|Year : 2011 | Volume
| Issue : 4 | Page : 275-281
Natural radioactivity in the volcanic field north of Sana'a, Yemen
EE Saleh1, AI El-Mageed2, AH El-Kamel2, A Abbady3, S Harb3
1 Department of Physics, Faculty of Education Toor El-Baha, Aden University, Yemen
2 Department of Physics, Faculty of Science, Assiut University, Egypt
3 Department of Physics, Faculty of Science, South Valley University, Egypt
|Date of Web Publication||17-Jan-2013|
A I El-Mageed
Department of Physics, Faculty of Science, Assiut University
Source of Support: None, Conflict of Interest: None
The level of natural radioactivity in rocks and soil of 32 samples collected from locations at North Sana'a in Yemen was measured. Concentrations of radionuclides in rocks and soils samples were determined by gamma-ray spectrometer using HPGe detector with specially designed shield. The average radioactivity concentrations of 226 Ra, 232 Th and 40 K were determined and expressed in Bq kg−1 . The results showed that these radionuclides were present in concentrations of 21.79 ± 3.1, 19.5 ± 2.6 and 399.3 ± 16 Bq kg−1 , respectively, for rocks. For soil, the corresponding values were 48.2 ± 4.4, 41.7 ± 4.5 and 939.1 ± 36 Bq kg−1 . Also, the radiological hazard of the natural radionuclides content, radium equivalent activity, total dose rates, external hazard index and gamma activity concentration index of the (rocks/soils) samples in the area under consideration were calculated. The data were discussed and compared with those given in the literature.
Keywords: Natural radioactivity, radiological effects, rock, soil
|How to cite this article:|
Saleh E E, El-Mageed A I, El-Kamel A H, Abbady A, Harb S. Natural radioactivity in the volcanic field north of Sana'a, Yemen. Radiat Prot Environ 2011;34:275-81
|How to cite this URL:|
Saleh E E, El-Mageed A I, El-Kamel A H, Abbady A, Harb S. Natural radioactivity in the volcanic field north of Sana'a, Yemen. Radiat Prot Environ [serial online] 2011 [cited 2020 Jun 6];34:275-81. Available from: http://www.rpe.org.in/text.asp?2011/34/4/275/106205
| 1. Introduction|| |
Measurements and studies of natural radioactivity in soil and rocks are very important to determine the amount of change of the natural background activity with time as a result of any radioactive release; monitoring of any release of radioactivity to the environments is important for environmental protection. 
Quaternary intraplate volcanism north of Sana'a covers an area of ~800 km 2 ; the volcanic can be divided into two phases on the basis of relative age: (1) subhorizontal, plateau-forming basalt flows; (2) a number of lava flows, up to 15-km long.  On the basis of morphology there is a clear distinction in relative age between the older plateau-forming flows and the large stratovolcanoes, and the considerably better preserved strombolian ash and spatter cones with their parasitic resent lava flows. This distinction is used to divide the volcanic samples into two suites, an old and young series. 
This paper presents details of natural radioactivity data in rocks and soil ( 226 Ra, 232 Th, and 40 K) and their radiological Hazard as radium equivalent activity, representative level index I r, external hazard index H ex and dose rate for intraplate volcanic field located in western Yemen, near Sana'a, which is one of such four intraplate volcanic fields in Yemen.
| 2. Experimental|| |
2.1 Geological outline
Quaternary intraplate volcanism North of Sana'a, Yemen covers an area of ~800 km 2 , located between Lat. 15°30' and 15°54'N and long. 43°57' and 44°15'E [Figure 1] and, although laterally extensive, is only a thin carapace (<200 m) with an estimated volume of 60 ± 20 km 3 . This is the volumetric equivalent of about five Oligocene flood basalt flows in this region, and some two orders of magnitude smaller than the total volume of preserved flood volcanism in Yemen.  The volcanic field comprises a number of sub-horizontal lava flows, each 5-20-m thick (younger series Region A); and, in some places Oligocene flood volcanic flows (older series Region B). 
2.2 Sampling and sample preparation
A total of 18 rocks and 14 surface soil samples have been collected randomly from the studied area. Rock sample was crushed to small pieces and grinded to be powder. Soil samples were collected with the only constraint that no sampling site should be taken close to a field boundary, a road, a tree or other obstruction. Surface soils were then taken from different places randomly within the marked and cleared area from the ground surface up to 2 cm and mixed together thoroughly in order to obtain a representative sample of that area. Each sample (rock/ soil) was dried in an oven at 105°C and sieved through a 100 mesh which is the optimum size enriched in heavy mineral.  The samples were packed in plastic containers with dimensions of 75 mm in diameter and 90 mm height. The samples were weighed and stored for a minimum period of 1 month to allow daughter products to come into radioactive equilibrium with their parents 226 Ra and 232 Th and then were counted for 480-720 min depending on the concentration of the radionuclides.
2.3 Experimental setup
Each sample was measured with a gamma-ray spectrometer consisting of an HPGe setup and multichannel analyzer 8192 channel. The detector used is coaxial closed end, closed facing window geometry with vertical dipstick (500-800 μm). The HPGe detector is p-type with the following specifications: resolution (FWHM) at 122 keV, 57 Co is 1100 eV and at 1.33 MeV 60 Co is 2.00 keV - relative efficiency at 1.33 MeV 60 Co is 30c/o. The detector is shielded in a chamber of four layers starting with Plexiglas (10-mm thick), copper (30-mm thick), lead (100-mm thick) and finally cadmium (3-mm thick). This shield serves to reduce different background radioactivity.
The emitted X-rays from lead, which contains radioactive impurities due to antimony impurities, can be absorbed by lining the inside of the shield with a graded layer of 0.05-inch cadmium and 0.25 inch Perspex.  To minimize the effect of the scattered radiation from the shield, the detector is located in the center of the chamber. Then the sample was placed over the detector for at least 10 h. The spectra were either evaluated with the computer software program Maestro (EG and G ORTEC), or manually with the use of a spread sheet (Microsoft Excel) to calculate the natural radioactivity. 226 Ra activity of the samples was determined via its daughters ( 214 Pb and 214 Bi) through the intensity of the 295.22 keV, 351.93 keV, for 214 Pb Gamma-lines and 609.31 keV, 1120 keV, 1764.49 keV, for 214 Bi Gamma-lines. 232 Th activity of the sample was determined from the daughters ( 228 Ac), ( 212 Pb) and ( 208 Ti) through the intensity of 209.25 keV, 338.32 keV, 911.2 keV, 968.97 keV gamma-lines for ( 228 Ac), ( 212 Pb) emissions at 238.63 keV and ( 208 Ti) emissions at 583.19 keV, 2614 keV gamma-lines. 40 K activity was determined from the 1460.7 keV emissions gamma-lines.
2.4 Calculation of the radiological effects
The most widely used radiation hazard index Ra eq is called the radium equivalent activity. The radium equivalent activity is a weighted sum of activities of the 226 Ra, 232 Th and 40 K radionuclides based on the assumption that 370 Bq kg−1 of 226 Ra, 259 Bq kg−1 of 232 Th and 4810 Bq kg−1 of 40 K produce the same gamma-ray dose rate.  Radium equivalent activity can be calculated from the following relation suggested by Beretka and Mathew. 
Where ATh is the specific activity of 232 Th in Bq kg−1 , ARa is the specific activity of 226 Ra Bq kg−1 , AK is the specific activity of 40 K in Bq kg−1 .
Another radiation hazard index called the representative level index, I r, is defined from the following formula. ,
Where: ARa, ATh and AK having the same meaning as in Eq. (1).
External hazard index due to the emitted gamma rays of the samples are calculated and examined according to the following criterion:
Where: ARa , ATh and AK are the activity concentrations of 226 Ra, 232 Th and 40 K, respectively. The calculated average external hazard index was found to be less than unity.
The total air absorbed dose rate (nGy.h−1 ) due to the mean activity concentrations of 226 Ra, 232 Th and 40 K (Bq kg−1 ) can be calculated using the formula of Beck et al.,
Where: A Ra , A Th and A K are the mean activity concentrations of 238 U, 232 Th and 40 K, respectively, in (Bq kg−1 ). Beck et al. (1972) derived this equation for calculating the absorbed dose rate in air at a height of 1.0 m above the ground from measured radionuclides concentrations in environmental materials.
| 3. Results and Discussion|| |
Quaternary intraplate volcanism North of Sana'a has a sequence ranging from Plio-Quaternary age, which are composed of older plateau-forming basalt flows (region B); number of recent lava flows up to 15-km long (region A). 
[Table 1] illustrates the specific activity of the natural radionuclide ( 226 Ra, 232 Th, and 40 K) in the sample rocks and soil collected from the area under investigation. The specific activities are given throughout the paper in Bq kg−1 dry weight. The table also lists the type of rock. The mean activity of 226 Ra was found to range from 21.1 ± 4 to 33.3 ± 2.4 Bq kg−1 with an average value of 26.6 ± 2.9 Bq kg−1 in rocks (region A) and from 10.3 ± 1.3 to 22.1 ± 2.6 Bq kg−1 with an average value of 16.98 ± 3.4 Bq kg−1 (region B). The corresponding values are from 23.9 ± 4.5 to 69.3 ± 4.6 Bq kg−1 with an average value 48.2 ± 4.4 Bq kg−1 for soil.
|Table 1: Activity Concentrations of 226Ra, 232Th and 40K (Bq kg−1) of Rocks and Soils Samples|
Click here to view
232 Th activity concentration in rock samples ranged from 19.7 ± 1 to 30.8 ± 2.5 Bq kg−1 with an average value of 23.2 ± 2.8 Bq kg−1 (region A) and from 10.5 ± 1 to 26.9 ± 4 Bq kg−1 with an average value of 15.1 ± 2.5 Bq kg−1 (region B).
For soil the corresponding values are from 17.5 ± 4 to 52.6 ± 5 Bq kg−1 with an average value of 41.7 ± 4.5 Bq kg−1 .
40 K values ranged from 418.4 ± 15 to 573.7 ± 22 Bq kg−1 with an average value of 515.6 ± 18 Bq kg−1 in rocks samples (region A) and from144.7 ± 8.5 to 467.8 ± 19 Bq kg−1 with an average value of 283.8 ± 13.6 Bq kg−1 , whereas the corresponding value for soil are from 504.9 ± 21 to 1229.5 ± 45 Bq kg−1 with an average value of 939.1 ± 36 Bq kg−1 .
Baker et al. (1997)  divided the rocks into two subsets on the basis of relative age into younger and older series. The younger series include a greater proportion of more silica-undersaturated and alkaline rocks than the older series. It is worth noting that Baker et al. (1997)  stated that light rare earth elements, Th, U and K among other trace elements are generally higher in abundance in the young series compared to the old series at a specific MgO content. This is reflected by the results of the present study [Table 1] showing activity concentration averages for the younger rocks nearly twice as those for the older ones.
Natural radioactivity is present in soil and content of those may be changed by anthropogenic activities (like the use of fertilizers) and transported from one place to another into the ground by the rain or by wind. For this reason, results show an increase of activity concentration of 226 Ra, 232 Th and 40 K for soil rather than rocks.
The mean activity concentrations in soil for 226 Ra and 232 Th in the present study are 48 and 41 Bq kg−1 , which agree with the worldwide average concentrations of these radionuclides in soils 40 Bq kg−1 for 226 Ra and 232 Th as reported by UNSCEAR.  Although the mean activity concentration of 40 K in this study is 939 Bq kg−1 , this value is higher than the activity concentration (370 Bq kg−1 ) reported by UNSCEAR. 
3.1 Comparison of activity concentrations with similar studies
The activity concentrations of 226 Ra, 232 Th and 40 K in rocks and soil samples from studied area were compared with those from similar investigations in other countries and summary results were given in [Table 2]. Similarly, Bellia et al.,  reported the natural radioactivity in volcanic island (Ustica) Italy and the results showed the concentration of 226 Ra, 232 Th and 40 K in the trachybasalt are higher than those in the basalt rocks, also the activity concentrations for soil are higher than those in the rocks. The activity concentration for natural radionuclides in rocks and soil in the present study are higher than in literature (Patra et al.,  El-Aydarous,  Tzortzis et al.,  Narayang et al.,  ). When we compare the results in this study for basalt rocks with the results for Granite rocks reported by (El-Arabi et al.,  Nagdia,  Ahmed et al.,  ) we noted the activity concentration for natural radionuclides for granite rocks are much higher than the activity concentrations in basalt rocks, this may be due to the granite rocks mainly consist of magnetite, biotite, plagioclase, quartz, microcline and potassium feldspar minerals which accumulate in limited uranium and thorium. El-Arabi et al.,  also investigated variation of radium and thorium with the associated elements, and found the increase in the concentration of 226 Ra and 232 Th with increase in the Fe 2 O 3 , FeO, Al 2 O 3 , TiO 2 , SiO 2 , Na 2 O and K 2 O elements.
|Table 2: Concentrations of 226Ra, 232Th and 40K in Rocks and Soils of the Present Work and other Studies|
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[Table 3] shows the comparison of radium equivalent Ra eq , representative level index I r, external hazard index H ex and radiation dose rate for rocks and soils from quaternary volcanism Sana'a Yemen with the results in many countries (Akhtar et al.,  Bellia et al.,  Abbady,  Yaboah et al.,  Patra et al.,  Khaled,  Abdul Jabbar et al.,  Ziqiang et al.,  Malanca et al.,  and Mustapha, et al.,  ). It can be seen that values of radiological parameter and radiation dose rate for rocks obtained from this study fall within the lowest side of all reported values from other countries except in the case of Egypt (Bir El-Sid). In contrast, values of radium equivalent Ra eq , representative level index I r, external hazard index H ex and radiation dose rate for soil obtained in this study match with those of other countries.
|Table 3: Radium Equivalent Activity, Representative Index Ir, External Hazard Index Hex and the Dose Rate in Air of the Present Work and other Studies|
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| 4. Conclusions|| |
Rock samples and surface soil of the Quaternary intraplate volcanism North of Sana'a (Yemen) were measured for their radioactivity content. The results show that the mean concentration values of 226 Ra, 232 Th and 40 K in rocks are 26.6 ± 2.9, 23.2 ± 2.8 and 515.6 ± 18 Bq kg−1 for region A, respectively, and 16.98 ± 3.4, 15.1 ± 2.5 and 283 ± 13.6 Bq kg−1 for region B, respectively, while that of surface soil were 48.2 ± 4.4, 41.7 ± 4.5 and 939.1 ± 36 Bq kg−1 , respectively. Also, the results showed that the radioactivity of region A is higher than the radioactivity of region B. On the other hand, the radioactivity of soil samples is higher than the radioactivity of rock samples.
The mean values of radium equivalent activity (Ra eq ), representative level index, I r , and external hazard index H ex for rocks samples under investigation are 82.2, Bq kg−1 0.3 and 0.2, respectively, while that of surface soil were 180.1 Bq kg−1 , 0.7 and 0.5, respectively.
The results indicate that the dose rate at 1 m above the ground from terrestrial sources in all samples under investigation were 38.39 and 86.89 nGy h−1 for rocks and surface soil, respectively. These values agree with the worldwide average concentrations of these radionuclides in soils reported by UNSCEAR,  which is the range 24-160 nGy h−1 . These values present no hazards to humans.
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[Table 1], [Table 2], [Table 3]