Bragg-Peak Luminescence of Liquid Water under High-LET Carbon-Ion Irradiation: Constraints from Nitrate and Dissolved-Gas Perturbations

Abstract

The origin of the faint luminescence emitted by liquid water below the Cherenkov threshold─whether it is mediated by chemically evolving radiolytic intermediates or originates from a prompt physical excitation of the medium─has remained unresolved since its first observation by Belcher in 1953. We examine the Bragg-peak luminescence of pure water under 290 MeV u–1 12C-ion irradiation using two independent perturbations: NaNO3 addition (0–1000 mM) and replacement of dissolved air with N2. At the Bragg peak, where the carbon-ion velocity is far below the Cherenkov threshold and even the most energetic δ-rays cannot produce Cherenkov radiation, neither perturbation produces a large suppression of the observed luminescence; the concentration-dependent Bragg-peak position shift is quantitatively accounted for by the increased solution density. At 1 M NaNO3, the mean nitrate spacing is comparable to the characteristic track-core dimension, so nitrate scavenging can compete directly with precursor relaxation and intratrack reactions on subpicosecond time scales rather than being excluded spatially. The weak nitrate dependence therefore constrains a dominant contribution from longer-lived, readily scavenged radiolytic intermediates but does not exclude an ultrafast chemical pathway. Independently, the observations are compatible with a dielectric energy-loss framework in which Im[−1/ε(ω)] governs electronic excitation of the medium. The ∼8 eV feature provides a plausible, but nonunique, microscopic doorway to the emitting manifold, while contributions from higher-energy excitations followed by relaxation cascades cannot be excluded.