The $0^{+}\to 0^{+}$ positron double-$\beta$ decay with emission of two   neutrinos in the nuclei $^{96}$Ru, $^{102}$Pd, $^{106}$Cd and $^{108}$Cd
  Theoretical results for %the $0^{+}\to 0^{+}$ positron double-$\beta$ decay with emission of two neutrinos in the nuclei $^{96}$Ru, $^{102}$Pd, $^{106}$Cd and $^{108}$Cd are presented. The study employs the Hartree-Fock-Bogoliubov model to obtain the wave functions of the parent and daughter nuclei, in conjunction with the summation method to estimate the double beta decay nuclear matrix elements. The reliability of the intrinsic wave functions of $^{96,102}$Ru, $% ^{96}$Mo, $^{102,106,108}$Pd and $^{106,108}$Cd nuclei are tested by comparing the theoretically calculated spectroscopic properties with the available experimental data. Calculated half-lives $T_{1/2}^{2\nu}$ of $% ^{96}$Ru, $^{102}$Pd, $^{106}$Cd and $^{108}$Cd nuclei for 2$\nu $ $\beta ^{+}\beta ^{+}$, 2$\nu $ $\beta ^{+}EC$ and 2$\nu $ $ECEC$ modes are presented. The effect of deformation on the nuclear transition matrix element $M_{2\nu}$ is also studied. 
P. K. Raina, A. Shukla, S. Singh, P. K. Rath and J. G. Hirsch