The products of the ${}^{203,205}\mathrm{Tl}({}^{50}\mathrm{Ti},2n)$ fusion-evaporation reactions were studied using the recently commissioned Argonne Gas-Filled Analyzer at Argonne National Laboratory. Two $\alpha$-decay activities with energies of 9210(19) and 9246(19) keV and half-lives of ${42}_{-14}^{+42}$ and $24.4_{-4.5}^{+7.0}$ ms were observed which were followed by the known $\alpha$ decays of ${}^{247}\mathrm{Md}$ and ${}^{243}\mathrm{Es}$. They are interpreted as originating from the $1/2^{-}\left[521\right]$ and $7/2^{-}\left[514\right]$ single-proton Nilsson states in the hitherto unknown isotope ${}^{251}\mathrm{Lr}$. From the measured $Q_{\alpha }$ values the $1/2^{-}$ level was placed 117(27) keV above the $7/2^{-}$ level in ${}^{251}\mathrm{Lr}$ in contrast to ${}^{255}\mathrm{Lr}$ where the $1/2^{-}$ level is the lowest. Also, the $\alpha$ decay of ${}^{253}\mathrm{Lr}$ was studied in more detail and a new $\alpha$ line at 8660(20) keV was found and a new half-life value of 2.46(32) s for an isomeric state in ${}^{253}\mathrm{Lr}$ was measured. The ${}^{251,253,255}\mathrm{Lr}{Q}_{\alpha }$ values were compared with predictions of various mass models. The relative energies of the $1/2^{-}\left[521\right]$ and $7/2^{-}\left[514\right]$ single-proton Nilsson states in ${}^{251,253,255}\mathrm{Lr}$ isotopes were compared with results of the cranking shell model with pairing treated using the particle-number-conserving method. The level separation and, in particular, the level order change between ${}^{251}\mathrm{Lr}$ and ${}^{255}\mathrm{Lr}$ was reproduced only when the hexacontetrapole deformation ${\varepsilon }_6$ was included in the calculations.

• Received 30 June 2022
• Accepted 11 October 2022

DOI:https://doi.org/10.1103/PhysRevC.106.L061301