6o Τακτικό Εθνικό Συνέδριο Μετρολογίας, Αθήνα, 13-14 Μαΐου 2016
Uncertainty induced to ground radioacticity measurements by sampling definitional errors
The typically used uncertainty components regarding the estimation of ground isotopes concentrations are usually based only on the laboratory analyses procedures metrological parameters; however, further and important uncertainty components could be due to the definitional errors of the measurands. Within this study, three categories of such errors, both of random and also systematic nature are identified, namely: (i) definitional error due to the spatial distribution of the sampling scheme, (ii) definitional error due to a single point estimation of the true concentration distribution, and (iii) definitional error pertaining to the ground usage history at the exact sampling point. The category (i) errors, are evaluated based on the sampling scheme of about 1200 different sampling points, which was used by the NTUA Nuclear Engineering Laboratory for the mapping of natural or artificial isotopes concentration on the Greek continental surface soils. The employed methodology could be also extended to other similar studies. The category (ii) errors, are evaluated through the comparison of results coming from a specific methodology to results deduced by more simplified alternative methodologies. For this purpose, results on surface and at depth samplings coming from a strictly defined and performed methodology were used. The sampling ground was of well-known usage history; the sampling scheme was based on a cartesian 60x60m grid with nodes at 20m orthocanonical distances; the uncertainty due to this sampling method and the related laboratory analyses have already been published. In this way, the additional uncertainty due to the use of rather approximate methodologies is assessed; any such change of methodology introduces changes to the definition of the sampling and measurement. Finally, the category (iii) errors are qualitatively, and, as practically as possible, quantitatively assessed based on specific assumptions on the ground usage history (e.g. undisturbed, pasture, cultivated etc.). For this purpose, surface and at depth ground samplings coming from a strictly defined and performed methodology at 15 sites were used. Characteristic examples of the influence of initial assumptions for the ground history on uncertainty are emerging and discussed. This study (a) presents the strengths and the limitations of surface ground sampling schemes and the way to optimize them, if required, aiming at the minimization of the intrinsic uncertainty of the results, (b) gives rise to understanding on which simplifying compromise could be acceptable for the replacement of a costly and strict but probably non-practical- sampling methodology by another more approximate, mainly in the case of wide scale measurements and (c) shows the contribution of an erroneous initial assumption for the ground history to the intrinsic uncertainty of the result.