This is a kind of reversal of beta decay. Whereas in beta decay a neutron turns into a proton and expels an electron, electron capture transforms a proton into a neutron by neutralising its charge. So the atomic number drops by one, while the mass number remains the same. Potassium 40 (atomic number 19) decays to argon 40 (atomic number 18) by this means. And there are various other ways in which nuclei can be radioactively transformed into other nuclei.
