Continuous measurements of soil radon under regular field conditions

Continuous soil radon measurements were performed in the frame of an EU-radon network using the Clipperton II detector. It has been found that in some periods, soil radon levels obtained with one Clipperton II probe are very different from those obtained with another probe placed at the same depth but a short distance apart. Sudden fluctuations of soil radon levels in a given site have been observed as well. To determine whether this behaviour corresponds to the nature of radon gas in the soil or to the detection technique, various experiments have been performed. It has been observed that when different detectors are exposed at the same regular field conditions, their discrepancy is lower than 11%. When the soil radon concentration is approximately constant, the statistical fluctuations of the Clipperton probes are lower than 15%. The application of the Fourier-filtering algorithm to the rough data, removing all frequency components smaller than 1/24 h21 , has led to a complete correlation between different Clipperton II probes. The time response of the probes has been investigated in a specific experiment at the laboratory. It has been found that the response of the probes to a sudden change of radon concentration is controlled by the diffusion process along the bottom tube of the probe. Therefore, this study shows that the experimental data can be attributed to the natural behaviour of soil radon