CsI endcap photon detector for a K+ → π+ ν bar ν experiment at BNL

The authors have constructed an endcap photon detector with undoped Cesium Iodide (CsI) crystals for K+ → π+ν bar ν experiment E787 at Brookhaven National Laboratory (BNL). To reject backgrounds it is essential to have a photon detecting system with high efficiency. Good resolution for timing and energy is critical for reducing accidental vetoes in a high-counting-rate environment. The endcap detector is located in a 1 T magnetic field. Fine-mesh photomultiplier tubes are attached directly to the crystals for efficient light collection, and the fast-decay component of the CsI light output is selected by optical filters. The tube gains are monitored with the light from a xenon flash lamp and are monitored with the light from a xenon flash lamp and from light pulsers. The pulse shapes are recorded by transient digitizers based on charged coupled devices (CCDs), which enables precise determination of timing of endcap signals. The construction and performance of the detector are described