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 maintaining demand a clean environment. Generally, clean environments are classified into classes 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 using soft or hard walls that isolate the room 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, so the workers have to use gloves, hats, pants, shoes, and gowns that are designed for this purpose. Laminar hoods is another method to get a clean environment, they are easy to maintain, 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 has a dimension of 10x10cm and has a distance of 3.5 mm 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.

2- 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.

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.

3-A Readout plate:

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.

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:

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).

[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

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.

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

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