Representation of the energies of the isobars. The single-beta decay ()—green arrows—between 76Ge and 76Se is energetically forbidden, hence leaving double beta ()—pink arrow—as the only decay channel. The two mass parabolas exist because of the pairing interaction that lowers the energy of even —even nuclei with respect to odd —odd nuclei. For odd nuclei there is a single mass parabola, and all single-beta transitions are energetically allowed (taken from J. Menendez's PhD thesis).
