CRYOGENIC SUPPLY FOR THE GERDA EXPERIMENT

In the GERDA experiment (GERmanium Detector Array for the search of neutrinoless double beta decay of 76Ge) germanium diodes are suspended in a superinsulated cryostat filled with 70 m3 of liquid argon. The cold medium is required since the diodes have to be operated at low temperatures, and furthermore for shielding against background radiation. For the same reason the whole experiment will be placed in the underground laboratories in the Gran Sasso mountains, Italy. In order to avoid any detrimental perturbation inside the dewar vessel, the liquid-argon (LAr) inventory in the main tank will be kept in a subcooled state at a working pressure of 0.12 MPa absolute at the surface. At the TU Dresden an appropriate cryogenic arrangement was designed to match these requirements. Liquid nitrogen (LN2) is used as a cooling fluid. Special care was taken to cope with the narrow temperature span between the LAr boiling temperature and triple point. In the proposed solution a subcooler located close to the cryostat neck provides a stable LAr convection inside the main tank. The working pressure is adjusted with a controlled, slightly elevated temperature level at the liquid-vapor interface.