Study Of Shielding Design For SANS At CSNS

Neutron scattering technology is a powerful tool in detecting the microstructures of materials. The small angle neutron scattering (SANS) instrument is one of the three neutron science instruments that are under construction on phase one of the China Spallation Neutron Source (CSNS) project, and it will become a great platform for research and development of neutron scattering technology. The design and optimization of the radiation shielding system is very important to the construction and operation of the instrument, as it guarantee the radiation safety of personnel, bring convenience to engineering works and save costs. The shielding design of this large and complex instrument is usually needed for Monte Carlo (MC) calculation. In this thesis, sophisticated variance reduction techniques are implemented based on the characters of SANS instrument, thus the study of shielding design have been made by the efficient MC calculation.The particle transport Monte Carlo code FLUKA is employed to carry out the calculations. The application of FLUKA in simulating radiation transport through bulk shielding materials (concrete and iron) is studied and verified at first. By solving one-dimensional deep penetrating problem, the use of FLUKA variance reduction techniques is also validated. Then based on the characters of SANS structure and neutron source term, a very efficient MC calculation method is developed for the shielding calculation. In this method, the geometry importance, source biasing and two-step thecniques are used in combination.Beam line shielding calculations are performed considering both scenarios of closed versus open TO chopper. The shielding parameters are determined according to the calculated radiation dose rate distributions. Attenuation curves of different shielding configurations are calculated and the shielding thickness is optimized accordingly. The shielding on the top of the sample chamber is further optimized for the mechanical operations. The TO chopper can suppress the background by shielding the incident high energy neutrons with its rotor. The proper size of the rotor is estimated to guarantee the realization of the function of TO chopper. Radioactivity of the rotor is calculated to provide a valuable reference for its material selection and maintenance work. The basic design scheme of the beam stop is also discussed, and the current design can settled for the requirement of radiation safty, while suppressing the backscattering neutron background.