Volume 52, Numéro 4, October-December 2017
|Page(s)||277 - 280|
|Publié en ligne||8 septembre 2017|
Estimation of radiation hazard indices from syenite building rocks in the South-western region of Cameroon
Nuclear Physics Laboratory, Faculty of Science, University of Yaoundé I,
P.O. Box 812,
2 National Radiation Protection Agency, P.O. Box 33732, Yaoundé, Cameroon
* Corresponding author: email@example.com
Accepted: 7 July 2017
Radium equivalent (Raeq) and external hazard index (Hex) were estimated based on 226Ra, 232Th and 40K measurements in syenite building rocks collected from various sampling sites of Lolodorf area, South-western region of Cameroon to assess the radiation hazards due to the use of these rocks in dwellings construction. The analysis has been carried out by γ-ray spectrometry using a high purity germanium (HPGe) detector. Raeq varied from 186 ± 4 to 2564 ± 24 Bq · kg−1, while Hex varied from 0.5 ± 0.01 to 6.93 ± 0.06. The corresponding mean values are 1463 ± 930 Bq · kg−1 and 3.95 ± 2.51 in Awanda subdivision while in Madong subdivision, these values are 683 ± 553 Bq · kg−1 and 1.84 ± 1.49, respectively. Absorbed dose rate ranged from 98 ± 2 to 1171 ± 11 nGy · h−1 while annual effective dose ranged from 0.60 to 7.18 mSv · year−1. The average absorbed dose rates are 670 ± 420 nGy · h−1 and 313 ± 245 nGy · h−1 in Awanda and Madong, respectively. These average values give rise to a mean annual effective dose of 4.11 mSv · year−1 and 1.92 mSv · year−1. All syenite building rocks have shown Raeq activity concentrations higher than the limit set in the Organization for Economic Cooperation and Development (370 Bq · kg−1), equivalent to external gamma dose of 1.5 mSv · year−1. The obtained values of the annual external effective dose in Awanda and Madong are higher (nine-fold and four-fold, respectively) than the worldwide mean annual effective dose (0.48 mSv · year−1). The syenite rocks in the corresponding localities can be considered as high radiation's material not recommended for dwellings construction.
Key words: radium equivalent / external hazard index / syenite building rock / absorbed dose / effective dose
© EDP Sciences 2017
Syenite is a coarse-grained igneous rock, similar in appearance and composition to granite. Unlike granite, it contains little or no quartz. They are occasionally substituted for granites as building stones. Syenite rock is used worldwide in the glass and ceramic industries. The use of syenite in ceramic industry makes it an important source of radiation exposure. It is known that the syenite rock contains various concentrations of uranium, thorium, radium and their daughters products (Beyala Ateba et al., 2010, 2011). This rock is internationally traded in large quantities. The mining of syenite rock for international trading and its processing and use in the manufacture of glass, therefore, redistributes the radioactive trace elements throughout the environment. Considerable attention has been given to possible exposures of humans to ionizing radiation from external and internal environmental sources. This attention is due to the fact that external radiation exposures from natural radionuclides contribute about one third of the average annual dose to humans from all radiation sources with a great variability due to rock contents and/or altitude (UNSCEAR, 1982, 1993, 2000).
Ele Abiama et al. (2010) and Beyala Ateba et al. (2010, 2011) studied the background radiation exposure to the public from the Lolodorf area. This study concluded that the concerned region could be said to have a high natural background radiation due to the high concentrations of radium and thorium in soils and rocks samples. Radon and thoron have been measured in some houses of Bikoue and Ngombas of Lolodorf area by Saïdou et al. (2015). The results revelled high concentrations of thoron.
Since the syenite is used in dwellings construction in south region of Cameroon, the aim of the present study is to determine radium equivalent activities and the external hazard index in order to decide whether they are acceptable according to the OECD criterion (OECD, 1979).
The radium equivalent activity concentrations, the external hazard index, the outdoor absorbed dose rate and the annual effective dose are reported and discussed in the light of the international recommendations. The radiation hazards arising due to the use of syenite building rocks reported in this study can be included into baseline map of radioactivity background levels in Cameroon, which will contribute to improve available data worldwide. The knowledge of radiation level is required by the building construction corporation to adopt preventive measures to prevent from the harmful effects of ionizing radiation.
The localities of Awanda and Madong can be considered as public areas with intense human activities covering all sectors of the economy (agricultural lands, schools…). Awanda and Madong were thus considered for sampling and include respectively five and six inhabited areas. Each area where five samples have been collected randomly is considered as sampling site. Twenty-five and thirty syenite building rocks were thus collected in Awanda and Madong, respectively (Fig. 1). All samples were brought to the laboratory where they were dried overnight at a temperature of 250 °C, crushed into small pieces and homogenized. The samples were transferred to polyethylene bottles of 87 cm3 volume and stored for more than 30 days to reach secular equilibrium amongst 226Ra, 232Th, and their short-lived decay products.
Geological and radiometric map of Lolodorf area.
A γ-ray spectrometer consisted with an n-type HPGe detector (CANBERRA model GR3019) and a multi-channel analyzer (CANBERRA Inspector 2000) was used for the measurement. The energy resolution of 1332.5 keV γ-ray line from 60Co was found to be 1.85 keV at full width of half-maximum (FWHM) with the relative efficiency of 34.9%. The detector was lead shielded (CANBERRA model 777) with 1 mm thick Sn and 1.5 mm thick Cu graded liner of the inside to absorb characteristic X-rays of Pb. Measurement of γ-ray energy range was adjusted to 0–2000 keV and calibrated using a mixed gamma-nuclides source of 241Am, 109Cd, 57Co, 139Ce, 137Cs, 88Y and 60Co. Samples were measured for 6 hours and the obtained γ-ray spectra were analyzed by a γ-ray spectrum analysis software CANBERRA Genie 2000.
Radiation measurements were performed for quantitative determination of radionuclides present in the samples. The main objective was the determination of the specific activity concentration of 226Ra, 232Th and 40K using γ-ray spectrometry. The activity concentrations of natural radionuclides in samples were calculated following the method described by Beyala Ateba et al. (2010).
The radium equivalent Raeq was estimated by using Beretka and Mathew (1985) relation. According to Stranden (1976) and Krisiuk et al. (1971), it is assumed that 370 Bq · kg−1 of 226Ra, 259 Bq · kg−1 of 232Th and 4810 Bq · kg−1 of 40K produce the same gamma ray dose rate. A radium equivalent of 370 Bq · kg−1 in building materials will produce an exposure of about 1.5 mSv · year−1 to the inhabitants (UNSCEAR, 1982).
The model proposed by Krieger (1981) was used for the calculation of external hazard index Hex.
The total air absorbed dose rate (nGy · h−1) due to the mean specific activity concentrations of 226Ra, 232Th and 40K was calculated using the formula given by UNSCEAR (1993).
In order to estimate the annual effective dose rates, the conversion coefficient from the absorbed dose in air to the effective dose (0.7 Sv · Gy−1), the outdoor occupancy factor (0.2) and the indoor occupancy factor (0.8) proposed by UNSCEAR (2000) were used. Since the objective of the study is to estimate radiological hazards from building rocks, it is very important to take into account both components of annual effective dose.
Tables 1 and 2 present the concentrations of 226Ra, 232Th and 40K, the absorbed dose rate, the radium equivalent activity concentrations and the external hazard index due to syenite building rocks collected from five and six sampling sites of Awanda and Madong, respectively.
Maximum and minimum radium equivalent activity values are 2564 ± 24 Bq · kg−1 and 186 ± 4 Bq · kg−1 calculated in building rock samples from sampling sites AW-02 and AW-03, respectively. The Raeq of syenite rocks from four sampling sites of Awanda, namely (AW-01, AW-02, AW-04 and AW-05) and four sampling sites of Madong (MA-01, MA-02, MA-04 and MA-05) were higher than the international accepted value of 370 Bq · kg−1.
According to Tables 1 and 2, absorbed dose rates estimated at a height of 1 m above the ground surface ranged from 98 ± 2 to 1171 ± 11 nGy · h−1 with average of 670 ± 420 nGy · h−1 and 313 ± 245 nGy · h−1 in the villages of Awanda and Madong, respectively. Raeq activity concentrations varied from 186 ± 4 to 2564 ± 24 Bq · kg−1, while Hex varied from 0.5 ± 0.01 to 6.93 ± 0.06. The corresponding mean values are 1463 ± 930 Bq · kg−1 and 3.95 ± 2.51 in Awanda while in Madong these values are 683 ± 553 Bq · kg−1 and 1.84 ± 1.49, respectively. Annual effective doses ranged from 0.60 to 7.18 mSv · year−1 with mean values of 4.11 mSv · year−1 and 1.92 mSv · year−1 in Awanda and Madong respectively. These values are about nine-fold and four-fold higher, respectively than the resulting worldwide mean annual effective dose.
Similar studies focused on radiation hazard indices from cement, tiles, gravel and bricks have been done in the world (Amrani and Tahtat, 2001; Kumar et al., 2003; Ngachin et al., 2007; Mehdizadeh et al., 2011). From the radiological point of view, the results of the various radiation hazard indices for the corresponding studies are all lower than the maximum permissible levels.
According to the study of radioactivity levels in granite of Gable Gattar II in the north eastern desert of Egypt (El-Shershaby, 2002), radium equivalent and external hazard index were found to be 28317 Bq · kg−1 and 77, respectively. These values are typical for the region under study. According to the radiation hazard indices, syenite rocks examined in the framework of this study are not acceptable for use as building material as defined by the OECD criterion.
Analytical result of rock samples from Awanda.
Analytical result of rock samples from Madong.
The radium equivalent activity concentrations and the external hazard indices obtained for the syenite building rocks in this study were higher than the criterion limit. Therefore, the use of these materials in dwellings construction is considered to be unsafe for inhabitants according to OECD (1979). The health hazard of inhabitants due to natural background radiation from syenite rocks used in dwellings construction in South-western region of Cameroon can be significant. The data obtained in this work are reference values to be used as a data baseline for drawing a radiological map of the region.
The authors thank the International Atomic Energy Agency (IAEA), which has provided financial support through the technical cooperation project CMR 0003 (Grant number CMR 06006).
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Cite this article as: Beyala Ateba JF, Owono Ateba P, Simo A, Ben-Bolie GH, Ekobena HF, Abega CR, Mvondo S. 2017. Estimation of radiation hazard indices from syenite building rocks in the South-western region of Cameroon. Radioprotection 52(4): 277–280
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