Single GEM Measurement Matteo Alfonsi,Gabriele Croci and Bat-El Pinchasik June 25 th 2008 GDD Meeting 1

In the last presentation.. 2

Disagreements between measurements of these days and literature 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

Picture of the GEM used last time and of a Standard Stock GEM GEM used last timeStock GEM (measured this week) Dimensions measured again with Rui’s microscope: they are STANDARD GEM in all aspects!! 4

Stock GEM Parameters Standard GEM (10x10 cm 2 active area) –Pitch 140 µm –Outer(Inner) Hole Diam. 70(50) µm –Thickness 50 µm Wide Drift Gap Geometry –2 mm Induction gap –13.5 mm drift gap Standard Gas Ar/CO 2 70%/30% 8.9 keV X-Rays from the side 13.5 mm 2 mm X-Rays 5

Pulse Height Spectrum The Gain is evaluated using (Rate Method) Energy Resolution: 23% Ed = Ei = 3kV/cm ∆V GEM = 520 V Flux = 1.1kHz (beam from side) 6

Voltage Scan Drift Field = 3 kV/cm; Induction Field = 3 kV/cm, Flux = 2.5MHz 7 G ≈ 500V !!!

Drift Scan 8 GEM Voltage = 500 V; Induction Field = 3 kV/cm two weeks ago: 3x3 cm 2 GEM beam on cathode

Induction Scan Drift Field = 3.0 kV/cm; GEM Voltage = 500 V 9

Gain measurement using 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 We reversed also the polarity of our HV supply to cross-check measurement keV X-Rays V drift  Keithley V TopGEM to HV V BottomGEM = 0 V anode = 0 E drift E GEM V TopGEM : 0

Gain from ionization 11 G ≈ 500V !!! Ionization current about 170pA Another comment: you start to collect all ionization already at 0.03 ÷ 0.04 kV/cm.  In the drift scan the transparency slope for low fields can’t be explained only by recombination

Simplified setup without using anode: electrons collected on bottom I drift = I prim [1 + f coll (G − 1) f fb ] I top = I prim [ − 1 + f coll + f coll (G − 1) (1 − f fb )] I bottom = − I prim f coll G 12 I prim : ionization current f coll : collected fraction of primary electrons f fb : fraction of ions reaching the cathode G : absolute gain 8.9 keV X-Rays V drift < 0 V TopGEM  Keithley V BottomGEM > 0 V anode = 0 E drift E GEM I drift I top I bottom Not measured N.B.: Induction field reversed, so the electronic charge is collected on bottom

13 Ionization current Drift scan with GEM Voltage = 20 V (no multiplication for sure!) Induction Field = kV/cm X-Rays ? ? ?? The slight rise of ion current on cathode can be explained as edge effects (no walls on active area borders!): as drift field increases, ions produced outside the active area are “pulled” inside Ionization current about 3.7nA (maximum allowed I XRAY ≈ 4mA used now) Current on bottom ≈ 0 (Transparency ≈ 0)

Transparency and Gain Voltage scan with Drift Field = 1.0 kV/cm Induction Field = kV/cm No multiplication up to V GEM ~ 140  You can measure here transparency disentangled from gain and compare it with simulation!!

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X-Rays ? ? ??

Ionization Current Measurement Ionization Current on Drift Electrode Top GEM Current (when measurable) No X-Rays We found a mean value for the ionization current around 150 pA 17

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 18

Simulation Comparison Simulations ????? 19