Difference between revisions of "Detector Description"

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=GEM Detector Design=
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=GEM Detector Design and Structure=
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==Clean Environment==
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The GEM preamplifier described in section XXXX was used to increase the signal amplitude of the ionization chamber. Primary electrons are liberated by an ionizing particle intersecting an ionization chamber that has a 90/10 Ar/CO2 gas; the cathode's electric field accelerates the electrons towards the GEM preamplifier.  As described in section XXX, a single GEM preamplifier can increase the number of liberated electron by three orders of magnitude via secondary ionization.<ref = "chechik"> R. Chechik, A. Breskin, G. P. Guedes, D. Mörmann, J. M. Maia, V. Dangendorf, D. Vartsky, J. M. F. Dos Santos, and J. F. C. A. Veloso, Recent Investigations of Cascaded GEM and MHSP detectors, IEEE Trans. Nucl. Sci. 2004 </ref>  Using three pre-amplifiers will increase the signal amplitude making it measurable.  A high voltage divider circuit is used to establish the electric fields for each preamplifier using a single power supply channel, and secondary electrons are guided towards a segmented charge collector.
  
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Detectors are very sensitive devices to the atmosphere particulates. Most of the methods for detector construction and maintaining demand a laminar flow hood or a clean room that accommodates  clean environment, Generally, the clean environment are classified depending on the size and number of existing particulates in the room. Also, the clean rooms maintain keeping the number of particulates low by restricting the number of objects that enter the room, in addition to specifying the workers to be clean by provding them with gloves, hats and gowns that are designed specially for this clean rooms. 
 
  
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The GEM based neutron detector consists of :
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==Detector Structure==
  
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1- A cathode; a square copper plate has a dimension of 10x10cm and has a distance of 1.2 cm from the top of the fist GEM card. The cathode design allows to raise the potential on its surface up to 5 kV (in air) without any discharge effect.
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The triple GEM detector is composed of  three GEM preamplifiers, a cathode and an anode. A GEM preamplifier is a 50 micron thick kapton foil clad on both sides with 5 microns of copper. A staggered pattern of 50 micron diameter holes, equally spaced by distances comparable to the hole diameter, is chemically etched into the copper clad foil of a 140 um pitch distance over an area of 10x10cm.  <ref name = "Souli"> F. Sauli, et al, NIM A 386, 531 (1997) </ref>  The detector contains three GEM preamplifiers mounted on square plastic frames separated by a vertical distance of 2.8 mm and placed parallel to the cathode as shown in the figure below.
  
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2- A neutron sensitive material; in our case U-233 have been used  as a sensitive material and has relatively a high fission cross in the desired energy range, U-233 is coated on the top of a metal plate in circular disk that has a diameter of 3cm and fixed as a part of the detector cathode's surface.
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[[File:GEM_Detector_original.png | thumb |alt=Example alt text|Fig.1 shows the original GEM detector design.| center | 200px]] 
  
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3- GEM cards; the detector has three GEM cards in square plastic frames, they have  dimension of 10x10cm and are placed parallel to the cathode as shown in the figure below.
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The cathode is a square copper plate that is 10x10cm and is 3.5 mm away from the top of the first GEM card. This cathode design is capable of being set to be at a potential voltage of 5 kV (in the air) without any discharge. The charge collector (readout anode) is constructed of 50-80 micron wide strips that are insulated to determine the location of the collected electrons, and are arranged to allow equal charge sharing on the upper (x coordinate) and lower (y coordinate) charge collector layers <ref name= readout>Physik Department E18, Technische University Munchen, 2D readout Plane, 21 of Jun.2012. http://www.e18.ph.tum.de/research/compass/gempixelgem-tracking-detectors </ref>.
  
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[[File:GEM_Detector_original.png | 600px]]
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[[File:Charge_collector.jpg  | thumb |alt=Example alt text|Fig. shows the charge collector dimensions and arrangement <ref name = readout/>.| center| 200px]]
  
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All the above 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  kapton window to reduce the energy loss of incident particles entering the chamber.  The figures below show top, bottom, and side views of the detector's chamber design.
  
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Insert drawing with dimensions so someone can build a copy of your detector.
 
  
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Each GEM preamplifier is made of a copper clad kipton plate  of a thickness 50 um and an area of 10x10cm, It has holes with a diameter of 50 um and 10 um rim which are chemically etched in an equilateral triangular pattern with 140 um pitch distance.
 
  
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4- A sealed chamber; All the previous components exist in a 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 closed cavity around the GEM cards away from the surrounding atmosphere. The purpose of sealant is to insulate the chamber's gas from the surrounding atmosphere gas components. Also, the chamber has a square window of kipton of dimension of 13x13cm.
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[[File:GEM_top.png | 200px]]  [[File:GEM_bottom.png | 200px]]  [[File:GEM_sides.png | 200px]]
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=Modifying the GEM detector as neutron sensitive detector=
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The GEM’s original design was modified to convert it to a neutron-sensitive detector. As mentioned previously in Section YYY, fissionable material, inserted inside the chamber, may be used to indirectly detect neutrons by detecting the ionization caused by fission fragments released into the chamber if a neutron induced fission event occurs. The cathode design has a 3 cm diameter coating of U-233 with a 30-40 um thickness. The kapton window height was increased 2.5 mm to accommodate an increase in 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 which had enough area to cover the fissionable material was attached in the space between the cathode and the first GEM card.  The shutter could be opened or closed from outside the chamber.  When the shutter is closed, it covers the U-233 coating and stops the emitted fission fragments ionizing the gas beyond the shutter.  When the shutter is open, the ionization due to  fission fragments produces a signal.  
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  What about alpha and beta particles.
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The shutter has the ability to stop the fission fragments that are emitted from U-233 coating. Having the  U-233 as a source for alpha particles, the QDC charge spectrum showed a difference in case the shutter was open and when it was close, such a test proved the ability of the FR4 shutter to stop (or partially stop) the emitted alpha particles from U-233 coating. The figure below shows the charge spectra in case of shutter open and closed as the detector's operating voltage is 2.6 kV and 2.9 kV for GEM and cathode successively.
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{|
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|-
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| [[File:2.6_2.9kV_4.5.png | thumb |alt=Example alt text|charge collected as the voltage is 2.60 2.90 kV for GEM and cathode successively| 200px]]
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|}
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Since the fission fragments are heavier ions than the alpha particles, a closed shutter should stop them. Such an ability is important to distinguish the fission fragments' signal from the other particles' signals in a heavy radiation environment created in an operating accelerator or reactor.
 +
 
  
  
  
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The figures below show top, bottom and side view of the detector design.
 
  
  
  
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[[File:GEM_top.png | 200px]]  [[File:GEM_bottom.png | 200px]]  [[File:GEM_sides.png | 200px]]
 
  
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=Modifying the GEM detector as neutron sensitive detector=
 
  
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The GEM original design is modified to convert it to a neutron sensitive detector. One of changes is in the cathode design, it has a circular  coating of U-233 with a diameter of 3 cm and 40-50 um thickness. Also the distance of the cathode from the first GEM becomes 8 mm instead of 3.5 mm. The kepton window's height increased to 2.5 mm, it helps to avoid any direct contact with the cathode when it is located to the new height. The figures below show the modified detector design. An FR4 shutter  covers the coating area, if the shutter is close, it will stop the fission fragment ionization, when it is open, the ionization of the fission fragments will be cuase a signal that will be collected by the detector's trigout. The detector trigout is connected to a high pass filter to decrease the level of the detected noise as mentioned in details in the next section.
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The figures below show the modified components of the detector.  
  
 
{|boardclip |
 
{|boardclip |
 
|-
 
|-
 
|[[File:GEM_design.png | thumb |alt=Example alt text|Adding modifications to GEM design.| 100px]] ||[[File:modified_GEM_pic.png | thumb |alt=Example alt text|Modifying the cavity size by the increasing the height of kapton window. | 100px]]
 
|[[File:GEM_design.png | thumb |alt=Example alt text|Adding modifications to GEM design.| 100px]] ||[[File:modified_GEM_pic.png | thumb |alt=Example alt text|Modifying the cavity size by the increasing the height of kapton window. | 100px]]
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|-
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||
 
|[[File:GEM_shutter_open.png | thumb |alt=Example alt text|Detector's shutter is open.| 100px]]|| [[File:GEM_shutter_close.png | thumb |alt=Example alt text|Detector's shutter is close.| 100px]]
 
|[[File:GEM_shutter_open.png | thumb |alt=Example alt text|Detector's shutter is open.| 100px]]|| [[File:GEM_shutter_close.png | thumb |alt=Example alt text|Detector's shutter is close.| 100px]]
 
|}
 
|}
  
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=High voltage divider=
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=High voltage divider circuit=
  
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GEM preamplifiers are connected with high voltage divider circuit. as shown in the figure below:
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A high voltage divider circuit is used to proportionally apply voltages on both faces of each GEM preamplifier as defined by the resistors used in the circuit.  The first GEM preamplifier that an electron encounters as it traverses the detector is set to have the most negative voltage.
 +
The GEM preamplifiers are connected to a high voltage divider circuit that specifies the electron multiplication and supports (manages) their transfer to the charge collector. As  mentioned previously,  The applied voltage on  GEM cards determines all the detector's properties, such as the order of electron multiplication, and the electron collection by the readout plate. A high voltage divider circuit was designed for these purposes as shown in the figure below: <ref = "SergeHV">  Pinto, Serge . Gas Electron Multipliers Development of large area GEMS and spherical GEMS. Diss. Mathematisch-Naturwissenschaftliche Fakultät , 2011 </ref>
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{|
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|[[Image:GEM_HV_Dist_Net.jpg | thumb |alt=Example alt text| Figure shows the HV-divider circuit.| 100px]]
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|}
  
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<center>[[Image:GEM_HV_Dist_Net.jpg | 60px]]</center>
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The HV circuit also provides the cathode with a voltage up to 3.6 kV to produce an electric field  to drift  most of the  electrons, which are primarily produced by ionization. It provides the GEM cards with voltage between the top and bottom of each one and allows the voltage to decrease gradually to reach the minimum value between the third GEM's sides. So, the main advantages of the circuit design are to  provide a voltage for electron multiplication, and to guide most of the drift electrons to the charge collector.
  
 +
The circuit provides a trigger signal through a high pass filter connected to the third GEM electrode. One of the ways to get the detector signal is by a high pass filter that is in contact with the bottom side of the third GEM card; a positive signal is detected by the oscilloscope, since all the electrons are leaving the last GEM electrode toward the charge collector. The job of the high pass filter is to block all the low signal frequencies to provide a signal that can clearly be detected by a 50 ohm terminated oscilloscope.
  
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The ciruit provides the cathode with the highest voltage to drift most of the electrons primarily produced by ionization, then gradually the voltage decreases to have the least value on the bottom side of the third GEM card, which will help to guide all the electrons to the grounded readout plate. 
 
  
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The table shows the measurements of voltage between the sides of the GEM electrodes, and voltage between each side and the ground that are provided by the HV-voltage divider circuit shown above.
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{| border="1" cellpadding="4"
 +
|-
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| <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>
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|-
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| 2550 || 2579 ||  2259 ||304 || 1671|| 1394 || 279 ||  818|| 570 ||245 
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|-
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| 2600 || 2630 ||  2303 ||310 || 1704|| 1421 || 285 ||834|| 581 || 250
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|-
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| 2650 || 2680 || 2348 || 316|| 1737||  1449  || 290 || 850|| 592 || 255
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|-
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| 2700 || 2731 || 2393 ||322 || 1770|| 1476 ||296 ||866|| 603 || 260
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|-
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| 2750 || 2781 ||  2373|| 328 || 1803|| 1503 || 302 ||882|| 614 ||264
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|-
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| 2800 || 2832 ||  2482|| 332 || 1836|| 1530|| 307 || 898|| 625 || 269
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 +
|}
  
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=Construction environment=
  
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Detectors are very sensitive devices, its components are connected to high voltage power supplies in a relatively thin chamber with Ar/CO2 90/10 gas mixture. As a result, the method of construction and maintaining demands a laminar flow hood that accommodates a clean environment from any source of dust in the atmosphere or any surrounding objects, Also the gloves are used to avoid any particle transfer to the detector's components through the steps of its construction or testing. 
  
  
  
 +
<references/>
  
 
Go Back [https://wiki.iac.isu.edu/index.php/Performance_of_THGEM_as_a_Neutron_Detector#Detector_Description]
 
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Latest revision as of 02:56, 27 March 2014

GEM Detector Design and Structure

The GEM preamplifier described in section XXXX was used to increase the signal amplitude of the ionization chamber. Primary electrons are liberated by an ionizing particle intersecting an ionization chamber that has a 90/10 Ar/CO2 gas; the cathode's electric field accelerates the electrons towards the GEM preamplifier. As described in section XXX, a single GEM preamplifier can increase the number of liberated electron by three orders of magnitude via secondary ionization.<ref = "chechik"> R. Chechik, A. Breskin, G. P. Guedes, D. Mörmann, J. M. Maia, V. Dangendorf, D. Vartsky, J. M. F. Dos Santos, and J. F. C. A. Veloso, Recent Investigations of Cascaded GEM and MHSP detectors, IEEE Trans. Nucl. Sci. 2004 </ref> Using three pre-amplifiers will increase the signal amplitude making it measurable. A high voltage divider circuit is used to establish the electric fields for each preamplifier using a single power supply channel, and secondary electrons are guided towards a segmented charge collector.


Detector Structure

The triple GEM detector is composed of three GEM preamplifiers, a cathode and an anode. A GEM preamplifier is a 50 micron thick kapton foil clad on both sides with 5 microns of copper. A staggered pattern of 50 micron diameter holes, equally spaced by distances comparable to the hole diameter, is chemically etched into the copper clad foil of a 140 um pitch distance over an area of 10x10cm. <ref name = "Souli"> F. Sauli, et al, NIM A 386, 531 (1997) </ref> The detector contains three GEM preamplifiers mounted on square plastic frames separated by a vertical distance of 2.8 mm and placed parallel to the cathode 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 10x10cm and is 3.5 mm away from the top of the first GEM card. This cathode design is capable of being set to be at a potential voltage of 5 kV (in the air) without any discharge. The charge collector (readout anode) is constructed of 50-80 micron wide strips that are insulated to determine the location of the collected electrons, and are arranged to allow equal charge sharing on the upper (x coordinate) and lower (y coordinate) charge collector layers <ref name= readout>Physik Department E18, Technische University Munchen, 2D readout Plane, 21 of Jun.2012. http://www.e18.ph.tum.de/research/compass/gempixelgem-tracking-detectors </ref>.

Example alt text
Fig. shows the charge collector dimensions and arrangement <ref name = readout/>.

All the above 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 kapton window to reduce the energy loss of incident particles entering the chamber. The figures below show top, bottom, and side views 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 Section YYY, fissionable material, inserted inside the chamber, may be used to indirectly detect neutrons by detecting the ionization caused by fission fragments released into the chamber if a neutron induced fission event occurs. The cathode design has a 3 cm diameter coating of U-233 with a 30-40 um thickness. The kapton window height was increased 2.5 mm to accommodate an increase in 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 which had enough area to cover the fissionable material was attached in the space between the cathode and the first GEM card. The shutter could be opened or closed from outside the chamber. When the shutter is closed, it covers the U-233 coating and stops the emitted fission fragments ionizing the gas beyond the shutter. When the shutter is open, the ionization due to fission fragments produces a signal. 


 What about alpha and beta particles.

The shutter has the ability to stop the fission fragments that are emitted from U-233 coating. Having the U-233 as a source for alpha particles, the QDC charge spectrum showed a difference in case the shutter was open and when it was close, such a test proved the ability of the FR4 shutter to stop (or partially stop) the emitted alpha particles from U-233 coating. The figure below shows the charge spectra in case of shutter open and closed as the detector's operating voltage is 2.6 kV and 2.9 kV for GEM and cathode successively.

Example alt text
charge collected as the voltage is 2.60 2.90 kV for GEM and cathode successively

Since the fission fragments are heavier ions than the alpha particles, a closed shutter should stop them. Such an ability is important to distinguish the fission fragments' signal from the other particles' signals in a heavy radiation environment created in an operating accelerator or reactor.





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

A high voltage divider circuit is used to proportionally apply voltages on both faces of each GEM preamplifier as defined by the resistors used in the circuit. The first GEM preamplifier that an electron encounters as it traverses the detector is set to have the most negative voltage. The GEM preamplifiers are connected to a high voltage divider circuit that specifies the electron multiplication and supports (manages) their transfer to the charge collector. As mentioned previously, The applied voltage on GEM cards determines all the detector's properties, such as the order of electron multiplication, and the electron collection by the readout plate. A high voltage divider circuit was designed for these purposes as shown in the figure below: <ref = "SergeHV"> Pinto, Serge . Gas Electron Multipliers Development of large area GEMS and spherical GEMS. Diss. Mathematisch-Naturwissenschaftliche Fakultät , 2011 </ref>

Example alt text
Figure shows the HV-divider circuit.

The HV circuit also provides the cathode with a voltage up to 3.6 kV to produce an electric field to drift most of the electrons, which are primarily produced by ionization. It provides the GEM cards with voltage between the top and bottom of each one and allows the voltage to decrease gradually to reach the minimum value between the third GEM's sides. So, the main advantages of the circuit design are to provide a voltage for electron multiplication, and to guide most of the drift electrons to the charge collector.

The circuit provides a trigger signal through a high pass filter connected to the third GEM electrode. One of the ways to get the detector signal is by a high pass filter that is in contact with the bottom side of the third GEM card; a positive signal is detected by the oscilloscope, since all the electrons are leaving the last GEM electrode toward the charge collector. The job of the high pass filter is to block all the low signal frequencies to provide a signal that can clearly be detected by a 50 ohm terminated oscilloscope.


The table shows the measurements of voltage between the sides of the GEM electrodes, and voltage between each side and the ground that are provided by the HV-voltage divider circuit shown above.

[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

Construction environment

Detectors are very sensitive devices, its components are connected to high voltage power supplies in a relatively thin chamber with Ar/CO2 90/10 gas mixture. As a result, the method of construction and maintaining demands a laminar flow hood that accommodates a clean environment from any source of dust in the atmosphere or any surrounding objects, Also the gloves are used to avoid any particle transfer to the detector's components through the steps of its construction or testing.


<references/>

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