The CRESST Facility

Due to the low event rate anticipated for dark matter particle-nucleus elastic scattering, an extremely low background environment is required. Not only dark matter particles but also muons, neutrons, electrons, photons and alpha particles will interact in the detector. These can come from cosmic rays, as well as natural and induced radioactivity near the detector. These background signals, if not suppressed, would occur much more frequently than the signals expected from dark matter particles. Thus, to shield against cosmic radiation, the setup is installed in a deep underground site under the Gran Sasso massiv in Italy, in average covered by 1400 meters of rock. Secondly, ambient radioactivity originating from the surroundings is suppressed as much as possible by multiple passive shielding layers. This passive shielding is composed of 14cm of radiopure copper directly surrounding the experimental volume, followed by 20cm of Bolidean lead with a low ²¹⁰Pb activity of 35Bq/kg. The entire shielding is enclosed in an air tight aluminium container (the radon-box) which is constantly flushed with nitrogen gas and maintained at a slight overpressure in order to prevent radon from penetrating the shielding. A neutron moderator of 50cm polyethylene is placed outside the radon-box. With the moderator installed, the remaining neutron flux would be dominated by neutrons induced by muons in the lead of the shielding. Such a background is suppressed by the muon veto system installed inside the neutron moderator.
The shielding and the detectors themself are produced from materials which are carefully selected and stored underground to avoid an activation by cosmic rays.

Since CRESST detectors operate at about 15mK, the main part of the underground facility is a cryostat the design of which combines the requirements of low temperature with those of low background. To avoid any line of sight between detectors and non-radiopure materials, a design has been chosen in which a low background cold box housing the detectors is well separated from a commercial dilution refrigerator. Thus, the dilution unit of the cryostat and the dewars containing cryogenic liquids do not extend into the experimental volume.

The low temperature of the dilution refrigerator is brought to the detectors via a 1.5m long cold finger. An additional 20cm thick lead shield (Plombum lead with a ²¹⁰Pb activity of only 3.6Bq/kg) inside a copper can (which transmits the cooling power) is placed between the mixing chamber and the cold finger. This shield, combined with another one at liquid nitrogen temperature surrounding the cold finger, serves to block radiation coming from the dilution refrigerator into the experimental volume.

The cold box consists of five concentric radiation shields which surround the experimental volume and the cold finger:

• a room temperature vacuum can
• a first shield thermally anchored to the liquid nitrogen dewar of the refrigerator
• an inner vacuum can sunk at the temperature of the liquid helium dewar
• an inner radiation shield at 600mK
• an innermost radiation shield at 80mK

To reduce the effect of external vibrations, the cryostat hangs from a 20cm thick wood plate which rests on air dampers. To reduce the effect of vibrations created inside the cryostat by boiling cryogenic liquids, the detectors in the cold box are mounted onto a spring loaded support hanging from the cold finger.

A three level building houses the whole setup. A two level Faraday cage which surrounds the experiment has been chosen large enough so that all work on the low-background components can be performed inside the cage. In order to provide clean conditions while mounting detectors, the ground floor of the Faraday cage is equipped as a class 100 clean room. The upper level of the Faraday cage is outside the clean room and allows access to the top of the cryostat and to the electronics so that maintenance can be done without entering the clean environment. The gas handling and the pumping system of the cryostat, as well as the data acquisition system, are located outside the Faraday cage. On the third floor there is a small chemistry laboratory and a laminar flow area where detectors are prepared before being mounted in the cryostat.