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).
As a result, by reducing the atomic number by two, double electron capture transforms the nuclide into a different element.
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.
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.
However, if the lepton number is not conserved, or equivalently the neutrino is its own antiparticle, another kind of process can occur: the so-called neutrinoless double electron capture.