The tagging detector of the CP-violation experiment NA48 at CERN

The CP-violation experiment NA48 at CERN aims at measuring direct CP-violation in the decays of neutral kaons into π+π− and π0π0. The experiment uses simultaneous, almost collinear beams of neutral KL0 and KS0 mesons, which are produced on two different targets. KS0 events are “tagged” by measuring the times of the protons that fly towards the KS0 production target. In order to sustain the high rate of 107protons/s in this beam without suffering from too many pile-up events, the detector consists of 24 separate scintillators (12 horizontal and 12 vertical), each of which sees only a small fraction of the beam. Their signals are digitized by specially developed 1GHz FADCs. The exact time of a pulse is established offline by a fit procedure. The detector and its electronics have been successfully used during test beams in 1994–1996 and during the first physics run of NA48 in 1997. The inefficiency of the detector has been measured to 2×10−4. Sources of background have been identified to allow for off-line correction. The mechanical setup of the detector, the 1GHz FADC and results from the test and data runs are presented.