Difference between revisions of "Detector Description"

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=GEM Detector Design and Structure=
 
=GEM Detector Design and Structure=
 
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  You should first describe the basics of the gas electron multiplier  then describe the ionization chamber equipped with GEM foils, then describe the assembly in a clean room environment.
 
 
 
 
 
 
The GEM preamplifier described in section XXXX was used to increase the signal amplitude of the ionization chamber.  The strong electric fields of the GEM foils cause secondary ionization as the primary ionization event is transported towards the cathode.  
 
The GEM preamplifier described in section XXXX was used to increase the signal amplitude of the ionization chamber.  The strong electric fields of the GEM foils cause secondary ionization as the primary ionization event is transported towards the cathode.  
 
A single GEM preamplifier attains an electron multiplication up to <math>10^3 </math>, on the other hand, Adding a rim of 10um for each hole and increasing the number of cards to three GEM cards increase the multiplication to <math>10^4-10^5 </math>. In the detector chamber,  three GEM cards have been installed to provide multiplication that raise the signal amplitude over the noise level at relatively low voltage applied between the top and bottom side of each GEM preamplifier.
 
A single GEM preamplifier attains an electron multiplication up to <math>10^3 </math>, on the other hand, Adding a rim of 10um for each hole and increasing the number of cards to three GEM cards increase the multiplication to <math>10^4-10^5 </math>. In the detector chamber,  three GEM cards have been installed to provide multiplication that raise the signal amplitude over the noise level at relatively low voltage applied between the top and bottom side of each GEM preamplifier.

Revision as of 22:19, 31 October 2013

GEM Detector Design and Structure

The GEM preamplifier described in section XXXX was used to increase the signal amplitude of the ionization chamber. The strong electric fields of the GEM foils cause secondary ionization as the primary ionization event is transported towards the cathode. A single GEM preamplifier attains an electron multiplication up to [math]10^3 [/math], on the other hand, Adding a rim of 10um for each hole and increasing the number of cards to three GEM cards increase the multiplication to [math]10^4-10^5 [/math]. In the detector chamber, three GEM cards have been installed to provide multiplication that raise the signal amplitude over the noise level at relatively low voltage applied between the top and bottom side of each GEM preamplifier.


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.

Example alt text
Fig.1 shows the original GEM detector design.

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.


Example alt text
Fig. shows the charge collector wire's dimensions and arrangement.

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.


GEM top.png GEM bottom.png GEM sides.png

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.

Example alt text
Adding modifications to GEM design.
Example alt text
Modifying the cavity size by the increasing the height of kapton window.
Example alt text
Detector's shutter is open.
Example alt text
Detector's shutter is close.

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

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).

[math] V_{source} \pm 1 [/math] [math] V_{G1T} \pm 1 [/math] [math] V_{G1B} \pm 1 [/math] [math] \Delta V_1 \pm 1 [/math] [math] V_{G2T} \pm 1 [/math] [math] V_{G2B} \pm 1 [/math] [math] \Delta V_2 \pm 1[/math] [math] V_{G3T} \pm 1 [/math] [math] V_{G3B} \pm 1 [/math] [math] \Delta V_3 \pm 1 [/math]
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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