포털 모니터의 배경 방사선 평가를 위한 자연방사선 모델 개발
Development of Natural Radiation Model for Evaluation of Background Radiation in Radiation Portal Monitor
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In ports and airports, radiation portal monitors (RPM) are deployed to detect illicitradioactive materials. Detected gamma rays in a RPM include background radiation and radiationfrom a freight. As a vehicle moves through the RPM, the vehicle causes the fluctuations in thenatural background radiation signal, which ranges of up to 30%. The fluctuation increases theuncertainty of detection signal and can be a cause of RPM false alarm. Therefore, it is importantto evaluate background radiation as well as radiation from a container. In this paper, a naturalbackground radiation model was developed to evaluate RPM. To develop natural backgroundradiation model, a Monte Carlo simulation was performed and compared with experimentalmeasurements from a RPM for 40K, 232Th series, and 235U series, which are major sources ofnatural background radiation. For a natural radiation source, we considered a cylindrical soilvolume with 300 m radius and 1 m depth, which was estimated as the maximum range affectingthe RPM by MCNP6 simulation. The volume source model was converted to surface source byusing MCNP SSW card for computational efficiency. The computational efficiency of the surfacesource model was improved to approximately 200 times better than that of the volume sourcemodel. The surface source model is composed of a hemisphere with 20 m radius in which the RPMand container are modelled. The natural radiation spectrum from the simulation was best fitted tothe experimental measurement when portions of 40K, 232Th series, and 235U series were 0.75, 0.0636,and 0.0552 Bq·g-1, respectively. For gross counting results, the difference between simulation andexperiment was around 5%. The background radiation model was used to evaluate backgroundsuppression from a 40 ft container with 7.2 m·s-1 speed. In further study, background modelsand freight models for RPM in real container ports will be developed and applied to optimize theoperating system of port security.
Abstract
In ports and airports, radiation portal monitors (RPM) are deployed to detect illicitradioactive materials. Detected gamma rays in a RPM include background radiation and radiationfrom a freight. As a vehicle moves through the RPM, the vehicle causes the fluctuations in thenatural background radiation signal, which ranges of up to 30%. The fluctuation increases theuncertainty of detection signal and can be a cause of RPM false alarm. Therefore, it is importantto evaluate background radiation as well as radiation from a container. In this paper, a naturalbackground radiation model was developed to evaluate RPM. To develop natural backgroundradiation model, a Monte Carlo simulation was performed and compared with experimentalmeasurements from a RPM for 40K, 232Th series, and 235U series, which are major sources ofnatural background radiation. For a natural radiation source, we considered a cylindrical soilvolume with 300 m radius and 1 m depth, which was estimated as the maximum range affectingthe RPM by MCNP6 simulation. The volume source model was converted to surface source byusing MCNP SSW card for computational efficiency. The computational efficiency of the surfacesource model was improved to approximately 200 times better than that of the volume sourcemodel. The surface source model is composed of a hemisphere with 20 m radius in which the RPMand container are modelled. The natural radiation spectrum from the simulation was best fitted tothe experimental measurement when portions of 40K, 232Th series, and 235U series were 0.75, 0.0636,and 0.0552 Bq·g-1, respectively. For gross counting results, the difference between simulation andexperiment was around 5%. The background radiation model was used to evaluate backgroundsuppression from a 40 ft container with 7.2 m·s-1 speed. In further study, background modelsand freight models for RPM in real container ports will be developed and applied to optimize theoperating system of port security.
참고문헌 (16)
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