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

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1m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.

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.