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E i s given by: . f -1 - -1) 1- p d ( Q E log E / E ~ ' (68) and t h e average energy of t h e r e c o i l [email protected] of t h i s atom is: I Using Eqs. ei) I . e average; e n e r p of-; (h) tpe average ran t h e ith generation f r e c o i l atoms. (1)Passage of' Fission Fragments Through Matter, P a r t ' . I I , C ' 1) -- " . -. 397 439 fi 1 Ir secondary 800 Fe tertiary 6400 2. 12x103 I. 4 ~ 1 0 - ~ 2 . 85 2 . 6~lO-~ 1 2600 6. 84~103 A useful rule of thumb f o r primary r e c o i l s i s that f o r A l a r g e r t h a n 50, a neutron makes about A c o l l i s i o n s i n l o s i n g 99% of i t s energy and t h a t t h e average energy of t h e r e c o i l s i s about 1x i n i t i a l neutron A energy.

Since, a s we havg seen, a l l the energy i n t h i s part of t h e path goes i n t o d i s p l a c e g e n t s , and s i n c e fw$her as we w i l l show l a t e r , most of t h e energy of t h e p r i m a 7 r e c o i l s goes i n t o secondary and t e r t i a r y displacements, t h i s t procedure w i l l n o t a f f e c t t h e f i n a l result. The average energy of t h e primary r e c o i l s i n t h i s region w i l l now be found. It i s well known t h a t t h e percent loss of energy p e r c o l l i s i o p of a neutron with an atom remains c o n s t a n t , on t h e average, throughout t h e p a t h of t h e neutron.

The number of c o l l i s i o n s i n a t h i c k n e s s dx is: = n No 6 d x 91 where rn i s t h e neutron e l a s t i c c r o s s s e c t i o n and is about 5 barxis f o r Fe. , , in of primary displacements and i f I t h e average energy of t h e displacements the foregoi - . -. ...... I i l r s . . . . -.. - ... - _- - -- then give the totpl number of displacements per primary recoil. Although the primary recoils all move into the forward hemisphere, the secondaries , and so f o r t h w i l l not and we may assume that the flux o i atoms will be roqhly isotropic.

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Atomic Displacements Prod by Fission Fragments, Neutrons in Matter [declassified]


by Thomas
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