Microsecond and nanosecond isomers populated in fission reactions

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, I^π=(25/2⁺) and 2434 keV, I^π=19/2^- states have measured half-lives of T_1/2=200(30) μs and 8.2(2) ns respectively. The 2614+Δ keV, I^π=(27/2⁺) and 2486 keV, I^π=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 πd_5/2 or πg _7/2 configuration, in ¹²¹Sb and ¹²³Sb respectively, coupled to aligned (h_11/2)² neutrons. The I^π=19/2^- isomeric state in ¹²¹Sb is proposed to have a νh_11/2 ⊗ νd_3/2 ⊗ 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 g_7/2) ⊗π(g_9/2)² configuration which is observed systematically in the even-Z N=57 isotones.