Just Beyond the S-Process Termination Point: Nucleosynthesis of Lead–Bismuth Cyclic Reactions

We examine cyclic nuclear reactions in the lead–bismuth (Pb–Bi) system near the s-process termination point. We present a numerical investigation of the isotopic evolution and decay heat generation in an extended 60-isotope nuclear reaction network under continuous 30 keV neutron irradiation (1014–1028 n cm−2 s−1) using the Chebyshev Rational Approximation Method (CRAM). The network accounts for 88 transitions, utilising a hybrid data approach that combines neutron capture cross-sections from EAF-2010 and TALYS with fundamental decay properties from the ENDF/B-VIII.0. Our simulations reveal two distinct evolutionary regimes. At moderate fluxes (1014–1020 n cm−2 s−1), the system establishes a steady cyclic loop driven by the α-decay of Po210, successfully reproducing the s-process termination isotopic distribution (Pb208≫209Bi>207Pb>206Pb), characteristic of low-metallicity AGB stars. As the flux exceeds 1022 n cm−2 s−1, the classical balance breaks down, propelling mass flow toward heavier trace isotopes and suggesting a potential transition into the intermediate neutron capture (i-process) regime. Heat density calculations demonstrate that while the energy release of the core cycle plateaus near 105 W cm−3, the extended chain drives an energy surge to over 108 W cm−3 at 1024 n cm−2 s−1 before the system enters an unstable transient state at extreme fluxes.