Readout of a TPC with the MEDIPIX2 CMOS chip as direct anode

Saclay/Dapnia P. Colas

Y. Giomataris

NIKHEF A. FornainiH. van der GraafJ. TimmermansJ. Visschers

CERN/MEDIPIX2 E. Heijne

Univ. Twente/MESA+ J. Schmitz

DriftSpace

GEM foils

MediPix CMOS pixel sensorBrass spacer blockPrinted circuit boardAluminium base plate

Our GEM-equipped TPCWe have constructed a small test TPC equipped with three GEM foils whichcan be read out by means of the MEDIPIX2 CMOS pixel sensor.The GEM foils were obtained from the CERN/Sauli/GEM group;hole-to-hole distance (hexagonal geometry): 140 µm, hole diameter 85 µm,fiducial surface 100 mm x 100 mm, thickness 50 µm.The drift volume (vol. 100x100x100 mm3) is surrounded by square wire loops,spaced 6.3 mm, put at decreasing potential. Three GEM foils are placed 7.4 mmbehind the plane of the bottom wire loop; the distance between GEM foils is 1.6 mm.The anode plane, at ground potential, is 6.6 mm below the third GEM foil.

Top Drifter                      5700 V

Bottom Drifter                3473 V

Top GEM 1                     2813 V

Bottom GEM 1               2462 V

Top GEM 2                     1876 V

Bottom GEM 2               1524 V

Top GEM 3                       938 V

Bottom GEM 3                 586 V

In the base plate of the chamber, a hole was cut out for the MEDIPIX2 chip: its pixel surfacewas flush with the (anode plane) base plateplane. The MEDIPIX2 chip contains 256 x 256square pixels with pitch 55 µm x 55 µm givinga total fiducial sensitive area of14.08 mm x 14.08 mm. Each pixel is equippedwith a low-noise charge preamp, discriminator,two threshold DACs, a 13-bit counter andcommunication logic.Since a triggering system had not beenimplemented, we operated the MEDIPIX2sensor by enabling the counters manually,and stop the counting after a pre-set timeinterval (0.1 - 10 s). After that, the counts ofeach pixel are read out.

Drift length: 100 mmDistance between GEMs: 1.6/2.6 mmDistance bottom GEM/MEDIPIX: 6.6 mm

First events, recorded on March 29, 2003.Drift space irradiated with 55Fe quanta

No source; exposed 0.01 s No source; exposed 2 s No source; exposed 2 s

No source; exposed 0.1 s 90Sr source; exposed 0.01 s

Fiducial field:14 x 14 mm2

Collected ionisationin 14 x 14 x 100 mm3

during exposure time

Feb 9, 2004

Dean Karlen: VRVS meeting Sept 17, 2003

August 2002 Jeju, KoreaDean Karlen: calculation of diffusion constants;Kirsten Sachs: measurment: offset 500 m in drift versus diffusion

Sept 2003: VRVS meeting:Dean Karlen: Diffusion/defocussing too large at small drift distance

Induced charge effect not ruled out

Anode strip Scope

55Fe Test Pulse (edge)

Possible explanation for charge widening:

• Electrons end up in low E-field region• Electrons are either ‘caught’ by drift field or diffuse to low field regions• Electrons may migrate to regions associated with adjascent GEM holes

Conclusions

- We have readout a TPC with triple GEM by means of a monolitic pixel chip as direct anode;

- The distribution of the electrons leaving a GEM hole is much wider than the hole pitch. This may be explained by a large fraction of the avalanche not traveling straight to the anode, but arriving much later, after an effective de-focusing effect.

- Due to the charge spread, the signal per pixel is very small, and the combination of three GEMs and the MediPix can not detect single primary electrons;

- We expect much better results from a Micromegas/Pixel sensor combination

Thanks to:

NIKHEF:Wim GotinkJoop RovenkampJouriAd Berkien,Frank de ZwartEdward Berbee,Ton Boerkamp

Saclay:Arnoud

Short term plans:

- Micromegas/Pixel sensor tests;- Construction of Micromegas in post-wafer process;- definition of TimePix CMOS chip