1. Chevron / Zigzag Pad Designs for Gas Trackers
Bo Yu
Instrumentation Division

2. 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

3. 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

4. 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

5. 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

6. Capacitive Charge Division
(a) Single Intermediate Strip method (b) Two Intermediate Strip method
G.C. Smith, et al., IEEE TNS vol. 35 (1988), p409.

7. Interpolating Cathode Patterns with MWPCs

8. Linearity vs. Readout Pitch

9. 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

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

11. 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 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

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

13. “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.

14. Intermediate Strip Patterns
Other interpolating pad designs and their x-ray uniform irradiation responses
Two Intermediate Strips
Single Intermediate Zigzag

15. 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)

16. 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)

17. 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)

18. 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

19. The Completed Anode Pad/ASIC Board

20. 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