Influence of solar activity on ambient dose equivalent H*(10) measured with thermoluminescent dosimeters in Slovenia

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 H*(10) measured by TL does not include the dose contribution from neutrons.

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 r stands for correlation coefficient, i_min and i_max for the first and the last year of the time interval under investigation, respectively, H*(10)_i and n_i for ambient dose equivalent and mean number of sunspots over the year i, respectively, H ∗<!-- ∗ --> ( 10 ) ¯<!-- ¯ --> and n ¯<!-- ¯ --> $\overline{H^{*}(10)} \text { and } \bar{n}$for mean annual ambient dose equivalent and mean annual sunspot number over the time interval (i_min–i_max), and σ[H^*(10)] and σ(n) for the dispersion of ambient dose equivalents and the number of sunspots over the observed period, respectively. To evaluate the correlation coefficient, it is important to choose the time interval over which Chernobyl contamination does not interfere with the solar cycle, that is, when its influence on ambient dose equivalent is lower than annual dose equivalent fluctuations. Therefore, the first year i_min starts with local ambient dose equivalent minimum, after which dose equivalent starts to rise, which is a trend opposite to the decreasing trend of Chernobyl contamination. This criterion removes the influence of the Chernobyl contamination. For the last year of the time interval, i_max, we took 2019 for all measurements.

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 H*(10).

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 H *(10) around NPP Krško was 0.763±0.057 and 0.765±0.057 mSv, respectively, whereas in the rest of Slovenia it was 0.849±0.046 and 0.791±0.059 mSv, respectively. The lowest H*(10) values corresponding to the highest solar activity in 2002 were measured as late as 2004, most probably because of the influence of the residual contamination with the Chernobyl fallout, which increased dose equivalents in 2002 and 2003, so the minimum tends to be overestimated. From the maxima measured in 2009 and 2019 it follows that the highest H*(10) corresponding to no sunspots is 0.835±0.045 mSv for NPP Krško and 0.916±0.048 mSv for the rest of Slovenia. Maximal annual H *(10) averted by the solar activity is calculated as the difference between the maximal H *(10) and the minimum measured when the solar activity is maximal. During the 2014 solar maximum, the averted H *(10) at NPP Krško and the rest of Slovenia was 0.072±0.073 and 0.067±0.066 mSv, respectively, whereas during the 2003 solar maximum, it was 0.070±0.073 and 0.125±0.077 mSv, respectively.

On the other hand, H*(10) due to secondary cosmic rays excluding neutrons was 0.316 and 0.317 mSv at 155 m.a.s.l. during the solar minimum in 2019 and 2009, respectively and 0.305 and 0.297 mSv during the solar maximum in 2014 and 2003, respectively. EXPACS calculations of the averted H*(10) at NPP Krško of 0.011 and 0.020 mSv during the solar maximum of 2014 and 2003, respectively is considerably lower than the averted H *(10) from TLD measurements.

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 H*(10) corresponding to the quiet sun was 0.845±0.045 mSv, implying an averted dose equivalent of 0.132±0.070 mSv in 2014 and 0.180±0.067 mSv in 2004. Model calculations, however, give averted dose equivalents of 0.033 mSv for 2014 and 0.094 mSv for 2004.

The sum of annual mean natural H *(10) contributions from soil in the rest of Slovenia was 0.66 mSv (21), which was constant over time, while the cosmic-ray contribution was about 0.31 mSv, which exceeds the measured annual ambient dose equivalents. Therefore, it is not likely that the disagreement between the doses averted by the solar activity originates from an overestimation of the contribution of the charged-particle component of the cosmic rays to the measured ambient dose equivalents, but rather points to an underestimation of the influence of solar activity in EXPACS calculation.