Rain-induced increase in background radiation detected by Radiation Portal Monitors

• Published in: Journal of environmental radioactivity Vol. 137; pp. 137 - 141
• Main Authors: Livesay, R.J., Blessinger, C.S., Guzzardo, T.F., Hausladen, P.A.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01-11-2014; Elsevier
• Subjects: Analysis methods; Applied sciences; Atmospheric pollution; Background Radiation; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental radiation; Exact sciences and technology; Gamma Rays; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Pollution, environment geology; Portal monitors; Radiation Monitoring - methods; Radioisotopes; Radon; Radon Daughters - analysis; Rain; Tennessee; Tennessee; Radioisotopes; Environmental radiation; Radon; Background radiation; Portal monitors; Performance evaluation; Natural origin pollution; Radioactive pollution; Natural radioactivity; Wet deposition; Air radioactivity; Radioactive fall out; Radioactivity measurement; Measuring instrument; Carcinogen; Radon 222; Gamma radiation; Lead 214; Air pollution; Portable equipment; Bismuth 214; Daughter product
• ISSN: 0265-931X; 1879-1700
• DOI: 10.1016/j.jenvrad.2014.07.010

A complete understanding of both the steady state and transient background measured by Radiation Portal Monitors (RPMs) is essential to predictable system performance, as well as maximization of detection sensitivity. To facilitate this understanding, a test bed for the study of natural background in RPMs has been established at the Oak Ridge National Laboratory. This work was performed in support of the Second Line of Defense Program's mission to enhance partner country capability to deter, detect, and interdict the illicit movement of special 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, which has been well documented in the field of environmental radioactivity, originates primarily from the wet-deposition of two radioactive daughters of 222Rn, namely, 214Pb and 214Bi. In this study, rainfall rates recorded by a co-located weather station are compared with RPM count rates and high-purity germanium spectra. The data verify that these radionuclides are responsible for the largest environmental background fluctuations in RPMs. Analytical expressions for the detector response function in Poly-Vinyl Toluene have been derived. Effects on system performance and potential mitigation strategies are discussed.
•Analytical response-function of Radiation Portal Monitors to wet-deposition of radioactive radon-daughters.•Spectral proof of constituent radio-isotopes in rain (using HPGe), directly correlated with RPM response.•Direct proof of the independence of RPM rain-response and atmospheric pressure.•Future study will be able to estimate global/seasonal activity-density of rain from RPM responses.•Future automation of analysis will include filters based on characteristic RPM response.