Radon effects in ground gamma-ray spectrometric surveys

The effect of emanation radon is shown to be a significant problem for estimating uranium concentration from ground-based gamma-ray spectrometric surveys. Radon gas (a daughter product in the U-238 decay series) escapes from rocks and soils near the Earth's surface into the lower atmosphere. Under early morning, still-air conditions, radon concentrates as a thin layer near the Earth's surface. If ground radiometric surveying is undertaken before this radon layer is mixed into the lower atmosphere, large errors in U concentration estimates result. This paper shows the effect of early-morning radon accumulation on a gamma-ray spectrometric survey near Boorowa, NSW. Paddocks surveyed in the early morning show much higher apparent uranium concentrations than those surveyed later in the day. We demonstrate the radon diurnal effect by monitoring the equivalent U concentration at a fixed site over several weeks. Typically, there is a build-up of radon near the Earth's surface overnight. Radon concentration reaches a maximum at about one hour after sunrise, before slowly dispersing over a period of 2-3 hours. By reporting both the Boorowa survey and the radon monitoring results in the same units of equivalent uranium concentration, we demonstrate that the anomalous effects seen in the Boorowa survey are consistent with that of a radon diurnal. The monitoring data also show the effect of rainfall on apparent U concentrations. Rain deposits radioactive daughter products of atmospheric radon onto the ground, resulting in a significant increase in apparent U concentration. These short-lived daughter products decay to insignificant concentrations within about 3 hours. Ground surveys should not be conducted during or within 3 hours of the cessation of rain, or under early-morning, still-air conditions. Also, because high moisture concentrations in soil reduces the gamma-ray fluence rate at the Earth's surface, surveys should not be conducted after prolonged or heavy rainfall where the soil profile is saturated.