The behaviour of beta rays in electric and magnetic fields shows them to consist of charged particles that are identical with electrons. The beta particles emitted by the radioactive substances found in nature are negatively charged; i.e. to say, they are ordinary electrons.

When the energy requirement for the elimination of apha particle cannot be met, the nuclei of high atomic number emit beta particles,i.e., negative electrons. Because of the existence of radioactive b-particle emitters, it was thought, at one time , that atomic nuclei actually contain electrons , but this view was discarded after the discovery of neutron. The present theory is that b-particle emission accompanies the conversion of a neutron into a proton, when the conditions are such that the energy required to remove a neutron from a particular nucleus is numerically less than gained by the addition of a proton to the residual nucleus. The process taking place may be written as

                        Neutron ® proton + electron + neutrino

Mass                1                 1             0               0

Charge             0                 +             -               0

Spin                 ^+/-1/2           ^+/-1/2       ^+/-1/2        ^+/-1/2

It will be noted that an additional particle, called neutrino, is postulated as accompanying the conversion of a neutron into a proton plus an electron, which is immediately emitted as a negative b-particle. The neutrino is not required to conserve either mass or charge, but it is necessary to satisfy conversion of spin. The neutron, proton and electron each have spin quantum numbers of +1/2 or –1/2 , and the conversion is not possible  without an additional particle of similar spin.