Experimental techniques and computational methods used at the Frascati neutron generator in order to determine the neutron source spectrum

FNG is a deuteron electrostatic accelerator o of beam energy 260 KeV producing 1011 neutrons/s with energy 14 MeV from m T(d,n)α or 109 neutrons/s with energy 2.5 MeV from D(d,n)3He reactions. The absolute neutron intensity at FNG is measured by the associated particle method; accuracy is better than 5%. The neutron source spectrum is calculated using an accurate source routine which is compiled inside MCNP5 transport code. This routine takes into account all the physical aspects of the neutron production from beam-target interaction, including ions scattering, fusion cross-section anisotropy, etc. The FNG source routine is now included in the SINBAD database, where several benchmark experiments performed at FNG are available. The source routine can be compiled inside MCNP5. In the MCNP geometry the target holder and the used detectors dimensions and materials are also described. The latest version of FNG source routine, not yet distributed, works for deuteron beam energy up to 10 MeV and for thick or thin, solid or gas target options. The source routine has been recently validated for FNG with accurate neutron spectroscopy measurements using diamond detectors. Diamond detectors have been recently demonstrate to be a very high energy resolution fast neutron spectrometer for the energy range 8-20 MeV (FWHM 0.4% constant in the energy range). Validation of the FNG source routine is also in progress using the IRMM Van-der-Graaff of Geel (Belgium) for deuteron beam energy up to 5 MeV. Other mature source routine codes have been developed by other laboratories, i.e. TARGET code from PTB or NeuSDesc form JRC-Geel and they permit to obtain accurate calculated source spectra. Spectra in the whole energy range are measured at FNG using activation foil unfolding techniques. The first guess input spectrum is calculated u using MCNP5+source routine.