Radiocarbon is produced in the atmosphere as a result of...

the interaction of cosmic rays with nitrogen in the atmosphere. When cosmic rays collide with nitrogen atoms, they can cause a reaction that transforms the nitrogen into carbon-14, a radioactive isotope of carbon. This carbon-14 then combines with oxygen to form carbon dioxide, which is taken up by plants during photosynthesis. Animals then consume these plants, incorporating the carbon-14 into their bodies. As a result, all living organisms contain a small amount of carbon-14, which decays over time at a known rate. This decay process is used in radiocarbon dating to determine the age of organic materials.

Cosmic radiation