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

The performance of a novel integrated photosensor for use in a xenon gas pr oportional scintillation detector is described. Earlier integrated photo-se nsor designs were limited in charge gains due to the onset of electrical br eakdown, which was ascribed to optical positive feedback from scintillation photons produced in the charge amplification stage. The present design use s a gas electron multiplier (GEM) composed of a 50 mu m thick Kapton film w ith copper-plated electrode surfaces on both sides and perforated with 200 mu m holes at a 300 mu m pitch. The front surface is made photosensitive wi th a 150-nm-thick CsI him. When an appropriate voltage is applied between t he copper electrodes, the resulting electric field directs photoelectrons p roduced on the front surface through the holes in the GEM and onto a wire c hamber where charge amplification occurs. Optical positive feedback is esse ntially eliminated since the charge amplification stage is optically de-cou pled 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 g ains up to about 10(3) take place. However, the measured effective quantum efficiency, namely, the number of photoelectrons traversing the GEM holes p er incident 170 nm scintillation photon, as measured under present conditio ns, is only about 1%. A discussion of the results is presented. (C) 1999 El sevier Science B.V. All rights reserved.