A study of the non-uniformity of the PMT photocathode response and its influence on the results obtained in different scintillation counting experiments

Abstract

This work presents an investigation on the non-uniformity of the response of the photocathode of two photomultiplier tubes (PMTs) and its potential influence on the results of scintillation measurements. An experimental system was developed which allows studies of the non-uniformity of the photocathode response and its influence on the shape of alpha spectra obtained after pulse-shape discrimination. The system is presented together with tests of Hamamatsu R7600U-200 and H11934-203 PMTs. A degradation of the counting efficiency and a shift of the peak position towards smaller values were identified in the measurements near the edges of the photocathode where the energy resolution is significantly worse compared to the central region of the PMT. Such non-uniformity also affects absolute activity measurements based on the free parameter model, i.e., the Triple to Double Coincidence Ratio and the CIEMAT/NIST efficiency tracing methods. A significant non-uniformity of the response of a PMT would interfere with the assumption of the traditional free parameter model of a Poisson distribution of the number of emitted photons. Experimental results are presented in the paper, together with a discussion of their practical influence on scintillation and absolute activity measurements.