gem chambers at bnl the detector from cern, can be configured with up to 4 gems the detector for pad...
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![Page 1: GEM Chambers at BNL The detector from CERN, can be configured with up to 4 GEMs The detector for pad readout and drift studies, 2 GEM maximum](https://reader035.vdocuments.site/reader035/viewer/2022070306/55173bb9550346f5558b6180/html5/thumbnails/1.jpg)
GEM Chambers at BNL
The detector from CERN, can be configured with up to 4 GEMs
The detector for pad readout and drift studies, 2 GEM maximum.
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Energy Resolution of the Double GEM Detector
5.4 keV collimated x-ray, Ar+20% CO2. FWHM ~ 17%
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Gas Gain vs. Photon Flux
Ar+20% CO2, 5.4 keV x-rays (~1mm2), Ed=1kV/cm, Et=4kV/cm, Ei=5kV/cm, Qa~0.2pC
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Double GEM Gas Gain Uniformity
Collimated 5.4keV x-ray (~1mm2), scanned the detector with a 1mmx1mm grid, over 9cmx9cm area.
90 100 110 120 130 140 150
Relative Amplitude
Pulse height histogram of all entries on the map
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Ion Feedback in Single GEM Chamber
Ion Feedback is defined as the current ratio between the window and the anode: fi = - Iw / Ia
7.0)(i
d
a
w
E
E
I
I
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Ion Feedback in a Double GEM Chamber
Other factors:
•Induction field
•GEM voltage
•Transfer field
•Asymmetry in the two GEM’s gains
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Ion Feedback in a Triple GEM Chamber
The reduced second transfer field also results in large reduction in the effective gain (~ a factor of 5)
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Ion Feedback under 1%
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“Line Response” of a Fine Zigzag Pattern
0
1000
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7000
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10000
2000 3000 4000 5000 6000 7000 8000
Reconstructed Position [µm]
Co
un
t
5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals, center of gravity algorithm
Overall rms position error: 93µmIncluding ~ 100µm fwhm x-ray photoelectron range,
100µm beam width, and alignment errors.
-100
-80
-60
-40
-20
0
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3000 3500 4000 4500 5000 5500 6000 6500 7000
Reconstructed Position [µm]
Po
sit
ion
Err
or
[µm
]
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Intermediate Strip Patterns
Single Intermediate Zigzag
Two Intermediate Strips
Other interpolating pad designs and their x-ray uniform irradiation responses
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Summary
• Double GEM demonstrated very good energy resolution with collimated x-
ray beam. It also exhibits somewhat a large gain variation over 9cmx9cm
area.
• The Double GEM detector’s gain has a dependence on photon flux, even
down at kHz/mm2 range. The amount of gain change varies over a GEM
detector.
• With a reasonable drift field (~1kV/cm), it is difficult to keep the ion
feedback rate under 10% for double GEM, 2% for triple GEM. To reduce
the ion feedback to below 1%, 5 GEM planes are needed.
• Interpolating pad readout for GEM with better than 100µm resolution in
one direction possible (@ 2mm pitch) with 5.4keV x-rays, with minimal
diffusion.
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Further R&D Topics
• Detailed simulations to determine the acceptable ion feedback
• Spatial variation of the GEM gain – If the variation is stable over time, it can be corrected by calibration.
• Degradation of energy resolution with intermediate strip readout
• Dependence of gas gain on flux– Difficult to correct
• Join multiple GEM foils– TPC’s active area is larger than CERN’s GEM foil capacity
• Drift properties of the TPC gas
• GEM Operation in pure CF4
– Reached a gas gain of 600 on a triple GEM before HV instability
• Aging Study– Should we do our own study or rely on others’ results?
• Integration of TPC & HBD: – Design of the field cage, its optical transparency.