Highly sensitive gamma-spectrometers of GERDA for material screening:   Part I
  The GERDA experiment aims to search for the neutrinoless double beta-decay of 76Ge and possibly for other rare processes. The sensitivity of the first phase is envisioned to be more than one order of magnitude better than in previous neutrinoless double beta-decay experiments. This implies that materials with ultra-low radioactive contamination need to be used for the construction of the detector and its shielding. Therefore the requirements on material screening include high-sensitivity low-background detection techniques and long measurement times. In this article, an overview of material-screening laboratories available to the GERDA collaboration is given, with emphasis on the gamma-spectrometry. Additionally, results of an intercomparison of the evaluation accuracy in these laboratories are presented. 
D. Budj\'a\v{s} (1), C. Cattadori (2), A. Gangapshev (3), W. Hampel   (1), M. Heisel (1), G. Heusser (1), M. Hult (4), A. Kl...