Shielding optimization of Am-Li neutron in new pile laboratory-neutron multiplicity counter

In order to reduce Am-Li neutron background that had affected active neutron multiplicity assay for highly enriched uranium, optimization shielding of Am-Li was simulated for New Pile Laboratory-Neutron Multiplicity Counter (NPL-NMC). A cylindrical high density polyethylene was chosen as neutron shield with the radius of 15 cm. By comparing simulation results with the shielding to experimental results without the shielding, the Am-Li neutron background detection efficiency was significantly reduced, from 15.77% to 1.94%, reducing by about 87.7%. But shielding effect on fission neutron count was relatively small, only about 2.4% lower than the original. Reducing of the background neutron counts significantly improved the mass measurement sensitivity of uranium components. Mass measurement limit of uranium components dropped to 2.6 kg from the original about 6.4 kg with 3000 s counting time. Meanwhile, the measurement accuracy of uranium components was significantly improved in NPL-NMC, which could be increased by 50% under the same measurement conditions.