Neutral-Pion Gamma Rays from the Galaxy and the Interstellar Gas Content

Knowledge of the total y-ray production rate per H atom from the decay of 7TO mesons produced in interstellar cosmic-ray interactions is essential for determining the possible amount of interstellar H2. This production rate is recalculated here using the latest accelerator data on 7rO production in p-p interactions up to 1500 GeV. A simple but accurate approximation used here resolves the past disagreement over the magnitude of this rate. An upper limit is obtained of (1.51 + 0.23) x 10-25 1, consistent with the observed upper limit of 1.6 x 10-25 1 Subject headings: cosmic rays - gamma rays - interstellar matter