Cobalt 60 is produced in a nuclear reactor by bombarding  59 Co with neutrons. It has a half life of 5.3 years and decays by negative beta emission to metastable  60 Ni. This rapidly releases a gamma ray of either 1.17 or 1.33 MeV to reach a stable state. The source typically has an activity of 185 – 370 MBq, giving a dose rate at 80 cm of 1 – 2 Gy/minute. The Cobalt 60 source is usually replaced before a single half life has elapsed. Gamma ray treatment machines are known as teletherapy units. The components of a cobalt-60 machine are: A radioactive source, in this case  60 Co, which is housed in a steel capsule. Source housing, which includes the primary beam collimator to prevent unwanted radiation emission. The source housing can also shift the source to allow gamma rays to exit the unit through the collimator aperture. A gantry (in isocentric machines) to allow the source to rotate around a fixed position. The SAD is usually 80 or 100 cm. The gantry is typically attached