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Naturally occurring iron (₂₆Fe) consists of four stable isotopes: 5.845% of Fe (possibly radioactive with a half-life over 4.4×10 years), 91.754% of Fe, 2.119% of Fe and 0.286% of Fe. There are 28 known radioactive isotopes and 8 nuclear isomers, the most stable of which are Fe (half-life 2.6 million years) and Fe (half-life 2.7 years).

Much of the past work on measuring the isotopic composition of iron has centered on determining Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, although applications to biological and industrial systems are beginning to emerge.

List of isotopes

1. Fe – 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.
8. Believed to decay by ββ to Cr with a half-life of over 4.4×10 a
9. Lowest mass per nucleon of all nuclides; End product of stellar nucleosynthesis

Iron-54

Fe is observationally stable, but theoretically can decay to Cr, with a half-life of more than 4.4×10 years via double electron capture (εε).

Iron-56

Fe is the most abundant isotope of iron. It is also the isotope with the lowest mass per nucleon, 930.412 MeV/c, though not the isotope with the highest nuclear binding energy per nucleon, which is nickel-62. However, because of the details of how nucleosynthesis works, Fe is a more common endpoint of fusion chains inside supernovae, where it is mostly produced as Ni. Thus, Ni is more common in the universe, relative to other metals, including Ni, Fe and Ni, all of which have a very high binding energy.

The high nuclear binding energy for Fe represents the point where further nuclear reactions become energetically unfavorable. Because of this, it is among the heaviest elements formed in stellar nucleosynthesis reactions in massive stars. These reactions fuse lighter elements like magnesium, silicon, and sulfur to form heavier elements. Among the heavier elements formed is Ni, which subsequently decays to Co and then Fe.

Iron-57

Fe is widely used in Mössbauer spectroscopy and the related nuclear resonance vibrational spectroscopy due to the low natural variation in energy of the 14.4 keV nuclear transition. The transition was famously used to make the first definitive measurement of gravitational redshift, in the 1960 Pound–Rebka experiment.

Iron-58

Iron-58 can be used to combat anemia and low iron absorption, to metabolically track iron-controlling human genes, and for tracing elements in nature. Iron-58 is also an assisting reagent in the synthesis of superheavy elements.

Iron-60

Iron-60 is an iron isotope with a half-life of 2.6 million years, but was thought until 2009 to have a half-life of 1.5 million years. It undergoes beta decay to cobalt-60, which then decays with a half-life of about 5 years to stable nickel-60. Traces of iron-60 have been found in lunar samples.

In phases of the meteorites Semarkona and Chervony Kut, a correlation between the concentration of Ni, the granddaughter isotope of Fe, and the abundance of the stable iron isotopes could be found, which is evidence for the existence of Fe at the time of formation of the Solar System. Possibly the energy released by the decay of Fe contributed, together with the energy released by decay of the radionuclide Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of Ni present in extraterrestrial material may also provide further insight into the origin of the Solar System and its early history.

Iron-60 found in fossilised bacteria in sea floor sediments suggest there was a supernova in the vicinity of the Solar System approximately 2 million years ago. Iron-60 is also found in sediments from 8 million years ago. In 2019, researchers found interstellar Fe in Antarctica, which they relate to the Local Interstellar Cloud.

The distance to the supernova of origin can be estimated by relating the amount of iron-60 intercepted as Earth passes through the expanding supernova ejecta. Assuming that the material ejected in a supernova expands uniformly out from its origin as a sphere with a surface area of 4πr. The fraction of the material intercepted by the Earth is dependent on its cross-sectional area (πR_earth) as it passes through the expanding debris. Where M_ej is the mass of ejected material.$${\displaystyle M_{\text{Fraction intercepted }}={\frac {\pi R_{\text{Earth }}^{2}}{4\pi r^{2}}}M_{ej}}$$Assuming the intercepted material is distributed uniformly across the surface of the Earth (4πR_earth), the mass surface density (Σ_ej) of the supernova ejecta on Earth is: $${\displaystyle \Sigma _{ej}={\frac {M_{\text{Fraction intercepted }}}{A_{\text{surface,Earth }}}}={\frac {M_{ej}}{16\pi r^{2}}}}$$The number of Fe atoms per unit area found on Earth can be estimated if the typical amount of Fe ejected from a supernova is known. This can be done by dividing the surface mass density (Σ_ej) by the atomic mass of Fe. $${\displaystyle N_{60}=\left({\frac {M_{ej,60}/m_{60}}{16\pi r^{2}}}\right)}$$The equation for N₆₀ can be rearranged to find the distance to the supernova.$${\displaystyle r={\sqrt {\frac {M_{ej,60}}{16\pi m_{60}N_{60}}}}}$$An example calculation for the distance to the supernova point of origin is given below. This calculation uses speculative values for terrestrial Fe atom surface density (N₆₀ ≈ 4 × 10 atoms/m) and a rough estimate of the mass of Fe ejected in a supernova explosion (10 M_☉). $${\displaystyle r={\sqrt {\frac {10^{-5}M_{\odot }}{16\pi \left(60m_{p}\right)N_{60}}}}}$$$${\displaystyle r=3\times 10^{18}m=100pc}$$More sophisticated analyses have been reported that take into consideration the flux and deposition of Fe as well as possible interfering background sources.

Cobalt-60, the decay product of iron-60, emits 1.173 MeV and 1.333 MeV as it decays. These gamma-ray lines have long been important targets for gamma-ray astronomy, and have been detected by the gamma-ray observatory INTEGRAL. The signal traces the Galactic plane, showing that Fe synthesis is ongoing in our Galaxy, and probing element production in massive stars.

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: Iron". CIAAW. 1993.
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. ^ Bikit, I.; Krmar, M.; Slivka, J.; Vesković, M.; Čonkić, Lj.; Aničin, I. (1998). "New results on the double β decay of iron". Physical Review C. 58 (4): 2566–2567. Bibcode:1998PhRvC..58.2566B. doi:10.1103/PhysRevC.58.2566.
5. N. Dauphas; O. Rouxel (2006). "Mass spectrometry and natural variations of iron isotopes". Mass Spectrometry Reviews. 25 (4): 515–550. Bibcode:2006MSRv...25..515D. doi:10.1002/mas.20078. PMID 16463281.
6. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
7. Fewell, M. P. (1995). "The atomic nuclide with the highest mean binding energy". American Journal of Physics. 63 (7): 653. Bibcode:1995AmJPh..63..653F. doi:10.1119/1.17828.
8. R. Nave. "Mossbauer Effect in Iron-57". HyperPhysics. Georgia State University. Retrieved 2009-10-13.
9. Pound, R. V.; Rebka Jr. G. A. (April 1, 1960). "Apparent weight of photons". Physical Review Letters. 4 (7): 337–341. Bibcode:1960PhRvL...4..337P. doi:10.1103/PhysRevLett.4.337.
10. "Iron-58 Metal Isotope". American Elements. Retrieved 2023-06-28.
11. ^ Vasiliev, Petr. "Iron-58, Iron-58 Isotope, Enriched Iron-58, Iron-58 Metal". www.buyisotope.com. Retrieved 2023-06-28.
12. Rugel, G.; Faestermann, T.; Knie, K.; Korschinek, G.; Poutivtsev, M.; Schumann, D.; Kivel, N.; Günther-Leopold, I.; Weinreich, R.; Wohlmuther, M. (2009). "New Measurement of the Fe Half-Life". Physical Review Letters. 103 (7): 72502. Bibcode:2009PhRvL.103g2502R. doi:10.1103/PhysRevLett.103.072502. PMID 19792637.
13. "Eisen mit langem Atem". scienceticker. 27 August 2009. Archived from the original on 3 February 2018. Retrieved 22 May 2010.
14. Belinda Smith (Aug 9, 2016). "Ancient bacteria store signs of supernova smattering". Cosmos.
15. Peter Ludwig; et al. (Aug 16, 2016). "Time-resolved 2-million-year-old supernova activity discovered in Earth's microfossil record". PNAS. 113 (33): 9232–9237. arXiv:1710.09573. Bibcode:2016PNAS..113.9232L. doi:10.1073/pnas.1601040113. PMC 4995991. PMID 27503888.
16. Colin Barras (Oct 14, 2017). "Fires may have given our evolution a kick-start". New Scientist. 236 (3147): 7. Bibcode:2017NewSc.236....7B. doi:10.1016/S0262-4079(17)31997-8.
17. Koll, Dominik; et., al. (2019). "Interstellar Fe in Antarctica". Physical Review Letters. 123 (7): 072701. Bibcode:2019PhRvL.123g2701K. doi:10.1103/PhysRevLett.123.072701. hdl:1885/298253. PMID 31491090. S2CID 201868513.
18. Ertel, Adrienne F.; Fry, Brian J.; Fields, Brian D.; Ellis, John (20 April 2023). "Supernova Dust Evolution Probed by Deep-sea 60Fe Time History". The Astrophysical Journal. 947 (2): 58–83 – via The Institute of Physics (IOP).
19. Harris, M. J.; Knödlseder, J.; Jean, P.; Cisana, E.; Diehl, R.; Lichti, G. G.; Roques, J. -P.; Schanne, S.; Weidenspointner, G. (2005-04-01). "Detection of γ-ray lines from interstellar 60Fe by the high resolution spectrometer SPI". Astronomy and Astrophysics. 433 (3): L49–L52. arXiv:astro-ph/0502219. Bibcode:2005A&A...433L..49H. doi:10.1051/0004-6361:200500093. ISSN 0004-6361.
20. Wang, W.; Siegert, T.; Dai, Z. G.; Diehl, R.; Greiner, J.; Heger, A.; Krause, M.; Lang, M.; Pleintinger, M. M. M.; Zhang, X. L. (2020-02-01). "Gamma-Ray Emission of 60Fe and 26Al Radioactivity in Our Galaxy". The Astrophysical Journal. 889 (2): 169. arXiv:1912.07874. Bibcode:2020ApJ...889..169W. doi:10.3847/1538-4357/ab6336. ISSN 0004-637X.

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

Isotopic compositions and standard atomic masses from:

• de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
• Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051.
• "News & Notices: Standard Atomic Weights Revised". International Union of Pure and Applied Chemistry. 19 October 2005.

Half-life, spin, and isomer data selected 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
• 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.

Further reading

• J. M. Nielsen (1960). The Radiochemistry of Iron (PDF). National Academy of Sciences/National Research Council.

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• Isotopes of iron
• Iron
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