THE GABBRO AND SERPENTINIZED PERIDOTITE OF BONASSOLA (BRACCO-LEVANTO OPHIOLITE, ITALY) - AN EXTREMELY LOW NATURAL RADIATION AREA TO IMPROVE IN SITU GAMMA SPECTROMETRY

摘要

Outcrops of dunite, serpentinized peridotite and clinopyroxene-rich coarse gabbro around Bonassola (Bracco-Levanto ophiolite, NW Italy) have been analyzed for natural radionuclides using γ-spectrometry. The very low activity concentrations of natural radionuclides in the dunite outcrop (＜ 9.0 Bq kg~(-1) ~(226)Ra; ＜ 3.5 Bq kg~(-1) ~(228)Ac and ＜ 25 Bq kg~(-1) ~(40)K) indicate that this site is characterized by a very low natural radiation for both cosmic (being at sea level) and terrestrial contributions. Before starting a field radiometric survey it is important to assess the local background, i.e. the g radiation that does not originate from rock, in order to correct the measurements for the local background. Contrary to the general practice regarding the background radiation in γ-spectrometry measurements, the investigated mafic-ultramafic outcrops has been used to estimate the detection limits for in situ measurement of ~(238)U, ~(232)Th and ~(40)K using sodium iodine (NaI) scintillator detectors. These results have proved useful to quantify the minimum detectable amount of activity (MDA) of ~(238)U, ~(238)Th and ~(40)K for the Nal detector; the MDA points at sub ppm levels for ~(238)U and ~(232)Th and 0.01 wt% K_2O. This study also permits to assess the radiological risk of dimension stones that are petrographically similar to the serpentinized ultramafics occurring in the Bracco-Levanto ofiolite. The comparison of the activity concentration index I, defined by the EU Basic Safety Standards Directive and depending on the activity concentrations of ~(226)Ra, ~(232)Th and ~(40)K, with other stones used as building material, indicates that the Bonassola mafic-ultramafic rocks provide a negligible contribution to the indoor gamma radiation dose.