Electron Capture

A minor mode ... competing with positon emission

Positron emission versus electron capture
The emission of a positron and the capture of an electron are twin reactions which both result in the diminution of the number of protons by 1 (from Z to Z-1) and the production of a neutrino. In the formula, the positron observed at the final stage of the beta decay appears as an electron in the initial stage of the capture. When comparing the energies, we can see that the 0.511 MeV rest mass energies of the electron and positron cancel each other out, revealing that the electron capture saves 1.022 MeV on the beta decay.

Electron capture is a comparatively minor decay mode caused by the weak force. The best-known example is of potassium 40, as 11% of the nuclei of that isotope of potassium present in our body decay by electronic capture.

The electron's capture trigger the emission of an invisible neutrino by the nucleus.

The capture of an electron has the same effect on a nucleus as the emission of a positron: one of the constituent protons transforms into a neutron, and the total electric charge diminishes by 1. Electron capture, along with beta-positive decay, is Nature's way of guaranteeing that no nucleus becomes too proton-heavy. Beta-negative decay has no competitor on Earth however, since the capture of positrons would occur in an antimatter world of science fiction.

The electron to be captured is taken from the group of electrons circling the nucleus, a process that turns out to be more difficult than it seems. Most of the electrons orbit the nucleus at a large radius compared to the nucleus. Even the innermost electron K-layer electrons are far from very small volume of the nucleus where the weak forces responsible for the capture operate. This explains why electron capture is difficult and therefore rare.

The difficulty of electron capture
The capture of an electron occurs much less frequently than the emission of a positron. The reactions are similar from the point of view of the weak forces responsible for both, but whereas beta decay can occur spontaneously, electron capture requires the absorption of the electron in the nucleus. The short distance over which the weak forces act dictate that the electron must come into contact with a proton's constituent quark to transform into a neutrino. The probability that an electron, even one belonging to the innermost 'K' shell, would find itself inside the nucleus is very low indeed (for potassium 40, the volume of the nucleus is less than a billionth of the K layer volume).

However, this mode of transformation is more economical from an energy perspective than positron emission; electron capture becomes the only viable process when very little energy is available.

Decay of this type often passes unseen, as the neutrino that carries away the released energy is impossible to detect. The recoiling nucleus also barely moves, with the few microns that it covers being too small to be observed.

These events would go unnoticed if it were not for the restructuring that the nucleus and electron shells both undergo. Electrons are usually captured from the inner K layer, leaving 'holes' behind them. An atom with a gap in its electron structure starts to rearrange itself, emitting X rays in the process or Auger’s electrons. Such a capture may also leave the nucleus at a higher energy level than it was at before, causing it to release gamma rays in an attempt to return to the ground state.

As a result, electron capture particular decay mode is very hard to detect. This particular decay mode was discovered only in 1937 by the American physicist Luis Alvarez (1911-1988), some forty years after the discovery of beta-negative radioactivity and only a few years after the observation of the positron and beta-positive decays.

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