INSTR'20

The Drift Chamber of the MEGII experiment

25 Feb 2020, 12:00

20m

Invited Oral Tracking and vertex detectors Tracking and vertex detectors

Speakers

Mr Gianluigi Chiarello (INFN Roma1) Dr Giovanni Francesco Tassielli (INFN Lecce & University of Salento)

Description

The MEG experiment at the Paul Scherrer Institut searches for the charged Lepton Flavor Violating $\mu^{+}\rightarrow e^{+}\gamma$ decay. MEG has already determined 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 in 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 positron tracker is a unique volume, cylindrical drift chamber, with the axis parallel to the muon beam. The external radius ($30~cm$) of the chamber is constrained by the available space inside the COBRA magnet, while the internal radius ($17~cm$) is large so that low energy positrons, less than $45~MeV/c$, are swept out of the magnet by the gradient field without crossing the sensitive volume. With the new tracking system layout the main advantages are that the positrons with a momentum greater than $45~MeV/c$ will be tracked as close as possible to the Timing Counter system (TC) by using a very small amount of material, 1.45 10$^{-3}$ X$_{0}$, allowing to increase: the positron reconstruction efficiency, the postiron momentum and vertex resolutions and to the positron timing matching resolution. The single drift cell is approximately square, with a $20~\mu$m gold plated W sense wire surrounded by $40~\mu$m silver plated Al field wires in a ratio of 5:1. For equalizing the gain of the innermost and outermost layers, two guard wires layers ($50~\mu$m silver-plated Al) have been added at proper radii and at appropriate high voltages. The total number of wires amounts to 11904. Due to the high wire density ($12 wires/cm^{2}$) and the stringent precision requirements on the wire position and uniformity of the wire mechanical tension (better than $0.5~g$) impose 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 basic idea is to create a multi-wires plane, by soldering the wires between two $40~\mu$m thick custom wire-PCBs. Despite to the conceptual simplicity of the building strategies, to ensure the electrostatic stability of the drift cells and meet the requirements on the uniformity of the wire mechanical tension for all the multi-wires plans necessary for the construction of the CDCH. All these constraints require the use of an automatic wiring system (called wiring robot). The CDCH is the first drift chamber ever designed and built in a modular way, in fact, it is built by overlapping along the radius, alternatively, multi-wires plane and PEEK spacers, to set the proper cell width, in each of the twelve sectors, between the spokes of the rudder wheel shaped end-plate. A carbon fiber support structure guarantees the proper wire tension and encloses the gas volume. At the innermost radius, an Al Mylar foil separates the drift chamber gas volume from the helium filled target region. We describe the CDCH design and construction (wiring procedure and assembly procedure). The wiring phase at INFN-Lecce, the choice of the wires, their mechanical properties and a material budget estimation are presented. The assembly and sealing at INFN-Pisa are then describe, before the preparation of the endcaps services.

Presentation Materials

Slides

MEGII-CDCH-INSTR20.pdf