Development of GEM Detectors for Muon Tomography

1
Development of GEM Detectors for Muon Tomography
N. Leioatts, A. Quintero-Segovia, T. Garlick, M.
Hohlmann - Florida Institute of Technology
Introduction 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.

• Amplifier
• 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.

• Improvements
• New lid with welded feedthroughs
• Can provide power for a second amplifier

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.
A
C
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.
B
D
Left Results from System Pressure Testing with
Original Lid Design Above Output to GEM foils
versus Supply Voltage
A) GEM foil before stretching C) Drift Cathode
being mounted B) GEM foil after stretching
D) Complete Stack