  The measurement of environmental neutrons is particularly important in the
search for new physics, such as dark matter particles, because neutrons
constitute an often-irreducible background source. The measurement of the
neutron energy spectra in the sub-MeV scale is technically difficult because it
requires a very good energy resolution and a very high $\gamma$-ray rejection
power. In this study, we used a super-fine-grained nuclear emulsion, called
Nano Imaging Tracker (NIT), as a neutron detector. The main target of neutrons
is the hydrogen (proton) content of emulsion films. Through a topological
analysis, proton recoils induced by neutron scattering can be detected as
tracks with sub-micrometric accuracy. This method shows an extremely high
$\gamma$-ray rejection power, at the level of $5 \times 10^7 ~
\gamma/\rm{cm}^2$, which is equivalent to 5 years accumulation of environmental
$\gamma$-rays, and a very good energy and direction resolution even in the
sub-MeV energy region. In order to carry out this measurement with sufficient
statistics, we upgraded the automated scanning system to achieve a speed of 250
g/year/machine. We calibrated the detector performance of this system with 880
keV monochromatic neutrons: a very good agreement with the expectation was
found for all the relevant kinematic variables. The application of the
developed method to a sample exposed at the INFN Gran Sasso surface laboratory
provided the first measurement of sub-MeV environmental neutrons with a flux of
$(7.6 \pm 1.7) \times 10^{-3} \rm{cm}^{-2} \rm{s}^{-1}$ in the proton energy
range between 0.25 and 1 MeV (corresponds to neutron energy range between 0.25
and 10 MeV), consistent with the prediction. The neutron energy and direction
distributions also show a good agreement.
