Optimized thermal moderators for Compact Accelerator-driven Neutron Sources

Compact Accelerator-driven Neutron Sources (CANS) have the advantage (compared to researchreactors or spallation sources) that the primary neutrons are emitted from a volume well below 1 dm3.The thermal moderator is used to change the energy of the primary neutrons (typically in the MeVrange) down to the 100 meV range, where they are useful for the structural investigation of matter.This moderation process takes place by multiple scattering events with the nuclei of the moderatormaterial(s). To be able to extract neutron beams efficiently from the thermal moderator, it isimportant not to dilute the primary neutron cloud too much, but to keep it confined and dense forthe time of the neutron pulse length desired.The different materials useful for building a moderator-reflector assembly around a target of a CANSdiffer in their absorption probability, scattering power, and energy transfer during a single neutronscattering event. The scattering leads to energy loss and confinement (by a randomized flight directionof the neutron after the scattering), while absorption and diffusion out of the moderator -reflectorregion are the main mechanisms of intensity decay. Some typical materials are e.g. light water orpolyethylene as hydrogen rich materials that lead to fast energy transfer (complete thermalizationwithin 7 μs) and a good confinement of the thermal neutron cloud (about 8 cm FWHM), but thethermal neutrons decay with a time constant below 200 μs due to the nuclear absorption by thehydrogen nuclei. Beryllium or lead show a much weaker scattering probability and a lower energytransfer, which lead to slower moderation, a larger size of the thermal neutron cloud, but a longerlifetime due to an absorption probability that is several orders of magnitude lower.We try to optimize the geometry and the combination of materials in a way to be able to provide theextraction of several neutron beams from a single moderator-reflector assembly with a suitable pulsetime structure either for thermal neutron instruments or for feeding cold neutron sources insertedinto the thermal moderator assembly.