The article deals with the detection efficiency of UV-photon detectors consisting of Thick Gas Electron Multipliers (THGEM) coated with CsI photocathode, operated in atmospheric Ne/CH4 and Ne/CF4 mixtures. We report on the photoelectron extraction efficiency from the photocathode into these gas mixtures, and on the photoelectron collection efficiency into the THGEM holes. Full collection efficiency was reached in all gases investigated, in some cases at relatively low multiplication. High total detector gains for UV photons, in excess of 105, were reached at relatively low operation voltages with a single THGEM element. We discuss the photon detection efficiency in the context of possible application to RICH.

The detection of primary scintillation light in combination with the charge or secondary scintillation signals is an efficient technique to determine the events "t=0" as well as particle / photon separation in large mass TPC detectors filled with noble gases and/or condensed noble gases. The aim of this work is to demonstrate that costly photo-multipliers could be replaced by cheap novel photosensitive gaseous detectors: wire counters, GEM's or glass capillary tubes coupled with CsI photocathodes. We have performed systematic measurements with Ar, Kr and Xe gas at pressures in the range of 1-50 atm as well as some preliminary measurements with liquid Xe and liquid Ar. With the gaseous detectors we succeeded in detecting scintillation light produced by 22 keV X-rays with an efficiency of close to 100%. We also detected the scintillation light produced by β's (5 keV deposit energy) with an efficiency close to 25%. Successful detection of scintillation from 22 keV gamma's open new experimental possibilities not only for nTOF and ICARUS experiments, but also in others, like WIMP's search through nuclear recoil emission.

The operation of single-, double-and triple-THGEM UV-detectors with reflective CsI photocathodes (CsI-THGEM) in Ne/CH 4 and Ne/CF 4 mixtures was investigated in view of their potential applications in RICH. The studies were carried out with UV, x-rays and βelectrons and focused on the maximum achievable gain, discharge probability, cathode excitation effects and long-term gain stability. Comparative studies under similar conditions were made in CH 4 , CF 4 and Ne/CF 4 , with a MWPC coupled to a reflective CsI photocathode (CsI-MWPC). It was found that at counting rates ≤ 10 Hz/mm 2 the maximum achievable gain of CsI-THGEMs is determined by the Raether limit; at counting rates > 10 Hz/mm 2 it dropped with rate. In all cases investigated the attainable CsI-THGEM gain was significantly higher than that of the CsI-MWPC, under similar conditions. Furthermore, the CsI-THGEM UV-detector suffered fewer cathode-excitation induced effects as compared to CsI-MWPC and had better stability at high counting rates. KEYWORDS: Micropattern gaseous detectors; Photon detectors for UV, visible and IR photons;Avalanche-induced secondary effects; Cherenkov detectors. 1. High gains (> 10 5 ) are reachable with single-or cascaded-THGEM electrodes in selected gases; mixtures of choice could be Ne with a small addition of quenchers (CH 4 , CF 4 ) [3,4]. Due to the exponential nature of single-photoelectron pulse-height distributions, and taking into account signal-over-threshold considerations, a high detector gain is an important factor in improving single-photon detection efficiency. 2. A THGEM can operate in poorly quenched gas mixtures as well as in gases emitting UV light (e.g. noble gases [5], CF 4 [6,4]). This permits conceiving windowless detectors (same detector and radiator gas, e.g. like in [7]), with simpler layout and larger Cherenkov-photon detection yields. 3. In intense-background environment, THGEMs can operate in the so-called "Hadron-Blind mode" with zero or reversed electric field above the photocathode [8]; this significantly reduces particle-induced ionization signals [7]. CsI-THGEMs are currently considered for the upgrade of the RICH systems at the 10]. The reasons to replace the CsI-MWPCs [11] in these experiments are numerous: The CsI-MWPCs operate at relatively low gas gains (~10 4 ), limited by avalanche photon-feedback. Furthermore, there is evidence that at high counting rates the MWPCs suffer from feedback-related discharges, followed by difficulties to restore the operating voltage, up to 1-day periods [12]. This could have resulted from a cathode excitation process [13], in which the surface is modified under discharge-induced ion bombardment, with