A high speed disintegration electron (i.e., negatron or positron) spontaneously emitted from an atomic nucleus as a form of radiation. Electrons (negative or positive) do not exist in nuclei; they are created at the moment of emission and are accompanied by the transition of a neutron into a proton, or a proton into a neutron. The atomic number of the nuclide is changed by a +1 or a –1 with no significant change in atomic weight. A beta particle can penetrate only a few millimeters of rock.