Detector Description
GEM Detector Design
Clean Environment
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.
Detector Structure
The triple GEM detector consists of :
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.
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.
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.
3-A Readout plate;
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.
The figures below show top, bottom and side view of the detector design.
High voltage divider
GEM preamplifiers are connected with high voltage divider circuit. as 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)
The ciruit provides the cathode with the a voltage of 3.6 kV, so it provides an electric field that drift most of the electrons that are primarily produced by ionization, also the cicuit provides the GEM preamplifiers with voltage between the top and bottom of each card, then the voltage gradually decreases to have he least value between the sides of the third GEM card, The circuit design advantage is to provide enough voltage for electron multiplication, also to guide most of the drift electrons to the grounded readout plate.
The following table shows the voltages measurements between the sides of the GEM preamplifiers, and voltage between each side 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 |
Modifying the GEM detector as neutron sensitive detector
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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