Double electron capture

Double electron capture is a decay mode of atomic nucleus. For a nuclide (A, Z) with number of nucleons A and atomic number Z, double electron capture is only possible if the mass of the nuclide of (A, Z−2) is lower.

In this mode of decay, two of the orbital electrons are captured by two protons in the nucleus, forming two neutrons. Two neutrinos are emitted in the process. Since the protons are changed to neutrons, the number of neutrons increases by 2, the number of protons Z decreases by 2, and the atomic mass number A remains unchanged. By changing the number of protons, double electron capture transforms the nuclide into a new element.

Example:

In most cases this decay mode is masked by more probable modes (single electron capture etc.), but when all these modes are forbidden or strongly suppressed, double electron capture becomes the main mode of decay. There exist 35 naturally occurring isotopes that can undergo double electron capture. There is, however, only one confirmed observation of this process (for barium-130). One reason is that the probability of double electron capture is enormously small (the theoretical predictions of half-lives for this mode lies well above 10²⁰ years). A second reason is that the only detectable particles created in this process are X-rays and Auger electrons that are emitted by the excited atomic shell. In the range of their energies (~1–10 keV), the background is usually high. Thus, the experimental detection of double electron capture is more difficult than that for double beta decay. Double electron capture can be accompanied by the excitation of the daughter nucleus. Its de-excitation, in turn, is accompanied by an emission of photons with energies of hundreds of keV.

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