Naturally occurringcobalt, Co, consists of a single stableisotope,⁵⁹Co (thus, cobalt is amononuclidic element). Twenty-eightradioisotopes have been characterized; the most stable are⁶⁰Co with ahalf-life of 5.2714 years,⁵⁷Co (271.811 days),⁵⁶Co (77.236 days), and⁵⁸Co (70.844 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 19meta states, of which the most stable is^58m1Co with a half-life of 8.853 h.

The isotopes of cobalt range inatomic weight from⁵⁰Co to⁷⁸Co. The maindecay mode for isotopes with atomic mass less than that of the stable isotope,⁵⁹Co, iselectron capture and the main mode of decay for those of greater than 59 atomic mass units isbeta decay. The maindecay products before⁵⁹Co areiron isotopes and the main products after arenickel isotopes.

Radioisotopes can be produced by variousnuclear reactions. For example,⁵⁷Co is produced bycyclotron irradiation of iron. The main reaction is the (d,n) reaction⁵⁶Fe +²H → n +⁵⁷Co.^[4]

One of the terminal nuclear reactions in stars prior tosupernova produces⁵⁶Ni. Following its production,⁵⁶Ni decays to⁵⁶Co, and then⁵⁶Co subsequently decays to⁵⁶Fe. These decay reactions power the luminosity displayed inlight decay curves. Both the light decay andradioactive 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 forSN 1987A. Between 600 and 800 days after SN1987A occurred, the bolometric light curve decreased at an exponential rate withhalf-life values from τ_1/2 = 68.6 days to τ_1/2 = 69.6 days.^[6] The rate at which the luminosity decreased closely matched the exponential decay of⁵⁶Co with a half-life of τ_1/2 = 77.233 days.

Cobalt-57 (⁵⁷Co or Co-57) is used in medical tests; it is used as aradiolabel for vitamin B₁₂ uptake. It is useful for theSchilling test.^[7]

Cobalt-60 (⁶⁰Co or Co-60) is used inradiotherapy. It produces twogamma rays with energies of 1.17MeV and 1.33 MeV. The⁶⁰Co source is about 2 cm indiameter and as a result produces ageometricpenumbra, making the edge of theradiation field fuzzy. The metal has the unfortunate habit of producing fine dust, causing problems with radiation protection. The⁶⁰Co 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 whereLinacs are common.

Cobalt-60 (⁶⁰Co) is useful as a gamma ray source because it can be produced in predictable quantities, and for its highradioactivity simply by exposing natural cobalt toneutrons in a reactor.^[8] The uses for industrial cobalt include:

• Sterilization of medical supplies andmedical waste
• Radiation treatment offoods for sterilization (coldpasteurization)^[9]
• Industrialradiography (e.g., weld integrity radiographs)
• Density measurements (e.g., concrete density measurements)
• Tank fill height switches

⁵⁷Co is used as a source inMössbauer spectroscopy of iron-containing samples. Electron capture by⁵⁷Co forms an excited state of the⁵⁷Fe 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.

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean;Wapstra, Aaldert Hendrik (2003),"The NUBASE evaluation of nuclear and decay properties",Nuclear Physics A,729:3–128,Bibcode:2003NuPhA.729....3A,doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean;Wapstra, Aaldert Hendrik (2003),"The NUBASE evaluation of nuclear and decay properties",Nuclear Physics A,729:3–128,Bibcode:2003NuPhA.729....3A,doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center."NuDat 2.x database".Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.).CRC Handbook of Chemistry and Physics (85th ed.).Boca Raton, Florida:CRC Press.ISBN 978-0-8493-0485-9.

• Isotopes of cobalt
• Cobalt
• Lists of isotopes by element

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