Abstract: The GERmanium Detector Array (Gerda) experiment aims for the discovery of neutrinoless double beta decay (0νββ) decay in 76Ge. It uses HPGe detectors enriched in the isotope 76Ge, which are directly immersed into liquid argon (LAr). In second phase (Phase II) of Gerda, the radio-pure cryogenic liquid acts not only as cooling medium for the detectors and passive shielding but also as active shielding. Phase II started data taking in Dec 2015 with the design goal of increasing the sensitivity to T 0ν1/2 = O1026 yr by reducing the background by one order of magnitude. Due to the active veto system detecting LAr scintillation light, the superior energy resolution and an improved background recognition, the initial release of Phase II already showed a background rate in the energy region of interest (ROI), after pulse shape discrimination (PSD) and liquid argon veto cuts, in the range of a few counts/(ROI·ton·yr). This made Gerda the first 0νββ experiment being background free up to its design exposure of 100 kg·yr. With the latest data release in mid 2018, comprising a total exposure of 82.4 kg·yr, Gerda remained in the background free regime and it is the first experiment to surpass a median sensitivity on the half-life of 1026 yr for 0νββ decay.

In this talk, a full analysis of the background is presented where all available information on the background has been incorporated in order to develop a detailed background model describing the decomposition of the measured energy spectrum. For the first time, the single- and two-detector data have been combined in a multivariate Bayesian fit approach. Additionally, the background model focuses further on two prominent features in the energy spectrum: the α events dominating the high energy part of the spectrum and the count rates of the potassium γ-lines at 1525 keV and 1461 keV. Thanks to the granularity of the detector array, a study of the coincident events in the two-detector data which can provide further information regarding the location of contaminations has been integrated. Using the background model, important information on the main sources and their locations contributing to the background around
the Q-value of the decay (Qββ) can be deduced. Besides, the spectral shape of the total background around Qββ can be extracted. Both are crucial input informations for reliable results on the 76Ge 0νββ signal search.