Improving Computational Efficiency in Monte Carlo Simulation for Radiation Portal Monitor

\nAt the border and airports, a radiation portal monitor (RPM) system has been installed and is operated to detect illegal radioactive materials in cargo containers. To evaluate the RPM's performance and determine the optimized alarm criteria in various conditions, the Monte Carlo simulation technique is considered essential; however, the calculation efficiency has to be improved when we are dealing with complex geometries and a wide range of source term (e.g., a few hundred meters in diameter). In the present study, the applicability of the various techniques to improve calculation efficiency was evaluated. For example, as a simple but highly effective way, a simple box was used to surround the complex geometry of the RPM, resulting in 1.75 times of improvement in the calculation efficiency. Additionally, when the simple box and Surface Source Write and Read (SSW/SSR) techniques were combined, the computational efficiency increased by 374 times. These results could significantly enhance the calculation efficiency of the Monte Carlo simulation, not only for the RPM but also for other radiation detectors in radiation transport applications.\n