NUCLEAR-STRUCTURE CALCULATION OF BETA+BETA+, BETA+ EC AND EC EC DECAY MATRIX-ELEMENTS

Nuclear matrix elements for double positron emission (beta+beta+), positron emission/electron capture (beta+/EC) and double electron capture (EC/EC) in the 2nubetabeta decay mode and for beta+beta+ and beta+/EC decay in the 0 nubetabeta mode are calculated for the experimentally most promising isotopes Ni-58, Kr-78, Ru-96, 106Cd, 124Xe, 130Ba and Ce-136 within pn-QRPA. We point out that the matrix element for the 2 nubeta+/EC decay differs from the 2 nubeta+beta+ matrix element, an effect not considered previously. For the neutrino accompanied decays our calculation predicts for the beta+/EC and the EC/EC mode half lives which are shorter typically by 4 7 orders of magnitude than those for the double positron emission. However, even for the best candidates typical values for 2nubeta+/EC (2nuEC/EC) are still in the range of approximately 10(22) ((some) 10(21)) years. For 0nubetabeta decay we have calculated all matrix elements relevant for both, the mass mechanism and the right-handed currents for the first time complete. A detailed discussion of the differences between the 0nubeta+beta+, the 0nubeta+/EC and 0nubeta-beta- decay is given.