  The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground
laboratory (LNGS) of INFN is searching for neutrinoless double beta decay of
76Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the
Q-value of the decay, Q_bb. 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_bb. The main parameters needed for the neutrinoless double
beta decay 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_bb with a
background index ranging from 17.6 to 23.8*10^{-3} counts/(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-bb
is dominated by close sources, mainly due to 42K, 214Bi, 228Th, 60Co and alpha
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 gamma peaks, the energy spectrum can be fitted in an energy range
of 200 kev around Q_bb with a constant background. This gives a background
index consistent with the full model and uncertainties of the same size.
