The full setup of the Heidelberg-Moscow double β decay experiment is presented. This experiment gives at present the most stringent upper bound, improving the neutrino mass limit into the sub-eV range. Out of 19.2 kg of 86% ${\mathrm{enriched}}^{76}$Ge five crystals were grown with a total mass of 11.51 kg. Since February 1995 all five detectors, corresponding to 10.96 kg active mass, are in regular operation in the Gran Sasso underground laboratory, four of them in a common shield. No signal is observed for the neutrinoless double β decay (0νββ). The measured data from the first three enriched detectors with a statistical significance of 13.60 kg yr result in a new half-life limit of ${\mathit{T}}_{1/2}$${(0\mathrm{}}^{+}$→$0^{+}$)>7.4×${10}^{24}$ yr (90% C.L.). With this limit a Majorana mass of the neutrinos larger than 0.6 eV (90% C.L.) is excluded. From the data taken in the previously operated setup with three enriched detectors in a common shielding and a statistical significance of 10.58 kg yr new results are extracted for the two neutrino double β decay (2νββ) ${\mathrm{of}}^{76}$Ge. The procedure of a quantitative and model-independent description of the background via a Monte Carlo simulation is outlined in some detail. The combined result is ${\mathit{T}}_{1/2}^{2\mathrm{\nu }}$=[${1.77}_{\mathrm{-}0.01}^{+0.01\mathrm{}}$${\mathrm{}\left(\mathrm{stat}\right)\mathrm{}}_{\mathrm{-}0.11}^{+0.13\mathrm{}}$(sys)]×${10}^{21}$ yr. Further on the results concerning new Majoron models and the impact on SUSY parameters are briefly reviewed. Future improvements on the background with the application of digital pulse shape analysis are discussed and an outlook on the future of ββ research is given.

• Received 20 December 1995

DOI:https://doi.org/10.1103/PhysRevD.55.54