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 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 For 2-3 ch. The position resolution of the centroid is limited by the signal to noise ratio of the electronics, and the readout pitch

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

Measure the Charge Spread on the Anode Plane 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 s ~ 0.2mm

“Line Response” of a Coarse Zigzag Pattern 5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals

“Line Response” of a Fine Zigzag Pattern 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 Other interpolating pad designs and their x-ray uniform irradiation responses Two Intermediate Strips Single Intermediate Zigzag

Chevron Readout with GEMs Fine “Zigzag” pattern 2 mm x 10 mm pads Scan with X-rays 50 mm x 8 mm 100 mm steps Scan perpendicular to pads Integrate signal along pad (8 mm) R.Wilcox, B Azmoun (BNL)

Interpolating Pad Readout No floating strips Floating strip patterns 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 R.Wilcox, B Azmoun (BNL)

Schematic View of the LEGS TPC HV Cathode Plane Double GEM planes Digital readout board Interpolating anode pad plane with front-end ASICs (7296 channels)

Layout of the Anode Pad / ASIC Board Pad size: ~2mmx5mm, 22 rows Total # 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