A Statistical Discovery of a Stability Critical Point at N/Z = 1.55 in Heavy Nuclides

Whether the stability of heavy nuclides undergoes a smooth transition with increasing neutron number, or whether a definite critical N/Z value exists, has remained an open question in nuclear physics. The shell model explains additional stability at local magic numbers, and the liquid drop model describes the overall trend of nuclear stability evolving smoothly with N/Z, but neither predicts a global stability cliff and decay mode switching at a specific N/Z value. Using experimental data for 577 heavy nuclides (Z > 82, N > 126) from the ENDF/B-VIII.0 database, this paper identifies a statistically highly significant stability cliff at N/Z = 1.55 through piecewise linear regression and the Chow test (p < 0.000001). Before this critical point, alpha-decay dominates absolutely; after it, beta- decay surges to dominance and the spontaneous fission channel is forcibly opened. The same critical value is cross-validated across five independent dimensions: the precipitous decline in logarithmic half-life, the forced switching of decay modes, systematic differences in half-life distributions, the opening conditions of the spontaneous fission channel, and robustness to the choice of analysis parameters. The existence of this structural break is a purely empirical finding derived from publicly available data and standardized statistical methods, independent of any theoretical framework. A possible physical interpretation in terms of nucleon correlation network saturation is briefly discussed.