NEUTRINOLESS MODES OF DOUBLE-BETA DECAY

Three types of nuclear double beta decays without emission of neutrinos are compared; (1) capture of an atomic electron and emission of a positron, (2) emission of two positrons and (3) emission of two electrons. Captures from the K and L(I) shells are taken in to account. Half-lives for the 0+ --> 0+ transitions are calculated by using relativistic wave functions. They are shorter than those obtained by using solutions of the Schrodinger equation. For the case of one positron emission, the 124Xe decay is expected to be the shortest half-life, if there is no anomalous cancellation of the nuclear matrix elements. It is also discussed that the transition rate for the capture of double atomic electrons is smaller than the one for the single atomic electron capture.