Radiological sources are used throughout the world for medical and industrial purposes. Until the 1950s, only naturally occurring radioactive materials, such as radium-226, were available for use in radiological sources. Since then, sources containing radioactive material produced artificially in nuclear reactors and accelerators have become widely available, including cesium-137, cobalt-60, and iridium-192. Sealed sources vary in size from the size of a grain of rice to rods up to several inches in length.