Measurement of low light flux by photon counting

In order to measure low light flux with a photomultiplier tube it is necessary to know the properties of the dark current of the tube. In this paper the dark-current statistics and other properties are measured for an International Telephone and Telegraph FW 130 photomultiplier both cooled and at room temperature. The tube was found to have a constant non-thermal residual count rate. The pulse height distribution of these non-thermal dark-count pulses was shown to be the same as for the room-temperature dark current and to correspond to electrons emitted from the cathode. The dark-count pulses were shown to have a Poisson counting distribution both at room temperature and when the tube was cooled. Finally, having demonstrated that the cooled dark current was random, a low light flux giving 0·043 photoelectrons/s was detected in the presence of a cooled dark current (at maximum efficiency) of 0·459 counts/s.