The production and moderation of neutrons at an accelerator-based neutron source is done at a sophisticated Target-Moderator-Reflector (TMR) unit. There, neutrons are produced by nuclear reactions of protons within a suitable target material e.g. Be, Ta, W at energies below the spallation threshold creating neutrons with relativity low energy. For scientific applications these neutrons are then further reduced in energy by moderator materials to energies at around 25 meV or below. A reflector increases the neutron flux within the moderator system but also affects the neutron pulse structure. The extractable neutron brilliance depends on the material selection, the geometry of the TMR unit, the arrangement of the extraction channels as well as the proton pulse structure.

In the development of high-current accelerator based pulsed neutron sources (HiCANS) for neutron scattering, analytics and imaging different proton pulse frequencies and pulse length are available to satisfy best different sets of instruments. At such a neutron source a TMR unit can be operated with a long proton pulse producing a long neutron pulse or an intermediate proton pulse producing a short neutron pulse and therefore needs to be optimized for the appropriate time-averaged brilliance.

In the proposed project the optimization of a TMR unit with Monte Carlo methods (MCNP, FLUKA, PHITS) will be done to find suitable material, geometries and extraction channel arrangements for different combinations of instruments operated at a corresponding neutron pulse. It will take as reference the conceptual design of the High Brilliance neutron Source (HBS) to be improved. The work proposed will include:

• Monte Carlo simulations of different moderator – reflector material combinations as well as varying the dimensions of the moderator and the reflector for a predefined extraction channel arrangement. The influence of the different combinations on the neutron pulse structure and the peak as well as the time-averaged brilliance shall be investigated.
• Varying the extraction channel arrangement with and without the operation of a cold source for a fixed moderator – reflector setup. The influence of the extraction channel on each other for the neutron pulse shall be investigated.

Developing an optimized TMR setup for different suits of instruments for various proton pulse frequencies. The aim is to develop a realistic performance of the TMR unit.