 When energetic protons enter the atmosphere they create isotopes by reactions with the major components; the most important of these is carbon-14 (14C), which is created when secondary neutrons react with nitrogen.14C, which has a half-life of 5,730 years, reacts with oxygen to form carbon dioxide which is taken up by plants; dating wood by its 14C content is the basis of radiocarbon dating.If wood of known age is available the process can be reversed.