Measurement of the neutron flux in the CPL underground laboratory and simulation studies of neutron shielding for WIMP searches

Searches for weakly interacting massive particles (WIMPs) can be carried out based on the detection of nuclear recoil energy in CsI(Tℓ) crystals. It is crucial to minimize the neutron background as well as to fully understand the remaining background sources through adequate shielding when using the pulse shape discrimination method for WIMP detection. We have measured the neutron flux at 350 m minimum depth, where the CheongPyung underground laboratory (CPL) is located, to be 3.00 ± 0.02 (stat.) ± 0.05 (syst.) × 10−5 neutrons/cm2/s with the neutron energy in the range 1.5<En<6 MeV. Using the GEANT4 simulation, we have demonstrated that the neutron flux can be reduced sufficiently for dark matter searches with 30 cm of polyethylene passive shield and 20 cm of BC501A liquid scintillator active shield. The neutron induced background at a few keV energy deposit in CsI crystal is less than 0.001 counts/keV/kg/day. An active shield not only reduces the neutron background but can also reduce the uncertainty in the neutron background estimation.