Neutron activation experiments on chromium and tantalum in the NPI p-7Li quasi-monoenergetic neutron field

The work aims to make the study of neutron activation cross section data up to 35MeV neutron energy using the reaction 7Li(p,n). This reaction produces the high-energy quasi-monoenergetic neutrons with some tail to lower energies. The cross section data for neutron energies higher than 20MeV are not well known. The data are needed for the development of the International Fusion Material Irradiation Facility (IFMIF) to test materials for fusion power plants technology and for the comparison to nuclear reaction models as well. The variable-energy proton beam of NPI cyclotron is used for the production of the quasi-monoenergetic neutron field using the thin lithium target. The carbon backing for the Li target is utilized as the beam stopper. The stable operation of the source is achieved under cooling of the Li foil and the carbon backing separately. The system permits to produce neutron flux density of about 109 n/cm2/s in peak at 30MeV neutron energy. We used the alloy of 70% chromium and of 30% nickel instead of pure chromium foils because of bad mechanical properties of pure chromium material. The chromium and tantalum samples of 15mm in diameter and of 0.75mm thickness were activated. All investigated samples were stacked in packets containing the “reference” dosimetry foils of gold. The nuclear spectroscopy methods were used to obtain the activities of produced isotopes. Spectra at HPGe detectors were measured at different cooling times. The most important data obtained for neutron energies of 20 to 35MeV are described and discussed. The analysis of resulting specific activities and reaction rates is carrying out in terms of C/E ratio (E measured reaction rate, C the predicted value of this observable calculated for the neutron spectrum). The neutron flux and spectra across the Cr(+Ni) and Ta samples are calculated using the modified data of the 7Li(p,n) reaction measured by other authors. Activation cross section data for selected reactions of neutrons were taken from EAF-2005 library. The preliminary results are discussed.