Characterization of thin LGAD sensors designed for beam monitoring in proton therapy

A fast 144-channel proton counter prototype, designed for monitoring the fluence rate of clinical proton beams, is based on a thin Low Gain Avalanche Detector (LGAD), segmented into 146 strips (114 μm width, 26214 μm length, 180 μm pitch). The layout of the sensor was designed in the framework of the Modeling and Verification for Ion beam Treatment planning (MoVe-IT) project in collaboration with Fondazione Bruno Kessler (FBK, Trento, Italy) and fourteen wafers were produced and delivered by FBK in 2020. In this paper, we present the laboratory characterization of the sensors performed on the entire wafer at FBK, right after production, and at the University of Turin after cutting the sensors using a probe station connected with a power device analyzer for static electrical tests and an infrared picosecond laser to study the dynamic properties. In addition, one sensor was tested with the clinical proton beam at National Center for Oncological Hadrontherapy (CNAO, Pavia, Italy). The results obtained from the test at FBK and UNITO facilities demonstrated that the cut did not affect the yield production. The static electrical tests proved that the MoVe-IT-2020 sensors production was of very high quality. The width of the inter-strip dead region measured was 80.8 μm. 22% larger than the distance of the gain layers, and has a small dependence on laser intensities. A preliminary beam test at CNAO showed good separation between signal and noise in the LGAD strip, which allows counting properly the protons by selecting the optimal signal threshold.