Shielding of the GERDA experiment against external gamma background

The GERmanium Detector Array (GERDA) experiment will search for neutrinoless double beta decay of Ge-76 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 Q(beta beta)(Ge-76) = 2039 keV to a level of about 10(-4) cts/(keV kg y). (C) 2009 Elsevier B.V. All rights reserved.