Standard Single GEM: transport
properties
Matteo Alfonsi & Gabriele Croci
GDD Meeting
September, the 30th 2008
1
“Inverted Ei Setup”
Vdrift < 0
Idrift
8.9 keV X-Rays
Itop
Ibottom
Not measured
Case 1) Electron drift in GEM holes
Edrift
VTopGEM  Keithley
EGEM
VBottomGEM > 0
N.B.: Induction field always reversed, so the
charge is collected on bottom
Vanode= 0
Case 2) Ions : Vdrift > 0 and VBottomGEM < 0 !!!!!!!
2
Electrons Drift Scans
3
Electrons Drift Scans for higher GEM voltages
4
Ions Drift Scans
5
Electrons – Ions Comparison @ 20V
6
Electron GEM Voltage Scan
Edrift = 1 kV/cm
7
Ions GEM Voltage Scan
Edrift = 1 kV/cm
8
“Standard Setup”
Vdrift < VTopGEM
Idrift
8.9 keV X-Rays
Itop
Ibottom
Edrift
VTopGEM < - 600 V
EGEM
VBottomGEM = - 600 V
Ianode
Anode  Keithley
9
Ions Drift Scan in Std Setup (Low ΔVGEM)
10
Ions Drift Scan in Std Setup (High ΔVGEM)
 Still reasonable @ ΔVGEM = 300V
Ions ionization current
But @ ΔVGEM = 500V we
discovered that few photons
(about 1 every 2400 if you
assume G = 600) are
converted in the holes
(this is our hypothesis..)
Electrons ionization current
11
Comparison Ions Electrons in Std Setup (Low ΔVGEM)
Electrons
Ions
12
Possible explanation
Cu X-Rays
Ions produced in the
drift gap far from the
GEM foil are more
likely focused in the
centre of the hole and
pass through, because
they are characterized
by a lower diffusion
GEM Electric Field with
high potential
difference on GEM
electrodes
13
Comparison Ions Electrons in Std Setup ( ΔVGEM = 500V)
Electrons
Ions
14
Comparison of “Inverted Ei
Setup” with the “Standard
Setup” with Induction field at 3
kV/cm
15
Electron Drift Scans Comparison (no Gain)
16
Ions Drift Scans Comparison (no Gain)
17
Ions Drift Scans Comparison (300 V)
18
Charging up problems
19
At the same time Itop goes from 7.1 nA
to 5.8 nA
20
Time Scans, Edrift = 1kV/cm, ΔVGEM =50V, Einduction = 3 kV/cm
1) On Before
Ions Current
2) Ions and
On Before
Electrons Current
Variation ~ 30%
Variation ~ Few %
Ions Current
3) Electrons, Ions
and On Before
4) Ions,Electrons,
Ions and On Before
Electrons Current
21
Simulations
22
Ions Drift simulation:
ED = 1kV/cm , VGEM = 20V , inverted EI = -0.1kV/cm
1000 events generated
Ions starting from a track
passing above the hole,
along the diameter of it.
Ions starting
points shown.
Z-start = 90 µm
Color
represents the
ending place
NO IONS SHOULD PASS
FROM MEASUREMENTS
Ions ending place (%)
Anode  0.8 %
Bottom GEM  29.3 %
Kapton 0.8 %
Top GEM  68.5 %
Other  0.6 %
23
Spare Slides
24
Drift Scan comparison (500V)
VGEM = 500 V
Einduction = 3 kV/cm
VGEM = 500 V
Einduction = - 2.5 kV/cm
25