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 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. This paper provides a new answer to this question, starting from the Encrypted Chain Fracture Model. The core mechanism of this model is that neutrons within an atomic nucleus must be contained by the proton encrypted chain network in order to exist stably. The containment capacity of the encrypted chains has an upper limit determined by geometric arrangement and symmetric pairing constraints, from which it can be semi-quantitatively estimated that this upper limit falls within the N/Z≈1.5 to 1.6 interval. Using experimental data for 577 heavy nuclides (Z>82, N>126) from the ENDF/B-VIII.0 database, a statistically highly significant stability cliff is identified at N/Z=1.55 through piecewise linear regression and the 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. The emergence of magic numbers and the existence of the superheavy stability island can be explained within the unified physical framework of encrypted chain nested closure. The theoretical foundation of the Encrypted Chain Fracture Model—the stable configuration of sealed nodes in constraint network dynamics—has been rigorously established through mathematical theorems in independent work. All analyses in this paper use publicly available data and standardized statistical methods and are independently reproducible.