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| Conference Presentation (Invited) | FZJ-2020-03116 |
2020
Abstract: The Jülich High Brilliance Neutron Source (J-HBS) will provide neutrons to differentinstruments by utilizing optimized Target-Moderator-Reflector (TMR) assemblies inthe form of multiple target stations with different pulse structures. These TMR unitsproduce neutrons of the required wavelengths by using different combinations ofmoderators and reflectors. For instruments that require long wavelength neutrons,cryogenic materials are used to slow thermal neutrons down to the cold energyrange. These so-called “cold moderators” are optimized with regard to neutronbrilliance by choosing the appropriate material and dimensions.Two commonly used cold moderators, liquid hydrogen and solid mesitylene, weretested in optimized compact geometries at the Big Karl facility of the synchrotronCOSY at the Forschungszentrum Jülich by conducting neutron time-of-flight (TOF)measurements. The effects of temperature (mesitylene) and ortho-para ratio(hydrogen) on the moderators’ performance were investigated in systematicexperimental parameter studies. The efficiencies of the used cold moderators werecompared and neutron transport simulation models have been validated.With the aim to shift the neutron energy spectrum to even lower values, a coldmoderator system is currently being developed, which allows the use of solidifiedmoderators at operating temperatures below 10 K. For this system, methane will beused in phase II, as it is one of the most promising candidates at such lowtemperatures due to its low-lying energy levels, which allow the transfer of smallamounts of energy. However, the use of solid methane is challenging from anengineering perspective due to its low thermal conductivity and poor radiationresistance.In this presentation, the previously used cryogenic systems will be described and theresults of the measurements at Big Karl will be presented. Furthermore, preliminarydesign aspects of the planned “10 K” cryostat will be discussed.
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