The drift chamber of the MEG II experiment

The MEG experiment at the Paul Scherrer Institut searches for the charged-Lepton-Flavor-Violating µ+ → e+γ decay. MEG has already set the world best upper limit on the branching ratio: BR<4.2×10−13 @ 90% C.l. An upgrade (MEG II) of the whole detector has been approved to obtain a substantial increase of sensitivity. Currently MEG II is completing the upgrade of the various detectors, an engineering run and a pre-commissioning run were carried out during 2018 and 2019. The new positron tracker is a unique volume, ultra-light He based cylindrical drift chamber (CDCH), with high granularity: 9 layers of 192 square drift cells, ∼6-9 mm wide, consist of ∼12000 wires in a full stereo configuration. To ensure the electrostatic stability of the drift cells a new wiring strategy should be developed due to the high wire density (12 wires/cm2), the stringent precision requirements on the wire position and uniformity of the wire mechanical tension (better than 0.5 g). The basic idea is to create multiwire frames, by soldering a set of (16 or 32) wires on 40 µm thick custom wire-PCBs. Multiwire frames and PEEK spacers are overlapped alternately along the radius, to set the proper cell width, in each of the twelve sectors defined by the spokes of the rudder wheel shaped end-plates. Despite to the conceptual simplicity of the assembling strategies, the building of the multiwire frames, with the set requirements, imposes a use of an automatic wiring system. The MEG II CDCH is the first cylindrical drift chamber ever designed and built in a modular way and it will allow to track positrons, with a momentum greater than 45 MeV/c, with high efficiency by using a very small amount of material, 1.5x10−3 X0. We describe the CDCH design and construction, the wiring phase at INFN-Lecce, the choice of the wires, their mechanical properties, the assembly and sealing at INFN-Pisa and the preliminary commissioning activities at PSI.