Speaker
Description
Background:
Ring Imaging Cherenkov (RICH) detectors are indispensable for particle identification in experiments such as LHCb and Belle II, including their planned high-luminosity upgrades. These systems require photon sensors that combine high detection efficiency, precise timing, and robustness in magnetic fields. Silicon photomultipliers (SiPMs) meet these criteria due to their compactness, high gain, and sub-100 ps single-photon time resolution. Their main limitation, however, is sensitivity to neutron irradiation, which alters device properties and leads to sharply increased dark count rates (DCR), thereby threatening long-term stability in harsh radiation environments.
Methods:
A broad set of SiPMs from multiple manufacturers — including SensL, Hamamatsu, Ketek, FBK, Advansid, and Broadcom — were irradiated at the TRIGA nuclear reactor of the Jožef Stefan Institute to fluences between 10⁹ and 10¹³ neq/cm², covering the range expected in future RICH detectors. Characterisation was carried out before and after irradiation using:
• current–voltage (I–V) and capacitance–voltage (C–V) scans,
• dark count rate (DCR) measurements,
• single-photon timing resolution (SPTR) studies, and
• determination of full depletion voltage from C–V curves.
Measurements were performed across a wide temperature range (+20 °C to −180 °C) with a stabilised cryogenic setup. Post-irradiation annealing at elevated temperatures was also conducted to assess the potential recovery of performance.
Results:
The presentation will cover:
• Design of the irradiation and characterisation campaign,
• Experimental results for multiple SiPMs, including I–V, C–V, DCR, depletion voltage, and SPTR performance across fluences.
Neutron irradiation shifted the depletion voltage and altered the internal electric field, impacting key operating parameters. Breakdown voltage increased, leakage currents rose, and the DCR degraded significantly. Despite these effects, timing performance was robust: whenever single photons could be resolved, the SPTR remained ~90 ps FWHM, even after exposure up to 10¹³ neq/cm².
Room-temperature operation became unfeasible beyond 10¹⁰ neq/cm², but cooling successfully mitigated radiation-induced degradation. At 10¹² neq/cm², stable operation was possible around −140 °C, while at 10¹³ neq/cm², functionality was only recovered at liquid-nitrogen temperature. Annealing partially reduced leakage currents and DCR, typically lowering the cooling requirement by ~20 °C, but did not restore room-temperature performance or further improve timing.
Conclusions:
This study provides essential benchmarks for the integration of SiPMs into next-generation RICH detectors at the HL-LHC and Belle II upgrades. It demonstrates that neutron damage alters electric field and operating parameters — increasing breakdown voltage, shifting depletion voltage, and raising DCR — yet excellent timing precision is preserved with appropriate cooling. By testing devices from various major manufacturers, this work highlights both the common challenges and the feasibility of using SiPMs as fast photon detectors in high-radiation environments.
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