A Comprehensive Empirical Study of the Critical Point at N/Z = 1.55 in Heavy Nuclides: Five Independent Dimensions of Evidence

Whether a universal critical N/Z value governs the stability and decay modes of all heavy nuclides has remained an open question. Using public nuclear databases (ENDF/B-VIII.0, NuDat 3.0, ENSDF), this paper identifies N/Z = 1.55 as such a critical point through five independent empirical analyses. (1) Piecewise linear regression and the Chow test on 577 heavy nuclides reveal a highly significant structural break in logarithmic half-life at N/Z = 1.55 (p < 0.000001). (2) The doubly magic 208Pb chain and two non-magic control chains (At, Fr) demonstrate that shell stabilization fails universally once N/Z exceeds 1.55. (3) A survey of 32 nuclides with competing alpha and beta- decay channels shows a rapid phase transition in decay mode dominance at N/Z = 1.55. (4) In the supercritical region, beta-dominated half-lives obey a power-law scaling T1/2 proportional to (N - 1.55Z)^(-gamma) with gamma approximately 6.5, verified across four isotopic chains. (5) Significant spontaneous fission (branching ratio > 0.1%) occurs exclusively at N/Z no less than 1.55, growing exponentially above this threshold. These five independent lines of evidence establish N/Z = 1.55 as a universal structural break subordinating local shell effects. A possible physical interpretation in terms of nucleon correlation network saturation is briefly discussed.