Seasonal variations of ambient dose equivalent rate on the streets of Saint Petersburg

The gamma dose rate in air at any given outdoor location is not constant throughout the year. These fluctuations of the dose rate should be taking into account when assessing external radiation dose to humans. The aim of this study was to determine the seasonal variability of ambient dose equivalent rate on the streets of St. Petersburg. Materials and Methods: A portable gamma spectrometer-dosimeter, carried in a backpack on the operator, was used to measure gamma spectra in situ. Measurements were conducted in summer and winter on the streets located in the central part of the city. For comparison, spectra were measured on paths and lawns of the city parks. Results and Discussion: On average, ambient dose equivalent rates from natural radionuclides on the streets in winter were 6 % lower than those in summer. The difference was statistically significant (Wilcoxon test for the pair samples, p < 0.01). More pronounced differences between winter and summer were found in parks: with a snow cover depth of approximately 20 cm, the ambient dose equivalent rate on lawns and paths decreased by 32 % and 29 %, respectively. The differences between the seasons were statistically significant (p < 0.05). The relatively small decrease in the ambient dose equivalent rate on the city streets in winter is explained not only by snow removal from sidewalks and roadways but also by the fact that in this location some gamma radiation comes from radionuclides in the walls of buildings. Conclusion: The seasonal variations in the ambient dose equivalent rate from natural radionuclides on the city streets are extremely small. A coefficient of 0.95 can be used to estimate the magnitude of the reduction in the corresponding rate of external exposure to humans on the streets in winter compared to summer.