Trigger-Induced Mechanical Resonances of Gating Grid Wires in the Multi-Wire Proportional Chambers of the ALICE TPC

A crucial issue in the design of modern multi-wire proportional chambers (MWPCs) is to avoide mechanical instabilities of thin and long wires that are held at high potentials [1]. Still, these structures are mechanically very sensitive and event-related (random) oscillations have been reported for drift tubes [2,3], where the gas ionisation originates the driving force.Gated MWPCs may exhibit a similar effect, recently observed at the ALICE TPC: the trigger-related change of electrostatic potential of the gating grid wires lead to a periodic residual electrostatic force. Albeit rather small (about 10−6 Ncm−1) it may give rise to sizeable (a few μm) resonant oscillations of the wires. The oscillations result in systematic pick-up signals, with an amplitude comparable to those of minimum ionising particles.An exhaustive set of measurements, at the detector as well as at a dedicated laboratory set-up with an open chamber, is carried out and forms the subject of this writing.