- Email: [email protected]
26. A time of flight method for the measurement of total cross-sections for fast neutrons
LOW AND MEDIUM ENERGY SCATTERING
1137
w i t h s u f f i c i e n t l y s m a l l e n e r g y s p r e a d t o resolve t h e s c a t t e r i n g r e s o n a n c e s . O f f - r e s o n a n c e a n g u l a r d i s t r i b u t i o n s h a v e b e e n a n a l y z e d in t e r m s of p o t e n t i a l s c a t t e r i n g p h a s e shifts, w h i c h in s e v e r a l cases s h o w u p i n t e r e s t i n g a n o m a l i e s . D e t a i l e d a n a l y s i s of t h e r e s o n a n c e s c a t t e r i n g allows o n e t o m a k e spin a n d p a r i t y a s s i g n m e n t s . F r o m t h e 18 r e s o n a n c e s a n a l y z e d in t h i s fashion, t y p i c a l causes will be discussed a n d a s u m m a r y of all t h e r e s u l t s will b e p r e s e n t e d . I) W i l l a r d , B a i r and K i n g t o n , Phys. Rev. 98 (1955) 669. 2) F o w l e r , J. L. and J o h n s o n , C. H., Phys. Rev. 98 (1955) 728. 3) C o h n , H. O. and F o w l e r , J. L., Bull. Am. physl Soc. I (1956) 175.
26. A t i m e of f l i g h t m e t h o d for t h e m e a s u r e m e n t of total c r o s s - s e c t i o n s for fast n e u t r o n s . P. HUYER, C. C. JONKER, J. VAN DEN BERG a n d J. RETHMEIER. N a t u u r k u n d i g L a b o r a t o r i u m d e r Vrije U n i v e r s i t e i t , A m s t e r d a m , N e d e r l a n d . A t i m e of f l i g h t m e t h o d for t h e d e t e r m i n a t i o n of t o t a l cross-sections h a s b e e n realized by detecting neutrons twice along their path by stilbene scintillation counters. The t i m e i n t e r v a l b e t w e e n t h e pulses of t h e t w o c o u n t e r s is m e a s u r e d b y a d e l a y e d coincidence c i r c u i t a n d d e t e r m i n e s t h e e n e r g y of t h e n e u t r o n s b e t w e e n t h e c o u n t e r s . I n t h e first s c i n t i l l a t i o n c r y s t a l t h e n e u t r o n s are s c a t t e r e d i n t o t h e e n e r g y region selected b y t h e d e l a y of t h e c o i n c i d e n c e circuit, a n d t h e r e f o r e t h e m e t h o d is i n d e p e n d e n t of t h e e n e r g y s p e c t r u m of t h e a v a i l a b l e n e u t r o n source. W i t h t h e s c a t t e r e r b e t w e e n t h e c o u n t e r s , t o t a l cross-section m e a s u r e m e n t s of s u l p h u r h a v e b e e n p e r f o r m e d in t h e e n e r g y r a n g e 0.5 M e V t o l . l . M e V as a c h e c k o n t h e m e t h o d . T h e a v e r a g e e n e r g y r~sGlution w a s 2 0 % 6btaine~l w i t h a ' t i m e r e s o l u t i o n of 6 t o 9 X 10 -9 sec a n d a d i s t a n c e b e t w e e n t h e c o u n t e r s of I m. A full a c c o u n t will b e p u b l i s h e d s h o r t l y in P h y s i c a .
Ic. S C A T T E R I N G OF P H O T O N S 27. I n e l a s t i c n u c l e a r s c a t t e r i n g of p h o t o n s b y llSIn. O. V. BOGDANKEVICH, L . E . LAZAREVA a n d F. A. ~ICOLAEV. L e b e d e v P h y s i c a l I n s t i t u t e , Moscow, U.S.S.R.
T h e yield of t h e l l S I n (7, 7') lXSIn* r e a c t i o n h a s b e e n m e a s u r e d as a f u n c t i o n of s y n c h r o t o n e n e r g y of 5 M e V t o 27 MeV. T h e a c t i v a t i o n c u r v e for t h e p h o t o - e x c i t a t i o n of 115In* was m e a s u r e d w i t h a s c i n t i l l a t i o n c o u n t e r , w h i c h d e t e c t e d t h e 7-rays e m i t t e d in t h e t r a n s i t i o n f r o m t h e m e t a s t a b l e t o t h e g r o u n d s t a t e ( b y = 335 KeY, ~ = 4.5 h). T h e flux of X - r a y s , - i m p i n g i n g o n t h e s a m p l e , w a s d e t e r m i n e d b y m e a s u r i n g t h e ioniz a t i o n w i t h i n t h e air c a v i t y of a i o n i z a t i o n c h a m b e r w i t h t h i c k a l u m i n i u m walls (7.5 cm.). T h e cross-section c u r v e of t h e 1151n (7, 7') l l S I n * r e a c t i o n was c o m p u t e d b y t h e p h o t o n difference m e t h o d . T h e cross-section h a s a m a x i m u m a t 8.6 4- 0.5 M e V ; t h e w i d t h a t h a l f - m a x i m u m is 2.6 MeV. T h e cross-section a t t h e m a x i m u m is 1.9 4- 0.3 m b . B e g i n n i n g w i t h E ,-- 16 M e V t h e cross-section r e v e a l s a second rise r e a c h i n g a b o u t 17 m b a t 27 MeV. T h e i n c r e a s e of cross-section was c o n f i r m e d b y a series of c o n t r o l experiments. T h e r a t i o of t h e cross-section l e a d i n g t o t h e g r o u n d a n d isomeric s t a t e s s h o u l d n o t c h a n g e v e r y m u c h w i t h p h o t o n energy. T h e r e f o r e t h e s h a r p rise of t h e cross-section Physica XXII
72
1137
w i t h s u f f i c i e n t l y s m a l l e n e r g y s p r e a d t o resolve t h e s c a t t e r i n g r e s o n a n c e s . O f f - r e s o n a n c e a n g u l a r d i s t r i b u t i o n s h a v e b e e n a n a l y z e d in t e r m s of p o t e n t i a l s c a t t e r i n g p h a s e shifts, w h i c h in s e v e r a l cases s h o w u p i n t e r e s t i n g a n o m a l i e s . D e t a i l e d a n a l y s i s of t h e r e s o n a n c e s c a t t e r i n g allows o n e t o m a k e spin a n d p a r i t y a s s i g n m e n t s . F r o m t h e 18 r e s o n a n c e s a n a l y z e d in t h i s fashion, t y p i c a l causes will be discussed a n d a s u m m a r y of all t h e r e s u l t s will b e p r e s e n t e d . I) W i l l a r d , B a i r and K i n g t o n , Phys. Rev. 98 (1955) 669. 2) F o w l e r , J. L. and J o h n s o n , C. H., Phys. Rev. 98 (1955) 728. 3) C o h n , H. O. and F o w l e r , J. L., Bull. Am. physl Soc. I (1956) 175.
26. A t i m e of f l i g h t m e t h o d for t h e m e a s u r e m e n t of total c r o s s - s e c t i o n s for fast n e u t r o n s . P. HUYER, C. C. JONKER, J. VAN DEN BERG a n d J. RETHMEIER. N a t u u r k u n d i g L a b o r a t o r i u m d e r Vrije U n i v e r s i t e i t , A m s t e r d a m , N e d e r l a n d . A t i m e of f l i g h t m e t h o d for t h e d e t e r m i n a t i o n of t o t a l cross-sections h a s b e e n realized by detecting neutrons twice along their path by stilbene scintillation counters. The t i m e i n t e r v a l b e t w e e n t h e pulses of t h e t w o c o u n t e r s is m e a s u r e d b y a d e l a y e d coincidence c i r c u i t a n d d e t e r m i n e s t h e e n e r g y of t h e n e u t r o n s b e t w e e n t h e c o u n t e r s . I n t h e first s c i n t i l l a t i o n c r y s t a l t h e n e u t r o n s are s c a t t e r e d i n t o t h e e n e r g y region selected b y t h e d e l a y of t h e c o i n c i d e n c e circuit, a n d t h e r e f o r e t h e m e t h o d is i n d e p e n d e n t of t h e e n e r g y s p e c t r u m of t h e a v a i l a b l e n e u t r o n source. W i t h t h e s c a t t e r e r b e t w e e n t h e c o u n t e r s , t o t a l cross-section m e a s u r e m e n t s of s u l p h u r h a v e b e e n p e r f o r m e d in t h e e n e r g y r a n g e 0.5 M e V t o l . l . M e V as a c h e c k o n t h e m e t h o d . T h e a v e r a g e e n e r g y r~sGlution w a s 2 0 % 6btaine~l w i t h a ' t i m e r e s o l u t i o n of 6 t o 9 X 10 -9 sec a n d a d i s t a n c e b e t w e e n t h e c o u n t e r s of I m. A full a c c o u n t will b e p u b l i s h e d s h o r t l y in P h y s i c a .
Ic. S C A T T E R I N G OF P H O T O N S 27. I n e l a s t i c n u c l e a r s c a t t e r i n g of p h o t o n s b y llSIn. O. V. BOGDANKEVICH, L . E . LAZAREVA a n d F. A. ~ICOLAEV. L e b e d e v P h y s i c a l I n s t i t u t e , Moscow, U.S.S.R.
T h e yield of t h e l l S I n (7, 7') lXSIn* r e a c t i o n h a s b e e n m e a s u r e d as a f u n c t i o n of s y n c h r o t o n e n e r g y of 5 M e V t o 27 MeV. T h e a c t i v a t i o n c u r v e for t h e p h o t o - e x c i t a t i o n of 115In* was m e a s u r e d w i t h a s c i n t i l l a t i o n c o u n t e r , w h i c h d e t e c t e d t h e 7-rays e m i t t e d in t h e t r a n s i t i o n f r o m t h e m e t a s t a b l e t o t h e g r o u n d s t a t e ( b y = 335 KeY, ~ = 4.5 h). T h e flux of X - r a y s , - i m p i n g i n g o n t h e s a m p l e , w a s d e t e r m i n e d b y m e a s u r i n g t h e ioniz a t i o n w i t h i n t h e air c a v i t y of a i o n i z a t i o n c h a m b e r w i t h t h i c k a l u m i n i u m walls (7.5 cm.). T h e cross-section c u r v e of t h e 1151n (7, 7') l l S I n * r e a c t i o n was c o m p u t e d b y t h e p h o t o n difference m e t h o d . T h e cross-section h a s a m a x i m u m a t 8.6 4- 0.5 M e V ; t h e w i d t h a t h a l f - m a x i m u m is 2.6 MeV. T h e cross-section a t t h e m a x i m u m is 1.9 4- 0.3 m b . B e g i n n i n g w i t h E ,-- 16 M e V t h e cross-section r e v e a l s a second rise r e a c h i n g a b o u t 17 m b a t 27 MeV. T h e i n c r e a s e of cross-section was c o n f i r m e d b y a series of c o n t r o l experiments. T h e r a t i o of t h e cross-section l e a d i n g t o t h e g r o u n d a n d isomeric s t a t e s s h o u l d n o t c h a n g e v e r y m u c h w i t h p h o t o n energy. T h e r e f o r e t h e s h a r p rise of t h e cross-section Physica XXII
72