With the optimized detectors being much smaller than the detectors operated in CRESST-II, also a new holding scheme is required.
6.2.1 Requirements for the Detector Holder
The new detector holder must fulfill certain requirements:
In order to veto background induced by surface-αevents the module housing needs to be fully-scintillating. In CRESST-II phase 2, different alternative designs with a fully-scintillating detector holders were successfully operated and showed that they are able to efficiently vetoα-induced backgrounds. The new design is based on one particular design, namely the detector holder with the absorber crystal held by CaWO4 sticks. For more details see e.g. [69].
Mechanical stress on the CaWO4 crystals causes stress-relaxation events, which are low-energetic events without a light signal. As such events can appear in the region of interest for dark-matter search, they need to be prevented. Such events were observed in the past due to a too tight clamping of the crystal [120] as well as in modifications of the clamps with scintillating plastic materials [63]. Another source for no-light events was observed due to a strong pressure on a roughened crystal surface, which might cause micro-fractures of the crystal [69]. This can be prevented or at least reduced by polishing all crystal surfaces in areas where pressure is applied to the crystal.
In order to avoid external backgrounds all materials must be as radiopure as possible.
The light collection and transportation to the light detector has to be ensured for a maximization of the detected light.
The thermal coupling of the crystal via the holding structure must not be too strong in order to allow an operation of the detector as cryogenic calorimeter.
The detectors have to be held in a stable position at mK temperatures in order to minimize mechanical vibrations, that can induce additional noise.
The modules are mounted in the carousel, which was designed for 33 cylindrical modules with an absorber crystal mass of ∼ 300 g. As the plan for future phases of CRESST is to mount 100 detector modules in the present CRESST setup [84], it is desired to design the outer dimensions of the module that allow for close packing.
light detector (with TES) block-shaped target crystal
CaWO4 iStick (with holding clamps) reflective and scintillating housing
Figure 6.1:Schematic drawing of the small detector module. The phonon detector is depicted on the left side, while the light detector is shown on the right side of the housing. Both detectors are equipped with TES, which are depicted as red rectangles. The two detectors are enclosed by a scintillating and reflective surrounding. The phonon detector is held by scintillating CaWO4
sticks, which are pressed against the crystal from outside the module by bronze clamps. The iStick TES are located outside the housing (red rectangles). The light detector is also held by CaWO4sticks.
6.2.2 Fully-Scintillating Housing
In the new holding scheme both detectors, the crystal as well as the light detector, are held by CaWO4 sticks that are pressed against the crystal with bronze clamps from outside the housing. These bronze clamps maintain their flexibility also at mK temperature and, thus, can keep the detectors in a stable position without introducing mechanical stress. A schematic drawing of the module is shown in figure 6.1, while a photograph of an opened module is shown in figure 6.2.
Due to the small size and low weight of the CaWO4 crystal, three CaWO4 sticks are enough to hold it in position. Two sticks are pressed against the bottom of the crystal, while one is pressed against the top. The sticks have a polished spherical ending to provide a point-like contact to the absorber crystal (see figure 6.3a). They have a length of 12 mm and a diameter of 2.5 mm. In order to prevent sidewards motions of the crystal, the latter features small hutches, where the sticks are pressed into. With this holding scheme, the crystal is held stable enough to rotate the module in any direction, which simplifies the mounting of the complete module.
Additionally, also the light detector is held by three CaWO4 sticks. Similar to the crystal, it is held by two sticks on the bottom and one stick on the top of the detector.
In order to stabilize the detector they are rounded concavely, such that the thin wafer cannot move sideways.
All copper parts are covered with scintillating and reflective foil on the inside of the housing. Thus, all parts inside the housing are either scintillating or active. The
target crystal clamp pressing on
CaWO4 iStick copper holder
light detector CaWO4 stick
Figure 6.2: Photograph of an assembled detector module, where two sides of the housing are opened. The target crystal with its TES is visible in the back. In the foreground the brownish SOS light detector with its small TES can be seen. The darker circle around the TES is a silicon dioxide layer, that is evaporated on the light detector below the tungsten film in order to improve the film quality. Both detectors are held by CaWO4 sticks. All three CaWO4
sticks holding the light detector are visible. Only one of the bottom iSticks holding the phonon detector and the top iStick outside the copper housing can be seen. The clamps pressing the top sticks are visible outside the copper housing. The copper holder is surrounding the module on all sides. Each side can be opened individually. The reflective and scintillating foil is glued to the copper parts.
reflective and scintillating foil encloses the detectors completely except for openings where the sticks and the bondwires are fed through. The foil is cut in a way that such openings are minimized.
The top and the bottom of the housing are each made of a copper structure, with holes where the sticks are fed through. Additionally, the clamps to hold the sticks are screwed to these structures and the bond pads are glued onto them. The sides are each closed with a copper sheet with a thickness of 2 mm. These copper sheets are screwed to the top and bottom part and can be opened individually. A photograph of a fully-mounted detector module with two sides opened is shown in figure 6.2. The closed module with the outer dimensions of 55×40×35 mm2 is block shaped, which allows a close packing of the modules.