Critical-Point Boundary for the Nuclear Quantum Phase Transition Near A=100 from Mass Measurements of 96,97Kr

Mass measurements of Kr-96,Kr-97 using the ISOLTRAP Penning-trap spectrometer at CERN-ISOLDE are reported, extending the mass surface beyond N = 60 for Z = 36. These new results show behavior in sharp contrast to the heavier neighbors where a sudden and intense deformation is present. We interpret this as the establishment of a nuclear quantum phase transition critical-point boundary. The new masses confirm findings from nuclear mean-square charge-radius measurements up to N = 60 but are at variance with conclusions from recent gamma-ray spectroscopy.