We are developing a novel high-brightness atomic beam,
comprised of a two-body exotic atom called muonium (M = μ
+ +
e
-), for next-generation atomic physics and gravitational
interaction measurements. This M source originates from a thin sheet
of superfluid helium (SFHe), hence diagnostics and later
measurements require a detection system which is operational in a
dilution cryostat at temperatures below 1 K. In this paper,
we describe the operation and characterization of silicon
photomultipliers (SiPMs) at ultra-low temperatures in SFHe
targets. We show the temperature dependence of the signal shape,
breakdown voltage, and single photon detection efficiency,
concluding that single photon detection with SiPMs below
0.85 K is feasible. Furthermore, we show the development of
segmented scintillation detectors, where 16 channels at 1.7 K
and one channel at 170 mK were commissioned using a
muon beam.
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