Feasibility study for radionuclide identification using multi-array plastic scintillator and energy weighted algorithm of radiation portal monitors: A Monte Carlo study

Radiation portal monitor (RPM) systems have been deployed worldwide to monitor the inflow of illegal radionuclides at border crossings. Large-sized plastic scintillation detector has been widely used for RPM system, but due to the components of the plastic scintillator of low Z-number material the poor energy resolution shows the broad energy spectrum without clear photo peak identifying the radionuclides. The aim of this study is to develop an RPM based on the scintillator array for enhancing the detection efficiency and energy resolution. Using a Monte Carlo (MC) method, an RPM comprising the multi-array hexagonal scintillator was simulated with varied diameters of 8-24 cm and thicknesses of 5-25 cm, and the performance was assessed with four radionuclides of ¹³⁷Cs, ⁶⁰Co, ²²⁶Ra, and ⁴⁰K. To determine the optimal scintillator dimensions, the generation and collection efficiency of optical photons were assessed. The performance of the multi-array scintillator was evaluated by comparing it with the conventional single scintillator in terms of the gamma-collection efficiency and the energy resolution with the full width at half maximum (FWHM) of the energy weighted spectrum. The plastic scintillator of 8 cm diameter and 15 cm thickness was determined to have the optimal dimensions out of the 25 geometry conditions. The multi-array plastic scintillator showed an increased optical photon-collection probability, up to 35% more than a conventional plastic scintillator, while the FWHM decreased by more than 20%, resulting in an improved energy resolution. Based on this MC-based design and optimization study, a proof-of-principle detection system is currently being built and an experimental study to demonstrate its superior accuracy in the identification of radionuclides will also be conducted in the future.