section_path: Characteristics/Isotopes
section_name: Isotopes
paragraph_text: Isotopes
  
  Iron has four stable isotopes: Fe (5.845% of natural iron), Fe (91.754%), Fe (2.119%) and Fe (0.282%). Twenty-four artificial isotopes have also been created. Of these stable isotopes, only Fe has a nuclear spin (− 1 ⁄ 2). The nuclide Fe theoretically can undergo double electron capture to Cr, but the process has never been observed and only a lower limit on the half-life of 4.4×10 years has been established.
  
  Fe is an extinct radionuclide of long half-life (2.6 million years). It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide Ni. Much of the past work on isotopic composition of iron has focused on the nucleosynthesis of Fe through studies of meteorites and ore formation. In the last decade, advances in mass spectrometry have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work is driven by the Earth and planetary science communities, although applications to biological and industrial systems are emerging.
  
  In phases of the meteorites Semarkona and Chervony Kut, a correlation between the concentration of Ni, the granddaughter of Fe, and the abundance of the stable iron isotopes provided evidence for the existence of Fe at the time of formation of the Solar System. Possibly the energy released by the decay of Fe, along with that released by Al, contributed to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of Ni present in extraterrestrial material may bring further insight into the origin and early history of the Solar System.
  
  The most abundant iron isotope Fe is of particular interest to nuclear scientists because it represents the most common endpoint of nucleosynthesis. Since Ni (14 alpha particles) is easily produced from lighter nuclei in the alpha process in nuclear reactions in supernovae (see silicon burning process), it is the endpoint of fusion chains inside extremely massive stars. Although adding more alpha particles is possible, but nonetheless the sequence does effectively end at Ni because conditions in stellar interiors cause the competition between photodisintegration and the alpha process to favor photodisintegration around Ni. This Ni, which has a half-life of about 6 days, is created in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the supernova remnant gas cloud, first to radioactive Co, and then to stable Fe. As such, iron is the most abundant element in the core of red giants, and is the most abundant metal in iron meteorites and in the dense metal cores of planets such as Earth. It is also very common in the universe, relative to other stable metals of approximately the same atomic weight. Iron is the sixth most abundant element in the universe, and the most common refractory element.
  
  Although a further tiny energy gain could be extracted by synthesizing Ni, which has a marginally higher binding energy than Fe, conditions in stars are unsuitable for this process. Element production in supernovas greatly favor iron over nickel, and in any case, Fe still has a lower mass per nucleon than Ni due to its higher fraction of lighter protons. Hence, elements heavier than iron require a supernova for their formation, involving rapid neutron capture by starting Fe nuclei.
  
  In the far future of the universe, assuming that proton decay does not occur, cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into Fe nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting all stellar-mass objects to cold spheres of pure iron.