Diffuse galactic gamma ray lines

The origin and observability were studied of diffuse gamma ray line emission from the galaxy. It was found that such lines could be formed by nuclear excitation interactions of low energy cosmic rays with both interstellar gas and dust grains. The gamma ray emission lines from deexcitation of grain nuclei are sharp with Doppler widths of the order of 10 kev or less; the lines from gas nuclei are also relatively sharp with widths of the order of 100 kev for the most intense line; and the lines from cosmic ray nuclei are broad with widths of the order of several hundred kev. A detailed evaluation is presented of the production rate of the 4.44 Mev line for a variety of assumed cosmic ray spectra. Results are compared with reported galactic gamma ray line intensities and it is concluded that the measurements are consistent with a low energy cosmic ray density which increases toward the galactic center in proportion to the molecular gas density.