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Development of GEM Detectors for Muon Tomography
N. Leioatts, A. Quintero-Segovia, T. Garlick, M. Hohlmann - Florida Institute of Technology
Introduction
Amplifier
GEM (Gaseous Electron Multiplier) detectors utilize an electron
avalanche to detect charged particles with high spatial precision.
The initial particle (in our case a cosmic ray muon) enters the
system and ionizes the gas, setting a minimal number of electrons
free. In the presence of an electric field (~5kV/cm) the electrons
drift toward the readout while the positive ion “tails” are repelled.
The electrons then enter an acceleration region in the foils where
their increased energy causes them to excite more electrons. Finally
these electrons are read out as a voltage on a printed circuit board.
The foils are decoupled from the readout which makes the detector
versatile and inexpensive.
Oscillations in the amplifier caused by
capacitance in the readout motivated our new
readout geometry

Readout moved inside detector to reduce noise
and shorten cable length before amplification

Shown below is our amplifier output (channel 2)
versus test pulse (channel 1) in the old (left) and
new (right) geometries

Sparking
Setup
A triple Gem consisting of three layers of foils is used for higher gain.
Amplification occurs in the region between the foils, where the electric field
reaches 60 KeV. Electrons entering this region are accelerated and in turn
ionize more gas in the next drift region. 25 primary electron-ion pairs per cm
are created in Argon at NTP with total electron-ion pairs numbering 100 per
cm(PDG 28.7). With a gain of 20 per foil we achieve a total gain of 8000
yielding 80,000 primary electrons at the readout plane. Readout is currently
a single channel with an area of 0.3 cm x 0.7 cm and a charge integrating
amplifier takes care of signal output.
Construction
The GEM foils come unmounted from the factory and must be
stretched tight to keep the electric field constant across the
plane. They are then glued to the frames and mounted in sets
of 3 on the stack creating a drift field in between the
acceleration regions which are created by the foils themselves.
Improvements
New lid with welded feedthroughs
 Can provide power for a second amplifier

Output of HV circuit
4500.000
A
Output Voltage of HV Circuit
4000.000
C
3500.000
Gem 3+
3000.000
Gem 3Gem 2+
2500.000
Gem 22000.000
Gem 1+
Gem 1-
1500.000
DC
1000.000
500.000
0.000
0.000
2.000
4.000
6.000
8.000
10.000
Input Voltage to Power Supply
Gas System Testing
B
D
3.00
Gauge Pressure (kPa)
2.50
2.00
Pressure Test 1
1.50
Pressure Test 2
Pressure Test 3
1.00
0.50
Above: Output to GEM foils versus Supply Voltage
0.00
0
200
400
Time (s)
A) GEM foil before stretching
B) GEM foil after stretching
C) Drift Cathode being mounted
D) Complete Stack
Left: Results from System Pressure Testing with
Original Lid Design
600
12.000
14.000
Conclusion and Future Work
Detector construction is ongoing at Florida Institute
of Technology. Refined structure has led to a
better gas tight system and improved readout
electronics. After commissioning with the 55Fe
source the current detector will be tested for muon
sensitivity before building a 30 cm x 30 cm and
ultimately a 100 cm x 100 cm detector. A more
precise readout capable of 50 micron resolution will
be implemented to utilize the GEMs' spatial
precision.