YRAST SPECTROSCOPY OF THE N = 48 NUCLEUS KR-84

More than 20 new levels of 84Kr populated via the 82Se(α, 2nγ) reaction have been established up to spin J=14ℏ at an excitation energy of Ex = 7.65 MeV by in-beam γ-ray and conversion-electron spectroscopy. For 31 excited states the mean lifetimes have been measured by applying Doppler-shift and pulsed-beam γ-ray timing methods. The dominant structure of the Jπ = 12+ isomer at 5373 keV (T 1 2 = 44 ± 2 ns) has been inferred from the comparison of the experimental g-factor of g(12+) = +0.17 ± 0.02 with estimates for different 4qp configurations to be the stretched 4qp configuration π(f 5 2-1, p 3 2-1)ν(g 9 2-2). This interpretation of the 12+ state has been confirmed by shell-model calculations with 88Sr as the core. Some enhancement of the E2 strength observed for the 102+ → 82+ transition might point to the alignment of two g 9 2 protons. Most of the negative-parity states could be grouped into two band-like sequences, built on top of the two lowest-lying 5- states, with ΔJ = 2 and ΔJ = 1, respectively, and B(E2) values of about 10 W.u. Some arguments allow the two-neutron hole configuration ν(g 9 2-1, p 1 2-1) to be ascribed to the 51- state and the ΔJ = 2 level sequence, whereas for the 52- state and the ΔJ = 1 level pattern a two-proton configuration π(f 5 2-1 or p 3 2-1, g 9 2) might be predominant. The highest-lying levels of 84Kr observed up to spin 14ℏ at an excitation energy of 7.65 MeV were found to have odd parity and to be connected by fast M1 transitions. © 1990.