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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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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You should first describe the basics of the gas electron multiplier  then describe the ionization chamber equiped with GEM foils, then describe the assembly in a clean room environment.
 
  
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==Detector Structure==
  
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==Clean Environment==
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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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Gaseous detectors, like most detectors, are constructed in environments that minimize the presence of contaminants. Most of the methods for detector construction and maintenance demand a clean environment.  Generally, clean environments are classified depending on the size and the number of existing particulates in the room; each class maintains its environment by filtering the air that enters the room, by keeping air circulation in a specific rate, and by using soft or hard walls that isolate the room from the surrounding environment. Clean rooms have restrictions on the type of materials for the equipment and furniture, and have special instructions for the workers who can carry contaminants: the workers have to use gloves, hats, pants, shoes, and gowns that are designed for this purpose. Laminar hoods are another method to get a clean environment: they are easy to maintain, have less restrictions to keep them clean,and usually provide enough space for building and maintaining small size detectors.
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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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==Detector Structure==
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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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The triple GEM detector consists of :
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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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1- A cathode; 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.
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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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2- GEM cards; the detector has three GEM cards in square plastic frames, they are 10x10cm and are placed parallel to the cathode as shown in the figure below.
 
  
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[[File:GEM_Detector_original.png | thumb |alt=Example alt text|Fig.1 shows the original GEM detector design.| 200px]] 
 
  
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[[File:GEM_top.png | 200px]]  [[File:GEM_bottom.png | 200px]]  [[File:GEM_sides.png | 200px]]
  
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Each GEM preamplifier is made of a copper-clad kipton plate which is 50 um thick and has an area of 10x10cm. It has holes with a diameter of 50 um; each hole has a 10um rim which is chemically etched in an equilateral triangular pattern with a 140 um pitch distance.
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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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3-A charge collector (readout anode); it 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.
 
  
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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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[[File:Charge_collector.jpg |Charge_collector.jpg | thumb |alt=Example alt text|Fig. shows the charge collector wire's dimensions and arrangement| 100px]]
 
  
  
  
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4- A sealed chamber; All the previous components exist in a chamber that consists of two ertalyte plastic sheets which 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 window for incident particles.
 
  
  
  
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The figures below show top, bottom and side view of the detector design.
 
  
  
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The figures below show the modified components of the detector.
  
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[[File:GEM_top.png | 200px]] [[File:GEM_bottom.png | 200px]] [[File:GEM_sides.png | 200px]]
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{|boardclip |
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|-
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|[[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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|[[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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|}
  
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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.
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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.
  
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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 circuit 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.
 
  
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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).
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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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=Modifying the GEM detector as neutron sensitive detector=
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=Construction environment=
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The GEM’s original design was modified to convert it to a neutron sensitive detector. The cathode design has a 3 cm diameter coating, and 40-50 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.
 
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The figures below show the modified components of the detector.
 
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{|boardclip |
 
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|[[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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|[[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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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. 
  
  
  
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<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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