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Since CRESST detectors operate at
about 15mK, the main part of the facility
at Gran Sasso is a
cryostat whose design 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. A 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
The cold
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
seals.
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.
The CRESST passive shielding is composed of 14cm of radiopure copper
directly surrounding the cold box hosting 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.
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.
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