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Photon detection and localization with GEM

We describe the performance of a photon detector based on the gas electron multiplier (GEM), with a CsI photocathode deposited on the first electrode of a cascade of three or four. The excellent position and multi-hit resolution achieved makes it suitable for RICH applications. A special readout board, providing three projections for each coordinate, permits to resolve ambiguities for multi-hit events. A similar device has been used to detect the gas scintillation in xenon; we observed both primary and field-enhanced (secondary) scintillation with soft X-ray sources. This offers a possible way to eliminate the parallax error in detectors with thick conversion volumes.