Extending the low- Z “border” of the A=100 region of deformation with precision mass spectrometry of Kr96−<…

The onset of collective nuclear behavior in the $N=60$, A≈100 region is examined through high-precision mass measurements of ^96-98Kr, performed with the ISOLTRAP mass spectrometer at ISOLDE, CERN. Our results for ^96-97Kr agree with previous measurements, with our new ⁹⁷Kr Penning-trap mass value being three times more precise. The mass value of ⁹⁸Kr 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.