Radon emanation measurements of the GERDA inner detector

The GERDA-Experiment is designed to search for the neutrinoless double beta decay of 76Ge. To minimize background bare Ge-diodes enriched in 76Ge will be operated in liquid argon. The radioactive noble gas isotope 222Rn represents a dangerous source of background, if it is contained in the liquid argon. Monte-Carlo simulations show that a 222Rn content on the order of 10 mBq in the Gerda cryostat corresponds to a background count rate of ∝10-4 counts/(keV.kg.a) around the Q-value of the ββ-decay (2039 keV). Therefore, the 222Rn emanation of all components of the inner detector (cryostat and attached lock) must be measured to identify and eliminate possible radon sources. We use ultra-low background miniaturized proportional counters for the 222Rn emanation tests. If necessary a Mobile Radon Extraction unit (MoREx) is available for the pre-concentration of 222Rn from large gas samples. With this technique we have tested small items to be installed in the inner detector. We have also performed a screening of the entire cryostat. In this talk the results of these measurements are presented and the implications for the GERDA-experiment are discussed.