Radiation damage of FBK Silicon Photomultipliers for HEP applications

Silicon Photomultipliers (SiPMs) are photo-detectors currently experiencing a significant development in several fields: from medical applications to LiDAR, homeland security, analytical instrumentation, High Energy Physics (HEP). In particular, this work deals with SiPMs in HEP with a focus on their tolerance to the radiation. In fact, in HEP sensors are usually exposed to a large amount of radiation and they are expected to survive in such an environment. After a detailed introduction on the main SiPM characteristics, the main mechanisms of damage in SiPMs are addressed. They mainly consist into the generation of defects into the bulk, resulting in an increase of the noise, and in the surface layer, resulting in the generation of fixed charge in the oxide and defects at interface between the oxide and the silicon and, as a consequence, in an increase of the leakage current. This work aims to verify these theoretical assumptions through three irradiation test campaigns, where FBK SiPMs are exposed to the effects of both ionizing and non-ionizing radiation at medium-high doses. The first irradiation test took place at the Laboratori Nazionali del Sud (LNS) in 2019, where SiPMs were exposed to increasing 62 MeV proton fluences up to about 10^14 n_eq/cm^2. The main SiPMs parameters were assessed after irradiation and some information about the damage into the bulk of the sensors were derived. In fact, the noise was observed to increase significantly with respect to the non-irradiated SiPM, as expected from theory. Information were then extracted about activation energy, and a preferred spatial localization of the defects inside the single microcell was also observed. During the second irradiation test, FBK SiPMs were irradiated at the Trento Protontherapy Center with 74 MeV protons at increasing fluences, up to 6.4x10^11 n_eq/cm^2, using a custom setup which allowed direct reverse current-voltage measurement shortly after each irradiation step, reducing the impact of the annealing on the results. SiPMs were characterized extracting DCR, breakdown voltage and other key parameters from reverse current measurements. Then, the several technologies under test were compared using figures of merit such as Signal-to-Noise Ratio (SNR) and energy resolution, in X-rays or gamma-rays spectroscopy, assumed as possible applications. The NUV-HD technologies with smaller cell sizes showed the best best performances, in terms of energy resolution, SNR and noise level after annealing. The third irradiation campaign aimed to test the tolerance of FBK SiPMs to ionizing radiation through an X-rays irradiation at the TIFPA facility in Trento up to a dose of 100 kGy. SiPMs were irradiated using the same custom setup used in the second irradiation test, performing reverse current measurement after each irradiation step. After a 30-day annealing, samples irradiated at 100 kGy were fully characterized and compared to the non-irradiated ones. RGB-HD was the only technology to show a decrease of the PDE due to border effects in the electric field inside the microcells of the SiPM, while the NUV-HD-cryo was the only technology not showing variations neither in the noise nor in the PDE.