Angular momentum in the6He+209Bireaction deduced from isomer ratio measurements

Abstract

The angular momentum distribution of the compound nucleus is a fundamental characteristic of the reaction dynamics and can provide insight into reactions involving neutron- or proton-rich projectiles. Specifically, following the fusion of ${}^{6}\mathrm{He}$ with ${}^{209}\mathrm{Bi}$ (at center-of-mass energies of 18 to 27 MeV), ${}^{212}\mathrm{At}$ is formed by the evaporation of three neutrons from the compound nucleus. The decay process leaves the residual ${}^{212}\mathrm{At}$ in either the ground state ${(J}^{\ensuremath{\pi}}{=1}^{\ensuremath{-}},$ ${T}_{1/2}=314$ ms) or a metastable state ${(J}^{\ensuremath{\pi}}{=9}^{\ensuremath{-}},$ ${T}_{1/2}=119$ ms). The ratio of the number of residual ${}^{212m}\mathrm{At}$ to the total number of ${}^{212}\mathrm{At}$ residual nuclei is sensitive to the original momentum distribution of the compound nucleus. The measured isomer ratio is consistent with that predicted by standard models. This agreement is observed even at the lower energies where the measured three neutron evaporation cross section is greatly enhanced compared to model calculations. While the inclusion of coupling to the neutron-transfer channels improves the agreement with the observed cross-section data somewhat, the predicted isomer ratio then diverges from the measured ratio.