Speaker
Description
In the ISIS Neutron and Muon Source Target Station 2, a decoupled solid methane moderator is used to convert fast neutrons coming from the target into cold, lower energy neutrons, due to its high efficiency compared to liquid hydrogen moderators. The operational temperature of the solid methane moderator is kept to 47K [1] using a helium cooling loop inside the methane vessel. However, it has been proven challenging to sustain this temperature, and after two user-cycles (55-65 mAh) the temperature degrades to 60K, due to a complicated combination of effects. At the same time, there has recently been an undertaking to introduce a gadolinium poisoning foil to improve time of flight resolution for the WISH instrument, which could be beneficial for future instruments. In addition, it was hoped that the cooling could be improved using a new type of heat exchanger. Colder methane temperatures (approximately 30 to 35 K) would produce more neutrons at longer wavelengths, typically 6 – 10 Å, for the WISH diffractometer instrument.
This paper will report on the performance of the new designs during commissioning from June 2025 to April 2026. The first iteration of the new heat exchanger design (Mk2) was not as effective as the original design proposal, partly through a discrepancy in the calculations but also due to further complex phenomena. While Mk2 started at a much higher temperature than the previous design, it did not deteriorate as quickly and lasted two user-cycles. Subsequently, an alternative design (Mk3) was commissioned in February 2026. Both new heat exchangers were designed to have a higher flow rate of helium to aid their ability to cool the moderator. However, on increasing the inlet pressure during the first round of commissioning, no significant effect was observed. This has led to the determination of the real flow rates in previous moderators and aided in the effort to understand other mechanisms within the moderator. Furthermore, more knowledge has been gained in both the cryogenic system operation and moderator neutronic performance. Importantly, the commissioning of the new designs has confirmed the design choices for the WISH II Instrument, part of the Endeavour project, as well as opening the possibility of redesign with different requirements.
Author(s): Mark Telkman, Paul Morgan
References
[1] O. Kirichek, C. Lawson, G. Draper, D. Jenkins, D. Haynes and S. Lilley, "Solid Methane Moderator: thermodynamics and chemistry," Journal of Neutron Research, vol. 22, no. 2-3, pp. 281-286, 2020.