Iron-55 decays via electron capturetomanganese-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-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 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 used, in 2016,[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]
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.[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]
^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.
^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. ISBN978-0-8031-0603-1.
^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. S2CID91254200.
