CsI films are known to be efficient photoconvertors, intensively investigated for UV-photon imaging devices. The article reviews the production and characterization techniques of CsI photocathodes and their photoemission properties in vacuum and gas media, in charge collection and multiplication modes. The important roles of surface phenomena, gas and electric fields are described. The stability of the films is discussed.

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CsI UV photocathodes: history and mystery

A model of electron transport in alkali halides, below 10 eV, is described. It is based on theoretically calculated microscopic cross sections of electron interactions with lattice phonons. Both acoustic and optical scatterings are taken into account, the former being also treated as a quasielastic process that randomizes the electron motion. Monte Carlo calculations based on the model simulate the UV‐induced photoelectron emission from CsI. The calculated quantum efficiency and energy spectra are in good agreement with experimental data, in the photon energy range of 6.3–8.6 eV. The probability for an electron to escape from CsI, NaCl, and KCl is provided as a function of its energy and creation depth. A comparison is made between our approach and other phenomenological models.

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Low-energy electron transport in alkali halides

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 CH4 and under a detector gain of 105, for several substrate materials. We compare the ageing results with that of CsI photocathodes coated with thin NaF protective films.

/pdf/study-of-csi-photocathodes-volume-resistivity-and-ageing-104ahx7moy.pdf

Study of CsI photocathodes: volume resistivity and ageing

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.

/pdf/field-enhancement-of-the-photoelectric-and-secondary-uqqdanngwg.pdf

Field enhancement of the photoelectric and secondary electron emission from CsI

We present the results of our latest investigations, made in the laboratory and in a RICH detector, of the dependence of the UV quantum efficiency (QE) of CsI on the gas and the electric field. We confirm that in charge collection mode the QE in a gas is generally lower than in vacuum due to electron backscattering; in charge multiplication mode the QE increases with the field and reaches the vacuum value at high gas gains. The lowest photoemission under low electric field at the photocathode surface was observed in He-based gas mixtures; in Ar-hydrocarbon mixtures it approaches that of pure hydrocarbons. In CH4 and in CH 4 i-C 4 H 10 the QE value is practically independent of the field and is close to that in vacuum. The implications of CsI-based UV-detectors, particularly to fast RICH devices, are discussed.

/pdf/field-dependent-photoelectron-extraction-from-csi-in-3zc6gdiven.pdf

Field-dependent photoelectron extraction from CsI in different gases

CsI photocathodes of 30 × 30 cm 2 size have been operated in a fast RICH detector composed of a NaF radiator and a multiwire proportional chamber with a cathode pad readout. Results are presented from tests with a 3 GeV c proton beam. A Cherenkov angular resolution of 8 mrad is obtained by detecting a mean of 8 photoelectrons per ring. Methods are described for counting the Cherenkov photons and evaluating the differential quantum efficiency of the CsI photocathode. The results obtained with photons impinging at a large angle on the CsI layer are presented and discussed in comparison with higher quantum efficiencies observed under different experimental conditions.

/pdf/fast-rich-detector-with-a-cesium-iodide-photocathode-at-1n6gukmrnx.pdf

Fast RICH detector with a cesium iodide photocathode at atmospheric pressure

The ALICE collaboration proposes to investigate central ion-ion collisions (PbPb) at the Large Hadron Collider operated in the ion mode. The core of the detector covers 1.8 units of rapidity centered at midrapidity. The maximum expected rapidity density is 8000 charged particles. The proposed detector is planned to measure the momentum, charge and mass of the majority of the particles in the range of transverse momentum of 0.05–2.0 GeVc. In the range of momenta where charged particle identification, by means of ionization losses, is not possible (by the TPC, and/or silicon detectors), i.e. above 0.8 GeV/c for pions and kaons, the ALICE detector envisages two alternatives currently actively investigated — a time of flight wall or a RICH detector. The RICH detector under study is of the proximity focusing type with a liquid C6F14 radiator, using a MWPC with pad readout equipped with a solid photocathode for UV light detection.

/pdf/a-fast-rich-detector-for-particle-identification-in-alice-at-2h5ttts8hn.pdf

A. DiMauro

Papers

Fast RICH detector with a cesium iodide photocathode at atmospheric pressure

A fast RICH detector for particle identification in ALICE at LHC

A RICH detector as particle identification detector in ALICE