Feasibility study of pure NaI and CsI directly coupled to PMTs at 77K for dark matter search

在室温和液氮温度下，用两个峰值强度分别为470和300 nm的LED测量了PMT-R8778（AR）的量子效率比，结果表明，PMT-R8778（AR）在室温和液氮温度下的量子效率比非常接近于1。用Ba ~（133）、Cs ~（137）、Co ~（60）和K ~（40）γ射线源测量了NaI和CsI晶体在RT和LMT下的光产额。在两种晶体中观察到在LNT下的光产额与在RT下的光产额相比显著增加。在LNT下CsI晶体的光电子产额达到~18 keV。如此高的产率使得有可能达到比在RT下利用CsI（Tl）的KIMS暗物质实验低3倍的能量阈值，并且还增加闪烁脉冲形状分析的信号/背景辨别能力，抑制暗物质或中微子背景的关键技术之一，NaI晶体在LNT处的光产额由于其未抛光的侧表面和与PMT的不完美的光学耦合而不那么高。NaI晶体的尺寸也太小，无法捕获100 keV以上的伽马射线。这使得使用普通伽马射线源校准系统变得困难。手套箱足够大，可以组装晶体和光电倍增管，但太小，无法容纳整个真空室。部分组装必须在手套箱外进行，导致NaI晶体表面部分湿润。所有这些都可以在未来轻松改进。

A vacuum chamber hosting two photomultiplier tubes (PMTs) and a pure NaI or CsI crystal was built and operated at both room and liquid nitrogen temperatures (RT & LNT).The ratio of the quantum efficiency of PMT R8778(AR) at RT & LNT was measured to be very close to 1 using 2 LED with peak intensities at 470 and 300 nm, respectively. The light yields of the NaI and the CsI crystal at RT & LMT were measured using Ba133, Cs137, Co60 and K40 gamma-ray sources. Significant increase of the light yield at LNT compared to that at RT was observed in both crystals. A yield of ~18 photoelectrons/keV was reached in the CsI crystal at LNT. Such a high yield makes it possible to reach an energy threshold 3 times lower than that of the KIMS dark matter experiment, which utilizes CsI(Tl) at RT, and also to increase the signal/background discrimination power of scintillation pulse shape analysis, one of the key techniques to suppress the background for dark matter or neutrino-nucleus coherent scattering detection.The light yield of the NaI crystal at LNT was not as high due to its unpolished side surface and imperfect optical coupling to the PMT. The dimension of the NaI crystal was also too small to capture gamma-rays above 100 keV. This made it difficult to calibrate the system using common gamma-ray sources. The glove box was large enough to assemble the crystal and PMTs, but too small to house the whole vacuum chamber. Part of the assembly had to be done outside of the glove box, leading to partially moisturized surface of NaI crystal. All of these can be easily improved in the future.