Double electron capture

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Double electron capture is a

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.^[2]

Example:

130 56Ba

+

2  e−

→

130 54Xe

+

2  ν e

Rarity

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: ⁷⁸
₃₆Kr, ¹²⁴
₅₄Xe, and ¹³⁰
₅₆Ba.^[3]

One reason is that the probability of double electron capture is stupendously small; the

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.^[citation needed]

Modes with positron emission

If the mass difference between the mother and daughter atoms is more than two masses of an electron (1.022 

When the mass difference is more than four electron masses (2.044 MeV), the third mode, called

This mode of decay has never been observed experimentally, and would contradict the Standard Model if it were observed.

See also

• Double beta decay
• Neutrinoless double beta decay
• Beta decay
• Neutrino
• Particle radiation
• Radioactive isotope

References