Fast neutron reaction rate and spectrum measurement

The results of the first series of experiments performed in the neutron spectrum generated by the converter facility NISUS are presented. The neutron spectrum inside the facility was measured by means of the foil activation technique. Absolute reaction-rates of 8 threshold detectors and several capture reactions were measured. Irradiated foils were gamma counted on a calibrated Ge(Li) detector and their absolute activities evaluated. The fast fission ratio was measured inside the facility by both the integral counting technique and the La-140 counting technique. The absolute reaction-rates were used with an average set of differential cross-section data to determine the fast neutron spectrum. Three unfolding techniques have been employed and the resultant unfolded spectra are compared. Only threshold detectors were used for the spectrum unfolding and hence the resultant spectrum is valid above 1 MeV. The agreement between different techniques is assessed and the final spectrum envelope has a spread of ±12% at low energies (1 MeV to 3 MeV) decreasing to ±6% at middle energies and ±9% at high energies (above 8 MeV). The results are compared to theoretical predictions and also to the results of spectrum measurements using Li-6 sandwich detectors. A comparison is also made between the experimental spectral indices obtained in NISUS and those obtained in a similar facility.