Complex sentence: Some kinds of cosmic radiation causes a nuclear reaction with nitrogen-14 in the upper atmosphere of the Earth, creating the 14 C. This decays back to 14 N after a half-life of several thousand years.
Split into simpler sentences: Some kinds of cosmic radiation causes a nuclear reaction with nitrogen-14 in the upper atmosphere of the Earth, creating the carbon-14. This radioisotope decays back to nitrogen-14 after a half-life of several thousand years.