| Mass Number: | 55 |
| Symbol: | Fe |
| Num Neutrons: | 29 |
| Num Protons: | 26 |
| Decay Product: | Manganese-55 |
| Decay Mass: | 55 |
| Decay Symbol: | Mn |
| Decay Mode1: | Electron capture |
| Decay Energy1: | 0.00519 |
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 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 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]
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-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]