1. Development of Gas Electron Multiplier Detectors for Muon Tomography
M. Abercrombie, A. Quintero, A. Menendez, K. Gnanvo, M. Hohlmann
Physics and Space Science Department
Florida Institute of Technology
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

2. Overview Why GEMs? How GEMs work Assembly process Improvements
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

3. Why GEMs? Compact design, versatile
Better spatial resolution than similar techniques ( µm)
Relatively low cost
Uses naturally occurring, ubiquitous cosmic ray muons, easily penetrate high z materials
Well suited for muon tomography of potential nuclear contraband.
My talk today is going to be about using GEM detectors for muon tomography as a means for detecting shielded nuclear contraband…Muons easily penetrate, good for detection high z materials
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

4. GEM Foils
GEM foils are made of a thin sheet of Kapton covered on both sides by copper
High density of tiny, chemically etched holes (~100 per mm2)
Foils stretched, framed,
and glued; delicate and
tedious process
When stretching must ensure uniformity.
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

5. Testing the Foils Cleanroom N2 gas flow Voltage of 500V applied
Expect a leakage
current of less
than 5nA
5 volume exchanges in 3 hours. Conducted in our Class 10,000 cleanroom. Causes of leakage current (to be too high), want to ensure that sparking doesn’t occur (why?) Apply voltage using _____?
Image taken at CERN
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

6. How GEMs Work Multiple foils stacked inside the detector
Sealed and put under Ar/CO2 gas mixture
Voltage applied to the foils creates a uniform electric field between foils
Why do we use 3? What the Ar/Co2 does. Important that the foils are flat and precisely spaced so that the correct electric field is present. Explain the pictures!
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

7. How GEMs Work Muon ionizes the Ar/CO2 gas in the detector
Electrons from ionization process move toward the readout plane
Accelerated by high electric field, excites more electrons
Cascade of electrons
Readout in 2 dimensions
What are the electric fields, and why are they those particular values?
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

8. Our Set Up at Florida Tech
High voltage supply, HV circuit board, 10 x 10 cm detector, oscilloscope, preamp, gas flow. What does the HV Circuit board do?
HV Power Supply
HV Circuit Board
10 cm x 10 cm detector
Preamp
Oscilloscope
Gas Flow
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

9. Our Set Up at CERN Prototype for a Muon Tomography station
determine incoming and
outgoing paths
angle of deflection gives
target composition
Four 30 cm x 30 cm GEM
detectors
3 cm x 3 cm x 3 cm target
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

10. Results from CERN: Gain Curve
Gain defined as ratio of collected charges at readout to primary charge
Logarithmic behavior
The gain in GEM detectors depends on geometry of the holes, external fields and gas mixture. The gain of the detectors is defined as ratio of collected charges with the readout to primary charge, this is done measuring the collected current at a known radiation flux. A logarithmic behavior with a gas gain up to 2  104 was obtained as expected. Used a 8.04 keV collimated X-ray generator and GDD-CERN’s lab electronic to calculate the gain of one of the six detectors made.
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

11. Results from CERN: Uniformity Test
Test for consistent readout across the detector
Position (cm)
Average Peak (ADC)
1 (0)
1434
2 (4)
1395
3 (7.8)
1341.5
4 (11.6)
1342
5 (15.5)
1566
6 (19.5)
1498
7 (21.1)
1384
Total Average
Stan Dev.
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

12. Results from CERN: Spectra
Energy spectrum of Cu X-rays in blue
Cosmic ray muon pulse height distribution in red (5 hours)
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

13. Improvements Goal: Avoid tedious framing procedure
How: Use honeycomb spacers between the foils
will ensure that the GEM foils are flat
will not require that the foils be framed
Would be a significant improvement in the construction of GEM detectors. What are the honeycomb spacers made out of?
M. Abercrombie - Florida Academy of Sciences - March 19, 2010

14. Future Work Obtain a signal with our 10 cm x 10 cm detector
Test the performance of honeycomb spacers between foils
Construction of a 30 cm x 30 cm detector with honeycomb spacers
Learn more at:
M. Abercrombie - Florida Academy of Sciences - March 19, 2010