The Interstellar Spectrum of Cosmic Rays

Recent measurements of the angular distribution of cosmic gamma rays have been used to give information about the spectrum of cosmic ray protons and heavier nuclei in interstellar space. Attention is devoted particularly towards gamma rays from directions well away from the galactic centre where the predominant production mechanism for quanta above 100 MeV appears to be by way of neutral pions generated in cosmic ray-interstellar gas nucleus collisions. The distribution of cosmic rays both in and away from the galactic plane is a crucial problem in the analysis and it is examined in some detail. It is concluded that the spectral form for the interstellar spectrum (within about 2 kpc) suggested by Comstock, Hsieh & Simpson gives a good fit to the data. The intensity of protons at 3 GeV is $$\tilde{\lt}\,2\cdot7\,\times\,{10}^{-1}\,\text{m}^{-2}\,\text{s}^{-1}\,\text{sr}^{-1}\,\text{MeV}-1$$.