© EDP Sciences 2018

1 Introduction

All building materials contain radionuclides of natural origin and are considered to be the main sources for the radiation exposures of individuals living in dwellings. The use of materials with elevated radioactivity in building construction can increase the indoor exposure to ionizing radiation. In recent years, the study of radioactivity has attracted increasing interest in various building materials in European American, Asian, Indian and some African countries. Results are available online (Ackers et al., 1984; Aders et al., 1985; Beretka and Mathew, 1985; Ettenhuber and Lehmann, 1986; Pinnock, 1991; Tufail et al., 1992; Malanca et al., 1993; Hayumbu et al., 1995; Bou-Rabee and Bem, 1996; Ahmad et al., 1997; Khatibeh et al., 1997; Ahmed and Hussein, 1998; Mantazul et al., 1998; Alam et al., 1999; Ibrahim, 1999; Amrani and Tahtat, 2001; Rizzo et al., 2001; Stoulos et al., 2003; Ademola and Farai, 2005; Dabayneh, 2007; Ngachin et al., 2007; Brigido Flores et al., 2008; Faheem and Mujahid, 2008; Turhan et al., 2008; Cosma et al., 2009; Mavi and Akkurt, 2010; Baykara and Karatepe, 2011; El-Taher, 2012; Ravisankar et al., 2012; Xinwei et al., 2012; El-Mageed et al., 2014; Raghu et al., 2015; Li et al., 2016). Unfortunately, no studies have been conducted to assess the level of radioactivity in building materials in Chad. Thus, Chad does not have standards or guidelines for the maximum level of radioactivity in building materials. However, a study carried out in 1970 and funded by the United Nations Development Programme (UNDP) with the support of experts from the International Atomic Energy Agency (IAEA) revealed the existence of radioactive and toxic heavy metals in Mayo-Kebbi region (Oyamta et al., 2013). Boreholes carried out in the soil and subsoil have proven the existence of uranium (U), chromium (Cr), nickel (Ni) and copper (Cu). A cement plant was built in the studied area, gold mining is still done by craftsmen, the prospecting of uranium and oil was intensified. Small-scale mining of lime by craftsmen, gravel and laterite is sustained. Thus, naturally occurring radioactive materials (NORM) measurements in construction materials in Chad have become an imperative. They will contribute to the development of safety standards for the health protection of the population and workers, and will allow the development of national guidelines used for providing recommendations, for the use and management of raw building materials.

As reported by the United Nations Scientific Committee on the effects of atomic radiation (UNSCEAR) (UNSCEAR, 2000), the worldwide average value for outdoor gamma absorbed dose rate in air due to terrestrial sources is 54 nGy h⁻¹ and absorbed dose rate in air inside homes is usually higher than outside (∼20% on average, but sometimes much more) due to the contribution of construction materials. Natural radionuclides in building materials contribute to radiation exposure in two ways; external and internal exposure. External exposure comes mostly from direct gamma radiation emitted from the decay of radionuclides, of the uranium ²³⁸U series, thorium ²³²Th series and potassium ⁴⁰K. In the ²³⁸U series, the decay chain segment starting from radium ²²⁶Ra is radiologically the most important and, therefore, reference is often made to ²²⁶Ra instead of ²³⁸U. The internal exposure is due to alpha particles resulting from the decay of radon ²²²Rn and its progeny. Radon is a chemically inert gas which is colorless, odorless, and highly radioactive. When radon is inhaled, the alpha particles dose is delivered directly to the bronchial tissue, creating a potential for radiogenic lung cancer. Taking in account that people spend more than 80% of their time indoors, the duration of exposure to internal and external radiation from building materials is extended.

In this work, a high purity germanium detector (HPGe) was used to measure the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in various building materials (cement, sand, gravel, and soil bricks) collected in the Mayo-Kebbi region (Fig. 1). The results were used to assess the potential radiological risk associated with the building materials by evaluating the radium equivalent activity (Ra_eq), the external radiation hazard index (H_ex), the internal radiation hazard index (H_in), the indoor air absorbed dose rate (D_in), the outdoor air absorbed dose rate (D_out), the activity utilization index (AUI), the annual effective dose E, the annual gonadal dose equivalent (AGDE), the representative level index (RLI), as well as, the excess lifetime cancer risk (ELCR). The activity concentrations and radium equivalent activity were compared to other studies done elsewhere in the world. Other results were compared to the global average value established by the United Nations Scientific Committee on the effects of atomic radiation (UNSCEAR, 1977, 1988, 1993, 2000, 2010). We also checked whether some results meet the criteria of the Organization for Economic Cooperation and Development (OECD).

For the first time, data will be available on the levels of radioactivity in samples of building materials collected in Chad. These data will be stored in the national database of the Chadian Radiation Protection and Nuclear Safety Agency, and will serve as the baseline level for natural radionuclides in the study area and will contribute to the establishment of a radiological map of the whole country. The recommendations mentioned in the conclusion will allow the Chadian government to regulate the use of construction materials in the study area.

Figure 1 Map showing the location of Mayo-Kebbi Ouest region (provided by Google Map).

2 Materials and methods

2.1 Study area

The region of Mayo-Kebbi Ouest is one of the 23 regions of Chad, and its capital is Pala (09°21′ 50.721″N, 14°54′34.864″E). It covers an area of 12,479 km² and had 564,470 inhabitants in 2009 (INSEED, 2009). This region has a long history of mineral prospecting and mining. A cement plant was built there, the exploitation of gold is artisanal, and the prospection of uranium and oil has been intensified. Artisanal mining of lime, gravel and laterite is supported. These mining activities are performed in a disordered way, and without respecting the international standards (ISO/TC 82/SC 7) that can help minimize the potential long-term damage from mining activities, thus enhancing the quality of life of residents living in a mining area. Our research covers the three departments of the region.

2.2 Sampling and conditioning

According to the June 2013 final report of the Third Survey of Consumption and the Informal Sector in Chad (ECOSIT3, 2013), 82.4% Chadian households lives in dwellings whose walls are made of traditional materials or unsustainable. 57.7% of households lives in dwellings with soil bricks, made of mud kneaded with straw and water and called “banco”. 24.7% of households lives in dwellings with straw walls. These proportions are even higher, in areas such as Mayo-Kebbi which are very far from the capital of the country. Only 9.3% of households occupy housing, with walls in semi hard. Households living in dwellings with hard walls construction (stone / blockwork or concrete / cement) are rare and represent only 2.5%. A wealthy family will add cement to the exterior of a finished wall, protecting it from erosion and ensuring its strength. 91.8% of dwellings occupied by households have clay floors. Few households occupy dwellings whose floor is concrete, cement or tile (6.6%). These data from ECOSIT3 show that the most commonly used building materials in Chad in general and in Mayo-Kebbi in particular, classified in descending order of their use are: soil bricks (banco), gravel, sand, straw and cement.

A total of 19 samples from four types of building materials (soil bricks, gravel, sand, and cement), which are representative of the building materials of the region were collected to measure activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K. These sites are displayed in Table 1.

Soil bricks samples were taken from inhabited houses. What we call one soil bricks sample, is in fact a mixture of soil taken from five different houses separated from one another by a distance of at least 300 m. Thus, the 11 samples of studied soil bricks, were collected in 55 different houses, covering the entire research area. In each house, the procedure was as follows: soil was taken from the floor and from the four walls of the house. For the floor, a square of 1 meter of side was drawn on the ground. At the middle of a square, as at each corner, was dug a vertical hole of about 5 cm deep and at the bottom of each hole, a handful of soil was taken. For the walls, a brick was randomly located on each side of the wall about 1 meter above the ground, in the middle of the brick, a horizontal hole about 5 cm deep was dug and a handful of earth was taken from the bottom of the hole. Then, we closed the holes with the mud collected in the vicinity of the house.

Samples of sand and gravel were taken from large heaps of sand and gravel stock for sale, near the pond of Pala, and Lake of Léré, from where they were extracted. These two sites provide the largest quantity of sand and gravel used in the study area for the construction of houses. The cement was taken directly from the manufacturing plant at the bagging point.The cement plant is the only plant in Chad. It provides most of the cement used in the area for building construction.

Samples were packed on-site, in polythene bags and correctly catalogued, tagged and coded according to the type of sample and the location of the sampling site. The samples were brought to the sample preparation Unit of the Nuclear Technology Section, Institute of Geological and Mining Research in Yaoundé, Cameroon, where they were dried at 90 °C in an oven during 24 hours, and then they were crushed and sieved to obtain a fine powder with grain size less than 1 mm. The cement did not undergo this treatment, because it was already in the form of a powder at the moment of collection. The powders thus obtained were sealed in plastic bags and transported to iThemba LABS, in South Africa, where they were placed again in an oven heated at 105 °C for 48 hours to completely remove moisture. Then they were cooled in a moisture-free atmosphere, weighed and stored in 100 mL plastic polyethylene cylindrical containers. The containers were hermetically sealed for 4 weeks to prevent the escape of ²²²Rn gas and ²²⁰Rn. This allows ²²⁶Ra and ²³²Th and their short-lived daughters to reach secular equilibrium.

2.3 Gamma spectrometry detection system and efficiency calibration

The measurements described in this study were carried out using a High Purity Germanium (HPGe) detector in a low-background setup located at the Environmental radioactivity laboratory (ERL) of iThemba LABS in South Africa. The detector is basically a p-type GC4520 Canberra coaxial detector. It has a relative efficiency of 45% and a nominal resolution of 2.2 keV FWHM at 1332 keV, and a Peak to Compton Ratio (P/C) of 54. Its endcap diameter is 83 mm. A digital electronic card (DSA) was used to couple the detection system to a PC equipped with Genie 2000 analysis software, covering the range of gamma energy emitted between 50 and 3000 keV. The detector is housed in a 10-cm thick cylindrical lead shield to reduce as much as possible background radiation. The internal interface of the main lead shield is covered with a 2-mm copper liner in order to absorb X-rays generated in the lead.

Energy and efficiency calibrations of gamma-ray spectrometry systems were performed using the radionuclide specific efficiency method to reduce the uncertainty in gamma-ray intensities and the influence of the summation of coincidence and self-absorption effects of emitting gamma photons (Ingersoll, 1983). The energy calibration was done using a standard IAEA-RGTh-1 (3250 Bq kg⁻¹ thorium ore) standard reference source prepared in a 1-liter filled Marinelli beaker. Three samples of U (RGU-1), Th (RGTh-1) and K (KCl) were prepared in a 100-mL pill bottle and used as standard sources for efficiency calibration. The detection efficiency of the pill bottle of the samples was determined by first calculating the absolute efficiency while using ²²⁶Ra, ²³²Th and ⁴⁰K spectrum lines. Absolute photo-peak efficiency was calculated for gamma-ray spectrum lines for the ²²⁶Ra, ²³²Th and ⁴⁰K using the following equation: $$\epsilon =a{E}^{-b}\text{,}$$(1) where E is the gamma-ray energy in keV. The power fit parameters a and b were obtained to be 4.4676 and 0.731, respectively. The absolute efficiency curve is presented in Figure 2.

Figure 2 Efficiency calibration curve for pill-bottle geometry.

3 Results and discussion

3.1 Activity concentration

All samples were counted for 12 h (43200 s), using the Genie 2000 data acquisition software provided by Canberra. The activity concentrations of each radionuclide were evaluated using the equation (2) below: $$A_i\left(\text{Bq}{\mathrm{k}\mathrm{g}}^{-1}\right)=\frac{N_i}{\epsilon \left(E\right)\gamma tm},$$(2) where N_i is the net gamma count in a photopeak (background corrected), ε(E) the detector efficiency as function of gamma-ray energy, γ the number of gamma per disintegration of the given nuclide at energy E (the absolute transition probability of gamma-decay), m the sample mass (kg) and t the counting time (s).

For radionuclides with more than one photopeak in the spectrum, the activity concentration was calculated using weighted average method. The activity concentration of parents was calculated using the weighted average of the daughters’ activity concentrations being in secular equilibrium. The gamma-ray emissions of ²¹⁴Pb (295.2, 351.9 keV) and ²¹⁴Bi (609.3, 1120.2, 1764.5 keV) were assumed to represent the activity concentration of ²³⁸U, and the gamma-ray lines of ²²⁸Ac (338.4, 911.1, 968.9 keV) and Tl (583.1, 2614.7 keV), were used to represent the activity concentration of ²³²Th. The activity concentration of ⁴⁰K was determined using its gamma-ray line at 1460.8 keV.

The potential radiological hazards associated with the building materials samples were assessed by calculating the radium equivalent activity and some radiological hazards index.

Table 2 presents the range and the average values of the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K measured in various types of building materials from Mayo-Kebbi region. Figure 3 is a graphical representation of the results that are in Table 2. It can be observed that ⁴⁰K contributes more to the specific activity compared to ²²⁶Ra and ²³²Th isotopes. The highest average activity concentration of ⁴⁰K is 839.54 ± 8.98 Bq kg⁻¹ in soil bricks sample from Madajang and the lowest average is 4.28 ± 1.96 in Pala gravel sample. The ²²⁶Ra average activity concentration vary from 0.56 ± 0.37 in Léré sand sample to 434.88 ± 7.11 Bq kg⁻¹ in soil bricks sample collected from Zabili. For ²³²Th, the average activity concentrations range from 1.30 ± 0.60 Bq kg⁻¹ in Pala gravel sample to 50,61 ± 1.51 Bq kg⁻¹ in soil bricks sample collected from Madajang. It is well known that the average worldwide activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K are 40, 40 and 400 Bq kg⁻¹, respectively (UNSCEAR, 2000). Based on these criteria, high activity concentrations (> 40 Bq kg⁻¹) of ²²⁶Ra were found in soil bricks sample. The ²²⁶Ra concentration in soil bricks from Zabili is more than 10 times higher than the corresponding worldwide average value and the ²²⁶Ra concentration in soil bricks from Madajang is 64.4 Bq kg⁻¹. High activity concentrations of ²³²Th (51 Bq kg⁻¹) and ⁴⁰K (789 and 840 Bq kg⁻¹) were respectively found in soil bricks samples from Zabili and Madajang. The ⁴⁰K activity concentration measured in soil bricks from Madajang is more than twice the corresponding worldwide average value which is 400 Bq kg⁻¹.

Figure 3 Graphic view of the activity concentrations Ai (Bq kg−1) of the samples.

3.2 Assessment of the radium equivalent activity (Ra_eq)

The distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in building materials is not uniform. Therefore, the complete and real radioactivity levels of ²²⁶Ra, ²³²Th and ⁴⁰K in building materials can be assessed using a common radiological index called the radium equivalent activity (Ra_eq)_. The radium equivalent activity is a weighted sum of activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K radionuclides based on the assumption that 370 Bq kg⁻¹ of ²²⁶Ra, 259 Bq kg⁻¹ of ²³²Th and 4810 Bq kg⁻¹ of ⁴⁰K produce the same radiation dose rate (Ngachin et al., 2007; Guembou Shouop et al., 2017a, 2017b). The radium equivalent activity was calculated using the following equation (3) (Koblinger, 1984; Ngachin et al., 2007; El-Galy et al., 2008; El-Taher, 2012; Guembou Shouop et al., 2017a, 2017b): $$R{a}_{eq}=A_{Ra}+\left(1.43\times A_{Th}\right)+\left(0.077\times A_K\right),$$(3) where A_Th, A_Ra and A_K are the specific activities of ²³²Th, ²²⁶Ra and ⁴⁰K in Bq kg⁻¹, respectively.

The calculated values of the radium equivalent (Ra_eq) of all the samples analyzed are given in Table 2, and the graphical view is shown in Figure 4. These values range from 7.74 Bq kg⁻¹ (Pala-1: gravel) to 511.40 Bq kg⁻¹ (Zabili: soil bricks). The results summarized in Table 3 show that the values of Ra_eq with the exception of the Zabili soil brick samples are below the maximum allowable value of 370 Bq kg⁻¹ according to the UNSCEAR (2000) report, which is equivalent to an external dose of 1 mSv/y (NEA-OECD, 1979; EC, 1999). The Zabili soil brick sample presents a significant radiological hazard compared to all other building materials analyzed.

Figure 4 Radium equivalent activity (Bq kg−1) as a function of location of the building material samples analysed.

3.3 Comparison of specific activities and Ra_eq of samples from Mayo-Kebbi region, with those found in similar studies in other countries of the world

Table 4 compares the reported values of the activity concentrations of radionuclides and their radium equivalent activity for selected building materials, obtained in other countries in comparison to those determined in this study. As shown from these table, the radioactivity and the radium equivalent activity in building materials varied from one country to another, which can be attributed to differences in the contents of radioactive minerals and in the geological, geochemical and geographical origins of the raw materials, among other factors. Radium, thorium and potassium are not uniformly distributed in soil or rocks, from which building materials are derived, but the activity concentrations vary, often greatly, over a distance of some meters. The measured values of radium, thorium and potassium contents show only the average radioactivity in building materials used in Mayo-Kebbi Ouest region not over the entire Chad. It is also important to point out that the other values are not the representative values for the countries mentioned but for the regions from where the samples were collected. From these comparisons, it can be seen that the construction materials of the Mayo-Kebbi region in Chad, have on average specific activities and radium equivalent activity much lower than most of the countries with which the comparison was made.

3.4 Absorbed gamma dose rate (D)

If some building materials have a high activity concentration, they may increase indoor and outdoor radiation exposure as well as the internal and external exposure of inhabitants. The absorbed dose rate measures the energy deposited in a medium by ionizing radiation per unit mass and per time unit. The larger the absorbed dose the higher the hazard.

Outdoor absorbed dose rates (D_out): the guidelines provided by the (UNSCEAR, 2000) report allows to assess the outdoor absorbed gamma dose rates in air, due to terrestrial gamma rays, coming from disintegration of the nuclides ²²⁶Ra, ²³²Th and ⁴⁰K, present in building materials. Those nuclides are supposed to be equally distributed in ground, and the absorbed dose rate measurement is supposed to have been making at 1.0 m above the ground level. The (D_out) was calculated using the following equation by (Shams et al., 2015). $$D_{out}\left(\mathrm{n}\mathrm{G}\mathrm{y}{\mathrm{h}}^{\mathrm{-}\mathrm{1}}\right)=0.427A_{Ra}+0.662A_{Th}+0.043A_K,$$(4) where A_Ra, A_Th and A_K have been defined previously. The dose conversion factors 0.43 nGy h⁻¹ Bq⁻¹ kg⁻¹ for ²²⁶Ra, 0.666 nGy h⁻¹ Bq⁻¹ kg⁻¹ for ²³²Th and 0.047 nGy h⁻¹ Bq⁻¹ kg⁻¹ for ⁴⁰K are from (Ngachin et al., 2007). In the UNSCEAR and European Commission reports, the dose conversion coefficient was calculated for the centre of the standard room. The dimension of the room is 4 m × 5 m × 2.8 m. The thickness of the walls, floors and the ceiling and density of the structure are 20 cm and 2350 kg m⁻³ (concrete), respectively.

The third column of Table 5 shows that the Outdoor absorbed dose rates range from 3.4 nGy h⁻¹ in Gravel from Pala, to 223.2 nGy h⁻¹ in soil bricks from Zabili, with mean value of 31.5 nGy h⁻¹. The world average value is 60 nGy h⁻¹, which means that most samples are below this value, except the soil brick sample from Zabili, which is four times higher than the world average value.

Indoor absorbed dose rates: according to UNSCEAR (2000), the building materials act as sources of radiation and also as shields against outdoor radiation. In general, the indoor-outdoor ratio range is relatively narrow and reflects the fact that building materials are usually of local origin and that their radionuclide concentrations are similar to those in local soil. Since essential data on average radon accumulation in the indoor atmosphere of houses, in the study area, are not yet available, we assessed the indoor absorbed dose rates, based on the fact that, the worldwide average gamma dose rate indoors is 1.4 times higher than outdoors (UNSCEAR, 2000; Asaduzzaman et al., 2015). $$D_{in}\left(\mathrm{n}\mathrm{G}\mathrm{y}\mathrm{.}{\mathrm{h}}^{\mathrm{-}\mathrm{1}}\right)=1.4D_{out}\mathrm{.}$$(5)

The calculated indoor gamma dose rates for the nineteen (19) samples are presented in Table 5. The fourth column of this table show that the Indoor absorbed dose rates range from 4.834 nGy h⁻¹ in Gravel from Pala, to 312.521 nGy h⁻¹ in soil bricks from Zabili, with mean value of 44.069 nGy h⁻¹. The population-weighted world average is 84 nGy h⁻¹ (UNSCEAR, 2000), which implies that the indoor absorbed dose rate of the soil brick samples from Madajang is 1.34 times higher than the world average value, and that from Zabili, is 3.72 times higher than the world average value (Fig. 5).

Figure 5 Values of E (mSv y−1) of all the samples.

3.5 Annual effective dose (E)

The annual effective dose is estimated from the dose rate. Using the result of Outdoor and Indoor absorbed dose rates calculated above, annual effective dose was estimated as follow (Mahmoud et al., 2014): $$E\left(\mathrm{n}\mathrm{S}\mathrm{v}{\mathrm{y}}^{\mathrm{-}\mathrm{1}}\right)=\left(D_{out}O{F}_{out}+D_{in}O{F}_{in}\right)T.CC,$$(6) where E (nSv y⁻¹) is annual effective dose, D_out (nGy h⁻¹) and D_in (nGy h⁻¹) are mean outdoor and indoor absorbed dose rates, T (h) is time to convert from year to hour (8760 hours), OF_out and OF_in are outdoor and indoor occupancy factors (20% and 80% for outdoor and indoor, respectively) and CC is conversion coefficient (0.7 × 10⁻⁶ Sv per Gy for adults) reported by UNSCEAR to convert absorbed dose in air to the effective dose equivalent in human (UNSCEAR, 2000).

The calculated annual effective dose values for nineteen (19) samples are presented in Table 5.

The fifth column of Table 5 shows the annual effective dose values ranging from 0.028 mSv y⁻¹ in Gravel from Pala, to 1.807 mSv y⁻¹ in soil bricks from Zabili, with mean value of 0,255 mSv y⁻¹. It should be noted that the average value of the annual effective dose is less than the maximum value of 1 mSv y⁻¹ recommended by ICRP (ICRP, 1991; EC, 1999). However, Zabili soil brick samples have an annual effective dose greater than this limit, and 7 times higher than the average value calculated for the entire study area.

3.6 Annual gonadal dose equivalent (AGDE)

The annual gonadal dose equivalent (AGDE) is used to evaluate the potential effects of the specific activities of ²²⁶Ra, ²³²Th, and ⁴⁰K on certain important organs, such as reproductive organs (gonads), bone marrow and bone cells. In the samples, it was calculated using the following relation (Shams et al., 2015): $$AGDE\left(\mathrm{\mu }\mathrm{S}\mathrm{v}{\mathrm{y}}^{\mathrm{-}\mathrm{1}}\right)=3.09A_{Ra}+4.18A_{Th}+0.314A_K,$$(7) where A_Ra, A_Th and A_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K respectively in Bq kg⁻¹. The numbers 3.09, 4.18 and 0.314 are the respective conversion factors that transform the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K into total dose received by the organs of interest.

From Table 5 and Figure 6, the values of AGDE ranged from 23,927 μSv y⁻¹ in Gravel from Pala to 1572,2 μSv y⁻¹ in soil bricks from Zabili, with mean value of 214,9 μSv y⁻¹. This mean value is lower than the world worldwide mean value which is between 316.68 and 415.65 μSv y⁻¹ (Shams et al., 2015). But we observe that the first three samples of Table 5, collected at Madajang and Zabili exceed the world mean value, indicating that the hazardous effects of the radiation are not negligible. Special attention should be given to these two areas.

Figure 6 Values of Dout (nGy h−1), Din (nGy h−1), and AGDE (μSv y−1), for all samples.

3.7 External and internal radiological hazard index

External hazard indices (H_ex): the external radiological risks associated with construction materials are assessed by calculating the External Risk Index (Hex), which is given by the following formula (El-Taher, 2012; El-Mageed et al., 2014): $$H_{ex}=\frac{A_{Ra}}{370}+\frac{A_{Th}}{259}+\frac{A_K}{4810},$$(8) where A_Ra, A_Th and A_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in Bq kg⁻¹. Its numerical value must be less than unity, to consider that the external radiological hazards are negligible. H_ex = 1 is a corresponding quantity to the upper limit of Ra_eq (370 Bq kg⁻¹) (Ngachin et al., 2007; Guembou Shouop et al., 2017a, 2017b).

The calculated external hazard index values are presented below in Table 6 and Figure 7. The values of H_ex is ranged from 0.021 for gravel from Pala to 1.382 for soil bricks from Zabili. Except the soil bricks of Zabili, the external hazard indexes of other studied samples are less than unity, and accordingly they can safely be used for construction unlike those from Zabili.

Internal hazard indices (H_in): for individuals living in the dwellings, Radon and its short-lived product can be hazardous to their respiratory organs. Internal hazard indices H_in is used to quantify the internal exposure to radon and its daughter products. H_in is given by the following equation (Brigido Flores et al., 2008): $$H_{in}=\frac{A_{Ra}}{185}+\frac{A_{Th}}{259}+\frac{A_K}{4810}\mathrm{.}$$(9)

For the safe use of a material in the construction of dwellings, H_in should also be less than unity (Raghu et al., 2015).

The calculated values of the internal hazard index for the studied samples are given below in Table 6 and Figure 7. Considering that the value of H_ex and H_in must not exceed the unit limit, it appears that only soil bricks from Zabili which have H_in = 2.56, do not meet this criterion. For all other samples, the internal hazard indices are lower than the unity. So, soil bricks from Zabili should be handled or used with caution to avoid excessive exposure of the people to radiation. The use of building materials from other localities have no immediate negative health implications on the inhabitants, but it is necessary to observe and study the long-term cumulative effects on the inhabitants of those localities.

Figure 7 Radiation hazard indices.

3.8 Activity utilization index (AUI)

The activity concentrations of natural radionuclides in building materials affect the absorbed dose inside, especially for massive houses made of different materials such as stones, bricks, concretes or granites. This is explained by the fact that, the building materials act as sources of radiation and also as shields against outdoor radiation, thereby radiation emitted by sources outdoors is efficiently absorbed by the walls (UNSCEAR 2000 Report). As a result, indoor air dose rates will be higher than the natural radionuclide concentrations used in building materials. By applying the appropriate conversion factors, an Activity utilization index (AUI) is constructed to facilitate the calculation of air dose rates, from different combinations of the three nuclides ²²⁶Ra, ²³²Th and ⁴⁰K in building materials samples. (AUI) is given by the following expression taken from (Shams et al., 2015). $$AUI=\left[\left(\frac{A_{Ra}}{50}\right)f_{Ra}+\left(\frac{A_{Th}}{50}\right)f_{Th}+\left(\frac{A_K}{500}\right)f_K\right]w_m,$$(10) where f_Ra = 0.462, f_Th = 0.604 and f_K = 0.041 are the fractional contributions to the total dose rate in air due to gamma radiation from the actual concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K respectively. w_m is the fractional use of building materials in the dwelling with the activity characteristic. In this study, we assume that we are in the case of full utilization of the typical masonry, and we take w_m = 1.

The values of the activity utilization index calculated for the studied samples are given in Table 6 and Figure 7. They range from 0.042 for sand from Lere to 4.634 for soil bricks from Zabili, with an average value of 0.461. This average value is less than 2 and corresponds to an annual effective dose less than 0.3 mSv y⁻¹, indicating that building materials from Mayo-Kebbi region (Chad) are safe. But the case of soil bricks from Zabili, with AUI = 4.634 are to be avoided. It is advisable not to use them to build houses.

3.9 Representative level index (RLI)

The representative level index is used to estimate the level of gamma radiation hazard associated with the natural radionuclides in specific building materials. It is a screening tool for identifying materials that may be hazardous to health when used for the construction of buildings. It is correlated with the annual dose due to the excess external gamma radiation caused by superficial material. Values of RLI ≤ 1 correspond to dose rate ≤ 0.3 mSv y⁻¹ whereas RLI ≤ 3 correspond to dose rate ≤ 1 mSv y⁻¹. RLI is calculated using equation based on (NEA-OECD, 1979) formula: $$RLI=\left(\frac{A_{Ra}}{150}\right)+\left(\frac{A_{Th}}{100}\right)+\left(\frac{A_K}{1500}\right)\mathrm{.}$$(11)

The safety value for this index must be ≤ 1.

The representative level index for building material samples are displayed in Table 6 and Figure 7. The calculated RLI varies from 0.053 for gravel from Pala to 3.441 for soil bricks from Zabili, with an average of 0.471. It is clear that this average value does not exceed the upper limit of the RLI, which is unity. Therefore, according to the dose criterion above, bricks from Zabili with RLI = 3.441 (≥ 3) should not be used as a building materials, since these values correspond to dose rates higher than 1 mSv y⁻¹.

3.10 Excess lifetime cancer risk (ELCR)

The Excess lifetime cancer risk (ELCR) can be defined as the probability that an individual can develop cancer over his lifetime due to exposure level to radiation. The ELCR has been calculated using the following equation (Chandrasekaran et al., 2014). $$ELCR=E\left(\mathrm{m}\mathrm{S}\mathrm{v}{\mathrm{y}}^{\mathrm{-}\mathrm{1}}\right)DL\left(70\mathrm{y}\right)RF\left(0.05\times {10}^{-3}\mathrm{m}\mathrm{S}{\mathrm{v}}^{\mathrm{-}\mathrm{1}}\right),$$(12) where E is the annual effective dose equivalent, DL is the duration of life (70 years average) and RF is the risk factor, i.e. fatal cancer risk per mSv. For stochastic effects, the ICRP  Publication 106 used a value of RF = 0.05 × 10⁻³ mSv⁻¹ in any given population. The worldwide recommended value of ELCR is 0.29 × 10⁻³ (Chandrasekaran et al., 2014; Ugbede and Echeweozo, 2017).

The values of ELCR obtained for the studied samples are summarized in the last column of Table 6 and on the graphical representation of Figure 7. ELCR values ranged from 0.058 × 10⁻³ to 3.794 × 10⁻³ with average value of 0.535 × 10⁻³. The average of the excess lifetime cancer risk is 1.84 times greater than the upper recommended value of 0.29 × 10⁻³. Building materials with such high ELCR values can lead to radiation hazards, and the risk of developing cancer by people living in this environment is very high, so they should be avoided for construction.

4 Conclusion

With high purity γ-ray spectrometry system, the activity concentrations of ²³⁸U (²²⁶Ra), ²³²Th and ⁴⁰K in building materials collected from Mayo-Kebbi region (Chad) were measured. The materials studied in this research show that some of the building materials used in the region are not safe in terms of radiological hazard. All types of cement analysed show low levels of radioactivity, even when compared to levels in other countries around the world. Gravel and sand samples also show low levels of activity and therefore negligible radiological risk. The value of radiation hazard parameters of soil bricks, especially from Zabili presents significant radiological exposure risks. The soil bricks from Zabili exhibit an annual effective dose of 1.807 mSv y⁻¹ which is about 2 times higher than the maximum value recommended by ICRP, especially if we considered that it is a single source of radiological exposure. The average of the excess lifetime cancer risk is 1.84 times greater than the upper recommended value of 0.29 × 10⁻³, and soil bricks, from Zabili have an excess lifetime cancer risk, 13 times greater than the upper recommended value. Building materials with such high excess lifetime cancer risk values, can lead to radiation hazards. It may be due to the presence of relatively higher activity concentration of ²³⁸U in the soil of this location, where prospecting for uranium was already done by an international mining company. Thus, it is recommended that soil bricks in the region, especially from Zabili should not be used as building materials. From a radiation protection point of view, additional regulations will be needed on building materials from the Mayo-Kebbi region in Chad.

Acknowledgments

The authors are grateful to the Abdus Salam International centre for theoretical physics (ICTP) for its support through the OEA-AC-71 project at the Centre for atomic molecular physics and quantum optics (CEPAMOQ) of the University of Douala (Cameroon). This work was supported in part by the National Reasearch Foundation (NRF) of South Africa, using the facilities and technical support available at NRF/iThemba LABS.

References

All Tables

All Figures

	Figure 1 Map showing the location of Mayo-Kebbi Ouest region (provided by Google Map).
In the text

	Figure 2 Efficiency calibration curve for pill-bottle geometry.
In the text

	Figure 3 Graphic view of the activity concentrations Ai (Bq kg−1) of the samples.
In the text

	Figure 4 Radium equivalent activity (Bq kg−1) as a function of location of the building material samples analysed.
In the text

	Figure 5 Values of E (mSv y−1) of all the samples.
In the text

	Figure 6 Values of Dout (nGy h−1), Din (nGy h−1), and AGDE (μSv y−1), for all samples.
In the text

	Figure 7 Radiation hazard indices.
In the text