Design Of Moderator-Collimator Based On Compact D-D Neutron Generator And Simulation Study Of Thermal Neutron Imaging

As an important nondestructive testing technology,thermal neutron radiography has great potential in fields like aerospace,industry,scientific research,national defense.To further broaden thermal neutron radiography's application,a portal miniaturized neutron imaging system is urged to be developed.Now,Lanzhou University's compact D-D neutron generator has the capacity of being designed into a manageable miniaturized neutron source,which laid a valid foundation for the development of miniaturized thermal neutron imaging.Normally,the energy of the fast neutron generated by the D-D neutron generator is 2.5 MeV,it has to be moderated to obtain the needed thermal neutron beam.This paper will base on Lanzhou University's compact D-D neutron generator to simulate and design a moderator-collimator which is going to be employed in thermal neutron radiography,preparing for the next step of developing portal miniaturized neutron imaging system on the basis of compact D-D neutron generator.According to neutron moderation theory,this paper built the model of D-D neutron source,geometry,and materials in MCNP5,targeting materials like polyethylene?PE?,water?H₂O?,heavy water?D₂O?,graphite?C?,etc.the simulations of neutron transportation were conducted.By studying materials'neutron-moderating behavior,PE and D₂O were selected as the main moderator,among which the thickness of PE is suggested at the range of 5-8 cm while the D₂O is at least 10 cm.Initially,the basic structure of the neutron moderator-collimator was determined based on the structure of the D-D neutron generator.This moderator-collimator was a multi-layered cube,whose each layer was designed as the main moderator,neutron collimator,neutron absorber,and?absorber from the bottom up,respectively.After determining the structure,the model of neutron source and moderator-collimator were constructed in MCNP,with PE and D₂O being the materials of the main moderator separately,neutron transportation's Monte Carlo simulation of four material combinations were performed and the optimization of moderator-collimator's structure was completed from the simulation.By comparing the feature of the collimated neutron beam,this paper proposed two material combinations:scheme 1-2 adopts D₂O as the main moderator,the corresponding moderator-collimator has the geometry of60×60×34 cm³,and the specific material of layers are 15 cm-D₂O+12 cm-PE+5 cm-?PE+8%B?+2 cm-Pb;scheme 2-2 adopts PE as the main moderator,the corresponding moderator-collimator has the geometry of 60×60×29 cm³,and the specific material of layers are 6 cm-PE+16 cm-PE+5 cm-?PE+8%B?+2 cm-Pb.Ultimately,the simulation of thermal neutron imaging was carried out in Geant4,the simulation model of the imaging system was constituted as follows:neutron beam+image quality indicator+⁶LiF?ZnS?scintillation screen+photon-counting plane.In this part,the parallel monoenergetic thermal neutron beam and parallel collimated thermal neutron beam were used to create the images which show that image taken by parallel monoenergetic thermal neutron beam was clearer,and its spatial resolution was better than 50?m calculating by circular slit edge line spread function's?LSF?half-width.Meanwhile,the image created by the latter was relatively vague,its spatial resolution was worse than 50?m.Though better spatial resolution could be achieved in the simulation model,in the experiments,the neutron beam is not strictly parallel and the neutron fluence might not be uniform,those factors would all influence the quality of the image.