operating the gridpix detector in dark matter search experiments … · dual-phase tpcs for dm...

Dual-phaseTPCs for DM

GridPix asalternativereadout

Challenges &studies

Summary

Operating the GridPix detectorin Dark Matter search experiments

T 106.4

Rolf Schon

Nikhef, AmsterdamDetector R&D

March 30, 2011

Rolf Schon (DPG 2011 Karlsruhe) GridPix in DM search experiments 1 / 13

Challenges &studies

Summary

Search for Dark Matter

• Dark Matter contributes touniverse’s energy density.

• Different approaches to detectWIMP interactions with ordinarymatter:

Atoms4.6%

DarkMatter23%

DarkEnergy72%

Solid state bolometers

• CDMS Ge/Si

• CRESST CaWO4/ZnWO4

• DAMA/LIBRA NaI(Tl)

• EDELWEISS Ge

• . . .

Noble liquids

• ArDM LAr

• WArP LAr

• XENON LXe

• ZEPLIN LXe

• . . .

Challenges &studies

Summary

Search for Dark Matter

• Dark Matter contributes touniverse’s energy density.

• Different approaches to detectWIMP interactions with ordinarymatter:

Atoms4.6%

DarkMatter23%

DarkEnergy72%

Solid state bolometers

• CDMS Ge/Si

• CRESST CaWO4/ZnWO4

• DAMA/LIBRA NaI(Tl)

• EDELWEISS Ge

• . . .

Noble liquids

• ArDM LAr

• WArP LAr

• XENON LXe

• ZEPLIN LXe

• . . .

Challenges &studies

Summary

Dual-phase Time Projection Chamber I

Example DARWIN

• DARWIN design report†

• Combination of Ar and Xe in a multi-target experiment

† L. Baudis et al. DARWIN: dark matter WIMP search with noble liquids,

arXiv:1012.4764v1, 2010.

Challenges &studies

Summary

Dual-phase Time Projection Chamber II

Example XENON

TopPMTArray

BottomPMTArray

LiquidXe

Cathode

proportional (S2)

direct (S1)

GammaE

Challenges &studies

Summary

Detection of S2

To date

LiquidXe

Anodeproportional (S2)

TopPMTArray • Ionization electron drifts into gas phase

• Proportional amplification in gas gap

• Scintillation due to electron avalanche

• Detection of scintillation light by PMTs

An alternative method

• Ionization electron drifts into gas phase

• Direct detection of electron avalanche

⇒ one energy conversion step less!

? ? ?GasXe

LiquidXe

Challenges &studies

Summary

Detection of S2

To date

LiquidXe

? ? ?GasXe

LiquidXe

Challenges &studies

Summary

Detection of S2

To date

LiquidXe

? ? ?GasXe

LiquidXe

Challenges &studies

Summary

GridPix – a micro pattern gas detector

• Amplification grid integrated onto pixelated readout chip(Timepix: 256× 256 pixels on 14× 14 mm)

• Pitch of 55 µm ⇒ high x-y resolution

• Time information ⇒ micro TPC with little material

Gas, as detection material for tracking detectors, has severaladvantages with respect to Si:

! Gas can be exchanged: radiation damage does not occur.! By applying a strong electric field, noiseless gas amplification is

possible, resulting in arbitrarily large charge signals and thisreduces the required power for the input circuits, and thus therequired materials for cooling.

! The source capacitance, seen at the input of preamplifiers, isminimal.

! Gas is light.! Gas is generally cheap.! Gas detectors are little sensitive for (background) X-rays,

gammas and neutrons.! There is no bias current in gaseous detectors.! Gaseous detectors do not require to be operated in a cooled

environment.! d-rays can be recognized and do not cause an error in the track

position measurement.

Gaseous detectors have two disadvantages: discharges occur,potentially destroying (parts of) the detector, and chamber ageing.

4. The SiProt protection layer

In proportional chambers, sparks (discharges) may occur. If, inthe avalanche area, the density of primary electrons exceeds acertain value, an avalanche may turn into a streamer and initiate adischarge. Due to these discharges, many Medipix-2 chips, appliedin GridPix detectors, get destroyed. We noticed that the chips didsurvive if the GridPix detector was filled with He-based gases. Dueto its low mass, the density of primary electrons is a factor "3lower with respect to Ar-based gas mixtures. It may be possible toconstruct and operate these detectors such that discharges do notoccur, but the reliability of GridPix detectors, in general, would bemore acceptable if the detectors could be made intrinsically sparkproof.

Discharges have two damaging effects on chips:

! The hot plasma may evaporate the structure of the top layer ofthe chip.

! The pixel circuitry may be exposed to a very large charge,causing a local breakdown.

We have designed a protection circuit in the form of a pn-junction, in 130nm CMOS technology, which can absorb a chargeinjection of 10pC by harmless dissipation [SiProt]. This circuit isapplied in the pixels of the Medipix-3 chip [5]. However, theMedipix-2 and TimePix chips, are not equipped with thisprotection circuit, and they were estimated to fail when exposedto charges in excess of 2–6pC.

We made Medipix-2, TimePix and PSD-46 chips dischargeproof by covering them with a high-resistivity layer of hydro-genated amorphous silicon (a-Si:H). Such a-Si Protection (SiProt)layer should prevent surface damage due to evaporation by theplasma. Electrons created in a discharge would not enter the pixelinput pad but would stay on the surface of the (high resistivity)layer. This reduces the electric field directly above the surfacecharge, eventually quenching the discharge.

In a first attempt we applied a 3mm layer of a-Si:H. We noticedthat this GridPix chamber worked as before: effects like a reducedgas gain or a widening of the charge distribution were notobserved. The chamber worked well for weeks in a He/isobutane80/20 mixture with a gas gain of "104. After replacing the gasmixture by Ar/isobutane 80/20, the Medipix-2 chip died within8h. After this we applied layers of 20mm a-Si:H, and chips havenot broken down ever since. Layers of 15mm a-Si:H have shown tobe adequate for Ar mixtures.

Recently, silicon nitride (Si3N4) has shown to be a promisingalternative for a-Si:H: a 7mm-thick layer has shown to protectadequately a TimePix chip. This material is commonly used in chiptechnology.

5. Chamber ageing

Gaseous detectors have often shownworsening in performancewith time. A common form of chamber ageing is the deposit ofhigh-resistivity polymers onto the electrodes of the detector. Thisdeposit is about proportional to the collected charge per unitsurface of the electrode. In the past, a large variety in the speed ofageing has been observed in relation to a large variety ofcompounds. Until today, not a single compound has beenidentified positively causing ageing, in spite of numerousdedicated experiments.

In line with measurements, MPGDs are expected to berelatively insensitive for ageing for three reasons: (a) the chargedeposit is diluted over a large anode surface (compare the surfaceof thin anode wires in proportional chambers); (b) the gas gain inMPGDs is in the order of 103–104 (cf. 105 of typical proportionalchambers) and (c) the electric field at the anode is usually muchless strong in MPGDs. The energy (temperature) of the avalancheelectrons is substantially lower in MPGDs.

At Nikhef, a irradiation facility with a 90Sr source with astrength of 5GBq is in operation. In addition, a GridPix detector inwhich a strong primary charge from photo-electrons can becreated by means of the irradiation of a metal cathode mesh withUV light is in preparation. This GridPix detector is sensitive forageing compounds. A (heated) container is included in the closedgas system, allowing the testing of their ageing properties ofcompound traces from outgassing materials.

6. New developments

It has been noticed that walls of gaseous volumes contribute inthe generation of primary charge. In solid matter, the energytransfer between minimum ionizing particles (MIPs) and theelectrons close to the surface of the wall facing the gaseous

ARTICLE IN PRESS

Fig. 1. The Integrated Grid (InGrid) on top of a 20mm SiProt layer, on top of aTimePix chip.

H. van der Graaf / Nuclear Instruments and Methods in Physics Research A 607 (2009) 78–80 79

More on Timepix:T 108.5, Fr 15:05(COBRA)

Challenges &studies

Summary

5. Chamber ageing

6. New developments

ARTICLE IN PRESS

Challenges &studies

Summary

5. Chamber ageing

6. New developments

ARTICLE IN PRESS

Challenges &studies

Summary

GridPix – how it looks in real life

Challenges &studies

Summary

GridPix – the principle I

-Vcathode

-Vgrid

0 V anode (chip)

Path of acharged particle

0 V > -Vgrid > -Vcathode

Gas detector – which gas?

• Usually a mixture of gases:• often noble gas, e.g. Ar, He• organic quenching gas, e.g. iC4H10, dimethylethylene

Challenges &studies

Summary

GridPix – the principle II

-Vcathode

-Vgrid

0 V anode (chip)

Electrons

------------ ------

Detection efficiency

• Single electron detectionefficiency > 95 %

• Beneficial for low rate ofprimary ionization

Challenges &studies

Summary

GridPix – the principle II

-Vcathode

-Vgrid

0 V anode (chip)

Electrons

------------ ------

Detection efficiency

• Single electron detectionefficiency > 95 %

• Beneficial for low rate ofprimary ionization

Challenges &studies

Summary

Purity: operation in pure gas and maintaining it

Operation in pure noble gas

• Gas gain in pure Ar, Xe sufficiently high?

Outgassing constraints

• Impurity level has to be minimized(especially radioactive impurities)

• Tests in outgassing chamber ongoing

Challenges &studies

Summary

Operation in a cryogenic environment

Gas gain in cold (dense) gas

LXe LAr LN

−108 ◦C −186 ◦C −196 ◦C

• Operation confirmed for −73 ◦C in Ar/iC4H10 90/10and −50 ◦C in non-mixed Ar (Ar 4.7: 99.997 % purity)

• Further studies at Nikhef with Ar 6.0 and Xe

• Test planned in ArDM test cryostat at CERN.

Thermal stress

• Preliminary dummy tests in LN (suddenimmersion) proved destructive.

• Further tests with gradual cool down

Challenges &studies

Summary

LXe LAr LN

−108 ◦C −186 ◦C −196 ◦C

Thermal stress

Challenges &studies

Summary

LXe LAr LN

−108 ◦C −186 ◦C −196 ◦C

Thermal stress

Challenges &studies

Summary

LXe LAr LN

−108 ◦C −186 ◦C −196 ◦C

Thermal stress

Challenges &studies

Summary

• Possibility of an alternative readout of ionization signal indual-phase TPCs; to use complementary to PMTs

• GridPix offers direct detection of electrons.

• Tests on thermal stress: ongoing

• Study of performance (gas gain)• operational down to −50 ◦C so far X• at cryogenic level: Nikhef setup finalized soon

• Tests in LAr cryostat (CERN): soon

Challenges &studies

Summary

Challenges &studies

Summary

Challenges &studies

Summary

Thanks to you for your attention!

operating the gridpix detector in dark matter search experiments … · dual-phase tpcs for dm...

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