Final answer:
In beta decay, a neutron within an unstable nucleus transforms into a proton and an electron (beta particle) is emitted, which is the source of electrons in this process.
Step-by-step explanation:
The source of the electrons produced in beta decay is not the escape of a pre-existing electron from the nucleus, but rather the transformation of a neutron into a proton and a beta particle (electron). This process occurs in unstable nuclei where the neutron to proton ratio is too high and results in the emission of an electron from the nucleus at high speed, along with an antineutrino. The emitted electron, known as a beta particle, increases the atomic number by one while the mass number remains unchanged. Theories that combine quantum mechanics and relativity describe the production and annihilation of particles such as these electrons.