Measurements of photomultiplier single photon counting efficiency for the Sudbury Neutrino Observatory

A knowledge of the absolute efficiency of the photomultiplier tubes (PMTs) used in the Sudbury Neutrino Observatory (SNO) is important for modelling t he behaviour of the detector. While the quantum efficiencies for the PMTs a re known, the efficiency in single-photon counting mode (which includes col lection efficiency and counting efficiency) is of more relevance. A calibra tion of the 8 in. Hamamatsu R1408 photomultipliers in this mode requires a known flux of incident light at the very low level of 10(-16) W cm(-2) (250 photons cm(-2) s(-1)). A method of performing such measurements, utilising the extreme linearity obtainable with silicon photodiodes, is presented in this paper. The results of efficiency measurements on four PMTs are given and a comparison is made with previous results obtained using a Cherenkov s ource of known intensity. (C) 1999 Elsevier Science B.V. All rights reserve d.