  At high energies, the hadronic origin of gamma rays from supernova remnants
is still debated. Assuming the observed gamma-rays from the Wolf-Rayet
supernova remnant Cas A are due to accelerated protons and ions, we predict the
nuclear de-excitation line emission arising from interactions with the heavy
elements in the supernova ejecta. This illustrative example highlights the
importance of MeV gamma ray observations of the hadronic fingerprint of cosmic
rays. The lines could be observed in the MeV band with a future space mission
such as GRIPS which has been proposed to ESA as an all-sky survey mission with
gamma-ray, X-ray and near-infrared telescopes. In MeV gamma rays, its
sensitivity will improve by a factor of 40 compared with previous missions.
