Consider a sample of pure beta-active material.

Beta particles are electrons (or positrons) that do not exist inside the nucleus but are created at the instant of decay when a neutron converts into a proton (or vice versa).

The energy released in beta decay is shared between the beta particle and the antineutrino (or neutrino). Thus, beta particles are emitted with a continuous spectrum of energies, not a single discrete energy.

The antineutrino has a very small, non-zero mass. Even if considered massless, it carries energy and therefore possesses momentum ($p = \dfrac{E}{c}$).

In beta decay, the mass number of the nucleus remains unchanged while the atomic number changes by one unit. Therefore, the parent and daughter nuclei have the same mass number and are isobars.