  We report on the properties of thick GEM-like (THGEM) electron multipliers
made of 0.4 mm thick double-sided Cu-clad G-10 plates, perforated with a dense
hexagonal array of 0.3 mm diameter drilled holes. Photon detectors comprising
THGEMs coupled to semi-transparent CsI photocathodes or reflective ones
deposited on the THGEM surface were studied with Ar/CO2 (70:30), Ar/CH4 (95:5),
CH4 and CF4. Gains of ~100000 or exceeding 1000000 were reached with single- or
double-THGEM, respectively; the signals have 5-10 ns rise times. The electric
field configurations at the THGEM electrodes result in an efficient extraction
of photoelectrons and their focusing into the holes; this occurs already at
rather low gains, below 100. These detectors, with single-photon sensitivity
and with expected sub-millimeter localization, can operate at MHz/mm2 rates. We
discuss their prospects for large-area UV-photon imaging for RICH.
