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 ^192-198Pt 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 (¹⁹⁸Pt). 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-Ω 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. (authors)