P-1a-12 1 Natural gamma radiation in air versus soil nature in Portugal

Since 1985 the portuguese population exposure to natural gamma radiation is the object of a on-going study, in order to assess the natural gamma radiation doses all over the country. For the primary surveys a G-M based portable dose rate meter was used, in order to have a general picture of natural gamma dose rates outdoors in different regions, mainly those corresponding to highly populated areas (1) The program was then extended all over the country and these data was further associated to data of an indoors survey on natural gamma dose rates, made by termoluminescent dosimetry, based on LiF square rods (TLD-100) in order to evaluate the effective dose to portuguese population due to natural gamma radiation (2). A correction for the cosmic radiation component for different latitudes and altitudes was introduced after a cooperation for the Radiation Atlas – Natural Sources of Ionizing Radiation in Europe (3). So, the first Radiological Map for portuguese population exposure to natural terrestrial gamma radiation was established and the effective dose for every district (administrative division) was evaluated resulting 0.61mSv.y –1 for the whole country. It should be noticed that for some regions indoors values were obtained by calculation upon the proposed (4) average relation 1.2 indoors /outdoors dose. The analysis of outdoors survey results showed that northern regions had higher dose rate levels than southern ones and a quite different geological nature For instance, districts of Viana do Castelo, Braga, Porto