Measurement of the 9Be(n,2n) 8Be reaction cross section in the 9Be(d,n) thick-target neutron spectrum

Abstract

4He production has been measured by an isotopic dilution mass spectrometry method, for the irradiation of Be-metal samples by fast neutrons from the 9Be(d,n) source reaction, corresponding to 7-MeV deuterons incident on a thick Be-metal target. The 58Ni(n,p) 58g+mCo dosimetry reaction was employed as a reference standard for measurement of the neutron flux. The integral cross section for the 9Be(n,2n) 8Be(2 4He) reaction ( 8Be breaks up very quickly into two α-particles) was deduced by applying a correction of 9.4% to account for the 9Be(n,α) 6He reaction contribution to the total 4He yield from neutrons on Be. The experimental integral reaction cross-section ratio of 9Be(n,2n)2 4He to 58Ni(n,p) 58g+mCo in this spectrum was found to be 1.217, with an error of 3.5%. This is in good agreement with the corresponding calculated ratio of 1.161 which is based on an average of results obtained by considering four distinct representations of the 9Be(d,n) neutron spectrum and ENDF/B-VI values for the reaction cross sections. The scatter of the calculated results amounts to 0.5%. It is small because the shapes of the response functions, R( E) = ø( E) σ( E), for these two reactions in this neutron spectrum are very similar. The observed C/E is 0.954. This is very good agreement, considering the various uncertainties involved, including those for the differential cross sections from ENDF/B-VI. This integral measurement is sensitive to the differential reaction cross section in the range 3–6 MeV. The uncertainty attributable to the 58Ni(n,p) 58g+mCo reaction differential cross section in this range is estimated to be 3.5%. Therefore, the results of the present investigation indicate that ENDF/B-VI represents the 9Be(n,2n)2 4He reaction differential cross section quite well over this energy range.