| General | |
|---|---|
| Symbol | 55Fe |
| Names | iron-55, 55Fe, Fe-55 |
| Protons(Z) | 26 |
| Neutrons(N) | 29 |
| Nuclide data | |
| Half-life(t1/2) | 2.737 years |
| Decay products | 55Mn |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Electron capture | 0.00519 |
| Isotopes of iron Complete table of nuclides | |
Iron-55 (55Fe) is aradioactive isotope ofiron with anucleus containing 26protons and 29neutrons. It decays byelectron capture tomanganese-55 and this process has a half-life of 2.737 years. The emittedX-rays can be used as an X-ray source for various scientific analysis methods, such asX-ray diffraction. Iron-55 is also a source forAuger electrons, which are produced during the decay.
Iron-55 decays viaelectron capture tomanganese-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 emittingAuger electrons of 5.19 keV, with a probability of about 60%,K-alpha-1X-rays with energy of 5.89875 keV and a probability about 16.2%,K-alpha-2X-rays with energy of 5.88765 keV and a probability of about 8.2%, orK-betaX-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 themanganese-55 after the electron capture have been used as a laboratory source of X-rays in variousX-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 asX-ray fluorescence instruments.[4] TheExoMars mission ofESA used, in 2016,[5][6] such an iron-55 source for its combinedX-ray diffraction/X-ray fluorescence spectrometer.[7] The 2011 Mars missionMSL used a functionally similar spectrometer, but with a traditional, electrically powered X-ray source.[8]
The Auger electrons can be applied inelectron capture detectors forgas chromatography. The more widely usednickel-63 sources provide electrons from beta decay.[9]
Iron-55 is most effectively produced by irradiation of iron withneutrons. The reaction (54Fe(n,γ)55Fe and56Fe(n,2n)55Fe) of the two most abundant isotopesiron-54 andiron-56 with neutrons yields iron-55. Most of the observed iron-55 is produced in these irradiation reactions, and it is not a primary fission product.[10] As a result ofatmospheric nuclear tests in the 1950s, and untilthe test ban in 1963, considerable amounts of iron-55 have been released into thebiosphere.[11] People close to the test ranges, for exampleIñupiat (Alaska Natives) and inhabitants of theMarshall Islands, accumulated significant amounts of radioactive iron. However, the shorthalf-life and the test ban decreased, within several years, the available amount of iron-55 nearly to the pre-nuclear test levels.[11][12]