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

在K^+→π^+νν^0衰变产生的π^+动量大于π^+动量的运动学区域，对稀有K^+→π^+π^-衰变进行了研究，这是主要的背景源。作为日美合作项目之一的BNL-E787实验，在这个运动学区域寻找静止态Kaons衰变的K^+→π^+νν^^-，发现了一个候选事件。我们现在正在准备E949实验，作为日美合作项目。E949实验旨在发现大约十个K^+→π^+νν^-衰变事件。为此，寻找π^+动量较低的运动学区域是非常重要的。在这个运动学区域，π^0衰变产生的光子倾向于向入射K^+光束的方向发射。我们做了一个高统计模拟来证实这一点。这使得我们提高了入射光束方向上的光子探测效率。该区域的光子探测器应能在1特斯拉的强K束流和磁场中正常工作。作为这类光子探测器的候选者，我们设计了以下两种方法：1)具有可在强磁场中使用的精细网状光电倍增器的纯CsI晶体；2)将带有波长移位光纤的铅和塑料闪烁体夹层计数器读出到普通光电倍增器中。为了找到抑制K+→π^+π^0本底的最佳光子探测器设计，通过计算机模拟和原型探测器的性能测试，对两种设计进行了研究。

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.