Detector Description

GEM Detector Design

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.

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.

Fig.1 shows the original GEM detector design.

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 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.

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

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:

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.

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

Modifying the GEM detector as neutron sensitive detector

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.

The figures below show the modified components of the detector.

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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