Iron-58

Main isotopes of iron

isotope	NA	half-life	DM	DE (MeV)	DP
⁵⁴Fe	5.85%	is stable with 28 neutrons
⁵⁵Fe	syn	2.73 y	ε	0.231	⁵⁵Mn
⁵⁶Fe	91.75%	is stable with 30 neutrons
⁵⁷Fe	2.12%	is stable with 31 neutrons
⁵⁸Fe	0.28%	is stable with 32 neutrons
⁵⁹Fe	syn	44.6 d	β⁻	1.565	⁵⁹Co
⁶⁰Fe	syn	2.6×10⁶ y	β⁻	3.978	⁶⁰Co

Standard atomic weight (A_r)

55.845(2)

Naturally occurring iron (₂₆Fe) consists of four stable isotopes: 5.845% of ⁵⁴Fe (possibly radioactive with a half-life over 3.1×10²² years), 91.754% of ⁵⁶Fe, 2.119% of ⁵⁷Fe and 0.282% of ⁵⁸Fe. There are 24 known radioactive isotopes and their half-lives are shown below. See Brookhaven National Laboratory Interactive Table of Nuclides for a more accurate reading.

Much of the past work on measuring the isotopic composition of Fe 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.

⁵⁴Fe is observationally stable, but theoretically can decay to ⁵⁴Cr, with a half-life of more than 3.1x10²² years via double electron capture (εε).

The isotope ⁵⁶Fe is 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. However, because of the details of how nucleosynthesis works, ⁵⁶Fe is a more common endpoint of fusion chains inside extremely massive stars and is therefore more common in the universe, relative to other metals, including ⁶²Ni, ⁵⁸Fe and ⁶⁰Ni, all of which have a very high binding energy.

The isotope 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. The transition was famously used to make the first definitive measurement of gravitational redshift, in the 1960 Pound-Rebka experiment.

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