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]
Re
lativ
e P
uls
e H
eig
ht [
a.u
.]
A
B
C
D
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
0
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2000 3000 4000 5000 6000 7000 8000
Reconstructed Position [µm]
Cou
nt
5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals
“Line Response” of a Fine Zigzag Pattern
0
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2000 3000 4000 5000 6000 7000 8000
Reconstructed Position [µm]
Co
un
t
-100
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-20
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3000 3500 4000 4500 5000 5500 6000 6500 7000
Reconstructed Position [µm]
Po
sit
ion
Err
or
[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
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