Detector Assembly

Introduction

This page provides a guide to assemble a Triple 10x10 cm2 GEM detector. Please keep in mind that the following instructions are preliminary and under review .

Note that the assembly has to be done in a clean room classified as ISO 7 (class 10,000) . You must wear: a vest, a head cover, a shoe cover, a mask and the gloves.

At first it is very important to choose the configuration of the new detector. This is determinated by the gap spacing between each foil. The tipical CMS configuration is the (3,1,2,1) mm, where these numbers represent the distance in mm between the drift foil, the GEM1 foil (G1), the GEM 2 foil (G2) and the GEM3 foil (G3).

Now you can go trough the following paragraphs where all the steps to built a new Triple GEM detector are described in details.

The GEM kit

The kit contains:

• 1 sealed box with 3 GEM foils and 1 drift foil (it can be opened only inside the clean room);
• 1 readout board in its black electrostatic protection bag;
• 1 green epoxy gas frame with O-ring grooves at both sides and three gas inlets;
• 1 cover for the gas box;
• 1 kapton protection gas window;
• 2x Viton O rings;
• 2xM3 gas connectors;
• 2x gas connector adapters;
• 4x nylon screws and nuts;
• Washers (0.5 mm thick) for z spacing between the GEM foils and drift;
• 28x M3 metallic screws;
• 28x M3 nuts;

Before starting

Check that you dispose of the following equipment before opening the GEM box:

• a soldering station;
• a HV tester or a power supply that can at least provide 300 V with a current reading of nA;
• a capacitance tester;
• a screwdriver or an hexagonal key;
• scissors;
• a razorblade;
• cleanroom napkins;
• ethanol, this is not mandatory, demineralized water can also be used.

It is very important to keep the GEMs as little as possible in open air, even inside the cleanroom, to avoid any dust or humidity accumulation. You must avoid to directly touch by hands (or by gloves) the surface of the GEM foil. Clean by ethanol the working desk.

Now you can extract the GEM foils from the box one at time taking care of never put the foils in contact with another object apart the clean room proof napkins.

Now you are ready to start the quality check of each foil:

• make sure by eye that looks not damaged. Never try to clean the GEM foils by any means. If the foils seem to have suffered or are dirty due to the transportation and/or handling, the assembly should not be performed and the foils should be sent back for cleaning.

• perform the leakage current test link.

If all the 3 foils pass the leakage test you can proceede to the assembly of the detector.

At first you can separate, by using the scissors, the top and bottom fanouts. Cut as close as possible to the edge of the foil, while being careful not the accidentally cut into one of the contacts.

Assembly

Preparation of the gas box

1) At first provide to clean the following components:

• green frame: make sure that any dust remnant from the drilling process has been removed. To do so, pass a screw inside every hole and remove any dust systematically using an air blower. Use also the blower to clean inside the O-ring grooves.
• O-rings: gently polish the rings using clean room proof napkins and ethanol. IMPORTANT: this cleaning is not sufficient if you plan to do aging studies.

2) Then make sure that the O-rings fit inside the frame grooves (a lot of patience is required).

3) Insert the gas connectors in the green frame. Since the connectors are fragile, do not screw them too tightly into the frame.

4) Insert the gas in/outlets, but before flush the holes once more using the blower to make sure that they are completely dust-free. Close the third gas inlet with a M3 screw.

Preparation of the readout board

The readout board contains 10 conductive pads of which only 7 are used to power the drift and the bottom and the top of each GEM foil.

So the board will have a specific orientation. This has to be determined considering that inside the detector the total voltage can reach values of ~5000 V. As a consequence, in order to avoid the probability to generate sparks, the voltage should grow from one fanout to the other, such that the potential gradient is minimized. In the figure you can see the connection for the fanout in the CMS standard configuration of the Triple GEM.

The first plan of the stuck is the readout board:

1) Extract the readout from the black electrostatic bag

2) Ensure that the plastic protection covers the active area. Clean the board with the air blower.

3) Insert from the bottom the four nylon screws into the four innermost holes of the readout board. These will be used to keep the GEMs and the drift foils into their respective positions. Make sure that the screws are perpendicular to the plane. They should be long enough to accommodate all the foils included in your GEM-design.

4) Finally, according to your detector design, choose and prepare the 7 pads by placing a drop of soldering on them. These drops will be used to solder the fanout of the drift and GEM foils.

GEM placement

Now you are ready to build your Triple GEM stack. Note that a GEM foil is mounted on a frame with a thickness of 0.5 mm on both sides. This has to be taken into account to achieve the desidered spacing. In addition you can use the plastic spacers (washers) with a thickness of 0.5 mm.

1) Insert the washers on the nylon screws according to the design you have in mind.

2) Remove the protection on the readout board and position the first GEM foil (G3) on the readout plane on top of the washers.

3) Place the GEM with the two fanouts on the right position with respect to the readout board.

4) Before soldering, protect your GEM with a cover or use, for example, the drift plane. Make sure that this protection do not touch the GEM directly.

5) Solder your fanout at the proper place to the readout pads.

6) Check the capacitance and leakage current after the soldering stage and compare with the values taking during the leakage test. The two sets of values should agree.

- If however you observe a larger leakage current, this can be due from some dust deposit during the soldering or the handling of the GEM.

- If you measure a higher capacitance value after the soldering, this suggests that you don’t have a good connection.

7) Insert the washers for the second gap according to your design and place the second GEM foil on top. As before, you should take care to protect the top face of the GEM. Solder the foil and perform the leakage and capacitance test and the leakage test of both G2 as G3.

8) Place the washers for the first transfer gap and insert the G1 foil. Solder and retest all three GEMs.

9) Finally, insert the washers for the drift gap, slide the drift foil on top. Make sure to place the copper face on the bottom and the Kapton face on top.

10) Close the stack with nylon nuts. Firmly tighten them (by hand is fine). The foils have to be stretched and parallel to the readout plane. Check that all the foils are kept horizontally, that the gaps concord to your design and that the foils are properly stretched.

11) Cut the part of the nylon screws that are sticking out of the stack.

12) Perform a last quick leakage test of all three GEMs.

Closing the gas box

Before closing the gas box, take care of cleaning the Kapton window from the remaining dust, using a clean room napkin and ethanol. Close the gas box as follows:

1) Place the two O-rings in the grooves of the green frame and position the green frame on top of the readout board. Take care that the gas connectors may not interfere with HV lines and read out lines.

2) Make sure that the rubber bands form a seal in the grooves of the wall. The 28 holes in the wall should match the holes in the base panel.

3) Place the CLEANED Kapton foil on top of the green frame.

4) Close the stack using the cover. While doing this, make sure that the O-rings stay in place. Fix the frame by tightening the 28 screws from the bottom of the readout.

5) Finally close the gas connectors off using the plastic tubes to avoid the entering of the dust from the gas line.

Now the detector is ready to go outside the clean room. And we are ready to build the high voltage circuit to supply the detector.

-- FedericaPrimavera - 20 Aug 2014