  It is crucial to identify the ultrahigh-energy cosmic-ray (UHECR) sources and
probe their unknown properties. Recent results from the Pierre Auger
Observatory favor a heavy nuclear composition for the UHECRs. Under the
requirement that heavy nuclei survive in these sources, using gamma-ray bursts
as an example, we predict a diagnostic gamma-ray signal, unique to nuclei - the
emission of de-excitation gamma rays following photodisintegration. These gamma
rays, boosted from MeV to TeV-PeV energies, may be detectable by gamma-ray
telescopes such as VERITAS, HESS, and MAGIC, and especially the next-generation
CTA and AGIS. They are a promising messenger to identify and study individual
UHE nuclei accelerators.
