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A gamma-ray spectrometry survey with NaI(Tl) (ø75 mm × 75 mm) has been performed on a large scale to determine the distribution of 40K, 238U and 232Th in soil and rocks in Zhuhai, a southern Chinese city located in Guangdong Province. The survey sampled 970 sites which covered an area of more than 100km2. The average activities of 40K, 238U and 232Th were found 655±338, 85.8±31.6, and 159.8±49.0 Bq.kg−1 in soil or cement of pavement in the urban area; 832±455, 87.3±41.6, and 179.3±40.7 Bq.kg−1 in weathered granite; 634±76.8, 35.2±14.6 and 70.2± 18.6 Bq.kg−1 in the Quaternary sediments in Doumen District. The average 232Th activity (159.8±49.0 Bq.kg−1) is above the average in both China (49 Bq.kg−1) and the world (30 Bq.kg−1). Terrestrial air-absorbed dose rates calculated from the above activities are 85.08±15.63, 183.33±40.80 nGy.h−1 respectively over the Quaternary sediments and weathered granite, and 163.49±30.15 nGy.h−1 in the urban area. The average calculated air-absorbed dose rate is about 12.63% larger than the value measured by a portable plastic scintillator dosemeter. The 40K, 238U and 232Th activities in surface medium are mainly originated from biotite granites, as indicated by a strong correlation between the radioactivity level and geological lithology in the surveyed area.
Wang et al. (Sat,) studied this question.