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| Home > Publications database > Harnessing the Complementary Properties of Methane and Parahydrogen for the Design of an Intense Very Cold Neutron Source for the High Brilliance Source |
| Poster (Invited) | FZJ-2025-02977 |
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2025
Abstract: Very cold neutron (VCN) sources present an exciting opportunity for scientists to access unprecedented length and time scales, and achieve improved resolution in neutron experiments. VCNs are defined over a wide spectral range, from 1 meV (9 Å) down to a few hundred neV (> several 100 Å). Recent advancements in the development of thermal scattering kernels for candidate very cold neutron (VCN) moderator materials have opened opportunities for exploring conceptual designs of VCN sources tailored to emerging high-intensity compact accelerator-driven neutron sources (HiCANS) like the High Brilliance Neutron Source (HBS). In contrast to the expansive moderator designs typical of large reactor and spallation sources, HiCANS, with a smaller source, necessitate compact moderator solutions. At the HBS, a hydrogen-rich moderator is required to effectively slow neutrons to the VCN energy range within the limited volume that aligns with the target footprint of the HBS. Methane is a promising candidate to serve as a VCN moderator since it possesses a desirable low-lying rotor mode at ~ 1 meV, which serves as a pathway for neutron moderation to VCN energies. However, thin geometries are required to limit neutron absorption. Owing to the characteristic dip in the total neutron scattering cross section of liquid parahydrogen (l-pH₂), below the thermal energy scale, thermal neutrons are efficiently converted to cold neutrons, and cold neutrons are subsequently easily transmitted in l-pH₂. Informed also by earlier studies, a moderator concept, harnessing the properties of both methane l-pH₂ has been conceived, wherein a thin, large-surface area plate of methane is boosted by a parahydrogen backing. A geometrical optimisation of the moderator concept was conducted in PHITS and the results shall be presented and compared against the low dimensional l-pH₂. Perspectives are also provided for instruments which may be optimally fed with this moderator at HiCANS facilities.
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