Uni Karlsruhe (TH)
Institut für Experimentelle
Kernphysik
Magnetic Field Studies of a
Time Projection Chamber with GEM-Technology
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BERKELEY LAB
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B. Ledermann , M. Ball , J. Kaminski , S. Kappler ,
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Th. Müller , M. Ronan , P. Wienemann
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DESY
b)
a) IEKP, Karlsruhe, Germany
b) DESY, Hamburg, Germany
c) LBNL, Berkeley, USA
A GEM-TPC for the future Linear Collider:
The GEM-Technology:
The Gas Electron Multiplier (GEM) consists of a very thin, two-side
metal-clad polymer foil (56 µm total thickness), which is perforated with a
high density of photolithographically etched holes. As illustrated in Figure 1,
these holes are placed in a triangular pattern with a typical pitch of 140 µm
and a typical diameter of 70 µm.
electrons
ions
1
Figure 1: Electron microscope photograph of a
´Standard Geometry´ GEM
One proposal for the future Linear Collider
project is the TESLA project at DESY
(Hamburg, Germany). The central tracker
of choice is the Time Projection Chamber
(TPC). Parameters of the TESLA-TPC:
Radius 161.8 cm; Length: 2x 250cm;
Favoured gas mixture: Ar:CH4:CO2-93:5:2
(so-called TDR-Gas)
6
5
copper
kapton
copper
70 µm
2
4
3
Figure 2: Schematic
view of the electric field
lines in a GEM, showing
also the principle of ion
feedback suppression
copper
kapton
copper
3
Principle of a GEM-TPC:
As seen in the right picture, an ionizing particle creates a track of electron-ion-pairs. The electrons drift with
constant velocity along the field lines
towards the endplate, where they are
amplified by the GEMs and finally
collected by the pads. With The 2D
information from the readout plus the drift time,
a precise 3D image of the track can be reconstructed
readout electronics
4
5
140 µm
Applying a potential difference (usually 300-500 V) between upper and
lower metal layer creates a strong electric field inside the holes and makes
proportional gas amplification possible. Due to the electric field
configuration, electrons are mostly released into the volume below, the
backdrifting ions are mostly absorbed by the upper metal layer
suppressed ion feedback
STAR readout electronics:
320 channels
The low probability for electrical
sampling rate: 19.66 MHz
discharges of GEMs in the gas mixture and their insensitivity to aging
grant stable operation even in the
harsh radiation environment of
hadronic beam experiments.
GEMs and
pad structure
y
Track
Gas mixture:
typically Ar:CH4:CO2-93:5:2
(so-called TDR-Gas)
25 cm
Figure 4: Different cluster width for
different magnetic fields; definiton of a, b
Variation of the sampling rate effects not only
the long. spatial resolution, but also, through
Transverse and longitudinal spatial
cross-correlation, the transverse spatial resolution. resolution worsen due to diffusion
and due to pad-size limitation.
186.3cm
Measurements in high magnetic fields:
To evaluate the performance of a GEM-TPC in high magnetic fields our
TPC prototype was put inside the former compensation magnet of ZEUS
(at DESY), a superconducting solenoid with a 28cm bore and a magnetic
field of up to 5.5 T.
Measurement of diffusion coefficients: A magnetic field parallel to the
0.4
0.3
0.2
s
2
= s 02 + D 2 x
s 0 » 450 m m
0.1
0
20
15
10
5
spatial resolution in µm
transverse diffusion coefficient in µm/ cm
squared cluster width in mm²
electric field forces the drifting electrons
on a helix and thus the electron diffusion
and the resulting cluster size is reduced
(Figure 4). Measured diffusion coefficients are in good agreement with
MagBoltz simulations.
transverse spatial resolution for different sampling rates
Transverse diffusion coefficients
Squared cluster width at B = 2T
0.5
450
MagBoltz simulation
400
Experimental data
100
0
100
80
60
300
250
200
1
2
3
4
5
TDR 4T 19cm
0
0
Bernhard Ledermann
1
2
3
4
5
6
7
alpha-cut in °
600
i*ts_length
2*pad_length
550
500
450
600
TDR 4T 19cm
2
4
6
8
10
12 14
16
20
25
0
staggered geometry
3and1 geometry
500
combs geometry
400
300
200
5
10
15
20
25
drift distance in cm
18
alpha in °
ledermann@iekp.fzk.de
0
5
10
15
20
beta in °
staggered geometry
800
3and1 geometry
750
combs geometry
2 mm
700
650
600
450
12
0
25
850
500
expected for the pad-size
limited region: 2mm = 577µm
100
300
longitudinal spatial resolution for different pad geometries
550
350
50
0
15
drift distance in cm
To find the best suiting pad geometry three special geometries (see Figure 5) were tested
in the magnet. The results show that the 3and1 geometry has advantages in transverse
resolution and the comb geometry in longitudinal resolution.
400
0
10
transverse spatial resolution for different pad geometries
longitudinal spatial resolution for different angles
100
20
5
Testing different pad geometries at B = 4T in the pad-size limited region:
50
150
40
600
100
magnetic field in T
350
700
400
150
spatial resolution in µm
120
800
500
0
i*pad_width
2*pad_length
160
200
drift distance in cm
400
180
120
transverse spatial resolution for different angles
spatial resolution in µm
spatial resolution in µm
140
200
250
25
450
220
Sampling rate: 12 MHz
Sampling rate: 16 MHz
Sampling rate: 19.66 MHz
Sampling rate: 25 MHz
Sampling rate: 33 MHz
Sampling rate: 36 MHz
140
300
The following figures show the dependence of the transverse spatial resolution on the
angle a (Figure 4). By application of hard cuts on a you can reach transverse spatial
resolutions of 50-60 µm.
Also shown is the depenb=arcsin(
)
a=arcsin(
)
dence of the longitudinal
spatial resolution on the
angle b. Steps in resolution can be explained by
inclusion of one more pad
or time slice to the track.
160
Sampling rate: 12 MHz
Sampling rate: 16 MHz
Sampling rate: 19.66 MHz
Sampling rate: 25 MHz
Sampling rate: 33 MHz
Sampling rate: 36 MHz
240
longitudinal spatial resolution for different sampling rates
350
Dependencies of spatial resolution on different angles at B = 4T:
transverse spatial resolution for different angle-cuts
a
readout boards:
32x8 pads of 1.27x12.5 mm (normal pads)
30x12 pads of 2x6 mm (different pad geometries)
Amplification:
2 GEMs
typical gain: 3000
scintillator 2
x
6 mm
scintillator 1
t
b
spatial resolution in mm
µ
spatial resolution in µm
solenoidal
magnetic
field
drift volume
Advantages of a GEM-TPC:
- Intrinsically suppression of ions released into the drifting region (~10-3)
- Almost evanescent distortions due
to ExB-effects
- Higher granularity
- Small signal in transverse direction
- Truly 3D-detector (no ambiguities)
- Minimum amount of material
- Good dE/dx resolution (~5%)
TPC prototype:
length: 25cm
inner diameter: 20cm
spatial resolution in mm
Figure 3: Schematic
view of magnet
setup at DESY
diffusion
0
5
10
15
400
20
25
drift distance in cm
0
5
10
15
20
25
drift distance in cm
Figure 5: Three different pad geometries: staggered, 3and1, combs
http://www-ekp.physik.uni-karlsruhe.de/~lederman
IEEE, Rome 2004