Mapping Ground Exposure Rate and External Hazard Index in Part of Basement Complex Rock of North Central Nigeria

Abstract

In this work, a preprocessed aero radiometric survey data of geologic survey map series of Nigeria basement complex sheet 145 was used to map the spatial distribution of ground level concentration of the three radioelements that contribute significantly to the emission of gamma rays, notably potassium (40K), thorium (232Th) and uranium (238U) and estimating the external hazard associated with human exposure due to the elements. The elemental average concentrations of 40K, 232Th and 238U obtained from the survey data are 1.37485%, 24.5297ppm and 5.5491ppm respectively. The relative concentrations of the three elements are in line with the global trend where Th records the highest mean concentration, followed by U with then K recording the least. Except for K, the values of the other elements were found to be above the global crustal abundance ranges of 2 - 2.5% for 40K, 8 – 12 ppm for 232Th and 2 – 3 ppm for 238U. The mean ground level external dose rate was found to be 213.2 ±21.2 nGyh-1. This is about four times higher than the global average terrestrial dose rate of 55 nGyh-1. The computed external hazard index however gives an average value of 0.329, which is within the acceptable limit (<1). External hazard index above this limit were however recorded at three points, spanning a small area of about 0.096 km2. In situ and ground geophysical measurements are therefore