Weak magnetism in two neutrino double beta decay

We have extended the formalism for the two-neutrino double beta decay by in cluding the weak-magnetism term, as well as other second-forbidden correcti ons. The weak magnetism diminishes the calculated half-lives in similar to 10%, independently of the nuclear structure. Numerical computations were pe rformed within the pn-QRPA, for Ge-76, Se-82, Mo-100, Te-128 and Te-130 nuc lei. Not one of the second-forbidden corrections modifies significantly the spectrum shapes. The total reduction in the calculated half lives varies f rom 6% up to 32%, and strongly depend on the nuclear interaction in the par ticle-particle S=1,T=0 channel. We conclude that the higher order effects i n the weak Hamiltonian cannot be observed in the two-neutrino double beta e xperiments. (C) 1999 Elsevier Science B.V. All rights reserved.