Wavelength Dependence of Photo Multiplier. Single Electron Response Amplitude Spectra

A study of the 56 AVP photomultiplier has shown that the single electron response amplitude spectrum depends on the wave length of the detected photons. In fact, for 6700 A light a high quality spectrum i.e. presenting a strong maximum, may be obtained by applying well chosen voltages to the first dynode and by using the positive focusing bias recommended by the manufacturer. A continuous degradation of the spectrum is observed when the wavelength decreases from 6700 Å to 3100 Å; for blue light there is no maximum anymore. It was found that for an illumination of the central area of the photocathode, the spectra for blue light may be markedly improved by running the focusing electrode at a negative bias with respect the photocathode. Results concerning peak to valley type figures of merit of the spectra are shown vs. focusing voltage and for various wavelengths the implications of these observations are discussed. It seems that these phenomena are not specific of the 56 AVP as analogous results were observed on the recent 2106 phototube.