The characteristics of radon and thoron concentration from soil gas in Shenzhen City of Southern China

Radon (²²²Rn) and thoron (²²⁰Rn) from soil gas are very significant factors that can affect the indoor radon level in the first floor or in the basement. China is one of the countries with the highest thorium content in the world. Therefore, it is very significant to study ²²²Rn/²²⁰Rn concentration in the soil in Shenzhen City (SC). A ²²²Rn/²²⁰Rn survey was performed using a portable radon monitor (model RAD7) at 69 sites, covered a total area of 1800 km² in 2013 to get the original data for radon risk estimation in SC. The average values of ²²²Rn and ²²⁰Rn concentration of soil gas of the total 69 locations are 86 ± 72 kBq·m⁻³ and 118 ± 85 kBq·m⁻³, respectively. ²²²Rn/²²⁰Rn concentrations are related to geological lithology. ²²²Rn concentrations vary from 40 to 370 kBq·m⁻³ and from 15 to 118 kBq·m⁻³ in weathered granite products and sediments, respectively, while ²²⁰Rn concentrations are from 103 to 435 kBq·m⁻³ and 2.2 to 96 kBq·m⁻³. The higher ²²⁰Rn values were mainly observed at the sites covered by the weathered granite products. Comparing with the areas of high ²²²Rn concentration, the areas of high ²²⁰Rn values are larger. The distribution of ²²²Rn concentration in the vertical direction displays an exponential distribution mode, but there is no rule of ²²⁰Rn concentration. The investigation suggests that people should pay attention to ²²⁰Rn contribution in the radon mapping of SC, as well as in the indoor radon survey.