  The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground
laboratory (LNGS) of INFN is searching for neutrinoless double-beta
($0\nu\beta\beta$) decay of $^{76}$Ge. The technological challenge of GERDA is
to operate in a "background-free" regime in the region of interest (ROI) after
analysis cuts for the full 100$\,$kg$\cdot$yr target exposure of the
experiment. A careful modeling and decomposition of the full-range energy
spectrum is essential to predict the shape and composition of events in the ROI
around $Q_{\beta\beta}$ for the $0\nu\beta\beta$ search, to extract a precise
measurement of the half-life of the double-beta decay mode with neutrinos
($2\nu\beta\beta$) and in order to identify the location of residual
impurities. The latter will permit future experiments to build strategies in
order to further lower the background and achieve even better sensitivities. In
this article the background decomposition prior to analysis cuts is presented
for GERDA Phase II. The background model fit yields a flat spectrum in the ROI
with a background index (BI) of $16.04^{+0.78}_{-0.85} \cdot
10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched BEGe data set and
$14.68^{+0.47}_{-0.52} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the
enriched coaxial data set. These values are similar to the one of Gerda Phase I
despite a much larger number of detectors and hence radioactive hardware
components.

Nuclear Experiment;Instrumentation and Detectors