用于LZ暗物质实验和未来稀有事件探测的放射性纯钛的鉴定
Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event Searches
摘要：
LUX-ZEPLIN（LZ）实验将在双容器低温恒温器中使用含有总共10吨液体氙的探测器来寻找暗物质粒子的相互作用。低温恒温器的大质量和接近活性检测器体积要求使用具有极低固有放射性的材料。我们报告进行放射性测定活动，以确定合适的金属，限制放射性纯生产的因素的测定，和选择钛的建设LZ低温恒温器和其他探测器组件。该钛的活性测定结果为$^{238}$ U $_{e}$ ~ $<$ 1.6~mBq/kg、$^{238}$ U $_{l}$ ~ $<$ 0.09~mBq/kg、$^{232}$ Th $_{e}$ ~ $=0.28\pm 0.03$ ~mBq/kg、$^{232}$ Th $_{l}$ ~ $=0.25\pm 0.02$ ~mBq/kg、$^{40}$ K~ $<$ 0.54~mBq/kg和$^{60}$ Co~ $<$ 0.02~mBq/kg（68\% CL）。这种低的固有活性，这是有史以来报道的钛最低的一些，使其能够用于未来的暗物质和其他罕见事件的搜索。已经进行了蒙特卡罗模拟，以评估LZ低温恒温器的预期背景贡献。在1,000天的5.6吨基准质量的WIMP搜索暴露中，低温恒温器将仅贡献$0.160\pm0.001$（stat）$\pm0.030$（sys）计数的平均背景。
英文摘要：
The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a detector containing a total of 10 tonnes of liquid xenon within a double-vessel cryostat. The large mass and proximity of the cryostat to the active detector volume demand the use of material with extremely low intrinsic radioactivity. We report on the radioassay campaign conducted to identify suitable metals, the determination of factors limiting radiopure production, and the selection of titanium for construction of the LZ cryostat and other detector components. This titanium has been measured with activities of $^{238}$U$_{e}$~$<$1.6~mBq/kg, $^{238}$U$_{l}$~$<$0.09~mBq/kg, $^{232}$Th$_{e}$~$=0.28\pm 0.03$~mBq/kg, $^{232}$Th$_{l}$~$=0.25\pm 0.02$~mBq/kg, $^{40}$K~$<$0.54~mBq/kg, and $^{60}$Co~$<$0.02~mBq/kg (68\% CL). Such low intrinsic activities, which are some of the lowest ever reported for titanium, enable its use for future dark matter and other rare event searches. Monte Carlo simulations have been performed to assess the expected background contribution from the LZ cryostat with this radioactivity. In 1,000 days of WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute only a mean background of $0.160\pm0.001$(stat)$\pm0.030$(sys) counts.
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