Answer to Question #12507 Submitted to "Ask the Experts"

While performing a recent survey in northern Montana, I was conducting background readings outside a structure of concern. The readings ranged from 10–14 microRoentgens per hour (µR h^-1) with a Ludlum 44-2 NaI probe.¹ These numbers are consistent with background exposures I would expect. While surveying a structure (vertical steel piping and partially covered tin roof), however, we noticed that typical indoor measurements of piping (8–12 µR h^-1) coincidentally elevated to 11–16 µR h^-1 during heavy cloud cover and a heavy rainstorm.

Upon completion of the survey, a revisit to the original background reading location showed readings now of 16–22 µR h^-1 with heavy cloud cover and saturated ground soils. The instrument response was checked with a check source before the work began and at completion, and check source readings were identical. Could the climatic change between the beginning of the survey and completion of the survey have impacted the background gamma readings and why?

Your inference that changes in the weather might have produced elevations in the background readings are most legitimate. The phenomenon of "radon washout" from the atmosphere is well-documented during periods of precipitation. In particular, the short-lived progeny of radon, especially lead-214 (²¹⁴Pb) and bismuth-214 (²¹⁴Bi), are carried to the surface of the earth and produce measurable increases in the gamma background.

Additionally, radon and progeny concentrations in air may increase under other atmospheric conditions, especially during episodes of temperature inversions in the atmosphere. Such inversions are often enhanced by significant cloud cover, which tends to trap heat released from the earth, causing an inverse temperature profile at low altitudes. Such inversions trap pollutants, including radon and progeny near the surface of the earth, adding to an increased background level that may be aggravated by precipitation that carries progeny to the ground.

Another weather-related factor affecting radon concentrations is barometric pressure. As pressure decreases, as it often does under adverse weather conditions, the release rate of radon from the earth is increased near the surface of the earth, producing potentially higher airborne radon and progeny concentrations.

Finally, the NaI probe that you are using is significantly photon-energy dependent in its response. Depending especially on what photon energies were used to calibrate the instrument, the background readings could be enhanced.

George Chabot, PhD

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1} Although the Health Physics Society endorses the use of International System (SI) units, some instruments still provide measurements in traditional units (in this situation, µR h^-1).