59Co is the only stable cobalt isotope and the only isotope that exists naturally on Earth. Twenty-two radioisotopes have been characterized: the most stable, 60Co, has a half-life of 5.2714 years; 57Co has a half-life of 271.8 days; 56Co has a half-life of 77.27 days; and 58Co has a half-life of 70.86 days. All the other radioactive isotopes of cobalt have half-lives shorter than 18 hours, and in most cases shorter than 1 second.
Cobalt-60 is used in radiotherapy. It produces two gamma rays with energies of 1.17 MeV and 1.33 MeV. The 60Co source is useful for about 5 years but even after this point is still very radioactive, and so cobalt machines have fallen from favor in the Western world where linacs are common.
Cobalt-60 is useful as a gamma ray source because it can be produced in predictable quantities, and for its high radioactivity simply by exposing natural cobalt to neutrons in a reactor. The uses for industrial cobalt include:
- Sterilization of medical supplies and medical waste
- Radiation treatment of foods for sterilization (cold pasteurization)
- Industrial radiography (e.g., weld integrity radiographs)
- Density measurements (e.g., concrete density measurements)
- Tank fill height switches.
5 minute measurement with a High Purity Germanium (HPGe) radiation detector.