MEASUREMENT OF THE BE-9(N,2N)BE-8 REACTION CROSS-SECTION IN THE BE-9(D,N) THICK-TARGET NEUTRON-SPECTRUM

He-4 production has been measured by an isotopic dilution mass spectro metry method, for the irradiation of Be-metal samples by fast neutrons from the Be-9(d,n) source reaction, corresponding to 7-MeV deuterons incident on a thick Be-metal target. The Ni-58(n,p)Co-58g+m dosimetry reaction was employed as a reference standard for measurement of the n eutron flux. The integral cross section for the (Be(n,2n)Be(2He))-Be-9 -Be-8-He-4 reaction (Be-8 breaks up very quickly into two a-particles) was deduced by applying a correction of 9.4% to account for the (Be(n ,alpha)He)-Be-9-He-6 reaction contribution to the total He-4 yield fro m neutrons on Be. The experimental integral reaction cross-section rat io of Be-9(n,2n)2He-4 to Ni-58(n,p)Co-58g+m in this spectrum was found to be 1.217, with an error of 3.5%. This is in good agreement with th e corresponding calculated ratio of 1.161 which is based on an average of results obtained by considering four distinct representations of t he Be-9(d,n) neutron spectrum and ENDF/B-VI values for the reaction cr oss sections. The scatter of the calculated results amounts to 0.5%. I t is small because the shapes of the response functions, R(E) = phi(E) sigma(E), for these two reactions in this neutron spectrum are very si milar. The observed C/E is 0.954. This is very good agreement, conside ring the various uncertainties involved, including those for the diffe rential cross sections from ENDF/B-VI. This integral measurement is se nsitive to the differential reaction cross section in the range 3-6 Me V. The uncertainty attributable to the Ni-58(n,p)Co-58g+m reaction dif ferential cross section in this range is estimated to be 3.5%. Therefo re, the results of the present investigation indicate that ENDF/B-VI r epresents the Be-9(n,2n)2He-4 reaction differential cross section quit e well over this energy range.