Radioactive emission of beta particles (energetic electrons) occurs when a nucleus bound neutron becomes a proton. This causes the emission of an electron and anti-neutrino.
Because the mass of a neutron is greater than that of a proton free, it also decays in the same way, i.e. into an electron, proton and anti-neutrino.
With the emission of a prototron and a neutrino, a bound proton can also become a neutron. This conversion is only possible if a bound proton within the nucleus. It can be converted into a neutron or a positron and a neutrino.
Although electrons (positrons), are not found in the nucleus of the cell, they are created spontaneously during decay and are emitted.