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Iron-55 (55Fe) is a radioactive isotope of iron with a nucleus containing 26 protons and 29 neutrons. It decays by electron capture to manganese-55 and this process has a half-life of 2.737 years. The emitted X-rays can be used as an X-ray source for various scientific analysis methods, such as X-ray diffraction. Iron-55 is also a source for Auger electrons, which are produced during the decay.

Iron-55, 55Fe
Name, symbolIron-55, 55Fe
Nuclide data
Half-life2.737 years
Decay products55Mn
Decay modes
Decay modeDecay energy (MeV)
Electron capture0.00519
Isotopes of iron
Complete table of nuclides


Iron-55 decays via electron capture to manganese-55 with a half-life of 2.737 years.[1] The electrons around the nucleus rapidly adjust themselves to the lowered charge without leaving their shell, and shortly thereafter the vacancy in the "K" shell left by the nuclear-captured electron is filled by an electron from a higher shell. The difference in energy is released by emitting Auger electrons of 5.19 keV, with a probability of about 60%, K-alpha-1 X-rays with energy of 5.89875 keV and a probability about 16.2%, K-alpha-2 X-rays with energy of 5.88765 keV and a probability of about 8.2%, or K-beta X-rays with nominal energy of 6.49045 keV and a probability about 2.85%. The energies of the K-alpha-1 and -2 X-rays are so similar that they are often specified as mono-energetic radiation with 5.9 keV photon energy. Its probability is about 28%.[2] The remaining 12% is accounted for by lower-energy Auger electrons and a few photons from other, minor transitions.


The K-alpha X-rays emitted by the manganese-55 after the electron capture have been used as a laboratory source of X-rays in various X-ray scattering techniques. The advantages of the emitted X-rays are that they are monochromatic and are continuously produced over a years-long period.[3] No electrical power is needed for this emission, which is ideal for portable X-ray instruments, such as X-ray fluorescence instruments.[4] The ExoMars mission of ESA is planned to use, in 2018,[5][6] such an iron-55 source for its combined X-ray diffraction/X-ray fluorescence spectrometer.[7] The 2011 Mars mission MSL used a functionally similar spectrometer, but with a traditional, electrically powered X-ray source.[8]

The Auger electrons can be applied in electron capture detectors for gas chromatography. The more widely used nickel-63 sources provide electrons from beta decay.[9]


Iron-55 is most effectively produced by irradiation of iron with neutrons. The reaction (54Fe(n,γ)55Fe and 56Fe(n,2n)55Fe) of the two most abundant isotopes iron-54 and iron-56 with neutrons yields iron-55. Most of the observed iron-56 is produced in these irradiation reactions, and it is not a primary fission product.[10] As a result of atmospheric nuclear tests in the 1950s, and until the test ban in 1963, considerable amounts of iron-55 have been released into the biosphere.[11] People close to the test ranges, for example Iñupiat (Alaska Natives) and inhabitants of the Marshall Islands, accumulated significant amounts of radioactive iron. However, the short half-life and the test ban decreased, within several years, the available amount of iron-55 nearly to the pre-nuclear test levels.[11][12]


  1. ^ Georges, Audi (2003). "The NUBASE Evaluation of Nuclear and Decay Properties". Nuclear Physics A. 729 (1): 3–128. Bibcode:2003NuPhA.729....3A. CiteSeerX doi:10.1016/j.nuclphysa.2003.11.001.
  2. ^ Esam M. A. Hussein (2003). Handbook on radiation probing, gauging, imaging and analysis. Springer. p. 26. ISBN 978-1-4020-1294-5.
  3. ^ Preuss, Luther E. (1966). "Demonstration of X-ray Diffraction by LiF using the Mn Kα X-rays Resulting From 55Fe decay". Applied Physics Letters. 9 (4): 159–161. Bibcode:1966ApPhL...9..159P. doi:10.1063/1.1754691.
  4. ^ Himmelsbach, B. (1982). "Portable X-ray Survey Meters for In Situ Trace element Monitoring of Air Particulates". Toxic Materials in the Atmosphere, Sampling and Analysis. ISBN 978-0-8031-0603-1.
  5. ^ "The ESA-NASA ExoMars Programme Rover, 2018". ESA. Retrieved 2010-03-12.
  6. ^ "The ExoMars instrument suite". ESA. Retrieved 2010-03-12.
  7. ^ Marinangeli, L.; Hutchinson, I.; Baliva, A.; Stevoli, A.; Ambrosi, R.; Critani, F.; Delhez, R.; Scandelli, L.; Holland, A.; Nelms, N.; Mars-Xrd Team (March 12–16, 2007). An European XRD/XRF Instrument for the ExoMars Mission. 38th Lunar and Planetary Science Conference. Lunar and Planetary Science Conference (1338). League City, Texas. p. 1322. Bibcode:2007LPI....38.1322M.
  8. ^ Chemistry & Mineralogy (CheMin), NASA
  9. ^ D.J. Dwight; E.A. Lorch; J.E. Lovelock (1976). "Iron-55 as an auger electron emitter : Novel source for gas chromatography detectors". Journal of Chromatography A. 116 (2): 257–261. doi:10.1016/S0021-9673(00)89896-9.
  10. ^ Preston, A. (1970). "Concentrations of iron-55 in commercial fish species from the North Atlantic". Marine Biology. 6 (4): 345–349. doi:10.1007/BF00353667.
  11. ^ a b Palmer, H. E.; Beasley, T. M. (1965). "Iron-55 in Humans and Their Foods". Science. 149 (3682): 431–2. Bibcode:1965Sci...149..431P. doi:10.1126/science.149.3682.431. PMID 17809410.
  12. ^ Beasley, T. M.; Held, E. E.; Conard, R. M.E. (1965). "Iron-55 in Rongelap people, fish and soils". Health Physics. 22 (3): 245–50. doi:10.1097/00004032-197203000-00005. PMID 5062744.

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