iron-55

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.

==Decay== 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. The remaining 13% is accounted for by capture from shells higher than K, resulting in lower-energy photons and electrons.