Post on 29-Mar-2015
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.
Energy Resolution of the Double GEM Detector
5.4 keV collimated x-ray, Ar+20% CO2. FWHM ~ 17%
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
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
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
Ion Feedback in a Double GEM Chamber
Other factors:
•Induction field
•GEM voltage
•Transfer field
•Asymmetry in the two GEM’s gains
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)
Ion Feedback under 1%
“Line Response” of a Fine Zigzag Pattern
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
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
20
40
60
80
100
3000 3500 4000 4500 5000 5500 6000 6500 7000
Reconstructed Position [µm]
Po
sit
ion
Err
or
[µm
]
Intermediate Strip Patterns
Single Intermediate Zigzag
Two Intermediate Strips
Other interpolating pad designs and their x-ray uniform irradiation responses
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.
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.