Radiation Protection Dosimetry Advance Access originally published online on June 26, 2007
Radiation Protection Dosimetry 2008 128(2):213-216; doi:10.1093/rpd/ncm327
© The Author 2007. Published by Oxford University Press. All rights reserved
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Determination of uranium, thorium and potassium activity concentrations in soil cores in Araba valley, Jordan
M. Abusini1,*,
K. Al-ayasreh1 and
J. Al-Jundi2
1 Department of Physics, Al-albayt University, PO Box 928125, Al-Mafraq, Jordan
2 Department of Physics, The Hashemite University, Zarqa, Jordan
* Corresponding author: abusini{at}aabu.edu.jo
Received February 28, 2007, amended May 8, 2007, accepted May 19, 2007
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ABSTRACT
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Soil samples were collected from six different locations in Araba valley, situated between Aqaba port and Dead sea. The samples have been analysed by using gamma-ray spectrometry. From the measured gamma-ray spectra, activity concentrations are determined for 238U, 232Th and 40K. The mean activity concentration for 238U, 232Th and 40K was found to be in the range 19 ± 1.4 to 38.7 ± 3, 14.3 ± 0.8 to 35 ± 3.2 and 94 ± 18.9 to 762 ± 47.4 Bq kg–1, respectively. These results indicate that the mean concentrations of 238U, 232Th and 40K in the populated Araba valley are lower than those in other populated areas. On the other hand, the concentrations of the major oxides (Al2O3, SiO2, K2O, CaO and Fe2O3) in the samples were determined using wavelength dispersive X-ray fluorescence. High potassium and iron content in some samples might be attributed to the active faults, which refer to the Dead sea transform fault.
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INTRODUCTION
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Radioactivity of soil environment is one of the main sources
of exposure to humans. Hence it is important to know its distribution,
gamma radiation from radionuclides which are characterised by
half-lives comparable to the age of the earth, such as
40K and
radionuclides from the
238U and
232Th series. Their decay products
represent the main external source of radiation to the human
body. More specifically, natural radioactivity and the associated
external exposure due to gamma radiation depend primarily on
the geological and geographical conditions, and appear at different
levels in the soil of each region in the world
(1–4). Higher
radiation levels are associated with igneous rocks, such as
granite and lower levels with sedimentary rocks. However, some
shales and phosphate rock have relative high content of those
radionuclides
(5,6). Therefore, measurements of natural radioactivity
in soil are of a great interest for many researchers throughout
the world, which led to worldwide national surveys in the last
two decades
(7–10). This study complements a few other
studies, which were conducted at different locations in Jordan
(11–13).
In this paper, we measure the specific activity of the naturally
occurring radionuclides
238U,
232Th and
40K in soil cores obtained
from six locations of Araba valley, along with the concentrations
of various chemical oxides in the collected samples. The results
will be used to establish a baseline map for that area. This
map will be used as a reference information to assess any change
in the radioactivity background level due to the change in the
topography of the location, other developments and settlement
around it, or any artificial influences on the environment.
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MATERIALS AND METHODS
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Araba valley situated between the Aqaba port and the Dead sea
(Figure
1). Soil samples were collected from transects
perpendicular to the traffic flow along the Araba valley–Aqaba
way. The area of the Araba valley was divided into six locations,
eight samples from each location depths (0–10 and 10–20
cm) were collected. The sampling sites were located at distance
of 30 m from the roadside curb. Soil samples crushed, oven dried
at a temperature of 70°C for 24 h and sieved through a 0.2
mm mesh. About 10 g of the collected samples were taken for
chemical analysis, the concentration of Al
2O
3, SiO
2, K
2O, CaO
and Fe
2O
3 compounds in the samples were determined using wavelength
dispersive X-ray fluorescence spectrometer. The remaining portions
of each sample were then packed in a standard Marinelli beaker
that was hermetically sealed, dry-weighed and stored for about
4 weeks to allow equilibration of
226Ra with
222Rn.
238U and
232Th were assumed to be in equilibrium with their progeny
(8,14).
The background with empty beaker was subtracted.
The gamma-ray activities were measured using a low-level counting
system of a high-purity germanium computer-controlled detector
by conventional electronic, of 20% relative efficiency. The
resolution was 2 keV for the 1333 keV
60Co. The detector was
shielded in a 10 cm thick lead internally lined with 2 mm copper
foil. A reference sample from Canberra Industries, Inc, type
MGS-5, was used for efficiency calibration in the same geometry
as the measured for 20 h. The gamma-ray lines of
212Pb(238.6
keV),
208TI(583 keV) and
228Ac(911.2 keV) were used to determine
the activity of
232Th, whereas the ray lines of
214Pb(295.2
keV and 351.9 keV) and
214Bi(609.3 KeV and 1120.3 KeV) were
used to determine the activity of
238U. The activity of
40K
is based on the detection of its 1460.8 KeV gamma-ray.
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RESULTS AND CONCLUSION
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Table
1 represents the activity concentrations obtained
by direct gamma-ray spectrometry measurements of the soil samples.
Activity of
238U,
232Th and
40K is reported throughout this
paper in Bq kg
–1 dry weight. The mean activity concentration
of
238U was found to be in the range 19 ± 1.4 to 38.7
± 3 Bq kg
–1, while for
232Th it is in the range
14.3 ± 0.8 to 35 ± 3.2 Bq kg
–1.On the other
hand, the activity of
40K was found to be in the range 94 ±
18.9 to 762 ± 47.4 Bq kg
–1. The results show a
high activity of
40K in Locations 1 and 2. The obtained results
are comparable to the worldwide average concentration of these
radionuclide in soil reports by the UNSCEAR
(1), which are 40
Bq kg
–1 for
238U and
232Th and 370 Bq kg
–1 for
40K.
The small variation between the mean activity concentration
obtained for
238U and
232Th in all locations may be ascribed
to the fine particle nature of soil, since the distributions
of elements were found to be very particle size-dependent
(15).
It is also noticed that the mean activity concentrations of
238U,
232Th and
40K are slightly decreased with depth. This
may be explained by the variation of the spatial movement of
the fine particles, which is expected to be faster than leaching
downward and also to the differences of the sample's grain size.
The gamma dose rate (
D) in nGy h
–1 in the outdoor air
at 1 m above the ground is calculated using the following equation
by the UNSCEAR
(1):
where
CK,
CU and
CTh are the average activity concentrations of
40K,
238U and
232Th in Bq kg
–1, respectively.
The air-absorbed dose rate due to the gamma-ray emission from the samples ranges between 25.2 and 65.8 nGy h–1.The average absorbed dose rate values calculated for each site is shown in Table 2.
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Table 2. The average absorbed dose rate in (nGy h–1) caused by 40K, 238U, 232Th and the total absorbed dose obtained from 40K, 238U and 232Th.
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The mean concentrations of
238U,
232Th and
40K in the populated
Araba valley and other studies conducted nationally and internationally
are shown in Table
3, obviously demonstrate that the average
activity concentrations obtained in this study are lower than
presented in other studies. It may be noted that the higher
activity concentration of
40K obtained in the surface soil samples
of Araba valley is 762 Bq kg
–1, yet it is still in the
range of UNSCEAR report
(16), which is 140–850 Bq kg
–1.
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Table 3. Reported values of gamma activity in soil (Bq kg–1), from work conducted worldwide and results obtained in this study.
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The concentration of the major metal oxides in the samples is
given in Table
4; no enriched area was observed. Samples
of high potassium and iron content are further characterised
by decreased silicate content, which might be attributed to
the active faults which refer to the Dead sea transform faults
(17).
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SUMMARY
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The obtained results confirm some facts in agreement with many
reported previously. They are comparable to the worldwide average
concentration of these radionuclide in soils reported by UNSCEAR
(1).
The highest activity was found in Locations 1 and 2. The variation
of the activity concentration in these sites is ascribed to
the fine particle nature of soil, since the distribution of
elements was found to be very particle size-dependent. It follows
from the comparison of the measured activities that the geographical
position, geological structure and the geomorphology of the
surrounding area are also influence the results.
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ACKNOWLEDGEMENT
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The authors would like to thank Al-albayt University and The
Hashemite University for their financial support.
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