Low-frequency noise in a hybrid photomultiplier tube

Abstract The noise in a hybrid photomultiplier tube is investigated. Photoelectrons produced at the photocathode are accelerated towards a silicon P-I-N diode. They create electron-hole pairs in the depletion region of the diode and produce an amplified current of gain M , which is not due to avalanche multiplication by the internal field in the diode, but multiple ionisations of the primary photoelectrons. The gain increases linearly with an acceleration voltage, V PC , beyond a threshold voltage, which is necessary for photoelectrons to penetrate the top layer of the material. Above 1 Hz, we observed only shot noise. The shot noise is the sum of two contributions: (i) full shot noise due to photon absorption in the diode, because the photocathode is partially transparent; (ii) a specific white noise due to the multiplication of primary photoelectrons. It is shown that the multiplication mechanism gives rise to a white noise S 1 ∝ M 3 ∝ V PC 3 .