Quantum Efficiency Of CsI Photocathodes Research Articles

Quantum efficiency and stability of alkali halide UV photocathodes in the presence of electric field

It was shown previously that CsI sensitivity can be substantially reduced by intense UV irradiation. In the present study we measure the influence of negative and positive electric fields on the degradation of CsI photocathode sensitivity when it is subject to a large dose UV irradiation. The UV sensitivity of positively biased CsI photocathodes did not decay as fast as in case of negatively biased films. Thus we found that negative (photoelectron-extracting) electric field not only increases quantum efficiency of CsI photocathodes (as reported previously by Buzulutskov et. al., J. Appl. Phys. 77 (1995) 2138), especially at long wavelengths, but also substantially reduces the long term performance of the photocathode under intense UV irradiation. Electron-repelling electric field (generally used in MCP detectors with opaque photocathodes), on the other hand, significantly reduces aging of the photocathode under UV irradiation.

Role of the substrate reflectance and surface-bulk treatments in CsI quantum efficiency

Abstract We have experimentally investigated the following aspects related to the quantum efficiency of CsI photocathodes: the type of substrate, the film thickness and the effect of a “bulk treatment” during the film growth. We discovered that, using a high reflectivity aluminium substrate, the photoemission of very thin CsI film is enhanced. Our study also revealed that photocathodes become less sensitive to moisture when a negative bias voltage is applied to the substrate during the film deposition process.

Study of the quantum efficiency of CsI photo-cathodes exposed to oxygen and water vapour

The operation of CsI photocathodes in gaseous detectors requires special attention to the purity of the applied gas mixtures. We have studied the influence of oxygen and water vapour contaminations on the performance of CsI photocathodes for the ALICE HMPID RICH prototype. Measurements were done through comparison of Cherenkov rings obtained from beam tests. Increased levels of oxygen and water vapour did not show any effect on the performance. The results of this study found a direct application in the way of storing CsI photocathodes over long periods and in particular in the shipment of the HMPID prototype from CERN to the STAR experiment at BNL.

Measurements of the quantum efficiency of CsI photocathodes in a multiwire proportional chamber

The quantum efficiency of reflective CsI photocathodes in a multiwire proportional chamber (MWPC) has been measured. The MWPC is operated with methane at room temperature and atmospheric pressure and a gas gain of about 10 5. Two different methods are used to determine the quantum efficiency (QE). The first uses Cherenkov radiation of β-particles in a 1 cm thick NaF window of the CsI-MWPC and measures the average ratio of the CsI quantum efficiency to a standard QE s (λ) used in simulation. The second method uses a monochromator and compares the count rates of the CsI-MWPC with the count rates of a Philips XP2020Q photomultiplier. Results are presented for the dependence of the quantum efficiency on the photocathode production procedure, on the substrate material, on the thickness of the CsI layer, on exposure time to ambient air, on healing of the photocathodes, on flushing them with methane and on the cumulative charge collected.

Incident angle effect on the quantum efficiency of CsI photocathodes

A systematic investigation of the effect of incident angle of the incoming UV light on the quantum efficiency of CsI photocathodes in the spectral range 140–220 nm is presented. The measurements were made in vacuum. A decrease of the photocurrent with an increase of the photon angle of incidence is observed, reaching a relative loss of 30% for an incidence angle of 55° at a wavelength of 210 nm.

Electric field effects on the quantum efficiency of CsI photocathodes in gas media

We have measured the quantum efficiency (QE) of CsI photocathodes as a function of the electric field strength in a parallel-plate geometry, in CH 4, C 2H 6 and i-C 4H 10 both in charge collection and multiplication modes. It was found that in the collection plateau the QE value in gases is lower compared to that of vacuum and is independent of the field; in gas media the QE starts to increase at the transition between collection and multiplication modes and reaches the vacuum value at high gas gain. We explain this effect by a decrease of the electron-molecule elastic backscattering while entering the multiplication mode. We conclude that the electric field effects observed here, would also apply for other photocathodes and gas mixtures. An enhancement of the QE after microdischarges was observed and is discussed in detail.

CsI and some new photocathodes

A discussion of the possible sources of discrepancies in the measurements of the quantum efficiency of CsI photocathodes is presented. We propose that the major causes for disagreements in QE are due to the QE dependence on the current density extracted from the photocathode, on the electric field, and on the temperature of the photocathode. Preliminary results on TMAE enhanced GaAs and Si, plus TMAE protected CsTe and SbCs photocathodes, operated in gas, are also presented.

Quantum efficiency and ageing studies for bulk and porous CsI photocathodes in a MWPC

The quantum efficiency of CsI photocathodes operating in a proportional gas chamber is measured in the wavelength range 185–220 nm. The tests are carried out with methane gas at atmospheric pressure using a pulsed UV lamp. Two different techniques are used to produce the photocathodes. The CsI is either evaporated under high vacuum or under low pressure of argon. The quantum efficiency of the photocathodes produced by the first technique is measured to be 9% at 190 nm while it is enhanced by a factor of 1.3 and 2 at 190 nm and 200 nm respectively for the photocathodes prepared with the second method. No significative drop of the quantum efficiency is observed over a total period of one month.