1. Single GEM Measurement
Matteo Alfonsi,Gabriele Croci and Bat-El Pinchasik
June 11th 2008
GDD Meeting

2. Parameters Standard GEM 3 cm x 3 cm surface area
Pitch 140 µm
Hole Diameter 70 µm
Thickness 50 µm
Standard Gaps Geometry
2 mm Induction gap
3 mm drift gap
Standard Gas Ar/CO2 70%/30%
8.9 keV X-Rays

3. Picture of the GEM used and of a Standard Stock GEM
The focus used for the two pictures was different.
From optical microscope measurement (by eyes) we found that the hole diameter is the same for both.
We still miss the measurement with Miranda’s camera microscope (out of order).

4. Pulse Height Spectrum
The rate used in the following current measurements is higher, around a value of 250 kHz/mm2
The Gain is evaluated using (Rate Method)

5. Drift Field = 1.9 kV/cm; Induction Field = 3 kV/cm
Voltage Scan
Drift Field = 1.9 kV/cm; Induction Field = 3 kV/cm

6. GEM Voltage = 500 V; Induction Field = 3 kV/cm
Drift Scan
GEM Voltage = 500 V; Induction Field = 3 kV/cm

7. Drift Field = 1.9 kV/cm; GEM Voltage = 500 V
Induction Scan
Drift Field = 1.9 kV/cm; GEM Voltage = 500 V

8. Literature comparison
J. Benlloch et al. /Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 410)
S Bachmann et al. Nucl. Instrum. Methods Phys. Res., A,
438(CERN-EP ):376– p, Apr 1999.

9. Simulation Comparison
Simulations
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10. An other method to measure Effective Gain
We tried to measure the Gain without measuring the rate (as in the usual method) but measuring the ionization current
We used this setup to measure the ionization current
We fixed the rate to a value where we can read the ionization current
8.9 keV X-Rays
Vdrift = 0
Edrift
VTopGEM < 0
EGEM
VBottomGEM = 0
Vanode= 0
We read the ionization electron current on the drift electrode using Keithley 6517A electrometer

11. Ionization Current Measurement
Top GEM Current (when measurable)
No X-Rays
Ionization Current on Drift Electrode
We found a mean value for the ionization current around 150 pA

12. GAIN Measurement: Two Methods Comparison
Rate Method
Ed = 1.9 kV/cm ; Ei = 3 kV/cm
Ionization Method
Ed = 1.7 kV/cm ; Ei = 3 kV/cm
It seems that the two results are compatible

13. New detectors Coming.. Standard Bulk Micromegas THGEM “Closed”
Geometry: Amplification gap = 128 µm
THGEM “Closed”

14. THGEM Closed THGEM Copper FiberGlass BackPlane
Conductive Glue
Copper
FiberGlass BackPlane
THGEM (geometry pitch 0.7 mm, hole diameter 0.3 mm, thickness 0.6) closed on one side. Glue a kapton foil on the bottom of the THGEM with conductive glue. Glue a copper readout electrode to this kapton foil (anode) and then glue a fiberglass support to this anode.

15. Testing Standard Bulk Micromegas in air (with Paul Colas)
Vmesh (V)
I (nA)
200 2
300
400
2.1
450
2.2
500
2.3
600
2.7
650
2.75
700
2.95
800
3.1
830
First Spark, than stable
870
Last Stable Value before having many sparks
Mesh Photos
The gas that will be used to test this detector (under Paul’s suggestion) is
Ar/CF4/Iso 95%/3%/2%
and the maximum voltage that will be applied to the mesh (being Vanode =0) is 360V
Vanode = 0