Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component .
Supposing that this component is made of nuclei , two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated which , added to the standard cosmic ray spectra , produce the observed ionization rates .
The clearest evidence of the presence of such low-energy nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the interstellar medium would be a detection of nuclear \gamma -ray line emission in the range E _ { \gamma } \sim 0.1 - 10 MeV , which is strongly produced in their collisions with the interstellar gas and dust .
Using a recent \gamma -ray cross section compilation for nuclear collisions , \gamma -ray line emission spectra are calculated alongside with the high-energy \gamma -ray emission due to \pi ^ { 0 } decay , the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E _ { \gamma } = 0.1 GeV .
Our predicted fluxes of strong nuclear \gamma -ray lines from the inner Galaxy are well below the detection sensitivies of INTEGRAL , but a detection , especially of the 4.4-MeV line , seems possible with new-generation \gamma -ray telescopes based on available technology .
We predict also strong \gamma -ray continuum emission in the 1-8 MeV range , which in a large part of our model space for low-energy cosmic rays exceeds considerably estimated instrument sensitivities of future telescopes .