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In the CRESST experiment scintillating calcium tungstate (CaWO4) is used to search for dark matter. If a particle hits a CaWO4-crystal and deposits some energy in the crystal, the temperature of the crystal is increased and scintillation light is produced. The ratio between scintillation light and temperature rise depends on the type of particle. This behavior can be used to distinguish between different types of particles and, thus, suppress background events.
The majority of the background events (beta and gamma radiation; cosmic muons) interact mainly with the electron system of the detector material. However, WIMPs are supposed to scatter off the nuclei. Since the amount of scintillation light generated by an interaction with the elctrons is about 10-40 times larger than for an interaction with nuclei, the majority of the background events can be suppressed by a simultaneous measurement of the temperature rise and the scintillation light generated by an energy deposition in a CaWO4-crystal.
For the simultaneous measurement of scintillation light and temperature rise a CRESST detector module consists of two low-temperature detectors mounted inside a reflective and scintillating housing. One of the two low-temperature detectors, the phonon detector, is based on a CaWO4-crystal with a mass of about 300 g. This phonon detector is used to measure the temperature rise induced by the energy deposition of an incident particle.
The other detector, the light detector, is based on a silicon crystal or a silicon-coated sapphire crystal and is used to measure the scintillation light emitted by the phonon detector's CaWO4-crystal. Since the light detector is also a low-temperature detector, it measures the temperature rise of the silicon induced by the absorption of scintillation light.
Both, the phonon and the light detector are mounted together inside of an reflective housing to increase the fraction of scintillation light absorbed in the light detector. The housing is also scintillating in order to identify rare radioactive decays.
Both, the phonon and the light detector are low-temperature detectors. A low-temperature detector consists of an absorber crystal (CaWO4 for the phonon detector, Si for the light detector) and a sensitive thermometer to measure the temperature rise induced by an energy deposition. For CRESST detectors superconducting Transition Edge Sensors (TES) are used as thermometers. A TES is a superconducting thin metal film (e.g. tungsten) evaporated onto the absorber crystal.
If a particle interacts with the absorber crystal and deposits some energy inside the crystal, this energy is converted into phonons (heat). After a short time, these phonons are spreaded throughout the complete crystal. Phonons entering the TES are increasing the temperature of the thin metal film. The TES is operated in the steep phase transition between superconducting and normalconducting state. Thus, a small rise in temperature leads to a large rise in electrical resistance of the TES. This change in resistance is converted into a voltage signal using a SQUID (Superconducting Quantum Interference Device). Data acquisition electronics is used to read out, digitalize and store this voltage signal.