HV stability and aging phenomena observed in the drift chamber system of the MEG experiment

Abstract

The MEG experiment at the Paul Scherrer Institut, searching for the charged lepton flavour violating decay μ+→e+γ, took physics data from 2009 until 2013. The drift chamber system was part of the dedicated positron spectrometer that was designed to ensure a precision measurement of 52.8 MeV positrons and consisted of 16 individual drift chamber modules. The individual detector modules were operated with a He/C2H6 (50:50) gas mixture to combine a long radiation length with good high voltage stability properties ensured by the high hydrocarbon fraction.During long-term operation several aging phenomena were observed. A continuous decrease of the electron multiplication factor was caused by a continuously growing layer on the node wires, most likely caused by polymerisation of the hydrocarbon in high gain and high rate operation. The aluminum coating of the cathodes showed damages most probably due to ion impact, in some cases the layer even peeled off. In addition, some drift chamber modules suffered from Malter effect, showing remaining and fluctuating currents during beam of periods, most likely caused by remaining photo resist on the cathode foils.