Neutron shell effects in the (n, 2n) cross-sections at 14 MeV

Abstract

The systematics of the available total (n, 2n) reaction cross-sections for 14 MeV neutrons have been investigated for even-neutron target nuclei. The existence of neutron shell effects is found and discussed on the background of the statistical theory. Minima in the (n, 2n) cross-sections at the magic neutron numbers N=28 and 50 for even and N=20 for odd-proton target nuclei can be accounted for by similar trends in the Q values. Cross-section maxima at the closed neutron shells N=82 and 126 for even and N = 28, 50, 82 and 126 for odd-proton target nuclei can be understood as an effect of shell closure on the nuclear level density, the relative decrease of which on passing across a closed shell is larger for higher excitation energies. The (n, n′γ) reaction is the principal one competing with the (n, 2n) process at 14 MeV in the medium and heavy mass regions. Since for the (n, n′γ) reaction the residual excitation energies lies in the region up to 12 MeV, whereas for the (n, 2n) reaction, it does not exceed 4 MeV, and since the relative change in the level density is greater at the higher excitation energies, the competition between (n, n′γ) and (n, 2n) reactions favours the latter in the region of closed neutron shells and thus accounts for the observed maxima in the (n, 2n) cross-section systematics.