Response of End-Window Photomultiplier Tubes as a Function of Temperature

This paper reports a study of the response of commercial end-window photomultipliers to light of various wavelengths, in the 4000 to 7000 A interval, as a function of temperature. All tubes studied were of nominal 2-inch diameter, and included various standard tubes made by DuMont and RCA, low-temperature modifications of standard tubes, and the recently-developed multialkali-photocathode tubes. Most of the experiments were performed with the tube operating as a multiplier; in a series of auxiliary experiments, the dynode string was shorted to the anode so that the effect of cooling the cathode could be studied separately. In a typical tube the response to blue light increases somewhat with decreasing temperature, whereas the response to red light decreases rapidly; these effects are associated with changes in the photocathode spectral sensitivity. At low temperatures (~-100°C or below) the response falls sharply for all wavelengths, an effect attributed to the increased resistivity of the semiconducting photocathode. This effect is not observed in tubes whose photocathode is covered with a semitransparent metallic backing. The temperature dependence of spectral sensitivity in multialkali-photocathodes (RCA C-7261) is significantly different from that of a Cs-Sb photocathode.