Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)

The Peking University Neutron Imaging Facility (PKUNIFTY) is a Radio Frequency Quadruple (RFQ) accelerator based system. The fast neutrons are produced by 2 MeV deuterons bombarding beryllium target. The moderator, reflector, shielding and collimator have been optimized with Monte-Carlo simulation to improve the neutron beam quality. The neutrons are thermalized in water cylinder of Φ26×26 cm² with a polyethylene disk in front of Be target. The size of deuteron beam spot is optimized considering both the thermal neutron distribution and the demand of target cooling. The shielding is a combination of 8 cm thick lead and 42 cm thick boron doped polyethylene. The thermal neutrons are extracted through a rectangular inner collimator and a divergent outer collimator. The thermal neutron beam axis is perpendicular to the D⁺ beam line in order to reduce the fast neutron and the γ ray components in the imaging beam. When the neutron yield is 3×10¹² n/s and the L/D is 50, the thermal neutron flux is 5×10⁵ n/cm²/s at the imaging plane, the Cd ratio is 1.63 and the n/γ ratio is 1.6×10¹⁰ n/cm²/Sv.