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We report on the measurements of the fluxes and spectra of the environmental fast neutron background at the China Jinping Underground Laboratory (CJPL) with a rock overburden of about 6700 meters water equivalent, using a liquid scintillator detector doped with 0.5% gadolinium. The signature of a prompt nuclear recoil followed by a delayed high energy γ-ray cascade is used to identify neutron events. The large energy deposition of the delayed γ-rays from the (n, γ) reaction on gadolinium, together with the excellent n-γ discrimination capability provides a powerful background suppression which allows the measurement of a low intensity neutron flux. The neutron flux of (1.51±0.03 (stat.)±0.10 (syst.))×10 −7 cm −2 s −1 in the energy range of 1 -10 MeV in the Hall A of CJPL was measured based on 356 days of data. In the same energy region, measurement with the same detector placed in a one meter thick polyethylene room gives a significantly lower flux of (4.9 ± 0.9 (stat.) ± 0.5 (syst.)) × 10 −9 cm −2 s −1 with 174 days of data. This represents a measurement of the lowest environmental fast neutron background among the underground laboratories in the world, prior to additional experiment-specific attenuation. Additionally, the fast neutron spectra both in the Hall A and the polyethylene room were reconstructed with the help of GEANT4 simulation. 1 Main contributor underground site is produced in the rock through the spontaneous fission of 238 U and the (α, n) reactions of light nuclei bombarded by the α-particles emitted in the U/Th decay chains. An additional neutron background is taken along with the infrastructure and the experimental setup of laboratories. Nuclear recoils and other interactions due to the environmental neutron background can restrict experimental sensitivities or bring false positive signals in the studies of rare phenomena. Therefore, the understanding of neutron spectrum and identifying neutron sources are substantial issues for background reduction and guide the design of shielding systems for the next generation of large-scale experiments.The China Jinping Underground Laboratory

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In this paper, we apply the binary Bell polynomial approach to high-dimensional variable-coefficient nonlinear evolution equations. Taking the generalized (2+1)-dimensional KdV equation with variable coefficients as an illustrative example, the bilinear formulism, the bilinear Bäcklund transformation and the Lax pair are obtained in a quick and natural manner. Moreover, the infinite conservation laws are also derived.

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