Study of fast neutron detector for COSINE-100 experiment

A monitoring system for fast neutrons is planned in the COSINE experiment, a dark matter experiment with NaI crystals. We pursued several R&D approaches for a neutron detector using a liquid scintillator (LS). A pulse shape discrimination (PSD) technique is used for the identification of neutron events and the PSD properties of two different LS were compared. A good separation power between neutrons and $\gamma$ has been achieved for energies between 200 keVee to 1500 keVee. The combination of alumina adsorption, filtration, and water extraction is effective in purifying the LS, which leads to a reduction in the $\alpha$ contamination by $^{210}$Po of more than a factor of two. The measured activities of the internal $\alpha$ are 0.36$\pm$0.04~mBq/kg and 0.21$\pm$0.03~mBq/kg before and after purification, respectively.