Operation principles and properties of the multi-GEM gaseous photomultiplier with reflective photocathode D.M¨ormann * , A. Breskin, R. Chechik, C. Shalem Department of Particle Physics, The Weizmann Institute of Science, 76100 Rehovot, Israel Abstract Gas Electron Multipliers with a reflective photocathode deposited on the surface of the first multiplying element are very attractive devices for photon detection and imaging over large area at moderate cost. They combine production and operation simplicity, high sensitivity to single photons, fast time response and accurate local- ization. In this work we present in detail the mechanisms governing the operation of these photon detectors. The results of electron extraction, transfer, multiplication and detection processes in this multi-element structure are presented and analyzed. We discuss the role of important elements and parameters influencing the detector’s operation and performance: the GEM geometry, the choice of the different electric fields and the gas mixture. Key words: GEM, gaseous photon detectors, CsI photocathode, gaseous multipliers PACS: 29.40.Cs, 29.40.Gx, 85.60.Gz 1 Introduction Gas Avalanche Photomultipliers (GPMTs) based on cascaded Gas Electron Multipliers [1] (GEM) coupled to semi-transparent photocathodes have been extensively studied for UV-photon imaging. Due to minimal photon-feedback and limited ion-feedback effects in these multiplying structures [2–5], they operate reliably, with single-photon sensitivity, at gains exceeding 10 6 , in at- mospheric pressure gases, including noble gas mixtures [2] and CF 4 [6]. These * Corresponding author. Email: dirk.moermann@weizmann.ac.il Preprint submitted to Elsevier Science 18 March 2004