209Bi(n,γ)210gBi快中子截面离线测量的本底放射性问题

Background]:Lead bismuth eutectic alloy (LBE:44.5 wt.%Pb+55.5 wt.%Bi) is an important coolant candidate for fourth-generation reactors due to its good physical and chemical properties. However, the risk of radioactive polonium radiation protection caused by liquid lead-bismuth alloy as the main coolant of the reactor has become an important problem in the research and development of lead-bismuth reactors. The 209Bi(n,)210gBi reaction is a key source term for 210Po generation in lead-bismuth reactors, and accurate measurement of the 209Bi(n,)210gBi reaction cross-section is very important for assessing the risk of radioactive polonium release in lead-bismuth reactors.[Purpose]: The objective of this study is to investigate the self-background radioactivity issues in bismuth targets during the cross-section measurement of 209Bi(n,)210gBi reactions, specifically focusing on the concentration levels and source tracing analysis of 210Po in bismuth targets. Furthermore, the research evaluates the practical impact of this background radiation on cross-section measurements based on real-world experimental conditions. Finally, recommendations for material selection criteria of bismuth targets are provided to optimize measurement accuracy. [Methods]: The analysis of polonium background concentration in bismuth targets is based on the spontaneous deposition technique. By leveraging the differences in electrochemical potential sequences between the substrate electrode and polonium ions (as well as other interfering ions), a high enrichment of polonium on a silver substrate material is achieved. This process effectively separates interfering elements. The prepared nanoscale thin-film source is then measured and analyzed in a low-background alpha detection system for subsequent evaluation. [Results]: The experimental results show that the concentration of radioactive Po in pure bismuth material varies greatly. The traceability analysis confirms that the target material contains different amounts of 210Pb, 210Bi, and 210Po and does not meet the condition of radioactive equilibrium. The influence of radioactive polonium in the target material on the measurement of the average energy position of the fission reactor 209Bi(n,)210gBi cross section was evaluated using Talys-2.0. [Conclusions]: The theoretical analysis results show that the background level of radioactive polonium in low-background target material can be controlled below 1% of the irradiation yield, which will effectively improve the precision of the 209Bi(n,)210gBi cross section measurement.