Status of the Heidelberg-Moscow ββ experiment with 76Ge

Abstract

The status of the HEIDELBERG-MOSCOW double beta decay experiment using 16.9 kg of 86% enriched 76Ge is presented. From this material 12 kg are available in form of five grown crystals. Three detectors with a total mass of 6.02 kg are in regular operation in the GRAN SASSO underground laboratory. With a total 76Ge exposure of 91.63 mol·y we derive the following half life limits for the neutrinoless double beta (0τββ) decay: T 1 2 (0 + → 0 +) > 1.9·10 24 y and T 1 2 (0 + → 2 +) > 8.2·10 23 y with 90% C.L. In the transition to the ground state a stagnation of the half life limit in time is visible. By this limit a Majorana mass of the neutrinos larger than 1.1 eV is excluded. The background at 2 MeV is B = 0.3 counts/kg·y·keV for the total array. From the data taken with the second enriched detector a half life of T 1 2 = (1.42 ± 0.03 stat ± 0.13 syst )·10 21 y for the two neutrino double beta (2τββ) decay of 76Ge is extracted. This result represents with an 76Ge exposure of 19.3 mol·y the first high statistics measurement of this rare nuclear decay mode. From the same set of data a half life of 1.7·10 22 y is excluded for the neutrinoless decay with majoron emission (0τχββ), leading to an upper limit for the neutrino-majoron coupling of <g τχ < < 1.8·10 −4 with 90% C.L.