The paper deals with the study of natural radioactivity in the rocks of Coorg district, Karnataka state. The samples were analyzed by NaI(Tl) based gamma ray spectrometer. The activity of 232 Th varies from 16.46 Bq kg À1 to 160.84 Bq kg À1 with mean value of 79.05 Bq kg À1. The activity of 40 K varies from 96.72 Bq kg À1 to 933.68 Bq kg À1 and that of 226 Ra was found to vary from BDL to 34.36 Bq kg À1. A correlation coefficient of R ¼ 0.81 was found between 232 Th activity and 40 K activity. The correlation between 226 Ra activity and 232 Th activity is R ¼ 0.59. The correlation between 40 K and 226 Ra is R ¼ 0.51. The Radium Equivalent Activity (Ra eq) ranges from 46.78Bq kg À1 to 315.55Bq kg À1. The absorbed gamma dose rate (D) ranges from 21.28 nGy h À1 to 141.45 nGy h À1 , with an average of 73.37 nGy h À1. The radionuclides activity was compared with world average values. The external Hazard index (H ex) varies from 0.12 to 0.84. The internal hazard index (H in) is in the range 0.15e0.89. The hazard indices were found to be within safety limits.

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