The idea behind this type of detector is quite simple: if some amount of energy is deposited in a crystal, then its temperature will increase by a certain amount which is given by the heat capacity. On a microscopic level, temperature can be seen as a movement of the atoms in the crystal lattice - the higher the temperature, the faster the atoms move around their fixed position in the crystal. This implies that there is a lowest temperature when all atoms are standing still. This absolute zero is the zero-point of the Kelvin temperature scale where 0 K corresponds to -273.15 C.
When going close to this absolute zero temperature, the tiniest amount of movement induced by a particle hitting an atom in the detector will cause a dramatic measurable temperature increase, which would not be noticeable at room temperature. But even then, the typical amount of energy expected from a dark matter interaction only causes a temperature increase of a millionth of a degree. This requires a very sensitive device to detect temperature changes.

One such device is a so-called transition edge sensor (TES). These consist of a thin film of a material that becomes superconducting below a certain critical temperature. This means that below that critical temperature, an electric current can flow through the material without resistance and above the critical temperature, the material behaves as a normal electric conductor with an ohmic resistance.
The transition between the superconducting and normal conducting state happens in a very narrow temperature region, in which the electrical resistance increases rapidly with increasing temperature. If one now stabilizes the thin superconducting film at a temperature within this narrow transition region with a heater, each small temperature increase leads to a sudden measurable increase in resistance which can be read out as voltage pulses with sensitive electronics.

With CRESST-II, the simultaneous read-out of the deposited energy in the main absorber and an associated light signal was introduced to identify and suppress backgrounds due to radioactive contaminations. This required the change from sapphire absorbers to a scintillating absorber material.
Scintillation is the property of a material to emit light when radiation or highly energetic particles are absorbed. Related phenomena are fluorescence and phosphorescence which are known from glow-in-the-dark toys or the glowing effect under black light. After some research, CaWO₄ was chosen as the default material. It has several desirable properties:

•   high scintillation light output at low temperatures
•   high density
•   large atomic number, which enhances the expected event rate in some scenarios

The CaWO₄ main absorbers are then paired with a second cryogenic detector which detects the scintillation light. These light detectors are typically made from a thin sapphire wafer with a thin layer of SiO₂ to enhance the light absorption.

Over the years the design of CRESST detector modules has evolved based on our experience and depending on the WIMP mass range we are focussing on. CRESST-I started out with cubic sapphire absorbers, CRESST-II continued with cylindrical CaWO₄ absorbers and CRESST-III went back to smaller cuboid absorbers using a variety of different materials. Besides these standard designs, various other geometries have been tested throughout the years.

CRESST-II Modules

CRESST-II modules consist of a cylindrical CaWO₄ crystal that has a diameter and height of 40mm and weighs about 300g. This is paired with a circular light detector. They are held with flexible bronze clamps in a light-tight copper housing. To maximize the light collection, the module is clad with a highly reflective foil which is itself scintillating. This allows us to identify background events due to radioactive decays happening on the surfaces of the absorber crystal or the foil.

CRESST-II Detector Module.