  Several experiments have been conducted in the YangYang Underground
Laboratory in the Republic of Korea such as the search for dark matter and the
search for neutrinoless double-beta decay, which require an extremely low
background event rate due to the detector system and the environment. In
underground experiments, neutrons have been identified as one of the background
sources. The neutron flux in the experimental site needs to be determined to
design a proper shielding system and for precise background estimation. We
measured the neutron spectrum with a Bonner sphere spectrometer, with Helium-3
($^{3}$He) proportional counters. The neutron flux at the underground
laboratory was so low that the radioactive decays from the radioisotopes
contained in the detector created a significant background interference to the
neutron measurement. Using Monte Carlo simulations, the intrinsic $\alpha$
background distribution due to the radioactive isotopes in the detector
materials, was estimated. The neutron count rate of each Bonner sphere was
measured from the pulse height spectrum of the $^{3}$He proportional counter,
after subtracting the $\alpha$ particle background. The neutron flux and the
energy spectrum were determined using the unfolding technique. The total
neutron flux measured was (4.46 $\pm$ 0.66) $\times$ $10^{-5}$ $\rm{cm^{-2}
s^{-1}}$, and the thermal and fast neutron flux (in the range 1 to 10 MeV) were
(1.44 $\pm$ 0.15) $\times$ 10$^{-5}$ $\rm{cm^{-2} s^{-1}}$ and (0.71 $\pm$
0.10) $\times$ 10$^{-5}$ $\rm{cm^{-2} s^{-1}}$, respectively.

Nuclear Experiment