Operating a GridPix detector in Dark Mattersearch experimentsRolf Schon, Gijs Hemink, Maarten van Dijk, Matteo Alfonsi, Niels van Bakel, Patrick Decowski rolfs@nikhef.nl

Dark Matter and the energy density of the universe

Figure 1: Visible baryonic matter is but asmall contribution to the energy densityof the universe [1].

Luminous baryonic matter is not enoughto explain rotation curves of galaxiesand clusters of galaxies that have beenmeasured by astronomers. These obser-vations point to a contribution of so-farundetected non-luminous, “dark” mat-ter to the energy density of the universe(see Fig. 1).

One promising candidate for dark mat-ter is the so-called Weakly InteractingMassive Particle (WIMP) which onlyinteracts through gravitation and theweak force [2]. The event rate for colli-sions with ordinary matter is expected to be as low as 0.1 events/kg/day (with energydeposition of O(10 keV)) [3].

For the detection of WIMPs, a high-mass target is required to provide a sufficient totalcross-section. Moreover, the background level has to be controlled very well becauseof the low expected event rate.

The XENON dark matter search experimentThe XENON collaboration uses xenon as the target material in a dual-phase time projection chamber (TPC), i.e. alayer of gasous xenon on top of liquid xenon (LXe). Similarly, argon can be used instead of xenon.

Figure 2: Detection principle of the XENON TPC [4].

WIMPs scatter elastically from the Xe nucleiwhich allows a direct detection due to the nu-clear recoil. The recoil results in scintilla-tion and ionization of the Xe molecules. Theprompt scintillation is recorded by PMTs at topand bottom of the TPC (signal S1). The ioniza-tion electrons are drifted out of the liquid phaseby an electric field. In the gas region an elec-tron avalanche is produced by an amplificationfield leading to a second scintillation signal S2detected by the top PMT array.

The relation S2/S1 is different for electrons,gammas, and nuclear recoils. This provides anexcellent tool to discriminate the backgroundproduced by radioactive decay.

The GridPix detector and what it has to offer

Figure 3: The integrated grid on the pixel chip.

The GridPix is a micro-pattern gasdetector composed of a Microme-gas-like amplification grid on topof a pixelated Timepix chip. Thegrid is integrated by CMOS post-processing technology (see Fig. 3).

When radiation passes through thedrift region between the cathodeand grid, the gas atoms are ion-ized and the freed electrons drift to-wards the grid. Due to an ampli-fication field between grid and an-ode (i.e. the chip) those electronsare multiplied into avalanches. Thecharge of such an electron cloud isthen large enough to be detected by the pixels of the chip. These processes are sketchedin Fig. 4:

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Figure 4: Principle of the GridPix detector.

The gain of the amplification depends on the gas (mixture) used inside the detector.With typical gases (Ar/iC4H10, He/iC4H10, CO2/DME etc.) a gain of several thousandhas been achieved. This leads to a single electron detection efficiency of over 95%.

The GridPix may offer an improvement to the XENON experiment because the elec-trons drifted out of the liquid phase of the TPC can be detected directly without the needfor the second scintillation.

Cold temperature tests of GridPix

Figure 5: GridPix’s test vessel inside theNLR cryo-chamber.

First tests of operating GridPix in cryogenic environment were car-ried out at the National Aerospace Laboratory (NLR). For two gasesthe following temperatures were reached during cool-down (seeFig. 5) with confirmed operation:

Gas composition Ar/iC4H10 90/10 mixture pure ArTemperature −73 ◦C −50 ◦C

Although the tests did not reach the LXe temperature of −108 ◦C,GridPix is expected to be still operational at lower temperatures.This will be the subject of future studies at Nikhef.

To operate a GridPix in a LXe cryostat there are two issues:

• keep outgassing and radioactive impurities to a minimum,

• withstand thermal stress due to the cryogenic environment.

The outgassing will be minimized by careful selection of materialsfor the components (printed circuit boards PCB, electronics, glueetc.) and by testing their outgassing properties under ultra-highvacuum (UHV) with a mass spectrometer (see Fig. 6). Preliminaryresults show good outgassing qualities of GridPix.

The thermal stress on material and joints is under investigation. Asan example, a data cable and a GridPix detector have been sub-merged in LN2 (T = −196 ◦C). The data cable still performed wellafter the test whereas the grid of the detector partly peeled off due to the temperature shock (see Fig. 7).

Figure 6: GridPix in an UHV chamber at Nikhef. Figure 7: Ripped grid after an LN2 test.

Open questions and OutlookAs the achievable gas gain in a gaseous detector depends on the gas pressure, it is necessary to study GridPix’s performanceat noble liquid conditions. For that purpose a cryostat will soon be set up at Nikhef which can be filled with Xe or Ar.

A further development will be to test GridPix in an LAr environment (T = −186 ◦C) in the ArDM test cryostat at CERN.This will allow for predictions on the performance. Furthermore, LAr is significantly colder than LXe so that it provideseven tighter constraints on GridPix’s performance and mechanical robustness.

The goal is to implement GridPix into the top of a dual-phase TPC, such as proposed in the DARWIN design study. However,the aim is not to replace the complete PMT array, but to complement it to improve the detection susceptibility for ionizationelectrons (S2 signal). This will lead to a more reliable discrimination of electron recoil background from the signal and thusto cleaner results.

The ultimate aim is to detect WIMPs and to establish whether they eyplain the observed dark matter abundance.

References[1] N. Jarosik et al. Seven-year Wilkinson Microwave Anisotropy Probe

(WMAP) Observations, arXiv:1001.4744v1, 2010.[2] G. Steigman and M.S. Turner. Cosmological constraints on the properties of

weakly interacting massive particles, in Nuclear Physics B, 1985.[3] J. Angle et al. First results from the XENON10 Dark Matter Experiment at

the Gran Sasso National Laboratory, arXiv:0706.0039v2, 2007.[4] E. Aprile. Direct Searches for Dark Matter, Talk given at EPS–HEP, Krakow,

July 2009.

AcknowledgementsContributions of the helping hands and minds of a lot of people made this work possible, especially those of Maarten van Dijk, Gijs Hemink,Matteo Alfonsi, Martin Fransen, Harry van der Graaf, Berend Munneke, and Patrick Decowski.

This work is supported in part by the Marie Curie project MC-PAD of the European Union.

The Marie Curie projectParticle Detection