Low-lying structures in the Gamow-Teller strength functions for the double-beta-decaying nucleiGe76,Se82,Te128, andTe130

Excitation-energy distributions of transition strength to $1^{+}$ states excited via the (p,n) reaction at 134.4 MeV on targets of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$, ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$, and ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}$ were measured for excitation energies up to 25 MeV. Structures observed in the neutron spectra with forward-peaked (ΔL=0) angular distributions were identified as $1^{+}$ states, except for the isobaric analog transitions. The total $1^{+}$ strength in these reactions was extracted by normalizing the intensity in the $1^{+}$ peaks to the Fermi transition strength observed in the isobaric analog state. The Gamow-Teller strength observed in $1^{+}$ peaks above a fitted polynomial background is typically 55% of the sum rule obtained by assuming that the strength of ${\beta }^{+}$ transitions is negligible. The portion of this strength found at excitation energies less than that of the Gamow-Teller giant resonance varied from 15% for ${}_{}{}^{128}{}_{}{}^{}\mathrm{Te}$ to 38% for ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$. Experimental results are compared with predictions of a shell model that includes a pairing force and a long-range Gamow-Teller force in both parent and daughter nuclei. A comparison of the strength functions of the tellurium isotopes is made; this comparison is relevant in determining whether double-beta decay without neutrino emission (0ν decay) is observed in these isotopes.