Rain-Induced Increase in Background Radiation Detected by Radiation Portal Monitors

A complete understanding of both the steady state and transient natural background in radiation portal monitors (RPMs) is essential to predictable system performance as well as maximization of detection sensitivity. To facilitate this understanding, a test bed (the Portal Monitor Test Bed) for the study of steady state and transient natural background in RPMs has been established at Oak Ridge National Laboratory in support of the mission of the Second Line of Defense Program to detect the illicit movement of nuclear material. In the present work, transient increases in gamma-ray counting rates in RPMs due to rain are investigated. The increase in background activity associated with rain has been well documented in the field of environmental radioactivity and originates from the atmospheric deposition of two radioactive daughters of 222Ra, namely 214Pb and 214Bi. In this work, RPM count rates are directly compared with rainfall rates recorded by a co-located weather station. The data verify that these radionuclides are responsible for the dominant transient natural background fluctuations in RPMs.