Accidental alpha-event correction in the measurement of environmental neutrons

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.