Abstract:
The outdoor terrestrial gamma radiation due to 4oK, 238U and 232Th have been
measured in 175 locations in the oil producing areas (Niger Delta) of Nigeria using
the technique of in-situ gamma spectroscopy.
The net count per second (cps) was computed after a count of 1000s for the
gamma energy lines of 1.460 MeV, 1.760 MeV and 2.615 MeV that were used as the
indices for the determination of specific activities of 4oK, 238U and 232Th, respectively .
.The specific activities obtained ranged between 13.4 ± 0.7 Bq kg" and 127.8 ± 5.8 Bq
kg" for 4oK, 10.8 ± 2.0 Bq kg" and 26.7 ± 1.5 Bq kg" for 238U and 11.0 ± 2.2 Bq kg"
. and 30.5 ± 1.9 Bq kg" for 232Th with an arithmetic mean values and standard
deviations of 34.8 ± 20.4 Bq kg", 16.2 ± 3.7 Bq kg" and 24.4 ± 4.7 Bq kg" for 4oK,
238U and 232Th respectively. No artificial radionuclide was found at any of the
locations surveyed.
The outdoor gamma radiation absorbed dose rates in air due to each
radionuclide was calculated from the mean values of the activity concentrations at each
location. The values ranged between O. 6 nGy h-I and 5.4 nGy h-I with a mean of 1.5 ±
0.9 nGy h-I for 4oK, 4.6 nGy h-I and 11.5 nGy h-I with a mean of 6.9 ± 1.6 nGy h-I for
238U and 7.3 nGy h-I and 20.3 nGy h-I with a mean of 16.3 ± 3.1 nGy h-I for 232Th. The
combined dose rate for the region ranged between 13.5 ±,2.0 and 33.2 ± 3.0 with a
mean value and standard deviations of24.7 ± 1.9 nGy h-I
The estimated annual outdoor effective dose equivalent to people in the region
ranged between 29.1 j..1SVfl and 39.5 j..1SVfl with a mean of 32.1 j..1SVfl with a
mean of 35.0 j..1Svfl for the urban dwellers, and between 25.8 ~tSVfl and 63.4 uSv
y"1with a mean of 47.0 j..1Svfl for the rural dwellers. The estimated average annual
outdoor effective dose equivalent to people; n the region is 32.1 j..1SVfl. Using the
projected population for the region of 23.18 million for the year 2000, the estimated
collective effective dose equivalent for the region is 7.4 man -s-v'. The expected
number of people per thousand in the region at risk of incurring radiation-induced
cancer per year was found to be 16.
.
