Design and performance of the GERDA low-background cryostat for operation in water

In searching for the neutrinoless double-beta decay of 76Ge the GERmanium Detector Array (GERDA) experiment at the INFN Laboratori Nazionali del Gran Sasso has achieved an unprecedented low background of well below 10-3 cts/(keV·kg·yr) in the region of interest. It has taken advantage of the first realization of a novel shielding concept based on a large cryostat filled with a liquid noble gas that is immersed in a water tank. The germanium detectors are operated without encapsulation in liquid argon. Argon and water shield the environmental background from the laboratory and the cryostat construction materials to a negligible level. The same approach has been adopted in the meantime by various experiments. This paper provides an overview of the design and the operation experience of the 64 m3 liquid argon cryostat and its associated infrastructure. The discussion inludes the challenging safety issues associated with the operation of a large cryostat in a water tank