A prompt gamma neutron activation analysis system with a 252Cf neutron source for on-line cement analysis has been simulated with the MCNP code.The results indicate that the optimum arrangement is a Bi shield of 20-mm thickness,a polyethylene moderator of 50-mm thickness,a source-to-sample distance of 70 mm,and cement samples of 1200 mm×600 mm×170 mm.To absorb thermal neutrons and suppress low-energy γ-rays,the optimum-sized sheets are 150 mm×7 mm Cd,and 150 mm×15 mm Pb. A prompt gamma neutron activation analysis system with a 252Cf neutron source for on-line cement analysis has been simulated with the MCNP code. The results indicate that the optimum arrangement is a Bi shield of 20-mm thickness, a polyethylene moderator of 50-mm thickness, a source-to-sample distance of 70 mm, and cement samples of 1200 mm × 600 mm × 170 mm. To absorb thermal neutrons and suppress low-energy γ-rays, the optimum-sized sheets are 150 mm × 7 mm Cd, and 150 mm × 15 mm Pb.