Cosmogenic Neutrons
Cosmogenic neutrons, neutrons produced from cosmic radiation in the Earth's atmosphere or surface, and those produced in particle accelerators can be significantly higher energy than those encountered in reactors. Most of them activate a nucleus before reaching the ground; a few react with nuclei in the air. The reactions with nitrogen-14 lead to the formation of carbon-14, widely used in radiocarbon dating.
Read more about this topic: Neutron Radiation