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Electron capture (sometimes called inverse beta decay) is a decay mode for isotopes that will occur when there are too many protons in the nucleus of an atom and insufficient energy to emit a positron; however, it continues to be a viable decay mode for radioactive isotopes that can decay by positron emission. If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden and electron capture is the sole decay mode. For example, Rubidium-83 will decay to Krypton-83 solely by electron capture (the energy difference is about 0.9 MeV).

In this case, one of the orbital electrons, usually from the K or L electron shell (K-electron capture, also K-capture, or L-electron capture, L-capture), is captured by a proton in the nucleus, forming a neutron and a neutrino.

p  e  →  n  νe

Note that a free proton cannot normally be changed to a free neutron by this process. The proton and neutron must be part of a larger nucleus. Since the proton is changed to a neutron, the number of neutrons increases by 1, the number of protons decreases by 1, and the atomic mass number remains unchanged. By changing the number of protons, electron capture transforms the nuclide into a new element. The atom moves into an excited state with the inner shell missing an electron. When transiting to the ground state, the atom will emit an X-ray photon (a type of electromagnetic radiation) and/or Auger electrons.

Contents

History

The theory of electron capture was first discussed by Gian-Carlo Wick in a 1934 paper, and then developed by Hideki Yukawa and others. K-electron capture was first observed by Luis Alvarez, in vanadium-48. He reported it in a 1937 paper in the Physical Review.[1][2][3] Alvarez went on to study electron capture in gallium-67 and other nuclides.[1][4][5]

Reaction details

Examples:
2613Al  e  →  2612Mg  νe
5928Ni  e  →  5927Co  νe
4019K  e  →  4018Ar  νe

Note that it is one of the initial atom's own electrons that is captured, not a new, incoming electron as might be suggested by the way the above reactions are written. Radioactive isotopes which decay by pure electron capture can, in theory, be inhibited from radioactive decay if they are fully ionized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by the r-process in exploding supernovae, are ejected fully ionized and so do not undergo radioactive decay as long as they do not encounter electrons in outer space. Anomalies in elemental distributions are thought to be partly a result of this effect on electron capture.

Chemical bonds can also affect the rate of electron capture to a small degree (generally less than 1%) depending on the proximity of electrons to the nucleus.[1]

Around the elements in the middle of the periodic table, isotopes that are lighter than stable isotopes of the same element tend to decay through electron capture, while isotopes heavier than the stable ones decay by electron emission.

Common examples

Some common radioisotopes that decay by electron capture include:

Radioisotope Half-life
7Be 53.28 d
37Ar 35.0 d
41Ca 1.03E5 a
44Ti 52 a
49V 337 d
51Cr 27.7 d
53Mn 3.7E6 a
57Co 271.8 d
56Ni 6.10 d
67Ga 3.260 d
68Ge 270.8 d
72Se 8.5 d

For a full list, see the table of nuclides.

References

  1. ^ a b pp. 11–12, K-Electron Capture by Nuclei, Emilio Segré, chapter 3 in Discovering Alvarez: selected works of Luis W. Alvarez, with commentary by his students and colleagues, Luis W. Alvarez and W. Peter Trower, University of Chicago Press, 1987, ISBN 0226813045.
  2. ^ Luis Alvarez, The Nobel Prize in Physics 1968, biography, nobelprize.org. Accessed on line October 7, 2009.
  3. ^ Nuclear K Electron Capture, Luis W. Alvarez, Physical Review 52 (1937), pp. 134–135, doi:10.1103/PhysRev.52.134 .
  4. ^ Electron Capture and Internal Conversion in Gallium 67, Luis W. Alvarez, Physical Review 53 (1937), p. 606, doi:10.1103/PhysRev.53.606.
  5. ^ The Capture of Orbital Electrons by Nuclei, Luis W. Alvarez, Physical Review 54 (October 1, 1938), pp. 486–497, doi:10.1103/PhysRev.54.486.

External links

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