Mitigation of synchrotron radiation background at the CEPC interaction region for the Vertex Detector

Abstract Synchrotron radiation (SR) background is a significant challenge for the Vertex Detector (VTX) at the proposed Circular Electron Positron Collider (CEPC). This study presents a comprehensive mitigating system designed to mitigate this background under Higgs operation mode. A simulation framework was developed to evaluate the combined SR generated from the last bending dipoles, final focusing quadrupoles, and the detector solenoid, assuming a transverse beam distribution up to 10σ. This framework was used to systematically design and optimize a mitigating system comprising an external mitigating wall, an internal collimating mask, and a gold coating on the central Be beam pipe. The performance of the final configuration — a single 4 mm-high W mask at z = -1.9 m, an external mitigating wall, and a 10 μm Au layer on the Be pipe — was validated. Simulations predict this system suppresses the total SR background to a VTX hit density of 0.048 MHz/cm 2 on the innermost layer, a value well within the operational tolerance and providing a critical reference for the engineering design of the CEPC interaction region.