Spectra and Ionization Efficiencies of Charged Decay Particles in Kilonova Ejecta

Abstract

Ionization by radioactive decay products including $α$-particles, $β$-decay electrons, and fission fragments plays a central role in determining the nebular-phase ionization state and spectra of kilonovae. In this work, ionization cross sections, stopping powers, thermalization histories, and particle degradation spectra are calculated self-consistently for $α$-particles and fission fragments propagating through expanding kilonova ejecta. The treatment includes interactions with bound and free electrons, charge evolution of fission fragments, adiabatic energy losses, and particle spectra obtained from the continuous slowing-down approximation and Spencer--Fano formalism. The work per ion pair is evaluated for a range of ejecta compositions and ionization states. Despite the large differences in mass, charge, and injection energy between $β$-electrons, $α$-particles, and fission fragments, the resulting work per ion pair is found to be remarkably similar across all decay channels and target species considered. In particular, heavy-element ions exhibit nearly identical normalized ionization efficiencies for all decay products. This robustness arises because the ionization cross sections and stopping powers are governed by the same underlying collision physics, causing the ratio between ionization and energy loss to remain approximately constant over the relevant energy range. The results imply that the ionization state of late-time kilonova ejecta depends only weakly on the dominant radioactive decay channel, even in ejecta where $α$-decay and fission dominate the heating budget.