New thermal treatment for MONK

The treatment of thermal neutron scattering is an important part of a Monte Carlo programme. This is particularly so for criticality calculations where the keff can be signally effected by the shape and effective temperature of the calculated thermal neutron flux. MONK has at present a special treatment for hydrogen bound in water and uses a free gas model for all other nuclides. This paper describes a new thermal treatment for MONK which uses the neutron scattering laws in the S(α, β) form from any libraries in the ENDFB6 format. The new S(α, β) data is preprocessed into marginal probability distributions which are fitted to a given accuracy by piecewise exponential functions. For hydrogen in water a maximum error in these distributions of 3% can be achieved with under 20K words of processed data. This new work is being funded in part by BNFL as part of the NCD Collaboration. (Author)