  The NUCLEUS experiment aims to measure coherent elastic neutrino nucleus
scattering of reactor anti-neutrinos using cryogenic calorimeters. Operating at
an overburden of 3 m.w.e., muon-induced backgrounds are expected to be one of
the dominant background contributions. Besides a high efficiency to identify
muon events passing the experimental setup, the NUCLEUS muon veto has to
fulfill tight spatial requirements to fit the constraints given by the
experimental site and to minimize the induced detector dead-time. We developed
highly efficient and compact muon veto modules based on plastic scintillators
equipped with wavelength shifting fibers and silicon photo multipliers to
collect and detect the scintillation light. In this paper, we present the full
characterization of a prototype module with different light read-out
configurations. We conclude that an efficient and compact muon veto system can
be built for the NUCLEUS experiment from a cube assembly of the developed
modules. Simulations show that an efficiency for muon identification of >99 %
and an associated rate of 325 Hz is achievable, matching the requirements of
the NUCLEUS experiment.
