We report here on the final results of a search for ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$ double-$\beta$ decay conducted in the Gotthard underground laboratory. The detector consists of an array of eight high-purity natural germanium crystals totaling 1095 ${\mathrm{cm}}^3$ fiducial volume. The accumulated data set represents a sensitivity of 10.0 kg yr. No indication of neutrinoless double-$\beta$ decay was found. The measured half-life limits are $T_{\frac{1}{2}}(0^{+}\to 0^{+})>\mathrm{}6.0\mathrm{}\left(3.3\right)\mathrm{}\times {10}^{23}$ yr for the transition to the ground state and $T_{\frac{1}{2}}(0^{+}\to 2^{+})>\mathrm{}1.4\mathrm{}\left(0.65\right)\mathrm{}\times {10}^{23}$ yr for the transition to the first excited state at 68% (90%) C.L. From these results we derive an upper limit for the Majorana mass of the neutrino in the range of 1.8 to 6.7 eV depending on matrix-element calculations. The same results allow limits to be set for the right-handed-current parameters: $〈\lambda 〉<3.6\mathrm{}\times {10}^{-6\mathrm{}}$ and $〈\eta 〉<2.2\mathrm{}\times {10}^{-8\mathrm{}}$.

• Received 11 October 1991

DOI:https://doi.org/10.1103/PhysRevD.45.2548