Data Discovery and Physical Mechanism of the Stability Critical Point in the Heavy Nuclide Region: The Precipitous Decline at N/Z=1.55 and the Emergent Upper Limit of Encrypted Chain Containment Capacity

Whether there exists a definite critical N/Z value for the stability of heavy nuclides has remained an open question. 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 Chow test (p<0.000001). Before this critical point, α-decay dominates absolutely; after it, β⁻ decay surges to dominance and the spontaneous fission channel is forcibly opened. This paper proposes that the physical origin of this critical value lies in the emergent upper limit of encrypted chain containment capacity within the Encrypted Chain Fracture Model—when N/Z exceeds this threshold, the proton encrypted chain network can no longer contain all neutrons, triggering a global switch in decay modes. The emergence of magic numbers and the existence of the superheavy stability island can be unified within the same physical framework. All analyses use publicly available data and standardized statistical methods and are independently reproducible.