Radioactivity measurements in volcano-tectonic area for geodynamic process study

In the last ten years we carried out several radioactivity investigations in the aetnean area, a peculiar site characterized by both tectonic and volcanic features. In particular, continuous measurements in-soil radon gas carried out from 2001 until 2006 in the eastern flank of Mt. Etna, while several volcanic events occurred, showed a possible correlation between radon concentration and geodynamic activity, in particular magma uprising. We report in particular on the survey performed in order to determine vertical radon concentration profiles at different depths in sites near active faults in order to extract radon diffusion coefficients for the different sites. Moreover laboratory analysis allowed determining radionuclide contents (via γ-spectroscopy) and radon exhalation rate (via Can-technique) for different rock samples from the monitored sites. This study represents a contribution to better define the radon transport process through fractured media, in particular in volcanic area. ∗E-mail: daniela.morelli@ct.infn.it EPJ Web of Conferences DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences , 2012 , epjconf 20122 / 24 05009 (2012) 405009 SIF This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20122405009

Radon diffusion and transport through different media is a complex process and is affected by several factors [12][13][14][15].Even if in the last years several radon measurements were done for geophysical investigations, origin and mechanism of the radon anomalies and their relationship to geodynamical events (seismic or volcanic ones) are, however, poorly understood.
According to proposed models [2,4] radon anomalies are related to the mechanical opening of new cracks, widening or closing of old cracks or, alternatively, to compression mechanisms [16].The diffusion coefficient of radon in the rocks changes significantly in these models, an is affected by several factors.Only a fraction of radon atoms can reach the Earth's surface and can exhale to atmosphere before decaying.
With the aim to better define the transport process of radon through fractured media, we reconstructed vertical profiles at different depths near active faults, determining in-soil radon diffusion coefficients in the Aetnean sites.Laboratory measurements were carried out on different rock samples in order to extract the exhalation rate.

Experimental procedure
The investigated area is located on Mt.Etna, a complex volcano (more than 3300 m high and 40 km in diameter) with four summit craters (SC), more than 300 lateral vents and cones on its flanks, and with several fault systems.Since the actual volcanic activity prevalently concerns the eastern flank and since our previous data [9,[17][18][19][20][21], continuously collected from 2001 to 2004 in the Etnean area, showed for the NE flank a possible correlation between the radon trend and eruptive events, we carried out an investigation of insoil radon levels in the eastern region (see fig. 1), in particular near the villages of Vena (V) [NE, 825 m a.s.l.], Cugno di Mezzo (CM) [E, 1400 m a.s.l.],Santa Venerina (SV) [SE, 400 m a.s.l.].
All the three sites are located near fault systems that play an important role when an eruption is approaching.In particular, Vena is near the crossing point between the Pernicana Fault (PF) and the Timpe della Naca Faults (NF).Cugno di Mezzo is located on the southern edge of the eastern caldera named "Valle del Bove", characterized by very active faults.S. Venerina is located close to two systems, the Timpe Faults System (TFS) and the S. Tecla Fault (STF).The three sites were simultaneously investigated for a long time under the same meteorological conditions, in order to extract vertical radon profiles and the diffusion coefficient.

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Gamma-ray spectroscopy in laboratory and exhalation rate analysis on rock samples collected in the investigated sites were performed, too.

Radon diffusion coefficient
In order to determine the vertical profile of the radon concentration at different depths, in-soil measurements were carried out using nuclear track detectors (SSNTD-CR39).The detectors (active area: 25 × 25 mm 2 ) were placed inside a diffusion chamber that allows only 222 Rn entry.At each measurement point detectors were vertically allocated, in 20 cm distance intervals, from 20 to 100 cm of depth, inside a closed PVC tube (l = 100 cm, φ = 6.3 cm), uniformly pierced along all its length in order to allow the radon entry.In order to avoid saturation effects the CR-39 detectors were replaced every two weeks.
After the exposure the CR-39 detectors were chemically etched and read by an optical microscope (connected to a CCD camera and a personal com-

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Table 1: In soil average Rn concentration at one meter depth; the diffusion coefficient D is extracted from the fit, for the three sites (V, CM, SV) (see text).

Site
Rn D puter) by means of a semiautomatic procedure for discriminating the tracks according to minor and major axis and area.The vertical profile of the radon concentration was determined by fitting the measured values to an exponential relation [22]: where C ∞ is the radon concentration at profound depth, D is the diffusion coefficient, λ the radon decay constant and z the depth.
In figure 2 the radon concentration vertical profiles, obtained in the investigated sites, are plotted.Data refer to average values of in-soil radon concentration at each depth over the investigation period.In table 1 the values of the diffusion coefficient D obtained by data fit are reported.

Radionuclide amount
Rock samples collected from the investigated sites and together with other rock samples (sedimentary and metamorphic) were radiometrically characterized by means of γ-ray spectroscopy, performed using a high-purity germanium (HPGe) detector with a relative efficiency of 30%.
In table 2 is reported the radionuclide concentration.The specific activity concentration of 238 U was determined using the gamma line of 214 Bi at 609 keV and 1120 keV.For 232 Th, the specific activity was calculated using the gamma line of 228 Ac at 911 keV.The specific activity concentration of 40 K was estimated directly by its gamma line at 1461 keV [23].
The radionuclide distribution for different kinds of rock is shown in fig. 3. Higher uranium and thorium concentration is observed in magmatic rocks.

Radon exhalation rate
The radon exhalation rate was determined by means of the Can Technique [24] using CR39 detectors.The rock samples, dried at 80 • C and reduced to about 300 g mass, were allocated in sealed cylindrical cans (φ = 8.6 cm; h = 10.5 cm).A CR-39 detector was fixed, inside its diffusion chamber, on the top inside each can and exposed for detecting alpha particles from the decay of radon exhaled from the sample in the remaining volume of the can.After three months exposure the detectors were etched and read for track counting.
The mass exhalation rate ε M was determined by the expression [25]: where C is the radon concentration measured by the track detector [Bq m −3 ], V is the free volume of the can, λ is the radon decay constant; t is the exposure time, M is the sample mass.The obtained values of the exhalation rates, for the rock samples, are shown in fig. 4. Generally, for volcanic rocks a correlation is found between 238 U concentration and exhalation rate.

Conclusions
In order to better define the radon transport process through fractured media, an investigation was performed in sites located in the geodynamically active area on the eastern flank of Mt.Etna.
The survey was performed using different methodologies for both short and long-term measurements, and for reconstructing vertical radon profiles at different depths near active faults, in order to determine the diffusivity of radon.
Moreover, laboratory measurements on rock samples, collected from the investigated sites, allowed to extract the radon exhalation rate under controlled physical conditions.
The comparison with results obtained using sedimentary and metamorphic sample rocks has shown that magmatic rocks have higher radionuclide amounts and higher exhalation rate.
By comparing in-soil Radon concentration values, the Radon diffusion coefficient, the amount of uranium, and the exhalation rate a good correlation was found, representing a further contribution to define radon transport process, allowing in particular to shed light on the role of radon as precursor of volcanic activity.

Figure 2 :
Figure 2: Vertical radon concentration profiles in the Vena, Cugno di Mezzo and Santa Venerina sites.