Medical physics Question:

A radioactive isotope with a half life of 5.26 years that was heavily used for external radiation therapy before the popularization of linear accelerators. It is a beta emitter that decays to Ni-60 and gives off two gamma rays of average energy 1.25 MeV in the process. This is the therapeutic part of the beam. One major disadvantage of Cobalt compared to Linear Accelerators is a wider penumbra at the field edge. It is seldom used for external beam treatments but is still employed in the Gamma Knife stereotactic system.