Iron-55 | |
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General | |
Name, symbol | Iron-55,55Fe |
Neutrons | 29 |
Protons | 26 |
Nuclide data | |
Half-life | 2.737 years |
Decay products | 55Mn |
Decay mode | Decay energy |
Electron capture | 0.00519 MeV |
Iron-55 or 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.
Contents |
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 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 5.889 keV and a probability about 16.2 %, [[K-alpha]-2] X-rays with energy of 5.888 keV and a probability of about 8.2 %, or K-beta X-rays with nominal energy of 6.49 keV and a probability about 2.85 %. The energies of these 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 MSL09 will use 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.[3]
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-55 is produced in these irradiation reactions, and it is not a primary fission product.[9] 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.[10] People close to the test ranges, for example Eskimos 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.[10][11]