Measurement of Natural Radioactivity in Lagoon Sands Used in Construction in the District of Abidjan, Côte d’Ivoire

Sand is an important natural material for the construction of houses, work buildings and other public spaces. This work, which is one of the first contributions to the environmental quality of construction materials, concerns the measurement of natural radioactivity in the lagoon sands collected in the district of Abidjan. Nineteen (19) samples of these sands are analyzed by gam-ma-ray spectrometry equipped with HPGe detector. The mean values obtained for 226 Ra, 232 Th and 40 K are respectively 7.76 ± 1.84 Bq·kg −1 , 5.21 ± 1.36 Bq·kg −1 , and 217.31 ± 5.03 Bq·kg −1 . The estimated average value of radium equivalent (Raeq) is 31.94 Bq·kg −1 . The results show that the average values obtained are far lower than the global limits of 35, 30, and 400 Bq·kg −1 for the concentrations of 226 Ra, 232 Th and 40 K, respectively, and 370 Bq·kg −1 for the equivalent radium established by the United Nations Scientiﬁc Committee on the Effects of Atomic Radiation (UNSCEAR). Therefore, the use of the analyzed lagoon sand samples in the different construction sectors should not cause serious radiological effects on the populations living in the District of Abidjan. Our results provide new data on building materials radioactivity in Côte d’Ivoire and all over the World. They can also be used as a reference for future work.


Introduction
Exposure to ionizing radiation from sources of radioactivity in the environment represents one of the major health risks for human beings. The most important natural radionuclides which could cause radiation protection problems are uranium ( 238 U and 235 U), thorium ( 232 Th), their decay products, and potassium ( 40 K) [1]. Construction materials generally contain different amounts of radionuclides which depend both on the nature of the soil and on the geological position in a geographic area. Therefore, construction materials represent a permanent source of ionizing radiation because people spend more time (estimated at 80% indoors) in built environments (homes, workplaces and entertainment spaces) [2].
In this context, great interest has been given to determining the levels of radioactivity in building materials in many countries [3]- [10]. In Côte d'Ivoire, the number of construction projects is in perpetual growth in recent years. This is the result of strong demographic growth. As a result, sand extraction from the lagoon has increased considerably, particularly in the District of Abidjan to supply the construction market. At present, knowledge of the radiological impacts related to construction materials is limited because little study has been dedicated to them compared to other environmental studies. In this context, the objective of this work is to assess the levels of natural radioactivity in the lagoon sand samples collected from the District of Abidjan.

Study Area
The District of Abidjan is located between latitudes 5˚10' and 5˚38' North and longitudes 3˚45' and 4˚21' West and occupies an area of 2119 km 2 . Its population density is estimated at 2221 inhabitants/km 2 . According to the recent census, its population is estimated at 4,707,404 inhabitants, with a growth rate of 3.7% [11].
Geologically, the lithology of the study area consists of clayey sand, medium sands and coarse sands resting on a granite and schist base [12]. From a hydrological point of view, the district of Abidjan is covered by a dense hydrographic network made up of three main lagoons (parallel to the Atlantic Ocean and intersecting the coast) EBRIE AGHIEN and POKOU as well as many rivers ( Figure 1). A large amount of sandy sediment has developed at the bottom of these lagoons which makes this region one of the most important sand production regions in Cote d'Ivoire.

Sampling and Preparation Equipment
The materials analyzed during this work are nineteen (19)   Marinelli beakers (SG 500) for the conditioning of the samples, an oven to dry these samples, a ball mill, and a sieve of 2 millimeters mesh.

Equipment Analysis
The radioactivity of the sand samples is measured using the gamma-ray spectrometer installed at the laboratory of the Radiation Protection Institute (RPI) of the Ghana Atomic Energy Commission (GAEC). The measurement chain includes the measurement chamber, a liquid nitrogen cooling system (77 K), a High Purity Germanium (HPGe) coaxial detector model GX 4020-7500 SL (Canberra) with a resolution of 2 keV at 1332 keV ( 60 Co). There is also a high voltage power supply (4000 V) necessary for the collection of the electric charges deposited in the detector. A cylindrical lead shielding 10 cm thick makes the contributions of ambient radiation reduction possible. Moreover, there is a multichannel analyzer (MCA) connected to a computer with an acquisition and processing software (GENIE 2000, Canberra). For the calibration, we used standard multi sources composed of 241 Am, 109 Cd, 139 Ce, 113 Sn, 137 Cs, 57 Co, 60 Co, and 88 Y.

Samples Collection
The nineteen sand samples are collected from the main suppliers and at the con-World Journal of Nuclear Science and Technology struction sites. Each sample is a combination of five (5) incremental samples taken at random from five points in the sand stock from a depth of approximately 15 cm. Two (2) kilograms of each sample is collected and packaged in contamination-free polyethylene bags, adequately labeled in situ to facilitate their exact identification in the preparation laboratory.

Preparation Methods
In laboratory, all samples are dried at 105˚C in an oven until all moisture has been removed. A constant dry weight of the samples is obtained after 24 hours.
The samples are then crushed and sieved (at 2 mm) in order to get rid of the unwanted fractions. After their reduction to a homogeneous powder, the samples are conditioned in Marinelli type geometry (SG 500), sealed against radon for 30 days before the measurements in order to allow a secular equilibrium.
This equilibrium corresponds to at least seven half-lives of 222 Rn (3.81d) between the radionuclides 226 Ra and 232 Th and their respective descendants [13].

Radioactivity Measurement
The radioactivity measurement of the samples was carried out at the laboratory of GAEC. One (1) kilogram dry mass sand samples are measured for 10 hours using a Gamma-ray spectrometer. During the measurement, the HPGe detector mounted on the cryostat is cooled with liquid nitrogen (77 K) in order to decrease the resistivity of the material and reduce the thermal background noise. The detector is also surrounded by a cylindrical lead castle of 10 cm thick to attenuate the background noise in the measured spectrum due to natural ambient radioactivity. Spectra analysis are performed using Genie 2000 (Canberra) software. The spectrum of each sample gives the number of counts recorded per second as a function of the energy of the gamma rays emitted by the radionuclides. Background measurements of the laboratory are also carried out in order to obtain net counts for each sample. Thus, an empty Marinelli beaker is measured to find the background noise generated by the ambient natural radioactivity. This background noise is then subtracted from the activities of each radionuclide in the measured samples.
The energy and the calibration efficiency of the detector are determined using a standard multi-source with energies between 60 and 2000 keV. This makes it possible to take into account the desired radionuclides and reduce the counting losses linked to the coincidence summing and self-absorption effects of gamma rays. The energy and calibration efficiency of the detector, respectively, are used to identify the radionuclides detected in the samples. Thus, we can determine their concentrations [14].

Activity Concentration
The activity concentrations of radionuclides in the sand samples are calculated using the equation below.

Radium Equivalent Activity (Raeq)
The distribution of radionuclides 226 Ra, 232 Th and 40 K in terrestrial materials is not uniform. The risk induced by the radioactivity existing in a building material is evaluated by different models among them the equivalent radium (Ra eq ). Ra eq is defined as the weighted sum of the activities of 226 Ra, 232 Th, and 40 K, according to Equation (3).

Activity Concentration
The activity concentration results acquired for the nineteen (      the world average, we can notice that the average concentration of 232 Th found (5.21 ± 1.36) Bq·kg −1 is lower (6 times) than the UNSCEAR limit (2000) of 30 Bq·kg −1 [2] shown in Figure 2. Comparing with the mean values measured in other countries (Table 2), it can be seen that the mean value of 232 Th in this work is similar to 5 Bq·kg −1 measured in Kenya but lower than the value of 3.2 Bq·kg −1 found in Egypt. However, the value of 119.42 Bq·kg −1 measured in India is more than 20 times higher than in our work. The activity concentrations of 226 Ra obtained are slightly higher than those of 232 Th ( Figure 5). From the results of the samples shown in Figure 3, one can notice that the concentrations of 226 Ra vary in all measurements. This variation is from (1.9 ± 0.05) Bq·kg −1 in the SAB-2 sample to (15.3 ± 3.75) Bq·kg −1 in the SAB-5 sample, and none is below the detection limit. The differences observed between the concentrations of 226 Ra are probably dependent on the geological origin of the samples. In addition, according to Figure 3, we notice that the average value of the concentration of 226 Ra (7.76 Bq·kg −1 ) is lower (5 times) than the standard imposed by UNSCEAR (2000) which is estimated by 35 Bq·kg −1 [2]. The comparison of the values with those of other studies made on sand ( Table 2) allows noticing that the average value obtained in this work is lower than 37 Bq·kg −1 measured in the USA and 44 Bq·kg −1 recorded in Turkey but slightly higher than the value of 3.7 Bq·kg −1 obtained in Australia. According to Figure 5, the concentrations of 40 K in the samples are the highest. This could be explained by the fact that originally the 40 K radionuclide is the most abundant in the earth's crust [18]. As for the 226 Ra and the 232 Th, the values of 40 K generally vary between (160.74 ± 1.05) Bq·kg −1 in the sample SAB-2 to (296.91 ± 7.93) Bq·kg −1 in SAB-14 while fluctuating according to the sampling points. Figure 4 shows the variation of 40 K concentration in sand samples. As can be seen, all the levels shown in this graph are well above the detection limit of 0.15 Bq·kg −1 . In addition, as can also be seen from the data in Figure 4 some values are different from point to point while others are similar. These deviations could indicate a difference in the geological origin of these samples. Moreover, the comparison at the world level shows that the average value 217.31 Bq·kg −1 obtained with regard to the (19) samples studied is twice above the limit of UNSCEAR (2000) quantified at 400 Bq·kg −1 [2]. Moreover, by comparing the average value with those of some other countries cited in Table 2

Radium Equivalent Activity (Raeq)
To demonstrate the relationship between all these measured concentrations and their radiological impact, the radium equivalent activity (Ra eq ) is estimated using  Figure 6 represents the variation of Ra eq in samples. It reveals marked differences in radium equivalent between some samples. These differences are linked to the differences observed in the activity concentrations of radionuclides according to the sampling points. Moreover, all the values of (Ra eq ) obtained in this study are lower than the average value recommended by UNSCEAR [2] which is 370 Bq·kg −1 . Therefore, the use of the analyzed sands in the construction works could not cause significant radiological risks in the short term.

Conclusion
Gamma-ray spectrometry is used to measure the activity concentrations of 226 Ra, World Journal of Nuclear Science and Technology negligible. However, the radiological risks due to cumulative exposure dose should be assessed considering long-term exposure. In perspective, future studies will be oriented towards the determination of the long-term carcinogenic effects and the contribution of these sands to indoor radon.