Development of Real-Time 1-MeV Equivalent Neutron Fluence Monitor Based on SiPD for COMET Experiment

The COMET experiment at J-PARC aims to search for the charged lepton flavor violating process of neutrinoless mu-e conversion. The goal sensitivities of its Phase-I and Phase-II experiments are respectively to be improved by a factor of 100 and 10000 to the current limit. To achieve those sensitivities, very high intensity beam from J-PARC is used in both Phase-I and Phase-II experiments. According to the simulation studies, 1 MeV equivalent neutron fluence of ∼10¹² n/cm² in the COMET Phase-I was expected. Thus, the radiation tolerant parts have been selected and countermeasure against single event upset in FPGA has been considered. In addition to those investigations, we have developed the real-time neutron fluence monitor in order to promote those studies with high precision, and this monitor is expected to install near detector region in Phase-I experiment for the purpose of one of the environment monitors. The result of the monitor in Phase-I experiment is also assumed to be used for the studies for Phase-II experiment. We adopted the simple silicon photodiode (SiPD) produced by Hamamatsu for the monitor because the Si was well-known that the leakage current increased with the damage from neutron and it was available for the monitor. We have developed the prototype of the monitor based on the SiPD with the thickness of 320 μm and the size of 5 × 5 mm2 and 7 × 7 mm2, and neutron irradiation campaign was performed. In this test, it was observed that the increase of the leakage current depended on the neutron fluence. We found that the monitor can be utilized with the accuracy of ∼20% even if the annealing effect and detailed temperature correction were not considered. The detailed performance evaluation of the monitor is ongoing. In this paper, we describe the development of the realtime 1-MeV equivalent neutron fluence monitor. The prospect of the monitor is also described.