B.K. Singh

Sealed GEM photomultiplier with a CsI photocathode: ion feedback and ageing

Abstract We present the performance of a sealed gaseous photomultiplier consisting of a cascade of 3 or 4 Gas Electron Multiplier (GEM) elements coupled to a semitransparent CsI photocathode, in Ar/CH 4 (95/5). A few-month stability study of the photocathode in a sealed mode is presented. Increasing the number of GEMs in cascade substantially reduces the ageing of the detector under strong irradiation. The ion feedback to the photocathode has probably a minor effect on the ageing rate.

Advances in gas avalanche photomultipliers

Abstract Gas avalanche detectors, combining solid photocathodes with fast electron multipliers, provide an attractive solution for photon localization over very large sensitive areas and under high illumination flux. They offer single-photon sensitivity and the possibility of operation under very intense magnetic fields. We discuss the principal factors governing the operation of gas avalanche photomultipliers. We summarize the recent progress made in alkali-halide and CVD-diamond UV-photocathodes, capable of operation under gas multiplication, and novel thin-film protected alkali-antimonide photocathodes, providing, for the first time, the possibility of operating gas photomultipliers in the visible range. Electron multipliers, adequate for these photon detectors, are proposed and some applications are briefly discussed.

A gas avalanche photomultiplier with a CsI-coated GEM

We describe the operation principle and properties of a CsI-coated GEM photodetector. This type of detector performs photon detection with reflective photocathodes, which are easy to produce and have high quantum efficiency. In the proposed configuration, the detector is practically free of avalanche-induced photon feedback effects. The influence of the GEM voltage and the electric fields close to the CsI-GEM electrode on the photon detection efficiency is studied. The conditions for obtaining full extraction of photoelectrons from the photocathode and their transfer through the GEM apertures are presented.

CsBr and CsI UV photocathodes: New results on quantum efficiency and aging

Abstract We report on the photoemission properties of 300xa0A thick transmissive- and 5000xa0A thick reflective UV-sensitive CsBr photocathodes. Following post-evaporation heat treatment at 70°C the absolute quantum efficiency is 35% at 150xa0nm, with a red boundary cut-off at about 195xa0nm. Extensive aging studies of CsBr and CsI photocathodes, under high photon flux and under ion bombardment in gas avalanche multiplication mode, were carried out for the first time without exposure to air. The results are compared with the previously published data on CsI aging and the methodology of the aging tests is discussed in details.

GEM-based gaseous photomultipliers for UV and visible photon imaging

We present the current status of our research on GEM-based gaseous photomultipliers. Detectors combining multi-GEM electron multipliers with semi-transparent and reflective photocathodes are discussed. We present recent progress in extending the sensitivity of these detectors into the visible range. We demonstrate the long-term stability of an argon-sealed bi-alkali photo-diode and provide preliminary results of a gas-sealed Kapton-GEM detector with a bi-alkali photocathode. The problem of ion-induced secondary electron emission is addressed.

Composite photocathodes for visible photon imaging with gaseous photomultipliers

Abstract We report on the results of coating K–Cs–Sb visible photocathodes, with thin CsBr and CsI protective films, in view of their possible combination with gas electron multipliers. Absolute quantum efficiency, photoelectron transport through the coating film, the effect of exposure to oxygen and to air and the photocathode behavior under intense photon flux are presented. The coated K–Cs–Sb photocathodes have typically maximum quantum efficiency values of 5–6% at 312xa0nm. They can withstand exposure to 150xa0Torr of oxygen for more than half an hour. Appropriate electron multipliers and possible applications are briefly discussed.

Photoelectron transport in CsI and CsBr coating films of alkali antimonide and CsI photocathodes

The transport of low-energy (<3u2009eV) photoelectrons in CsI and CsBr films was investigated by direct photoionization in the film and by photoinjection of electrons from underlying K–Cs–Sb, Cs3Sb, and CsI photocathodes. Photoelectron energy distributions and the photoyield dependence on film thickness were studied, assisted by in situ x-ray photoelectron spectroscopy surface characterizations. A model describing electron transport through the coating film was used, which correlated well with experimental results from the various material combinations, coating thickness, and photon energies. The model provides valuable information on the interface potential barrier of theses systems, as well as quantum-yield attenuation length and photoelectron energy distributions.

Progress in GEM-based gaseous photomultipliers

Abstract We discuss recent progress in gaseous photomultipliers (GPMTs) comprising UV-to-visible spectral range photocathodes (PCs) coupled to multiple Gas Electron Multipliers (GEM). The PCs may be either semitransparent or reflective ones directly deposited on the first-GEM surface. These detectors provide high gain, even in noble gases, are sensitive to single photons, have nanosecond time resolution, and offer good localization. The operation of CsI-based GPMTs in CF 4 opens new applications in Cherenkov detectors, where both the radiator and the photosensor operate in the same gas. The latest results on sealed visible-light detectors, combining bialkali PCs and Kapton-made GEMs are presented.

On the efficient operation of a CsI-coated GEM photon detector

We report on the ecient operation of a CsI-coated GEM photon detector. We describe its operation mode and the dependence of the single electron detection e�- ciency on the electric Þelds. Conditions for obtaining full eciency of photoelectron extraction and their focusing into the GEM apertures, in 1 atmosphere CH4, are presented. The quantum eciency of the CsI-coated GEM is 35% at 150nm.

Sealed gas UV-photon detector with a multi-GEM multiplier

A sealed gas photon detector with a gas electron multiplier (GEM) cascade coupled to a semitransparent CsI photocathode has been studied. High gains, reaching 10/sup 6/, and fast anode signals, of a width of 15 ns, were obtained with this GEM-photomultiplier in Ar-CH/sub 4/ (95/5), in a photon counting mode. The lifetime of this first prototype detector under high photon flux and large anode currents is provided.