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10 cm x 10 cm Single Gas Electron Multiplier (GEM) X-ray Fluorescence Detector for Dilute
Elements
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2014 J. Phys.: Conf. Ser. 493 012018
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17th Pan-American Synchrotron Radiation Instrumentation Conference (SRI2013)
IOP Publishing
Journal of Physics: Conference Series 493 (2014) 012018
doi:10.1088/1742-6596/493/1/012018
10 cm x 10 cm Single Gas Electron Multiplier (GEM) X-ray
Fluorescence Detector for Dilute Elements
E H Shaban1, D P Siddons2 and D Seifu3
1
Southern University, Electrical Engineering, Baton Rouge, LA 70813
Brookhaven National Laboratory, NSLS, Upton, NY 11973,
3
Morgan State University, Physics Department, Baltimore, MD 21251
2
Email: [email protected]
Abstract. We have built and tested a 10 cm x 10 cm single Gas Electron Multiplier (GEM) X-ray
detector to probe dilute amounts of Fe in a prepared sample. The detector uses Argon/Carbon
Dioxide (75/25) gas mixture flowing at a slow rate through a leak proof Plexi-glass enclosure held
together by O-rings and screws. The Fluorescence X-ray emitted by the element under test is
directed through a Mylar window into the drift region of the detector where abundant gas is
flowing. The ionized electrons are separated, drifted into the high electric field of the GEM, and
multiplied by impact ionization. The amplified negatively charged electrons are collected and
further amplified by a Keithley amplifier to probe the absorption edge of the element under test
using X-ray absorption spectroscopy technique. The results show that the GEM detector provided
good results with less noise as compared with a Silicon drift detector (SDD).
1. Introduction
We used a 10 cm x 10 cm rectangular standard single Gas Electron Multiplier (GEM) to build a
fluorescence detector [1-3]. Fluorescence X-ray is a characteristic secondary X-ray emitted from an
element when it is bombarded by high energy X-ray beamline. Fluorescence X-ray experiments are used
in X-ray fluorescence spectroscopy to analyze samples in ceramics, metals, geology and forensic science.
There are a number of commercially available fluorescence detectors such as Lytle[4], Passivated
Implanted Planar Silicon (PIPS) [5], and Silicon Drift Detector (SDD) detector [6]. In this poster we are
developing another fluorescence detector using a GEM that was made and provided by 3M Corporation
[7].
The intent is to use the GEM as an amplification medium of the primary electrons ionized by the low
number of fluorescence X ray photons emitted from dilute elements in order to be able to identify and
characterize the element. The low number of X-ray fluorescence photons ionize a detecting medium of gas
that result in a very low number of the primary electrons. In order to detect the signal from the low count
of the primary electrons an amplifier is required to amplify the signal above the background noise level
present. In this experiment we used the GEM holes as a confined path for the electrons that are drifted by
an electric field. Within the GEM holes path, a very high and controllable electric field is applied to
accelerate the primary electrons in the presence of a detecting gas. The highly accelerated electrons by the
strong electric field produce more electrons by impact ionizations with the neutral gas atoms. An
avalanche of electrons is formed that provides a signal that can be easily detected, amplified using 428
Keithley current amplifier, and processed to provide a better high signal to noise (S/N) ratio absorption
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
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1
17th Pan-American Synchrotron Radiation Instrumentation Conference (SRI2013)
IOP Publishing
Journal of Physics: Conference Series 493 (2014) 012018
doi:10.1088/1742-6596/493/1/012018
edge plot. This is a continuation to our previous experiment to build a fluorescence detector using a 3-inch
diameter GEM that was donated by 3M corporation a few years ago [2]. In this experiment we have built a
larger solid angle measuring 10 cm x 10 cm single GEM fluorescence detector. Figure 1 shows a
schematic of the experiment. The detector is placed at right angle with respect to the test sample and the X
ray beamline axis.
Mylar window
X ray
Argon Gas
V1
0
GEM1
V2
V3
0
0
PCB
Amplifier
Figure 1: A sketch of a single GEM detector showing the Mylar window, the GEM and the PCB
2. Experiment
The GEM gas detector is formed from three regions known as the drift, amplification, and collection or
induction regions. Figure 1 shows the single GEM gas detector that was built using Plexi glass machined
in a rectangular shape measuring 19 cm x 19 cm with an open X-ray window measuring 10 cm x 10 cm,
that has the same size as the GEM dimensions. The window entrance of the detector is covered by a one
sided Aluminized Mylar placed a distance of about 2.5 cm away from the GEM upper conducting copper
electrode. The GEM is separated from the collection region or the printed circuit board (PCB) using a
Plexi glass flange about 0.3 cm thick. The bulk of the gas, Argon/Carbon dioxide (75/25) mixture, is
allowed to enter and leave the drift region at a slow rate. The tightly enclosed detector allows the gas to
fill the whole detector with the detecting gas with a controllable inlet and outlet passage located in the
drift region. The drift electric field is applied between the Aluminized Mylar surface, termed as the
cathode, and the upper copper conducting side of the GEM. The polarity of the applied voltage in this and
subsequent regions is made such that the electron drift towards the GEM upper electrode and out of the
GEM lower electrode towards the conducting copper coated PCB that acts as an anode and a virtual
ground. In the drift region the ionized heavy charges are drifted towards the Mylar. A relatively very high
electric field, as compared to that at the drift region electric field, is applied between the two copper
electrodes of the GEM. This field can be changed to control the amplification of the primary ionized
electrons or the gain of the GEM detector.
The printed circuit board (PCB) or the anode collects the abundant electron avalanche driven out form
the GEM amplification region. The electric field magnitude in the three regions can be controlled by an
adjustable High Voltage DC sources. The most critical electric field is within the GEM electrodes. It
should be adjusted to avoid gas breakdown due to the spark formation that can damage the GEM. All
parts of the detector are machined and held together using O-rings and screws to form a gas leak proof
2
17th Pan-American Synchrotron Radiation Instrumentation Conference (SRI2013)
IOP Publishing
Journal of Physics: Conference Series 493 (2014) 012018
doi:10.1088/1742-6596/493/1/012018
enclosure. The electron charge is collected from the PCB and fed to an external 428 Keithley current
amplifier and then processed to display the absorption edge of the sample under test. We have used a two
dimensional Maxwell Software to plot both the potential lines and the electric field lines in the three
regions of the GEM detector. Figure 2 and 3 show the potential lines and the electric field strength in the
GEM amplification region only. In this region the primary electrons drifting from the drift region enter the
GEM holes and are amplified by impact ionization.
Figure 2: The equi-potential lines within the
GEM and the Kapton Dielectric
Figure 3: The electric field lines within the GEM
3. Results
A prepared dilute sample of Fe about (3-5%) is made at the Center for Advance Microstructures and
Devices (CAMD) at Louisiana State University (LSU). The test sample was measured using the GEM
detector and a Silicon Drift Detector (SDD) that is regularly used at the Double Crystal Monochromator
(DCM) at CAMD. We found that the closer the GEM detector entrance to the sample the less noise scans
we obtained. The single scans that we obtained improved dramatically from the one we first obtained
when the detector was at a distance. The scan in Figure 4 is the best scan we obtained using the GEM
detector. This is to demonstrate the ability of each detector to provide a single scan for comparison.
Figure 5 is the same as Figure 4 but enlarge to compare the noise level in each detector.
Figure 4: EXAFS scans for a dilute sample using the GM and SDD detectors
3
17th Pan-American Synchrotron Radiation Instrumentation Conference (SRI2013)
IOP Publishing
Journal of Physics: Conference Series 493 (2014) 012018
doi:10.1088/1742-6596/493/1/012018
Figure 5: An enlarged diagram of figure 4 to compare the noise in the GEM & SDD detectors
Acknowledgement
This work is supported by NSF award DMR-1126444, and partly by LEQSF-EPS (2012)- PFUND-290.
This work was done in collaboration with NSLS, the Instrumentation Department at Brookhaven National
Lab, NSLS workshop, and Morgan State University. We thank the students Goings, Leonard, Driggs,
Berthe, Bamagu, and Carriere for the help. We thank CAMD/LSU for providing the beamtime at DCM
hutch, Dr. Roy, Merchant, and Jiles for the help & assistance, and 3M Corporation for providing the
GEMs.
References
[1]
Sauli F 1997 Nucl. Instr. And Meth. A 386 531
[2]
Shaban E H, Siddons D P and Kuczewski A, 2007 Nucl. Instr. And Meth. A 582 (220), 186
[3]
Bouclier R, Capeans A, Dominik W, Hoch M, Labbe J C, Million G, Ropelewski L, Sauli F
and Sharma A 1997 IEEE Transaction on Nuclear Science, Vol. 44, No. 3, 646.
[4]
www.ssrl.slac.stanford.edu/mes/xafs
[5]
www.canberra.com/product/detector/pips-detector-technologoes.asp
[6]
www.hitachi-hitec-science.us
[7]
www.3m.com/wps/portal/3M/en-US/Austin/plant
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