In a radioactive decay of a nucleus, an electron is also emitted. This may happen due to the fact that:

Radioactive decay involving electron emission is known as beta-minus (β−) decay. In this process, a neutron within the atomic nucleus transforms into a proton [2]. This conversion is governed by the weak interaction, where a down quark changes into an up quark [3]. To conserve electric charge, an electron (the beta particle) must be created and emitted because the neutral neutron becomes a positively charged proton [4]. Additionally, to satisfy the conservation of lepton number and momentum, an antineutrino is simultaneously produced and emitted [4]. It is important to note that electrons do not exist inside the nucleus prior to the decay; they are created at the exact instant of the transformation [1]. Conversely, the conversion of a proton into a neutron results in the emission of a positron (beta-plus decay), not an electron [2].

Sources

1. [1] https://www2.lbl.gov/abc/wallchart/chapters/03/2.html
2. [2] https://www.energy.gov/science/doe-explainsbeta-decay
3. [3] https://neutrinos.fnal.gov/sources/beta-decay/
4. [4] https://www.sciencedirect.com/topics/engineering/beta-decay