The onset of collective nuclear behavior in the ${ N=60}$, ${ A\sim100}$, 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.
Categories: nucl-ex