# Isotopes of iron

Natural iron ($_{26}$Fe) consists of four stable isotopes: 5.845% $^{54}$Fe (possibly radioactive with half-life >4.4×10$^{20}$ years), 91.754% $^{56}$Fe, 2.119% $^{57}$Fe and 0.286% $^{58}$Fe. There are 28 known radioisotopes and 8 nuclear isomers, the most stable of which are $^{60}$Fe (half-life 2.6 million years) and $^{55}$Fe (half-life 2.7 years).
Much of the past work on measuring the isotopic composition of iron has centered on determining $^{60}$Fe 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, though applications to biological and industrial systems are beginning to emerge.

## List of isotopes

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | Natural abundance (mole fraction) |
| Nuclide | Excitation energy | Excitation energy | Excitation energy | Half-life | Decay mode | Daughter isotope | Spin and parity | Normal proportion | Range of variation |
| $^{45}$Fe | 26 | 19 | 45.01547(30)# | 2.5(2) ms | 2p (70%) | $^{43}$Cr | 3/2+# | | |
| $^{45}$Fe | 26 | 19 | 45.01547(30)# | 2.5(2) ms | β$^+$, p (18.9%) | $^{44}$Cr | 3/2+# | | |
| $^{45}$Fe | 26 | 19 | 45.01547(30)# | 2.5(2) ms | β$^+$, 2p (7.8%) | $^{43}$V | 3/2+# | | |
| $^{45}$Fe | 26 | 19 | 45.01547(30)# | 2.5(2) ms | β$^+$ (3.3%) | $^{45}$Mn | 3/2+# | | |
| $^{46}$Fe | 26 | 20 | 46.00130(32)# | 13.0(20) ms | β$^+$, p (78.7%) | $^{45}$Cr | 0+ | | |
| $^{46}$Fe | 26 | 20 | 46.00130(32)# | 13.0(20) ms | β$^+$ (21.3%) | $^{46}$Mn | 0+ | | |
| $^{46}$Fe | 26 | 20 | 46.00130(32)# | 13.0(20) ms | β$^+$, 2p? | $^{44}$V | 0+ | | |
| $^{47}$Fe | 26 | 21 | 46.99235(54)# | 21.9(2) ms | β$^+$, p (88.4%) | $^{46}$Cr | 7/2−# | | |
| $^{47}$Fe | 26 | 21 | 46.99235(54)# | 21.9(2) ms | β$^+$ (11.6%) | $^{47}$Mn | 7/2−# | | |
| $^{48}$Fe | 26 | 22 | 47.980667(99) | 45.3(6) ms | β$^+$ (84.7%) | $^{48}$Mn | 0+ | | |
| $^{48}$Fe | 26 | 22 | 47.980667(99) | 45.3(6) ms | β$^+$, p (15.3%) | $^{47}$Cr | 0+ | | |
| $^{49}$Fe | 26 | 23 | 48.973429(26) | 64.7(3) ms | β$^+$, p (56.7%) | $^{48}$Cr | (7/2−) | | |
| $^{49}$Fe | 26 | 23 | 48.973429(26) | 64.7(3) ms | β$^+$ (43.3%) | $^{49}$Mn | (7/2−) | | |
| $^{50}$Fe | 26 | 24 | 49.9629880(90) | 152.0(6) ms | β$^+$ | $^{50}$Mn | 0+ | | |
| $^{50}$Fe | 26 | 24 | 49.9629880(90) | 152.0(6) ms | β$^+$, p? | $^{49}$Cr | 0+ | | |
| $^{51}$Fe | 26 | 25 | 50.9568551(15) | 305.4(23) ms | β$^+$ | $^{51}$Mn | 5/2− | | |
| $^{52}$Fe | 26 | 26 | 51.94811336(19) | 8.275(8) h | β$^+$ | $^{52}$Mn | 0+ | | |
| $^{52m}$Fe | 6960.7(3) keV | 6960.7(3) keV | 6960.7(3) keV | 45.9(6) s | β$^+$ (99.98%) | $^{52}$Mn | 12+ | | |
| $^{52m}$Fe | 6960.7(3) keV | 6960.7(3) keV | 6960.7(3) keV | 45.9(6) s | IT (0.021%) | $^{52}$Fe | 12+ | | |
| $^{53}$Fe | 26 | 27 | 52.9453056(18) | 8.51(2) min | β$^+$ | $^{53}$Mn | 7/2− | | |
| $^{53m}$Fe | 3040.4(3) keV | 3040.4(3) keV | 3040.4(3) keV | 2.54(2) min | IT | $^{53}$Fe | 19/2− | | |
| $^{54}$Fe | 26 | 28 | 53.93960819(37) | Observationally Stable | Observationally Stable | Observationally Stable | 0+ | 0.05845(105) | |
| $^{54m}$Fe | 6527.1(11) keV | 6527.1(11) keV | 6527.1(11) keV | 364(7) ns | IT | $^{54}$Fe | 10+ | | |
| $^{55}$Fe | 26 | 29 | 54.93829116(33) | 2.7562(4) y | EC | $^{55}$Mn | 3/2− | | |
| $^{56}$Fe | 26 | 30 | 55.93493554(29) | Stable | Stable | Stable | 0+ | 0.91754(106) | |
| $^{57}$Fe | 26 | 31 | 56.93539195(29) | Stable | Stable | Stable | 1/2− | 0.02119(29) | |
| $^{58}$Fe | 26 | 32 | 57.93327358(34) | Stable | Stable | Stable | 0+ | 0.00282(12) | |
| $^{59}$Fe | 26 | 33 | 58.93487349(35) | 44.500(12) d | β$^−$ | $^{59}$Co | 3/2− | | |
| $^{60}$Fe | 26 | 34 | 59.9340702(37) | 2.62(4)×10$^6$ y | β$^−$ | $^{60}$Co | 0+ | trace | |
| $^{61}$Fe | 26 | 35 | 60.9367462(28) | 5.98(6) min | β$^−$ | $^{61}$Co | (3/2−) | | |
| $^{61m}$Fe | 861.67(11) keV | 861.67(11) keV | 861.67(11) keV | 238(5) ns | IT | $^{61}$Fe | 9/2+ | | |
| $^{62}$Fe | 26 | 36 | 61.9367918(30) | 68(2) s | β$^−$ | $^{62}$Co | 0+ | | |
| $^{63}$Fe | 26 | 37 | 62.9402727(46) | 6.1(6) s | β$^−$ | $^{63}$Co | (5/2−) | | |
| $^{64}$Fe | 26 | 38 | 63.9409878(54) | 2.0(2) s | β$^−$ | $^{64}$Co | 0+ | | |
| $^{65}$Fe | 26 | 39 | 64.9450153(55) | 805(10) ms | β$^−$ | $^{65}$Co | (1/2−) | | |
| $^{65}$Fe | 26 | 39 | 64.9450153(55) | 805(10) ms | β$^−$, n? | $^{64}$Co | (1/2−) | | |
| $^{65m1}$Fe | 393.7(2) keV | 393.7(2) keV | 393.7(2) keV | 1.12(15) s | β$^−$? | $^{65}$Co | (9/2+) | | |
| $^{65m2}$Fe | 397.6(2) keV | 397.6(2) keV | 397.6(2) keV | 418(12) ns | IT | $^{65}$Fe | (5/2+) | | |
| $^{66}$Fe | 26 | 40 | 65.9462500(44) | 467(29) ms | β$^−$ | $^{66}$Co | 0+ | | |
| $^{66}$Fe | 26 | 40 | 65.9462500(44) | 467(29) ms | β$^−$, n? | $^{65}$Co | 0+ | | |
| $^{67}$Fe | 26 | 41 | 66.9509300(41) | 394(9) ms | β$^−$ | $^{67}$Co | (1/2-) | | |
| $^{67}$Fe | 26 | 41 | 66.9509300(41) | 394(9) ms | β$^−$, n? | $^{66}$Co | (1/2-) | | |
| $^{67m1}$Fe | 403(9) keV | 403(9) keV | 403(9) keV | 64(17) μs | IT | $^{67}$Fe | (5/2+,7/2+) | | |
| $^{67m2}$Fe | 450(100)# keV | 450(100)# keV | 450(100)# keV | 75(21) μs | IT | $^{67}$Fe | (9/2+) | | |
| $^{68}$Fe | 26 | 42 | 67.95288(21)# | 188(4) ms | β$^−$ | $^{68}$Co | 0+ | | |
| $^{68}$Fe | 26 | 42 | 67.95288(21)# | 188(4) ms | β$^−$, n? | $^{67}$Co | 0+ | | |
| $^{69}$Fe | 26 | 43 | 68.95792(22)# | 162(7) ms | β$^−$ | $^{69}$Co | 1/2−# | | |
| $^{69}$Fe | 26 | 43 | 68.95792(22)# | 162(7) ms | β$^−$, n? | $^{68}$Co | 1/2−# | | |
| $^{69}$Fe | 26 | 43 | 68.95792(22)# | 162(7) ms | β$^−$, 2n? | $^{67}$Co | 1/2−# | | |
| $^{70}$Fe | 26 | 44 | 69.96040(32)# | 61.4(7) ms | β$^−$ | $^{70}$Co | 0+ | | |
| $^{70}$Fe | 26 | 44 | 69.96040(32)# | 61.4(7) ms | β$^−$, n? | $^{69}$Co | 0+ | | |
| $^{71}$Fe | 26 | 45 | 70.96572(43)# | 34.3(26) ms | β$^−$ | $^{71}$Co | 7/2+# | | |
| $^{71}$Fe | 26 | 45 | 70.96572(43)# | 34.3(26) ms | β$^−$, n? | $^{70}$Co | 7/2+# | | |
| $^{71}$Fe | 26 | 45 | 70.96572(43)# | 34.3(26) ms | β$^−$, 2n? | $^{69}$Co | 7/2+# | | |
| $^{72}$Fe | 26 | 46 | 71.96860(54)# | 17.0(10) ms | β$^−$ | $^{72}$Co | 0+ | | |
| $^{72}$Fe | 26 | 46 | 71.96860(54)# | 17.0(10) ms | β$^−$, n? | $^{71}$Co | 0+ | | |
| $^{72}$Fe | 26 | 46 | 71.96860(54)# | 17.0(10) ms | β$^−$, 2n? | $^{70}$Co | 0+ | | |
| $^{73}$Fe | 26 | 47 | 72.97425(54)# | 12.9(16) ms | β$^−$ | $^{73}$Co | 7/2+# | | |
| $^{73}$Fe | 26 | 47 | 72.97425(54)# | 12.9(16) ms | β$^−$, n? | $^{72}$Co | 7/2+# | | |
| $^{73}$Fe | 26 | 47 | 72.97425(54)# | 12.9(16) ms | β$^−$, 2n? | $^{71}$Co | 7/2+# | | |
| $^{74}$Fe | 26 | 48 | 73.97782(54)# | 5(5) ms | β$^−$ | $^{74}$Co | 0+ | | |
| $^{74}$Fe | 26 | 48 | 73.97782(54)# | 5(5) ms | β$^−$, n? | $^{73}$Co | 0+ | | |
| $^{74}$Fe | 26 | 48 | 73.97782(54)# | 5(5) ms | β$^−$, 2n? | $^{72}$Co | 0+ | | |
| $^{75}$Fe | 26 | 49 | 74.98422(64)# | 9# ms [>620 ns] | β$^−$? | $^{75}$Co | 9/2+# | | |
| $^{75}$Fe | 26 | 49 | 74.98422(64)# | 9# ms [>620 ns] | β$^−$, n? | $^{74}$Co | 9/2+# | | |
| $^{75}$Fe | 26 | 49 | 74.98422(64)# | 9# ms [>620 ns] | β$^−$, 2n? | $^{73}$Co | 9/2+# | | |
| $^{76}$Fe | 26 | 50 | 75.98863(64)# | 3# ms [>410 ns] | β$^−$? | $^{76}$Co | 0+ | | |
| This table header & footer: view ||||||||||
* ↑ $^m$Fe – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Modes of decay: EC:Electron capture IT:Isomeric transition n:Neutron emission p:Proton emission
* ↑ Bold symbol as daughter – Daughter product is stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ Believed to decay by β$^+$β$^+$ to $^{54}$Cr with a half-life of over 4.4×10$^{20}$ a
* ↑ Lowest mass per nucleon of all nuclides; End product of stellar nucleosynthesis

## Iron-54

$^{54}$Fe is observationally stable, but theoretically can decay to $^{54}$Cr, with a half-life of more than 4.4×10$^{20}$ years via double electron capture (εε).

## Iron-56

$^{56}$Fe is the most abundant isotope of iron. It is also the isotope with the lowest mass per nucleon, 930.412 MeV/c$^2$, 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, $^{56}$Fe is a more common endpoint of fusion chains inside supernovae, where it is mostly produced as $^{56}$Ni. Thus, $^{56}$Ni is more common in the universe, relative to other metals, including $^{62}$Ni, $^{58}$Fe and $^{60}$Ni, all of which have a very high binding energy.
The high nuclear binding energy of $^{56}$Fe represents the point where further nuclear reactions become energetically unfavorable. Therefore 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 $^{56}$Ni, which subsequently decays to $^{56}$Co and then $^{56}$Fe.

## Iron-57

$^{57}$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. The transition was famously used to make the first definitive measurement of gravitational redshift, in the 1960 Pound–Rebka experiment.

## 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. Iron-58 is also an assisting reagent in the synthesis of superheavy elements.

## Iron-60

Iron-60 has a half-life of 2.6 million years, 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 $^{60}$Ni, the granddaughter isotope of $^{60}$Fe, and the abundance of the stable iron isotopes could be found, which is evidence for the existence of $^{60}$Fe at the time of formation of the Solar System. Possibly the energy from the decay of $^{60}$Fe contributed, together with the energy from the decay of the radionuclide $^{26}$Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of $^{60}$Ni in extraterrestrial material may also provide further insight into the origin of the Solar 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. Iron-60 is also found in sediments from 8 million years ago. In 2019, researchers found interstellar $^{60}$Fe in Antarctica, which they relate to the Local Interstellar Cloud.
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 4πr$^2$. The fraction of the material intercepted by the Earth is dependent on its cross-sectional area (πR$^2$$_{Earth}$) as it passes through the expanding debris. Where M$_{ej}$ is the mass of ejected material.$M_{\text{Fraction intercepted }}={\frac {\pi R_{\text{Earth }}^{2}}{4\pi r^{2}}}M_{ej}$Assuming the intercepted material is distributed uniformly across the surface of the Earth (4πR$^2$$_{Earth}$), the mass surface density (Σ$_{ej}$) of the supernova ejecta on Earth is: $\Sigma _{ej}={\frac {M_{\text{Fraction intercepted }}}{A_{\text{surface,Earth }}}}={\frac {M_{ej}}{16\pi r^{2}}}$The number of $^{60}$Fe atoms per unit area found on Earth can be estimated if the typical amount of $^{60}$Fe ejected from a supernova is known. This can be done by dividing the surface mass density (Σ$_{ej}$) by the atomic mass of $^{60}$Fe. $N_{60}=\left({\frac {M_{ej,60}/m_{60}}{16\pi r^{2}}}\right)$The equation for N$^{60}$ can be rearranged to find the distance to the supernova.$r={\sqrt {\frac {M_{ej,60}}{16\pi m_{60}N_{60}}}}$An example calculation for the distance to the supernova point of origin is given below. This calculation uses speculative values for terrestrial $^{60}$Fe atom surface density (N$_{60}$ ≈ 4 × 10$^{11}$ atoms/m$^2$) and a rough estimate of the mass of $^{60}$Fe ejected by a supernova (10$^{-5}$ M$_☉$). $r={\sqrt {\frac {10^{-5}M_{\odot }}{16\pi \left(60m_{p}\right)N_{60}}}}$$r=3\times 10^{18}m=100pc$More sophisticated analyses have been reported that take into consideration the flux and deposition of $^{60}$Fe as well as possible interfering background sources.
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 $^{60}$Fe synthesis is ongoing in our Galaxy, and probing element production in massive stars.