Development of microlens-enhanced SiPMs for a radiation-hard scintillating fibre tracker at LHCb

The LHCb experiment at CERN's Large Hadron Collider (LHC) is currently taking data with a newly developed Scintillating Fibre (SciFi) Tracker read out with multichannel silicon pho- tomultipliers (SiPMs). The innermost fibre modules, exposed to the highest radiation, will need to be exchanged at the end of the current run, as part of Upgrade Ib (2026–2028). For this consolidation work and in view of Upgrade II (2032–2034) this thesis work focusses on the development of microlens-enhanced SiPMs with increased photon detection efficiency (PDE). A simulation study comparing the expected increase in light detection between SiPMs without and with microlenses has been conducted to guide possible implementations, and has been validated by a simulation based on a commercial software. Microlenses have been deposited on available, non-customised SiPMs. The light yield of a short fibre mat has been measured and the results of different SiPM surfaces compared. For microlens-enhanced SiPMs with a fill factor of 50.0%, an improvement of 32% at a low overvoltage of $\Delta \text{V}$ = 1V over SiPMs with a residual layer is seen. This increase drops to 19% at a higher overvoltage of $\Delta \text{V}$= 4V. For SiPMs with a larger fill factor of 82.4% no significant difference is observed. For Upgrade II, cryogenic cooling is foreseen to mitigate radiation-induced ageing effects. Irradiated detectors with an advanced technology have been tested, and a reduction in dark count rate of five orders of magnitude between operation temperatures of –40 ◦C and –196 ◦C (liquid nitrogen) has been measured. Customised SiPMs, combining a high PDE and low crosstalk probability in the cryogenic technology have been produced and initial promising results are presented.