Effects of deformation on neutron total cross sections of even-ANd and Sm isotopes

Abstract

Neutron total cross sections of $^{144}\mathrm{Nd}$ and $^{148}\mathrm{Sm}$ and total cross-section differences for the pairs of isotopes $^{144,142}\mathrm{Nd}$, $^{146,144}\mathrm{Nd}$, $^{148,144}\mathrm{Nd}$, $^{150,144}\mathrm{Nd}$, $^{150,148}\mathrm{Sm}$, $^{152,148}\mathrm{Sm}$, and $^{154,148}\mathrm{Sm}$ have been measured at 25 energies from 0.75 to 14 MeV. Each sample consisted of about 40 g of powdered oxide with an isotopic enrichment >94%. For each isotopic pair the total cross-section difference was determined directly by measuring the ratio of transmitted flux for matched samples. This difference was found to have an oscillatory behavior when plotted as a function of incident neutron energy. The oscillations have been explained as a deformation effect qualitatively in terms of the nuclear Ramsauer effect and quantitatively using a deformed optical model potential. Adding two neutrons to a nucleus in this mass region was found to have a different effect on the neutron total cross section than adding two protons. A coupled channel analysis was performed using as input data the present total cross sections, low-energy neutron scattering parameters, and elastic and inelastic (${2}^{+}$) differential cross sections for neutron scattering at 4 and 7 MeV. Excellent total cross-section fits were obtained for the Sm isotopes; however, the total cross-section fits for the Nd isotopes were only fair. Quadrupole deformation parameters have been deduced for all the isotopes studied. The sensitivity of neutron total cross-section differences to this parameter and to the real isospin term of the nuclear potential is discussed.NUCLEAR REACTIONS $^{142,144,146,148,150}\mathrm{Nd}$, ${E}_{n}=0.75\ensuremath{-}13.89$ MeV, $^{148,150,152,154}\mathrm{Sm}$, ${E}_{n}=0.75\ensuremath{-}14.49$ MeV; measured ${\ensuremath{\sigma}}_{n,\mathrm{tot}}(E)$. Enriched targets. Deduced coupled-channel optical-potential parameters, quadrupole deformation parameters.