Gamma rays are the highest-energy forms of light in the electromagnetic spectrum and they can have over a billion times the energy of the type of light visible to the human eye. Gamma rays can be created in several different ways: a high-energy particle can collide with another particle, a particle can collide and annihilate with its anti-particle, an element can undergo radioactive decay, or a charged particle can be accelerated. In this animation, we see a high-energy photon collide with a free electron, which causes the creation of a gamma-ray.
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 220.127.116.11.0