用于COSINE-100U的升级型NaI(Tl)晶体封装的开发及初始性能
Development and Initial Performance of an Upgraded NaI(Tl) Crystal Encapsulation for COSINE-100U
AI总结：
该研究开发了用于COSINE-100U的升级型NaI(Tl)晶体封装，通过优化光耦合结构提升光产额、降低表面本底，修复了问题晶体，增强了低质量暗物质探测灵敏度。
AI中文摘要：
COSINE-100实验旨在利用低本底NaI(Tl)探测器检验DAMA/LIBRA的年度调制信号声称。针对COSINE-100U升级项目，我们开发了一套新型晶体封装系统，以在保持探测器长期稳定性的同时提升光收集效率，进而增强对低质量暗物质的探测灵敏度。该升级设计取消了COSINE-100所用的石英光学窗，通过2毫米厚的硅胶光学垫将光电倍增管（PMTs）直接耦合至晶体端面，减少了光学界面数量；对于更大尺寸的晶体，其边缘被倒成斜面，以更高效地将闪烁光引导至3英寸高量子效率PMTs上。性能研究采用了室温下COSINE-100U的2462小时（102.6天）数据，以及用于直接本底对比的、2023年3月运行末期采集的698小时（29.1天）COSINE-100数据。全部8块晶体的光产额均高于COSINE-100，范围为15.8至27.7个光电子/千电子伏（p.e./keV），其中6块超过20 p.e./keV；测得的体α本底率低于COSINE-100的值，且与内部210Pb的预期时间演化一致，而1至2兆电子伏（MeV）的表面α本底率则大幅降低。该升级还修复了此前因光学性能不佳被排除在COSINE-100物理分析之外的两块晶体，独立验证测试表明，该封装在低温液体闪烁剂中长期浸泡过程中仍保持机械强度和光学稳定性。本文介绍了该封装设计、室温探测器性能，以及在研烈山（Yemilab）设施实现的表面相关本底降低效果。
英文摘要：
The COSINE-100 experiment was designed to test the DAMA/LIBRA annual-modulation claim using low-background NaI(Tl) detectors. For the COSINE-100U upgrade, we developed a new crystal-encapsulation system to increase light-collection efficiency while preserving long-term detector stability, thereby improving sensitivity to low-mass dark matter. The upgraded design eliminates the quartz optical windows used in COSINE-100 and directly couples the photomultiplier tubes (PMTs) to the crystal end faces through 2-mm-thick silicone optical pads, thereby reducing the number of optical interfaces. For the larger crystals, the crystal edges were beveled to guide scintillation light more efficiently onto 3-inch high-quantum-efficiency PMTs. The performance study uses 2462~h (102.6~days) of room-temperature COSINE-100U data and, for direct background comparisons, reference COSINE-100 data acquired near the end of operation. 698~h (29.1~days) of COSINE-100 data acquired near the end of operation in March 2023. All eight crystals showed higher light yields than in COSINE-100, with values ranging from 15.8 to 27.7~p.e./keV; six crystals exceeded 20~p.e./keV. The measured bulk-$α$ rates were lower than the COSINE-100 values and consistent with the expected time evolution of internal $^{210}$Pb, while the 1--2-MeV surface-$α$ rates were substantially reduced. The upgrade also restored two crystals that had previously been excluded from the COSINE-100 physics analysis because of poor optical performance. Independent validation tests demonstrated that the encapsulation remains mechanically robust and optically stable during long-term immersion in liquid scintillator at low temperature. This paper presents the encapsulation design, the room-temperature detector performance, and the reduction in surface-related backgrounds achieved at the Yemilab facility.
