Spectroscopic studies beyond the $N=152\mathrm{}$ neutron gap: Decay of ${}_{101}^{255}\mathrm{Md}$ and ${}_{101}^{256}\mathrm{Md}$

The isotopes ${}^{255}\mathrm{Md}$ and ${}^{256}\mathrm{Md}$ were produced by the irradiation of ${}^{253}\mathrm{Es}$ with 35–45 MeV $\alpha$ particles by $(\alpha ,n)$ and $(\alpha ,2n)$ reactions and were removed from the target by a helium jet system. $\alpha ,$ $\gamma ,$ and $\alpha$-$\gamma$ coincidence spectra were measured with Si and Ge(Li) detectors. From the EC decays of ${}^{255}\mathrm{Md}$ and ${}^{256}\mathrm{Md},$ levels in ${}^{255}\mathrm{Fm}$ and ${}^{256}\mathrm{Fm}$ were deduced. Favored $\alpha$ decay of ${}^{255}\mathrm{Md}$ was found to populate the ${7/2}^{-}\left[514\right]$ single-particle state in ${}^{251}\mathrm{Es},$ thus establishing the ${7/2}^{-}\left[514\right]$ as the ${}^{255}\mathrm{Md}$ ground state. Several $\gamma$ rays were observed in the ${}^{256}\mathrm{Md}$ $\alpha$-$\gamma$ coincidence spectrum. ${}^{256}\mathrm{Fm}$ is the heaviest nucleus in which excited intrinsic states have been identified.