Upgrade for Phase II of the GERDA experiment.

The GERDA collaboration is performing a sensitive search for neutrinoless double beta decay of 76Ge<inline-graphic></inline-graphic> 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 (100 kgyear<inline-graphic></inline-graphic>). It will reach thereby a half-life sensitivity of more than 1026<inline-graphic></inline-graphic> 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. [ABSTRACT FROM AUTHOR]