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 π ⁰π ⁰. The experiment uses simultaneous, almost collinear beams of neutral K _L⁰ and K _S⁰ mesons, which are produced on two different targets. K _S⁰ events are "tagged" by measuring the times of the protons that fly towards the K _S⁰ production target. In order to sustain the high rate of 10 ⁷ protons/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 1 GHz 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 1 GHz FADC and results from the test and data runs are presented.