The ultra light Drift Chamber of the MEG II experiment

The MEG experiment at the Paul Scherrer Institute searches for the charged Lepton Flavor Violating μ+→e+γ decay. MEG has already determined in a first data taking phase the world best upper limit on the branching ratio: BR(μ+→e+γ)<4.2×10−13 An upgrade of the whole detector has been approved to obtain a substantial increase in sensitivity. Currently MEG is in upgrade phases, this phase involves all the detectors. The new positron tracker is a high transparency single volume, fully stereo cylindrical Drift Chamber (CDCH), immersed in a non uniform longitudinal B-field, with length of 1.93 m, internal radius of 17 cm and external radius of 30 cm. It is composed of 9 concentric layers, divided into 12 identical sector of 16 drift cells. The single drift cell is approximately square, with a 20 μm gold plated W sense wire surrounded by 40 μm silver plated Al field wires in a ratio of 5:1. The total number of wires amounts to 11904 for an equivalent radiation length per track turn of about 1.5x10−3 X0 when the chamber is filled with a gas mixture of helium and iso-butane. Due to the high wire density (12wires∕cm2), the use of the classical feed-through technique as wire anchoring system could hardly be implemented and therefore it was necessary to develop new wiring strategies. The number of wires and the stringent requirements impose the use of an automatic system (wiring robot) to operate the wiring procedures. Several tests have been performed in different prototypes of the drift chamber, exposed to cosmic rays, test beams and radioactive sources, to fulfill the requirement on the spatial resolution to be less than 110 μm. In this paper we describe the CDCH construction and the first tests during the 2018 Engineering Run.