A standard closed cycle refrigerator (CCR) may be used on LAD. This is to be preferred to an orange cryostat whenever possible because it gives a much lower neutron background and is more reliable. The minimum temperature of the CCR is normally in the range 15-20K with the exact value depending on parameters such as the thermal load due to the sample. In order to achieve this minimum temperature it is essential to use a heat shield. The heat shield may be identified by its ‘top-hat’ appearance and one with vanadium windows should be used. The CCR is bottom-loading which means that it must be removed from the sample tank in order to change sample.

The CCR operates by forcing helium gas through a nozzle which gives a cooling effect. A compressor is situated at ground level underneath the LAD cabin for driving the helium gas and this must be switched on when cooling is required. It is possible for the compressor to crash, in which case it shows an orange ‘failure’ light. Before the compressor can be re-started in is necessary to reset it by pressing a micro-switch which is accessed via a hole on the left side of the front panel of the compressor (note that there are 2 micro-switches in this hole and one must find the correct switch by experimentation). One possible cause of a compressor crash is that not all necessary pieces of equipment have been connected, or that the compressor cooling water tap is turned off. Another possible cause of a crash is that the helium pressure within the compressor is inadequate. The helium pressure is indicated by a gauge on the front of the compressor and it should be 22 bar when the compressor is running or 22 bar when it is not running. If the helium pressure has fallen significantly below these values then the compressor must be re-charged. A helium gas cylinder mounted on wheels should be available somewhere in the area and instructions for its use are to be found on the top of the compressor.

With the compressor running at full cooling power the CCR heater should be able to take the sample temperature up to a maximum temperature in the region of 100-150K. For temperatures above this it is necessary to use a bypass valve fitted across the helium lines at the rear of the compressor. The purpose of the bypass valve is to reduce the helium pressure difference and hence to reduce the cooling power. A bypass valve setting in the region 80-120 is probably reasonable. Mains electricity power must be supplied to the bypass valve for it to function.

Last Updated 09 Nov 1998