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With several databases available, including two sets of in situ measurements of the ambient gamma dose rate and an airborne survey of K, Th, U in soil, Belgium is a favourable case for exploring the mapping methodology for terrestrial radiation. The first step is the harmonization of the different data sets, taking in situ measurements with an ion chamber as the reference. Corrections are necessary, based on the data themselves (a) to the measurements of permanent monitoring stations, (b) to the data calculated from airborne measurements of the soil activity, due in particular to the attenuation by the forest cover, and (c) to the other data calculated from the soil activity, due to the lower activity of the upper layer. After subtracting the cosmic contribution, a harmonized database of the terrestrial gamma dose rate (TGDR) based on 379 in situ measurements was built, together with a harmonized data set of 30134 TGDR values calculated from the concentrations of K, Th, U in soil deduced from the airborne survey. The two data sets are in good agreement with each other for all statistical characteristics that were examined like basic statistics, qq-plots, analysis of variance (ANOVA) or variograms, which validates the airborne-based data set by the link with in situ ion chamber measurements. ANOVA reveals the strong relation between TGDR and the soil class, which justifies the use of a soil map as the framework for developing the TGDR map. The variograms show the absence of residual spatial correlations within soil classes. The two harmonized TGDR data sets were mapped at the nodes of a kilometric grid by the moving average method within soil groups. There is a rather good agreement between the maps, confirming the equivalence between the two data sets and the validation of the airborne based one, which can obviously give more detail. After reducing the maps to a 10 km × 10 km grid, the two data sets were used to check the accuracy of the Belgian part of the European TGDR contained in the European Atlas of Natural Radiation.

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The European Atlas of Natural Radiation developed by the Joint Research Centre (JRC) of the European Commission includes maps of potassium K and thorium Th. With several different databases available, including data (albeit not calibrated) from an airborne survey, Belgium is a favourable case for exploring the methodology of mapping for these natural radionuclides. Harmonized databases of potassium and thorium in soil were built by radiological (not airborne) and geochemical data. Using this harmonized database it was possible to calibrate the data from the airborne survey. Several methods were used to perform spatial interpolation and to smooth the data: moving average (MA) without constraint, or constrained by soil class and by geological unit. Overall, there was a reasonable agreement between the maps on a 1 × 1 km2 grid obtained with the two datasets (airborne data and harmonized soil data) with all the methods. The agreement was better when the maps are reduced to a 10 km × 10 km grid used for the European Atlas of Natural Radiation. The best agreement was observed with the MA constrained by geological unit.

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A map of uranium concentration in soil has been planned for the European Atlas of Natural Radiation. This Atlas is being developed by the Radioactivity Environmental Monitoring (REM) group of the Joint Research Centre (JRC) of the European Commission. The great interest in uranium compared to other terrestrial radionuclides stems from the fact that radon (222Rn) is in the decay chain of uranium (238U) and that public exposure to natural ionizing radiation is largely due to indoor radon. With several different databases available, including data (albeit not calibrated) from an airborne survey, Belgium is a favourable case for exploring the methodology of uranium mapping. A harmonized database of uranium in soil was built by merging radiological (not airborne) and geochemical data. Using this harmonized database it was possible to calibrate the data from the airborne survey. Several methods were used to perform spatial interpolation and to smooth the data: moving average without constraint, by soil class and by geological unit. When using the harmonized database, it is first necessary to evaluate the uranium concentration in areas without data or with an insufficient number of data points. Overall, there is a reasonable agreement between the maps on a 1 km × 1 km grid obtained with the two datasets (airborne U and harmonized soil U) with all the methods. The agreement is better when the maps are reduced to a 10 km × 10 km grid; the latter could be used for the European map of uranium concentration in soil.

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The deviations of the distribution of Belgian indoor radon data from the log-normal trend are examined. Simulated data are generated to provide a theoretical frame for understanding these deviations. It is shown that the 3-component structure of indoor radon (radon from subsoil, outdoor air and building materials) generates deviations in the low- and high-concentration tails, but this low-C trend can be almost completely compensated by the effect of measurement uncertainties and by possible small errors in background subtraction. The predicted low-C and high-C deviations are well observed in the Belgian data, when considering the global distribution of all data. The agreement with the log-normal model is improved when considering data organised in homogeneous geological groups. As the deviation from log-normality is often due to the low-C tail for which there is no interest, it is proposed to use the log-normal fit limited to the high-C half of the distribution. With this prescription, the vast majority of the geological groups of data are compatible with the log-normal model, the remaining deviations being mostly due to a few outliers, and rarely to a "fat tail". With very few exceptions, the log-normal modelling of the high-concentration part of indoor radon data is expected to give reasonable results, provided that the data are organised in homogeneous geological groups.

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In the process of mapping indoor radon risk, an important step is to define geological units well-correlated with indoor radon. The present paper examines this question for the Walloon region of Belgium, using a database of more than 18,000 indoor radon measurements. With a few exceptions like the Carboniferous (to be divided into Tournaisian, Visean and Namurian-Westphalian) and the Tertiary (in which all Series may be treated together), the Series/Epoch stratigraphic level is found to be the most appropriate geological unit to classify the radon risk. A further division according to the geological massif or region is necessary to define units with a reasonable uniformity of the radon risk. In particular, Paleozoic series from Cambrian to Devonian show strong differences between different massifs. Local hot-spots are also observed in the Brabant massif. Finally, 35 geological units are defined according to their radon risk, 6 of which still present a clear weak homogeneity. In the case of 4 of these units (Jurassic, Middle Devonian of Condroz and of Fagne-Famenne, Ordovician of the Stavelot massif) homogeneity is moderate, but the data are strongly inhomogeneous for Visean in Condroz and in the Brabant massif. The 35 geological units are used in an ANOVA analysis, to evaluate the part of indoor radon variability which can be attributed to geology. The result (15.4-17.7%) agrees with the values observed in the UK.

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An inverse technique has been designed to unfold the x-ray tube spectrum from the measurement of the photons scattered by a target interposed in the path of the beam. A special strategy is necessary to circumvent the ill-conditioning of the forward transport algebraic problem. The proposed method is based on the calculation of both, the forward and adjoint analytical solutions of the Boltzmann transport equation. After testing the method with numerical simulations, a simple prototype built at the Operational Unit of Health Physics of the University of Bologna was used to test the method experimentally. The reconstructed spectrum was validated by comparison with a straightforward measurement of the X-ray beam. The influence of the detector was corrected in both cases using standard unfolding techniques. The method is capable to accurately characterize the intensity distribution of an X-ray tube spectrum, even at low energies where other methods fail.