Abstract: DiffMod is a simulation program for the evolution of a neutron ensemble in a thermal target – moderator – reflector assembly of a pulsed neutron source based on the statistical description of diffusion, scattering, moderation, and absorption processes. The spatial resolution, the energy resolution and the diffusion directions are strongly restricted to achieve calculation times in a realistic moderator – reflector assembly below 1 hour. In comparison with Monte-Carlo simulations describing the geometry and interactions between neutrons and moderator material exactly, we prove that the DiffMod approach can deliver intensities and pulse shapes that are exact within 10% compared to the Monte-Carlo simulations that require much more computing power. In addition, a time-resolved illustration of the spatial distribution of the neutrons at different energy levels is provided.
