Gamma-Ray Emission from the Orion Cloud Complex and the Ratio CO/H2

A new value for the density ratio of H2/(C-13)O is derived by making a comparison between the COS-B gamma-ray survey above 100 MeV and the CO survey of the Orion cloud complex. Results rely on the assumption that the gamma-ray emissivity above 100 MeV is uniform over the whole cloud complex and its surroundings, and has the local value derived by Lebrun et al (1981). Results are obtained for three regions covering the Orion complex, and for two regions outside the complex where CO radiation has not been detected, the number of observed gamma-rays is fully consistent with the sum of HI and isotropic background contributions.