Upgrade for Phase II of the Gerda experiment

The Gerda collaboration is performing a sensitive search for neutrinoless double beta decay of \(^{76}\hbox {Ge}\) at the INFN Laboratori Nazionali del Gran Sasso, Italy. The upgrade of the Gerda experiment from Phase I to Phase II has been concluded in December 2015. The first Phase II data release shows that the goal to suppress the background by one order of magnitude compared to Phase I has been achieved. Gerda is thus the first experiment that will remain “background-free” up to its design exposure (\(\hbox {100 kg}~\hbox {year}\)). It will reach thereby a half-life sensitivity of more than \(10^{26}\) year within 3 years of data collection. This paper describes in detail the modifications and improvements of the experimental setup for Phase II and discusses the performance of individual detector components.