UV radiation resistance and solar blindness of CsI and KBr photocathodes

A detailed study of the stability of CsI and KBr photocathodes under UV irradiation is presented. UV quantum efficiency degradation was found to be more pronounced at lower illumination intensity for the same accumulated dose and illumination wavelength. For an equal number of extracted photoelectrons in-band UV exposure led to a larger sensitivity decay as compared to out-of-band illumination. The angle of radiation incidence was not important for UV sensitivity degradation, while changes of visible light rejection (i.e., degradation of solar blindness) did depend on the incidence angle: the photocathodes illuminated at normal incidence were activated much faster than the films irradiated at grazing angle. We found that the increase of visible sensitivity can be characterized by the total accumulated dose and is independent of irradiation flux during UV activation. We also observed that heat annealing substantially improves the visible light rejection of CsI photocathodes.

I. Introduction

THE SENSITIVITY of imaging and spectroscopic detectors is often increased by photocathodes optimized for a particular wavelength range. Alkali halide photocathodes are currently widely used in various UV detecting devices [1]–[3][4][5][6] due to their high efficiency and relative stability under air exposure. Photoconversion efficiency is a crucial parameter determining the performance of the entire detecting device. At the same time, the stability of the photocathode sensitivity is essential for many applications where long operation time is required or large doses of UV irradiation are involved. The stability of UV quantum efficiency (QE) of alkali halide photocathodes under UV illumination was studied in recent papers [5], [7][8][9][10][11][12][13][14]. The current detailed study of the response of CsI and KBr photocathodes was performed with thin films of these materials deposited directly on the front surface of microchannel plates (opaque photocathodes). The ability of microchannel plates to detect almost every single photoelectron produced by the photocathodes allowed us to substantially extend the previous investigations with planar reflective photocathodes [11], where the sensitivity was limited by an electrometer measuring photocurrent from the samples. We have measured the stability of photoconversion efficiency in the UV range as a function of radiation wavelength, angle of incidence, flux rate, and dose (Section III). In addition, the dependence of visible light rejection (solar blindness) on the flux rate, wavelength, and angle of radiation incidence was determined (Section IV). We have also considered the influence of photocathode heat treatment on the efficiency of visible light rejection.