CSI ENDCAP PHOTON DETECTOR FOR A K-]PI(+)V(V)OVER-BAR EXPERIMENT AT BNL()

We have constructed an endcap photon detector with undoped Cesium Iodi de (CsI) crystals for K+ --> pi(+) nu ($) over bar nu experiment E787 at Brookhaven National Laboratory (BNL). To reject backgrounds it is e ssential to have a photon detection system with high efficiency, Good resolution for timing and energy is critical for reducing accidental v etoes in a high-counting-rate environment. The endcap detector is loca ted in a 1 T magnetic field. Fine-mesh photomultiplier tubes are attac hed directly to the crystals for efficient light collection, and the f ast-decay component of the CsI light output is selected by optical fil ters. The tube gains are monitored with the light from a xenon flash l amp and from light pulsers. The pulse shapes are recorded by transient digitizers based on charged coupled devices (CCDs), which enables pre cise determination of timing of endcap signals, The construction and p erformance of the detector are described.