Inside the BGO calorimeter two multi wire proportional chambers (MWPC) with increasing diameter allow the reconstruction of trajectories from charged particles. Each chamber is made of two coaxial cylindrical cathodes which is segmented into strips, heliocoidally wounded in opposite direction. The anode array consists of equally spaced wires stretched parallel to the cylinder axis in the middle of the active area (gas gap ≈ 8mm). The geometrical parameters are fine tuned to fit exactly inside the carbon structure of the BGO and around the target nose. The front end electronics (preamplifiers), which is provided by the Moscow collaborators, is not integrated in the chambers, but will be positioned very close (cable length about 150 cm) to one end of the chambers.
The 2 mm pitch of the wires, which have a diameter of 20 μm, results in an expected 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 by using a strip width of 4.5 mm. Finally, the resolution in the polar angle Θ, which depends on Δz and the relative radii of the two chambers, is expected to be Θ ≈ 1 degree with the planned geometry. 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 degrees to Θ = 163 degrees. The area between 8 degrees and 17 degrees is partly shadowed by flanges made from low density materials. This shadowing is almost negligible for neutral particles and can be evaluated with simulation.
The readout electronics after the preamplifiers is based on different VME modules for the wires and the strips. The wire hits are registered via VFB2 modules from ELB21 with a self-developed TDC firmware and the strip signals are measured via Sampling ADC modules from Wiener. The chamber gas is mixed from three components, 70% Argon, 29.5% Ethan, 0.5% Halocarbon 14 (CF4) inside a dedicated gas mixer in the vicinity of the detector. 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 chambers are completed and after a high voltage conditioning process at University of Pavia ready for installation in summer 2012. The preamplifiers from the Moscow group and the support structure build by the Rome group are available in autumn 2012 latest. The full setup at the experiment should be completed November 2012. Figure 3.45 shows the two chambers fully assembled (right) and with the inner cathode strips visible (left).