Nuclear beta decay is driven by the weak nuclear force featured by much weaker intensity (10-5 relative to the strong nuclear force), very short range (10-18m) and extremely low cross section (10-44m2), that operates at large typical time scale (10-8s). It occurs via the transformations of the nucleons (neutrons and protons) into each other when they are respectively in excess within the concerned nucleus with emission of either an electron and an anti-neutrino (for the neutron undergoing beta minus decay) or positrons and neutrinos (for the proton undergoing beta plus decay) , i.e., the transformation of a d-quark into a u-quark or vice-versa. In these weak nuclear processes, conservation laws of energy-momentum, spin, isospin, baryon and lepton numbers hold while parity conservation is violated in some cases. As nature wants it to be, nuclei having excitation energy in excess are unstable and release their this energy with evolving to more nuclear stability. All these effects have been pointed out experimentally and can be explained on theoretical grounds (see, e.g., the Fermi theory of beta decay).
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