Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1738688 | Journal of Environmental Radioactivity | 2011 | 17 Pages |
Large temporal variations of radon (222Rn) are often encountered in air in the geologic environment, at time scales from diurnal to annual. Interpretations as to the nature of these variations, unique to 222Rn, often invoke either above surface atmospheric variations, or the influence of subtle active geodynamic processes. So far the eventual geophysical drivers of the variation of 222Rn as well as its specific qualities enabling this temporal variation are not known. New insight on the temporal variation of 222Rn is gained by experimental simulation in confined air. Two short laboratory experiments, and one external experiment lasting over 3 years, were performed inside closed canisters and using natural and commercial 222Rn sources. Internal and external gamma and alpha detectors recorded variations of the radiation, up to around 20% of the equilibrium level. Radon signals of different time scale occurred with: a) periodic annual and semi-annual signals; b) non-periodic multi-day signals; c) periodic daily signals. Similar, related, inversely-related and dissimilar temporal patterns were manifested in the measured time series of the different sensors. Diurnal periodicity was dominated by the solar tide components S1, S2 and S3, exhibiting unlike relative amplitudes and different phases at the different sensors. A compound association occurs among the amplitudes and phases of the diurnal and seasonal periodicities of the daily 222Rn signal, linking the periodic phenomena to the rotation of earth around its axis and around the sun. 222Rn variation patterns in the frequency–time domain cannot be driven by the corresponding atmospheric variation patterns. These results, obtained under static and isolated conditions, are in disagreement with the expected radioactive equilibrium and its spatially uniform expression within and around the experimental volume. The external influence which drives the daily signals evolving from 222Rn inside the canister is non-atmospheric and seemed to be from a remote source and traversed a 5-cm thick lead shield. The similarities with observations on 222Rn signals from upper crustal levels imply that such an external influence, possibly as a component of solar irradiance, drives the 222Rn signals to a depth of at least 100 m. New combined prospects for the research are indicated in terms of the radioactive behavior of 222Rn in air and in terms of an above surface geophysical driver for this behavior.
► Experiments simulate radon signals in subsurface air at diurnal to annual scale. ► Signals of daily to semi-annual and annual scale reflect solar tidal periodicities. ► Dissimilar and compounded associations occur at different spatial directions. ► An above surface geophysical driver of radiation from radon is indicated. ► A component of solar irradiance probably drives radon signals to a depth of 100 m.