[en] In the inertial confinement fusion (ICF) experiment, down-scattered neutrons for fuel areal density measurement need to be separated from other background. The background on Shenguang Ⅲ facility is analyzed in detail by Monte Carlo for the DD fuel. As a result, the background is mostly from the process that primary neutrons scatter through vacuum chamber, surrounding diagnostic platforms, ceiling and floor. According to the analysis, appropriate materials and shielding solution are chosen. And the structure of the collimator is optimized by Monte Carlo. The design meets the experimental requirements. (authors)