Effects of Neutron Irradiation on Photomultiplier Tubes and Their Power Supplies

In the context of the Gamma-Flash program, whose objective is to investigate the emission of high-energy gamma rays and neutrons during thunderstorms, a neutron detection system, based on scintillators coupled to photomultiplier tubes (PMTs), has been developed. The neutrons emitted are generated by photoproduction reactions in the atmosphere, and they have a wide range of energies, from MeV down to meV. In this work, the effects of neutron irradiation on PMTs and their power supply are investigated to assess and quantify the effects of irradiation on dark-count rate and during data acquisition. The detection system was irradiated at the ChipIr beamline at ISIS Neutron and Muon Source, Didcot, U.K. During the irradiation of power supplies, an increase in dark-count rate has been observed, whose magnitude depends on the output voltages and the experimental parameters. In the specific case of our experiment, the dark-count rate went from an average value of 0 to 40 and 120 counts per second (counts/s) at the operating voltage of 1800 and 1900 V, respectively. Even more evident are the effects during the irradiation of the PMTs; in this case, the dark-count rate reaches about 4000 counts/s, high enough to hamper the measurements. These effects, despite their relevance to data acquisition and detector performance, do not produce long-term consequences on the electronic components. Beyond the Gamma-Flash program, this research is relevant, because the use of PMTs is common in many areas of scientific research and radiation detection applications. Our results highlight the need for great care in the interpretation of results in applications where PMTs and their power supplies may be exposed to high radiation fluxes. A proper design of the experiment or shielding strategies could prevent such unwanted behavior in data acquisition.