Calibration measurements with the multisphere method and neutron spectrum analyses using the Sand-II program

The multisphere method, a neutron detection technique, is used in combination with the Sand-II unfolding program for spectrum analyses. A 6LiI crystal is used as detector, placed in the centre of 15 spheres with diameters in the range of 0.05-0.45 m. The spheres were calibrated in the energy range from 100 keV-4 MeV with monoenergetic neutrons which were generated with a Van de Graaff accelerator. The count rates are corrected for the contribution of scattered neutrons. For the non-measured energies the efficiencies are derived from values given in the literature. For testing, the neutron spectra of an 241Am/Be and a 251Cf source were determined. The technique appeared to be suitable for the determination of the coarse structure of neutron energy spectra even at low flux densities of the order of 105 m-2 s-1. (orig.)