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