High pressure GEM operation aiming at thermal neutron detection

Experiments at the anticipated intense spallation neutron sources require neutron detection with typically millimeter position resolution. We are investigating the possibility to apply the Gas Electron Multiplier (GEM) for this. For efficient detection ³He gas at ~10 bar is required with in addition some stopping gas at a differential pressure of 3-5 bar, i.e. high enough to reduce the range of the neutron capture reaction products far enough to achieve the required resolution. Due to the large initial charge deposition in the gas, only a small gain (10-100) is necessary. However, at the high pressures involved, it is not clear if even this can be achieved. We present first results of experiments using a range of gases, pressures and in this case GEM hole sizes of 60 and 80 μm to tackle this problem. In an Ar/DME mixture we observed the highest gain, but this mixture does not meet the resolution criterion. The use of Xe (an effective stopping gas) in a Xe/DME mixture is ruled out as no signal can be detected at pressures above 2 bar