  Environmental neutrons are a source of background for rare event searches
(e.g., dark matter direct detection and neutrinoless double beta decay
experiments) taking place in deep underground laboratories. The overwhelming
majority of these neutrons are produced in the cavern walls by means of
intrinsic radioactivity of the rock and concrete. Their flux and spectrum
depend on time and location. Precise knowledge of this background is necessary
to devise sufficient shielding and veto mechanisms, improving the sensitivity
of the neutron-susceptible underground experiments. In this report, we present
the design and the expected performance of a mobile neutron detector for the
LNGS underground laboratory. The detector is based on capture-gated
spectroscopy technique and comprises essentially a stack of plastic
scintillator bars wrapped by gadolinium foils. The extensive simulation studies
demonstrate that the detector will be capable of measuring ambient neutrons at
low flux levels ($\sim$$10^{-6}\,\mathrm{n/cm^2/s}$) at LNGS, where the ambient
gamma flux is by about 5 orders of magnitude larger.
