Dipole Collectivity in Nuclei and the Cluster Model

Abstract

Measurements on enhanced E1 deexcitation transitions in light and heavy nuclei are discussed. In light nuclei these appear only from core-excited cluster collective states that have large alpha particle widths (θ2α) as well as large B(E2) deexcitation widths. They suggest the presence of an α+14C eluster dipole degree of freedom as suggested by Iachello and Jackson. In heavy nuclei the enhanced B(E1) are found in the light Ra-Th isotopes where small alpha particle hindrance factors for low-lying JΠ=1- states and large ground state reduced alpha particle widths are also found. In addition, the B(E1) show a smooth dependence on J and on isotopic number N. A phenomenon reminiscent of backbending is observed for E1 deexcitations in 218Ra. The data suggest the presence of a new low-lying collective dipole mode in both heavy and light nuclei. In heavy nuclei this may reflect either cluster states or stable octupole. Analysis of the data on Ra isotopes in terms of the Vibron cluster model yields a resonable fit with only one variable parameter.