Drift optimization for the photon detectors of the DELPHI forward ring imaging Cherenkov detector

Abstract The forward regions of the DELPHI experiment at LEP are equipped with ring imaging Cherenkov detectors, divided into 12 units in both end caps. Electrons are created through photoionization in the photon detector of such a unit. The electrons drift in a crossed electric and magnetic field to either side of the detector where they are detected by a multiwire proportional chamber. The electric field is divided into two halves meeting at an angle of 152° at the central division plane. This intersect consequently has to be carefully designed. Three different designs have been investigated, two with differently spaced metal electrodes and one with a thin glass volume degrader used as voltage interpolator. The distortions were measured with single photoelectrons and the field calculated in three dimensions. The smallest distortions, below 0.4 mm, were obtained with the interpolator.