Cryogenic operation of neutron-irradiated silicon photomultiplier arrays up to 1e14 neq/cm^2

Abstract

In the context of the Scintillating Fibre (SciFi) Tracker for the LHCb Upgrade 2, radiation-induced damage in silicon photomultipliers (SiPMs) has been studied over a wide temperature range, from room temperature down to 100 K. With the LHCb detector Upgrade 1, installed during the LHC's Long Shutdown 2 (LS2) (2019-2021), the first large-scale SciFi tracker read out by multichannel silicon photomultipliers (SiPMs) was constructed, installed, and has been operated ever since. A major challenge for the SciFi tracker is the neutron radiation at the SiPMs' location. At the end of the lifetime of the Upgrade 1 detector, the expected neutron fluence for the SiPMs will reach 6e11 neq/cm^2. Cryogenic operation is being investigated to mitigate even higher radiation-induced damage for Upgrade 2, where the total neutron fluence is expected to reach 3e12 neq/cm^2. A large set of custom SiPM arrays, varying in pixel size, electric field configuration, and doping implant concentration, developed by FBK and Hamamatsu, were tested after neutron irradiation. Characterisation was performed in a dedicated cryogenic test setup, where the operating temperature was varied over a wide range. Key performance parameters such as breakdown voltage, gain, dark count rate, optical crosstalk, and afterpulsing were characterised as functions of temperature, overvoltage, and neutron fluence. The result is a precise assessment of radiation damage for state-of-the-art technology from two leading SiPM manufacturers, allowing the results to be transferred to other SiPM applications.