Toward the Design of a Very Cold Neutron Source for the High Brilliance Source

In contrast to the moderator designs typical of large reactor and spallation sources, high-current, accelerator driven neutron sources, with smaller source dimensions, necessitate highly efficient and compact moderator solutions. At the High Brilliance Source (HBS), a hydrogen-rich moderator is required to effectively slow neutrons to the very cold neutron (VCN) energy range within the limited volume that aligns with the HBS target size. Methane, a well-established and highly efficient neutron moderator is a promising candidate to serve as a VCN moderator since it possesses a desirable low-lying rotor mode at ∽ 1 meV to facilitate neutron slowing. Liquid parahydrogen (pH2) is a known efficient cold neutron moderator since it is able to convert thermal neutrons to cold neutrons via a single interaction. A geometrical configuration combining methane, embedded in pH2 has been investigated to harness the complementary properties of both materials as a potential VCN moderator design for the HBS. Monte Carlo simulations using the PHITS particle transport code were conducted to evaluate the performance of the combined moderator concept when compared with a pure, low dimensional pH2 cold source.