CRESST-II: dark matter search with scintillating absorbers G. Angloher

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CRESST-II: dark matter search with scintillating absorbers

G. Angloher

^b*

, C. Bucci

^d

, C. Cozzini

^a

, F. von Feilitzsch

^c

, T. Frank

^b

, D. Hauff

^b

, S. Henry

^a

, Th. Jagemann

^c

, J. Jochum

^c

, H. Kraus

^a

, B. Majorovits

^a

, J. Ninkovic

^b

, F. Petricca

^b

, F. Pröbst

^b

, Y. Ramachers

^a

, W. Rau

^c

, W. Seidel

^b

, M. Stark

^c

, S. Uchaikin

^b

, L. Stodolsky

^b

, H.Wulandari

^c

a

Department of Physics, University of Oxford, Oxford OX1 3RH, United Kingdom

b

MPI für Physik, Föhringer Ring 6, 80805 Munich, Germany

c

Physikdepartment, TU München, James-Franck-Str. 1, 85748 Garching, Germany

d

Laboratori Nazionali del Gran Sasso, 67010 Assergi, Italy

In the CRESST-II experiment, scintillating CaWO

4

crystals are used as absorbers for direct WIMP (weakly

interacting massive particles) detection. Nuclear recoils can be discriminated against electron recoils by

measuring phonons and scintillation light simultaneously. The absorber crystal and the silicon light detector are

read out by tungsten superconducting phase transition thermometers (W-SPTs). Results on the sensitivity of

the phonon and the light channel, radiopurity, the scintillation properties of CaWO

4

, and on the WIMP

sensitivity are presented.

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1. INTRODUCTION The goal of the CRESST-II experiment is to improve the sensitivity on direct WIMP detection by active background discrimination. When scattering elastically on the absorber nuclei, WIMPs deposit energy causing a measurable temperature rise. In scintillating absorbers, the different light yield of electron and nuclear recoils can be used for active background discrimination. We have developed very sensitive cryogenic detectors to measure simultane-ously the temperature and the light signal caused by particle interactions in 300 g CaWO

4

crystals (Fig. 1). First runs with two complete detector modules each have been performed in the

Gran Sasso

underground laboratory during the last months.

2. SCINTILLATING CAWO

4

ABSORBERS

In CaWO

4

, tungsten increases the sensitivity for spin independent WIMP interaction ( A

²

, A = number of

nucleons). Crystal samples, however, differ considerably in radiopurity and light yield.

Figure 1. A detector module consists of a scintillating 300 g CaWO

4

crystal (phonon channel) and a Si wafer (light channel), both read out by a W-SPT.

The set up is

surrounded by

reflective foil.

2.1. Detector properties

Our detectors consist of cylindrical CaWO

4

crystals ( = 40 mm, h = 40 mm) read out by a W-SPT (6 x 4 mm

²

, 200 nm thick) located on the crystal surface.

Transition temperatures as low as 7 mK have been achieved by adjusting deposition temperature (~ 480 °C)

and by depositing a buffer layer of SiO

2

between the W film and the CaWO

4

crystal.

As

CaWO

4

is very sensitive to temperature

gradients, W

deposition,

photolithography and wet chemistry have to be done carefully. For W etching, a dilute mixture of NaH

2

PO

4

,

NaOH, and

Na

3

Fe(CN)

6

was used.

For detector

operation, each CaWO

4

crystal is held by six

Teflon clamps,

designed to reduce mechanical stress on the crystal. The resistance of the W- SPT (~ 0.3 ) is measured by passing a constant current through the read-out circuit in which the thermometer is in parallel with a shunt resistor (~ 0.05 ) and a SQUID input coil [1].

Thus, a rise in the W- SPT’s resistance raises the current in the SQUID input coil.

The temperature of the detector is controlled by a dedicated heater, consisting of a Au wire ( = 25 m) that is bonded to a Au pad in the center of the W- SPT and to Al contact pads to either side of the thermometer (Fig.

2). The temperature of the W-SPT is kept constant by applying a controlled voltage across the Au wire heater. Additionally, the heater is used to inject test pulses for energy calibration and stability monitoring [2].

Figure 2. Geometry and connection scheme of a W-SPT on CaWO

4

.

We obtained 100 % trigger efficiency for 2 keV heat pulses (baseline width 0.9 keV) and good energy resolution: E = 1.5 keV for 73 keV X- rays, E = 13 keV for 1.17 MeV ’s, and E

= 8 keV for 2.3 MeV

’s. Typically, the pulse shape can be described by 

rise

= 1.2 msec and 

dec

= 30 msec.

Contaminations from natural decay chains have been identified by their - decays. Whenever the temperature rise caused by -decays was

2 W

Au-pad Al-pad

thermal link (Au) bond wire

heater (Au)

electrical contact (Al)

CaWO

₄

W-SPT

Si-wafer

re fle ct iv e fo il

W-SPT

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Elsevier Science

beyond the dynamic range of the W-SPT,

precise energy

information was extracted from signal duration. At energies of few ten keV, where the WIMP signal is expected, a background count rate of ~ 10 electron recoils / (kg keV day) has been measured.

2.2. Scintillation properties

A simple photomultiplier set-up is used to invest-igate the room temperature scintillation properties of CaWO

4

. For irradiation with

⁶⁰

Co or

137

Cs, energy resolution and pulse height vary up to a factor of three.

At 1.17 MeV, an energy resolution of 5.4 % has been obtained for some CaWO

4

crystals.

Scintillation properties can be affected by crystal processing: A 50 % decrease in light yield has been observed after sensor deposition.

Deposition tempe- rature can affect the oxidation state of CaWO

4

that is known to influence the scintillation properties.

Also beveling the

crystal shows

significant impact:

Where-as light yield increased by a factor of 2, resolving power decreased by a factor of 4.

An important

improvement in energy resolution was obtained by grinding the crystal surface facing the light detector to a roughness of about 10 m.

⁶⁰

Co lines that previously appeared as single line could be separated after roughening. Surface roughening is known to reduce the influence of trapped light in crystals with high index of refraction (n

CaWO4

= 1.92). At low temperatures, the doublet structure in the light signal of electron recoils (Fig. 3) disappeared after roughening. Then, the absorption of 122 keV

’s in the CaWO

4

crystal was detected with a resolution of 17 % in the light channel.

3. LIGHT DETECTORS

In CaWO

4

typically less than 1.3 % of the absor-bed energy is transformed into light [3, 4]. The sensi-tivity of the light detector is of utmost importance.

Since photomultipliers

are not suitable for

technical and

radiopurity reasons, we decided to use cryogenic detectors consisting of silicon wafers (30 x 30 x 0.4 mm

³

) read out by dedicated W-SPTs [1].

Al phonon collectors that are attached to both sides of the tiny W-SPT increase the sensor’s sensitivity significantly. The light detector’s thermal coupling to the heat bath, and thus the calorimeter’s

integration time, can be adjusted to the scintillation time of CaWO

4

that is some

msec at low

temperatures [1].

The detector is held at its corners by four Teflon tongues.

Measurements of the baseline width gave an energy threshold that corresponds to the absorption of 2.8 keV X-rays in CaWO

4

.

Figure 3. According to a neutron calibration, nuclear recoils are expected below the dotted line when measuring phonons and scintillation light simulta-neously. The origin of the events in the nuclear recoil region around 120 keV is not yet understood.

4. UNDERGROUND RUNS

Several detector

modules, each

consisting of a CaWO

4

crystal and an associated light detector surrounded by reflective plastic foil (3M), have been run in the low background cryostat at LNGS. Its setup is described in [2]. The detector modules were mounted on a copper plate suspended on Cu/Be- springs to reduce microphonics.

Detectors are calibrated by irradiation with external

⁵⁷

Co and

⁶⁰

Co sources and by electric heater pulses.

In the most sensitive run, no nuclear recoil was detected between 15 keV and 100 keV (Fig. 3). Exposure was limited by cryostat failure to 0.98 kg days.

The corresponding

3 exposure:

0.98 kg days

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Elsevier Science

WIMP-sensitivity can be seen in Fig. 4.

In other runs, a varying number of events have been detected in the nuclear recoil region, their energy ranging till few hundred keV. The origin of these events has still to be clarified.

The experiment will be upgraded for operation of up to 33 detector modules. The long term goal is to reach a sensitivity of 10

^-8

picobarn for the WIMP-nucleon cross section.

Figure 4. Actual WIMP sensitivity of CRESST- II in comparison with other experiments [5, 6, 7].

ACKNOWLEDGME NTS

This work was supported by the DFG SFB 375 “Particle Astrophysics”, the EU

Network “Cryogenic Detectors” (contract ERBFMRXCT980- 167), the EU Network HPRN-CT-2002-00322 on Applied Cryo- detectors, BMBF, PPARC, and two EU

Marie Curie

Fellowships.

REFERENCES 1. F. Petricca et al.,

Light detector development for CRESST-II, to appear in the proceedings of the 10

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