CRESST - Detector Concepts

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


The CRESST experiment is a search for WIMP 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 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 apart from WIMPs interact in the detector 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. As one of the most important remaining backgrounds is neutrons, a 50cm thick polyethylene neutron shield is installed around the cryostat together with a muon veto. However, to reach the level of background suppression needed for the next generation of experiments 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 are being installed in CRESST II.

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 which gives a very efficient active background discrimination.

Therefore a system based on simultaneous detection of light and heat 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 including two readout channels for each module.

Each module consists of two cryogenic calorimeters:

  1. A phonon/heat detector to measure the total energy deposit. So far this has been a cylindrical 300g CaWO4 crystal with both, diameter and height, being 40mm.
  2. A second, smaller, calorimeter functioning as a light detector for the scintillation light.
The two detectors are mounted close to each other and are enclosed in a highly reflective housing for an efficient light collection.

  CRESST-II Detector Module

Detector module as used in CRESST-II with both a heat detector and a seperate light detector.

A CRESST Detector Module

One of the CRESST detector modules. When illuminated with ultraviolet light, the scintillating inner shield glows brightly.

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 (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.

  alternative

Schema 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.

Results

It has turned out that the two-channel detection system indeed performs very well in discriminating background events from WIMP candidates. From our data it was possible to derive strong limits on parameters of potential WIMPs which puts us among the most sensitive experiments in this field of research. Please check our publications to learn about these results in detail.



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