A. Buzulutskov

Photoelectron backscattering effects in photoemission from CsI into gas media

The photoemission from solid surfaces into gas is important in view of the application of solid photocathodes in fast-RICH devices. The photoemission from CsI into gas has been investigated in He-, Ar- and CH,-based gas mixtures as a function of the electric field at the photocathode surface. Measurements were made both in laboratory, with a UV source, and in a beam with a RICH detector. The results are interpreted in terms of current models of electron transport in gas. The electron collection efficiency, below gas ionization threshold, is reduced by backscattering. This phenomenon is particularly important in He-based gas mixtures. In CH, and Ar/CH, mixtures the backscattering is very low. At high electric field, under charge multiplication, a full collection efficiency, similar to that in vacuum, is obtained in all gases investigated. We discuss the parameters governing the choice of the gas mixture in this kind of photon detectors.

The scintillation of CF4 and its relevance to detection science

Abstract The scintillation properties of CF 4 are presented in comparison with those of Xe and CH 4 . Alpha-particle induced photon emission was measured with vacuum phototubes and with a CsI-based gaseous photomultiplier. The latter method provides an absolute sensitivity of such devices to particle-induced UV-photon background in CF4 and CH4 gaseous Cherenkov radiators. Integrated CF 4 scintillation yields over the range of 150–220 nm are, on the average, 315±95 to 242±60 photons/MeV, in the respective pressure range of 0.063 to 0.75 atm, compared to CH 4 which emits 0.06±0.01 photons/MeV at 1 atm. The total photon yield, integrated over the full emission spectrum of CF 4 (150–500 nm), is of the order of 1200 photons/MeV × 4 π. The primary scintillation photon yield of CF 4 is about 16(±5)% of that of Xe. No proportional secondary scintillation was observed in CF 4 . The avalanche-induced photon yield was measured to be of the order of 0.3 photons per electron. The implications of this considerable photon emission, are discussed.

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.

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.

STUDY OF CSI PHOTOCATHODES : VOLUME RESISTIVITY AND AGEING

Abstract Wire-chamber gaseous electron multipliers coupled to CsI photocathodes, provide interesting means for UV-photon imaging over a very large area. We report on the measurement of the CsI resistivity, important for the understanding of rate-dependent effects in CsI-based detectors. Photon counting at high rates is also closely connected to long-term photocathode stability. We present results on ageing of CsI photocathodes under high photon flux, with one atmosphere CH 4 and under a detector gain of 10 5 , for several substrate materials. We compare the ageing results with that of CsI photocathodes coated with thin NaF protective films.

The protection of KCsSb photocathodes with CsBr films

Abstract We report on photoemission properties of visible Kue5f8Csue5f8Sb reflective photocathodes coated with thin CsBr protective films. We present the absolute quantum efficiency and its stability under prolonged contact with oxygen. It is found that Kue5f8Csue5f8Sb photocathodes coated with 280 A thick CsBr films have 5% quantum efficiency at 312 nm and can withstand an exposure to 150 Torr of oxygen for half an hour. We discuss the possible role of good lattice constants matching between the photocathodes and the protecting film.

Field enhancement of the photoelectric and secondary electron emission from CsI

We have measured the electron emission from a CsI‐coated multiwire cathode, induced by ultraviolet photons and electrons, in vacuum at high electric fields. We found an enhancement in quantum efficiency of a factor of 1.5 at 160 nm, 3 at 185 nm, and 25 above 200 nm, at a field of 500 kV/cm. At the short wavelengths the amplitude of the effect is a linear function of the square root of the field strength. The enhancement of the electron‐induced secondary electron emission yield is dependent on the primary electron energy: for energies above 1 keV it varies by a factor of 2 to 10. A simple model of the field enhancement of the photoemission is suggested. Practical applications are discussed.

Evidence for thin‐film protection of visible photocathodes

NaI and CsI protective coatings on visible cesium‐antimony photocathodes have been studied. With NaI protective films the photocathodes are shown to withstand exposure to considerable doses of oxygen and dry air. This opens the way to their handling and operation in gas media.