One of the few methods of detecting cosmic rays in distant parts of the Galaxy is provided by the very new science of gamma-ray astronomy. When high-speed cosmic-ray protons collide with hydrogen atoms in the interstellar medium, nuclear reactions take place between them and a number of 7i°-mesons are created. These ?0 -mesons, or pions, are unstable and themselves decay almost immediately to produce a pair of gamma rays. These gamma rays have an average energy of about 100 MeV each, corresponding to a wavelength of 10-14m, and can pass right across the Galaxy with almost no hindrance by the interstellar medium.

The distribution of these gamma rays as a function of galactic longitude around the Milky Way, as measured by the SAS-2 satellite in 1973. Apart from a couple of subsidiary peaks which are due to gamma-ray emission from the Crab and Vela supernova remnants, the distribution of gamma rays is very similar to that of the interstellar hydrogen in the Galaxy. The measured intensity of the gamma rays in any direction is close to what is expected from the pion decay process if the intensity of cosmic-ray protons is about the same throughout the Galaxy as it is at the Earth. This is very encouraging support for the assumption that the cosmic rays we detect on Earth are typical of the Galaxy as a whole. As gamma-ray telescopes become more sophisticated in the next few years we may be able to locate sources of cosmic rays by this technique.

Comments are closed.