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Astrophysical sources of nuclei are expected to produce a broad spectrum ofisotopes, many of which are unstable. An unstable nucleus can beta-decayoutside the source into a single-electron ion. Heavy one-electron ions, thusformed, can be excited in their interactions with cosmic microwave backgroundphotons, in which case they relax to the ground state with the emission of agamma ray. Repetitive cycles of excitation and gamma-ray emission can producean observable feature in the gamma-ray spectrum with a maximum around 8 GeV(for iron). We find that the observed spectrum of Centaurus A is consistentwith a substantial flux of nuclei accelerated to 0.1 EeV. A characteristic5--10 GeV (iron) shoulder in the gamma-ray spectra of various sources can helpidentify astrophysical accelerators of nuclei or set upper limits on nuclearacceleration.

A gamma-ray signature of energetic sources of cosmic-ray nuclei