First results on 76 Ge neutrinoless double beta decay from CDEX-1 experiment

We report the first results on 76Ge neutrinoless double beta decayfrom stage one of the China dark-matter experiment(CDEX). A p-type point- contact high-purity germanium detector with a mass of 994 g has been installed to detect neutrinoless double beta decayevents, as well as to directly detect dark matter particles. An exposure of 304 kg d has been analyzed over a wide spectral bandfrom 500 keV to 3 MeV. The average event rate obtained was about 0.012 counts per keV per kg per day over the 2.039 MeV energy range. The half-life of 76Ge neutrinoless double beta decayderived based on this result is T 1/2 0ν >6.4×1022 yr (90% C.L.). An upper limit on the effective Majorana-neutrino mass of 5.0 eV has been achieved.