Neutron spectrum at the underground laboratory for the ultra low background experiment.

We measured the background neutron spectrum using a Bonner sphere spectrometer at the YangYang underground laboratory, where the vertical earth overburden is approximately 700 m. The neutron fluence rate and the energy spectrum were determined through the unfolding procedure. The measured neutron fluence rate was (0.242 ± 0.008) n cm(-2)h(-1) for k=1, and most of the neutrons were below 20 MeV.