Isotopes of iron

Isotopes of iron (₂₆Fe)

Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 54Fe 5.85% stable 55Fe synth 2.73 y ε 55Mn 56Fe 91.8% stable 57Fe 2.12% stable 58Fe 0.28% stable 59Fe synth 44.6 d β− 59Co 60Fe trace 2.6×106 y β− 60Co
Standard atomic weight Ar°(Fe)
	55.845±0.002 55.845±0.002 (abridged)

Naturally occurring iron (₂₆Fe) consists of four stable isotopes: 5.845% of ⁵⁴Fe (possibly radioactive with a half-life over 4.4×10²⁰ years), 91.754% of ⁵⁶Fe, 2.119% of ⁵⁷Fe and 0.286% of ⁵⁸Fe. There are 24 known radioactive isotopes, the most stable of which are ⁶⁰Fe (half-life 2.6 million years) and ⁵⁵Fe (half-life 2.7 years).

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

List of isotopes

Nuclide	Z	N	Isotopic mass (Da)	Half-life	Decay mode	Daughter isotope	Spin and parity	Natural abundance (mole fraction)
	Excitation energy							Normal proportion	Range of variation
45Fe	26	19	45.01458(24)#	1.89(49) ms	β+ (30%)	45Mn	3/2+#
					2p (70%)	43Cr
46Fe	26	20	46.00081(38)#	9(4) ms [12(+4-3) ms]	β+ (>99.9%)	46Mn	0+
					β+, p (<.1%)	45Cr
47Fe	26	21	46.99289(28)#	21.8(7) ms	β+ (>99.9%)	47Mn	7/2−#
					β+, p (<.1%)	46Cr
48Fe	26	22	47.98050(8)#	44(7) ms	β+ (96.41%)	48Mn	0+
					β+, p (3.59%)	47Cr
49Fe	26	23	48.97361(16)#	70(3) ms	β+, p (52%)	48Cr	(7/2−)
					β+ (48%)	49Mn
50Fe	26	24	49.96299(6)	155(11) ms	β+ (>99.9%)	50Mn	0+
					β+, p (<.1%)	49Cr
51Fe	26	25	50.956820(16)	305(5) ms	β+	51Mn	5/2−
52Fe	26	26	51.948114(7)	8.275(8) h	β+	52mMn	0+
52mFe	6.81(13) MeV			45.9(6) s	β+	52Mn	(12+)#
53Fe	26	27	52.9453079(19)	8.51(2) min	β+	53Mn	7/2−
53mFe	3040.4(3) keV			2.526(24) min	IT	53Fe	19/2−
54Fe	26	28	53.9396090(5)	Observationally Stable			0+	0.05845(35)	0.05837–0.05861
54mFe	6526.9(6) keV			364(7) ns			10+
55Fe	26	29	54.9382934(7)	2.737(11) y	EC	55Mn	3/2−
56Fe	26	30	55.9349363(5)	Stable			0+	0.91754(36)	0.91742–0.91760
57Fe	26	31	56.9353928(5)	Stable			1/2−	0.02119(10)	0.02116–0.02121
58Fe	26	32	57.9332744(5)	Stable			0+	0.00282(4)	0.00281–0.00282
59Fe	26	33	58.9348755(8)	44.495(9) d	β−	59Co	3/2−
60Fe	26	34	59.934072(4)	2.6×106 y	β−	60Co	0+	trace
61Fe	26	35	60.936745(21)	5.98(6) min	β−	61Co	3/2−,5/2−
61mFe	861(3) keV			250(10) ns			9/2+#
62Fe	26	36	61.936767(16)	68(2) s	β−	62Co	0+
63Fe	26	37	62.94037(18)	6.1(6) s	β−	63Co	(5/2)−
64Fe	26	38	63.9412(3)	2.0(2) s	β−	64Co	0+
65Fe	26	39	64.94538(26)	1.3(3) s	β−	65Co	1/2−#
65mFe	364(3) keV			430(130) ns			(5/2−)
66Fe	26	40	65.94678(32)	440(40) ms	β− (>99.9%)	66Co	0+
					β−, n (<.1%)	65Co
67Fe	26	41	66.95095(45)	394(9) ms	β− (>99.9%)	67Co	1/2−#
					β−, n (<.1%)	66Co
67mFe	367(3) keV			64(17) μs			(5/2−)
68Fe	26	42	67.95370(75)	187(6) ms	β− (>99.9%)	68Co	0+
					β−, n	67Co
69Fe	26	43	68.95878(54)#	109(9) ms	β− (>99.9%)	69Co	1/2−#
					β−, n (<.1%)	68Co
70Fe	26	44	69.96146(64)#	94(17) ms			0+
71Fe	26	45	70.96672(86)#	30# ms [>300 ns]			7/2+#
72Fe	26	46	71.96962(86)#	10# ms [>300 ns]			0+
This table header & footer:

• Atomic masses of the stable nuclides (⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe, and ⁵⁸Fe) are given by the AME2012 atomic mass evaluation. The one standard deviation errors are given in parentheses after the corresponding last digits.

Iron-54

⁵⁴Fe is observationally stable, but theoretically can decay to ⁵⁴Cr, with a half-life of more than 4.4×10²⁰ years via double electron capture (εε).

Iron-56

Main article: 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. However, because of the details of how nucleosynthesis works, ⁵⁶Fe is a more common endpoint of fusion chains inside supernovae, where it is mostly produced as ⁵⁶Ni. Thus, ⁵⁶Ni is more common in the universe, relative to other metals, including ⁶²Ni, ⁵⁸Fe and ⁶⁰Ni, all of which have a very high binding energy.

The high nuclear binding energy for ⁵⁶Fe represents the point where further nuclear reactions become energetically unfavorable. Because of this, 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 ⁵⁶Ni, which subsequently decays to ⁵⁶Co and then ⁵⁶Fe.

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. 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 is an iron isotope with 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 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. Possibly the energy released by the decay of ⁶⁰Fe contributed, together with the energy released by decay of the radionuclide ²⁶Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of ⁶⁰Ni present in extraterrestrial material may also provide further insight into the origin of the Solar System and its early history.

Iron-60 found in fossilised bacteria in sea floor sediments suggest there was a supernova in the vicinity of the Solar System approximately 2 million years ago. Iron-60 is also found in sediments from 8 million years ago. In 2019, researchers found interstellar ⁶⁰Fe 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 a surface area of 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.$$M_{\text{Fraction intercepted }}={\frac {\pi R_{\text{Earth }}^{2}}{4\pi r^{2}}}M_{ej}$$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: $$\Sigma _{ej}={\frac {M_{\text{Fraction intercepted }}}{A_{\text{surface,Earth }}}}={\frac {M_{ej}}{16\pi r^{2}}}$$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. $$N_{60}=\left({\frac {M_{ej,60}/m_{60}}{16\pi r^{2}}}\right)$$The equation for N₆₀ can be rearranged to find the distance to the supernova.$$r={\sqrt {\frac {M_{ej,60}}{16\pi m_{60}N_{60}}}}$$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 atoms²/m) and a rough estimate of the mass of ⁶⁰Fe ejected in a supernova explosion (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 ⁶⁰Fe 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 ⁶⁰Fe synthesis is ongoing in our Galaxy, and probing element production in massive stars.

References

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.

Further reading

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