Observation of a γ-decaying millisecond isomeric state in ¹²⁸Cd₈₀

A new high-spin isomer in the neutron-rich nucleus ¹²⁸Cd was populated in the projectile fission of a ²³⁸U beam at the Radioactive Isotope Beam Factory at RIKEN. A half-life of T_1/2 = 6.3(8) ms was measured for the new state which was tentatively assigned a spin/parity of (15^-). The experimental results are compared to shell model calculations performed using state-of-the-art realistic effective interactions and to the neighbouring nucleus ¹²⁹Cd. In the present experiment no evidence was found for the decay of a 18⁺ E6 spin-trap isomer, based on the complete alignment of the two-neutron and two-proton holes in the 0h_11/2 and the 0g_9/2 orbit, respectively, which is predicted to exist by the shell model. (authors)