Angular momentum in the He-6+Bi-209 reaction deduced from isomer ratio measurements - art. no. 047601

The angular momentum distribution of the compound nucleus is a fundamental characteristic of the reaction dynamics and can provide insight into reacti ons involving neutron- or proton-rich projectiles. Specifically, following the fusion of He-6 with Bi-209 (at center-of-mass energies of 18 to 27 MeV) , At-212 is formed by the evaporation of three neutrons from the compound n ucleus. The decay process leaves the residual At-212 in either the ground s tate (J(pi) = 1(-), T-1/2 = 314 ms) or a metastable state (J(pi) = 9(-), T- 1/2 = 119 ms). The ratio of the number of residual At-212m to the total num ber of At-212 residual nuclei is sensitive to the original momentum distrib ution 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 i s greatly enhanced compared to model calculations. While the inclusion of c oupling to the neutron-transfer channels improves the agreement with the ob served cross-section data somewhat, the predicted isomer ratio then diverge s from the measured ratio.