Dark matter search by means of high purity CaF_2 scintillators

我们已经修改了CANDLES III探测器系统，该系统最初是为了研究^ Ca的双β衰变而构建<48>的，以寻找暗物质候选WIMP。两次测量之间的差异是要检测的能量范围。在暗物质搜索中探测到的能量低于100 keV，而^ Ca的双β衰变的Q值<48>为4.3 MeV。该系统对低能量信号检测的要求是：良好的光收集效率、低背景环境和良好的检测效率.为了获得更好的光收集效率，我们研制了一种新型的导光系统，它由高反射率的镜面反射器组成。基于我们为CANDLES III系统开发的光子输运模拟程序，我们采用了光电倍增管、光导和消隐器的最佳配置来探测低能WIMP反冲信号.研究了CaF_2晶体中放射性杂质浓度与原材料的关系，并购买了新的CaF_2晶体.利用中子束研究了CaF_2的猝灭因子与Eu掺杂浓度的关系.为了找到猝灭因子依赖性的原因，我们测量了各种粒子束（γ，质子，Li）的闪烁发射谱。我们发现CaF2散射子在波长上有两个分量，并且它们的强度随入射粒子的不同而不同.为了有效地检测低能量信号，我们开发了一种智能触发系统，该系统使用光子在时间上的分布和PMT数。我们目前正在研究大坂大学改进的探测器的特性。研究结束后，我们将把探测器系统移到地下实验室，开始采集数据。

We have modified the CANDLES III detector system, which was originally constructed for the study of double beta decay of ^<48>Ca, to search for dark matter candidate WIMPs. The difference between two measurements is the energy range to detect. The energy to detect in dark matter search is below 100 keV, while the Q-value of the double beta decay of ^<48>Ca is 4.3 MeV. The requirements to detect low energy signals in this system are better light collection efficiency, low background environment and good detection efficiency.1. We developed a new light-guide system which consists of mirror reflector with high reflectivity to get better light collection efficiency. We employed an optimum configuration of photomultipliers, light-guides, and scintillators based on the photon-transportation simulation program which we developed for modified CANDLES III system to detect low energy WIMP recoil signals.2. We studied the dependence of radioactive impurities density inside the CaF_2 crystals on the raw materials and purchased additional ones which made from selected materials.3. We studied the dependence of the quenching factor of CaF_2 on the density of doped Eu using neutron beam.4. To find the cause of the quenching factor dependence, we measured the scintillation emission spectra with various particle beams (γ, proton, Li). We found that the CaF2 scintillators have two components in wavelength and the strength of them was different depending on the incident particle.5. To detect low energy signals efficiently, we developed an intelligent trigger system which uses photon distribution on time and PMT number.We are currently studying characteristics of the detector we modified at Osaka University. After that study we will move the detector system to underground laboratory and start data taking.