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 (271.8 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.

Isotopes of cobalt (27Co)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
56Co synth 77.236 d β+ 56Fe
57Co synth 271.811 d ε 57Fe
58Co synth 70.844 d β+ 58Fe
59Co 100% stable
60Co trace 5.2714 y β−100% 60Ni
Standard atomic weight Ar°(Co)

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]

List of isotopes
Nuclide
[n 1]		 Z N Isotopic mass (Da)
[n 2][n 3]		 Half-life
[n 4]		 Decay
mode
[n 5]		 Daughter
isotope
[n 6]		 Spin and
parity
[n 7][n 4]		 Natural abundance (mole fraction)
Excitation energy[n 4] Normal proportion Range of variation
47Co 27 20 47.01149(54)# 7/2−#
48Co 27 21 48.00176(43)# p 47Fe 6+#
49Co 27 22 48.98972(28)# <35 ns p (>99.9%) 48Fe 7/2−#
β+ (<.1%) 49Fe
50Co 27 23 49.98154(18)# 44(4) ms β+, p (54%) 49Mn (6+)
β+ (46%) 50Fe
51Co 27 24 50.97072(16)# 60# ms [>200 ns] β+ 51Fe 7/2−#
52Co 27 25 51.96359(7)# 115(23) ms β+ 52Fe (6+)
52mCo 380(100)# keV 104(11)# ms β+ 52Fe 2+#
IT 52Co
53Co 27 26 52.954219(19) 242(8) ms β+ 53Fe 7/2−#
53mCo 3197(29) keV 247(12) ms β+ (98.5%) 53Fe (19/2−)
p (1.5%) 52Fe
54Co 27 27 53.9484596(8) 193.28(7) ms β+ 54Fe 0+
54mCo 197.4(5) keV 1.48(2) min β+ 54Fe (7)+
55Co 27 28 54.9419990(8) 17.53(3) h β+ 55Fe 7/2−
56Co 27 29 55.9398393(23) 77.233(27) d β+ 56Fe 4+
57Co 27 30 56.9362914(8) 271.74(6) d EC 57Fe 7/2−
58Co 27 31 57.9357528(13) 70.86(6) d β+ 58Fe 2+
58m1Co 24.95(6) keV 9.04(11) h IT 58Co 5+
58m2Co 53.15(7) keV 10.4(3) μs 4+
59Co 27 32 58.9331950(7) Stable 7/2− 1.0000
60Co 27 33 59.9338171(7) 5.2714(6) y β−, γ 60Ni 5+
60mCo 58.59(1) keV 10.467(6) min IT (99.76%) 60Co 2+
β− (.24%) 60Ni
61Co 27 34 60.9324758(10) 1.650(5) h β− 61Ni 7/2−
62Co 27 35 61.934051(21) 1.50(4) min β− 62Ni 2+
62mCo 22(5) keV 13.91(5) min β− (99%) 62Ni 5+
IT (1%) 62Co
63Co 27 36 62.933612(21) 26.9(4) s β− 63Ni 7/2−
64Co 27 37 63.935810(21) 0.30(3) s β− 64Ni 1+
65Co 27 38 64.936478(14) 1.20(6) s β− 65Ni (7/2)−
66Co 27 39 65.93976(27) 0.18(1) s β− 66Ni (3+)
66m1Co 175(3) keV 1.21(1) μs (5+)
66m2Co 642(5) keV >100 μs (8-)
67Co 27 40 66.94089(34) 0.425(20) s β− 67Ni (7/2−)#
68Co 27 41 67.94487(34) 0.199(21) s β− 68Ni (7-)
68mCo 150(150)# keV 1.6(3) s (3+)
69Co 27 42 68.94632(36) 227(13) ms β− (>99.9%) 69Ni 7/2−#
β−, n (<.1%) 68Ni
70Co 27 43 69.9510(9) 119(6) ms β− (>99.9%) 70Ni (6-)
β−, n (<.1%) 69Ni
70mCo 200(200)# keV 500(180) ms (3+)
71Co 27 44 70.9529(9) 97(2) ms β− (>99.9%) 71Ni 7/2−#
β−, n (<.1%) 70Ni
72Co 27 45 71.95781(64)# 62(3) ms β− (>99.9%) 72Ni (6- ,7-)
β−, n (<.1%) 71Ni
73Co 27 46 72.96024(75)# 41(4) ms 7/2−#
74Co 27 47 73.96538(86)# 50# ms [>300 ns] 0+
75Co 27 48 74.96833(86)# 40# ms [>300 ns] 7/2−#
This table header & footer:
  1. mCo – Excited nuclear isomer.
  2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  5. Modes of decay:
    EC:Electron capture
    IT:Isomeric transition
    n:Neutron emission
    p:Proton emission
  6. Bold symbol as daughter – Daughter product is stable.
  7. ( ) spin value – Indicates spin with weak assignment arguments.

Stellar nucleosynthesis of cobalt-56

One of the terminal nuclear reactions in stars prior to supernova produces 56Ni. Following its production, 56Ni decays to 56Co, and then 56Co subsequently decays to 56Fe. These decay reactions 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 τ1/2 = 68.6 days to τ1/2 = 69.6 days.[5] The rate at which the luminosity decreased closely matched the exponential decay of 56Co with a half-life of τ1/2 = 77.233 days.

Use of cobalt radioisotopes in medicine

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.[6]

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 favor in the Western world where Linacs are common.

Industrial uses for radioactive isotopes

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.[7] The uses for industrial cobalt include:

  • Sterilization of medical supplies and medical waste
  • Radiation treatment of foods for sterilization (cold pasteurization)[8]
  • 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.

References
  1. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  2. "Standard Atomic Weights: Cobalt". CIAAW. 2017.
  3. Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
  4. Diaz, L. E. "Cobalt-57: Production". JPNM Physics Isotopes. University of Harvard. Archived from the original on 2000-10-31. Retrieved 2013-11-15.
  5. Bouchet, P.; Danziger, I.J.; Lucy, L.B. (September 1991). "Bolometric Light Curve of SN 1987A: Results from Day 616 to 1316 After Outburst". The Astronomical Journal. 102 (3): 1135–1146 – via Astrophysics Data System.
  6. Diaz, L. E. "Cobalt-57: Uses". JPNM Physics Isotopes. University of Harvard. Archived from the original on 2011-06-11. Retrieved 2010-09-13.
  7. "Properties of Cobalt-60". Radioactive Isotopes. Retrieved 2022-12-09.
  8. "Beneficial Uses of Cobalt-60". INTERNATIONAL IRRADIATION ASSOCIATION. Retrieved 2022-12-09.
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