First evidence for lepton number violation: The Ge-76 double beta experiment in Gran Sasso 1990-2003

Data acquisition in a long-running underground experiment has its specific experimental challenges, concerning data acquisition, stability of the experiment and background reduction. These problems are addressed here for the HEIDELBERG–MOSCOW experiment, which collected data in the period August 1990–May 2003. The measurement and the analysis of the data are presented. The duty cycle of the experiment was ~ 80%, and the collected statistics is 71.7 kg year. The background achieved in the energy region of the Q value for double beta decay is 0.11 events/kg year keV. The two-neutrino accompanied half-life is determined on the basis of more than 100 000 events. The confidence level for the neutrinoless signal has been improved to a 4σ level