A new integrated photosensor for gas proportional scintillation counters based on the gas electron multiplier (GEM)

Abstract

The performance of a novel integrated photosensor for use in a xenon gas proportional scintillation detector is described. Earlier integrated photo-sensor designs were limited in charge gains due to the onset of electrical breakdown, which was ascribed to optical positive feedback from scintillation photons produced in the charge amplification stage. The present design uses a gas electron multiplier (GEM) composed of a 50 μm thick Kapton film with copper-plated electrode surfaces on both sides and perforated with 200 μm holes at a 300 μm pitch. The front surface is made photosensitive with a 150-nm-thick CsI film. When an appropriate voltage is applied between the copper electrodes, the resulting electric field directs photoelectrons produced on the front surface through the holes in the GEM and onto a wire chamber where charge amplification occurs. Optical positive feedback is essentially eliminated since the charge amplification stage is optically de-coupled from the photocathode. The GEM also provides a small amount of charge gain, up to 3.3, before the electrons enter the wire chamber where charge gains up to about 10 3 take place. However, the measured effective quantum efficiency, namely, the number of photoelectrons traversing the GEM holes per incident 170 nm scintillation photon, as measured under present conditions, is only about 1%. A discussion of the results is presented.