radiochemistry, chemistry of radioactive substances (see radioactivity). Radioactive isotopes are very useful as tracers to study the mechanisms of complex organic reactions, since even minute amounts of these isotopes are easily detected by means of a Geiger counter or photographic film. For example, by feeding plants carbon dioxide that contains the radioisotope carbon-14 and by monitoring the carbon compounds through the plants' life cycle, the intermediate stages of the photosynthetic process can be determined. A method developed by W. F. Libby uses carbon-14 to date archaeological discoveries and other samples containing organic matter (see dating). Radioactive substances have also been used to investigate the properties of artificially produced elements and to measure the rates of electron transfers.