Low-energy neutron direct capture by12Cin a dispersive optical potential

Abstract

A dispersive optical potential for the interaction between low-energy neutrons and ${}^{12}\mathrm{C}$ nuclei is derived from a dispersion relation based on the Feshbach generalized optical model. The potential reproduces completely neutron total cross sections below 1.0 MeV and substantially reproduces the energy of the $3090\mathrm{keV}{(1/2}^{+})$ level in ${}^{13}\mathrm{C}$ which is of nearly pure ${2s}_{1/2}$ single-particle character. It is found that direct-capture model calculations with this potential explain quite successfully the observed off-resonance capture transitions to the ground ${(1/2}^{\ensuremath{-}}),$ $3090\mathrm{keV}{(1/2}^{+}),$ $3685\mathrm{keV}{(3/2}^{\ensuremath{-}}),$ and $3854\mathrm{keV}{(5/2}^{+})$ levels in ${}^{13}\mathrm{C}$ at neutron energies of 20--600 keV. Special emphasis is laid on the fact that in these model analyses, account should be taken of the spatial nonlocality of the neutron-nucleus interaction potential, in particular for negative energies.