Influence of octupole vibration on the low-lying structure of 251Fm and other heavy N=151 isotones

The structure of low-lying excited states in Fm-251, populated by the alpha decay of No-255, has been investigated by means of combined gamma 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) x 10(-3) W.u. These results, as well as the results of previous studies in N = 151 isotopes, are compared to theoretical calculations beyond themean-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.