Radioactive isotopes (radioisotopes) are isotopes which are atoms of the same element that are chemically identical but vary in the number of neutrons they contain. Radioactive isotopes can participate in reactions just like non radioactive elements. Autoradiography is a technique used to trace the movements of certain substances around plants. An example of this is carbon 14. Carbon 14, a radioactive isotope,is added to the carbon dioxide supply of the plant, to study the movement of products of photosynthesis. Positron Emission Tomography (PET) uses a camera to detect gamma rays emitted from the body and to pinpoint their exact location. Disease may be indicated if an organ takes up too much isotope, or not enough, or if there is an unusual pattern or rate of movement of the isotope through the body. It is known as 'nuclear' medicine because of the use of radioactive isotopes. Radioisotopes have been able to show us the inner workings and functions of plants and humans. They have also been used as diagnostic tools in medicine to show disease or illness. By being able to trace biochemical pathways, the use of radioisotopes has increased our knowledge of biological processes and chemical reactions in humans and plants. New technologies: Geiger counter and Autoradiography. A Geiger counter is a type of particle detector that measures ionizing radiation. It is used as a handheld radiation survey instrument. Autoradiography is a technique used to trace the movements of certain substances around plants. The movement of the radioisotope can be traced by taking an autoradiograph. An autoradiograph is produced when the plant is placed against photographic film. The areas of the plant in which the carbon 14 has accumulated can be seen as dark shadows on the film.