Micro-Hole and Strip Plate (MHSP) operation in CF4

The Micro-Hole and Strip Plate (MHSP) is a hybrid electron multiplier which combines the working principles of a Gas Electron Multiplier (GEM) and a Micro-Strip Gas Counter (MSGC). The compact double stage electron multiplication processes found in the MHSP enables the realisation of higher gas gain than the lone GEM operation. Thermal neutron detection using gas detectors involves the use of 3He gas with another suitable stopping gas, operated at elevated pressure to confine the products of the neutron-3He reaction. It is, however, well known that the gain of GEMs drops too sharply with increasing chamber pressure. We have pursued experimental work using the MHSP to infer the upper limits of thermal neutron position resolution. The stopping gas used during the present studies was Tetrafluoromethane (CF4) chosen due to its low X-ray and γ-ray efficiency, requiring only 2.6 bar to yield a position resolution of 1 mm. In the present studies, systematic measurements were taken to establish the gain versus CF4 pressure characteristics of the MHSP, in the 1 to 2.65 bar range. These studies demonstrated that it was possible to sustain gains above 104 and 300 with pressures of 1 and 2.65 bar, respectively. The advantage of having two multiplication stages in the MHSP separated by only few tens of microns, results in a less pronounced gain reduction with pressure, presenting higher gains than single or multiple GEMs. The energy resolution at 1 bar was 29% FWHM for the 5.9 keV Mn Kα line from a 55Fe source, which is a typical value for micro-patterned gas detectors operating in CF4