INVESTIGATION ON THE NATURAL RADIOACTIVITY LEVEL OF GROUND SURFACE IN DALIAN URBAN AREA

The contents of natural radionuclide such as ²³⁸U, ²³²Th and ⁴⁰K on the ground surface of Dalian City, Liaoning Province, are surveyed with the ARD γ-ray spectrometer. The results show that the average specific radioactivity of ²³⁸U is 23.09 Bq/kg (ranging from 1.24 to 151.91), ²³²Th 18.99 Bq/kg (0.41-92.57), and ⁴⁰K 386.39 Bq/kg(31.30-1095.50), which are all significantly lower than the national and world averages. On the basis of measured radionuclide concentration, the parameters including the absorbed dose rate of γ radiation(Dr) at 1 m above the ground, external exposure index (Ir), internal exposure index (I_Ra) and annual effective dose (Daed) are calculated, and then the natural radioactivity level are evaluated. The average Dr of 39.04 nGy/h is far lower than the national average (81.5 nGy/h) and world average (80 nGy/h). The average values of Ir and I_Ra are 0.23 and 0.12 respectively, much less than the national limit (1) for external exposure index of building materials. The Daed of 0.048 mSv is far below the world average annual effective dose (0.46 mSv) and limit of annual effective dose for external public exposure (1.0 mSv). The evaluation results show that the natural radioactive radiation of surface in Dalian urban area is at a safe level.