Effects of rainfall and temperature on background signal of radiation portal monitor.

Abstract

For homeland security, radiation portal monitors (RPMs) have been widely used to detect illegal radioactive materials at seaports, airports, nuclear facilities and other highly secured establishments. In general, commercial RPMs are based on a large plastic (i.e. PVT-polyvinyl toluene) scintillator detector and the associated electronics. In order to detect radioactive materials passing through the RPM, the alarm criterion should be set according to the background level, which depends on the operating location due to not only composition differences in soil and rocks but also weather variation (i.e. rainfall and temperature). It is well known that RPM background signal level increases with rainfall and that the PVT signal depends on the temperature due to scintillation light yield variation. In this study, the background signal level of two commercial RPMs (models: 4525-3800 and 7000, Ludlum), both of which are installed and operated at the Incheon and Donghae ports in Korea, was analysed by reference to a 3-year database of minute-to-minute RPM background signals and a rainfall-and-temperature database provided by the Korea Meteorological Administration (KMA). In terms of rainfall, the variation of the background signal level was examined with reference to the amount of rainfall. The averaged variation in the background signal level, which was as high as ~20% as a function of the amount of rainfall, was found to be dependent on a region's specific concentration of 222Rn in the atmosphere. In terms of temperature, the background signal level varied by ~4.7% within the temperature range of -5 to 30°C for the four studied sites (i.e. two sites per region for the Incheon and Donghae regions). Knowledge of the dependency of RPM background signal level on rainfall amount and temperature could be used to achieve more realistic estimation of the background radiation level for optimising commercial RPMs' alarm criteria.