Electron Induced Noise in Star Tracker Photomultiplier Tubes

The induced noise produced as a result of the impingement of electrons and X-rays on star tracker photomultiplier tubes has been investigated. Noise in this sense is anode current that is not due to a intended light signal, but which results from electron or X-ray irradiation. Anode current as a function of electron beam current density was measured for both shielded and unshielded RCA tubes 1P21, C70129C, and 8571. The anode current was found to vary linearly with electron beam current density. The shielded tubes were found to exhibit a 300-fold reduction in anode current density over the unshielded tubes. From this data it could be inferred that 2.6 MeV electrons are an order of magnitude more efficient in producing anode current than the X-rays resulting from stopping these electrons in 1/4-inch aluminum shields. The mechanism of this noise production was also studied, it was found that the major source of the noise is from luminescence in the glass envelope of the tubes, the remainder being due to recoil electrons in the area of the photocathode.