From quarks to nucleus
The beta decay of nuclei are the result of changes that occur at more elementary levels. For example, the transformation of a tritium nucleus (A) to helium-3 is caused by the transformation of one of the two tritium neutrons into a proton. The transformation of a neutron into a proton (B) is itself the result of a transformation of elementary particles that constitute the nuclear matter: quarks. Neutrons composed of two down quarks and one up quark and protons the opposite (two up and down). It is the transformation of a down quark into an up quark (C) that is the origin of the mutation of a neutron into a proton, thus the beta decay of tritium.