text: C decays by positron emission with a half-life of ca. 20 min. C is one of the isotopes often used in positron emission tomography.
  C decays by beta decay, with a half-life of 5730 years. It is continuously produced in the upper atmosphere of the earth, so it occurs at a trace level in the environment. However, it is not practical to use naturally-occurring C for tracer studies. Instead it is made by neutron irradiation of the isotope C which occurs naturally in carbon at about the 1.1% level. C has been used extensively to trace the progress of organic molecules through metabolic pathways.