Tests of Photomultipliers for Astronomical Pulse-Counting Applications

An apparatus is described for testing photomultiplier tubes to be used in an astronomical spectrophotometer. The wide differences in residual nonthermal dark current (after refrigeration) found among photomultiplier tubes with very similar rated characteristics require that each cell intended for faint-light astronomical applications be individually selected. The better CsSb and trialkali tubes which we tested have residual dark currents extrapolated to zero bias respectively in the range 2–6 and 15–30 counts/sec; unfortunately this residual component is usually also somewhat nonstatistical in character. Further general conclusions from our tests are presented, together with specific data for cells including relative-sensitivity scans of photosurfaces.