Transcript Slide 1
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.