|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).