Since 1930, industrial radiographers have utilized the high-energy gamma radiation emitted by radium to radiograph thick sections of metals. The radium sources were small by today's standards and exposure times were necessarily long. With World War II came nuclear reactors and nuclear fission producing many man-made isotopes whose cost, in comparison to radium, was very small. Industrial radiographers were the first to exploit these radiation sources. The first isotope to be introduced as a radiographic tool was cobalt-60, followed by iridium-192 and, more recently, cesium-137. These isotopes cover a large portion of the gamma spectra useful to industry as an inspection tool.