Cryogenic Operation of Cmos Spads as Candidate Photodetectors for Lhcb Rich Upgrade Ii

In the planned LHCb RICH (Ring Imaging Cherenkov Detector) upgrade II, silicon photomultipliers (SiPMs) are considered as one of the most promising photodetector candidates. They can meet all of the performance requirements of upgraded RICH detector, except for the neutron irradiation hardness. During the whole experiment run time, the photodetectors are expected to receive accumulated dose of about 1013 1-MeV neutron equivalent /cm2. For the SiPMs to keep sufficient performance after such a dose, a combination of mitigation steps will have to be taken, including cooling the photodetectors close to liquid nitrogen temperature. The spadRICH project is developing a CMOS single-photon avalanche diode (SPAD) based photodetector optimized for the application of the planned RICH detectors, with SPADs designed specifically for radiation hardness and cryogenic operation, including microlenses and on chip custom electronics. The latter is planned to include functions to mitigate radiation damage effects while meeting the other main performance requirements. A number of SPADs designed by EPFL AQUA in 55 nm, 110 nm and 180 nm CMOS technologies were characterized in a cryogenic experimental setup, measuring dark count rates, afterpulsing probability and response to low-level light illumination, from room to liquid nitrogen temperatures. Characterization was repeated with SPADs irradiated up to 1012 1-MeV neutron equivalent/cm2, with high temperature annealing and effects of microlens materials also studied. The characterization results are being used to optimize the design of spadRICH digital SiPM.