Understanding the radiation background at the north crossing point (NCP) in the tunnel of BEPCII is crucial for the performance safety of the High Purity Germanium (HPGe) detector, and in turn of great significance for long-term stable running of the energy measurement system. Therefore, as the first step, a NaI(Tl) detector is constructed to continuously measure the radiation level of photons as background for future experiments. Furthermore, gamma and neutron dosimeters are utilized to explore the radiation distribution in the vicinity of the NCP where the HPGe detector will be located. Synthesizing all obtained information, the shielding for neutron irradiation is studied based on model-dependent theoretical analysis.

The monocrystalline silicon neutron beam window is one of the key components of neutron spectrometers and thin circular plate.Monocrystalline silicon is a brittle material and its strength is not constant but is consistent with the Weibull distribution. The window is designed not simply through the average strength, but according to the survival rate. Bending deformation is the main form of the window, so dangerous parts of the neutron beam window is stress-linearized to the combination of membrane stress and bending stress. According to the Weibull distribution of bending strength of monocrystalline silicon based on a large number of experimental data, finally the optimized neutron beam window is 1.5mm thick. Its survival rate is 0.9994 and its transmittance is 0.98447; it meets both physical requirements and the mechanical strength.

The proton beam window (PBW) is one of the key devices of China Spallation Neutron Source (CSNS). It is the boundary between transport line and target. This paper will present a new PBW structure and detailed thermal-stress analysis. The energy deposition and scattering effect need to be low when the beam passes through the PBW, so proper selection of material and structure is important. According to the study of energy deposition, A5083-O is selected as the PBW material. A single-double layer structure is first proposed based on the study of cooling structures. Thermal analysis and structural optimization are discussed, and transient analysis is done to show the effect of the beam pulse. Besides, safety is confirmed for cases of cooling tunnel blockage, beam profile shrinkage, or centroid orbit offset. All these analyses show the newly designed PBW structure can meet the requirements of the CSNS well.