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Mode of radioactive decay

Double electron capture is a decay mode of an atomic nucleus. For a nuclide (A, Z) with a number of nucleons A and atomic number Z, double electron capture is only possible if the mass of the nuclide (A, Z−2) is lower. In this mode of decay, two of the orbital electrons are captured via the weak interaction 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 two, while the number of protons Z decreases by two, and the atomic mass number A remains unchanged. As a result, by reducing the atomic number by two, double electron capture transforms the nuclide into a different element. Example:
{| border="0"
| style="height:2em;"
|Link=yes barium ||+ ||2  ||→ ||Link=yes xenon ||+ ||2
|}

In most cases this decay mode is masked by other, more probable modes involving fewer particles, such as single electron capture. When all other modes are “forbidden” (strongly suppressed) double electron capture becomes the main mode of decay. There exist 34 naturally occurring nuclei that are believed to undergo double electron capture, but the process has been confirmed by observation in the decay of only three nuclides: 7836Kr, 12454Xe, and 13056Ba. One reason is that the probability of double electron capture is stupendously small; the half lives for this mode lie well above 10 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.

If the mass difference between the mother and daughter atoms is more than two masses of an electron (1.022 MeV), the energy released in the process is enough to allow another mode of decay, called electron capture with positron emission. It occurs along with double electron capture, their branching ratio depending on nuclear properties. When the mass difference is more than four electron masses (2.044 MeV), the third mode, called double positron decay, is allowed. Only six naturally occurring nuclides (78Kr, 96Ru, 106Cd, 124Xe, 130Ba, and 136Ce) plus the non primordial 148Gd and 154Dy are energetically allowed to decay via these three modes simultaneously.