Irina Bavykina, MPI f. Physik Investigation of ZnWO4 crystals as an absorbers in the CRESST Dark Matter search detector concept active background discrimination motivation for ZnWO4 absorber results 21/07/06 Irina Bavykina, MPI f. Physik

Motivation for Dark Matter searches on different astrophysical scales 21/07/06 Irina Bavykina, MPI f. Physik

WIMP direct detection requirements elastic scattering off nucleus: c low energy transfer to nucleus (< 40 keV) scattering events very rare: < 0.1 / (kg day) recoil best choice: low temperature calorimeters low energy threshold high target mass radiopure materials background discrimination 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Detector concept calorimeter model simple power-flux model heat bath absorber temperature sensor pulse height time ΔT τdec ΔT ~ ΔE/C τdec ~ C/G reality is more complicated: no thermal equilibrium decoupling of subsystems 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Readout of SPT SQUID output [V] Temperature [mK] 21/07/06 Irina Bavykina, MPI f. Physik

Main limitation: radioactive background active discrimination between electron recoils and nuclear recoils reflecting cavity silicon absorber scintillating dielectric absorber χ χ 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Quenching factor Quenching Factor = light output from electron recoil light output from nuclear recoil Irradiation with e- and  Irradiation with e-,  and n Energy in light channel [keVee] Energy in phonon channel [keV] Energy in phonon channel [keV] 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Open detector module 21/07/06 Irina Bavykina, MPI f. Physik

Motivation for different target materials discrimination technique can work with different dielectric scintillators BGO, BaF2, PbWO4, Al2O3, CaWO4, CdWO4, ZnWO4, etc. comparision of rates and spectra in different target materials to varify positive signal Important characteristics: light yield at low temperature (defines discrimination threshold) quenching factor (determination of nuclear recoil acceptance region ) high atomic mass (to take an advantage of large scalar cross section) surface properties, radio purity, phonon propogation properties ZnWO4 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Properties of ZnWO4 Properties CaWO4 ZnWO4 Density (g/cm3) 6.06 7.87 Melting point (°C) 1570 1200 Structure type Sheelite Wolframite Emission peak at 300K (nm) 420 480 Emission peak at 8K (nm) 510 (ACa=40, AZn=65) 19.3% more W nuclei ! 21/07/06 Irina Bavykina, MPI f. Physik

Light yield (H. Kraus et al, 2005) temperature dependence of the integrated luminescence yield Temperature [K] Intensity, a. u. Integrated light yield of ZnWO4 at 8K is ~ 10% higher 21/07/06 Irina Bavykina, MPI f. Physik

He3 / He4 dilution refrigerator 21/07/06 Irina Bavykina, MPI f. Physik

He3 / He4 dilution refrigerator Constant concentration of 6.5% of He3 in He4 at saturated vapor pressure even at 0K! This is finite solubility phenomenon. 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik ZnWO4 phonon detector Heat sink temperature [mK] SQUID output [V] ZnWO4 W-SPT SiO2 ZnWO4 crystal with W - SPT 110 115 120 130 140 125 145 150 135 70 30 60 50 40 20 10 80 Energy [keV] Counts SiO2 diffusion barrier: Tc≈ 7 to 25 mK (adjustable) TD (°C) 510 680 740 TC (mK) 200 55 22 Energy resolution: ΔEFWHM= (2.26 ± 0.14) keV @ E=122 keV 21/07/06 Irina Bavykina, MPI f. Physik

Measurement of the quenching factor of ZnWO4 Irradiation with e- and  Irradiation with e-,  and n Electron recoils Electron recoils Pulse height in light detector [keVee] Pulse height in light detector [keVee] Nuclear recoils Pulse height in phonon detector [keV] Pulse height in phonon detector [keV] 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Results Light to phonon ratio Energy in phonon detector [keV] 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik ZnWO4 quenching factor Energy range [keV] Quenching factor of neutrons 10 to 20 38.5 ± 1.6 20 to 40 12.5 ± 0.4 40 to 60 10.5 ± 0.1 60 to 80 10.7 ± 0.1 80 to 100 10.0 ± 0.1 100 to 120 11.1 ± 0.1 21/07/06 Irina Bavykina, MPI f. Physik

Irina Bavykina, MPI f. Physik Conclusions quenching factor of neutrons with ZnWO4 was measured for the first time measured light output below 150 keV is highly non-linear (could limit active background suppresion potential) ZnWO4 appeared to be suitable target material for CRESST ZnWO4 can be a complement to CaWO4 for the first multi-target detector capable of WIMP identification through material signature 21/07/06 Irina Bavykina, MPI f. Physik