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:
finger and the shields are made of radiopure copper which has been
electropolished after machining to remove residual surface contaminations and to
reduce the risk of recontamination. High purity lead is used for vacuum
- 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
The CRESST-II Cryostat in Gran Sasso. Click on the
image for a higher resolution image with description.
The CRESST Detector Modules are arranged in a support
structure which can hold up to 33 crystals, corresponding to 10kg of
A building three stories high hosts the CRESST Experiment
in the Gran Sasso Tunnel.