Shielding of the GERDA experiment against external gamma background

The GERmanium Detector Array (GERDA) experiment will search for neutrinoless double beta decay of Ge76 and is currently under construction at the INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The basic design of GERDA is the use of cryogenic liquid and water of high purity as a superior shield against the hitherto dominant background from external gamma radiation. In this paper we show by Monte Carlo simulations and analytical calculations how GERDA was designed to suppress this background at Qββ(Ge76)=2039keV to a level of about 10-4cts/(keVkgy).