Gain of a gas photomultiplier with CsI photocathode

A Gas PMT with the CsI photocathode was fabricated as a phototube of a glass vessel identical to a conventional vacuum PMT. A GEM and a Micromegas were installed in the phototube of the Gas PMT. The Gas PMT was operated with two different gas mixtures, Ar (90%) + CH4 (10%) and Ne (90%) + CF4 (10%), at the gas pressure of 1 atm. The gain of 105 was obtained for both the gas mixtures. It was found that the gain of the Gas PMT can be described using the Townsend ionization coefficient for the Micromegas. The imaging capability of the Ne gas mixture was checked by an optical imaging capillary (CP) gas detector, using X-ray beams. The image of an X-ray resolution chart indicates that the imaging resolution is determined by the diameter of the capillary, here 100 μm. The Ne gas mixture appropriate to the Gas PMT is well suited for a fine imaging detector.