A systematic study of the radioactive background in materials used in Ge-detector fabrication was conducted prior to the construction of an ultralow-background, 135 ${\mathrm{cm}}^3$ prototype detector. The background from primordial radioactivities in the new system was lower by factors of between 1.5×${10}^3$ and 2.5×${10}^4$ when compared to commercial low-background detectors. Data were collected for 8089 h with the detector located 1438 m underground, resulting in improved lower limits for the half-lives for both two-neutrino and neutrinoless ββ decay of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$ to the ground state of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Se}$ and for the neutrinoless ββ decay to the first excited state of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Se}$. The results of four recent theoretical calculations are compared in detail and used to extract limits on 〈$m_{\nu }$〉, the Majorana mass of ${\nu }_e$, and on the amplitudes of the couplings of right-handed Majorana neutrinos. The best combination of the lowest background data from recent experiments results in a new limit $T_{1/2}^{0\nu }$≳3×${10}^{23}$ yr, corresponding to 〈$m_{\nu }$〉<2.4 eV neglecting right-handed couplings or 2.8 eV including both right-handed neutrino couplings.

• Received 25 November 1985

DOI:https://doi.org/10.1103/PhysRevC.34.666