Microsecond and nanosecond isomers populated in fission reactions

Abstract

Fusion-fission reactions were induced by bombarding a thick 27Al target with 178Hf projectiles at a laboratory energy of 1150 MeV using the ATLAS accelerator at Argonne National Laboratory. The subsequent γ-ray decays were measured using the GAMMASPHERE germanium detector array. The beam was pulsed at two different ON/OFF cycles of 82.5/825 ns and 25/75 μs in order to observe the γ rays from the decay of isomeric states. In 121Sb 2721+Δ keV, Iπ=(25/2+) and 2434 keV, Iπ=19/2- states have measured half-lives of T1/2=200(30) μs and 8.2(2) ns respectively. The 2614+Δ keV, Iπ=(27/2+) and 2486 keV, Iπ=19/2 + states in 123Sb have measured half-lives of T 1/2=52(3) μs and 7.9(4) ns respectively. The positive parity isomers in these nuclei correspond to a πd5/2 or πg 7/2 configuration, in 121Sb and 123Sb respectively, coupled to aligned (h11/2)2 neutrons. The Iπ=19/2- isomeric state in 121Sb is proposed to have a νh11/2 ⊗ νd3/2 ⊗ πd 5/2 configuration. A previously unobserved isomer has been identified in 99Mo at an energy of 3010 keV, decaying with T 1/2=18(5) ns. This state is interpreted as an energetically favoured 3 quasi-particle alignment of ν(d5/2 g7/2) ⊗π(g9/2)2 configuration which is observed systematically in the even-Z N=57 isotones. Show more