This paper discusses collision processes between cosmicray primaries (protons and electrons) and the thermal photons of sunlight and starlight. In particular, electron-positron pair production and Compton scattering in interplanetary, intragalactic, and intergalactic space are treated in detail. It is found that the number of collisions between primary particles and thermal photons in single traversals of the solar system and the local galaxy is not sufficiently large to cause either appreciable energy loss to the particles or appreciable production of secondary pairs and energetic scattered photons. The same statement holds for the primary protons even on an intergalactic scale. On the other hand, energetic primary electrons may experience a sufficient number of Compton collisions in intergalactic space (travel time of the order 2×${10}^9$ years) to eliminate them effectively from the cosmic radiation reaching the neighborhood of the earth.

The stopping power for electrons of the observed interstellar radiofrequency spectrum is also estimated and found to be comparatively large.

• Received 20 November 1947

DOI:https://doi.org/10.1103/PhysRev.73.449