An Evaluation Of The Natural Radioactivity In The Soils Of Okahandja And Karibib, Namibia

ABSTRACT

The natural radioactivity and associated hazards in the soils of the towns of Karibib and Okahandja have been studied by gamma spectroscopy. Each town was divided into ten geographical areas and five soil samples were collected across each area. These samples were processed and placed in well-labelled 500 ml plastic bottles. A High Purity Germanium detector (HPGe) was used to measure the gamma ray spectra of the samples. The intensities of selected gamma lines were used to calculate the activity concentrations of the gamma emitting radionuclides 238U, 232Th, and 40K. The average activity concentrations of 238U, 232Th, and 40K in Karibib are 29.4 ± 5.8 Bq/kg, 49.0 ± 8.6 Bq/kg, and 824.3 ± 153.5 Bq/kg respectively while they are 40.9 ± 8.6 Bq/kg, 57.9 ± 19.4 Bq/kg, and 562.4 ± 125.4 Bq/kg respectively in Okahandja. Consequently, the average activity concentrations of 238U and 232Th are much higher in Okahandja than in Karibib, while the average activity concentration of 40K is much higher in Karibib than in Okahandja. These concentrations were used to calculate the absorbed dose rates and effective dose rates in air in the two towns. The average absorbed dose rate in air in Okahandja is 77.3 ± 18.0 nGy/hr while that in Karibib is 77.6 ± 10.9 nGy/hr. However, the resulting average effective dose rate in Okahandja and Karibib are the same and equal to 9.5 𝑥 10-2 mSv/yr. The average absorbed dose rate in air in each of the two towns is relatively high than the world average of 51 nGy/hr. However, the corresponding average effective dose rate in both towns is much below the recommended maximum permissible dose rate of 1.0 mSv/yr. This result implies that the two towns have normal background radiation. Also, the results of 163.0 ± 22.6 Bq/kg and 167.1 ± 40.1 Bq/kg obtained for the Radium equivalent activities, Raeq, in Karibib and Okahandja respectively are much lower than the recommended maximum value of 370 Bq/kg thus confirming that both towns have

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normal background radiation. Furthermore, the average external hazard indices determined for Karibib and Okahandja are respectively 0.4 ± 0.1 and 0.5 ± 0.1. These values are again far less than the recommended safe level of unity. This result further confirms that radiological hazard is negligible in Karibib and Okahandja. The results obtained in this study would be useful in establishing a baseline data that will serve as a reference to ascertain possible changes in environmental radioactivity due to nuclear and related activities in future.