The structure of low-lying excited states in 251 Fm, populated by the α decay of 255 No, has been investigated by means of combined γ and internal conversion electron spectroscopy. The values for the internal conversion coefficients for the 1/2 + → 5/2 + and 5/2 + → 9/2 − transitions have been measured. The determined M2/E3 mixing ratio and lifetime for the 5/2 + decay to the ground state allowed to determine the corresponding reduced transitions strengths of B(E3) = 18(6) W.u. and B(M2) = 3.0(6) × 10 −3 W.u. These results, as well as the results of previous studies in N = 151 isotopes, are compared to theoretical calculations beyond the mean-field approach, including the first QRPA calculations using the Gogny D1M parametrization for such heavy odd-N nuclei. The comparison points to the importance of accounting for the octupole vibrations for a proper understanding of the low-lying nuclear structure of some of the heaviest elements.
Decay spectroscopy of 250 No has been performed using digital electronics and pulse-shape analysis of the fast nuclear decays for the first time. Previous studies of 250 No reported two distinct fission decay lifetimes, related to the direct fission of the ground state and to the decay of an isomeric state but without the possibility to determine if the isomeric state decayed directly via fission or via internal electromagnetic transitions to the ground state. The data obtained in the current experiment allowed the puzzle to finally be resolved, attributing the shorter half-life of t 1/2 = 3.8 ± 0.3 μs to the ground state and the longer half-life t 1/2 = 34.9 +3.9 −3.2 μs to the decay of an isomeric state. 250 No becomes, thus, one of a very few examples of very heavy nuclei with an isomeric state living considerably longer than its ground state. This phenomenon has important consequences for the nuclear-structure models aiming to determine the borders of the island of stability of superheavy elements.
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