Neutron Moderation Theory Taking into Accout the Thermal Motion of   Moderating Medium Nuclei
  In this paper we present the analytical expression for the neutron scattering law for an isotropic source of neutrons, obtained within the framework of the gas model with the temperature of the moderating medium as a parameter. The obtained scattering law is based on the solution of the kinematic problem of elastic scattering of neutrons on nuclei in the L-system in the general case. I.e. both the neutron and the nucleus possess the arbitrary velocity vectors in the L-system. For the new scattering law the flux densities and neutron moderation spectra depending on the temperature are obtained for the reactor fissile medium. The expressions for the moderating neutrons spectra allow reinterpreting the physical nature of the underlying processes in the thermal region. 
V.D. Rusov, V.A. Tarasov, S.A. Chernezhenko, A.A. Kakaev, V.P. Smolyar
nucl-th