The MEG experiment took data at the Paul Scherrer Institute in the years 2009–2013 to test the violation of the lepton flavor conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavor violating decay μ+→e+γ: BR(μ+→e+γ) <4.2×10−13 at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of 6×10−14. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute (7×107 muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.
Ultra-thin metallic anode and cathode wires are frequently
employed in low-mass gaseous detectors for precision experiments,
where the amount of material crossed by charged particles must be
minimised. We present here the results of an analysis of the
mechanical stress and chemical corrosion effects observed in 40
and 50 μm diameter silver plated aluminum wires
mounted within the volume of the MEG II drift chamber, which caused
the breakage of about one hundred wires (over a total of
≈ 12000). This analysis is based on the careful inspection
of the broken wires by means of optical and electronic microscopes
and on a detailed recording of all breaking incidents. We present a
simple empirical model which relates the number of broken wires to
their exposure time to atmospheric relative humidity and to their
mechanical tension, which is necessary for mechanical stability in
the presence of electrostatic fields of several kV/cm. Finally we
discuss how wire breakages can be avoided or at least strongly
reduced by operating in controlled atmosphere during the mounting
stages of the wires within the drift chamber and by choosing a
25 % thicker wire diameter, which has very small effects on the
detector resolution and efficiency and can be obtained by using a
safer fabrication technique.
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