Isomers and oblate collectivity at high spin in neutron-rich Pt isotopes

Isomers and high-spin structures with rotation-aligned oblate configurations have been studied in several Pt isotopes. The 12(+) states in the even Pt isotopes from Pt192-198 are found to be metastable, and have (i(13/2))(2) neutron character. The progression of E2 transition probabilities from the 12(+) to 10(+) states across the Pt isotopic chain implies reduction in collectivity, followed by an abrupt decrease at N=120 (Pt-198). This behavior is quite distinct from the gradual decrease of B(E2) values near the respective ground states. A large contribution from aligned angular momentum, to the rotational sequences built on the 12(+) states, is visible. This is due to the relatively small crossing frequencies for nucleons in low-O orbitals at oblate deformation in comparison to higher values for prolate shapes. As a result, oblate rotation is found to be increasingly favored for higher neutron numbers.