Exposure of the general public, patients, and radiation workers to ionizing radiation must be limited to minimize the risk of harmful biological effects. In 1954, the National Committee on the Radiation Protection (NCRP) proposed a concept that radiation exposure should be kept as low as reasonably achievable (ALARA) concept. This concept is accepted by all regulatory agencies including International Commission on Radiological Protection (ICRP), the World Health Organization (WHO), and the European Commission (EC). When human body is exposed to ionizing radiation, it damages living systems by ionizing atoms composing of the molecular structures, causing abnormalities in the functioning of the living cell and consequently health issues. Rocks are the building blocks of the earth lithosphere (crust and mantle) embedded with long lived natural radionuclides (⁴⁰K, ²³⁸U (²²⁶Ra), ²³²Th) and other solid minerals.¹⁾ The concentrations of natural radionuclides depend on the local geological setting, the process of rock formation and lithological characteristics of a location.¹⁾

In Nigeria, there has been increase in the demand for dwellings across the major cities due to rapid population growth and urbanization. Majority of modern houses built in many cities in Nigeria contain 60~80% crushed rocks aggregate because of the strength and availability of the rocks in lieu of gravels that were popularly used in the past. However, the popular and general name for every quarried rock is granite, whereas there are various rocks crushed to different sizes or forms in commercial quantities to produce blocks for wall casting, flooring, external and internal decoration in various dwellings. Moreover, crushed rocks are processed to other products such as tiles, interlocks, bricks used for building construction. Despite the wide use of these rocks in building construction, study of the radioactivity levels of different types are not taken seriously; the building engineers and contractors are usually concerned with strength of rocks without due consideration of radiation emission from different types of the rocks. The radiological risk to individual in buildings constructed with crushed rocks may be high depending on the sources and the levels of natural radionuclides^2-4) have studied the radioactivity in rocks and soil matrices from parts of Ogun State, particularly Abeokuta identified as high background radiation area.⁵⁾ The aim of the present study is to measure the activity concentrations of ⁴⁰K, ²³⁸U (²²⁶Ra) and ²³²Th and geochemistry of major oxides: SiO₂, Al₂O₃, Fe₂O₃, P₂O₅, CaO, K₂O, TiO₂, MnO, MgO and Na₂O in rocks. The following objectives were targeted: i. to determine radiation emission from each type of the rocks ii. to estimate radiation hazard indices due to rocks iii. Correlate the activity concentrations of radionuclides and major oxides.

1. Study area

The Nigerian geological basement complex is located from between Latitude 4°N and 15°N and Longitude 3°E and 14°E between the Pan African mobile belt in-between the West African and Congo Craton.⁶⁾ Nigeria geology is broadly classified into three major litho-petrological components, which are, the Basement Complex, Younger Granites, and Sedimentary Basins. The Precambrian Basement Complex, is made up of the Migmatite-Gneiss Complex, the Schist Belts and the Older Granites. The Younger Granites comprise several Jurassic magmatic ring complexes centered on Jos and other parts of north-central Nigeria. The Sedimentary Basins, containing sediment fill of Cretaceous to Tertiary ages, comprise the Niger Delta, the Anambra Basin, the Lower, Middle and Upper Benue Trough, the Chad Basin, the Sokoto Basin, the Mid-Niger (Bida-Nupe) Basin and the Dahomey Basin.^6,7)

The basement complex of Southwestern Nigeria lies between latitudes 7°N and 10°N and longitudes 3°E and 6°E. The region is on the crystalline basement rocks comprising the amphibolite, migmatite gneisses, granites and pegmatite. Other important rock units found in region are the schist comprising biotite schist, quartzite schist, talc-tremolite schist, and the muscovite schist. The states of the southwestern part of Nigeria including Lagos, Osun, Oyo, Ogun, Ondo and Ekiti are situated on basement complex. In terms of lithological setting, Osun and Oyo States belong to crystalline basement complex region, Ondo and Ekiti State belong to post-cretaceous region that comprises shale and sandstone, Ogun State belongs to basement complex (undifferentiated) region and Lagos State belongs to geological area of post-cretaceous.⁸⁾

2. Sample collection

The geological map (Fig. 1) and features of rocks found in Ogun State have been carefully studied prior to the sample collection. Identification and classification of the rock along with physical examination of the rocks was carefully carried out by a geologist from Earth Sciences in Olabisi Onabanjo University. The weathered interface materials on the rocks were removed with sledge hammer and chisel before the samples were collected. The sampling was carried out randomly at 5 locations from each rock site. Ten different rocks were identified; five representative samples of each rock were collected to make a total of 50 samples for the study. The rock samples collected from each site were labeled for easy identification. The samples were then taken to the laboratory at the Department of Geology, University of Ibadan for crushing and pulverizing. The natural radioactivity levels in the samples were measured at the Radiation and Health Physics Research Laboratory at the Department of Physics, Federal University of Agriculture Abeokuta, while geochemical analyses of major oxides were performed at Geology Department, University of Ibadan.

Figure 1. Geological map of Ogun State showing the study areas

3. Sample preparation for spectrometry analysis

Each rock sample was crushed, pulverized and homogenized. The sample was then dried and sieved with a <0.16 mm mesh-size sieve before dried in an electric temperature-controlled oven at 110°C temperature for 4 hours to remove moisture. 200 g each of the dried samples was carefully weighed using an electronic balance with a sensitivity of 0.01 mg into a gas-tight radon impermeable, cylindrical polyethylene container of 2 cm uniform base diameter and sealed. The container was substantially fit to sit on the 5 cm×5 cm NaI (Tl) detector used for the study. The rock samples in the containers were then kept for 4 weeks to allow for a state of secular radioactive equilibrium between ²²²Rn and its short-lived decay products (²¹⁴Pb and ²¹⁴Bi).

4. Sample preparation for geochemical analysis

3 g of each pulverized rock sample was set aside for geochemical analysis. 0.2 g was taken with the aid of a weighing machine and digested with 5 mL of concentrated hydrogen fluoride (HF) and a mixture of prepared solution of nitric acid and hydrochloric acid (ratio 3:1). The sample was stirred and heated inside a fume cupboard containing water bath, the water was allowed to boil at 100°C before the counting time of two hours for the sample to be steamed. The sample was filtered into another graduated cylinder of 100 mL so as to have the stock solution for the analysis. Thereafter, the stock solution of the sample was diluted with distilled water and made up to 50 mL (representing stock solution, ×50-dilution factor). The dilution was done to prevent the analyzing machine from being damaged.

5. Determination of activity concentrations

A 5 cm×5 cm solid NaI (Tl) gamma-ray spectrometric manufactured by ORTEC and coupled to a Digital-based multi-channel analyzer (MCA) was used to count the activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th. The detector has a poor energy resolution of about 8% at energy of 0.662 MeV. This is considered adequate to distinguish the gamma energies of interest in the study. In addition, the photons emitted by the samples would sufficiently be discriminated if the emission probability and energy were high enough and the surrounding background continuum was low enough.

However, the activity concentration of ²¹⁴Bi determined from its 1.76 MeV gamma ray peak was chosen to provide an estimate of ²²⁶Ra in the rock samples, while that of the daughter radionuclide ²⁰⁸Tl determined from its 2.61 MeV gamma ray peak was chosen as an indicator of ²³²Th. The activity concentration of ⁴⁰K was determined from 1.46 MeV gamma-rays emitted during the decay of ⁴⁰K. The standard reference sample used for efficiency calibration was from Rocketdyne Laboratories California, USA, traceable to a mixed standard gamma source (Ref No 48722-356) by Analytic Inc., Atlanta, GA, USA.

Equation (1) shows the usual relationship between activity concentration and the count rate under the photo peak of a given gamma-ray spectrometry detector.²⁾

$$C=\frac{C_n}{{\epsilon }_p{I}_{\gamma }{m}_s}$$(1)

Where C is the activity concentration of the radionuclides (⁴⁰K, ²²⁶Ra and ²³²Th) in the sample (Bqkg^–1), Cn is the count rate under the photo peak, ε_p is the detector efficiency at a specific gamma-ray energy, I_γ is the absolute transition probability of the specific gamma-ray and m_s is the mass of the sample.

An empty container of the same geometry with sample container was counted for the same time to take care of the background radiation count and determination of the radionuclide detection limits. The detection limits (DLs) which describes the operating capability of the detector without the influence of any sample were determined using⁹⁾ model.

The detection limits (DLs) obtained in the present study were 0.12, 0.14 and 0.40 Bqkg^–1 for ⁴⁰K, ²²⁶Ra and ²³²Th respectively. The activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th less than the corresponding values of the DLs is referred to as below detection limit (BDL). One-half of each DL is considered for calculating the mean activity concentrations of the radionuclides and the radiological parameters.¹⁰⁾

6. Radiological assessments of the rock samples

6.1. Outdoor absorbed and effective dose rates

The quantity of absorbed dose is the amount of energy per unit mass absorbed by irradiated object. Absorbed dose is the energy responsible for damage in living organism. The absorbed dose rate (nGyh^–1) at 1 m above the ground in air is calculated using the expression given by equation (2).¹¹⁾

$$D_R=0.462A_{Ra}+0.64A_{Ra}+0.0417A_K$$(2)

Where D_R is the absorbed dose rate in nGyh^–1, A_Ra, A_Th and AK are the respective activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K measured in Bqkg^–1. However, annual effective dose is used to assess potential long-term effects that might occur in future due to ionizing radiation exposure of the general public. The annual effective dose E_D (mSvy^–1) to the public due to absorbed dose rate in air can be calculated using equation (3).¹²⁾

$$E_D=D_R\times 8760\times 0.2\times 0.7$$(3)

Where E_D is the effective dose in mSvy^–1, D_R (nGyh^–1) is the dose rate in air, 8760 is the time in hour for one year, 0.2 is the outdoor occupancy factor and 0.7 in the conversion factor.¹¹⁾

6.2. Radium equivalent activity (Ra_eq)

The radium equivalent activity (Ra_eq) is used as a common index to compare the specific activities of samples. It provides a useful guideline in regulating the safety standards on radiation protection of the general public and obtained as the sum of the weighted activities of ²²⁶Ra, ²³²Th and ⁴⁰K (Bqkg^–1) based on the estimation for which 10 Bqkg^–1 of ²²⁶Ra, 7 Bqkg^–1 of ²³²Th and 130 Bqkg^–1 of ⁴⁰K will deliver the same gamma dose rate.¹³⁾ The radium equivalent was calculated through the use equation (4).

$$R{a}_{eq}=C_{Ra}+1.43C_{Tn}+0.077C_K$$(4)

Where C_Ra, C_Th and C_K are the activity concentrations (Bqkg^–1) of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively.

6.3. External radiation hazard index (H_ex)

External hazard index (H_ex) is used to measure the external hazard due to the emitted natural gamma radiation. The external hazard index, H_ex estimates the potential radiological hazard posed by the different rock samples for the external gamma dose of materials to 1.5 mGy/year. It is another criterion to assess the suitability of a material. A safety criterion for materials used for building construction is that H_ex ≤1.¹²⁾ External hazard index is also calculated using equation (5).

$$H_{ex}=\frac{C_{Ra}}{370}+\frac{C_{Th}}{259}+\frac{C_K}{4810}$$(5)

Where C_Ra, C_Th, C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K respectively.

6.4. Internal radiation hazard index (H_in)

In addition to the external hazard index, there is also a threat to the respiratory organs due to ²²²Rn, the gaseous short-lived decay product of ²²⁶Ra. The internal hazard index (H_in) is defined generally to reduce the maximum permissible concentration of ²²⁶Ra to half the value appropriate for external exposure alone.¹⁴⁾ Internal exposure to radon and its progeny products is quantified by estimating the internal hazard index using the model provided by the equation (6).¹⁵⁾

$$H_{in}=\frac{C_{Ra}}{185}+\frac{C_{Tn}}{259}+\frac{C_K}{4810}$$(6)

Where C_Ra, C_Th, C_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K respectively. If the maximum concentration of ²²⁶Ra is one-half that of the normal acceptable limit, then H_in will be less than one. For safety precautions in the use of materials in the construction of dwellings, the criterion demands that H_in ≤1.

6.5. Representative gamma index (I_γ)

The gamma index (I_γ) is used as screening tool for identifying materials that might be a threat to human health. The representative gamma index (I_γ) used to estimate the level of γ - radiation hazard associated with the natural radionuclides in specific investigated samples. It is calculated using equation (7).¹⁶⁾

$$I_{\gamma }=\frac{C_{Ra}}{150}+\frac{C_{Tn}}{100}+\frac{C_K}{1500}$$(7)

Where C_Ra, C_Th and C_K, are the activity concentrations (Bqkg^–1) of ²²⁶Ra, ²³²Th and ⁴⁰K respectively.

1. Activity concentrations of the radionuclides in the rock samples

The activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th in different rock samples from Ogun State are presented in Table 1. The average activity concentration of ⁴⁰K, ²²⁶Ra and ²³²Th were 1,764.1±38.4, 36.9±21.5, 43.6±25.8 Bqkg^–1 respectively for Granite, 447.7±9.2, 27.1±1.8, 63±5.7 Bqkg^–1 respectively for Quartzite, 122.0±3.9, 16.0±3.9, 20.4±0.6 Bqkg^–1 respectively for Phosphate, 504.8±4.6, 26.3±14.8, 16.4±9.1 Bqkg^–1 respectively for Shale, 434.1±7.1, 14.2±0.6, 30.9±1.6 Bqkg^–1 respectively for Limestone, 274.7±4.5, 5.1±0.2, 13.4±3.8 Bqkg^–1 respectively for Sandstone, 1,086.8±2.0, 51.8±29.0, 18.7±0.4 Bqkg^–1 respectively for pegmatite, 1,753.0±33.9, 33.2±0.8, 68.8±1.2 Bqkg^–1 respectively for migmatite and 446.9±16.7, 28.8±16.1, 11.1±6.2 Bqkg^–1 for Clay stone. According to,¹²⁾ the recommended world average value of ⁴⁰K, ²²⁶Ra and ²³²Th for rocks are 500 Bqkg^–1, 30 Bqkg^–1 and 35 Bqkg^–1 respectively. From the result, granite, pegmatite and migmatite have values slightly higher than the recommended values, on the other hand, quartzite, phosphate, shale limestone and sandstone have average values lower than the¹²⁾ recommended values. The elevated level of natural radionuclide in granite, pegmatite and migmatite is because they are igneous rock, meanwhile other rocks fall into sedimentary and metamorphic rock. A similar report from the study area have been reported by several authors.^17-20) In addition, the results show that there are clear differences in concentration of ⁴⁰K, ²²⁶Ra and ²³²Th in various rock samples, this is graphically shown in Fig. 2 this implies that distribution of natural radionuclide greatly depends on rock type and consequently radiation risk due to individual rocks varies. As could be seen from Table 1, ⁴⁰K has the highest value in all the entire rock samples, however, ²³²Th was highest in pegmatite and the least was recorded in sandstone. The highest value of ²²⁶Ra was recorded in migmatite and quartzite with values 71±12 and 71±32 respectively. The trend of increment in the average concentration of ⁴⁰K indicated that granite>migmatite>pegmatite>quartzite>shale>clay stone>limestone>sandstone> phosphate whereas ²²⁶Ra decreased in the trend as from clay stone232Th was found as pegmatite>granite>migmatite>clay stone>quartzite>sandstone>phosphate>limestone>sandstone. The results show that natural radioactivity is more pronounced in the rocks that are igneous in nature this is an indication that igneous rocks have higher radiation risk when used for building construction even though it is the hardest types of rocks. The concentrations of the three natural radionuclides are independent of each other, there is no similarities in the concentration. ²²⁶Ra and ²³²Th have the most detrimental radiation effect on humans. Numerous studies undertaken by authors within and outside the world is shown in Table 2, 3^1,15,19-29) and are comparable with the present study.

Table 1 . Activity concentrations, absorbed and effective dose rates of natural radionuclides.

Rock type	40K (Bqkg–1)	232Th (Bqkg–1)	226Ra (Bqkg–1)	Outdoor absorbed gamma dose (nGyh–1)	Outdoor effective dose (mSv/yr)
Granite	1,790.1±58.7	BDL	BDL	33.5	0.04
	1,799.7±67.3	55.3±20.7	57.8±11.4	93.9	0.12
	1,701.4±67.7	42.9±11.7	66.8±32.2	86.6	0.11
	1,764.8±30.2	47.8±31.1	47.1±50.6	83	0.1
	1,764.3±75.1	38.2±21.9	46.3±32.1	76.5	0.09
Mean±σ	1,764.1±38.4	36.9±21.5	43.6±25.8	74.7±13	0.09±0.03
Quartzite	449.2±62.7	29.3±28.9	61.3±2.7	64.7	0.08
	460.8±17.4	27.8±7.1	65.4±22.8	66.9	0.08
	435.2±66.7	27.2±31.0	55.9±67.3	61.1	0.07
	444.9±54.1	24.4±20.7	71.4±32.3	66.4	0.08
	448.4±20.9	26.7±11.2	62.8±25.2	64.8	0.08
Mean±σ	447.7±9.2	27.1±1.8	63±5.7	64.7±09	0.07±0.01
Phosphate	121.8±11.1	10.2±10.6	21.3±9.1	21.2	0.03
	127.3±16.8	20.8±11.3	19.9±8.6	27.4	0.03
	116.2±19.8	15.3±4.9	20.6±4.3	24.3	0.03
	122.7±16.7	18.2±2.3	19.8±11.5	25.5	0.09
	122.1±11.8	15.7±9.4	20.3±9.4	24.6	0.03
Mean±σ	122.0±3.9	16.0±3.9	20.4±0.6	24.60±03	0.04±0.01
Shale	505.8±77.4	BDL	BDL	21.7	0.03
	500.2±42.3	30.5±11.8	20.2±9.4	49.2	0.06
	501.1±43.6	35.3±15.6	20.6±11.2	52.4	0.06
	511.9±63.5	33.1±18.4	20.5±4.5	51.5	0.06
	504.9±16.2	32.7±16.7	20.6±3.3	51	0.06
Mean±σ	504.8±4.6	26.3±14.8	16.4±9.1	45.1±16	0.05±0.01
Limestone	435.7±69.8	14.1±7.6	28.8±7.3	40.3	0.05
	438.2±45.4	13.8±4.5	30.4±2.1	40.9	0.05
	440.8±23.3	15.3±5.9	33.2±5.3	42.9	0.05
	422.3±50.6	14.1±3.4	30.8±2.9	40.9	0.05
	433.7±57.4	13.7±2.3	31.4±11.1	41.3	0.05
Mean±σ	434.1±7.1	14.2±0.6	30.9±1.6	41.2±09	0.05±0.00
Sandstone	273.7±96.1	5.2±2.1	10.7±5.0	20.2	0.02
	278.2±56.3	4.9±1.2	18.2±6.1	23.7	0.03
	267.8±81.7	5.3±1.1	16.3±9.2	21.8	0.03
	279.2±79.6	5.1±1.5	8.9±5.2	19.4	0.02
	274.7±44.7	4.9±1.3	13.1±7.2	21.3	0.03
Mean±σ	274.7±4.5	5.1±0.2	13.4±3.8	21.2±1.65	0.02±0.01
Pegmatite	1,085.8±92.1	BDL	BDL	45.9	0.06
	1,090.1±88.6	64.9±4.9	22.9±8.5	95.5	0.12
	1,086.7±61.8	67.2±9.8	23.4±3.6	96.9	0.12
	1,084.7±88.2	61.7±7.8	24.3±5.8	94.6	0.12
	1,086.6±20.7	65.3±7.8	22.7±8.6	95.7	0.12
Mean±σ	1,086.8±2.0	51.8±29.0	18.7±10.4	85.7±22.2	0.10±0.03
Migmatite	1,759.8±16.3	32.2±4.3	68.1±27.6	124.8	0.15
	1,762.4±34.1	33.6±5.2	67.3±1.4	125.8	0.15
	1,791.7±64.6	33.1±63	70.7±11.8	127.7	0.16
	1,698.4±50.6	34.4±2.4	69.3±4.8	123.3	0.15
	1,752.7±43.1	32.9±7.1	68.7±11.2	125.5	0.15
Mean±σ	1,753.0±33.9	33.2±0.8	68.8±1.2	125.4±23	0.15±0.01
Clay stone	431.2±14.5	BDL	BDL	18.6	0.02
	432.7±22.0	38.3±12.1	15.2±11.0	48.2	0.06
	451.8±16.2	34.2±15.3	11.9±10.2	45.2	0.06
	472.3±17.7	35.8±18.1	13.7±11.9	48.4	0.06
	446.7±21.4	35.7±11.2	14.4±3.4	47.2	0.06
Mean±σ	446.9±16.7	28.8±16.1	11.1±6.2	41.52±11	0.05±0.02

Table 2 . Comparison of activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th (Bqkg^–1) in some rocks from other places within Nigeria.

Location	Material type	226Ra	232Th	40K	References
Oguta	Soil	47.89	55.37	1,023	19)
Imo State	Soil sample	20.69	25.04	88.41	21)
Ogun	Rock	42.33	128.7	453.3	20)
Ogbomoso	Stone dust	27.87	16.69	175.85	22)
Ekiti	Concrete block	47.9	63.8	572.6	23)
Ekiti	Rock	18.7	39.8	351.1	23)

Table 3 . Average values of activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th in Bqkg^–1 in some rocks from different countries of the world.

Country	Material type	226Ra	232Th	40K	References
Slovak	Granitic rock	77.3	91.4	929.3	24)
Egypt	Granite	40	12.5	47.1	25)
Czech	Rock	386.2	55.0	1,244.0	1)
Bangladesh	Rock	25.5	37.4	884.0	26)
India	Granites	34.06	79.05	933.6	27)
Ghana	Granites	356	161	1,796.0	28)
Pakistan	Cement	111.2	33.2	199.1	15)
Kenya	Rock	195.6	409.5	915.6	29)

Figure 2. Activity concentrations ⁴⁰K, ²²⁶Ra and ²³²Th in different rock samples

2. Geochemistry of major oxides in the rock from the study area

The major element oxides composition of rocks from the study area measured (in weight %), are presented in the Table 4. A smooth and systematic variation in chemical composition of major elements in the rock samples were observed, this showed that the elemental composition of rocks depends greatly on magma composition, fractional crystallization process by which the rock is formed and geographical location.¹⁷⁾ The percentage composition of SiO₂ in the rock samples is above 50 wt% in all the rock samples from the study areas except in limestone, shale and phosphate. Al₂O₃ content in all the samples from the study areas ranged from 4.10~21.24 wt% while Fe₂O₃ and CaO content ranged from 0.39~7.5 wt%, 0.09~46.6 wt% respectively. In addition, sodium oxide (Na₂O) and potassium oxide (K₂O) are also present in at least 5 wt%. Other major oxides including TiO₂, P₂O₅, K₂O, MnO, MgO and Na₂O were depleted in the rock samples from the study area. This is a clear indication that samples analyzed in the present study were formed from igneous origin as a result of basement complex lithology of the area.

Table 4 . Major elemental oxides composition of rocks from the study areas (weight %).

Rocks	SiO2	Al2O3	Fe2O3	TiO2	CaO	P2O5	K2O	MnO	MgO	Na2O
Quarzite	90.10	4.10	1.70	-	1.30	-	-	-	-	-
Quartzite	89.10	4.90	1.80	-	1.39	-	-	-	-	-
Pegmatite	72.71	15.99	0.45	0.02	0.22	0.38	3.68	0.01	0.06	5.56
Pegmatite	72.07	15.16	0.39	0.01	0.09	0.28	8.42	0.05	0.10	2.76
Granite	69.37	13.98	3.86	0.01	2.58	0.38	3.58	0.21	0.09	3.01
Granite	59.21	9.86	12.97	0.97	5.99	0.81	3.38	0.16	0.07	2.13
Migmatite	68.38	15.87	4.50	0.55	4.07	0.01	2.89	0.11	2.31	4.03
Migmatite	57.22	16.23	7.18	0.84	6.56	0.21	1.39	0.10	3.81	3.38
Shale	47.54	20.68	4.49	2.66	2.31	0.02	0.92	0.01	7.08	0.64
Phosphate	31.70	12.44	0.26	0.86	10.09	31.66	2.43	0.04	6.49	0.65
Limestone	8.55	2.00	2.32	-	46.63	-	0.91	-	1.14	0.98
Limestone	8.65	2.10	2.72	-	47.63	-	0.51	-	1.04	0.38
Schist	65.38	15.87	4.50	0.55	4.07	0.13	2.89	-	2.31	4.03
Clay stone	56.38	21.24	7.65	1.04	3.45	0.23	0.94	0.02	1.46	1.28
Sandstone	57.18	21.24	7.65	1.06	3.45	0.23	0.94	0.02	0.46	1.28

3. Correlation between activity concentrations and major oxides in the rocks

The results of the Pearson correlation coefficients between the activity concentrations of ⁴⁰K, ²²⁶Ra, ²³²Th and major elemental oxide SiO₂, Al₂O₃, Fe₂O₃, TiO₂, CaO, P₂O₅, K₂O, MnO, MgO and Na₂O in rock samples are presented in Table 5. The analysis revealed that SiO₂ had significant positive correlation with ²²⁶Ra and ²³²Th at 0.05 levels. Similarly, MgO had a very strong positive correlation with ²³²Th at 0.01 levels while CaO had negative correlation with ²²⁶Ra and the remaining of the major element did not show any significant correlation with activity concentration. The positive correlation between SiO₂ and ²²⁶Ra and ²³²Th is an indication that the rock collected the study areas derived their origin or formed from either melting of the igneous material or sediment materials or a mixture. This further supports the strong association between SiO₂, ²²⁶Ra and ²³²Th which is an indication that their presence in the rock may be under similar geochemical influence.³⁰⁾ SiO₂ concentration exhibited high degree of negative correlation with CaO indicating calcite and quartz mineral in a rock sample may possibly not coexist. In contrast, there is a negative correlation between ⁴⁰K and (SiO₂, ²²⁶Ra, ²³²Th), implying that ²²⁶Ra and ²³²Th have dissimilar geochemical behavior with ⁴⁰K.

Table 5 . Pearson correlation matrix of activity concentrations and major oxides granite.

	K	Ra	Th	SiO2	Al2O3	Fe2O3	TiO2	CaO	P2O5	KO	MnO	MgO	Na2O
K	1
Ra	–0.806	1
Th	–0.602	0.845*	1
SiO2	–0.472	0.819*	0.886*	1
Al2O3	–0.231	0.479	0.321	0.708	1
Fe2O3	0.420	–0.743	–0.772	–0.955**	–0.828*	1
TiO2	–0.134	–0.331	–0.666	–0.731	–0.406	0.730	1
CaO	0.504	–0.814*	–0.796	–0.979**	–0.825*	0.971**	0.633	1
P2O5	0.520	–0.779	–0.790	–0.968**	–0.808	0.946**	0.601	0.989**	1
KO	–0.716	0.710	0.723	0.792	0.664	–0.844*	–0.415	–0.828*	–0.852*	1
MnO	0.755	–0.639	–0.763	–0.509	0.065	0.316	0.130	0.427	0.493	–0.565	1
MgO	–0.612	0.712	0.932**	0.693	0.018	–0.513	–0.485	–0.570	–0.599	0.585	–0.916*	1
Na2O	–0.458	0.679	0.660	0.908*	0.903*	–0.949**	–0.561	–0.964**	–0.973**	0.872*	–0.339	0.431	1

*Correlation is significant at the 0.05 level (two-tailed), **Correlation is significant at the 0.01 level (two-tailed)..

4. Absorbed and effective dose rates in rocks from the study area

Table 1 shows the absorbed dose and annual effective dose rates obtained in the rock samples from the study area. The mean absorbed dose in is 74±13 nGy/h (granite), 64±09 nGyh^–1 (quartzite), 24±11 nGyh^–1 (phosphate), 45±16 nGyh^–1 (shale), 41.2±09 nGyh^–1 (limestone), 21±10 nGyh^–1 (sandstone), 72 nGyh^–1 (pegmatite), 125±23 nGyh^–1 (migmatite) and 41±11 nGyh^–1 (claystone). These average values obtained in granite, quartzite, pegmatite, migmatite were higher than the world recommended average value of 59 nGyh^{–1 12)} whereas the average values obtained in sandstone, limestone, phosphate, shale were below the world recommended. The outdoor annual effective dose rates in all rock samples were slightly higher than the world average dose of 0.34 mSv/yr to individual from outdoor radiation exposure.³¹⁾

5. Radiological assessment of the rock samples from the study area

Radium equivalent, internal hazard and external hazard indices are parameters, used as a safety standard in radiation protection for the general public. Table 6, shows all the values Radium equivalent, internal hazard and external hazard indices in the rock samples. All the values of radium equivalent in the rock samples are within the recommended value of 370 Bq/kg. In addition, the external, internal and gamma hazard indices were less than unity except in granite, migmatite, pegmatite and quartzite as shown in Fig. 3.

Table 6 . Radium equivalent, internal, external hazard and gamma representative indices.

Rock type		Raeq (Bq/kg)	Hin	Hex	Iγ
Granite	Range	62.3~199.5	0.17~0.70	0.17~0.54	0.54~1.48
	Mean	156.18	0.542	0.42	1.176
	Std	54.38	0.22	0.15	0.37
Quartzite	Range	129.9~142.2	0.5~0.57	0.35~0.38	0.95~1.03
	Mean	137.66	0.542	0.37	1.00
	Std	4.78	0.03	0.01	0.03
Phosphate	Range	45.5~141.4	0.18~0.44	0.12~0.38	0.14~0.43
	Mean	70.74	0.248	0.188	0.332
	Std	39.85	0.11	0.11	0.11
Shale	Range	40.4~111.1	0.11~0.36	0.11~0.30	0.35~0.83
	Mean	94.36	0.3	0.254	0.718
	Std	30.28	0.11	0.08	0.21
Limestone	Range	83.2~88.9	0.30~0.33	0.22~0.24	0.63~0.67
	Mean	85.36	0.314	0.23	0.644
	Std	2.14	0.01	0.01	0.02
Sandstone	Range	39.0~48.4	0.13~0.18	0.11~0.13	0.31~0.37
	Mean	43.12	0.152	0.118	0.334
	Std	3.62	0.02	0.01	0.02
Pegmatite	Range	85.1~203.6	0.23~0.61	0.23~0.55	0.73~1.56
	Mean	177.54	0.53	0.478	1.374
	Std	51.71	0.17	0.14	0.36
Migmatite	Range	248.6~256.8	0.86~0.89	0.67~0.69	1.93~2.00
	Mean	252.4	0.87	0.68	1.966
	Std	3.04	0.01	0.01	0.02
Claystone	Min	96.1~103.2	0.29~0.32	0.26~0.28	0.73~0.78
	Mean	100.775	0.31	0.2725	0.76
	Std	3.32	0.01	0.01	0.02

Std: Standard deviation, Ra_eq: Radium equivalent, H_in: Internal hazard index, H_ex: External hazard index, I_γ: Gamma representative index..

Figure 3. Internal hazard index (H_in), External hazard index (H_ex), Gamma representative index (I_γ)

This study has measured the activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th and geochemistry of major oxides SiO₂, Al₂O₃, Fe₂O₃, P₂O₅, CaO, K₂O, TiO₂, MnO, MgO and Na₂O in different rock samples collected from Ogun State. The results of the analysis of data obtained from activity concentrations showed that the activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th are higher in granite, pegmatite, migmatite, and quartzite as a result of their similar characteristics of igneous origin but concentration of ⁴⁰K, ²²⁶Ra and ²³²Th are lower in shale, phosphate, clay stone, sand stone and limestone because they did not form from original rock but through either sedimentation or metamorphism. However, the activity concentrations of ⁴⁰K, ²²⁶Ra and ²³²Th in rocks depend on geology, rock-type and the mineral composition like SiO₂ which was found to be above 50 wt% in all the rock samples from the study areas except limestone. The mean annual effective dose due to radiation from rocks was comparable to the recommended safe limit, and the radiological hazard indices were slightly higher than the recommended international safe limits. Hence, the findings suggest that Ogun State could be described as a region having elevated background radiation. To avert potential radiation-related health issues, it is recommended to exercise care and subject the materials to international safety limits standard when building residences using rocks sourced from Ogun State. The results could be used by the government, local authorities can use to regulate the use of rocks with elevated radiation risk in building construction. Further investigation is also suggested in quarry sites to monitor radiation dose due to inhalation of dust by the workers and the public. It is recommended that residence in Ogun State should use home radiation monitoring instrument to monitor radon emanation from walls.

No potential conflict of interest relevant to this article was reported.

Olabamiji Aliu Olayinka (Doctor), Alausa Shamsideen Kunle (Professor)