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| Home > Publications database > A compact solid methane moderator system for the Jülich High Brilliance Neutron Source (HBS) prototype |
| Conference Presentation (Invited) | FZJ-2020-02364 |
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2020
Abstract: During the last year two cryogenic moderators for the Jülich High Brilliance Neutron Source (HBS) project were tested at the COSY facility at the Jülich Research Centre. Time-of-flight spectra were measured for solid mesitylene at different temperatures (24 K to 300 K) and liquid hydrogen for different ortho-para-ratios (25% to approx. 100% p-H2). An attempt to increase the cold neutron brilliance exists in lowering the temperature of the moderating material even below 20 K, a commonly used minimum temperature for solid cryogenic moderator materials. At temperatures below 20 K and corresponding energies, energy transfer mostly takes place by excitations of vibrational and rotational movements of the lattice molecules. Therefore, an effective cryogenic moderator has to allow sufficient low-energy modes. Solid methane shows a phase change below approximately 21 K from phase I to phase II, which leads to a change of free molecular rotations to three-fourths of hindered rotations. Due to energy transfer by inducing librational movements of the hindered molecules even down to low temperatures, methane in phase II is one of the most effective candidates for increasing the cold neutron brilliance. Simulations on different methane thicknesses and geometries were performed to find an effective way of dimensioning the phase II methane moderator to maximize cold neutron brightness while keeping the moderator vessel as compact as possible. Using a reentrant hole for extracting cold neutrons from the center of the moderator volume, as suggested in literature, didn’t prove profitable. A preliminary design for a new cryogenic system for using solid methane at an HBS prototype is currently being carried out. Besides different measures to keep the heat load onto the cold moderator as low as possible, one also has to examine possible dangers in using solid methane being irradiated for extended periods of time while at the same time keeping those periods of time as long as possible.
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