4.7 Monitoring the Moderator on GEM

GEM obtains its neutrons by viewing a moderator which contains liquid methane, CH₄. The methane should be maintained at a constant temperature of 109K. However, the moderator may not always perform ideally, particularly at the end of its life, and it can be useful to be able to monitor its performance. The moderator does not last indefinitely because radiation damage (ionisation of CH₄ molecules) results in the build-up of carbonaceous residue which gradually impedes the flow of the methane circulation. If the moderator temperature fluctuates then the flux shape changes and the measured parameters for a sample, especially the thermal parameters, are affected. If the methane flow is seriously impeded then the quantity of methane may be reduced, resulting in an overall loss of intensity.

Methane Moderator Temperature

If you are logged on to a VMS computer, then typing PPPMON will produce a window which shows the current methane moderator temperature, and some other parameters.

The log of the methane moderator temperature for a particular day can be plotted, either in Genie v2 by typing @g_f:tch4 , or in OpenGenie on a VMS computer by typing either tch4 or w1=tch4() . Shown here is a typical temperature log when the methane moderator is performing well.

Neutron/Proton Ratio

This may be plotted in Genie v2 by typing npcur for today's data or npold for data from a previous day.

Last updated on 28 Jul 2002 by Alex Hannon (a.c.hannon@rl.ac.uk)