In the simplest and most common interaction, a cosmic-ray proton strikes another proton. The protons survive the collision, but their interaction creates an unstable particle — a pion — with only 14 percent the mass of a proton. In 10 millionths of a billionth of a second, the pion decays into a pair of gamma-ray photons. More complex scenarios occur when cosmic-ray protons strike nuclei containing greater numbers of particles.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 188.8.131.52.0