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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.

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