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

Vladislav Todorov (1) , Philippe Cassette (1, 2) , Chavdar Dutsov (2) , Benoit Sabot (3) , Strahil Georgiev (1) , Krasimir Mitev (1)

1 Sofia University "St. Kliment Ohridski"
2 PSI - Paul Scherrer Institute
3 LNHB (CEA, LIST) - Laboratoire National Henri Becquerel (CEA, LIST)

We investigated the non-uniformity of the response of the photocathodes of some PMTs and their potential influence on the results of scintillation measurements. We have developed an experimental system which allows us to study 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 the first tests of a Hamamatsu R7600U-200 PMT. A degradation of the counting efficiency and a shift of the peak position towards small values was identified in the measurements near the edges of the photocathode where the energy resolution is significantly worse than the energy resolution at 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 (TDCR) method and the CIEMAT/NIST efficiency tracing method. The traditional free parameter model assumes a Poisson distribution of the number of emitted photons but the non-uniformity leads to a compound Poisson distribution, where the mean value of the distribution is a random variable. This introduces an extra variance of the calculated detection efficiency which should be taken into account in the measurement uncertainty budget. Experimental results are presented in the paper, together with their practical influence in scintillation measurements and absolute calibration of radionuclides.

Mots clés: photocathode; scintillation; instrumentation; radioactivity; metrology; ionizing radiation; alpha-rays; spectrum; pulse-shape discrimination; counting efficiency; non-uniformity; Primary activity measurement; Triple to Double coincidence Ratio (TDCR)

Domaines: Instrumentations et Détecteurs [physics.ins-det]

Vladislav Todorov, Philippe Cassette, Chavdar Dutsov, Benoit Sabot, Strahil Georgiev, et al.. A study of the non-uniformity of the PMT photocathode response and its influence on the results obtained in different scintillation counting experiments. NDIP20 - 9Th Conference on new developments in photodetection, Jul 2022, Troyes, France. pp.00371R1. ⟨cea-03830911⟩