Searching for ⁷⁶Ge neutrinoless double beta decay with the CDEX-1B experiment*

We operated a p -type point contact high purity germanium (PPCGe) detector (CDEX-1B, 1.008 kg) in the China Jinping Underground Laboratory (CJPL) for 500.3 days to search for neutrinoless double beta ( ) decay of 76 Ge. A total of 504.3 kg day effective exposure data was accumulated. The anti-coincidence and the multi/single-site event (MSE/SSE) discrimination methods were used to suppress the background in the energy region of interest (ROI, 1989–2089 keV for this work) with a factor of 23. A background level of 0.33 counts/ (keV kg yr) was realized. The lower limit on the half life of 76 Ge decay was constrained as , corresponding to the upper limits on the effective Majorana neutrino mass: 3.2–7.5 .