operation of a double phase pure argon large electron multiplier time projection chamber

Doktorandenseminar 2009 - Zurich 05 June 2009

Operation of a double phase pure argon Large Electron Multiplier Time

Projection Chamber

A. Badertscher, L. Knecht, D. Lussi, A. Marchionni, G. Natterer, P. Otiougova, F. Resnati, A. Rubbia, T. Viant

Institute for Particle Physics, ETH Zurich, Switzerland



IntroductionIntroductionDouble phase pure argon LEM-TPCLEM working principleSetup overviewLEM-TPC in detailDouble stage LEMAr purificationElectronicsOperation in GArOperation in LArOperation in double phasePMT signalConclusions

The detector presented in this talk is a liquid argon based Time Projection Chamber capable of charge multiplication.

Possible applications are:

• Giant detector for neutrino physics, proton decay and DM search A. Rubbia, arXiv:hep-ph/0402110, 2003

• Dark Matter imaging detector - A. Rubbia, J. Phys. Conf. Ser. 39 (2006) 129

Recent articles:

• A. Badertscher et al., arXiv:0811.3384, 2008


Double phase pure Ar LEM-TPCDouble phase pure argon LEM-TPC is a tracking and a calorimetric device that allows the amplification and the readout of charge signals from an Ar TPC working in double phase conditions.

• Free e- drift in LAr towards liquid-vapour interface.• e- are extracted to the vapour via extraction grids (Eliq ≈ 3kV/cm).• e- undergo multiplication in double stage LEM.• Multiplied charge induces signals on the segmented electrodes of top LEM and anode.

Anode

Cathode

LEM2

LEM1

Extraction

Grids

LAr level

IntroductionDouble phase pure argon LEM-TPCLEM working principleSetup overviewLEM-TPC in detailDouble stage LEMAr purificationElectronicsOperation in GArOperation in LArOperation in double phasePMT signalConclusions


LEM working principle

• e- are driven into the first LEM holes.• Townsend multiplication mechanism (G = ex, = Ape-Bp/E).• Double stage of multiplication: increase the gain maintaining low fields in each LEM.• Both LEMs were operating at the same voltage difference: G = GLEM1GLEM2 = GLEM

2 = e2x.• 3D view and energy measurement.

AnodeLEM2

LEM1

Grids

Cathode

Electron drift lines in double phase argon LEM-TPC Gain ~1000,

2x = 2mm: = 35 cm-1

From MAGBOLTZ: E ≈ 31 kV/cm

x



Setup overview

External bath

DetectorPurification circuit

HV supply

Vacuum pumps

Readout electronics

Input purification cartridge



LEM-TPC in detail

Signal plane

30 kV feedthrough

Signal cableHV connector

TPB coated

Level meters



Double stage LEM with anode

• Produced by standard PCB methods.• Double-sided copper-clad FR4 plates.• Precision holes by drilling.• Thickness: 1.6 mm.• Amplification hole diameter: 500 m.• Distance between centers of neighbouring holes: 800 m.• Segmented anode and LEM2 top plane: 2x16 strips 6 mm wide.

AnodeLEM1

LEM2

10x10 cm2

6 mm strips



Argon purification

Input LAr purification:• Custom made cartridge for LAr purification at detector input.

GAr purification circuit:• Heating resistors evaporate LAr in the detector.• A metal bellow pump pushes GAr into a flow meter and SAES getter (~48h to recirculate 1 volume).• Purified GAr condensates into the detector volume.


For a 10 cm drift at 1 kV/cm, contaminations better than 10 ppb (O2 equivalent) are needed.

To purify argon we use two purification stages:


Readout electronicsCustom made front-end charge preamp + shaper2 channel per chiprise time 0.6 s, fall time 2 s

Inspired by C. Boiano et al. IEEE Trans. Nucl. Sci. 52 (2004) 1931

In collaboration with CAEN, ADC and DAQ system development

• 12 bit 2.5 MS/s flash ADC.• Programmable FPGA.• Channel-by-channel trigger and global “trigger alert”.• 256 channel crate.• Chainable optical link.



• The gain is measured from 109Cd peak.• The electric field is calculated as the ratio of V across the LEM and the LEM thickness.

Operation in pure gas argon

55Fe and 109Cd sources positioned below the cathode grid

Deposited energy is proportional to the sum of the involved strips

Both anode and LEM signals can be used for the energy evaluation

55Fe (full peak) ~5.9 keV 6.9 kBq 29.3% FWHM

55Fe (escape peak) ~2.9 keV 6.9 kBq 42% FWHM

109Cd ~22.3 keV 0.5 kBq 24.7% FWHM

Pure argon gas operation, room temperature, 1.2 barIntroductionDouble phase pure argon LEM-TPCLEM working principleSetup overviewLEM-TPC in detailDouble stage LEMAr purificationElectronicsOperation in GArOperation in LArOperation in double phasePMT signalConclusions


Operation in pure gas argon

E (V/cm)

Anode LEM2 760

LEM2 ~14 103

LEM2 LEM1 590

LEM1 ~14 103

Drift 420

Pure GAr (Ar-60)

Room temperature

1.2 bar

Gain ~1000

LEM electrodes Anode electrodes

LEM electrodes

Anode electrodes



Operation with LEMs in liquid

LEM-TPC can be operated with the LEMs completely immersed in LAr without charge amplification.

Anode electrodes LEM electrodes

Anode electrodes

LEM electrodes

Gain = 1

Noise reduction via digital filter

S/N ≈ 80/5

Proof of LEMs transparency


E (kV/cm)

Anode LEM2 2.1

LEM2 ~27

LEM2 LEM1 1.7

LEM1 ~27

Drift 0.7


Operation in double phase

Radioactive sources were not suitable for cryogenic operation

Gain ~10Raw imagesS/N ≈ 800/10

Charge multiplication occurs in argon vapour: 87 K, 1 bar, ~3.4 denser than at STP LEM electrodes

Anode electrodes

LEM electrodes

Anode electrodes


E (kV/cm)

Anode LEM2 2.1

LEM2 ~26

LEM2 LEM1 1.5

LEM1 ~26

Drift 0.7


Preliminary analysis of cosmic tracks

Gain ≈ 10 Resolution ≈ 12%

In LAr MIP muon releases 2.1 MeV/cm @ 0.5-1 kV/cm ~10 fC/cm

Stopping power:Gauss-convolved Landau function fitted to the dQ/dx distribution (corrected for the free electron lifetime)

•3D reconstruction of each cosmic muon track.• Energy evaluation from anode electrodes signals.



Argon purity monitor

Improvement of the electron lifetime continuously purifying the argon

e [O2] = 300 ppb s

[O2] ≈ 7 ppb

LAr purity is monitored• from the time constant of the slow scintillating component (not sensitive if the purity is high)• from charge signal of long muon tracks



PMT signalPMT (TPB-coated) for monitoring extraction and LAr purity

Typical PMT signal from crossing muon

Scintillation light

Proportional light

Drift time

• Scintillation: primary light due to muon crossing the argon.• Proportional: produced by electrons in the high extraction field in the gas phase.



Electron velocity

From the maximal drift time we evaluated the electron velocity as a function of the drift field:Two different data sets and the empirical function (no fit) W. Walkowiak, Nucl. Instr. and Meth. A 449 (2000) 288



ConclusionsA novel kind of liquid argon TPC was developed and operated:double phase pure argon LEM-TPC acts as an imaging and calorimetric detector capable of higher energy sensitivity than standard LAr-TPC.

The detector can operate in pure argon gas as well as in liquid, simulating a standard LAr-TPC. In double phase operation a stable gain of 10 was achieved and cosmic muon tracks observed, higher gains are under study.Outlook:• Ongoing further characterization of 3 lt LEM-TPC.• Design and construct 1 m2 LEM for 1 ton LEM-TPC for ArDM experiment.

This work represents an important milestone in the realization of a very long drift (cost effective) LAr detectors for next generation neutrino physics, proton decay experiments and direct DM search with imaging device.


operation of a double phase pure argon large electron multiplier time projection chamber

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