Gating the MWPCs of the DELPHI Barrel RICH

The gating system of the MWPCs of the DELPHI Barrel RICH is essentially a means to prevent positive ions originating from avalanches in the chambers to escape into the Barrel RICH drift tubes. These ions would otherwise seriously affect the imaging quality of the Barrel RICH. We describe the gating grid, the gating pulser and results of tests at pressures of 1 and 1.3 bar in a magnetic field of 1.2 T and without. In the ungated (“closed”) mode, transfer of electrons and ions are 8% and 0% respectively; a further reduction of electron transfer is limited by power dissipation of the pulsing circuit. The occupation time of the readout electronics after the gating pulse is less than 1 μs. The test results are compared with those of electrostatic simulations. A triggering scheme is proposed.