Enhancing the photon detection efficiency of Silicon Photomultipliers for RICH at ALICE 3

The ALICE Collaboration is proposing a new apparatus, ALICE 3, planned to be installed for LHC Run 5, to investigate the Quark Gluon Plasma properties beyond current limits. Studies for the development of a Ring- Imaging Cherenkov detector for charged particle identification in the barrel region with ALICE 3 are ongoing. The proposed layout is a proximity-focusing RICH, using aerogel as Cherenkov radiator and a layer of Silicon Photomultipliers (SiPMs) for the photon detection. The coupling of a thin fused silica window with refractive index of approximately 1.47 at 400 nm to the SiPM for precise charged particle timing is also under study. Figure 1: Schematic view of the barrel RICH (bRICH) detector. The accompanying figure 1 shows the different components of RICH: the aerogel in blue, the gas in grey and the photodetector modules in orange. A limitation in the use of SiPMs comes from their limited radiation tolerance, leading to a dark count rate (DCR) increasing above the MHz/mm2 level
during operation at the expected operation temperature and recommended overvoltage with the expected fluence levels. A study on the possibility of using smaller SiPMs coupled with respect to the 2×2 mm2 baseline was done in order to reduce the dark count rate while retaining the same or even higher photon detection yield. A significant limitation to the SiPM photon detection efficiency (PDE) comes from reflections occurring at all SiPM interfaces among window, resin, passivation layer and silicon and leading to both photon losses and multiple-reflection background. Here, a study using a rough interface between the silicon window and the gas was conducted to reduce reflections.