Naturally occurring cobalt (₂₇Co) consists of a single stable isotope, ⁵⁹Co (thus, cobalt is a mononuclidic element). Twenty-eight radioisotopes have been characterized; the most stable are ⁶⁰Co with a half-life of 5.2714 years, ⁵⁷Co (271.8 days), ⁵⁶Co (77.27 days), and ⁵⁸Co (70.86 days). All other isotopes have half-lives of less than 18 hours and most of these have half-lives of less than 1 second. This element also has 11 meta states, all of which have half-lives of less than 15 minutes.

The isotopes of cobalt range in atomic weight from ⁴⁷Co to ⁷⁵Co. The main decay mode for isotopes with atomic mass less than that of the stable isotope, ⁵⁹Co, is electron capture and the main mode of decay for those of greater than 59 atomic mass units is beta decay. The main decay products before ⁵⁹Co are iron isotopes and the main products after are nickel isotopes.

Radioisotopes can be produced by various nuclear reactions. For example, ⁵⁷Co is produced by cyclotron irradiation of iron. The main reaction is the (d,n) reaction ⁵⁶Fe + ²H → n + ⁵⁷Co.^[4]

Cobalt-57 (⁵⁷Co or Co-57) is used in medical tests; it is used as a radiolabel for vitamin B₁₂ uptake. It is useful for the Schilling test.^[5]

Cobalt-60 (⁶⁰Co or Co-60) is used in radiotherapy. It produces two gamma rays with energies of 1.17 MeV and 1.33 MeV. The ⁶⁰Co source is about 2 cm in diameter and as a result produces a geometric penumbra, making the edge of the radiation field fuzzy. The metal has the unfortunate habit of producing fine dust, causing problems with radiation protection. 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 favour in the Western world where linacs are common.

Cobalt-60 (⁶⁰Co) 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.^[6] The uses for industrial cobalt include:

• Sterilization of medical supplies and medical waste
• Radiation treatment of foods for sterilization (cold pasteurization)^[7]
• Industrial radiography (e.g., weld integrity radiographs)
• Density measurements (e.g., concrete density measurements)
• Tank fill height switches

⁵⁷Co is used as a source in Mössbauer spectroscopy of iron-containing samples. Electron capture by ⁵⁷Co forms an excited state of the ⁵⁷Fe nucleus, which in turn decays to the ground state with the emission of a gamma ray. Measurement of the gamma-ray spectrum provides information about the chemical state of the iron atom in the sample.