The CRESST experiment searches directly for
dark matter particles via
their elastic scattering off 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 dark matter particle.
Such a search for very rare interactions with a low energy
deposit involves two major aspects: a
and a highly efficient suppression of backgrounds (since also many other particles apart from dark matter particles interact in the detector and have to be discriminated from
the interesting signals).
Due to the low event rate anticipated for dark matter particle-nucleus elastic scattering,
we require an extremely low background
environment. 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 (mainly copper and lead). The shielding and the detector
itself are made of materials which are carefully selected and stored underground
to avoid an activation by cosmic rays.
As one of the most important remaining
backgrounds originates from neutrons, a polyethylene
neutron shield with a thickness of 50cm is installed around the cryostat together with a muon veto.
However, to reach the level of background suppression needed, such measures are not sufficient in
themselves. Therefore, detector modules with a high degree of active background suppression involving scintillation light as well as heat detection were used in
are also installed in CRESST III.
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.
In the first phase of the CRESST
experiment (CRESST I), sapphire crystals were used
as target material. In these crystals, energy deposited by a particle interaction is visible as a heat signal which can be read out with a thermometer.
The detector modules for CRESST II and CRESST III exploit the fact that
most common backgrounds will produce a certain amount of light in a scintillating material,
while on the other hand the sought-for dark matter 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. In addition a small amount of the energy deposited is
emitted as scintillation light. A light absorber (usually silicon on sapphire) together with a second thermometer acts as light detector. Both detectors are mounted close to
each other and are enclosed in a scintillating and reflective housing for an efficient light collection.
As the amount of light produced differs for different kinds of particles, most common backgrounds (e.g. electron recoils, alpha events..) can be
eliminated. In this way a very efficient active background discrimination can be achieved using the so-called light yield defined as the fraction of light and heat energy as discrimination
In this schematic drawing the horizontal bands resulting from different event types are shown in the ligh yield - energy plane.
Per definition the light yield is normalized to 1 for gamma radiation with an energy of 122keV. Therefore, the light yield of betas and gammas
(electron recoils) is about 1. For alpha particles the light yield is about 0.22 and for nuclear recoils (O, Ca, W)
it is even lower (0.1 - 0.02).
(Eur. Phys. J. C (2014) 74 , arXiv:1401.3332).
Due to finite detector resolution, these bands can be separated to some extent.