SHELL-MODEL STUDIES OF THE DOUBLE-BETA DECAYS OF GE-76, SE-82, AND XE-136

The double beta decays of Ge-76, Se-82, and Xe-136 are studied in very large shell model spaces. The dimensions of the shell model spaces us ed in these calculations reach O(10(8)). The two neutrino matrix eleme nts obtained are in good agreement with the available experimental dat a. Assuming that the mass of the neutrino is smaller than 1 eV, we get the following upper bounds to the half-lives for the neutrinoless dec ays: T-1/2((0 nu))(Ge) > 1.85 x 10(25) yr, T-1/2((0 nu))(Se) > 2.36 x 10(24) yr, and T-1/2((0 nu)) 1/2 (Xe)> 1.21 x 10(25) yr.