The background in the 0νββ experiment Gerda = The background in the $$0\nu \beta \beta $$ 0 ν β β experiment GERDA

The GERmanium Detector Array (Gerda)experimentattheGranSassoundergroundlaboratory(LNGS)of INFN is searching for neutrinoless double beta (0νββ)decay of 76Ge. The signature of the signal is a monoen-ergetic peak at 2039 keV, the Qββ value of the decay. Toavoid bias in the signal search, the present analysis does notconsider all those events, that fall in a 40 keV wide regioncentered around Q . The main parameters needed for the ββ0νββ analysis are described. A background model was devel- 1232764 Page 2 of 25Eur. Phys. J. C (2014) 74:2764oped to describe the observed energy spectrum. The model contains several contributions, that are expected on the basis of material screening or that are established by the obser- vation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around Qββ with a background index ranging from 17.6 to 23.8 × 10−3 cts/(keV kg yr). A part of the data not considered before has been used to test if the predictions of the background model are consistent. The observed number of events in this energy region is consistent with the back- ground model. The background at Qββ is dominated by close sources, mainly due to 42 K, 214 Bi, 228 Th, 60 Co and α emitting isotopes from the 226Ra decay chain. The individual fractions depend on the assumed locations of the contaminants. It is shown, that after removal of the known γ peaks, the energy spectrum can be fitted in an energy range of 200 keV around Qββ with a constant background. This gives a background index consistent with the full model and uncertainties of the same size.