An in situ measurement of natural background radiation along the course of Imo River in Rivers State. The outdoor radiation exposure rate was measured in 20 randomly selected points along the river using well calibrated portable Geiger Muller and Scintillation detectors. Five measurements were made in each sampling point and an average value was used to determine the absorbed dose rate from natural background radiation. The absorbed dose rate was found to range from 104.4 and 217 nGyh-1 with mean value of 150.95nGyh-1 and the average annual effective dose of radiation was found to be 0.24mSvy-1 .The excess lifetime cancer risk determined ranges from 0.60 and 1.16 × 10-3 with mean value of 0.81. Though the annual effective dose equivalent are within the safe limit, the absorbed dose of radiation and the excess lifetime cancer risk exceeded the world accepted safe value of 89.0 nGyh-1 and 0.29 × 10-3 respectively. The study showed an area of slightly higher background radiation and caution need to be taken in the use of the river course. This study does not indicate any immediate radiological hazards for the general public, however long term exposure may lead to some radiation sickness.
