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DESCRIPTION OF MGWC's

The anode wires are gold plated tungsten wires, with a diameter of 5 µm or 10 µm held in a distance of 15  µm above the cathode plane (Fig. 1). Thin insulating strips, 40  µm wide, separate the anode wires from the cathode plane. These strips are made of polyimide having a dielectric constant of 2.9 and a resistivity of 10¹⁴ m. The anode-cathode distance (polyimide height) has been chosen to ensure small anode-cathode capacitance and limited electric field variation around the anode wires.

Figure 1: Schematic view of the MGWC.

The substrate, covered by the cathode, is not a very important element contrary to what it is for the MSGC's. However, in the case where the cathode plane is segmented in strips or pads to have a 2-D readout, the substrate dielectric constant must be as low as possible in order to minimize the cathode-cathode and cathode-ground capacitance and thus, to fight against cross-talk and electronic noise. Other limitations for the choice of the substrate, according to the experimental environment, could be the radiation length, the radiation resistance and the material natural radioactivity. In the present study, ceramic substrates (dielectric constant 10, resistivity 10¹² m) have been used. In the future, kapton (dielectric constant 3.5, resistivity 10¹⁴ m) and duroid (dielectric constant 2.94, resistivity 10¹² m) substrates will also be used which will give lower cathode-ground capacitance.

On top of the cathode plane, made of gold, the thin polyimide strips are photo-lithographically deposited. A frame wound with the anode wires and the desired pitch are aligned on top of the polyimide strips in order to have one wire at the middle of each strip. The wires are then soldered or glued with a conducting paste on golden pads at both ends. For the first prototype, the anode wires have been fixed on the pads with the help of a bonding machine without using a bonding wire. The ultrasonic energy melts the gold on both, wires and pads, forming a solid connection. Although the solidity of the connection has been tested by performing a bond pull strength test, an extra connection for each anode wire has been done behind the first one using again the bonding machine but this time in a more ``conventional'' way, i.e, bonds were realised on the anode wires using the wire of the bonding machine. In this way, very solid real solder is performed. A second prototype has proved that this second connection was not necessary speeding up considerably the chamber construction.

No silicon technology is used for the MGWC construction, with the benefit that large chambers (e.g 20 x 20  cm² ) can easily be constructed.

A prototype with a sensitive area of 2.6 x 2.6  cm² , 128 anode wires of 10 µm diameter with a pitch of 203.2  µm, has been build to test the detector feasibility and performance. The cathode plane was split into two large cathode strips, each with 64 anode wires above (Fig. 2). The ceramic substrate had a thickness of 0.7 mm.

Figure 2: Wafer of the first MGWC prototype.

  ...duroid     Rogers Corporation

Marcos Dracos Sat Apr 4 18:31:19 METDST 1998

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