Naturally occurring radioactivity at the Alps-Apennines transition

Field gamma-ray spectrometry was used for the quantitative assessment of radioactive elements and the expected radon flux of rocks cropping out at the Alps-Apennines transition zone (NW Italy). The spatial variation in radioelement concentration reflects, in general, the geochemical differences among the analysed rocks. Rocks of the ophiolitic complex clearly differ in K, U, and Th concentrations from sedimentary and metasedimentary rocks. The lowest U concentration (< 0.5 ppm) was found in tectonic units mainly formed by rocks of mafic composition. The average U concentration increases to 2.6-4.1 ppm in shales of different units and calc-schists, and it is highest (5.6 ppm) in the dolomitic rocks. Shales and phyllitic schists of pelagic origin show the highest Th concentrations. The expected radon exhalation from the decay of Ra-226 within the U-238 decay series is, consequently, lower in the ophiolitic rocks (0.7-2.2 Bq m(-2) h(-1)) and higher in calc-schists and dolomites (-18.4-20.7 Bq m(-2) h(-1)). The estimated radon flux is also significant in shales and phyllites (15.1 -18.4 Bq m(-2) h(-1)). (C) 2002 Elsevier Science Ltd. All rights reserved.