中国锦屏地下实验室极低通量本底中子的探测

The dark matter research is the major topics of the particle physics, cosmology, astrophysics currently. In dark matter search experiments, neutrons interact with detector material through nuclear recoil, the generated signal is very similar to that generated by the dark matter particles WIMPs in the detector. It is very difficult to distinguish them. Therefore, dark matter research experiment needs deep underground laboratory with ultra low background. And we need to measure the neutron background of the laboratory. Aim at the ultra low level of neutron background of China JinPing deep underground dark matter research laboratory, in this project we will study.(1) Using high pure materials, liquid scintillator loaded with Gd, to design and build a large volume fast neutron detector with high-sensitivity, high detection efficiency, and low self-background..(2) The liquid scintillator can separate the neutron and gamma according to their different pulse shape. And there is a special time interval between fast signal generated by neutron incident and slow signal generated by γ photons emitted after neutron has been captured by gadolinium, this can help to get better neutron and gamma separation performance..(3) The detector will be used to measure the very low flux neutron background of Jinping underground laboratory and provide important data of neutron background level for dark matter detection experiments.

暗物质的研究是当前粒子物理学、宇宙学、天体物理学等学科非常前沿的重大研究性课题，其瓶颈问题是实验上的突破。在暗物质探测实验中，本底中子与核碰撞在探测器介质中产生的信号与暗物质粒子WIMPs在探测器中产生的信号相似，无法区分。因此，暗物质探测实验需要本底极低的深地下实验室，并对实验室内的本底中子通量水平进行探测。针对中国锦屏地下暗物质探测实验室极低通量的本底中子水平，本项目拟研究（1）采用高纯度材料、载钆液体闪烁体，设计建造高灵敏度、高探测效率、自身放射性本底低的大体积快中子探测系统。（2）利用液闪本身的n/γ甄别特性，同时利用快中子在液闪中产生的快信号与中子慢化后被钆俘获释放的γ光子在液闪中产生的慢信号之间的时间关联，以实现高的n/γ分辨能力。（3）用该系统对锦屏地下实验室极低通量的本底中子进行测量，为暗物质探测实验提供本底中子通量水平数据。

设计并建造了低本底、高灵敏度、高探测效率的大体积液体闪烁体中子探测器，并对探测器进行了性能测试如能量刻度、效率刻度、n-gamma甄别等。使用此探测器对锦屏地下实验室大厅及聚乙烯屏蔽体内的中子本底进行了测量并得到了初步结果，测得大厅内中子通量在1-10 MeV能区为约1.50*10-7/cm2•s；在聚乙烯屏蔽体（1米厚）内的中子通量在1-10 MeV 能区为约4.27*10-9/cm2•s。如此低的中子本底水平为暗物质探测实验的开展提供了良好的环境。