THE ODD-A Ac NUCLEI — TESTS OF NUCLEAR MODELS

Abstract

The remarkable features of the spectra of the odd-A Ac isotopes from 215Ac (with 126 neutrons) through 231Ac (with 142 neutrons) serve as a sequence of examples for the application of a variety of nuclear models. As the shapes vary from spherical to spherical with strong octupole correlations to octupole-deformed, the corresponding symmetry breaking leads to increasingly complex spectra. The spectrum of 215Ac can be described in terms of the shell model configurations π(h9/2)4f7/2 and π(h9/2)4i13/2. However, even in this 126-neutron nucleus there is already a strongly enhanced E3 transition. The spectra of 217Ac and 219Ac can be interpreted in terms of the weak coupling of the h9/2 proton to the 216Ra and 218Ra cores, respectively. However, the existence of parity doublet bands (same spins but opposite parities) in 219At and the interleaving of positive and negative states in 218Ra clearly demonstrate the increasing importance of octupole deformation. Parity doublet bands are also present in the spectra of 221Ac. However, in both 219Ac and 221Ac the positive parity members of the parity doublets appear to cut off before reaching the low spin values expected from the negative parity bands. In 223Ac, 225Ac, and 227Ac the experimental spectra clearly show the existence of parity doublet bands built on the parity mixed configurations 5/2±(0.0; −0.2) and 3/2±(0.0; −0.3). The fact that the ordering of these two configurations changes for 223Ac results from decreasing quadrupole deformation. Other spectroscopic properties also confirm octupole deformation in these three isotopes. In 229Ac and 231Ac the only detailed but limited spectroscopic information comes from the (t, α) reaction on 230Th and 232Th. While it is consistent with the presence of parity doublets, much more detailed spectroscopic studies could give information about the possible coexistence of reflection asymmetry and reflection symmetry in these nuclei.