New Readout Methods for LAr detectors

P. Otyugova

20.10.2006

ETH Zurich, Telichenphysik

CHIPP Workshop on Neutrino physics

Liquid Argon TPCs detect the ionization charge to create the image of the event and the scintillation light can be used for triggering or T0 definition.

To detect the ionization charge in a large noble liquid detector very low noise charge preamplifier is required (challenging, costly).For example: in ICARUS detector the signal is only 15000 electrons for a minimum ionizing particle track with 3mm wire pitch. In this case to obtain a highsignal to noise ratio the equivalent noise charge has to be less then 1000 electrons.

Signal from a MIP recordedIn the Induction plane of T600ICARUS detector.

In 100 kton detector with 20 m drift In a field 1kV/cm the drift time is about 10 ms. With a 2 ms electron lifetime , the 6000 electron/mm signal is attenuated by:

It is too low for a readout as in ICARUS (See talk of A.Rubbia)

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e−t /τ ≈1/150

How can we solve this problem?

To compensate the attenuation and to improve signal-to noise ratioone can introduce a so-called double-phase amplification and readout.

e-

Ampl. + readout

race track electrodes

Liquid Ar

Gas Ar

Extraction grid

In this type of readout the electrons are extracted from the liquid by a grid into the gas phase where they are amplified and read out by a gas multiplier technique.

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σ=3mm- the spread from the diffusion on 20m drift distance. It gives a limit on a readout granularity.

References on a Double Phase detection method: Experimental Methods for Particle Dark Matter Detection. Dolgoshein et al., Fiz.El.Chast. Atom. Yad. 4 (1973) 167

Different types of gas avalanche detectors can be proposed for a charge readout: GEM, T-GEM, LEM, MicroMEGAS, ...

We are working on the R&D of a Large Electron Multiplier (LEM) made out of standard PCB technique.

This method potentially provides a several benefits:

1. Possibility to reach electron amplification gain up to 104.2. Improvement of signal to noise ratio which results in a possibility to detect events with a lower energy threshold or long drift paths. 3. Low cost and relatively easy mode of operation.

For Dark Matter applications, this method potentially provides additional benefits:

1. Improvement of position resolution as compared to PMT readout.2. Possibility to use very low background materials.3. Takes less space than PMTs.

What is the LEM?

Diameter ofthe hole: 500 microns.

Distance betweentwo holes: 800 microns.

LEM thickness:1.5mm.

For HV supply bothsurfaces are coveredwith copper electrodes.

LEM is a thick macroscopic GEM

For obtaining a necessary gain a double-stage LEM system is required.

Multiplication factor (average gain) ~104.The simulation is madefor 100% Ar gas at temperature T=87K and pressure1bar.

The detector will consist of 2 LEM stages with 3mm separation

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Image of the hole is made with magnification x500. Edge of the hole seems to be smooth. It helps us to avoid the corona discharges which can be caused by the sharp and not

uniform edges.

Experimental setup

Tests of a single-stage LEM at room temperature

The tests• LEMs of 3 different thickness were tested:

2.4mm, 1.6mm, 1.0mm• 2 radioactive sources were used Fe55 (5.8keV

X-rays), Cd109 (22keV X-rays)• The tests were made in a pressure range

from 1bar to 3.54bar.• The value of the drift field was ~ 100V/cm

which according to the simulation provides a 100% transparency for the drifting electrons

The typical spectrum (Fe55)

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Gain curves for 3 types of the LEMobtained with Fe55 source

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500

1000

1500

2000

2500

3000

3500

1600 1800 2000 2200 2400 2600 2800 3000 3200

Gain 2.4mm 1barGain 1mm 1barGain 1.6mm 1bar

Gain

Voltage (V)

The experimental setup

Tests of double-stage setup

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Double-stage LEM system under construction.

Before the tests chamber was evacuated down to a pressure of 2*10-4 mbar.

Tests of this setup were performed in a wide range of pressures and temperatures and now it is prepared to be tested in double-phase conditions.

The High Voltage system for the LEM setup

The setup is supplied through the voltage divider:

R01=R02=100MOhm

R11=R12=500MOhm

R21=R22=100MOhm

The setup inside the container

Shapes of the signals from double-stage LEM system on the output of the preamplifier and 50 Ohm input of theoscilloscope.

The average signal rise time is about 12s.These are signals from Fe55 radioactive source (5.8 keV),event rate is about 1kHz.

Gain curves at atmospheric pressure and room temperature

Transfer field:Etransf=360V/cm

Tests in gas at cryogenic temperatures

Here you can see a plot which illustrates a shift of gain curves withtemperature changes.

Nitrogen boiling point: 77K

Argon boiling point: 87K

E (V/cm)

Preparation for the double-phase test

To make these tests we have to modify our setup.

The invar-plates levelmeter is used to monitor the liquid level inthe setup.

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Prototype Stripped LEM readout

Here is the schematic viewof a LEM prototype withstrips.The prototype which weare going to test sooncontains 10 strips on each electrode.Strips on both sides of the PCB are perpendicular.

Design of a final LEM readout for ArDM Experiment

Number of channels-1024

•Full scale measurement of long drift (5 m), signal attenuation and multiplication

• Simulate ‘very long’ drift (10-20 m) by reduced E field & LAr purity

• High voltage test (up to 500 kV)

• Measurement Rayleigh scatt. length and attenuation length vs purity

• Design & assembly:

completed: external dewar, detector container

in progress: inner detector, readout system, …

e-

readout

race tracks

Flange with feedthroughs

LAr

Gas Ar

grid

8” PMTET 9357FLA

Extraction from LAr to GAr and LEM readout

Field shaping electrodes

8” PMTET 9357FLA

Extraction from LAr to GAr and LEM readout

Field shaping electrodes

ARGONTUBE

Field shaping rings(10mm spaced)

Greinacher chain

LEM

Bottom view

Top view

Light pulse source

Inner detector design

Outlook

1. Switching to a double-phase technique gives us a possibility to solve a problem of signal-to-noise ratio and signal attenuation in large cryogenic detectors.

2. It is planned to use LEM in many double-phase cryogenic detector projects: ArDM, GLACIER, ARGONTUBE.3. The preliminary tests showed us, that the multi-stage LEM provides gain of 104 in pure Ar gas at a pressure of 1bar and a temperature of 90K.4. Gain curves shifts correlated with the temperature changes can be explained with the fact that the gas density increases with decrease of the temperature.5. For the moment we are already prepared to start a double-phase test of our setup.