  In the present article we describe the calibration measurements of the
compact neutron spectrometer at certified PTB neutron source. The spectrometer
is based on two diamond detectors enclosing the Li-6 neutron converter. The
incident neutron energy is measured through the sum of energies of the two
conversion reaction products: t and alpha. The prototype used in this work was
based on two 4x4 mm2, 100 um thick single crystal CVD commercial diamond
plates. The measurements were performed in the fission spectrum domain at five
different neutron energies varying from 0.3 up to 2 MeV. The obtained data
showed a very good neutron energy reconstruction, i.e. in agreement with its
reference values within statistical uncertainties or with deviation below 11
keV in absolute scale. The energy resolution of the spectrometer was found to
be about 100 keV (RMS) with a small (about 9 keV/MeV) rising trend towards
higher energies. The measured detection efficiency was found to be compatible
with expectations based on analytical calculations and on Geant4 simulations
within 3% statistical and 4% systematic precision.
