The CRESST experiment is a search for
WIMP Dark Matter particles via
their elastic scattering on nuclei. The nuclei are in the absorber of a
cryogenic detector, capable of detecting the small energy of the recoiling nucleus which has been hit by an incoming WIMP.
Such a search for very rare interactions with a low energy
deposit involves two major aspects: a
sensitive detector
and a highly efficient suppression of backgrounds (since also many other particles interact in the detector apart from WIMPs and have to be discriminated from the interesting signals).
Background Reduction
Due to the low event rate anticipated for WIMP-nucleus elastic scattering,
we require an extremely low background
environment. Not only WIMPs 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 expected WIMP signals. 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. The shielding and the detector
itself are made of materials which are carefully selected and stored underground
to avoid activation by cosmic rays.
Since recent work has indicated that the most important remaining
background is neutrons, the CRESST II setup has
recently installed an additional 50cm thick polyethylene
neutron shield around the cryostat, together with muon veto panels.
However, to reach the level of background suppression needed for the next
generation of experiments such measures are not sufficient in
themselves, and in CRESST II a new type of detector module giving a
high degree of active background suppression is being installed. This involves
light as well as heat detection.
Detector Modules
Cryodetectors are extremely
sensitive and can measure the total energy
deposited by an interacting particle. To achieve the low, milliKelvin,
temperatures necessary to detect the low energies involved,
the detector is mounted in a
dilution refrigerator, which can
reach temperatures below 10 mK.
This system worked well in the first phase of the CRESST
experiment, with sapphire crystals
as the target material.
The detector modules for the second phase, CRESST II, exploit the fact that
most common backgrounds will produce some light in a scintillating material,
while on the other hand the sought-for WIMP induced recoils
will produce little or no light.
Thus detectors were developed based on
scintillating CaWO4 crystals as absorbers. In this crystal a particle
interaction produces mainly heat in the form of phonons, as for
sapphire. But in addition a small amount of the deposited energy is
emitted as scintillation light. Therefore when a second, smaller calorimeter
is added to detect this light, most common backgrounds can be eliminated
through their light signal. In tests this system was found to
give a very efficient active background discrimination.
Therefore a system based on simultaneous light and heat detection
is being installed in CRESST II. The CRESST II setup will consist of
up to 33 modules with both light and heat
detection, reaching up to 10kg of
active target mass. The system is read out by a 66-channel SQUID
system, two readout channels for each module.
Each module consists of two
cryogenic calorimeters:
- A phonon/heat detector to measuring the total
energy deposit. This is a cylindrical 300g CaWO4
crystal of both diameter and height 40mm.
- A second, smaller, calorimeter functioning as a light
detector for the scintillation light.
The two detectors are mounted close to each other and
enclosed in a highly reflective housing for efficient collection of
the light.
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The detector module as used in CRESST-II with
both a heat detector and a seperate light detector.
One of the CRESST detector modules. When illuminated with
ultraviolet light, the scintillating inner shield glows brightly.
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Cryogenic Calorimeters
A cryogenic calorimeter consists of an absorber and a temperature sensor in
thermal contact, weakly linked to a heat bath.
In an extremely simplified model the detector can be characterized
as an absorber with a heat capacity C. Then an energy deposition in the absorber δE
leads to a temperature rise δT of the detector
given by δT=δE/C. This relaxes back to its
equilibrium value via the thermal coupling to the heat
bath. The temperature rise is therefore a direct measurement of the
deposited energy.
In dielectric and semiconductor materials the heat capacity at low
temperatures is dominated by the phonon system in which C ∝ T3. At
millikelvin temperatures, due to the T3
dependence of the heat capacity, the energy deposition following a particle
interaction results in a measurable temperature rise.
The temperature sensors developed for CRESST are tungsten superconducting
phase transition thermometers (SPT) consisting of thin tungsten films
evaporated onto a surface of the absorbers. The thermometers are stabilized in the
transition from the normal conducting to the superconducting phase where a
small temperature rise leads to a relatively large increase in resistance,
making them extremely sensitive thermometers.
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Schema drawing of a CRESST-II calorimeter element
A typical transition curve. Since it is very steep,
a small change in temperature results in a measurable change of resistance.
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