Level structure in the transitional nucleus Fr215

Abstract

The level scheme of $^{215}\mathrm{Fr}$ is extended up to 55/2 $\ensuremath{\hbar}$ and 4.8 MeV excitation energy with the addition of 52 new $\ensuremath{\gamma}$-ray transitions. Previously established isomers and their half-lives, except for the ${47/2}^{+}$ state, are revisited. The discrepancy in the half-life of the ${39/2}^{\ensuremath{-}}$ state is resolved, and its half-life is revised to 11.4(14) ns. An overall good agreement is observed between the experimental results and the shell-model calculations performed using the CD-Bonn $NN$ interaction derived from the ${V}_{\text{low-}k}$ renormalization approach. A weak coupling of the odd proton to the even-even core is observed to account for the level structure at lower energies, which strongly resembles a decoupled nonrotational band. A new positive-parity sequence is also established which is observed to originate from the coupling of the ${i}_{13/2}$ proton at low excitation energy.