CylindricalDCH

The positron tracker is a unique volume, cylindrical wire drift chamber, with the axis parallel to the muon beam. The external radius of the chamber is constrained by the available room inside COBRA, while its length is dictated by the necessity of tracking positron trajectories until they hit the TC: about 180 cm of length.
The Drift Chamber is composed of 10 criss-crossing sense wire planes with wires extending along the beam axis with alternating stereo angles in order to reconstruct the coordinate along the axis of the chamber by combining the information of adjacent layers. The stereo angle varies from 8◦ in the outermost layers to 7◦ in the innermost ones. The field planes are common to the two ±stereo views and are stringed in both directions, creating a ground mesh between sense planes of alternating stereo view.

The number of anodic wires is 1920 while the cathode wires are ∼6400.

Schematic view

Schematic view of the Cylindrical Drift Chamber

The Drift cells have an almost square shape (eight field wires surrounding the central anodic wire). The side of each cell is 7 mm in order to guarantee a tolerable occupancy of the innermost wires, which are placed at roughly 18 cm from the beam axis where the rate is ∼1 MHz for a stopping rate of 7 × 107μ/s. Supposing a maximum drift time about 150 ns this configuration allow to have a ∼15% of occupancy of the innermost wires.