The projections shown in this chapter demonstrate that for the sensitivity in the low-mass region of the parameter space a reduction of the energy threshold is the crucial modification, while a reduction of background and an increase of exposure help to im-prove the limit further in the high-mass region. In order to explore new parameter space, the future strategy of CRESST-III is to operate detector modules with a thresh-old reduced to at least 100 eV in phase 1 [84]. Within the present work the new detector module for phase 1 with a reduced threshold is developed in order to gain sensitivity for low-mass dark matter particles.
In addition to the reduction of the threshold of the phonon detector smaller crystals are expected to be beneficial for the light detector performance. Due to the size reduc-tion of the crystal more light is expected to escape the crystal which may result in an increase of the detected light by a factor of 3 [84]. Moreover, also the size of the light detector can be adjusted to the crystal geometry, which corresponds to a reduction of the volume by a factor of 3. The latter is conservatively expected to translate in a reduction of light detector noise by a factor of 2 [84].
Ten of such small modules are operated in CRESST-III phase 1 which started to take data in September 2016. It is expected, that within one year an exposure of 50 kg days can be collected. The sensitivity expected for this exposure obtained with small
Figure 3.5:The expected sensitivities (1σC.L.) for a module with a reduced threshold of 100 eV (dark yellow), 50 eV (medium yellow), and 20 eV (light yellow) shown in the parameter space for elastic spin-independent dark matter nucleon scattering. All other parameters (concerning e.g light detector performance and background) are equal to the ones of TUM40. All projections are simulated for an exposure of 29 kg days in order to compare them to the projection for TUM40.
A lower energy threshold results in the low dark matter mass region in an improved sensitivity while at high dark matter masses no sensitivity enhancement is expected. For comparison the currently leading limits of different experiments (solid lines) and the neutrino floor for CaWO4
(gray-shaded area) are shown [37, 45, 49, 67, 74]. Note that the scale of the x-axis is different (extended down to dark matter particle masses of 0.1 GeV/c2 compared to 0.5 GeV/c2 before) compared to the plots shown previously.
modules is depicted in figure 3.6 for a threshold of 100 eV (dark orange) and a threshold of 20 eV (light orange). The measurements with a prototype module, discussed in chapter 7, indicate that a threshold of 20 eV is possible to achieve with the optimized detector module developed in the present work. Due to the reduced threshold new parameter space in the low-mass region can be explored. The improvement of the expected sensitivity in the low mass region is mainly caused by the decreased threshold, since the sensitivities are similar to the expected sensitivity shown in figure 3.5 where only the threshold is reduced. As expected, both projections only differ in the low-mass region, where the threshold dominates the sensitivity. The additional improvements of the light detector performance and the increased exposure enhance the sensitivity mainly in the mass region of&5 GeV/c2. With a small module having a threshold of 20 eV masses down to∼0.13 GeV/c2might be explored with CRESST-III phase 1. For a dark matter particle mass of 1 GeV/c2 the sensitivity is increased by more than four orders of magnitude.
For a further gain in sensitivity in CRESST-III phase 2 it is necessary to reduce the background. As already mentioned, it is planned to decrease the internal background of the crystals by a factor of 100 [84]. Additionally, the number of readout channels is planned to be increased, so that in total 100 of the small modules can be operated.
In this case it is foreseen to collect an exposure of 1000 kg days within two years of measurement time [84]. The expected sensitivity for this phase is illustrated in figure 3.6 as the two lowest projections for a threshold of 100 eV (red) and 20 eV (light red).
Compared to the sensitivity expected for CRESST-III phase 1 the sensitivity can be increased in the whole mass region by at least one order of magnitude due to the increase of exposure and the reduction of background. Compared to the current sensitivity CRESST-III phase 2 is expected to improve by six orders of magnitude for a dark matter particle mass of 1 GeV/c2. The expected sensitivities shown here are in good agreement to former projections [84].
To summarize, depending on the threshold of the small modules, CRESST-III will be able to explore the parameter space of dark matter down to masses of∼0.1 GeV/c2. Furthermore, with these detector modules and a large enough exposure the detection of coherent neutrino nucleus scattering might be in reach. In the remaining part of the present work the module foreseen for CRESST-III is discussed in detail.
Figure 3.6:The expected sensitivities (1σC.L.) for CRESST-III phase 1 (orange) and phase 2 (red) shown in the parameter space for elastic spin-independent dark matter nucleon scattering.
CRESST-III will operate small detector modules with a reduced threshold of at least 100 eV (dark colors) or even down to 20 eV (light colors). In the first phase an exposure of 50 kg days is aimed to be collected within one year. In this phase new parameter space in the low mass region can be explored, while the sensitivity for higher masses is only slightly increased. In the second phase the aim is to reduce the e−/γ-background by a factor of 100 and, additionally, to increase the number of operated detector modules to 100 in order to achieve an exposure of 1000 kg days within two years. These improvements are expected to result in an increase of the sensitivity in the whole mass range. For comparison the currently leading limits of different experiments (solid lines) and the neutrino floor for CaWO4(gray-shaded area) are shown [37, 45, 49, 67, 74].
Detector Model
In order to be able to optimize the detector performance a detailed understanding of the detector physics is required. The process that leads from an energy deposition to a thermal signal in the thermometer can be described within a thermal detector model, which is summarized in the present chapter. In order to model the detector behavior, the geometry of the detectors and especially that of the thermometers needs to be known (see section 4.1). The thermal model that describes the processes after an energy deposition which finally leads to the formation of the measurable signal is discussed in section 4.2. The detector is also affected by various noise sources (see section 4.3). The noise contribution measured in the different setups is investigated in section 4.4. For the determination of the detector performance the decisive parameter is the signal-to-noise ratio. Therefore, in section 4.5 it is discussed how signal and noise determine the sensitivity of a detector.
4.1 Components of the Detector Module
As mentioned in the previous chapter, a CRESST detector consists of a dielectric ab-sorber (CaWO4, sapphire or silicon) and a superconducting thin tungsten film. The latter is evaporated on the absorber and serves as a sensitive thermometer. An energy deposition in the absorber material is transformed into phonons which then can be transferred into the thermometer.
As the goal of the present work is to optimize the performance of the detectors, they are described in detail in the following.
4.1.1 Phonon Detector
In CRESST-II the absorber of the phonon detector typically is a scintillating CaWO4
single crystal. As explained before there exist different detector module designs (see section 2.6). In the conventional module the crystal is of cylindrical shape with a diameter and a height of ∼40 mm which corresponds to a mass of ∼300 g.
Two of the alternative detector designs mounted in CRESST-II phase 2 feature smaller crystals, in order to obtain the same outer dimensions of the module as the standard conventional design, which allows a proper mounting in the fixed geometry of the carousel. The cylindrical crystals of the modules with a silicon beaker as light detector have a mass of ∼ 195 g. For the module with the crystal held by CaWO4
sticks a cuboidal crystal has been used, as this shape is expected to have a higher light output [69, 80]. In order to fit into a standard module housing those crystals have to
tungsten film
heater bond wire 7.5
5.9
thermal link
Figure 4.1:Structure of the phonon detector TES used in CRESST-II phase 2. The tungsten film (dark gray) has a size of 7.5×5.9 mm2. The aluminum film (light gray) is used to contact the thermometer with bond wires. The bias supply wires provide a constant current for the thermometer. The thermal link is realized by a gold bond wire and couples the thermometer to the heat bath. The heater is used to stabilize the thermometer in its transition and to send heater pulses. All measures are given in millimeters. Image taken from [92].
be slightly smaller with a size of 40×32×32 mm3 corresponding to a mass of 249 g [81].
During the evaporation of the tungsten film for the thermometer, which happens at a temperature of ∼450°C and a low pressure of 10−8mbar, the oxygen content of the crystal is reduced which results in a reduction of its scintillation efficiency [59]. There-fore, most of the detectors are produced as composite detectors, where the thermometer is produced on a small carrier substrate (with a size of 20×10×1 mm3) which is then glued with a thin layer of epoxy resin to the big absorber crystal [57].
While the absorber crystals of the different module designs are slightly different, all phonon detector TES operated in CRESST-II phase 2 have the same size and design depicted in figure 4.1. The tungsten film (dark grey), which is evaporated either directly on the crystal or on a small carrier substrate, has a size of 5.9×7.5 mm2 and a thickness of 200 nm. On each of the two long sides an aluminum stripe (light grey), which is used to connect the thermometer via bond wires, is placed. The thermal coupling to the heat bath is realized with a gold bond wire of 25µm diameter, connected to the gold stripe (yellow) in the middle of the thermometer. The heater is realized by a gold bond wire, which is attached to the gold stripe in the middle of the thermometer. With a heater current controlled by a PID controller, the thermometer is kept in a certain point in its transition (see section 2.4).
4.1.2 Light Detector
In the majority of all modules operated in CRESST-II phase 2, the light absorber con-sists of an SOS (silicon-on-sapphire) wafer, which is a sapphire disk of 40 mm diameter
1
0.5 0.3
0.45
heater
thermal link tungsten film phonon collectors
Figure 4.2:Structure of the light detector TES as used in CRESST-II phase 2. The tungsten film (dark gray) has a size of0.45×0.3mm2. It features phonon collectors made of an aluminum film (light gray), which are also the contact pads for the bias supply wires. The thermal link consists of a thin gold film (yellow) and couples the thermometer weakly to the heat bath. The heater (left) is electrically separated from the thermometer film. It consists of a thin gold film with two aluminum contact pads. All measures are given in millimeters.
and of 0.46 mm thickness with a 1µm epitaxially grown silicon layer on one side1. The TES of the light detector covers a smaller area than the phonon detector TES.
In figure 4.2 the typical structure of the light detector TES is depicted. The actual thermometer film has a size of 0.45×0.3 mm2 (dark gray). To increase the amount of phonons absorbed by the thermometer it uses phonon collectors made of an aluminum film (light gray, see section 4.2.7). The thermal coupling is provided by a thin gold film (yellow) and couples the TES thermally to the heat bath of the cryostat. The heater is electrically and physically separated and consists of a small gold film with two aluminum bond pads. An electrically separated heater is advantageous as it reduces the observed noise [66]. Although, this heater is not electrically connected to the tungsten film, it works similar to the phonon detector heater, i.e. it allows to inject non-thermal phonons into the heater [66].