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Description
Silicon Photomultipliers (SiPMs) are under consideration for the outer timing layer of ALICE 3, the next-generation heavy-ion experiment at the LHC, which will replace the present ALICE experiment for LS4 (2034-2035) and beyond. While SiPMs are traditionally coupled to external scintillators or Cherenkov radiators, recent studies have shown that they can directly detect charged particles through Cherenkov light emitted in the few-hundred-micron-thick protective layer deposited on top of the device. A SiPM with such a protective layer on top can therefore combine the benefits of an external photon generator, producing a large number of photons, with a high level of simplicity and compactness, opening new possibilities for standalone charged-particle detection.
Beam tests were performed at the CERN PS using FBK SiPMs of different area (~3×3 mm² and 1×1 mm²) and protection layer thicknesses (450-2450 μm). The analog waveforms were recorded with an oscilloscope, and the effect of the resin layer was clearly observed as a transition from single-SPAD to multi-SPAD (Single Photon Avalanche Diode) signals per particle. In this contribution, we present an overview of these measurements, focusing on efficiency, time resolution, and noise rejection.
Efficiencies above 99% were achieved, far beyond expectations from the device fill factor. The high photon yield also enabled strong noise suppression: applying a threshold equivalent to 2-3 times the single-SPAD signal effectively rejected dark counts (DC) while preserving >99% of true signals. Time resolutions better than 20 ps were obtained, with performance improving as more SPADs were fired.
These results demonstrate the feasibility of exploiting SiPMs as efficient, low-noise, and ultra-fast particle detectors without the need for an external radiator. This novel operating mode opens the possibility to realize compact TOF systems for high energy and space applications or combined TOF+RICH system using a single SiPM-based technology or providing timing layers in next generation calorimetry to improve the shower identification. However, these applications would imply that radiation tolerance aspects of SiPM must be better investigated looking also to new technologies.
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