Integral Neutron Thermalization [annual summ rpt Oct 68 -

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G . , f o r a l u m i n u m ( 3 4 ) and l e a d . (35) 31 0 t 32 *0 - 64 0 - 0 0 Q 9 0 cg 8 P 8 cu 0 0 O0 A ' Shifts of the m a g n i t u d e i l l u s t r a t e d in F i g . 5. 1 could no doubt be d e t e c t e d by p r e c i s e d o u b l e - d i f f e r e n t i a l m e a s u r e m e n t s a t elevated t e m p e r a t u r e s . How- e v e r , i t is not a p r i o r i c l e a r how i m p o r t a n t s u c h s h i f t s a r e likely to be in t h e r m a l i z a t i o n calculations. requency s p e c t r u m f o r H, p (w), a s given by the H h a r m o n i c c e n t r a l f o r c e m o d e l .

0 2 5 eV T h e h i g h e r f r e q i i e n c i e s , which give r i s e t o t h e high Debye t e m p e r a t u r e s d e t e r m i n e d by s p e c i f i c h e a t m e a s u r e m e n t s , a r e a s s o c i a t e d with v i b r a - t i o n s of t h e l i g h t e r a t o m and a r e not r e l e v a n t to broadening of the heavy a t o m absorption lines. At low t e m p e r a t u r e s t h e c h e m i c a l binding of the h e a v y a t o m h a s a s i g n i f i c a n t effect o n the lineshape. An effective t e m p e r a t u r e m o d e l gives m u c h b e t t e r r e s u l t s than a p u r e f r e e g a s model, but is s t i l l significantly > 55 different f r o m exact r e s u l t s .

6 . 3 NUMERICAL RESULTS We h a v e c a l c u l a t e d the l i n e s h a p e f o r the 6. 65 eV r e s o n a n c e f o r using the code LAMB, a modified v e r s i o n of GASKET ( 1 ) , 2 t o e v a l u a t e the i n t e g r a l i n Eq. 6. 23. As a c h e c k o n t h e c o r r e c t n e s s of the 238 U i n UO modification and t h e a d e q u a c y of the i n t e g r a t i o n m e s h e s , s o m e s e p a r a t e c a l c u l a t i o n s w e r e done with a code w r i t t e n to evaluate the convolution i n t e g r a l , Eq.