环境中子测量中的偶然符合alpha事例修正

Accidental alpha-event correction in the measurement of environmental neutrons

摘要:

在暗物质直接探测实验中，中子是十分重要的环境本底. 利用快慢符合法，一个掺钆的液闪探测器被分别用于测量了世界最深的地下实验室—中国锦屏地下实验室的实验大厅及1 m厚聚乙烯房间内的快中子本底通量及能谱. 由于液闪无法通过信号波形区分中子与alpha事例，因此液闪中长寿命的U/Th核素衰变放出的alpha粒子是必须考虑的本底之一. 在长期平衡假设条件下，实验测得液闪内部alpha粒子的总计数率为 (0.548 ± 0.002) s-1，再通过偶然符合算法，可得到实验大厅356天总计2682个中子候选事例中，有6个偶然符合的alpha事例；聚乙烯房间173天总计44个中子候选事例中，有2个偶然符合的alpha事例. 偶然符合的alpha粒子对大厅的快中子通量测量结果影响可以忽略，而对聚乙烯房间中的通量结果约有4%的影响.

Abstract:

In the dark matter direct detection experiment, neutrons are important environmental background. In the China Jinping Underground Laboratory (CJPL), the world’s deepest underground laboratory, a gadolinium doped liquid scintillator (Gd-LS) detector was used to measure the fluxes and spectra of fast neutron background in the Hall and in a room with one meter thick polyethylene (PE) walls by fast-slow time coincident method. Liquid scintillators are incapable to discriminate neutrons and alpha-particles by their pulse shapes. Thus, the alpha-particle background from the long-lived U/Th nuclides inside the liquid scintillator has to be taken into account. Assuming secular equilibrium, the total rate of the alpha-decays in the detector was measured to be (0.548 ± 0.002) s-1. Then in the Hall, the number of random coincident alpha-events was determined to be 6 out of 2682 neutron candidates based on 356 days of data. And for the PE room, this value is 2 out of 44 neutron candidates with 173 days of data. The influence of the random coincident alpha-particles are negligible for the result of fast neutron flux in the Hall, but have around 4% for the measurement in the PE room.