l c o i n c i d e n c e s i s g i v e n by N = 2t Nn Nf , where т i s th e tim e r e s o l u t i o n o f th e c o i n c i d e n c e u n i t and N and ÍL. a re t h e cou n t r a t e s o f the n e u t r o n and
n I
f i s s i o n d e t e c t o r , r e s p e c t i v e l y . I t i s n e c e s s a r y t o c a l c u l a t e N f o r ea ch c h a n n e l o f th e m u l t i c h a n n e l a n a l y s e r b eca u se Nn v a r i e s from c h a n n e l to c h a n n e l . i s th e number o f e v e n t s p r o c e e d i n g an e v e n t i n th e i - t h c h a n n e l . The number o f e y s t e m a t ic - r a n d o m c o i n c i d e n c e s in th e i - t h chan
n e l i s g iv e n by
N1 = NJ
i= 3 + i
nw here т i s t h e ch a n n el w id th i n tim e , u n i t s , th e f i s s i o n r a t e p e r u n i t time and the number o f e v e n t s i n the j - t h c h a n n e l. The f a c t o r
n
o f 2 d i s a p p e a r s , b eca u se th e e v e n t s ca n n o t o c c u r i n the r e v e r s e time d i r e c t i o n .
This t y p e o f background d ep en d s on the sh a pe o f th e measured sp ectru m . I n g e n e r a l , t h e d i s t r i b u t i o n o f the s y s t e m a t i c - r a n d o m c o i n c i d e n c e s i s n e a r l y f l a t , s l i g h t l y d e c r e a s i n g w i t h i n c r e a s i n g c h a n n e l number. I f t h i s type o f background i s n e g l e c t e d th e v a lu e o b t a i n e d f o r T w i l l b e l o w , b eca u se t h e s y s t e m a tic - r a n d o m background i s im p o r ta n t o n l y i n the l o w - e n e r g y p a r t o f th e s p e c tr u m .
B. S c a t t e r e d background
The r e l a t i v e number o f s c a t t e r e d n e u t r o n s t h a t a r e d e t e c t e d depends s t r o n g l y on th e e x p e r i m e n t a l c i r c u m s t a n c e s . I t ca n be measured b y p l a c i n g a shadow cone betw een t h e s o u r c e and th e n e u t r o n d e t e c t o r t o s h i e l d a g a i n s t d i r e c t n e u t r o n s .
The d e t e c t i o n o f s c a t t e r e d n e u t r o n s i n c o i n c i d e n c e w i t h f i s s i o n
- 3 -
f r a g m e n ts w i l l d e c r e a s e th e v a l u e Of th e a v e r a g e n e u t r o n e n e r g y , and hence th e v a lu e o f T w i l l a l s o be lo w e r e d . / Assuming M a x w ellia n d i s
t r i b u t i o n , the a v e r a g e n e u tr o n e n e r g y / Ё / i s ^ T . / •
C. D ela y ed gamma r a y s
The e x i s t e n c e o f d e la y e d gamma r a y s i n the 2 52^ Of f i s s i o n p r o c e s s was p o in t e d o u t f i r s t by J o h a n sso n [10] . I t was l a t e r shown t h a t the d e t e c t i o n o f d e la y e d gamma r a y s c o u ld s t r o n g l y d i s t o r t the l o w - - e n e r g y p a r t o f th e n e u tr o n sp ectru m [б] . The background ca u sed by t h e s e d e l a y e d gamma r a y s can be measured a t s h o r t f l i g h t p a t h s . I f the spectrum d i s t o r t i o n ca u sed by th e d e t e c t i o n o f d e la y e d gamma r a y s i s n e g l e c t e d T w i l l a g a i n i b e too lo w .
No c o r r e c t i o n was made f o r d e la y e d gamma r a y s i n d a ta p u b l i s h e d b e f o r e J o h a n s s o n ’ s work. We h ave e s t i m a t e d th e m agnitude o f the d e l a y e d gamma r a y s u s in g th e r e c e n t d a t a o f John e t a l . [ l l ] , who measured the e n e r g y , h a l f - l i f e and i n t e n s i t y o f 144 gamma ra y s and the mass o f the e m i t t i n g f r a g m e n t s . For 0 . 2 < E< 2 MeV and t < 1 0 0 n s e c , the t o t a l gamma r a y en erg y o f u n r e s o lv e d l i n e s i s a t m ost 20 % o f t h a t o f r e s o l v e d l i n e s , s o t h a t o u r c a l c u l a t i o n s ca n g i v e a lo w e r l i m i t f o r the m agnitude o f the d e la y e d gamma r a y s .
3 . METHOD OF EVALUATION OF NEUTRON AND GAMMA RAY DISTRIBUTIONS We c a l c u l a t e d the e f f i c i e n c y o f d e t e c t i o n o f neutronB and gamma r a y s , t h e en ergy d i s t r i b u t i o n o f the f i s s i o n n e u tr o n s and the tim e d i s t r i b u t i o n o f d e la y e d gamma r a y s . Two d i f f e r e n t t y p e s o f s c i n t i l l a t o r were u s e d to d e t e c t the n e u t r o n s : o r g a n ic p l a s t i c s and 6I i - l o a d e d g l a s s s c i n t i l l a t o r s . S i n c e v e r y few d a t a were a v a i l a b l e on t h e d e t e c t i o n o f lo w - e n e r g y n e u tr o n s and gamma r a y s by o r g a n i c p l a s t i c s c i n t i l l a t o r s , the c a l c u l a t i o n s were c a r r i e d o u t a s f o r a n t h r a c e n e s c i n t i l l a t o r s , which have a s i m i l a r c h e m ic a l s t r u c t u r e and d e n s i t y . I t has b een shown t h a t the r e l a t i v e l i g h t o u tp u t f o r t h e d e t e c t i o n o f d i f f e r e n t charged p a r t i - ' d e s i s th e same b o t h f o r a n t h r a c e n e and o r g a n i c p l a s t i c s c i n t i l l a t o r s
O ] .
The e f f i c i e n c y o f n e u t r o n d e t e c t i o n was c a l c u l a t e d by th e f o l l o w in g fo r m u la :
where рд i s the m a c r o s c o p ic c r o s s - s e c t i o n o f (n ,p ) s c a t t e r i n g , Si i s the
- 4 -
t h i c k n e s s o f th e s c i n t i l l a t o r and E^h I s t h e e n e r g y t h r e s h o l d f o r n eu t r o n d e t e c t i o n . The v a l i d i t y o f t h i s sim p le fo rm u la h a s a l r e a d y b een proved e x p e r i m e n t a l l y [16] . pH f o r a n t h r a c e n e was c a l c u l a t e d from
the a p p r o p r ia t e t a b l e s [19] . The v a lu e o f E” h was d e t e r m in e d f o r e a c h m easurem ent.
The e f f i c i e n c y o f d e t e c t i o n o f the gamma ra y s was c a l c u l a t e d from d a t a o n th e gamma—r a y a b s o r p t i o n c o e f f i c i e n t s o f a n t h r a c e n e [15]
I n the i n v e s t i g a t e d e n e r g y range Compton s c a t t e r i n g i s t h e o n l y p o s s i b l e a b s o r p t i o n p r o c e s s and t h e r e f o r e th e e f f e c t o f the d e t e c t o r t h r e s h o l d on the e f f i c i e n c y c u r v e was a c c o u n t e d f o r i n th e same way a s i n n e u t r o n d e t e c t i o n :
S (E)
= Eo- f y
Et h E
The v a lu e o f E^h was d eterm in ed f o r each m easurem ent.
I n th e c a s e o f 4 * 6 Li g l a s s s c i n t i l l a t o r s an a b s o l u t e n eu tro n e f f i c i e n c y c u rv e was u sed [б] . The t h r e s h o l d f o r gamma-ray d e t e c t i o n i s about 0 . 8 MeV. We measured th e e f f i c i e n c y o f L i - g l a s s s c i n t i l l a t o r
СЛ pp
f o r the d e t e c t i o n o f uCo and Na gamma r a y s and found i t was 1 . 2 % , compared w i t h 1 # e f f i c i e n c y r e p o r t e d f o r ^ C o gamma r a y s [6] . Above 0 . 8 MeV d e l a y e d gamma r a y s are i n th e e n e r g y range o f 1 . 2 - 1 . 3 MeV o n l y
[ l l ] and t h u s the measured e f f i c i e n c y v a l u e s c o u ld be t a k e n i n t o a c c o u n t.
The n e u t r o n e n e r g y d i s t r i b u t i o n was assumed to be M a x w e llia n , w it h an e n e r g y p a r a m e te r o f T=1.5 MeV j i t was n o r m a liz e d to g i v e v = 3 . 7 7 n e u t r o n s p e r f i s s i o n . Taking i n t o a c c o u n t th e e f f i c i e n c y cu rv e and the f l i g h t p a t h , th e en erg y d i s t r i b u t i o n was tr a n s fo r m e d i n t o a
time d i s t r i b u t i o n .
The time d i s t r i b u t i o n o f d e la y e d gamma ra y s was c a l c u l a t e d from p u b l is h e d d a t a [ l l ] . The d i s t r i b u t i o n o f e a c h gamma r a y was n o r m a liz e d t o g iv e th e measured i n t e n s i t y p e r f i s s i o n . The "measured" d i s t r i b u t i o n o f d e la y e d gamma r a y s was g i v e n b y t a k in g i n t o a cco u n t t h e e f f i c i e n c y and summing up a l l th e gamma r a y s .
By com paring t h e c a l c u l a t e d d i s t r i b u t i o n s o f th e n e u t r o n s and d e la y e d gamma r a y s we c a n deduce th e m agnitude o f the d i s t o r t i o n e f f e c t caused by t h e d e t e c t i o n o f d e l a y e d gamma r a y s i n 2^2Cf f i s s i o n h e u t r o n spectrum m ea su rem en ts.
4 . DISCUSSION OF TOF MEASUREMENTS
The most im p o r ta n t d a ta on TOF measurements a re l i s t e d i n T a b le I I . The background e s t i m a t i o n s w ere made from t h e s e f i g u r e s .
А / Sm ith e t a l . (Jaj d id n o t r e p o r t an y data on th e t h r e s h o l d
- 5 -
e n e r g y o f n e u tr o n d e t e c t o r . Assuming d i f f e r e n t t h r e s h o l d e n e r g y v a l u e s f o r n e u t r o n d e t e c t i o n we c a l c u l a t e d th e measured n e u t r o n sp ectru m by th e method o u t l i n e d a b o v e . The b e s t agreem ent between t h e c a l c u l a t e d and the measured n e u t r o n sp ectru m was a c h i e v e d w ith a v a l u e o f = 100 keV. The 100 keV t h r e s h o ld e n e r g y f o r n e u t r o n s c o r r e s p o n d s t o a 5 keV t h r e s h o l d en erg y f o r gamma r a y s . / The l i g h t o u t p u t o f a n t h r a c e n e f o r 100 keV e n e r g y p r o t o n s i s e q u a l to th e l i g h t o u t p u t o f 5 keV en e r g y e l e c t r o n s [ l 7 , 18] . Gamma r a d i a t i o n w i t h an e n e r g y o f 5 keV l o s e i t s t o t a l e n e r g y i n th e s c i n t i l l a t o r [18] . / The c a l c u l a t e d n e u t r o n and de
la y e d gamma ray d i s t r i b u t i o n s can be s e e n in P i g . 2 . The background due to th e d e la y e d gamma rays i s r a t h e r im p o r t a n t . The number o f d e t e c t e d d e l a y e d gamma r a y s r e l a t i v e t o the number o f d e t e c t e d n e u t r o n s i s 10 %>
a t 0 . 2 MeV n e u t r o n e n e r g y , 3 - 6 $ a t 0 . 5 MeV and 1 % a t 2 MeV. Taking i n t o a c c o u n t t h i s c o r r e c t i o n , we g o t а 5 -Ю % h i g h e r v a lu e th a n the s i n g l e measured v a l u e f o r t h e tem p era tu re o f the M a x w e llia n d i s t r i b u t i o n .
The e f f e c t o f s y s t e m a t i c - r a n d o m c o i n c i d e n c e s was n o t c o n s i d e r e d , a lt h o u g h t h i s ty p e o f background a l s o ca n d e c r e a s e th e T v a l u e .
В / Bowman e t a l . [4] measured th e a n g u la r d i s t r i b u t i o n o f neu
t r o n s . The v a l u e , o f T was c a l c u l a t e d from the i n t e g r a t e d a n g u l a r d i s t r i b u t i o n . The s c a t t e r e d background was measured and th e s y s t e m a tic - r a n d o m background was c a l c u l a t e d , b u t t h e s e components w ere d i s r e g a r d e d in the d a t a a n a l y s i s . The a u t h o r s d e te r m in e d the background, i n d i f f e r e n t p a r t s o f the sp ectru m : in t h e random c o i n c i d e n c e r e g i o n , i n t h e v a l l e y b etw een the n e u t r o n and prompt gamma-ray peaks; and below 4 50 keV neu
t r o n e n e r g y , where the d e t e c t o r e f f i c i e n c y was 0 f o r n e u tr o n d e t e c t i o n . The background d i s t r i b u t i o n was c a l c u l a t e d from t h e s e v a l u e s f o r the whole sp ectru m , and thus t h e r e a l v a lu e o f background was d ete r m in e d w it h o u t any knowledge o f t h e e x i s t e n c e o f d e la y e d gamma r a y s .
С/ The r e s u l t s o f o u r c a l c u l a t i o n s 4f o r th e m easurem ents o f Condé e t a l . [5] w i t h a L i - g l a s s s c i n t i l l a t o r ca n be s e e n i n F i g . 3 . Above 200 keV th e number o f d e t e c t e d gamma ra y s i s n e g l i g i b l e . In meas
urem ents w it h a p l a s t i c s c i n t i l l a t o r th e t h r e s h o l d e n e r g y o f n eu tro n d e t e c t o r was 5 keV f o r gamma r a y s [9] , which c o r r e s p o n d s to 100 keV f o r n e u t r o n s , a s n o te d i n t h e d i s c u s s i o n o f S m ith ’ s m ea su rem en ts. The c a l c u l a t e d d i s t r i b u t i o n s a r e p l o t t e d i n P i g . 4 . The r e l a t i v e number o f d e t e c t e d d e la y e d gamma r a y s i s 5 a t 0 . 3 MeV and 2 # a t 1 MeV. The d i s t o r t i o n o f th e spectrum i s n o t n e g l i g i b l e .
There was no measurement o f s c a t t e r e d background and th e c o n t r i b u t i o n o f s y s t e m a tic - r a n d o m background was n ot c a l c u l a t e d . S i n c e a l l n e g l e c t e d background s o u r c e s d e c r e a s e th e T v a l u e , th e measured v a lu e o f T = 1 .3 9 MeV i s to o low.
D / Meadows [б] a n a l y s e d and d e term in e d a l l t y p e s o f background.
6
H is m aasurements show a s m a ll d e v i a t i o n from the M a x w e llia n shape b e
low 0 . 5 MeV and g a v e a r e s u l t o f T=1.565 MeV c a l c u l a t i n g b etw een 0 . 0 0 5 and 15 MeV, and T = 1 .5 9 2 b etw een 0 . 5 and 10 MeV.
Е / Zamyatnin e t a l . [7] to o k a l l t y p e s o f background i n t o a c count e x c e p t t h a t due to the d e l a y e d gamma r a y s . The d e t e c t o r t h r e s h o ld e n e r g y was 59 keV f o r gamma r a y s , which i s e q u a l to 3^-5 keV f o r n e u t r o
[4] . The c a l c u l a t e d d i s t r i b u t i o n s can be s e e n i n P i g . 5-The d i s t o r t i o n c a u s e d by th e d e t e c t i o n o f d e la y e d gamma ra y s i 3 n o t n e g l i g i b l e , t h e r e f o r e the v a l u e o f T =1.48 g i v e n by th e a u t h o r s i s somewhat lo w .
5. MEASUREMENTS WITH OTHER METHODS
When o t h e r methods a r e u sed to d e te r m in e n e u t r o n e n e r g i e s the background s o u r c e s d i f f e r somewhat from t h o s e i n TOP m ea su rem en ts. Im
p o r t a n t d i s t o r t i o n c a n be c a u s e d by the d e t e c t i o n o f d e la y e d n e u t r o n s a r i s i n g 2 - 2 0 s e c a f t e r f i s s i o n . I n the s p o n ta n e o u s f i s s i o n o f 2 52Cf 0 . 8 6 % o f a l l th e n e u t r o n s a r e d e la y e d [15] . A lth o u g h th e a v e r a g e en
e r g y o f t h e s e n e u t r o n s i s n o t known, the a v e r a g e e n e r g y o f 450 keV meas
ured f o r 235U and 2 ^9Pu [20] may be v a l i d f o r 2')2Cf t o o .
A /H ja lm a r e t a l . [1] measured the n e u tr o n sp e c tr u m w it h p h o to p l a t e s by com puting the n e u t r o n en e r g y from the t r a c k s o f r e c o i l e d p ro
t o n s i n e m u ls io n . Only 281 t r a c k s were recorded., and t h e low e n e r g y p a r t b e lo w 2 MeV was n o t c o n s i d e r e d . The s t a t i s t i c a l v a l i d i t y o f t h i s measurement i s d o u b t f u l .
В / The n e u t r o n d i s t r i b u t i o n r e p o r t e d by S m ith e t a l , [2] was measured above 2 MeV w it h p h o t o p l a t e s . The sp ectru m was e v a l u a t e d by Condé [5] I who g a v e a v a lu e o f T=1.57 MeV.
С / Bonner [5] measured th e n e u t r o n spectrum b y means o f a s p h e r e - m o d e r a t e d n e u tr o n s p e c t r o m e t e r [8] . N eu tron s were d e t e c t e d in a s m a l l 6L i l / E u / s c i n t i l l a t o r p l a c e d a t th e c e n t r e o f p o l y e t h y l e n e mode
r a t i n g s p h e r e s w i t h s i z e s r a n g i n g from 5 . 1 t o 30 cm i n d i a m e t e r . The c o u n t i n g - r a t e r a t i o o f s p h e r i c a l c o u n t e r s w i t h d i f f e r e n t s i z e s i s a s e n s i t i v e f u n c t i o n o f n e u t r o n e n e r g y . Assuming M a x w e llia n d i s t r i b u t i o n f o r t h e f i s s i o n n e u t r o n s th e measured r a t i o w i l l be a f u n c t i o n o f T o n l y . The b e s t v a lu e o f T computed from s i x in d e p e n d e n t m easurem ents w i t h
d i f f e r e n t d e t e c t o r p a i r s was T = 1 .3 6 7 . Background was measured w it h o u t th e 2 52Cf s o u r c e i n p o s i t i o n b u t the number o f s c a t t e r e d n e u t r o n s was n ot d e t e r m in e d , w h ich means t h a t the v a l u e o f T was lo w e r e d .
We c a l c u l a t e d the c o n t r i b u t i o n o f d e la y e d n e u t r o n s f o r one o f the d e t e c t o r p a i r B / 20 and 7 . 6 cm s p h e r e s / u s in g t h e e f f i c i e n c y c u r v e s g i v e n by B onner. T h is g i v e s a v a l u e o f T reduced by 30 keV. The d e c r e a s e i n T v a l u e due t o d e la y e d n e u t r o n s was l a r g e r f o r t h e o t h e r d e t e c t o r
- 7 -
p a i r s . The c o n t r i b u t i o n from t h e d e t e c t i o n o f prompt gamma r a y s i s n e g l i g i b l e .
6 . CONCLUSIONS
The e f f e c t o f d i f f e r e n t components o f the background on m eas- urementa o f f i s s i o n n eu tro n e n e r g y s p e c t r a o f Cf h ave been d i s c u s s e d and i t h a s been p o i n t e d out t h a t f a i l u r e t o a l l o w f o r them r e s u l t s i n a n e u t r o n spectrum w i t h a lo w e r e d tem p era tu re p a r a m e t e r . The n e g l e c t o f a l l [5] o r some [ 2 ,7 ] o f t h e com ponents due to d e t e c t e d gamma r a y s , s y s te m a tic -r a n d o m c o i n c i d e n c e s and s c a t t e r e d n e u t r o n s i n t i m e - o f - f l i g h t m easurem ents has meant th a t c a l c u l a t i o n s o f the M a x w e llia n tem p era tu re are to o low by 1 0 0 - 1 5 0 keY.
O th er measurements o b t a i n e d u s in g d i f f e r e n t m ethods [ 2 , 4 , 6 ] have p r e c i s e background c o r r e c t i o n s and g i v e a lm o s t i d e n t i c a l v a l u e s f o r t h e n e u t r o n 's p e c t r u m .p a r a m e t e r : 1 . 5 7 + 0 . 0 5 , 1 . 5 6 , 1 . 5 6 5 and 1 . 5 9 2 , r e s p e c t i v e l y .
I f the s p e c t r u m - d i s t o r t i n g e f f e c t s due t o t h e d i f f e r e n t back
grounds a r e p r o p e r l y a c co u n ted f o r , i t seems th a t t h e most p r o b a b le v a lu e o f th e M a x w ellia n tem p era tu re i s a b o u t 1 . 5 7 MeV. T his g i v e s a v a lu e o f 2 . 5 5 5 MeV f o r the a v e r a g e e n e r g y o f f i s s i o n n e u t r o n s from the s p o n t a n e o u s f i s s i o n o f 2^^Cf.
8 -
Table I .
R e s u l t s and methods o f n e u tr o n e n e r g y spectrum m easurem ents f o r 2^2 Cf s p o n t a n e o u s f i s s i o n
Л
A uthors Method
1
M axw ellian tem p era tu re
MeV
R e f .
E .H jalm ar e t a l . P h o t o p l a t e 1 402+^*®"^ ± . W _ 0 .o85 1
A .B .S m ith e t a l . P h o t o p l a t e 1 . 5 7 + 0 . 0 5 2
T i m e - o f - f l i g h t 1 . 4 2 + 0 . 0 5
T.W.Bonner "Bram blett" c o u n t e r 1 .3 6 7 + 0 . 0 3 0 3
H.R.Bowman e t a l . T i m e - o f - f l i g h t 1 . 5 6 4
H .Condé e t a l . T i m e - o f - f l i g h t 1 . 3 9 + 0 . 0 4 5
J.W.Meadows T i m e - o f - f l i g h t 1..592} 1 . 5 6 5 6
Y.S .Zamyatnin e t a l . T i m e - o f - f l i g h t 1 . 4 8 + 0 . 0 3 7
T a b le I I .
C h a r a c t e r i s t i c p a ra m eters o f th e e x p e r i m e n t a l a rra n g em en ts f o r TOP m easurem ents
A uth ors R e f .
En MeV
n e u t r o n
d e t e c t o r Z cm n d e t . t h r e s h o l d keY
L cm
En
EA .B .S m ith e t a l . [2] 0 . 2 - 3 P i l o t В
p l a s t i c
3 . 8 1 80
H.R.Bowman e t a l .
O]
0 . 5 - 6 P i l o t Вp l a s t i c
5 . 0 8 545 59 93
H .Condé e t a l . [5 ,9 ] 0 . 0 7 - 0 . 5 L i - g l a s s 0 . 9 5 800 25
0 . 3 - 7 NE-102
p l a s t i c
5 . 0 8 5 100
<T .W. Meadows & »—\
0 . 0 0 3 - 2 NE-905
L i - g l a s s
0 . 9 5 800 1 8 . 6
3 2 . 7 1 3 4 . 2
1 - 1 5 H y d r . l i q -
s c i n t i l l a t o r 2 . 3 5 850 4000 6500
1 3 8 . 6
Y u.S .Z a m y a tn in e t a l . [7] 0 . 4 - 6 p l a s t i c 3 . 0 59 50
Ед - measured e n e r g y range o f n e u t r o n s Л - t h i c k n e s s o f th e s c i n t i l l a t o r
En , E - t h r e s h o l d e n e r g y o f th e n e u t r o n d e t e c t o r f o r n e u t r o n and gamma r a y e n e r g i e s , r e s p e c t i v e l y L - f l i g h t p a t h
10 -
Пл. 1
R e p o r te d v a l u e s [ l - 7 ] o f th e M a x w ellia n te m p e r a tu r e T f o r t h e e n e r g y d i s t r i b u t i o n o f n e u t r o n s from 2 ^>2 C f f i B s i o n
11 -
C a lcu la ted d i s t r i b u t i o n s o f n eutrons and d ela y ed gamma rays f o r th e measurement o f Smith e t a l . [2]
12 -
P1&--Í
C a lc u la te d d i s t r i b u t i o n s o f n eu tron s and d elayed gamma ray s f o r th e measurement o f Condé e t a l . [5] w ith a L i - g la s s s c i n t i l l a t o r
- 13 -
P ig .4
C a lc u la te d d i s t r i b u t i o n s o f n eu tro n s and d e la y e d gamma r a y s f o r th e measurement o f Condé e t a l . [5] w ith a p l a s t i c s c i n t i l l a t o r
1 4
?Ак-.5
C alcu lated d is t r ib u t io n s o f neutrone and delayed gamma rays fo r the measurement o f Zamyatnin e t a l . [7]
- 15 -
Re f e r e n c e s
[1] E .H ja lm a r , H . S l ä t i s , S.G .Thom pson, A rklv f ö r F y s i k 1 0 , 3 5 7 , / 1 9 5 5 / [2] A . B . S m i t h , P . R . F i e l d s , J . H . R o b e r t s , P h y s . R é v . 1 0 8 . 4 1 1 , Л 9 5 у . [3] T.W.B o n n er , N u c l . P h y s . 2^, 1 1 6 , / L 9 6 1 /
[4] H .R .Bowman, S . В . Thompson, J . C . D . M i l t o n , W. J . S w i a t e c k i , P h y s . Rév.
126 . 2 1 2 0 , / 1 9 6 2 /
[5] H .Condé, G .D u r in g , A r k iv f ö r F y s i k 2 2 , 3 1 3 , / 1 9 6 5 / [6] J .W .Meadows, P h y s . R év. 1 5 7 . 1 0 7 6 , /1967/.
Г71 Y .S .Z a m y a tn in , N . I . K r o s k i n , A . K . M e ln ik o v , V . H . N e fe d o v , P r o c . 2 n d .C o n f
^ N u c l e a r D ata f o r R e a c t o r s , H e l s i n k i , 1 9 7 0 , IAEA Vien n a 1 9 7 0 , V o l . I I . p . l 8 3 . Г81 R . L . B r a m b l e t t , R . I . E w ing, T.W.B o n n e r , N u c l . I n s t r . and Meth.
2
, 1 / I 9 6 0 /[9] H .Condé, G.D u r in g , J . Hansen, A r k iv f ö r F y s i k 2 2 , 3 0 7 , / 1 9 6 5 / [10] S . A . E . J o h a n s s o n , N u c l . P h y s . 6 4 , 1 4 7 , Д 9 6 5 /
[11] W.John, F.W.Guy, J . J . W e s e l o w s k i , P h y s . Rev C2, 1451 / 1 9 7 0 /
[12] A . B . S m i t h , P . R . F i e l d s , A.M.F rie d m a n , P h y s . R év. 1 0 4 . 6 9 9 , Л 9 5 б / [13] S .C o x , P . F i e l d s , A .F ried m a n , R .S j o b l o m , A . S m i t h , P h y s . R ev.
1 1 2 . 960 / 1 9 5 8 /
[14] J . B . B i r k s , S c i n t i l l a t i o n c o u n t e r s . Pergamon P r e s s L t d ., London, 1 9 5 4 .
[15] J . H . N e i l e r , P . R . B e l l , i n * A l p h a - , B e t a - and gamma-ray s p e c t r o s c o p y ” e d . by K .S ie g b a h n . N o r t h - H o ll a n d P u b l . C o . , Amsterdam, / 1 9 6 5 / V o l . I . p . 2 4 5 .
[16] R .H o n e c k e r , H . G r ä s s l e r , N u c l . I n s t r . and M eth . 46 , 2 8 2 , / 1 9 6 7 /
[ 17] H .B . F r e y , W.M.Grim, W .M .P r esto n , T .S .G r a y , P h y s , R e v . 8 2 , 3 7 2 , Л 9 5 1 / [18] H .-H . K . - B . von S c h m e l i n g , Z e i t s c h r i f t f ü r P h y s ik 1 6 0 . 5 2 0 , / i 9 6 0 / [19] R .J .H o w e r to n , R e p o r t UCRL-5226 / R e v i s e d / , Л 9 5 9 /
[ 20] R . B a t c h e l o r , H .R .H yder, J . N u c l . E n e r g y 2 , 7 , / 1 9 5 6 /
[ 2 1] J . T e r r e l l , P r o c . C o n f . P h y s i c s and C h e m istr y o f F i s s i o n , IAEA V i e n n a , 1965 p . 3 .
ABSTRACT
The background s o u r c e s a r i s i n g i n d i f f e r e n t t e c h n i q u e s f o r m easuring f i s s i o n n e u t r o n e n e r g i e s were s t u d i e d i n d e t a i l and the b a c k ground due to th e d e t e c t i o n o f d e l a y e d gamma r a y s was c a l c u l a t e d . The p ro p o sed v a l u e o f the M a x w ellia n te m p e r a tu r e f o r the e n e r g y d i s t r i b u
t i o n o f n e u t r o n s from th e s p o n ta n e o u s f i s s i o n o f 2 52c f i s a b ou t T = 1 .5 7 MeV.
РЕЗЮМЕ
Подробно исследовались источники, возникающие при различных методах
измерения
энергии нейтронов деления и был вычислен фон, вызванный регистрацией запаздывающего гамма-излучения. Для температуры Максвеля энергетического распределения нейтронов спонтанного деления 252c f полу
чили значение Т ~ 1 . 5 7 м эв.
K iv o n a t
Különböző m ó d sz e r e k k e l t ö r t é n t h a s a d á s i n e u t r o n e n e r g ia sp e k tr u m -m é r é s e k n é l v i z s g á l t u k a l e h e t s é g e s h á t t é r f o r r á s o k a t s k i s z á m í t o t t u k a k é s l e l t e t e t t gammasugárzás d e t e k t á l á s á b ó l adódó h á t t e r e t . A 2 ^ 2 C f s p o n t á n h a s a d á s á n á l k e l e t k e z ő n eu tro n o k e n e r g i a e l o s z l á s á t l e i r ó M a x w e l l - e l o s z - l á s e n e r g ia p a r a m é t e r é n e k l e g v a l ó s z í n ű b b é r t é k e k é n t T = 1 . 5 7 MeV a d ó d o t t .
2017
P r i n t e d i n the C e n t r a l R e s e a r c h I n s t i t u t e f o r P h y s i c s , B u d a p e s t , Hungary
K ia d ja a KFKI K ö n y v t á r - K ia d ó i O s z t á l y O . v . : D r. Farkas I s t v á n n á
Szakmai l e k t o r : P á l l á G a b r i e l l a N y e l v i l e k t o r : Tim othy W ilk in s o n P éld á n y szá m : 2 1 5 Munkaszám: 5 4 0 0 K é s z ü l t a KFKI h á z i s o k s z o r o s í t ó j á b a n F e l e l ő s v e z e t ő : Gyenes Imre
