Neutron fluence map for the rotating target neutron source

Nine nickel, niobium, and gold dosimetry foils incorporated in a neutron irradiation experiment at the Rotating Target Neutron Source (RTNS) facility were analyzed to construct a three-dimensional neutron fluence map. The purpose of the map was to determine the exact neutron fluences received by several pure element specimens irradiated for helium generation studies. However, the map alone provides the most detailed profile yet obtained of the RTNS neutron fluence distribution at the highest flux region accessible for irradiation experiments. This report gives details of the map, and provides a dosimetry procedure for use in future irradiations requiring accurate fluence information. The map also provides precise relative cross sections for the Ni, Nb, and Au dosimetry reactions for 14.8-MeV neutrons. The /sup 58/Ni(n,p)/sup 58/Co cross section is significantly lower than the current published evaluation. An analytical expression was derived to describe the measured fluence distribution; this was compared with an existing mathematical model of the RTNS neutron fluence profile. The model was found to represent approximately the fluence profile for a range of source parameters, but it cannot be used for an accurate determination of the fluence variation in a given irradiation experiment.