Direct Measurement of the Quantum Counting Efficiency of RCA C31000E/F Photomultipliers at 6328 A

Direct measurement of the total quantum counting efficiency of the RCA C31000E/F shows that it is about 25 +/- 22% lower than the photocathode quantum efficiency in optimized commercial base units. Measurements with an EMI 9558 give similar results and with an Amperex 56TVP give similar although more erratic results due to its higher sensitivity to magnetic fields. The measurement techniques and conditions are described in detail. Diagnostic studies such as the question of dependence of counting efficiency on signal or background count rate are also presented.