SURPRISING FEATURES OF SIMPLE NUCLEAR SYSTEMS JUST ABOVE ~(132)Sn

摘要

The Advanced Time-Delayed Method was used to measure level lifetimes in ~(134)Sb, ~(135)Sb and ~(136)Te populated in the β decay of ~(134)Sn, βn decay of ~(136)Sn and via a chain of β decays from ~(136)Sn→~(136)Sb→~(1360Tc, respectively. High purity Sn beams were extracted at the ISOLDE separator at CERN using a novel production technique utilizing the molecular SnS~+ ions to isolate Sn from a strong isobaric contamination coming from less exotic fission products. In ~(134)Sb we have measured (all results reported below are preliminary) the lifetime of the 383 keV state as T_(1/2)26(5) ps and branchings for weak γ-rays de-exciting the 383 and 330 keV states. The latter were found much smaller than previously reported. Our value of B(M1;3-2~-) = 2.0(4) μ_N~2 is one of the fastest known MI rates at low excitation energy in all nuclei. In ~(135)Sb we have identified the missing 1/2~+ state at the energy of ?23 keV. From its lifetime, T_(1/2)=1.24(6) ns, we have determined a very collective B(E2;1/2+→5/2~+) = 13.0(11) W.u. We have measured lifetime for the 2_1~+ state in ~(136)Te, T_(1/2) = 41.6(84) ps, which implies a B( E2) value higher by ~20% than the one previously reported. Results of shell model calculations are presented and compared with experimental data.