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Physical And Radiological Characterisation Of Measuring Sites Within The Croatian Gamma Dose Rate Early Warning Network

The work described in this paper was done within the EU funded project “Upgrading the systems for the on- and off-line monitoring of radioactivity in the environment in Croatia in regular and emergency situations”. The existing system of early warning in case of nuclear accident in Croatia (SPUNN), managed by the State Office for Radiological and Nuclear Safety, includes 33 stations for measuring ambient gamma dose rate (GDR). The aim of the project was to determine appropriate correction factors that will allow the results from this network to be used not only for timely warning in case of nuclear accident but also in routine environmental monitoring to determine the background radiation. Additionally, in the case of fresh deposition due to radioactive contamination, the corrected values are better suited to be used as an input for support systems for decision making in the case of emergency (such as RODOS), as well as for international data exchange (EURDEP) or automatic interpolation and mapping of radiological data (INTAMAP). The response of the individual probes to natural or accidental radiation mostly depends on the geometry or topography, surrounding buildings, vegetation (trees) and the type of soil beneath the detector. In the case of measuring the dose rate, objects such as buildings act as a shield against gamma radiation and limit the field of vision of the detector. If we want to have representative values that can be compared with other measuring sites, it is clear that we need to define standard conditions that each location has to meet. This is true not only for the probes within the same network, but can also be applied more broadly, at the international level, since data exchange mechanisms for GDR data already exist across Europe. The response of each probe is not determined only by the physical features, it is also important to understand the radiological characteristics of the site. Radiological characterization was performed through measurements of gamma dose rate, in-situ gamma spectrometry and determination of cosmic and terrestrial components of natural radiation. Different soils contain different concentrations of natural uranium, thorium and radioactive potassium, so the level of radioactivity and the associated dose rate component depend on the type of soil or rock (geological structure). The contribution of secondary cosmic radiation depends primarily on altitude. As an additional benefit, we have also measured the concentration of artificial Cs-137 in the upper layer of the soil, which is a consequence of the Chernobyl accident. This can be useful to establish initial conditions in case of new radioactive contamination. All these measurements, as well as measurements with the reference instruments, were used to analyse the response of GDR probes. The paper will describe the tools and procedures that we used, as well as our findings, with complete evaluation of all probes and locations within the network. We have developed acceptance criteria and recommended measures to be taken for each station in order to standardize measurements.

Early warning network ; Gamma dose rate probes ; Site characterisatio

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