Moderator-Reflector Assembly for HBS

In a compact accelerator driven neutron source, the design of the moderator/reflector system is crucial to convert the fast neutrons produced at the compact target into a thermal or cold neutron cloud that can be extracted with a high brilliance. A quick moderation together with a small diffusion length in the thermal moderator is suitable to confine the thermal neutrons in a small space, yielding a high phase space density for extraction into a small space, e.g. to feed a neutron guide or a 1-dimensional cold neutron source. On the other hand, a strong confinement of the neutron cloud inside the thermal moderator has an influence to the neutron pulse length, which is important for the resolution of time-of-flight instruments. Increased absorption or leakage may result into shorter pulses, while longer pulses intended to accumulate more thermal neutrons in a larger volume can be achieved by diluting the hydrogen rich thermal moderator material. Therefore, the moderator/reflector system has to be adapted to the desired pulse length and will be different at the different target stations of the HBS facility to serve the proper resolution for different instrument types