The structure of low-lying excited states in ${}^{251}\mathrm{Fm}$, populated by the $\alpha$ decay of ${}^{255}\mathrm{No}$, has been investigated by means of combined $\gamma$ and internal conversion electron spectroscopy. The values for the internal conversion coefficients for the $1/2^{+}\to 5/2^{+}$ and $5/2^{+}\to 9/2^{-}$ 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 $B\left(E3\right)=18\left(6\right)$ W.u. and $B\left(M2\right)=3.0\left(6\right)\times {10}^{-3}$ 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.

• Received 6 March 2018

DOI:https://doi.org/10.1103/PhysRevC.97.054332