Use of hand-held gamma-ray spectrometry to assess decay of granite ashlars in historical buildings of NW Spain (Barbanza, Galicia)

Non-destructive methods are widely demanded in research on Cultural Heritage. In the last years, several techniques have been proposed, attempted and even routinely tried on research on assessing the deterioration of stone of Cultural Heritage buildings and monuments. We test here the use of hand-held gamma-ray spectrometry (GRS) to assess the decay of stone in two Heritage buildings. GRS is a non-destructive technique widely used in mineral exploration that provides an estimate of the environmental gamma radiation dose but also of the content of some radioisotopes of uranium, thorium and potassium in rocks and minerals. Such radioisotopes are present in very variable contents depending on the rock type. They have a different behavior when a stone overcome weathering processes, being usually the K and U leaching, while Th tends to be considered as an immobile element. We have tested GRS on two historical buildings built with granite blocks of the Peninsula of Barbanza (Galicia, NW, Spain): Santiago de Lampón and San Cristovo de Abaqueiro. The studied buildings show a mild deterioration in some façades, being an interesting study case. Measurements were carried out on the four façades of both buildings at on ashlars located at heights of 0.5, 1.5 and 2 m, above the ground, for comparing the possible effect of capillary water from underground on the weathering degree of the ashlars. The comparison of the content in K, U, and Th provides interesting results, that seems to be useful to assess the degree of decay of ashlars in buildings that shows slight differences in decay, being a non-destructive tool.