Determination of a Deuteron-Beryllium Neutron Source Spectrum by Multifoil Activation

The spectral neutron flux from a deuteron-beryllium neutron source, which is driven by a 19-MeV cyclotron deuteron beam and serves mainly for integral activation tests of fusion reactor structural materials, was determined by the multifoil activation method. Twenty-two selected threshold activation reactions were employed. An initial guess spectrum calculated by a Monte Carlo simulation was adjusted using the SAND-II code to be consistent with the measured reaction rates. The total neutron flux averaged over a 5 x 5 mm sample was found to be 2.52 x 10¹¹ n/scm^-2 at 10 A of deuteron beam with uncertainty of ~10%. The activation cross sections used were based on the FENDL/A-2.0 library. However, they were extrapolated beyond 20 MeV, the upper energy limit of that library, to the maximum neutron energy of 23.4 MeV and were modified where necessary. As a result, a self-consistent set of activation cross sections was obtained, which may also be used for the characterization of other neutron fields. The determined spectral neutron flux is to be used for analyses of integral activation tests of fusion reactor-relevant materials.