  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.
