Light Detector Geometry

Besides this effect further improvements are possible in the small module design. It is reasonable to match the size of the light absorber to the size of the crystal. In CRESST-II phase 2 a SOS light absorber with a diameter of 40 mm was used. For the optimized small crystal a light absorber with a size of 20×20 mm² is foreseen, which results in a reduction of the light absorber volume by a factor ofπ compared to the light detectors operated in CRESST-II.

For the light detector no adjustment of the TES structure is required. The change of the absorber size influences the thermalization of phonons and, thus, the fraction of phonons being absorbed in the thermometerε. The signal of the calorimetric ther-mometer is proportional to this fraction. In [66] it is shown that ε ≈ 5% for light detectors. Thus, the time constant for the thermalization due to absorptions in the thermometer τ_{f ilm} is 19 times longer than the time constant for the thermalization in the absorberτcrystal (see equation 4.18). Therefore, it is possible to estimate the change ofε caused by the adjustment of the light detector.

The reduction of the volume of the light absorber by a factor ofπimplies a reduction of the surface by the factor∼2π. Due to these changes, the thermalization of phonons in the crystal with the time constantτ_crystal is expected to become longer by a factor of 2 (see equation 4.16). Additionally, the time constant describing the thermalization due to absorption in the film τ_{f ilm} is expected to be shortened by a factorπ (see equation 4.17). This results inτ_{f ilm} being only three times larger thanτ_crystal in the small light absorber and, hence,εis expected to be around 25%. The enlargement ofεcorresponds

to an increase of the signal amplitude¹ of the non-thermal component by a factor of

∼4−5.

In conclusion, due the size reduction of the absorber crystal also the light detector improves its performance. On the one hand an additional light output of the crystal is expected and on the other hand the size reduction of the light absorber is expected to result in an increase of the signal amplitude. In total an increase of the signal amplitude by a factor of∼6−7 is expected.

1This does not completely translate into an increase of the signal height, since also the life-time of non-thermal phononsτn increases and, thus, the rise time of the pulse. As long as the decay time of the pulse does not change, the actual increase of the signal might be slightly lower.

The CRESST-III Low-Mass Dark Matter Detector Module

CRESST-III recently started to operate ten detector modules explicitly designed for low-mass dark matter searches. The optimizations of the detectors for a low threshold (< 100 eV) are discussed in chapter 5. Additionally, a major change of the detector geometry requires a new concept for the detector holder. In CRESST-II phase 2 three different fully-scintillating detector holders were operated successfully [76, 81] and, thus, they serve as conceptional basis for the new holding scheme. Moreover, with an ultra-low threshold new challenges arise for the holder. Backgrounds with even ultra-lower energy depositions have to be considered and new solutions to discriminate them have to be found.

In the present chapter an overview of the small module optimized for a low threshold is given. The detectors that are based on the optimization discussed in chapter 5 are described in section 6.1. The fully-scintillating detector holder for these optimized de-tectors is based on scintillating CaWO₄ sticks holding the detectors. The requirements and the concept for this detector holder are discussed in section 6.2. With a reduced energy threshold of the phonon detector even a fully-scintillating detector holder can cause backgrounds at lowest energies. In order to veto low-energetic events originating from energy depositions in the holding sticks, the sticks of the phonon detector are instrumented (see section 6.3).

6.1 Detectors

The design of the phonon detector results from the optimization discussed in chapter 5.

The CaWO4absorber crystal optimized for a low threshold has a size of 20×20×10 mm³, which corresponds to a mass of ∼ 24 g. It is equipped with a TES operated in the calorimetric mode (see figure 5.2). For CRESST-III phase 1 three phonon detectors are equipped with the larger version of the calorimetric TES structure designed within the present work (see figure 5.2a) and seven phonon detectors are equipped with the smaller version (see figure 5.2b). In both cases the tungsten film is evaporated directly on the crystal in order to avoid events occurring in a TES-carrier.

To enhance the light output, the crystal is roughened on five sides [59]. The side where the thermometer is placed needs to be polished as a polished surface improves the quality of the evaporated tungsten film. The absorber crystal has small hutches where the CaWO₄ holding sticks are pressed onto (see figure 6.1). In order to avoid micro-fractures of the crystal, these hutches are spherically polished.

The size of the light detector is adapted to the reduced size of the absorber crystal.

Therefore, the light detector consists of a SOS wafer with a size of 20×20 mm² and a thickness of 0.46 mm and is equipped with a small calorimetric TES (see figure 4.2).

Due to the reduction in size an increase of the signal height is expected. Additionally, an increased light output of the crystal is expected to increase the amount of detected light (see section 5.3).