CRESST - Detector Concepts

[ Background Reduction ] [ Detector Modules ] [ Cryogenic Calorimeters ]

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 sensitive detector 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).

Background Reduction

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 CRESST-II and are also installed in CRESST III.

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.

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 parameter:

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.

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 superconducting phase transition thermometers - transition edge sensors (TES) - 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.


Schematic drawing of a CRESST-II calorimeter element

A typical transition curve

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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