Evaluation of natural radioactivity levels and radiation hazards of Nkalagu limestone deposit, southeastern Nigeria

Levels of natural radioactivity due to ⁴⁰ K, ²³²Th, and ²²⁶Ra in limestone deposits and corresponding surface soil samples from the Nkalagu community of Ebonyi State were investigated. Gamma spectroscopic analysis using a NaI (TI) detector was employed to measure the activity concentrations (ACs) of the natural radionuclides in the collected samples. The average AC of ²²⁶Ra, ²³²Th, and ⁴⁰ K was found to be 64.62 ± 11.63, 54.27 ± 8.52, and 313.06 ± 25.33 Bq/kg, respectively, in limestone, whereas in the soil, the value was 60.80 ± 11.71, 44.97 ± 10.74, and 185.90 ± 18.20 Bq/kg, respectively. Except for ⁴⁰ K, the obtained mean ACs of the radionuclides were in excess of the worldwide averages reported by the United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR). The mean value of Ra_eq, H_ex, H_in, I_γ, and AUI was obtained as 166.33 Bq/kg, 0.45, 0.62, 1.18, and 1.28, respectively, in the limestone, and 139.42 Bq/kg, 0.38, 0.54, 0.98, and 1.12, respectively, in the surface soil. The radiation doses in limestone ranged from 120.78–168.89 nGyh⁻¹ and 63.86–88.88 nGyh⁻¹ for indoor and outdoor, respectively, whereas in the soil, the range was 104.47–129.77 nGyh⁻¹ and 54.60–68.02 nGyh⁻¹ for indoor and outdoor, respectively. The total annual effective dose ranged from 0.671 to 0.937 mSvy^–1 with an average of 0.800 mSvy^–1 in limestone and 0.579 to 0.720 mSvy^–1 with an average of 0.667 mSvy^–1 in the soil. Values of representative gamma index and activity utilization index, and indoor and outdoor radiation doses in limestone were above the recommended world average of ≤ 1, and 84 nGyh^–1 and 59 nGyh^–1, respectively, suggesting a restriction in the use of the limestone in construction of houses. The study therefore recommends that the use of limestone in building construction, especially in house interiors, should be discontinued.