Study of GEM Structures for a TPC Readout

M. Killenberg, S. Lotze, J. Mnich, A. Münnich, S. Roth, M. Weber

RWTH AachenOctober 2003

Experiments & numerical simulation

R&D on GEM for a TPC at Aachen

o Charge transfer in high magnetic fields: Collection of primary electrons ( dE/dx) Effective gain Ion feedback

o Gas studies: TPC gas: high drift velocity at low field High neutron background low H content Impact of gas on charge transfer in GEM

o Length & width of electron signal resolution

o Mechanics of GEM readout structure and long term TPC operation

Charge Transfer Measurements in Magnetic Field

5 T magnet at DESY

Charge transfer deduced From current measurements

o Triple GEM structure works in a large magnetic field

o B field improves some parameter (signal

height)

o Only small effect on collection of primary

electrons

Result of Measurements:

Langevin equation:

Aleph: B = 1.5 T @ = 9 Tesla: B = 4 T @ = 24

Impact on electron collection ?

dE/dx resolution

= cyclotron frequency = mean free time

Calculation of drift lines (no diffusion)

Collection of primary electrons

Simulation: Impact of Gas on Charge Transfer

Low diffusion gas ArCO2High diffusion gas ArCH4

Numerical simulation of diffusion with Garfield

Illustrative example:Drift paths of electrons randomly distributed over a GEM hole

Simulation of Gain in GEM Structures

Number of secondaries per primary electrons(single GEM)

Very broad distribution

Creation of secondariesmostly at edges

But there is no extractionat edges!

Gas amplication and electron extraction in a GEM

x electron createdx created & extracted

Primary electrons

Simulation of Gain in GEM Structures

Simulations allow optimisation of GEM readout structure

Comparison Measurements and Simulations

Measurement of Ion Feedback in Magnetic Field

TPC: Ion feedback into drift volume would distort electric field naturally suppressed in GEM structures

improves with magnetic field

Magnetic field [T]

Ion

fee

db

ack

Triple GEM setupOptimized for ion feedback

Width & Length of Signal

Comparison of signals: GEM versus wire readout

For optimal space resolutionmatch pad size to cluster size

Measurement of Charge Width in Magnetic Field

- Measurement of charge width after passing triple GEM stack- Reduction of diffusion in high magnetic fields

Fe55 source

ArCH4C02 92/5/3

2

20B2

0 cB1

σσ

Charge width is governed by primary ionisationand diffussion between GEM foils

Range of 2,68 keV electrons in Ar

Constrcution of a Triple GEM Readout Struture

Large (1.4 m3) TPC

Triple GEM structure

TPC operating with Triple GEM Readout

GEM readout is promising candidate to build a TPC with 100 m single point resolution in high B field

Conclusion & Outlook

o GEM readout structures successfully operated in 5 T field ion feedback & signal height improve no big loss of primary electrons cluster widths reduces as expected

o Long term stable operation of TPC with triple GEM structure

o Simulation tools in hand to optimize layout & operation parameter ion feedback <1% achieved

Next steps:

o Simulation of conditions for a TPC at the LC determine neutron backgound to choose gas and required ion feedback

o Build prototype TPC to demonstrate performance in test beam and magnetic field