CRESST II Phase 2

[ TUM40 Results ] [ Lise Results ]


The primary goal of CRESST-II Phase 2 was to confirm or exclude the possible WIMP interpretation of the excess above known backgrounds observed in CRESST-II Phase 1 (Eur. Phys. J. C (2011) 72). To reduce the background originating from surface events different strategies were carried out: New clamps were produced from material with highly increased radiopurity and all steps of assembling and mounting were carried out in de-radonized atmosphere to reduce the exposure to radon. Furthermore, 3 fully-active module designs were developed and installed.

The final module composition in the detector carousel was as follows:

Search for low mass dark matter particles with the detector TUM40

This module includes a CaWO4 crystal named TUM40 that was manufactured at TU München. The following figure shows the low energetic spectrum of this TUM40 crystal (blue) in comparison to a spectrum of a typical commercial crystal (black). In both crystals internal beta decays and gamma emissions are the main background source. In case of the commercial crystal this originates from the natural decay chains, whereas for TUM40 these contaminations are significantly reduced due to stringent raw material selection and a dedicated production facility. In this crystal the background is now dominated by lines originating from cosmogenic activation of tungsten isotopes.

In addition, TUM40 is one of the two modules installed where the CaWO4 absorber crystals are held by scintillating CaWO4 sticks successfully providing a veto function for surface events. Taking also into account its low threshold of about 600eV TUM40 is the module with the best overall performance of CRESST II Phase 2. A low threshold analysis was performed with the data obtained between August 2013 and January 2014 (Eur. Phys. J. C (2014) 74 , arXiv:1407.3146 ).

The two figures show the data of TUM40 in the light yield-energy plane (left) as well as an energy spectrum of the accepted events (right). In the left figure, the red lines indicate the central 80% band for tungsten recoils, the black lines for oxygen recoils. The yellow box is the region of interest for dark matter search. Its energy limit of about 40 keV is chosen since no significant contribution of recoil-energies is expected above 40keV. The upper light yield boundary is chosen in a way that 50 % of the oxygen recoil events are inside the region of interest.

In the right figure the energy spectrum of all events of TUM40 is shown in blue, whereas the accepted events (inside the region of interest) are marked in red in the insert showing the spectrum below 10keV. The x-ray lines originating of cosmogenic 179Ta are indicated.

Applying the Yellin optimum interval method an exclusion limit was calculated with 29kg days of data. In 2014 TUM40 achieved the best sensitivity below dark matter particle masses of 3GeV/c^2. At higher masses it challenges the positive signals of other experiments as well as the WIMP interpretation of the excess events of CRESST-II Phase 1 (see e.g., Eur. Phys. J. C (2011) 72).

Search for low mass dark matter particles with the detector Lise

Another low-threshold analysis was performed with the detector module Lise (conventional design) which had the lowest threshold (~0.3keV) in CRESST II Phase 2. The radiopurity of the commercial crystal installed was worse by a factor of ~2 below 40keV compared to TUM40. Due to the excellent threshold, however, the exclusion limit calculated with the Yellin optimum interval method for the whole data set of Lise (52kg days) is leading below ~1.5GeV/c^2 and extends - for the first time - down to the sub-GeV region (Eur. Phys. J. C (2016) 76 , arXiv:1509.01515 ).

Legend:

Conventional design:

In CRESST a module consists of a phonon detector (CaWO4 single crystal) and a light detector (Si or Si on Al2O3) which are mounted together in a reflective and scintilliating housing. Both detectors are equipped with a tungsten TES, which then is read-out with a squid system. Thus the heat and the simultaneously produced light signal can be recorded together.

In the conventional module both detectors are held by non-scintillating bronze clamps which might lead to a difficult background contribution due to surface alpha decays. Thus, to reduce this background large efforts were put into radon prevention.

Stick design:

One approach for an active suppression of 210Po decays is the replacement of the non-scintillating bronze clamps with scintillating CaWO4 sticks.

The left figure shows a schematic drawing of a module design with a block-shaped crystal (250 g) held by CaWO4 sticks. These sticks are fixed by clamps outside the scintillating housing. Pulse-shape analysis allows to discriminate events in the crystal from events in the sticks. The scintillation light is measured with a conventional light detector.

The right figure shows a photograph of the assembled module with crystal TUM40 inside.

Carrier design:

This design provides a fully scintillating holder which allows, as in previous design, to distinguish the background from surface alpha decays by using the additional light signal of the alpha as a veto. The module uses clamps covered with Parylene which was measured to be a good scintillator even at low temperatures. The coating is possible in this case as the clamps are not in contact with the target crystal, but only with a cylindrical CaWO4 disks which carries the TES. The design relies on the possibility to distinguish between events in the target crystal and events in the TES carrier as the latter might be caused by stress-relaxation events originating from the contact between the Parylene coated clamps and the crystal.

Schematic drawing (left) and photograph (right) of the carrier design. A roughened target crystal is glued to a polished carrier crystal. The crystal is held in position by three clamp pairs, only touching the carrier. The clamps are coated with scintillating Parylene. In the final module the crystal is enclosed in a reflective and scintillating housing.

Silicon beaker design

This design uses a silicon beaker, surrounding the cylindrical target crystal, as light absorber. As in the carrier design the target crystal is glued onto a carrier crystal. Non-scintillating clamps are attached to the carrier crystal. However, there is no line of sight between the target crystal and the clamp or any other non-active surface.

Schematic drawing (left) and photograph (right) of a module design with a Si beaker as light absorber. The crystal is held by clamps attached to the carrier, without any line of sight between target crystal and the non-scintillating clamps.



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