1. Study of non-linear response of a GEM read out with radial zigzag strips
A. Zhang, S. Colafranceschi, M. Hohlmann
Dept. of Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA
B. Azmoun, M. L. Purschke, C. Woody
Physics Dept., Brookhaven National Lab., Upton, NY 11973, USA
1. Motivation
2. Zigzag concept & Experimental setup
2 mm
Using zigzag strips to read out a GEM detector can significantly reduce the number of electronic channels (by a factor of three or more).
There can be a non-linear relation between measured hit position from charge sharing (centroid method) and true position. Charge-sharing readout designs may require a correction for such a non-linear response.
We aim to find a zigzag layout with minimal distortions and good spatial resolutions by studying the non-linear responses with different zigzag strip designs. The study is done with a 10×10 cm2 triple-GEM and a strongly collimated X ray gun (5.9 keV) on a high precision 2D motorized stage.
Collimator
0.5 mm
Measured coordinate
Zigzag concept:
Charge sharing on adjacent strips allows determining hit position using charge centroid.
Triple-GEM w/ zigzag readout. Gas gaps 3/1/2/1 mm; Ar/CO2 (70:30).
X ray collimator: 0.14 × 8 × 100 mm3, effective beam width ≈ 55 μm.
3. The zigzag structures and test boards
Type ① design
Type ② design
produced
Two types of zigzag designs:
① Tips on a strip don’t reach into centers of neighboring strips (poor interleaving).
② Tips on a strip reach into neighboring strip centers (good interleaving).
Blue lines indicate centers of each strip.
Zigzag strips run radially and allow measurement of azimuthal φ coordinate.
Two boards were produced with type ① design, with an angle pitch 1.37 mrad:
Radius 1420—1520 mm, 48 strips.
Radius 2240—2340 mm, 30 strips.
The design pushes the fabrication limits of PCB industry because of very fine tips and small gaps.
There are two sectors on each board with type ② design:
Left: 56 strips, angle pitch 4.14 mrad, radius 206—306 mm.
Right: 45 strips, angle pitch 1.37 mrad, radius 761—861 mm.
Three boards were produced by a PCB company (ACE). Board #2 is closest to what we have designed.
A fourth board was produced at CERN. Strips are on a kapton foil which is mounted on a honeycomb board. The outcome is comparable to board #2.
4. Experimental results of zigzag board scans with X ray
Results from scans across strips for the boards with type ① design
Results from scans across strips for improved boards with type ② design
Resolution studies with the CERN board (1.37 mrad pitch).
Residuals
2-strip events
σ = 80.2 μrad
Response is highly non-linear
Better linearity observed
Mean centroid
Mean centroid measured (CERN board)
4.14 mrad
angle pitch
σ = μrad
1.37 mrad
angle pitch
3-strip events
Mean strip multiplicity in clusters
Mean strip multiplicity (ACE #2 & CERN boards)
Spatial resolution (collimator effect subtracted using Geant4 simulation)
Spatial resolutions (µrad /
μm)
Pitch 4.14 mrad,
R≈229 mm
Pitch 1.37 mrad,
R≈784 mm
2-strip
3-strip
2 & 3-strip
ACE #1
266 / 61
371 / 85
328 / 75
56 / 44
69 / 54
60 / 47
ACE #2
288 / 66
480 / 110
384 / 88
57 / 45
97 / 76
84 / 66
ACE #3 -
572 / 131
140 / 110
CERN Board
397 / 91
393 / 90
92 / 72
71 / 56
4.14 mrad angle pitch
1.37 mrad angle pitch
≈3.3 mm
≈2.2 mm
0.5 mm
Results from scans along strips (CERN board)
0.5 mm
Mean centroid (X ray collimator is 8 mm across strips)
Board with 48 zigzag strips
Board with 30 zigzag strips
Residuals
Resolution for the 48-
strip board is 128 μrad
(188 μm) after non-linear response is corrected (≈181 μm after collimator effect is subtracted).
4.14 mrad angle pitch
1.37 mrad angle pitch
5. Conclusions
In the type 1 zigzag design, non-linear response clearly exists.
The response is much more linear in the type 2 zigzag design. No correction is needed and < 100 μm spatial resolution is achieved!
It is important to select a manufacturer to produce the zigzag strips with very high precision, since the designs are approaching the fabrication limits of the PCB industry.
There is no technical issue with producing very fine zigzag strips on a kapton foil. This allows a reduction of the readout material so that the overall radiation length of a GEM detector can be minimized.
σ = 128 μrad
Acknowledgement: this work is supported by BNL under the EIC eRD6 consortium.
IEEE Nuclear Science Symposium & Medical Imaging Conference • Strasbourg, France • Oct. 29 – Nov. 6, 2016