A method for evaluation of the master equation is developed, based on a simple approximation for the single scattering probability due to thermal displacements of lattice atoms. The results obtained are compared with computer simulations based on the binary collision model. The master equation including single scattering reproduces very well the results of computer simulations for a not too small depth. It is shown that with an appropriate choice of maximum scattering angle the diffusion approximation may be applied for perfect crystals.