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Gamma-radiation measurements and dose rate of the Aeolian Islands volcanics
by
Chiozzi Paolo
Dipartimento per lo Studio del Territorio e delle sue Risorse, Settore di Geofisica, Viale Benedetto XV, 5 I-16132 Genova, Italy
Coauthors: Pasquale Vincenzo, Verdoya Massimo, Furfaro Valeria
The gamma-ray activity related to the four major, naturally occurring, nuclides 40K, 235U, 238U, and 232Th is of growing interest in many branches of geophysics. In the environmental field, the interest provoked by such isotopes derives essentially from their danger to humans, which is mainly controlled by their amount in the ecosystem, their average life and chemical-physical properties. This determines the degree of scattering, concentration and, above all, of absorptivity of the radionuclides from the environment. It is well known that the magmatic activity in the volcanic arc of the Aeolian Islands has formed rocks with large concentrations of radioactive elements (uranium, thorium and potassium), which consequently produce high environmental radioactivity. Volcanic products cropping out on these islands display a wide compositional variability, ranging from calc-alkaline to shoshonitic and potassic. Calc-alkaline volcanics are the most common rocks. Shoshonitic and potassic lavas are present exclusively on Lipari, Vulcano and Stromboli. These rock associations show ages ranging from 1020 kyr to the present. We determined the U, Th and K concentrations and the air absorbed dose rate at numerous sites of the islands of the archipelago (Salina, Lipari, Vulcano, Stromboli, Filicudi and Panarea) with a NaI(Tl) gamma-ray spectrometer. Generally, U is spatially related to both Th and K and the Th/U ratio is on average 3.1-3.5. The magmatic evolution is reflected by the concentration of the three radioelements, as they are more abundant within the more felsic units of the volcanic series. The higher values of U (15.7-20.0 ppm) coincide with higher Th (48.3-65.9 ppm) and K (4.9-6.1 %) concentrations associated with rhyolitic rocks of Lipari and Vulcano. The air absorbed dose rate varies from 20 to 470 nGy/h. In Salina, Filicudi and Panarea, due to the abundance of rocks of mafic composition, the dose rate is lower than 120 nGy/h. This outdoor exposure value is typical of areas of normal background radiation. Critical levels of radiation from natural source are instead attained in the eastern part of Stromboli and Lipari and in the northern sector of Vulcano, where the higher absorbed dose rates (170-470 nGy/h) correspond to the most recent volcanic cycles and to the presence of obsidian lavas.
Date received: July 7, 2004
Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caon-05.