Investigations with Gaseous Electron Multipliers for use on the ISIS spallation neutron source

Abstract

Several investigations have been undertaken to ascertain the suitability of gaseous electron multipliers (GEMs) for use as a neutron detector on the ISIS spallation neutron source. Our initial investigations focused purely on whether these devices could be operated at the elevated pressure of 3 He and CF 4 necessary for 1mm position resolution (2.6 bars of CF 4 ). In fact we were able to operate the GEMs at suitable gains with 3.5 bars of CF 4 . However encouraging these results were, we found that the GEMs charged up over time, which we postulated was due to the kapton substrate. A similar problem was seen at the early stages of the development of the microstrip gas chamber (MSGC), a solution of which was to use the semiconducting glass Schott S8900 as the substrate. Therefore we focused our attention to the manufacture of a GEM structure on an S8900 substrate. Our first devices were manufactured from 1mm thick glass and exhibit gains in excess of 1×10 4 for a single GEM stage in an argon isobutane gas mixture, when illuminated with 55 Fe xrays. A small amount of charging under irradiation has been observed in a flowing gas mixture, but the GEMs quickly stabilise and track atmospheric conditions. Further measurements in a 3 He:CF 4 atmosphere will show how suited these devices are to the needs of ISIS.