Suppression of the two-neutrino double-beta decay by nuclear-structure effects

The nuclear matrix element for 2ν double-beta decay is calculated within the quasiparticle random-phase approximation. It is shown that the decay matrix element passes through zero as a function of the strength gpp of the particle-particle component of the spin-isospin polarization force, neglected previously. The analysis of electron capture/β+ decay rates for semimagic neutron-deficient nuclei suggests values for gpp in the very vicinity of this zero, which gives rise to long lifetimes. The qualitative features of nuclear ββ decay are illustrated with the example of an exactly soluble model.