UV-Grade Silicon Photomultipliers for Direct Counting of Low-Energy Electrons and Protons

This paper reports a novel application of silicon photomultipliers (SiPMs) for directly counting kiloelectron-volt-range electrons and protons. Current instruments for measuring low-energy ions and neutral atoms (<;30 keV) in space regularly use secondary electron multipliers, which pose practical limitations in small volume and low budget applications. A thin dead-layer SiPM, originally developed for vacuum-ultraviolet (VUV) photons, is tested for direct measurement of >keV particles replacing conventional detectors for certain applications in this paper. Our results demonstrated that the VUV-SiPM was sensitive above 1 keV both for electron pulses and protons. Using absolute beam monitor system, the proton detection efficiency of these SiPMs was evaluated. The efficiency was ~1% for 1-keV protons and plateaued above 5-6 keV at the expected 75% efficiency that agreed reasonably with the Monte Carlo numerical simulation results. The 75% plateau level was matching the open area ratio of the SiPM structure. The electron-pulse efficiency was also estimated by applying the same model, which shows ~1-keV detection limit and plateau above 3-4 keV. The agreement between the simulation and measurement shows that the dominating mechanism to trigger SiPM signals is the ionization loss of primary electrons and protons in the active volume of the pixel. An analytic investigation shows that the dark count rate can be significantly reduced with a double (or quadruple) coincidence measurement if multiple electrons can be produced from a single event.