Title : PH-dependent recoil stabilization in Iodates following the (n,gamma) process
Abstract:
The neutron capture reaction ¹²⁷I(n,γ)¹²⁸I offers a direct route to no-carrier-added ¹²⁸I, yet the chemical state of the product is dictated not by nuclear physics alone but by the subsequent hot-atom chemistry of the recoil fragment. In this work, we present a mechanistic evaluation of the pH-dependent chemical fate of ¹²⁸I recoil atoms generated in crystalline and aqueous iodate systems. De-excitation of the compound nucleus proceeds through prompt γ-ray emission, whose momentum transfer imparts recoil energies of approximately 100–300 eV to the daughter atom. Because this energy far exceeds the I–O bond dissociation energy, the recoiling ¹²⁸I atom emerges with its parent bonding environment completely disrupted, initiating a Szilard–Chalmers-type fragmentation cascade. Crucially, however, the experimentally observed retention, the fraction of ¹²⁸I recovered as iodate, is not fixed by the recoil event itself; rather, it is governed by the pH-mediated redox environment encountered during thermalization and throughout the subsequent diffusive lifetime of the fragment.
We analyze the reaction kinetics of the principal post-recoil fragments; atomic iodine (I⁰), iodide (I⁻), hypoiodite (IO⁻), and iodite (IO₂⁻) as explicit functions of hydronium and hydroxide ion concentrations. The recoil atom sheds its excess energy in discrete stages, traversing a sequence of charge and oxidation states before reaching thermal equilibrium. In crystalline matrices, thermalization proceeds within the constrained lattice cage, permitting partial recombination and annealing, whereas rapid solvation in aqueous media exposes fragments to bulk acid–base chemistry and renders retention acutely sensitive to pH.
In strongly acidic media (pH < 3), protonation of transient oxyiodine intermediates accelerates reductive pathways: hypoiodite is rapidly converted to hypoiodous acid (HIO), which undergoes disproportionation, while elevated hydronium activity promotes comproportionation between surviving iodate and reduced iodine species. The net consequence is diversion of recoil activity into two terminal sinks - volatile molecular iodine (I₂), which partitions out of the condensed phase, and stable iodide (I⁻), a thermodynamically favored low-valent reservoir. These sinks deplete the parent iodate fraction and significantly depress retention.
Conversely, in alkaline matrices (pH > 9), the abundance of OH⁻ ions kinetically and thermodynamically stabilizes the higher oxyiodine oxidation states. Base-catalyzed disproportionation of hypoiodite channels reduced fragments back toward iodate, while radiolytically generated hydroxyl radicals (⋅OH) systematically oxidize lower-valent iodine species up the oxidation ladder I⁰/I⁻ → IO⁻ → IO₂⁻ → IO₃⁻. The combined action of hydroxide stabilization and radiolytic oxidation restores ¹²⁸I activity to the parent chemical form, yielding retention that rises monotonically with pH.
By coupling these competing protonation, disproportionation, comproportionation, and radiolytic oxidation channels within a single kinetic framework, this study reconciles long-standing disparities in historical solution-phase retention data, much of which was acquired under inconsistently controlled acidity and dose conditions. The model establishes retention as a predictable, quantitative function of pH and irradiation parameters, transforming recoil stabilization from an empirical observation into a designable outcome. These findings carry direct implications for targeted isotope production and radiopharmaceutical purity: deliberate pH control during and immediately following irradiation can maximize the yield of the parent chemical form in Szilard–Chalmers enrichment schemes, enhancing specific activity, while simultaneously defining the conditions under which recoil-produced ¹²⁸I partitions into a single, well-characterized species.
Keywords:¹²⁸I; Szilard–Chalmers effect; neutron capture; recoil chemistry; iodate; retention; pH; radiopharmaceutical purity
