The correct answer is Cobalt - 60. 

• Cobalt - 60 radioisotopes are used in the treatment of blood cancer [leukemia].
• Cobalt - 60:
  • Cobalt-60 (60Co) is an artificial radioactive isotope of cobalt with a half-life of 5.2713 years.
  • It is generated artificially in nuclear reactors.
  • Intentional industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidic cobalt isotope 59 Co.
  • Measurable quantities are also generated as a by-product of typical nuclear power plant operation and may be detected externally when leaks occur.
  • In the hindmost case (in the absence of added cobalt) the incidental generator 60Co is largely the result of multiple stages of neutron activation of iron isotopes in the reactor's steel structures via the creation of its 59 Co precursor.
  • The easiest case of the latter would result from the activation of 58Fe.
  • 60Co pass off beta decay to the stable isotope nickel-60 (60Ni).
  • The activated nickel nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV.