Natural iron (₂₆Fe) consists of four stable isotopes: 5.85% ⁵⁴Fe, 91.75% ⁵⁶Fe, 2.12% ⁵⁷Fe and 0.28% ⁵⁸Fe. There are 28 known radioisotopes and 8 nuclear isomers, the most stable of which are ⁶⁰Fe (half-life 2.62 million years) and ⁵⁵Fe (half-life 2.7562 years).

Much of the past work on measuring the isotopic composition of iron has centered on determining ⁶⁰Fe variations due to processes accompanying nucleosynthesis (e.g., 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, though applications to biological and industrial systems are beginning to emerge.^[4]

List of isotopes

Iron-56

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 highest nuclear binding energy per nucleon, which is nickel-62.^[11] However, because of the details of how nucleosynthesis works, ⁵⁶Fe is a more common endpoint of fusion inside supernovae, where it is mostly produced as ⁵⁶Ni, which subsequently decays to ⁵⁶Co and then iron. Thus, ⁵⁶Fe is more common in the universe, relative to other heavy elements, including ⁶²Ni, ⁵⁸Fe, and ⁶⁰Ni, all of which have a comparably high binding energy.

Iron-57

⁵⁷Fe 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.^[12] The transition was famously used to make the first definitive measurement of gravitational redshift, in the 1960 Pound–Rebka experiment.^[13]

Iron-60

Iron-60 has a half-life of 2.62 million years,^[14] but was thought until 2009 to have a half-life of 1.5 million years. It undergoes beta decay to ⁶⁰Co, which then decays with the much shorter half-life of about 5 years to stable ⁶⁰Ni.

In phases of the meteorites Semarkona and Chervony Kut, a correlation between the excess 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.^[15] Depending on its original abundance, the energy from the decay of ⁶⁰Fe may have been significant, along with that of ²⁶Al, to the remelting and differentiation of asteroids and planetesimals after their formation. These nickel abundances in extraterrestrial materials may also provide further insight into the origin of the Solar System and its early history.

Live (interstellar) iron-60 was first identified in deep sea sediments in 1999.^[16] These are deep sea ferromanganese crusts, which are constantly growing, aggregating iron, manganese, and other elements.^[17] Iron-60 has been found in fossilized bacteria in sea floor sediments.^[18][19] In 2019, researchers found ⁶⁰Fe in Antarctica.^[20] Iron-60 shows two peaks in deep sea sediments, the first 1.7–3.2 million years ago and the second 6.5–8.7 million years ago. The peaks are related to the passage of the Solar System through the Local Bubble and likely the Orion–Eridanus Superbubble. These superbubbles were created by multiple supernovae.^[21] Traces of iron-60 have also been found in lunar samples.

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 surface area 4πr². The fraction of the material intercepted by the Earth is dependent on its cross-sectional area (πR 2
Earth ) as it passes through the expanding debris:

$${\displaystyle M_{\text{Fraction intercepted }}={\frac {\pi R_{\text{Earth }}^{2}}{4\pi r^{2}}}M_{ej}}$$

where M_ej is the mass of ejected material. Assuming the intercepted material is distributed uniformly across the surface of the Earth (4πR 2
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 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.

$${\displaystyle N_{60}=\left({\frac {M_{ej,60}/m_{60}}{16\pi r^{2}}}\right)}$$

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 ⁶⁰Fe atom surface density (N₆₀ ≈ 4 × 10¹¹ atoms/m²) and a rough estimate of the mass of ⁶⁰Fe ejected by a supernova (10×10⁻⁵ M_☉).

$${\displaystyle {\begin{aligned}r&={\sqrt {\frac {10^{-5}M_{\odot }}{16\pi \left(60m_{p}\right)N_{60}}}}\\r&=3\times 10^{18}m=100pc\end{aligned}}}$$

More sophisticated analyses have been reported that take into consideration the flux and deposition of ⁶⁰Fe as well as possible interfering background sources.^[22]

Cobalt-60, the decay product of iron-60, emits 1.173 MeV and 1.332 MeV gamma rays as it decays. These 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.^[23][24]

See also

Daughter products other than iron

• Isotopes of cobalt
• Isotopes of manganese
• Isotopes of chromium
• Isotopes of vanadium

References

Further reading

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