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.