  Astrophysical sources of nuclei are expected to produce a broad spectrum of
isotopes, many of which are unstable. An unstable nucleus can beta-decay
outside the source into a single-electron ion. Heavy one-electron ions, thus
formed, can be excited in their interactions with cosmic microwave background
photons, in which case they relax to the ground state with the emission of a
gamma ray. Repetitive cycles of excitation and gamma-ray emission can produce
an 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 consistent
with a substantial flux of nuclei accelerated to 0.1 EeV. A characteristic
5--10 GeV (iron) shoulder in the gamma-ray spectra of various sources can help
identify astrophysical accelerators of nuclei or set upper limits on nuclear
acceleration.
