Low-frequency noise in a hybrid photomultiplier tube

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, VPC, 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 &aM’a PK.