A method is presented by which the radial distribution of galactic high-energy gamma rays can be determined. It is shown that beyond the solar circle, the gamma ray intensity is proportional to the H I column density alone, and that the emissivity in the 70 MeV-5 GeV energy range is only about 15 percent lower than the local value. The radial distribution of the H I with which the gamma rays are correlated is examined and it is shown that a significant fraction of the gamma ray emission originates at large galactocentric distances, which implies the cosmic rays must be abundant in the outer galaxy.
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Bloemen et al. (1984) studied this question.