Feasibility study of a shielding-independent radiation portal monitor system for revealing 241-Am orphan sources in radiometric surveillance of scrap metal

Orphan radioactive sources inadvertently melted in steel plants are a health and economical hazard. Thus, regulation authorities impose strict control measurements to be performed at these plants. However, a recent series of incidents related to the melting of 241Am sources demonstrated the inadequacy of even state of the art systems in revealing this low-energy gamma ray emitter up to activities of the order of hundreds GBq. However, 241Am sources are characterized by a weak neutron emission which can be detected by proper monitor systems. This work provides a feasibility study of a shielding-independent neutron-sensitive portal monitor system to be installed in steel plants. Measurements were performed both with a certified 241Am shielded source and at the weighing area of a steel plant, to assess both the detector performance and the practical feasibility of the system implementation in work-place fields. A risk evaluation analysis demonstrated that with the current control systems 241Am sources up to 185 GBq could pass undetected with a potential exposure for workers equal to 1.7 mSv, consistent with recent accidents reports. The proposed technique could reveal 241Am activities down to 2 GBq, decreasing the worker exposure down to 24 μ Sv , without interfering with the standard plant operations.