Difference between revisions of "Detector Description"
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=GEM Detector Design and Structure= | =GEM Detector Design and Structure= | ||
− | The GEM preamplifier described in sectiton XXXX was used to increase the signal amplitude of the ionization chamber. The detector produces a signal when an ionization event happens. The cathode's electric field causes the primary electrons to drift toward the first GEM preamplifier. The potential of the GEM card produces a strong electric field which will cause electron multiplication when the primary electrons pass through the holes on its surface. Generally, a single GEM preamplifier attains an electron multiplication up to <math>10^3 </math>. On the other hand, increasing the number of cards to three may increase electron multiplication to <math>10^4-10^5 </math>. A potential divider circuit provides voltage that gradually drops to | + | The GEM preamplifier described in sectiton XXXX was used to increase the signal amplitude of the ionization chamber. The detector produces a signal when an ionization event happens. The cathode's electric field causes the primary electrons to drift toward the first GEM preamplifier. The potential of the GEM card produces a strong electric field which will cause electron multiplication when the primary electrons pass through the holes on its surface. Generally, a single GEM preamplifier attains an electron multiplication up to <math>10^3 </math>. On the other hand, increasing the number of cards to three may increase electron multiplication to <math>10^4-10^5 </math>. A potential divider circuit provides voltage that gradually drops to guide the secondary electrons toward a grounded, wire-segmented charge collector. The detector's chamber that has a thin kapton window that encloses its components with 90/10 Ar/CO2 gas. |
Revision as of 03:20, 1 November 2013
GEM Detector Design and Structure
The GEM preamplifier described in sectiton XXXX was used to increase the signal amplitude of the ionization chamber. The detector produces a signal when an ionization event happens. The cathode's electric field causes the primary electrons to drift toward the first GEM preamplifier. The potential of the GEM card produces a strong electric field which will cause electron multiplication when the primary electrons pass through the holes on its surface. Generally, a single GEM preamplifier attains an electron multiplication up to
. On the other hand, increasing the number of cards to three may increase electron multiplication to . A potential divider circuit provides voltage that gradually drops to guide the secondary electrons toward a grounded, wire-segmented charge collector. The detector's chamber that has a thin kapton window that encloses its components with 90/10 Ar/CO2 gas.
Detector Structure
You should describe the features of the detector using a paragraph and not an enumerated list.
First start with a description of the GEM foil and how the strong electric fields cause secondary ionization
After that you describe using several of the foils to further multiply the signal and introduce the anode and cathode to direct ionized electrons through the three amplification stages.
Finally you can describe the HV network used to supply a voltage to all the components and the ionization chamber which immerses them in the Ar/Co2 gas.
The triple GEM detector is based on three GEM preamplifiers, a cathode and an anode. The GEM preamplifier is made of a copper-clad kipton plate which is 50 um thick and has an area of 10x10cm, it has holes that are chemically etched with a diameter of 50 um with a 10um rim. The holes have a an equilateral triangular pattern with a 140 um pitch distance. The detector possesses three GEM cards in square plastic frames, they are placed parallel to the cathode and are separated by a vertical distance of 2.8 mm as shown in the figure below.
The cathode is a square copper plate that is a 10x10cm and has a distance of 3.5 mm from the top of the first GEM card. This cathode design allows a rise in potential on its surface up to 5 kV (in the air) without any discharge effect.The charge collector (readout anode) is constructed of 50-80 micron wide strips and are insulated to determine the location of the collected electrons. They are arranged to give equal charge sharing on the charge collector.
All the previous components exist in a sealed chamber that consists of two ertalyte plastic sheets; they are bolted together by a number of M3 plastic screws located around the detector window to form a well enclosed cavity. Also, the chamber has a 13x13cm kipton window for incident particles.The figures below show top, bottom and side view of the detector's chamber design.
Modifying the GEM detector as neutron sensitive detector
The GEM’s original design was modified to convert it to a neutron sensitive detector. As mentioned previously in the induced fission fragment section, the existence of a fissionable material is important to detect the neutron signal by detecting the ionization of one of the fission fragments. Therefore, the cathode design has a 3 cm diameter coating of U-233, and 30-40 um thickness. The kepton window had an increase in height of 2.5 mm, which helped to increase the distance of the cathode to 8 mm from the top of the first of GEM card (instead of 3.5 mm in the original design). An FR4 shutter was attached in the space between the cathode and the first GEM card, which had enough area to cover the coating area; it has a position controller that helps to open and close the shutter. When the shutter is closed, it covers the U-233 coating and stops the emitted fission fragments from causing ionization in the gas, but when the shutter is open, the ionization of the fission fragments produces a signal that indicates the existence of neutrons inside the chamber which the detector trigger detects for analysis by the DAQ system.
The figures below show the modified components of the detector.
High voltage divider circuit
The GEM preamplifiers are connected with high voltage divider circuit that determines the electron multiplication and transfer to the readout plate. The GEM cards are connected to the high voltage divider circuit that is shown in the figure below:
![GEM HV Dist Net.jpg](/./images/thumb/3/39/GEM_HV_Dist_Net.jpg/60px-GEM_HV_Dist_Net.jpg)
It provides the cathode with a voltage of 3.6 kV, and it produces an electric field which drifts most of the electrons that are primarily produced by ionization. The circuit also provides the GEM preamplifiers with voltage between the top and bottom of each card; the voltage gradually decreases to have the least value between the sides of the third GEM card. This design's advantage is to provide enough voltage for electron multiplication, and to guide most of the drift electrons to the grounded readout plate.
The following table shows the voltage measurements between the sides of the GEM preamplifiers, and voltage between each side and the ground using the HV-voltage divider circuit shown in figure (2).
2550 | 2579 | 2259 | 304 | 1671 | 1394 | 279 | 818 | 570 | 245 |
2600 | 2630 | 2303 | 310 | 1704 | 1421 | 285 | 834 | 581 | 250 |
2650 | 2680 | 2348 | 316 | 1737 | 1449 | 290 | 850 | 592 | 255 |
2700 | 2731 | 2393 | 322 | 1770 | 1476 | 296 | 866 | 603 | 260 |
2750 | 2781 | 2373 | 328 | 1803 | 1503 | 302 | 882 | 614 | 264 |
2800 | 2832 | 2482 | 332 | 1836 | 1530 | 307 | 898 | 625 | 269 |
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