Radiation Protection and Environment

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 43  |  Issue : 2  |  Page : 70--76

Assessment of environmental radiation exposure from soil radioactivity around the Southern area of Chad


Mistura B Ajani1, Peane P Maleka2, Iyabo T Usman3, Samafou Penabei4,  
1 Nuclear Structure Research Group (NSRG), School of Physics, University of Witwatersrand, Johannesburg, 2050; Department of Subatomic Physics, iThemba Laboratory for Accelerator Based Sciences, ZA-7129, South Africa
2 Department of Subatomic Physics, iThemba Laboratory for Accelerator Based Sciences, ZA-7129, South Africa
3 Nuclear Structure Research Group (NSRG), School of Physics, University of Witwatersrand, Johannesburg, 2050, South Africa
4 Department of Medical Physics, The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy

Correspondence Address:
Ms. Mistura B Ajani
Nuclear Structure Research Group (NSRG), School of Physics, University of Witwatersrand, Johannesburg 2050; Department of Subatomic Physics, iThemba Laboratory for Accelerator Based Sciences, P.O. Box: 722, Somerset West, ZA-7129
South Africa

Abstract

A study to ascertain the radioactivity levels of various radionuclides from soil samples collected in Chad using a high-resolution gamma ray spectrometry system is presented in this article. The activity concentrations are determined for the radionuclides:226Ra,214Pb,214Bi and228Ac,208Tl,212Pb following the decay of238U and232Th as well as40K,235U, and137Cs. The values of activity concentrations of238U,232Th, and40K in the soil samples ranged from 2 to 245, 2–40, and 20–454 Bq/kg, whereas235U and137Cs ranged from 0.8 to 21.7 and 0.3–3.8 (Bq/kg), respectively. In order to evaluate the radiological exposure of the natural radioactivity, the radium equivalent activity, external exposure index, internal exposure index, and annual effective-dose equivalent have been calculated which ranged from 27 to 465 (Bq/kg), 0.09–1.25, 0.13–2.38, and 0.09–1.65 (mSv/y), respectively. Correlation between238U versus232Th,40K versus238U, and40K versus232Th was investigated; the results showed good correlation for238U versus232Th and40K versus238U while40K versus232Th gives poor correlation. For the 20 samples collected and analyzed for this study, the results showed that average activity concentration of238U is relatively higher than the world average, while for both232Th and40K, it was relatively lower.



How to cite this article:
Ajani MB, Maleka PP, Usman IT, Penabei S. Assessment of environmental radiation exposure from soil radioactivity around the Southern area of Chad.Radiat Prot Environ 2020;43:70-76


How to cite this URL:
Ajani MB, Maleka PP, Usman IT, Penabei S. Assessment of environmental radiation exposure from soil radioactivity around the Southern area of Chad. Radiat Prot Environ [serial online] 2020 [cited 2020 Sep 23 ];43:70-76
Available from: http://www.rpe.org.in/text.asp?2020/43/2/70/293623


Full Text



 Introduction



Gamma radiation emitted from naturally occurring radioactive nuclides found in all soils is responsible for the internal and external exposure to the human body.[1] Human being receive up to 85% of annual dose exposure due to naturally occurring radioactive materials (NORMs), and the production of artificial radionuclides will increase the risk exposure.[2] Individuals are exposed in a different way to these various sources, in relation to their environment and way of living. Unregulated environmental exposure to radiation can lead to an increase in doses of ionizing radiation for the population, which requires an understanding of the environmental behavior of the different radionuclides and an estimation of their risks for humans. Natural sources are known to be the main contributor to the external dose of the world population.[1] These doses are generally correlated with relatively high activity concentrations of natural radionuclides such as238 U,232 Th, and40 K, which in turn depend on local geological and geographical conditions.[1],[3],[4] The present study examines the activity concentrations and the distribution of radionuclides present in the soil samples collected from Yapala, located in Mayo-Dallah one of the three regions of Mayo-Kebbi West in Chad [Figure 1]. The Mayo-Dallah is one of the sub-region that is rich in mineral resources.[5] Previously, mining activities were undertaken in a random way without following the international standards (ISO/TC82/SC7).[5] However, such activities are always accompanied by the disturbances of ecological factors in the environment. In 1970, a study was carried out which was funded by the United Nations Development Programme with the help of IAEA which revealed evidence of uranium in the region.[6] Additional studies to assess the radiological condition were done by Penabei et al.,[5] and IAEA,[2] whereby the studies showed the existence of uranium, thorium, and potassium for example. According to Penabei's research and the June 2013 final report of the Third Survey of Consumption and the Informal Sector in Chad (ECOSIT3, 2013), a significant number of households in the study area live in earthen brick dwellings, which are made in surrounding soil base. In order to provide information on the radionuclides present in the soils and the environmental pollution of the study area, a survey was conducted to quantify and qualify the indoor and outdoor concentration of natural concentrations (238,235 U,232 Th, and40 K). The results were used to evaluate the potential radiological risk associated with the soil samples by evaluating the radium equivalent activity, external radiation exposure index, and internal radiation exposure index. The result of the 20 collected samples, the activity concentrations of identified radionuclides are compared to the world average values reported and recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). These data were also used to investigate the correlation between the activity concentrations of238 U versus232 Th,238 U versus40 K, and232 Th versus40 K.{Figure 1}

 Materials and Methods



Study area, sample collection, and preparation

Twenty soil samples were collected from Yapala, located in Mayo-Dallah in the Southern area of Chad, about 73,520 hectares.[7] Each composite sample consisted of a mixture of five samples taken from an area of 3 m2 separated from each other. The spacing between the composite samples was randomized to cover the study site and to observe a significant local spatial variation in terrestrial radioactivity. Four samples were collected at the edges and one in the centre to form a composite sample. Samples labelled S1–S16 were from the former large open mining plains where mining is no longer undertaken at these locations, whereas samples S17–S20 were taken from new large open mining plains where activities are underway. Then, the collected samples were placed in a tightly sealed plastic bags and posted to iThemba LABS facilities in Cape Town, South Africa. The samples were prepared in Environmental Radioactivity Laboratory where they were crushed with mortar and pestle and sieved through a mesh to remove organic material and to obtain powder-like sample with smaller grain sizes. They were then transferred into 100 ml bottle, labelled and air-tight to inhibit any gaseous progenies from escaping from the bottle.

Sample counting

A hyper-pure germanium (HPGe) gamma-ray detector was used to count and determine the activity concentrations of various radionuclides identified in the samples. The HPGe detector specifications are as follows; GC4520 p-type Canberra detector system, crystal diameter of 6.25 cm and crystal length of 5.95 cm. The detector system is housed in a low background setup, 10 cm thick walls of Pb and covered in the inside with 2 mm thick Cu plates.[8] Standard nuclear electronics are used to process the detector signals. The pulses from the amplifier are collected and sorted using the ATOMKI Palmtop MCA instrument and software. Prior to sample counting cycle, which is during week days, the detector system is regularly calibrated for energy and efficiency at the beginning of each week. For this study, reference materials IAEA/RGU-1 (4938 Bq for238 U), IAEA/RGTh (3252 Bq for232 Th), and KCl (13910 Bq for40 K) were used for full-energy peak efficiency calibrations. Both reference materials and samples were prepared in 100 ml bottle to maintain similar counting geometry. The reference materials were counted for 1 h (3600 s) and the collected samples for 24 h.

Sample analysis

The data acquisition system and the radionuclide identification from the spectrum were done using ATOMKI Palmtop MCA software system. A background spectrum for an empty lead castle was counted for 3 days to determine the background level of HPGe detector, each peak was identified and subtracted manually from each sample measurement. Gf3 radware software was used to determine the area of the peak and then calculates peak emission rates (counts per se cond for the efficiency of the detector). After the conclusion of the measurement and the counting system, a full energy peak analysis was done for each spectrum obtained. The radionuclides of interest in the analysis include238 U members (214 Pb: 295, 352 keV,214 Bi: 609, 1120, 1764 keV),232 Th: members (228 Ac: 338, 911, 969 keV,208 Tl: 583, 2615 keV),40 K: 1461 keV,235 U: 144 keV and. In addition to that, anthropogenic137 Cs: 662 keV was also present. Details of how the analysis is done are shown in [Table 1] and [Table 2].{Table 1}{Table 2}

Radiological exposure assessments

Gamma dose rate

The gamma dose rates (D) for the measured samples were determined from the specific activity concentration, in addition to the associated radiological risks from the absorbed dose at 1 m above the ground was estimated using the following equation:[5]

Dout(nGy/h) = 0.043ARa + 0.666AT h + 0.047AK (1)

Where Dout is the absorbed dose rate in the air and A values are the measured activity of226 Ra,232 Th, and40 K in Bq/kg, respectively. According to (1) we calculate the Indoor absorbed dose rate based on the worldwide average gamma dose rate indoors of 1.4 times higher than outdoors:

Din(nGy/h) = 1.4 Dout (2)

Where Din is the indoor absorbed dose rate.

Radium equivalent dose (Raequ)

One of the most important parameter on radiological risk assessment is the radium equivalent activity. This quantity is calculated by:

Raequ= ARa+ 1.43ATh+ 0.077AK (3)

Where ARa, ATh, and AK are the activity concentration of226 Ra,232 Th, and40 K in Bq/kg. The highest value allowed for activity concentration of radium equivalent is 370 Bq/kg, which agrees to the effective dose limit of 1 mSv for the general public.[1],[9]

External exposure index (Hext)

The external radiological exposure index of the present soil samples is assessed by calculating the external exposure index to226 Ra using the following equation:[5],[9]

Hext= (ARa/370) + (ATh/259) + (AK/4810) ≤1… (4)

Where ARa, ATh, and AK are the same for the activity equivalent of radium.

Internal exposure index (Hext)

The exposure from the internal exposure of NORMs and TENORMs is expressed by the formula below:[5],[9]

Hint= (ARa/185) + (ATh/259) + (AK/4810) ≤1… (5)

If the highest activity concentration of226 Ra is half of the acceptable limit, therefore, the Hint will be lower than the unit.

Annual effective dose

Annual effective dose (AED) equivalent can be calculated by using the following equation:[5]

AED (nSv/yr) = ((Dout× OFout) + (Din× OFin)) × T × CF (6)

Where AED (nSv/yr) is the AED equivalent, Dout(nGy/h) and Din(nGy/h) is the outdoor and indoor absorbed dose mean, OFout and OFin are the occupancy factors of outdoor and indoor (0.2 and 0.8), T (h) is the conversion from year to hours (8760 h) and CF is the conversion factor (0.7 × 10 − 6 (Sv/Gy)) given by UNSCEAR[1] to convert absorbed dose in air to effective dose in human being.

 Results and Discussion



Activity concentration distribution of the samples

Example of daughter activities concentration obtained from measured sample is illustrated in [Figure 2], which are calculated using the formula from Alzubaidi et al.[10] The results in [Figure 2]a and b show the activity concentration calculated for terrestrial decay chains of238 U (daughter nuclides) and232 Th (daughter nuclides) for sample S1 while the straight line represent the weighted average of the data points. It can be seen in [Figure 2]a that the activities of226 Ra at an energy of 186 keV are higher than the activity concentrations of214 Bi and214 Pb. The results showed that there was a statistically significant variation between the226 Ra 186 keV line and the gamma lines of214 Bi and214 Pb. It should be noted that one significant difference between the226 Ra of 186 keV line and the gamma lines of214 Pb and214 Bi comes from the errors associated with the results of the 186 keV line which generally tend to be higher than those of the other two. [Figure 2]b shows that there is no significant variation between the gamma-ray lines. In addition,40 K,235 U, and137 Cs activity concentrations of radionuclide were directly measured using a single gamma line. The weighted average was calculated to achieve good estimate of activity concentration. [Table 3] illustrates the activity concentrations based on the weighted average values of238 U (214 Pb and214 Bi),232 Th (228 Ac and208 Tl),40 K235 U and137 Cs. It can be observed in [Table 3] that there is uniformity across the samples from S1 to S16 except for sample S17 to S20 that has high activity concentrations. The238,235 U,232 Th, and40 K values obtained from sample S1 to S16 range from 2 to 36, 2–8, and 20–282 Bq/kg, also, values from S17 to S20 range from 131 to 239, 14–40 and 150–455 Bq/kg. The relatively high concentration of40 K could be related to the agricultural activities in the region whereby potassium rich fertilizers are used. The highest value obtained in Sample S20 for137 Cs could be due to variations in many factors, such as surface topography or landscape of the area and factors that are related to soil, for instance, density and consequent soil movement, and chemical properties of the soil. Based on these, sample S1 to S16 are still within the world range value, but sample S17 to S20 of the238 U activity concentrations is relatively higher than world range values while232 Th and40 K are still within the world range values. The result obtained in[5] show consistent with the present study since they are both from Chad, although different regions.{Figure 2}{Table 3}

Correlations of activity concentration of238 U,232 Th,40 K

Activity correlations of sample S1–S20 for40 K versus232 Th,40 K versus238 U and238 U versus232 Th respectively, are illustrated in [Figure 3]. There is a positive correlation between238 U versus232 Th and40 K versus238 U with a correlation coefficient of R2 = 0.65 and R2 = 0.22. This correlation may be due to a similar indication of soil or sand chemical behavior and other artificial nuclides being distributed in the environment.[11],[12],[13] However, a poor correlation can be observed from40 K versus232 Th with a correlation coefficients of R2 = 0.09, the samples concentration of232 Th are associated to soil organic matter. A weak correlation between40 K and232 Th may be caused by a relatively high solubility or high moveable of potassium compared to thorium. The solid line in the figures represents the best fit which is approximately a linear relationship with a correlation coefficient.{Figure 3}

Exposure assessment

The evaluated radiation exposure indexes from the present study are illustrated in [Table 4]. Radium equivalent concentration range from 27 to 464 Bq/kg, while external, internal exposure and AED range from 0.07–1.24, 0.13–2.38 and 0.09–1.65 (mSv/y). Some of the samples are far below the permissible average value of 370 Bq/kg for Raequ as reported in[1] which relate to an effective dose of 1.5 mGy/a for the general public.[14] Sample S17 to S20 are relatively higher than the recommended values, this could be due to the fact that the samples were taken in a new dug hole where mining activities are underway. The activity concentration of radium equivalent is used to determine the radiation exposure indexes associated with the natural radioactivity (226 Ra,232 Th, and40 K) as explained in section 2.4, it is assumed that 370 Bq/kg of226 Ra, 259 Bq/kg of232 Th, and 4810 Bq/kg of40 K produce the same gamma dose rate.[15],[16] Before AED can be calculated, the absorbed dose rate needs to be calculated and apply a conversion factor of 0.7 × 10−6 Sv/Gy and the occupancy for dose out (Dout) and dose in (Din) are 0.2 and 0.8, respectively. AED average value is higher than the recommended world averages value of 0.48 mSv/y in 4 samples.[1]{Table 4}

 Conclusions



For this study, 20 soil samples were collected from Yapala, located in the Mayo-Dallah in Southern area of Chad in Central Africa. All samples were analyzed using HPGe gamma ray detector to identify and quantify radionuclides present in each sample. The concentrations of238 U,232 Th, and40 K ranged between 2–245, 2–40, and 20–454 Bq/kg, whereas235 U and137 Cs ranged from 0.8–21.7 and 0.3–3.8 (Bq/kg), respectively. This study revealed that the average activity concentration of238 U in Mayo-Dallah region is relatively higher than the world average value, while232 Th and40 K are lower. A good correlation was also observed between40 K versus238 U and238 U versus232 Th activity concentration while poor correlation was observed in40 K versus232 Th. The radium equivalent activity average value is 112 Bq/kg, and the average external exposure indexes, internal exposure indexes and AED are 0.30, 0.54 were less than the acceptable limit of unity indicating that the associated gamma radiation level was low. The main contributor to this dose is from sample S17 to S20, it is therefore recommended not to use soils sample S17–S20 as building materials.

Acknowledgments

The authors wish to express their sincere appreciation to Avuyile Bulala and Refilwe Setso for playing valuable role during sample preparation.

Financial support and sponsorship

This work is funded by National Research Foundation (NRF) through iThemba LABS.

Conflicts of interest

There are no conflicts of interest.

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