The background in the 0νββ experiment Gerda

The GERmanium Detector Array ( Gerda ) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta ( 0νββ ) decay of ^76 Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the Q_ββ value of the decay. To avoid bias in the signal search, the present analysis does not consider all those events, that fall in a 40 keV wide region centered around Q_ββ . The main parameters needed for the 0νββ analysis are described. A background model was developed 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 observation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around Q_ββ 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 background model. The background at Q_ββ is dominated by close sources, mainly due to ^42 K, ^214 Bi, ^228 Th, ^60 Co and α emitting isotopes from the ^226 Ra 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 Q_ββ with a constant background. This gives a background index consistent with the full model and uncertainties of the same size.