  Rare-event search experiments located on-surface, such as short-baseline
reactor neutrino experiments, are often limited by muon-induced background
events. Highly efficient muon vetos are essential to reduce the detector
background and to reach the sensitivity goals. We demonstrate the feasibility
of deploying organic plastic scintillators at sub-Kelvin temperatures. For the
NUCLEUS experiment, we developed a cryogenic muon veto equipped with wavelength
shifting fibers and a silicon photo multiplier operating inside a dilution
refrigerator. The achievable compactness of cryostat-internal integration is a
key factor in keeping the muon rate to a minimum while maximizing coverage. The
thermal and light output properties of a plastic scintillation detector were
examined. We report first data on the thermal conductivity and heat capacity of
the polystyrene-based scintillator UPS-923A over a wide range of temperatures
extending below one Kelvin. The light output was measured down to 0.8K and
observed to increase by a factor of 1.61$\pm$0.05 compared to 300K. The
development of an organic plastic scintillation muon veto operating in
sub-Kelvin temperature environments opens new perspectives for rare-event
searches with cryogenic detectors at sites lacking substantial overburden.
