Articles 35 and 36 of the EURATOM treaty (1) require from the European Union (EU) member states to monitor continuously the levels of environmental radioactivity, which is carried out in line with the Safety Standards Series No. RS-G-1.8 issued by the International Atomic Energy Agency (IAEA) (2). These include networks of thermoluminescence (TL) dosimeters measuring the ambient dose equivalent
Slovenia has two such networks. One comprises 66 TL dosimeters covering the 10 km perimeter around the Nuclear Power Plant (NPP) Krško reactor. The other consists of 50 TL dosimeters to cover the rest of the Slovenian territory (7). Both networks are run according to the Ionising Radiation Protection and Nuclear Safety Act (8) and the Rules on the Monitoring of Radioactivity (7).
The dosimeters are mainly exposed to ionising radiation from secondary cosmic rays (9), which are generated in the stratosphere and the upper troposphere by primary cosmic rays, and to gamma rays originating from radioactivity in the soil. Since the Chernobyl contamination has ceased to contribute to soil radioactivity, it has been nearly constant, with small variations owed to meteorological conditions. Namely, rain may increase it through the washout of radon daughters from the atmosphere (11) and snow may reduce with its cover. In other words, the contribution of natural radioactivity from soil to the annual exposure varies, but this variation does not affect semi-annual or annual measurements. The dose rate from cosmic rays, in turn, reflects the periodicity of solar activity, also known as the solar cycle (12). Annual solar activity is measured with annual mean number of sunspots (13) as a directly observed quantity. The aim of our study was to evidence that measured ambient annual dose equivalent
Environmental radiation dosimeters are designed to contain two 0.5 mm thick TL pellets of 5 mm diameter (6). They are placed on the top of a white plastic bottle, which is hung at 1.0 m above the ground. The luminescent material is CaF2 doped with Mn, which is produced by the Jožef Stefan Institute (JSI), Ljubljana, Slovenia (14).
Since these TL dosimeters are insensitive to neutrons, only charged particles and photons contribute to the measured ambient dose. Therefore, the total annual dose equivalent
Dosimeters are read with an in-house system IJS MR 200 C. The sensitivity of the measurements is 5 μSv and standard deviation under repeatable conditions 0.020 mSv at a dose of 0.40 mSv (6). The dosimeters have been type tested in the energy range from 40 keV to 1.2 MeV at the Laboratory for Dosimetry Standards of JSI, which is accredited by Slovenian Accreditation (15). As all dosimeters are calibrated individually, the uncertainty of the calibration constant is 5 % for individual component, and 2.5 % for the calibration field. The dosimeters are annealed by heating the pellets to 350 °C.
Our TL dosimeters are exposed at about 1 m above ground twice a year, from January to June and then from July to December. They are distributed to cover open terrain and rural and urban environments. The obtained annual ambient dose equivalents are the sum of semi-annual dose measurements.
Their reproducibility can be assessed from the scattering of semi-annual measurements at nine locations along the fence of the NPP Krško, 150–350 m from the reactor axis, under the assumption that all dosimeters there receive the same dose. This assumption stems from the fact that they are all positioned where soil is replaced by limestone gravel. The dispersion of dose rate equivalents measured during the same time interval is 5.9 % (16). This dispersion corresponds to the 5 % uncertainty of individual calibration constants.
Mean annual ambient dose equivalents from secondary cosmic rays were calculated with the Excel-based Program for calculating Atmospheric Cosmic-ray Spectrum (EXPACS) (17, 18, 19). It calculates fluxes of secondary cosmic rays, mainly of neutrons, electrons, and photons at a specified point in the atmosphere from the galactic (primary) cosmic rays and solar data (17). From these fluxes it calculates the effective dose, ambient dose equivalent, and absorbed dose in the air (17, 18, 19) taking into account the surrounding environment. For the surrounding environment, the “Ground” option was chosen with the water fraction of 0.2.
To assess quantitatively the influence of solar activity on ambient dose equivalent it is necessary to correlate it with the contribution of cosmic rays to ambient dose equivalent using the following equation:
where
Figures 1–3 show typical fluctuations in ambient dose equivalents and number of sunspots over time. Annual ambient dose equivalents in all three segments exhibit a decreasing trend since
Table 1 shows mean ambient dose equivalents around NPP Krško and the rest of Slovenia as well as the contribution from the secondary cosmic rays calculated with the EXPACS software for the elevation of 155 m.a.s.l., corresponding to the elevation of NPP Krško. All these measurements exclude the contribution of the neutron component, as TL dosimeters cannot detect it.
Mean annual ambient dose equivalents
Year | |||
---|---|---|---|
2002 | 0.791 | 0.792 | 0.297 |
2003 | 0.769 | 0.791 | 0.297 |
2004 | 0.765 | 0.793 | 0.279 |
2005 | 0.827 | 0.864 | 0.302 |
2006 | 0.783 | 0.846 | 0.310 |
2007 | 0.766 | 0.840 | 0.314 |
2008 | 0.822 | 0.868 | 0.314 |
2009 | 0.837 | 0.922 | 0.317 |
2010 | 0.793 | 0.886 | 0.317 |
2011 | 0.738 | 0.899 | 0.315 |
2012 | 0.806 | 0.881 | 0.313 |
2013 | 0.803 | 0.877 | 0.311 |
2014 | 0.763 | 0.849 | 0.305 |
2015 | 0.829 | 0.895 | 0.306 |
2016 | 0.814 | 0.902 | 0.311 |
2017 | 0.823 | 0.895 | 0.316 |
2018 | 0.829 | 0.870 | 0.316 |
2019 | 0.833 | 0.910 | 0.316 |
Table 2 shows correlation coefficients between annual mean number of sunspots and annual
Correlation between annual mean number of sunspots (
Location | Location | ||||
---|---|---|---|---|---|
NPP nine fence, locations mean value over | -0.28 | 2006 | Kredarica, 2515 m.a.s.l. | -0.69 | 2004 |
Krško locations mean value over three | -0.46 | 2006 | Trenta, 624 m.a.s.l. | -0.55 | 2003 |
Kusova Vrbina | -0.54 | 2000 | Stara Fužina, 547 m.a.s.l. | -0.53 | 2008 |
Amerika | -0.44 | 2003 | Bilje, 55 m.a.s.l. | -0.38 | 1995 |
Gmajnice | -0.38 | 2003 | Sečovlje, 2 m.a.s.l | -0.52 | 1992 |
The uncertainties of the means correspond to the dispersion of the readings under reproducible conditions of both semi-annual measurements (0.030 mSv at a value of the indication corresponding to the dose 0.4 mSv) averaged over the locations. Only the uncertainties due to the individual component of the calibration factor of the detectors are included in the analysis here; the uncertainty of the calibrating field is not considered, since it does not affect
Table 1 shows that mean annual dose equivalents during the lowest solar activity (around 2019 and 2009) were the highest: 0.833±0.062 and 0.837±0.063 mSv, respectively around NPP Krško and 0.910±0.068 and 0.922±0.069 mSv, respectively in the rest of Slovenia. At the highest solar activity around 2014 and 2002, annual
On the other hand,
A similar discrepancy can be observed for the measurements at Kredarica at the altitude of 2515 m.a.s.l.. Here the maximum annual ambient dose equivalents measured in 2019 and 2009 were 0.828±0.062 and 0.862±0.065 mSv, respectively, and the minimum dose equivalents measured in 2014 and 2004 were 0.713±0.053 and 0.665±0.050 mSv, respectively. Mean
The sum of annual mean natural
Our findings suggest that the contribution of cosmic rays to annual ambient dose equivalent can be easily identified through solar activity. We identified it quantitatively by calculating the correlation between mean annual number of sunspots and annual ambient dose equivalents measured at specific locations. Correlation coefficients turned out to be consistently below zero, implying a clear negative correlation between the two. Quantitative analysis of