Natural Radioactivity Concentration and Radiological Evaluation in Soil Samples Around Dangote Cement Factory Ibese, Ogun State, Nigeria

Background: Natural-occurring radioactive materials (NORMs) provide significant sources of human exposure to ionizing radiation but in certain cases, anthropogenic activities, like mining, have produced wastes that contain radiation above background levels in the environment, a situation that is of great concern for radiation protection. Around Dangote cement factory both mining and production have been on-going for some years, therefore there is need to evaluate the extent of the possible risk of the radionuclides to the health of the population in this study area. Measurements of radioactivity concentrations were carried out around Dangote Cement Factory Ibese. Samples of surface soil were measured using gammaray spectroscopy Nal (TI) scintillation detector. Results: Measurements showed that activity concentrations ranged from 18.33+ 1.91 Bqkg -1 to 29.14±4.4.2 Bqkg -1 , with an average of 23.40 Bqkg -1 for ( 238 U 226 Ra), 10.93±5.43 Bqkg -1 to 21.52±2.16 Bqkg -1 with an average of 16.50Bqkg -1 for 232 Th, and 291.78±15.50 Bqkg -1 to 338.60±3.922 Bqkg -1 with an average of 314.11 Bqkg -1 for 40 K. Similarly, the absorbed dose ranged from 28.63nGy/h to 38.24nGy/h with an average of 33.14nGy/h. The calculated annual effective dose ranged from 0.035mSv/y to 0.047mSv/y with an average of 0.040mSv/y. Conclusions: The average value of Radioactivity concentrations obtained for 238 U, 232 Th and 40 K are lower than the corresponding global values reported in UNSCEAR publication. The calculated absorbed dose and annual effective dose values are also less than the recommended safe levels.


Introduction
Man since formation of the earth is exposed to many diverse sources of radiations. These sources may be natural or as a result of human activities. The radiation from natural sources include cosmic radiation, external radiation from radionuclide in earth's crust and internal radiation from radionuclide's inhaled or ingested and retained in the body [1]. According to the United Nations Scientific Committee on effects of Atomic Radiation Report (UNSCEAR), the greatest contribution to mankind's exposure comes from natural background radiation [2]. Gamma radiations emitted from Natural Occurring Radioactive Materials (NORMS) such as uranium ( 238 U), thorium ( 232 Th), and potassium ( 40 k) are generally known as terrestrial background radiation and is the main external of irradiation of the human body. External exposures outdoors arise from terrestrial radionuclides present at trace levels in all soils. The specific levels are related to the geological and geographical conditions of those areas. Higher radiation levels are associated with igneous rock such as granite and lower levels with sedimentary rocks [2]. Also, the presence of NORMS in soil generally originates from the disintegrating rocks that are carried to soil by rain and flows [3]. Cement main constituents are clay, shale and limestone. These constituents of cement are found in the soil. Kim in 1995 reported the presence of radionuclide of thorium's and uranium series in limestone and also same traces concentration of thorium, uranium and potassium in black shale in some regions [4]. Radiation from soil environment is one of the main source of exposure to humans hence it is important to know the distribution of gamma radiation from radionuclide's such as 40 K and also from 238 U and 232 Th series [5]. The radionuclide's in limestone and shale and their overlying soil materials can become pollutant when present in greater levels than the natural concentrations. These higher concentrations in radionuclide in limestone and shale or cement raw materials may be detrimental to humans' health [6]. The presence of these radionuclides in different food crops grown on the soil around the cement factory may also constitute a health hazard. Gene damage is the greatest threat of radionuclide [7].
This study, assess the radioactivity concentrations in soil samples around Dangote Cement factory in Ibese, Ogun State, Nigeria and evaluate radiological hazard indices. The results obtained were compared with the previous results of some countries around the world.

Materials and Methods
Study Area. Ibese is locate in Yewa North local government Area, Egbado North, area of Ogun State, Southwest Nigeria. Its geographical coordinates are 6 0 58 1 0 11 North, 3 0 2 1 0 11 Exact. The people living these are predominantly farmers and traders. It's about 35km from Abeokuta the Ogun State Capital and 45km from Lagos.
The geology of Ibese and environs consists of Ewekoro formation which is marine and paleocene age. It consists of a limestone unit several meters in thickness which is overlain by a shale unit almost three times as thick as the limestone. In terms of regional geology, the study area belongs to the Eastern part of Dahomey Basin extending from the Volta Delta (South eastern Ghana) to the western flank of the Niger Delta in Nigeria [8]. Dangote cement factory is situated about a kilometer away from Ibese town.

Sample Collection and Processing
Samples were collected randomly during dry season of 2018. After the soil position where the soil samples are to be taken was determined, the ground was cleared of stones pebbles, vegetation and roots 2kg of soil sample were collected from a soil positioned about 10cm below the soil surface. A total of fifteen soil samples were taken and placed in a coded polythene bag. The fifteen soil samples were taking to the laboratory where they were first air dried and then grounded in to a fine powder of 200 µm in size after been oven dried at a temperature of 105°C for 8 hours. 100g of the homogeneous soil samples were then packed in polythene plastic, weighed and carefully sealed and stored for at least 4 weeks before counting to allow time for 238 U and 232 Th to reach equilibrium with their respective radionuclide daughters'.

Experimental and Calibration
Using a well calibrated Nal (TI) and well shielded detector couple to a computer resident quantum MCA2100R Multichannel analyzer for analyzer for 36000s.
An empty container under identical geometry was also counted for the same time. The 1460 KeV gamma-radiation of 40k was used to determine the concentration of 40 k in the sample. The gamma transition energy of 1764.5KeV 214 B I was used to determine the concentration of 238 U while the gamma transition energy of 2614 KeV 20 Th was used to determine the concentration of 232 Th while 137Cs was detected by its 661.6KeV gamma transition. The efficiency calibration of the detector was done using a reference standard dose, associated with the cement production. The energy calibration was also performed by using the peaks of the radionuclide's present in the standard sources. The channel scale was then converted to an energy scale. This produces energy calibration curve i.e energy versus channel.
The activity concentration of the radionuclides in the samples was calculated after decay correction using the expression: (1) Where A C is the activity concentration of the radionuclides in Bq/kg, M san is the mass sample (kg), N sam is the sample net count in peak range, F E is the gamma emission probability, µ (E) is the photo peak efficiency and T C is the counting time. The minimum detectable activity (MDA) for each radionuclide 238 U, 232 Th and 40 k was calculated using the equation: Where 1.645 is the coverage factor at 95% confidence level, N B is the background count at the region of interest, T C is the counting time, F E is the gamma emission probability, µ (E) is the photopeak efficiency and M is the mass of the sample. The MDA for each radionuclides were calculated as 0.30 Bq/kg for 238 U, for 0.12 Bq/kg for 226 Ra, 0.11 Bq/kg for 232 Th and 0.90 Bq/kg for 40 K. respectively.

Dose Rate Calculation
The absorbed dose rates at 1m above the ground are calculated by converting the activity concentration of 238 U, 232 Th and 40 K into dose by using the formula (UNSCEAR2000) below.
Where A k , A u and A Th are activity concentration of K, U and Th in each sample respectively

The Annual Effective Dose Rate Calculation
The annual effective dose was calculated from the absorbed dose rate by applying the dose conversion factor of 0.7SvGy -1 and an outdoor occupancy factor of 0.2 recommended by UNSCEAR [2]. Thus, annual effective dose was obtained using equation: Annual Effective Dose Rate (AEDR)=D (nGyh -1 ) x 8760 (hy -1 ) x 0.2 x 0.7SvGy -1 x 10 3 Where D is the absorbed dose rate in air and this calculation takes into account that the people spend 20% of their time outdoors.

External Hazard Index H EX
The external hazard index H ex is an assessment of the hazard of the natural gamma radiation. H ex for samples in this study was calculated using equation defined by Ghazwa et al., [9].
Where C Ra , C Th and C K are the activity concentrations of 226 Ra, 232 Th and 40 K in Bq/kg respectively. The maximum value of H ex equal to unity corresponds to the upper limit of Ra eq (370 Bq/kg).

Radium Equivalent Activity (Ra eq )
Ra eq is used to assess the gamma radiation hazards associated with materials that contain in 226 Ra, 232 Th and 40 K. It is assumed that 1 Bq/kg of 226 Ra, 0.7Bq/kg of 232 Th and 1.3 of 40 K produces the same gamma radiation dose rates. The Ra eq is given as: Ra eq =C Ra + (1.43 C Th ) + (0.077C K ) Where C Ra , C Th and C K are the average activity concentrations of 226 Ra, 232 Th and 40 K in Bq/kg respectively.

Calculation of Lifetime Cancer Risk
Lifetime cancer risk (ELCR) was calculated using equation below ELCR=AED X DL X RF Where DL is the life expectancy (The life expectancy for Nigeria in 2019 is 54.49) and RF is the risk factor (Sv -1 ), it is fatal cancer risk per Sievert. For stochastic effects from low dose background radiation, ICRP 103 suggested the value of 0.057 for the public exposure [10]. Activity concentrations in the soil samples measured in this study varied from 18.343qKg -1 to 29.14BqKg -1 with an average value of 23.4BqKg -1 as indicated in table 1.

Results and Discussion
Activity concentrations of 232 Th also ranged from 10.93BqKg -1 to 29.14BqKg -1 with an average of 16.47BqKg -1 . The activity concentrations of 40 k varied from 291.78 BqKg -1 to 338.60BqKg -1 with an average of 314.11 BqKg -1 .
The average values of activity concentrations of 238U, 232Th and 40K are lower than the 40 BqKg -1 for both 238 U and 232 Th and 370 BqKg -1 for 40K recommended by UNSCEAR (2000). It should be noted that despite the activity concentrations not higher than the recommended limit, they varied from one location to other as reported by UNSCEAR, (1993).
These average activity concentrations are found to be lower compared to values obtained elsewhere in Nigeria and in other countries in African. See table 2. The absorbed dose calculated ranged from 28.63nGg/h to 38.24nGg/h with a mean of 33.14nGy/h also the values of annual effective dose varied from 0.035mSv/y to 0.047mSv/y with an average of 0.04mSv/y. However, comparing the results with world wide data, the obtained value was lower than 6 nGyh -1 reported by UNSCEAR in 2000. Relative contribution to dose due to 40 K was 88.7% followed by the contribution of dose to 238 U and 232 Th as 6.6% and 4.6% respectively. The estimated values of Ra eq in the present study ranged from 63.54 to 80.90 Bqkg -1 are lower than the recommended maximum value of 370 Bqkg -1 by UNSCEAR. The external radiation hazard index (H ex ) calculated ranged from 0.159 to 0.221 which are lower than upper limit of unity. However, the value of this index must be less than unity in order to keep the radiation hazard insignificant. The estimated values of excess lifetime cancer risk (ELCR) for all the samples from Table 3 ranged from 1.08 x 10 -4 to 1.46 x 10 -4 which is lower than world average of 2.9 x 10 -4 [19].

Conclusion
In the present study, the results indicate that the natural radioactivity concentrations of 40 K, 232 Th and 238 U are relatively lower than the allowable limits recommended by UNSCEAR publication. The obtained results of absorbed dose rate, annual effective dose were found lower than recommended safety limits. Although the result in this study pose not treat to workers and people in the surrounding, it indicated the existence of radioactive materials in the environments where such activities are taking place. Also the ranged value of Ra eq activity and external health hazard index values were found to be lower than recommended safe limit values, this study still made available data helpful for future evaluations, in case gross contamination of the area.