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

建立了一个由两个光电倍增管(PMT)和一个纯NaI或CSI晶体组成的真空室，并在室温和液氮温度(RT&LNT)下运行，测量了PMT R8778(AR)在RT和LNT时的量子效率比非常接近于1，峰值强度分别为470 nm和300 nm。用Ba133、Cs137、Co60和K40四种伽马射线源测量了NaI和CsI晶体在RT&LMT时的光产额。两种晶体的LNT光产额都比RT光产额有了显著的提高。在LNT时，CsI晶体的产额为~18个光电子/keV。如此高的产额使其有可能达到比在RT利用CSI(Tl)的Kims暗物质实验低3倍的能量阈值，并提高了闪烁脉冲形状分析的信号/背景分辨能力，这是抑制暗物质或中微核相干散射探测背景的关键技术之一。由于NaI晶体的侧面未抛光以及与PMT的光学耦合不完善，LNT上的NaI晶体的光产额并不高。NaI晶体的尺寸也太小，无法捕获100keV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PMTs，但太小，无法容纳整个真空室。部分组装必须在手套箱外完成，导致NaI晶体表面部分潮湿。所有这些都可以在未来很容易地改进。

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.