The mass surface in the $A\sim 100$ region of the nuclear chart is extended by the measurement of the ${}^{98–100}$Rb isotopes with the Penning-trap mass spectrometer ISOLTRAP at ISOLDE/CERN. The mass of ${}^{100}$Rb is determined for the first time. The studied nuclides mark the known low-$Z$ frontier of the shape transition at $N=60$. To describe the shape evolution towards the krypton isotopic chain, a theoretical analysis is presented in the framework of the Hartree-Fock-Bogoliubov approach. The importance of the pairing interaction for describing the extent and strength of the region of quadrupole deformation is emphasized. A later transition to large prolate deformation or, alternatively, the predominance of oblate deformation is proposed as explanation for the different behavior of the krypton isotopes. Octupole collectivity is explored as a possible mechanism for the evolution of two-neutron separation energies around $N=56$.

• Received 1 September 2013

DOI:https://doi.org/10.1103/PhysRevC.88.054322