Nuclear radiations are used in biology because of their common property of ionizing matter. This makes their detection relatively simple, or makes possible the production of biological effects in any living cell. Nuclear radiations originate in atomic nuclei, either spontaneously, as in radioactive substances, or through interactions with neutrons, photons, and so on. Gamma radiation originates in atomic nuclei and constitutes one kind of nuclear radiation, but it is otherwise indistinguishable in its effects from x-radiation produced by extranuclear reactions. Because x-rays have been readily available for many years, they have been used more extensively in biology and medicine than gamma rays. Therefore, x-rays must be included in any discussion of the biological and medical uses of nuclear radiations.