  Understanding reflective properties of materials and photodetection
efficiency (PDE) of photodetectors is important for optimizing energy
resolution and sensitivity of the next generation neutrinoless double beta
decay, direct detection dark matter, and neutrino oscillation experiments that
will use noble liquid gases, such as nEXO, DARWIN, DarkSide-20k, and DUNE.
Little information is currently available about reflectivity and PDE in liquid
noble gases, because such measurements are difficult to conduct in a cryogenic
environment and at short enough wavelengths. Here we report a measurement of
specular reflectivity and relative PDE of Hamamatsu VUV4 silicon
photomultipliers (SiPMs) with 50 micrometer micro-cells conducted with xenon
scintillation light (~175 nm) in liquid xenon. The specular reflectivity at 15
deg. incidence of three samples of VUV4 SiPMs is found to be 30.4+/-1.4%,
28.6+/-1.3%, and 28.0+/-1.3%, respectively. The PDE at normal incidence differs
by +/-8% (standard deviation) among the three devices. The angular dependence
of the reflectivity and PDE was also measured for one of the SiPMs. Both the
reflectivity and PDE decrease as the angle of incidence increases. This is the
first measurement of an angular dependence of PDE and reflectivity of a SiPM in
liquid xenon.
