Extending the low-$Z$ "border'' of the $A=100$ region of deformation with precision mass spectrometry of ${}^{96-98}$Kr

The onset of collective nuclear behavior in the $N=60$, $A\sim 100$, region is examined through high-precision mass measurements of ${}^{96-98}$Kr, performed with the ISOLTRAP mass spectrometer at ISOLDE, CERN. Our results for ${}^{96-97}$Kr agree with previous measurements, with our new ${}^{97}$Kr Penning-trap mass value three times more precise. The mass value of ${}^{98}$Kr is measured for the first time. The new mass surface, together with comparisons to beyond-mean-field theoretical predictions, suggests that collectivity persists for the $Z=36$ isotopes, blurring the apparent ``low-$Z$ boundary’’ of this deformed region.