Radionuclides are chosen according to the type and character of the radiation they emit, intensity of emission, and the half-life of their decay. Common source radionuclides include cobalt-60,[1] iridium-192,[2] and strontium-90.[3] The SI measurement quantity of source activity is the Becquerel, though the historical unit Curies is still in partial use, such as in the USA, despite their NIST strongly advising the use of the SI unit.[4] The SI unit for health purposes is mandatory in the EU.