Study of bialkali photocathodes below room temperature in the UV/VUV region

Two photomultiplier tubes with bialkali photocathodes and fused silica windows (2 " Philips XP2020Q and 1-1/8 " Hamamatsu R1668) were studied between room temperature and -160 degrees C, illuminated in de mode in the UV/VUV wavelength range. The photomultipliers were operated as diodes, with direct measurement of the current of photoelectrons, in order to study in detail the effects associated to the photocathode. The influence of light intensity, applied voltage and illuminated area on the response as a function of temperature were investigated. The increase of the electrical resistivity of the emissive layer upon cooling proved to be responsible for a loss of linearity of the photocathode response, leading ultimately to the complete absence of signal at low temperature. A simplified model is presented that accounts for some experimental results. An unexpected increase of response below room temperature was also found at 170 nm.