In radioactive decay process, the negatively charged emitted  β-particles are

Beta decay can involve the emission of either electrons or positrons. The electrons or positrons emitted in a β-decay do not exist inside the nucleus. They are only created at the time of emission, just as photons are created when an atom makes a transition from higher to a lower energy state.In negative β-decay a neutron in the nucleus is transformed into a proton,an electron and an antineutrino.  Hence, in radioactive decay process.  the negatively charged emitted β-particles are the electrons produced as a result of the decay of neutrons present inside the nucleus.