D. Morelli

Comparison between different methodologies for detecting radon in soil along an active fault: The case of the Pernicana fault system, Mt. Etna (Italy)

Three different methodologies were used to measure Radon ((222)Rn) in soil, based on both passive and active detection system. The first technique consisted of solid-state nuclear track detectors (SSNTD), CR-39 type, and allowed integrated measurements. The second one consisted of a portable device for short time measurements. The last consisted of a continuous measurement device for extended monitoring, placed in selected sites. Soil (222)Rn activity was measured together with soil Thoron ((220)Rn) and soil carbon dioxide (CO(2)) efflux, and it was compared with the content of radionuclides in the rocks. Two different soil-gas horizontal transects were investigated across the Pernicana fault system (NE flank of Mount Etna), from November 2006 to April 2007. The results obtained with the three methodologies are in a general agreement with each other and reflect the tectonic settings of the investigated study area. The lowest (222)Rn values were recorded just on the fault plane, and relatively higher values were recorded a few tens of meters from the fault axis on both of its sides. This pattern could be explained as a dilution effect resulting from high rates of soil CO(2) efflux. Time variations of (222)Rn activity were mostly linked to atmospheric influences, whereas no significant correlation with the volcanic activity was observed. In order to further investigate regional radon distributions, spot measurements were made to identify sites having high Rn emissions that could subsequently be monitored for temporal radon variations. SSNTD measurements allow for extended-duration monitoring of a relatively large number of sites, although with some loss of temporal resolution due to their long integration time. Continuous monitoring probes are optimal for detailed time monitoring, but because of their expense, they can best be used to complement the information acquired with SSNTD in a network of monitored sites.

Radon as Earthquake Precursor

Earthquake predictions are based mainly on the observation of precursory phenomena. However, the physical mechanism of earthquakes and precursors is at present poorly understood, because the factors and conditions governing them are so complicated. Methods of prediction based merely on precursory phenomena are therefore purely empirical and involve many practical difficulties. A seismic precursor is a phenomenon which takes place sufficiently prior to the occurrence of an earthquake. These precursors are of various kind, such as ground deformation, changes in sea-level, in tilt and strain and in earth tidal strain, foreshocks, anomalous seismicity, change in b-value, in microsismicity, in earthquake source mechanism, hypocentral migration, crustal movements, changes in seismic wave velocities, in the geomagnetic field, in telluric currents, in resistivity, in radon content, in groundwater level, in oil flow, and so on. These phenomena provide the basis for prediction of the three main parameters of an earthquake: place and time of occurrence and magnitude of the seismic event. The most important problem with all these precursors is to distinguish signals from noise. A single precursor may not be helpful, the prediction program strategy must involve an integral approach including several precursors. Moreover, in order to evaluate precursory phenomena properly and to be able to use them confidently for predictive purposes, one has to understand the physical processes that give rise to them. Physical models of precursory phenomena are classified in two broad categories: those based on fault constitutive relations, which predict fault slip behavior but no change in properties in material surrounding the fault, and those based on bulk rock constitutive relations, which predict physical property changes in a volume surrounding the fault. Nucleation and lithospheric loading models are the most prominent of the first type and the dilatancy model is of the second type. During the past two decades efforts have been made to measure anomalous emanations of geo-gases in earthquake-prone regions of the world, in particular helium, radon, hydrogen, carbon dioxide. Among them radon has been the most preferred as earthquake precursor, because it is easily detectable. Radon is found in nature in three different isotopes:

Nuclear Track Detector Characterization via Alpha‐Spectrometry for Radioprotection Use

Solid Nuclear Track Detectors (SNTDs), CR‐39 type, are usually adopted to monitor radon gas concentrations. In order to characterize the detectors according to track geometrical parameters, detectors were irradiated inside a vacuum chamber by alpha particles at twelve energy values, obtained by different Mylar foils in front of a 241Am source. The alpha energy values were verified using a Si detector. After the exposure to the alpha particles, the detectors were chemically etched to enlarge the tracks, which were then analyzed by means of a semiautomatic system composed of an optical microscope equipped with a CCD camera connected to a personal computer to store images. A suitable routine analyzed the track parameters: major and minor axis length and mean grey level, allowing us to differentiate tracks according to the incident alpha energy and then to individuate the discrimination factors for radon alpha tracks. The combined use of geometrical and optical parameters allows one to overcome the ambiguity ...