|The difficulty of electron capture|
Weak forces are behind positron emission and electron capture. Electron capture occurs much less frequently than the emission of a positron. Whereas beta decay can occur spontaneously when energetically allowed, for an electron capture the weak forces requires that the electron come into close contact with a proton of the nucleus. 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).