Study area

Imo State University is in Owerri Metropolis in Imo State, South-Eastern Nigeria. The map of the study area is presented in Figure 1. The university is on a latitude of 5.5080°N and a longitude of 7.0423°E. The environmental catchment area of the study location is about 104 km², which cuts across Owerri Municipal, Owerri North, and Owerri West. IMSU accommodates an average daily population above 15 000, including students, staff, and visitors, as seen from the daily report at the school entrance gate. It is a famous institution in Owerri due to its location and population density. It is also at the base of the dip slope from Okigwe (the center of mining in Imo State) to Owerri. This suggests that most radionuclides washed off by water floods from the rocks and mining sites from Okigwe might settle in Imo State University and its environs.

Figure 1.

Map of the study location.

The study area is slightly sloped flat land, partly covered with sandy, loamy, stony, and clay soil. It has a moderately dry climate with an average monthly temperature of 24°C to 29°C, with rainfall from March to July and September to October. The dry season is dominated by harmattan breeze and moderate sun intensity from December to February and August.

Sample collection

Soil samples were collected from 30 sampling points in the natural, uncultivated grass-covered level areas within the region, as stipulated by International Atomic Energy Agency (IAEA) recommendations.³¹ The samples were collected on a sunny day in March 2022 after clearing each sampling point’s surface of stones, pebbles, vegetation, and roots. Each soil sample was collected at a depth of 15 cm beneath the surface and packed in a labeled polythene bag for preparation and gamma spectroscopic analysis.

Sample preparation

All collected soil samples were transported to the laboratory, and all visible unwanted materials were removed using a wooden tongue. The samples were first air-dried under ambient temperature (30°C) for 1 week and were later subjected to a higher temperature using a drying oven at 105°C to eliminate any moisture. Each sample was crushed using a mechanical potter and sieved with a 2.00 mm mesh to ensure sample homogeneity. About 0.2 kg of each homogeneous sample was weighed into a cylindrical container of negligible mass and correctly labeled with indelible ink for easy identification.^4,32 The cylindrical containers used for the package were washed with borehole water (5 times) and rinsed thoroughly with distilled water. The packaged samples were allowed in the laboratory for at least 4 weeks so natural radionuclides and their progenies could attain secular radioactive equilibrium.^4,8,32 Afterwards, the samples were gathered in a big plastic carrier and transported to the Radiation and Health Physics Research Laboratory, Department of Physics, University of Ibadan, Nigeria, for gamma-ray spectrometry analysis.

Gamma-ray spectrometry analysis

The gamma-ray spectrometry analysis was conducted at the radiation and health physics research laboratory, Department of Physics, University of Ibadan, Nigeria. Each sample was counted for a total of 36 000 seconds. The detector employed for the radioactivity measurements was a 76 mm × 76 mm NaI(Tl) detector crystal (802 Series, Canberra Inc.) coupled to a Canberra series 10 Plus multichannel analyzer (MCA) of model no: 1104 via a preamplifier. The system had an adequate lead shield that reduced the background radiation by 95%. The energy resolution of the NaI(Tl) detector was 8% at 0.662 MeV (¹³⁷Cs). As this study aimed to determine the activity concentration of ²³⁸U, ²³²Th, and ⁴⁰K radionuclides in the soil samples, the ²³⁸U and ²³²Th series and ⁴⁰K activities were considered as the amount of these radionuclides that would enter the air from the soil, so the energy values upon which the measurement was to be based were initially determined. The radionuclides (²³⁸U and ²³²Th) activity concentrations were determined using the gamma energy of their progenies that was noticed during the decay series as 1.760 MeV for ²²⁶Ra (²³⁸U) and 2.615 MeV for ²³²Th. The activity concentration of ⁴⁰K was determined using only its gamma energy of 1.460 MeV. The activity concentrations of radionuclides in the samples were estimated using equation (1).⁴

(1)

where C is the radionuclide activity concentration of soil samples given in Bqkg⁻¹, C_n represents the count rate under the corresponding peak, ϵ is the detector ’s efficiency at the specific gamma-ray energy, P_γgamma; is the absolute transition probability of the specific gamma-ray, M_s is the soil sample mass in kg, and t is the counting time in seconds.

A gamma-ray detector’s detection limit (DL) specifies its operational capabilities without the effect of the sample.³³ This calculation was performed using equation (2).

(2)

where t_b in the second represents the background counting time, C_b is the total background count in the corresponding peak, and is the conversion factor given in equation (1). The present study’s measurement system showed that soil samples’ detection limits were 16.96, 4.43, and 3.65 Bqkg⁻¹ for ⁴⁰K, ²³²Th, and ²³⁸U, respectively. Any activity concentrations lower than these numbers are considered below the detector’s detection limit (BDL).

Radiological hazard indices

Radium equivalent: Radium equivalent activity (Ra_eq) is one of the most widely used hazard indices in comparing radionuclides (²³⁸U, ²³²Th, and ⁴⁰K) in any material by a single quantity that considers the hazard associated with them.^18,29,31,34 The radium equivalent is calculated using equation (3).^8,35,36

(3)

where C_K, C_U, and C_Th are the activity concentrations in Bqkg⁻¹ of ⁴⁰K, ²³⁸U, and ²³²Th, respectively.

Absorbed dose rate

The absorbed dose rate in the air from exposure to natural primordial radionuclides was evaluated using the activity concentration of radionuclides. The absorbed dose rate (D [nGyh⁻¹]) in the air helps us quantify the amount of radiation absorbed by a body at 1 m above the ground due to ²³⁸U, ²³²Th, and ⁴⁰K. The absorbed dose rate in the air was calculated using equation (4) as given in the UNSCEAR 2000 report.^1,8,37

(4)

where C_K, C_U, and C_Th are the activity concentrations in Bqkg⁻¹ of ⁴⁰K, ²³⁸U, and ²³²Th, respectively.

Annual effective dose (µSvy⁻¹ ): it was calculated from the absorbed dose rate using equation (5)

(5)

where AED is the annual effective dose equivalent (µSvy⁻¹), and D is the absorbed dose rate in the air, is the conversion coefficient and 0.2 is the outdoor occupancy factor.^1,4,8

Radiological hazard indices (H_ex and H_in): The harmful gamma radiation effects caused by radionuclides present in these soil samples were estimated by calculating the different hazard indices. Although the total activity concentrations of the exact indication of total radiation hazards have been assessed, these hazard indices also guide the choice of the right materials for building hurts, bricks, and other materials for human habitation. Two hazard indices were employed: the external hazard index (H_ex) and the internal hazard index (H_in). Both indices were calculated using equations (6) and (7).⁸

(6)

(7)

Where C_U, C_Th, and C_K are as described in equation (3). Radiation hazard from soil samples is considered negligible if the value of both indices is less than unity.¹

Activity utilization index (AUI): This is a significant health index that estimates the excess external and indoor gamma radiation from soil and other building materials.⁸

(8)

Where C_U, C_Th, and C_K are as described in equation (3).

Excess lifetime cancer risk (ELCR): The excess lifetime cancer risk is evaluated using the equation (9)

(9)

Where AED is the annual effective dose equivalent as contained in equation (5),

LE is the life expectancy (assumed to be 70 years in this study),³⁴ and RF is the risk factor of fatal cancer per Sievert. According to the International Commission on Radiological Protection (ICRP) 103 report, the value of 0.057 was used for stochastic effect for the public.^38,39

Statistical analysis

The IBM Statistical Package for Social Science (SPSS-27) computer program was used to analyze data. The present study calculated and reported statistical parameters such as mean, maximum, and minimum. The values obtained were reported as means ± SD (standard deviation), and P < .05 was accepted for all comparisons. Pearson correlation was used to evaluate the level of correlation of activity concentrations obtained and absorbed dose, effective dose, and other radiological hazard indices, as presented in Table 5.

Activity concentration

The estimated activity concentration of naturally occurring radionuclides ²³⁸U, ²³²Th, and ⁴⁰K in soil samples were given in Table 1. The radioactivity concentration obtained from this study ranged from BDL to 31.53 Bqkg⁻¹ for ²³⁸U, from BDL to 37.15 Bqkg⁻¹ for ²³²Th and from 20.90 to 127.15 Bqkg⁻¹ for ⁴⁰K. The spatial distribution of the primordial radionuclides from the studied location is an indication that the evaluated radionuclides are not concentrated in one location, as the report showed a high value for one radionuclide and a low value for the others but unequally distributed within the campus of the institution. The average radioactivity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K found in soil samples were (20.32 ± 3.22, 22.55 ± 0.68, and 91.63 ± 1.54) Bqkg⁻¹, respectively. The activity concentration of radionuclides in soil samples from IMSU is within the report of Eke et al³⁴ on the top soil samples from Imo State Polytechnic. In addition, the mean activity concentration of natural radionuclides in soil samples at the 2 tertiary institutions in Owerri, as reported by Eke et al³⁴ is comparable. Although the activity concentration of radionuclides in soil samples from IMSU is far higher than the values reported by Popoola et al⁴⁰ the average values are lower than the world’s mean radioactivity concentration of (35, 45, and 420) Bqkg⁻¹for ²³⁸U, ²³²Th, and ⁴⁰K, respectively.¹ This is shown in Table 2, which compares the radioactivity concentration calculated in this study with other related studies within Nigeria and globally. The present finding showed that ⁴⁰K has the maximum average radioactivity concentration and mark maximum at all sampling points except at 2 points, IMSU 16 (Faculty of Law) and IMSU 22 (Agricultural Science Department). Potash feldspar minerals in the studied area may be responsible for the enhanced ⁴⁰K recorded in the present study. Since IMSU is at the base of the dip slope from Okigwe (the center of mining in Imo State) to Owerri. This suggests that migration of weathered radionuclides from the surrounding rocks might have settled in Imo State University and its environs, increasing the activity concentration in radionuclides of the studied area. The activity concentration of ⁴⁰K obtained from the present research corroborates the findings of Isinkaye.⁴¹

Table 1.

The estimated radioactivity concentration of ²³⁸U, ²³²Th and ⁴⁰K in Bqkg⁻¹.

Table 2.

Comparison of average natural radioactivity concentration and radiological parameters evaluated in this study with results from some related studies around the globe.

Similarly, the results of Egunyinka et al⁴² on the activity concentration of ⁴⁰K agree with the present study. Also, the soil radionuclides’ activity concentrations were comparable to other related studies conducted within and outside Nigeria.^4,43-51 Eke et al⁴ and Olagbaju et al⁵¹ obtained lower activity concentrations of ²²⁶Ra (²³⁸U) compared to the present study. Egunyinka et al⁴² and Ugbede et al⁴⁷ recorded higher activity concentrations of ²²⁶Ra (²³⁸U) compared to the present study.

Radiological dose assessment and radiological hazard indices

The dosimetry parameters, such as radium equivalent activity (Ra_eq), absorbed dose rate (D), and annual effective dose (AED), were recorded in Table 3. The calculated radium equivalent value ranges from 33.62 ± 3.46 to 81.66 ± 3.80 Bqkg⁻¹, with an average value of 59.62 Bqkg⁻¹. The estimated average value of Ra_eq obtained in this study is appreciably lower than the world recommended value of 370 Bqkg^{−1 1} and the result from other related studies, as shown in Table 2. The absorbed dose values in Table 3 were found in the range of 15.96 ± 1.62 to 36.91 ± 2.50 nGyh⁻¹ with an average of 26.86 nGyh⁻¹. The absorbed dose obtained in this study is lower than the world’s recommended value of 59 nGyh⁻¹ and within the range of other related studies, as shown in Table 2. The calculated AED value was shown in Table 3, the value ranged from 19.57 ± 1.99 to 45.27 ± 3.07 µSvy⁻¹. The average value for outdoor estimated was 33.07 µSvy⁻¹. The calculated AED was lower than the world’s recommended values of 480 µSvy⁻¹ (0.48 mSvy⁻¹),¹ as presented in Table 3. Also, the annual effective dose of soil samples from the present research is less than 1.0 mSvy⁻¹, the ICRP 2007 recommended value for the members of the public.^39,52

Table 3.

Calculated dosimetry quantities (radium equivalent (Bqkg⁻¹), absorbed dose ( ) and annual effective dose (µSvy⁻¹)).

The estimated radiological index parameters such as H_ex, H_in, AUI, and ELCR were recorded in Table 4. The values of the external hazard index (H_ex) and internal hazard index (H_in) ranged from 0.09 ± 0.01 to 0.22 ± 0.01 and 0.11 ± 0.00 to 0.31 ± 0.03, respectively, with associated average values of 0.16 and 0.22. The H_ex and H_in values estimated in this study were in line with the world’s recommended value of <1 (less than unity), as shown in Table 3. The activity utilization index shown in Table 4 has a value ranging from 0.23 ± 0.02 to 0.58 ± 0.04 with an average value of 0.42, which is in line with the results from other related studies and also less than the world recommended value of 2 as shown in Table 2. Figure 2 shows a strong relationship between the internal hazard index (H_in) and the external hazard index (H_ex). The excess lifetime cancer risk (ELCR) was calculated with estimated outdoor values of AED, recorded in Table 4. The range of ELCR recorded is 0.08 ± 0.01 (×10⁻³) to 0.18 ± 0.01 (×10⁻³), with the corresponding average value of 0.14 × 10⁻³. The ELCR value is lower than the world mean value of 0.29 × 10⁻³, as shown in Table 2. This implies that the radiation level measured in this study area may not pose serious radiological health hazards, and several research works have also confirmed that the health risk of exposure to low natural radiation doses may be insignificant.^53-56

Table 4.

Radiological hazard parameters (H_ex, H_in, AUI, and ELCR).

Figure 2.

Correlation between the internal hazard index and external hazard index.

Pearson’s correlation analysis

Table 5 presents the result of the correlation analysis. Pearson correlation analysis of activity concentrations of radionuclides, absorbed dose, effective dose, and the radiological parameters revealed strong relationships among these parameters. The findings revealed a significant correlation between the activity concentrations ²³²Th and all the radiological parameters. This implies that an increase in the activity concentrations of ²³²Th radionuclide in the soil sample of IMSU could result in exposure to the community. Similarly, there was a strong correlation between the activity concentrations ²³⁸U and the radiological parameters except for radium equivalent (Ra_eq) and activity utilization index (AUI). The results of the present study agreed with the findings of Mbonu and Ben⁵⁷ who reported the radiation hazard indices due to natural radioactivity in soil samples from Orlu, Imo State, Nigeria. The authors revealed that the activity concentrations of radionuclides in soil samples from the studied locations were below the safe limit. Even though the mean activity concentration of ⁴⁰K was more than ²³²Th and ²³⁸U, there was no significant correlation between it and the radiological parameters.

Table 5.

Correlation analysis between activity concentrations of radionuclides and their radiological parameters.