Study of Photon Detector in Beam for the Rare K^+ Decay at Rest Experiment

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The search for the rare kaon decay K^+→π^+νν^^- has been done in the kinematical region where the π^+ momentum is greater than the π^+ momentum from K^+→π^+π^0 decay which is the main background source. As one of the Japan-US Cooperative Projects, BNL-E787 experiment, searched for the K^+→π^+νν^^- from kaons decay at rest in this kinematical region and found one candidate event. We are now preparing E949 experiment as a Japan-US Cooperative Project. E949 experiment aims to find around ten events of K^+→π^+νν^^- decay. In order to do so, it is very important to search for the kinematical region where π^+ momentum is lower. In this kinematical region, photons from π^0 decay tend to be emitted to the direction of the incident K^+ beam. We have made a high statistics simulation to confirm it. This leads us to increase the photon detection efficiency in the direction of the incident beam. The photon detectors in this region should correctly work in the high intensity kaon beam and in the magnetic field of 1Tesla. As candidates of this kind of photon detector, the following two methods were designed.1) The pure CsI crystals with fine-mesh type photo-multipliers which can be used in the high magnetic field.2) The lead and plastic scintillator sandwich counter with wavelength shifter fiber read out to the normal photo-multipliers.In order to find the best design as the photon detector to suppress the K^+→π^+π^0 background, two designs were studied by computer simulations and performance tests of the prototype detectors.