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Iron-57

Main isotopes of iron
iso 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 (Ar)
  • 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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