Chevron / Zigzag Pad Designs for Gas Trackers

Bo Yu

Instrumentation Division

Outline

Overview of various interpolating readout methods Some results from BNL groups with GEMs An example of a TPC with “integrated front-end

electronics” Summary

Resistive Charge Division

AGS E814 DC1

J.L. Alberi and V. Radeka, IEEE Trans. Nucl. Sci. NS-23 (1976) 251

The spread (fwhm) of the induced charge on the cathode is about 1.6 times the anode to cathode distance

Geometrical Charge Division

Wedge and strip electrode patterns:

H.O. Anger, Instr. Soc. Am. Trans. 5 (1966), p311

C. Martin, et al., Rev. Sci. Instr. 52 (1981), p1067

Backgammon cathode

R. Allemand and G. Thomas, NIM. 137 (1976), p141

Zigzag pad/strip cathodes:

T. Miki, R. Itoh and T. Kamae, NIM. A236 (1985), p64.

E. Mathieson and G.C. Smith, IEEE TNS. vol 36 (1989), p305

Linearity of several chevron patterns

Patterns of (a) centered single chevron (b) displaced single chevron (c) centered one & a half chevron (d) displaced one & a half chevron (e) centered double chevron (f) displaced double chevron.Dashed line indicates anode wire position

Capacitive Charge Division

(a) Single Intermediate Strip method (b) Two Intermediate Strip method

G.C. Smith, et al., IEEE TNS vol. 35 (1988), p409.

Interpolating Cathode Patterns with MWPCs

Linearity vs. Readout Pitch

Interpolating Pad Readout with GEMs

i

iic Q

Qxx

Totalc Q

ENCwx 2

The spread of the electron cloud at the anode pad plane is mostly determined by the diffusion process. It is typically well under 1mm.

A rectangular pad with a width larger than the FWHM of the charge cloud will exhibit large position non-linearity.

A zigzag shaped pad with a zigzag periodicity under 1mm will give better position linearity. The width of the pad can be much larger than the charge cloud.

Pad width w periodicity

The position of the ionization center can be derived from the weighed average of the charge on the pads

The position resolution of the centroid is limited by the signal to noise ratio of the electronics, and the readout pitch

For 2-3 ch.

A Double GEM Chamber with Strip Anode PlaneThe detector used for most of the measurement, courtesy of Dr. Sauli from CERN.

Measure the Charge Spread on the Anode Plane

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

X-ray Position [mm]

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lativ

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uls

e H

eig

ht [

a.u

.]

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Most probable pulse height from a set of 4 anode strips at 0.4mm pitch as a function of x-ray position @ 0.1mm steps

3mm drift gap, 2mm transfer gap, 2mm induction gap, Ar+20% CO2

The charge spread is comparable to a Gaussian with ~ 0.2mm

“Line Response” of a Coarse Zigzag Pattern

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Reconstructed Position [µm]

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nt

5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals

“Line Response” of a Fine Zigzag Pattern

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Reconstructed Position [µm]

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Reconstructed Position [µm]

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sit

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[µm

]5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals, center of gravity algorithm, Argon+20% CO2, ~1cm deep

Overall rms position error: 93µm

Including ~ 100µm fwhm x-ray photoelectron range,

100µm beam width, and alignment errors.

Intermediate Strip Patterns

Single Intermediate Zigzag

Two Intermediate Strips

Other interpolating pad designs and their x-ray uniform irradiation responses

Fine “Zigzag” pattern

2 mm x 10 mm pads

Chevron Readout with GEMs

Scan perpendicular to padsIntegrate signal along pad (8 mm)

Scan with X-rays50m x 8 mm100 m steps

R.Wilcox, B Azmoun (BNL)

Pattern APE σ Resolution

Fine Chevron (no Floating Pads) 85.3 μm 108.0 μm 128.2 μm

Coarse Chevron (no Floating Pads) 34.3 μm 187.4 μm 183.8 μm

Fine Chevron (with Floating Pads) 42.2 μm 101.2 μm 97.6 μm

Coarse Chevron (with Floating Pads) 46.2 μm 106.2 μm 104.5 μm

Intermediate - - -

Straight Strips 59.6 μm 108.6 μm 113.3 μm

Interpolating Pad ReadoutNo floating strips Floating strip

patterns

R.Wilcox, B Azmoun (BNL)

Schematic View of the LEGS TPC

HV Cathode Plane Double GEM planes

Interpolating anode pad plane with front-end ASICs (7296 channels)

Digital readout board

Layout of the Anode Pad / ASIC Board

Pad size: ~2mmx5mm, 22 rowsTotal # of Channels: 7296

•High density interconnect traces with 0.006” width and spacing

•Gas tight construction: large number of blind vias

•Cylindrical geometry: no auto-routing of the traces

The Completed Anode Pad/ASIC Board

Summary

Interpolating readout provides an economical way of achieving good position resolution with fewer readout channels

It requires pulse height information (ADCs) for centroid reconstruction low noise, high dynamic range electronics

Double track resolution is degraded somewhat Chevron/zigzag readout has some “quirks” that the

designers need to be aware of Large area, high channel density, low mass pad plane is

challenging