Abstract
A new formalism based on the direct operational method** has been developed for numeri-cally calculating the fast neutron spectrum in an infinite homogeneous medium. By this method the fine structures of nuclear data such as the anisotropy of scattering cross sections in the high energy region can be easily taken into account. Application of this method to light water sys-tem has yielded the conclusion that: (1) the effect of anisotropic scattering in the center-of-mass system becomes significant near the source energy and near scattering resonance peaks, (2) the errors introduced by the G.G.-approximation are also significant under similar circumstances and (3) the contribution from inelastic scattering is negligible (>0.2 %) in the case of fission neu-tron source in light water system.
