Gamma radiotherapy using a cobalt bomb", radioactive cobalt-60 being the most common source of gamma radiation. The cobalt source is housed in the large treatment head, and gamma radiation released through a shutter when the machine is in operation. The gamma rays produced are at an energy of 1 million electron volts (1 meV), considerably less powerful than the 8-10 meV x- rays released from the other major radiotherapy tool, the linear accelerator. Gamma rays are typically used to treat localised secondary cancers, such as in cases of secondary bone disease. They may also be used as a palliative (pain reliever) in otherwise untreatable cancers."