Naturaliron (₂₆Fe) consists of four stableisotopes: 5.845%⁵⁴Fe (possibly radioactive withhalf-life >4.4×10²⁰ years),^[4] 91.754%⁵⁶Fe, 2.119%⁵⁷Fe and 0.286%⁵⁸Fe. There are 28 known radioisotopes and 8nuclear isomers, the most stable of which are⁶⁰Fe (half-life 2.6 million years) and⁵⁵Fe (half-life 2.7 years).

Much of the past work on measuring the isotopic composition of iron has centered on determining⁶⁰Fe variations due to processes accompanyingnucleosynthesis (i.e.,meteorite studies) and ore formation. In the last decade however, advances inmass spectrometry technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of thestable isotopes of iron. Much of this work has been driven by theEarth andplanetary science communities, though applications to biological and industrial systems are beginning to emerge.^[5]

⁵⁴Fe is observationally stable, but theoretically can decay to⁵⁴Cr, with a half-life of more than4.4×10²⁰ years viadouble electron capture (εε).^[4]

⁵⁶Fe is the most abundant isotope of iron. It is also the isotope with the lowest mass per nucleon, 930.412 MeV/c², though not the isotope with the highestnuclear binding energy per nucleon, which isnickel-62.^[7] However, because of the details of how nucleosynthesis works,⁵⁶Fe is a more common endpoint of fusion chains insidesupernovae, where it is mostly produced as⁵⁶Ni. Thus,⁵⁶Ni is more common in the universe, relative to othermetals, including⁶²Ni,⁵⁸Fe and⁶⁰Ni, all of which have a very high binding energy.

The high nuclear binding energy of⁵⁶Fe represents the point where further nuclear reactions become energetically unfavorable. Therefore it is among the heaviest elements formed instellar nucleosynthesis reactions in massive stars. These reactions fuse lighter elements like magnesium, silicon, and sulfur to form heavier elements. Among the heavier elements formed is⁵⁶Ni, which subsequently decays to⁵⁶Co and then⁵⁶Fe.

⁵⁷Fe is widely used inMössbauer spectroscopy and the relatednuclear resonance vibrational spectroscopy due to the low natural variation in energy of the 14.4 keV nuclear transition.^[8] The transition was famously used to make the first definitive measurement ofgravitational redshift, in the 1960Pound–Rebka experiment.^[9]

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.^[10][11] Iron-58 is also an assisting reagent in the synthesis of superheavy elements.^[11]

Iron-60 has a half-life of 2.6 million years,^[12][13] but was thought until 2009 to have a half-life of 1.5 million years. It undergoesbeta decay tocobalt-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 meteoritesSemarkona andChervony Kut, a correlation between the concentration of⁶⁰Ni, thegranddaughter isotope of⁶⁰Fe, and the abundance of the stable iron isotopes could be found, which is evidence for the existence of⁶⁰Fe at the time of formation of the Solar System. Possibly the energy from the decay of⁶⁰Fe contributed, together with the energy from the decay of the radionuclide²⁶Al, to the remelting anddifferentiation ofasteroids after their formation 4.6 billion years ago. The abundance of⁶⁰Ni inextraterrestrial material may also provide further insight into the origin of theSolar System and its early history.

Iron-60 found in fossilized bacteria in sea floor sediments suggest there was a supernova near the Solar System about 2 million years ago.^[14][15] Iron-60 is also found in sediments from 8 million years ago.^[16] In 2019, researchers found interstellar⁶⁰Fe inAntarctica, which they relate to theLocal Interstellar Cloud.^[17]

The distance to thesupernova 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 surface area 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.$${\displaystyle M_{\text{Fraction intercepted }}=\frac{\pi R_{\text{Earth }}^2}{4\pi r^2}{M}_{ej}}$$ Assuming the intercepted material is distributed uniformly across the surface of the Earth (4πR²_Earth), the mass surface density (Σ_ej) of the supernova ejecta on Earth is:$${\displaystyle {\mathrm{\Sigma }}_{ej}=\frac{M_{\text{Fraction intercepted }}}{A_{\text{surface,Earth }}}=\frac{M_{ej}}{16\pi r^2}}$$ The number of⁶⁰Fe atoms per unit area found on Earth can be estimated if the typical amount of⁶⁰Fe ejected from a supernova is known. This can be done by dividing the surface mass density (Σ_ej) by the atomic mass of⁶⁰Fe.$${\displaystyle N_{60}=\left(\frac{M_{ej,60}/m_{60}}{16\pi r^2}\right)}$$ 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 terrestrial⁶⁰Fe atom surface density (N₆₀ ≈ 4 × 10¹¹ atoms/m²) and a rough estimate of the mass of⁶⁰Fe ejected by a supernova (10^-5 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 theflux and deposition of⁶⁰Fe as well as possible interfering background sources.^[18]

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 forgamma-ray astronomy, and have been detected by the gamma-ray observatoryINTEGRAL. The signal traces theGalactic plane, showing that⁶⁰Fe synthesis is ongoing in our Galaxy, and probingelement production in massive stars.^[19][20]

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.

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