| General | |
|---|---|
| Symbol | 55Fe |
| Names | iron-55 |
| Protons (Z) | 26 |
| Neutrons (N) | 29 |
| Nuclide data | |
| Half-life (t1/2) | 2.7562 years [1] |
| Decay products | 55Mn |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Electron capture | 0.2312 [2] |
| Isotopes of iron Complete table of nuclides | |
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 with a half-life of 2.7562 years. This decay is to the ground state of the daughter, so emits only X-rays and Auger electrons. It is sometimes used as an X-ray source for various scientific analysis methods, such as X-ray diffraction and X-ray fluorescence.
Iron-55 decays via electron capture to manganese-55, after which the electrons around the nucleus rapidly adjust themselves to the lowered charge without leaving their shell, and shortly thereafter the vacancy (normally in the K shell) left by the 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 probability 60.1%, K-alpha-1 X-rays with energy of 5.89875 keV and probability 16.2%, K-alpha-2 X-rays with energy of 5.88765 keV and probability 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. [3] The remaining 13% is accounted for by capture from shells higher than K, resulting in lower-energy photons and electrons.
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. [4] No electrical power is needed for this emission, which is ideal for portable X-ray instruments, such as X-ray fluorescence instruments. [5] The ExoMars mission of ESA used, in 2016, [6] [7] such an iron-55 source for its combined X-ray diffraction/X-ray fluorescence spectrometer. [8] The 2011 Mars mission MSL used a functionally similar spectrometer, but with a traditional, electrically powered X-ray source. [9]
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. [10]
Iron-55 is most effectively produced by irradiation of iron with neutrons (in a nuclear reactor or detonation). The reactions (54Fe(n,γ)55Fe and 56Fe(n,2n)55Fe) of the two most abundant isotopes iron-54 and iron-56 with neutrons yield iron-55. Most of the observed iron-55 is produced by these irradiation reactions; it is not a primary fission product. [11]
As a result of atmospheric nuclear tests in the 1950s, and until the test ban in 1963, considerable amounts of iron-55 were released into the biosphere. [12] During this time, 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. [12] [13]