A 10−3 drift velocity monitoring chamber for the MEG II experiment

The MEG-II experiment searches for the lepton-flavor-violating decay: μ⟶e+γ. The reconstruction of the positron trajectory uses a drift chamber operated with a mixture of He and iC4H10 gas. It is crucial to provide a stable performance of the detector in terms of its electron transport parameters, avalanche multiplication, composition and purity of the gas mixture. In order to have a continuous monitoring of the quality of the gas injected, we plan to install a small drift chamber, with a simple geometry that allows to measure very precisely the electron drift velocity in a prompt way. The chamber is a small box with cathode walls, that determine a highly uniform electric field inside two adjacent drift cells. Along the axis separating the two drift cells, four staggered sense wires alternated with five guard wires collect the drifting electrons. The trigger is provided by two 90Sr radioactive sources placed on top of a two thin scintillator tiles telescope. The whole system is designed to give a response in few minutes about drift velocity variations at the 10−3 level. In this paper the development of the drift chamber is presented, with a particular focus on the details of its construction.