Continuum Scattering and Fission 

In ENDF/B, scattering from many closelyspaced levels or
multibody scattering such as (n,2n), (n,n'α),
and fission can be represented using a separable function of scattering
cosine and secondary neutron energy
where F is the probability that a neutron will scatter through a laboratory angle with cosine μ irrespective of final energy E'. It is obtained from MF=4. Similarly, g is the probability that a neutron's energy will change from E to E' irrespective of the scattering angle, and it is given in MF=5. Continuum reactions are easily identified by MT numbers of 649 and 91. The secondaryenergy distributions can be represented using a twodimensional tabulation or using an analytic law. The feed functions for continuum scattering are simply and the cosine omega is related to secondary energy E' by As discussed in the companion article in this volume "An Introduction to the ENDF Formats," the separability assumption is no longer considered adequate for reaction like (n,2n) or (n,n'α), because it doesn't describe the energyangle correlation that occurs for these reactions. As a result, the continuum approach is only found for old carryover materials and for minor actinides or fission products where neutron transport is a secondary concern. In these cases, the angular distribution is almost invariably given as isotropic in the laboratory system. However, the continuum approach remains the method of choice for fission, which is also modeled as isotropic in the laboratory frame. Therefore, in almost all cases of interest, the feed function reduces to
The integrals on the right hand side are returned by the GROUPR subroutine

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23 January 2013  T2 Nuclear Information Service  ryxm@lanl.gov 