Using a portable gamma-ray spectrometer to estimate the external exposure for a subway passenger

The subway (metro) is a popular form of public transportation system in many large cities. While in the subway territory (one of the locations in the city), a passenger is inevitably exposed to external irradiation from natural (terrigenous) sources of ionizing gamma radiation – 40 K, radionuclides of the 232 Th and 238 U series, which are present in the ground and building materials. The aim of this study was to estimate the external exposure for a metro passenger in St.-Petersburg (Russia). Materials and Methods: A portable gamma spectrometer-dosimeter, carried in a backpack by the operator, was used to measure the integral ambient dose equivalent rate and the effective activity concentration of natural radionuclides along 60 selected passenger routes in the metro. The effective dose rate of external exposure of a metro passenger was estimated using results of the measurements and the published conversion coefficient from ambient dose equivalent to the effective dose. Results and Discussion: The measured values of ambient dose equivalent rate varied from 65 to 214 nSv/h and averaged 92 nSv/h. Effective activity concentration of natural radionuclides ranged from 104 to 188 Bq/kg (average = 147 Bq/kg). The average ambient dose equivalent rate from natural radionuclides was 86 nSv/h (range from 61 to 107 nSv/h). The ratio of the ambient dose equivalent rate from natural radionuclides to the effective activity concentration varied slightly from 0.56 to 0.61 ((nSv/h)/(Bq/kg)) and averaged 0.58 ((nSv/h)/(Bq/kg)). The average effective dose rate of external exposure from natural radionuclides is conservatively estimated at 86 nSv/h for an adult passenger of the St. Petersburg metro. Conclusion: Comparison of the results of this work and literature data shows that the travel of a local resident or visitor in St. Petersburg using the metro, compared to walking on foot in surface urban locations, should not lead to additional external exposure from natural radionuclides present in the environment. In terms of ambient dose equivalent rate, the St.-Petersburg metro fully complies with modern sanitary and epidemiological requirements for public facilities.