The influence of biasing circuits on photomultiplier tube output noise spectra and transfer functions

An ITT type FW118 photomultiplier tube biased by using a high-voltage power supply and a resistor chain was found experimentally to have output noise spectra decreasing in magnitude as frequency increases, resembling in shape the 1/ noise spectrum. This lowering of the noise level as frequency increases was explained in terms of: 1) the shunting effect of the interelectrode impedance and the stray capacitances of the last stage, and 2) the gain fluctuation of the photomultiplier tube caused by the noise voltage developed across each biasing resistor. A method of flattening the noise spectra and improving the frequency response of the tube is also given in this paper.