Mass measurements of 96;97 Kr using the ISOLTRAP Penning-trap spectrometer at CERN-ISOLDE are reported, extending the mass surface beyond N ¼ 60 for Z ¼ 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 ¼ 60 but are at variance with conclusions from recent gamma-ray spectroscopy. DOI: 10.1103/PhysRevLett.105.032502 PACS numbers: 21.10.Dr, 64.70.Tg, 74.40.Kb Dynamical symmetries are an enlightening paradigm for describing the stucture of matter. Iachello and Arima have elegantly described the application of dynamic symmetries in nuclear physics by developing the Interacting Boson Model (IBM) [1]. The IBM allowed the classification of nuclear spectra in terms of U(6) group theory and predicted the occurrence of three dynamical symmetries: U(5), SU(3), and SO(6) [2]. More recent attempts to enlarge these concepts have resulted in the elaboration of criticalpoint symmetries, notably for nuclear shapes [3,4]. These are particularly interesting since they apply to interactions that are discontinuous and describe phenomena in terms of quantum phase transitions. Whereas classical phase transitions follow changes in temperature and pressure, the quantum phase transitions of nuclear shapes occur when neutrons and protons change their orbit occupation in the nucleus.The IBM formalism describes the ground-state band of a deformed nucleus and in the pure SU (3) Linking the atomic nucleus with dilute gases of ultracold atoms thus offers interesting possibilities for furthering our understanding of nuclear structure. Indeed, the ideas of phase transitions and condensates have been extended to fermionic matter in the form of an -particle condensation [6], which has profound implications in nucleosynthesis. Another example is the description of halo nuclides, where the alpha phase transition is favored over pairing in the dilute nuclear halo [7]-a striking parallel to Bose-Einstein condensates in dilute gases.Nuclear quantum phase transitions are linked to groundstate binding energies within the framework of the IBM [1] (for more detailed discussion in the experimental context, see also [8][9][10]). The ground-state binding energy reflects the net result of all interactions at work in the nucleus and its minimization is decisive for the occupational sequence of the nuclear orbitals. As such, nuclear deformation is highly visible from deep indentations of the mass surface. These features are the manifestations of a first-order quantum phase transition with the critical points defining where the phase transition starts and where it ends [11].In this Letter, we report new mass measurements in the Z ¼ 40 and N ¼ 60 region of the nuclear chart, where one of the most remarkable examples of nuclear shape transition (both for its intensity and its suddenness) is ...