Isotopes of cobalt

Naturally occurring cobalt, Co, 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 (271.81 days), 56Co (77.24 days), and 58Co (70.84 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 19 meta states, of which the most stable is 58m1Co with a half-life of 8.85hours.

The isotopes of cobalt range in atomic weight from 50Co to 78Co. The main decay mode for isotopes with atomic mass less than that of the stable isotope, 59Co, is electron capture to iron isotopes, and the main mode of decay for those with greater mass is beta decay to nickel isotopes.

List of isotopes

Stellar nucleosynthesis of cobalt-56

One of the terminal nuclear reactions in stars prior to supernova produces 56Ni. 56Ni then decays to 56Co, which then decays to 56Fe. These decays power the luminosity displayed in light decay curves. Both the light decay and radioactive decay curves are expected to be exponential. Therefore, the light decay curve should give an indication of the nuclear reactions powering it. This has been confirmed by observation of bolometric light decay curves for SN 1987A. Between 600 and 800 days after SN1987A occurred, the bolometric light curve decreased at an exponential rate with half-life values from 68.6 days to 69.6 days. The rate at which the luminosity decreased closely matched that expected of exponential decay of 56Co.

Cobalt-57

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.

57Co is used as a source of gamma rays 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.

Cobalt-60

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 2cm 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.[citation needed] 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 more usual.

Cobalt-60 (60Co) is useful as an industrial gamma ray source also: 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

See also

Daughter products other than cobalt

• Isotopes of nickel
• Isotopes of iron
• Isotopes of manganese
• Isotopes of chromium