MWPC
Inside the BGO two MWPCs are used for charged particle tracking. Each chamber has an inner and outer cylindrical wall made from 2mm thick Rohacell covered in 50 μm Kapton film. On the inside the walls are laminated with copper strips (0.1 μm thick, 4.5 mm wide and separated by 0.5 mm) which form the cathode. For the anode, 20 μm thick gold-
plated tungsten wires are stretched parallel to the beam direction. There is one anode wire every 2mm and the distance between anode and cathode is 4 mm. The
inner and outer strips are wound in opposite directions at ±45° with respect to the wires. The geometrical parameters are fine tuned to fit exactly inside the carbon structure of the BGO and around the target nose.
The 2 mm pitch of the wires, results in an azimuthal angular distribution of ΔΦ ≈ 2 degrees. The resolution along the longitudinal coordinate is determined by the strip width and the crossing angle between internal and external strips. The minimum resolution, can be achieved at an relative angle of 90 degrees of the strips. The expected resolution is Δz ≈ 300 μm. Finally, the resolution in the polar angle Θ, which depends on Δz and the relative radii of the two chambers, is ΔΘ ≈ 1°. The outer dimension of the full chamber set-up is length = 690 mm (active length = 520 mm), inner diameter 84.4 mm and outer diameter 187 mm. The acceptance for particle tracking in the laboratory system is from Θ = 8° to Θ = 163°. The chambers are mechanically connected to an individual holding structure on a rail system, which can be moved independently from the target system and the BGO detector. This is important to have enough flexibility for maintenance and target handling.
The MWPCs are operated at 2500V with a gas mixture of 69.5% argon, 30.0% ethane, and 0.5% Halocarbon 14 (CF4). To prevent current instabilities an adjustable amount of alcohol vapour is added to the mixture by passing the gas through alcohol in a tank.