  In the frame of direct dark matter search, the fast neutrons producing
elastic collisions are the ultimate background. The MIMAC (MIcro-tpc MAtrix
Chambers) project has developed a directional detector providing the
directional signature to discriminate them based on 3D nuclear tracks
reconstruction. The MIMAC team of the LPSC has adapted one MIMAC chamber as a
portable fast neutron spectrometer, the Mimac-FastN detector, having a very
large neutron energy range (10 keV - 600 MeV) with different gas mixtures and
pressures. The present paper shows its main features and functionality and
demonstrates its potential in the energy range from 1 MeV to 15 MeV at the
GENESIS neutron source facility of LPSC.
