Article

Natural radioactivity measurements and external dose estimation by car-borne survey in Douala city, Cameroon

¹ Nuclear Physics Laboratory, Faculty of Science, University of Yaoundé I, P.O. Box 812 Yaoundé, Cameroon
² Nuclear Technology Section, Institute of Geological and Mining Research, P.O. Box 4110 Yaoundé, Cameroon
³ Hirosaki University Graduate School of Health Sciences, Hirosaki City, Aoromi, Japan
⁴ National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan
⁵ Department of Radiation Physics, Institute of Radiation Emergency Medicine, Hirosaki University, Hirosaki City, Aomori 036-8564, Japan

^* Corresponding author: saidous2002@yahoo.fr

Received: 23 January 2018
Accepted: 27 July 2018

Abstract

A car-borne survey was carried out in Douala, the largest city in Cameroon to make a detailed distribution map of the absorbed dose rate in the city, to locate the high natural radiation areas useful later to carry out indoor radon, thoron, and thoron progeny measurements. Gamma-ray dose rates were measured using 3-in × 3-in NaI(Tl) detector. Activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K in soil from Douala city were determined by two methods: the first, using in situ gamma spectrometry and the second, at the laboratory using a NaI(Tl) detector. A heterogeneous distribution of absorbed dose rates in air was observed on the dose rate distribution map, and varies from 29 to 86 nGy h⁻¹ with an average of 50 nGy h⁻¹, lower than the world average value of 59 nGy h⁻¹. The activity concentrations with NaI(Tl) detector varied from 18 to 47 Bq kg⁻¹ for ²³⁸U, 21 to 54 Bq kg⁻¹ for ²³²Th, and 10 to 410 Bq kg⁻¹ for ⁴⁰K with averages of 29, 38, and 202 Bq kg⁻¹ respectively, for in situ measurements. They vary between 29–98 Bq kg⁻¹ for ²³⁸U, 29–92 Bq kg⁻¹ for ²³²Th, and 40 to 79 Bq kg⁻¹ for ⁴⁰K, with averages of 60, 57, and 56 Bq kg⁻¹ respectively for soil samples collected at Douala III subdivision. The highest value of the annual effective dose for in situ measurements by car was observed at Ndogbong and was found to be 0.7 mSv y⁻¹, higher than the world average value of 0.5 mSv y⁻¹.