High levels of terrestrial gamma radiation exposure in the Kopili Fault Zone on the eastern wedge of the Shillong Plateau, India

This study addresses the substantial terrestrial gamma radiation exposure and associated radiological risk in the Amsoi region, located in the seismically active Kopili Fault Zone (KFZ) on the periphery of Shillong Plateau’s gneissic complex. A portable monitoring device highly sensitive to gamma radiation, equipped with a NaI (Tl) scintillator, was used to quantify the terrestrial gamma dose rates in indoor and outdoor air. The recorded dose rates varied among house patterns, with mud houses having the highest. The calculated absorbed dose rates indoors and outdoors were found to be in the range of 157.9–362.5 nGy h⁻¹ and 163.7–336.2 nGy h⁻¹, respectively, which are much higher than the reported population-weighted global averages of 84 nGy h⁻¹ and 59 nGy h⁻¹. The indoor-to-outdoor ratio was also calculated and found to be in the range of 0.7–1.4. The elevated terrestrial gamma radiation could be attributed to the geological setting of the study area, located in the seismically active KFZ. The annual effective dose equivalents for indoor and outdoor environments were calculated and found to be in the ranges of 0.8–1.8 mSv and 0.2–0.4 mSv, respectively. The excess lifetime cancer risk was assessed by calculating the lifetime effective dose and was found to be in the range of 3.4 × 10^–3–7.3 × 10^–3, which is considerably higher than the global average of 1.45 × 10^–3. This study has revealed that the populations residing in this seismically active fault zone are living precariously under high terrestrial gamma radiation.