γ -ray production cross sections for proton and α-particle interactions with 12 C, 16 O, 24 Mg, and Fe have been measured in the energy range 5-25 MeV with proton beams and 5-40 MeV with α-particle beams. Isotopically pure foils of 24 Mg and foils of natural isotopical composition of C, MgO, and Fe have been used. γ -ray angular distributions were obtained with five high-purity Ge detectors with bismuth germanate Compton shields placed at angles of 45 • to 157.5 • . Cross sections for more than 50 different γ -ray transitions were extracted, and for many of them no data have been published before. Comparison of present data with data available in the literature shows mostly good to excellent agreement. In addition to the production cross sections, high-statistics, low-background line shapes of the 4.438 MeV 12 C γ ray from inelastic scattering off 12 C and spallation of 16 O were obtained. Comparison with nuclear reaction calculations shows that these data place interesting constraints on nuclear reaction models.

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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 γ-ray line emission in the range E γ ∼ 0.1-10 MeV, which is strongly produced in their collisions with the