Isotopes of iron explained

Naturally occurring iron (₂₆Fe) consists of four stable isotopes: 5.845% of ⁵⁴Fe (possibly radioactive with a half-life over years),^[1] 91.754% of ⁵⁶Fe, 2.119% of ⁵⁷Fe and 0.286% of ⁵⁸Fe. There are 24 known radioactive isotopes, the most stable of which are ⁶⁰Fe (half-life 2.6 million years) and ⁵⁵Fe (half-life 2.7 years).

Much of the past work on measuring the isotopic composition of iron has centered on determining ⁶⁰Fe 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.^[2]

List of isotopes

|-| rowspan=2|⁴⁵Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 19| rowspan=2|45.01458(24)#| rowspan=2|1.89(49) ms| β⁺ (30%)| ⁴⁵Mn| rowspan=2|3/2+#| rowspan=2|| rowspan=2||-| 2p (70%)| ⁴³Cr|-| rowspan=2|⁴⁶Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 20| rowspan=2|46.00081(38)#| rowspan=2|9(4) ms
[12(+4-3)&nbsp;ms]| β⁺ (>99.9%)| ⁴⁶Mn| rowspan=2|0+| rowspan=2|| rowspan=2||-| β⁺, p (<.1%)| ⁴⁵Cr|-| rowspan=2|⁴⁷Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 21| rowspan=2|46.99289(28)#| rowspan=2|21.8(7) ms| β⁺ (>99.9%)| ⁴⁷Mn| rowspan=2|7/2−#| rowspan=2|| rowspan=2||-| β⁺, p (<.1%)| ⁴⁶Cr|-| rowspan=2|⁴⁸Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 22| rowspan=2|47.98050(8)#| rowspan=2|44(7) ms| β⁺ (96.41%)| ⁴⁸Mn| rowspan=2|0+| rowspan=2|| rowspan=2||-| β⁺, p (3.59%)| ⁴⁷Cr|-| rowspan=2|⁴⁹Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 23| rowspan=2|48.97361(16)#| rowspan=2|70(3) ms| β⁺, p (52%)| ⁴⁸Cr| rowspan=2|(7/2−)| rowspan=2|| rowspan=2||-| β⁺ (48%)| ⁴⁹Mn|-| rowspan=2|⁵⁰Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 24| rowspan=2|49.96299(6)| rowspan=2|155(11) ms| β⁺ (>99.9%)| ⁵⁰Mn| rowspan=2|0+| rowspan=2|| rowspan=2||-| β⁺, p (<.1%)| ⁴⁹Cr|-| ⁵¹Fe| style="text-align:right" | 26| style="text-align:right" | 25| 50.956820(16)| 305(5) ms| β⁺| ⁵¹Mn| 5/2−|||-| ⁵²Fe| style="text-align:right" | 26| style="text-align:right" | 26| 51.948114(7)| 8.275(8) h| β⁺| ^52mMn| 0+|||-| style="text-indent:1em" | ^52mFe| colspan="3" style="text-indent:2em" | 6.81(13) MeV| 45.9(6) s| β⁺| ⁵²Mn| (12+)#|||-| ⁵³Fe| style="text-align:right" | 26| style="text-align:right" | 27| 52.9453079(19)| 8.51(2) min| β⁺| ⁵³Mn| 7/2−|||-| style="text-indent:1em" | ^53mFe| colspan="3" style="text-indent:2em" | 3040.4(3) keV| 2.526(24) min| IT| ⁵³Fe| 19/2−|||-| ⁵⁴Fe| style="text-align:right" | 26| style="text-align:right" | 28| 53.9396090(5)| colspan=3 align=center|Observationally Stable| 0+| 0.05845(35)| 0.05837–0.05861|-| style="text-indent:1em" | ^54mFe| colspan="3" style="text-indent:2em" | 6526.9(6) keV| 364(7) ns||| 10+|||-| ⁵⁵Fe| style="text-align:right" | 26| style="text-align:right" | 29| 54.9382934(7)| 2.737(11) y| EC| ⁵⁵Mn| 3/2−|||-| ⁵⁶Fe^[3] | style="text-align:right" | 26| style="text-align:right" | 30| 55.9349363(5)| colspan=3 align=center|Stable| 0+| 0.91754(36)| 0.91742–0.91760|-| ⁵⁷Fe| style="text-align:right" | 26| style="text-align:right" | 31| 56.9353928(5)| colspan=3 align=center|Stable| 1/2−| 0.02119(10)| 0.02116–0.02121|-| ⁵⁸Fe| style="text-align:right" | 26| style="text-align:right" | 32| 57.9332744(5)| colspan=3 align=center|Stable| 0+| 0.00282(4)| 0.00281–0.00282|-| ⁵⁹Fe| style="text-align:right" | 26| style="text-align:right" | 33| 58.9348755(8)| 44.495(9) d| β⁻| ⁵⁹Co| 3/2−|||-| ⁶⁰Fe| style="text-align:right" | 26| style="text-align:right" | 34| 59.934072(4)| 2.6×10⁶ y| β⁻| ⁶⁰Co| 0+| trace||-| ⁶¹Fe| style="text-align:right" | 26| style="text-align:right" | 35| 60.936745(21)| 5.98(6) min| β⁻| ⁶¹Co| 3/2−,5/2−|||-| style="text-indent:1em" | ^61mFe| colspan="3" style="text-indent:2em" | 861(3) keV| 250(10) ns||| 9/2+#|||-| ⁶²Fe| style="text-align:right" | 26| style="text-align:right" | 36| 61.936767(16)| 68(2) s| β⁻| ⁶²Co| 0+|||-| ⁶³Fe| style="text-align:right" | 26| style="text-align:right" | 37| 62.94037(18)| 6.1(6) s| β⁻| ⁶³Co| (5/2)−|||-| ⁶⁴Fe| style="text-align:right" | 26| style="text-align:right" | 38| 63.9412(3)| 2.0(2) s| β⁻| ⁶⁴Co| 0+|||-| ⁶⁵Fe| style="text-align:right" | 26| style="text-align:right" | 39| 64.94538(26)| 1.3(3) s| β⁻| ⁶⁵Co| 1/2−#|||-| style="text-indent:1em" | ^65mFe| colspan="3" style="text-indent:2em" | 364(3) keV| 430(130) ns||| (5/2−)|||-| rowspan=2|⁶⁶Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 40| rowspan=2|65.94678(32)| rowspan=2|440(40) ms| β⁻ (>99.9%)| ⁶⁶Co| rowspan=2|0+| rowspan=2|| rowspan=2||-| β⁻, n (<.1%)| ⁶⁵Co|-| rowspan=2|⁶⁷Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 41| rowspan=2|66.95095(45)| rowspan=2|394(9) ms| β⁻ (>99.9%)| ⁶⁷Co| rowspan=2|1/2−#| rowspan=2|| rowspan=2||-| β⁻, n (<.1%)| ⁶⁶Co|-| style="text-indent:1em" | ^67mFe| colspan="3" style="text-indent:2em" | 367(3) keV| 64(17) μs||| (5/2−)|||-| rowspan=2|⁶⁸Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 42| rowspan=2|67.95370(75)| rowspan=2|187(6) ms| β⁻ (>99.9%)| ⁶⁸Co| rowspan=2|0+| rowspan=2|| rowspan=2||-| β⁻, n| ⁶⁷Co|-| rowspan=2|⁶⁹Fe| rowspan=2 style="text-align:right" | 26| rowspan=2 style="text-align:right" | 43| rowspan=2|68.95878(54)#| rowspan=2|109(9) ms| β⁻ (>99.9%)| ⁶⁹Co| rowspan=2|1/2−#| rowspan=2|| rowspan=2||-| β⁻, n (<.1%)| ⁶⁸Co|-| ⁷⁰Fe| style="text-align:right" | 26| style="text-align:right" | 44| 69.96146(64)#| 94(17) ms||| 0+|||-| ⁷¹Fe| style="text-align:right" | 26| style="text-align:right" | 45| 70.96672(86)#| 30# ms
[>300&nbsp;ns]||| 7/2+#|||-| ⁷²Fe| style="text-align:right" | 26| style="text-align:right" | 46| 71.96962(86)#| 10# ms
[>300&nbsp;ns]||| 0+||

• Atomic masses of the stable nuclides (⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe, and ⁵⁸Fe) are given by the AME2012 atomic mass evaluation. The one standard deviation errors are given in parentheses after the corresponding last digits.^[4]

Iron-54

⁵⁴Fe is observationally stable, but theoretically can decay to ⁵⁴Cr, with a half-life of more than years via double electron capture (εε).^[1]

Iron-56

See main article: 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.^[5] 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 ⁵⁶Ni. Thus, ⁵⁶Ni 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 ⁵⁶Fe 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 ⁵⁶Ni, which subsequently decays to ⁵⁶Co and then ⁵⁶Fe.

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.^[6] The transition was famously used to make the first definitive measurement of gravitational redshift, in the 1960 Pound–Rebka experiment.^[7]

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.^[8] Iron-58 is also an assisting reagent in the synthesis of superheavy elements.^[9]

Iron-60

Iron-60 is an iron isotope with a half-life of 2.6 million years,^{[10] [11]} 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 ⁶⁰Ni, the granddaughter isotope of ⁶⁰Fe, and the abundance of the stable iron isotopes could be found, which is evidence for the existence of ⁶⁰Fe at the time of formation of the Solar System. Possibly the energy released by the decay of ⁶⁰Fe contributed, together with the energy released by decay of the radionuclide ²⁶Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of ⁶⁰Ni 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.^{[12] [13]} Iron-60 is also found in sediments from 8 million years ago.^[14] In 2019, researchers found interstellar ⁶⁰Fe in Antarctica, which they relate to the Local Interstellar Cloud.^[15]

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 4πr². The fraction of the material intercepted by the Earth is dependent on its cross-sectional area (πR²_earth) as it passes through the expanding debris. Where M_ej is the mass of ejected material.$$M\_=\backslash frac\; M\_$$Assuming the intercepted material is distributed uniformly across the surface of the Earth (4πR²_earth), the mass surface density (Σ_ej) of the supernova ejecta on Earth is: $$\backslash Sigma\_=\backslash frac=\backslash frac$$The number of ⁶⁰Fe atoms per unit area found on Earth can be estimated if the typical amount of ⁶⁰Fe ejected from a supernova is known. This can be done by dividing the surface mass density (Σ_ej) by the atomic mass of ⁶⁰Fe. $$N\_=\backslash left(\backslash frac\backslash right)$$The equation for N₆₀ can be rearranged to find the distance to the supernova.$$$$

r=\sqrtAn example calculation for the distance to the supernova point of origin is given below. This calculation uses speculative values for terrestrial ⁶⁰Fe atom surface density (N₆₀ ≈ 4 × 10¹¹ atoms²/m) and a rough estimate of the mass of ⁶⁰Fe ejected in a supernova explosion (10^-5 M_☉). $$$$

r=\sqrt

$$$$

r=3 \times 10^ m=100 p c

More sophisticated analyses have been reported that take into consideration the flux and deposition of ⁶⁰Fe as well as possible interfering background sources.^[16]

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 ⁶⁰Fe synthesis is ongoing in our Galaxy, and probing element production in massive stars.^{[17] [18]}

References

Isotope masses from:

Isotopic compositions and standard atomic masses from:

Half-life, spin, and isomer data selected from:

Further reading

• Book: J. M. Nielsen . 1960 . The Radiochemistry of Iron . National Academy of Sciences/National Research Council.

Notes and References

1. Bikit . I. . Krmar . M. . Slivka . J. . Vesković . M. . Čonkić . Lj. . Aničin . I. . New results on the double β decay of iron . Physical Review C . 1998 . 58 . 4 . 2566–2567 . 10.1103/PhysRevC.58.2566. 1998PhRvC..58.2566B .
2. N. Dauphas . O. Rouxel . 2006 . Mass spectrometry and natural variations of iron isotopes . . 25 . 4. 515–550 . 10.1002/mas.20078 . 16463281. 2006MSRv...25..515D .
3. Lowest mass per nucleon of all nuclides; End product of stellar nucleosynthesis
4. 10.1088/1674-1137/36/12/003. The Ame2012 atomic mass evaluation. 2012. Wang. M.. Audi. G.. Wapstra. A.H.. Kondev. F.G.. MacCormick. M.. Xu. X.. Pfeiffer. B.. Chinese Physics C. 36. 12. 1603–2014. 2012ChPhC..36....3M. 11858/00-001M-0000-0010-23E8-5 . 250839471 .
5. The atomic nuclide with the highest mean binding energy. American Journal of Physics. 1995AmJPh..63..653F. Fewell. M. P.. 1995. 63. 7. 653. 10.1119/1.17828.
6. Web site: R. Nave . Mossbauer Effect in Iron-57 . . Georgia State University . 2009-10-13.
7. Pound. R. V.. Rebka Jr. G. A. . April 1, 1960. Apparent weight of photons. Physical Review Letters. 4. 7. 337–341. 10.1103/PhysRevLett.4.337. 1960PhRvL...4..337P. free.
8. Web site: Iron-58 Metal Isotope . 2023-06-28 . American Elements . en.
9. Web site: Vasiliev . Petr . Iron-58, Iron-58 Isotope, Enriched Iron-58, Iron-58 Metal . 2023-06-28 . www.buyisotope.com . en.
10. New Measurement of the ⁶⁰Fe Half-Life . . 103 . 7. 72502 . 10.1103/PhysRevLett.103.072502. 19792637 . 2009PhRvL.103g2502R. 2009. Rugel. G.. Faestermann. T.. Knie. K.. Korschinek. G.. Poutivtsev. M.. Schumann. D.. Kivel. N.. Günther-Leopold. I.. Weinreich. R.. Wohlmuther. M..
11. Web site: Eisen mit langem Atem. 27 August 2009. scienceticker. 22 May 2010. 3 February 2018. https://web.archive.org/web/20180203071537/http://www.scienceticker.info/2009/08/27/eisen-mit-langem-atem/#more-5677. dead.
12. Belinda Smith. Ancient bacteria store signs of supernova smattering. Cosmos. Aug 9, 2016.
13. Peter Ludwig. etal. Time-resolved 2-million-year-old supernova activity discovered in Earth's microfossil record. PNAS. 113. 33. 9232–9237. Aug 16, 2016. 10.1073/pnas.1601040113. 27503888. 4995991. 1710.09573. 2016PNAS..113.9232L. free.
14. Colin Barras. Fires may have given our evolution a kick-start. New Scientist. 236. 3147. 7. Oct 14, 2017. 2017NewSc.236....7B. 10.1016/S0262-4079(17)31997-8.
15. Interstellar ⁶⁰Fe in Antarctica . Dominik . Koll . al. . et. . Physical Review Letters . 2019 . 123 . 7 . 072701 . 10.1103/PhysRevLett.123.072701. 31491090 . 2019PhRvL.123g2701K . 201868513 . 1885/298253 . free .
16. 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 . The Institute of Physics (IOP).
17. 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 . 10.1051/0004-6361:200500093 . astro-ph/0502219 . 2005A&A...433L..49H . 0004-6361.
18. 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 . 10.3847/1538-4357/ab6336 . free . 1912.07874 . 2020ApJ...889..169W . 0004-637X.