Global Journal of Physics
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ISSN: 2454-7042
Radioactivity Levels and dose evaluation in Some Environmental Rock
Samples from Taiz, Yemen
M. Al-Abyad1 , S. U. El-Kameesy2 , S.A. El-Fiki 2, M. N. Dahesh2,3
1
Physics department (Cyclotron Facility), Nuclear Research Cent re, Atomic Energy Authority, Cairo 13759,
Egypt
2
Physics department, Faculty of science, Ain Shams University, Cairo, Egypt
2,3
Ministry of higher education and scientific research- Republic of Y emen
Abstract
226
232
40
The specific activities of Ra, Th and K in a variety of rock samples from Taiz region, Yemen were investigated using
gamma ray spectroscopy technique to estimate the associated radiation hazard impacts. Furthermore, the X-ra y
fluorescence technique has been applied to detect the natural elements that may have industrial import ance. The mean
activity concentrations of 226Ra, 232 Th and 40K were found to be 65.58±1.38, 82.93±0.93 and 976.40±6.11 Bq kg-1
respectively. These values exceed the maximum international limits. Radium equivalent (Ra eq ), the external hazard index
(Hex), the internal hazard index (H in), the representative level index (I), dose rate, annual effective dose, excess lifetime
cancer risk (ELCR), annual gonadal dose equivalent (AGDE), emanation factor (F) and mass exhalation rate of radon (E Rn)
were estimated and discussed. Additionally, the X-ra y analysis showed that there are considerable concentrations of Fe,
Al, Zr and Ti have been observed.
Introduction
In rocks, natural radionuclides generate significant component of the background radiation exposure to h umans.
The main natural contributors to external exposure from gamma radiation are uranium 238U and thorium 232 Th series, in
addition to potassium 40K. Natural radioactivity measurements and studies in rocks and soil are very important to
determine the amount of change of the natural background activity with time as a result of any radioactive release [1].
The study of the concentrations of the natural radionuclides in rocks and soil permits to understand the radiation
effects of these elements due to the gamma-ray e xposure of human and irradiation of lung tissue caused by the inhalation
of radon and its daughters [2,3,4]. Therefore, it is important to estimate the radiation hazards arising due to the use of rock
and soil in the construction of dwellings [5, 6]. Se veral works have been performed to determine the natural radioactivity in
several zones in Yemen [7-15]. To our knowledge, there are no serious works have been published concerning the natural
radioactivity levels in Taiz region, Yemen. Therefore, the present work is devoted to study the radioactivity for different
types of rocks from Taiz region to assess the associated health hazards along with establishing a radiological baseline for
any future studies.
EXPERIMENTAL PROCEDURES AND METHODS
The study area
Fig.1 : The map of study area inTaiz region (Yemen).
Fig.2 : Location of samples in study area.
The study area is located in the Republic of Yemen, at Taiz region in the South -west of Sana’a (Fig.1). This area
located between latitudes (13°– 14°) longitudes (43°– 45°), 110 Km from the Red Sea and its height is about 3000 m
above sea level almost. Yemen’s land is covered with rocks whose ages date back to an era prior to the Cambrian. Some
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ISSN: 2454-7042
Cambrian rocks belong to a time before that era (that is about 3 billion years ago). Geologically speaking, Yemen
composes part of the Arabian Shield [16].
Sampling and sample preparation
Twenty-fi ve rock samples of different types such as Granite, Diorite, Rhyolite and Basalt were collected from the
study area as shown in Fig.2. Each sample is crushed to small pieces and grinded. Then the samples were weighed and
packed in cans (250 cm 3) for 6 weeks to reach secular equilibrium where the rate of decay of the daughters equals to that
of the parent [17]. The importance of this step is to ensure that radon confined within the volume and the daughters also
will remain in the sample.
Instrumentation and calibration
The prepared samples for the activity concentration measurements was assayed non -destructively using γ-ray
spectrometry and were performed by a HP(Ge) detector which is coupled to a PC -MCA. To reduce gamma ray
background a cylindrical lead shield with a fixed bottom and movable cover shielded the detector. The data acquisition
was performed by a multichannel analyser (MCA) using Gamma vision (Version 5.1, EG&G ORTEC) software program.
The HP(Ge) detector model Canberra GC-6020 of efficiency 60% and energy resolution of 2.4 keV full width at half
maximum (FWHM) for the 1332.5 keV gamma ray line of 60Co was used for activity measurements. The instrument was
calibrated by using standard source of known activity of 226Ra of the same geometry where known gamma ray energy
lines emitted were used for energy and efficiency calibration of the spectrometer. The absolute efficiency curve is
displayed in Fig.3.
Fig.3 : The absolute efficiency of the used HPGe detector.
Description of X-Ray Fluorescence System
The principle of X-ray fluorescence (XRF) technique is based on that when the individual atoms excited by an
external energy source, the x-ra y photons of a characteristic energy or wavelength will be emitted. After that, the elements
present can be identified and quantified by counting the number of photons of each energy emitted from a sample . The
XRF technique is widely used in the applications of science and industry.
In the present study, the XRF technique was used to determine the track element contents using PHLIPS
X’ Unique–II spectrometer with automatic sample charger PW, (30 positions). This instrument is connected to a computer
system using the X-40 program.
Activity concentrations
The average calculated activity concentration for 226Ra was based on the energy transitions of 295.1 keV (19.2%)
and 352.0 keV (37.1%) of 214Pb; 609.3 keV (46.1%), 1120 keV (15.1%) and 1764.5 keV (15.9%) of 214Bi. The
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ISSN: 2454-7042
corresponding activity concentrations for 232Th were calculated making use of the energy transitions 338.4 keV (12.4%)
and 911.2 keV (25.9%) of 228Ac; 583.19 keV (30.4%) and 2416 keV (35.6%) of 208Ti and 238.63 keV (43.3%) of 212Pb.
The activity concentration for 40K was determined from the emissions at 1460.7 keV (10.67%) gamma line. The
gamma transition with energies 92.8 keV (2.39%) and 1001 keV (0.84%) were used to determine the activity concentration
238
210
of U while the gamma transition 46.5 keV (4.05%) was used to determine the activity concentration of Pb. The activity
concentration is based on the following equation:
A= Np / (ε × η × m)
(1)
Where N p = the (cps) sample – (cps) background, ɛ the abundance of the gamma line in radionuclide, η the detector
efficiency of the specific γ-ray and m the mass of the sample (Kg).
The uncertainty of acti vity u (A) can be calculated by the following equation:
2
u 2 ( A) u ( N p ) u 2 ( ) u 2 (m)

 2 
A2
N p2

m2
(2)
Gamma-ray radiation hazard indices
The natural radioactivity of building materials is usually determined from 226Ra, 232Th and 40K contents. As the
activity of 226Ra or any of its daughters represents about 98.5% of that of 238U, the contribution from 238U could be replaced
with any of them. The gamma radiation hazards due to the specific radionuclides were assessed by four different indices;
radium equivalent (Ra eq ), external radiation hazard (H ex), the absorbed dose rate (D) and the annual effective dose rate.
Raeq can be calculated according to Beretka and Mathew (1985) as the following equation [18]:
Raeq = ARa + 1.43ATh + 0.077AK
226
232
(3)
40
-1
Where ARa, ATh and AK are the specific activities of Ra, Th and K in Bq kg respectively. The Ra eq is related to the
external gamma dose and internal dose due to radon and its daughters . The maximum value of Ra eq in building materials
must be less than 370 Bq kg -1 for safe used [18].
The external hazard index and internal hazard index (H ex and H in) are given by the following equations [18]:
Hex = ARa /370 + ATh/259 + AK /4810 ≤ 1
(4)
Hin = ARa /185 + ATh/259 + AK/4810 ≤ 1
(5)
Hex and H in must not exceed the limit of unity for the radiation hazard to be negligible.
Another hazard index called the representative level index (I γ) was determined for rock and soil samples according to the
following equation [18]:
Iγ = ARa /300 + ATh /200 +AK /3000
(6)
The representative level index should be less than unity.
-1
-1
-1
Absorbed dose rate (Gy h ) one meter above the ground (nGy h b y Bq kg ) due to natural radionuclides was
determined via the following equation [19]:
D = 0.462ARa + 0.604ATh + 0.0417AK
(7)
-1
The annual effective dose rate outdoor in units of mSv y is calculated by the following formula [20]:
-1
-1
-1
AEDE (outdoor) (mSv y ) = Dose rate (nGy h ) × 8760 h ×0.7 Sv Gy ×0.2×10
-6
(8)
-1
Where the 0.7 Sv Gy and 0.2 factors are the conversion factor from absorbed dose in air to effective dose a nd the
outdoor occupancy factor respectively (UNSCEAR) [1].
Additionally, the e xcess lifetime cancer risk (ELCR) was calculated using the following equation [21]:
ELCR = HR × DL × RF
(9)
Where H R is the annual effective dose equivalent, DL is the duration of life (70 years) an d RF is the risk factor (0.05 Sv-1).
The fatal cancer risk per Sievert is the definition of the risk factor.
The annual gonadal dose equivalent (AGDE) also was calculated using the following formula [22]:
AGDE (mSv/y) = (3.09ARa + 4.18ATh + 0.314AK) ×10 -3
(10)
Assuming an equilibrium state, the activity of radon is calculated by the following equation:
ARa = AD + ARn
(11)
226
Where ARa is the measured activity of Ra, AD is the measured activity of daughter elements 214 Pb (or 214Bi) and ARn is
the estimated activity of 222Rn, which can be expressed through the introduction of the radon emanation factor F, which is
defined as:
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ISSN: 2454-7042
F = ARn / AR a = (ARa – AD) / ARa
(12)
The mass exhalation rate or radon mass exhalation rate is product of the emanation coefficie nt and
production rate [23]. The mass exhalation rate ERn (Bq/kg.s) was determined by the following equation:
ERn = ARn x ARa x λRa
Rn
(13)
222
Where λRa is the decay constant of
222
-6
-1
Rn (2.1 × 10 s ).
Results and discussion
According to XRF results (table 1) Si in rocks is ranging from 51.94% to 62.66%. Also K contribution in the samples
is ranging from 9.32% to 16.25%. Additionally, Al is rangin g from 8.22% to 10.76% Fe is ranging from 9.04% to 24.16%
and Ti is ranging from 4204 ppm to 1.36%. The elements (Zr, Zn, Rb, Nb and Mn) also found in these samples. These
values indicate that the samples of the place under study can be considered as a pro duction area for many strategic
elements.
The activity concentrations of 226Ra, 232Th and 40K for all samples are presented in table 2. The values are given in
Bq/kg on a dry weight basis. The activity concentration of 226R is ranged from 27.54±1.01 to 234.49±3.13 Bq/kg with an
average 65.58±1.83 Bq/kg. The mean value of 226Ra is higher than the average international radioactivity levels, which is
232
33 Bq/kg [24, 25]. The acti vity concentration of Th is ranged from 15.82±0.40 to 415.31±2.47 Bq/kg with an average
232
82.93±0.93 Bq/kg. The mean value of Th is higher than the average international radioactivity levels, which is 45 Bq/kg
[24, 25]. The acti vity concentration of 40K is ranged from 257.91±2.61 to 1107.47±6.92 Bq/kg with an average 976.40±6.11
40
Bq/kg. The mean value of K is higher than the average international radioactivity levels, which is 420 Bq/kg [24, 25].
Table (1): Trace elements concentration for some samples in the area under investigation.
Element
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Si
61.43%
58.90%
62.66%
51.94%
57.79%
K
16.25%
14.38%
14.13%
9.32%
15.21%
Al
9.66%
9.21%
9.41%
8.22%
10.76%
Fe
9.04%
14.48%
10.61%
24.16%
12.52%
Zr
1.69%
5361ppm
7952ppm
3.35%
1.04%
Ti
4459ppm
7800ppm
4204ppm
1.36%
1.18%
Zn
1019ppm
1260ppm
1183ppm
1604ppm
1233ppm
Rb
6016ppm
1682ppm
2316ppm
1462ppm
1957ppm
Nb
6759ppm
609ppm
2001ppm
4350ppm
1415ppm
Mn
4972ppm
5553ppm
2620ppm
1616ppm
1.01%
Table 2: The activity concentrations of
226
Ra,
232
Th and
40
K in Bq kg-1.
Activity (Bq/kg)
NO
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Th-232
K-40
1
79.77±1.42
91.29±0.83
1094.74±5.42
2
59.79±2.36
61.45±1.53
1101.85±11.37
3
84.74±1.76
96.10±1.19
1027.52±7.38
4
234.49±3.13
415.31±2.47
887.36±7.71
5
82.60±1.81
105.60±1.28
1008.99±7.57
6
68.49±1.21
91.89±0.86
1104.33±5.67
7
84.06±1.08
93.10±0.73
1062.92±4.68
8
70.77±1.12
82.90±0.76
931.33±4.79
9
63.61±1.52
74.51±0.90
1028.08±6.29
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ISSN: 2454-7042
10
79.67±1.29
91.08±0.86
1005.16±5.42
11
50.66±1.22
77.67±0.86
1004.46±5.85
12
48.22±1.10
58.99±0.75
1096.15±4.72
13
37.52±0.95
56.80±0.68
610.43±4.19
14
40.73±1.19
44.75±0.77
1044.18±6.36
15
46.94±1.97
43.21±1.24
1035.22±10.41
16
77.49±1.35
85.58±1.02
993.78±5.86
17
61.56±1.00
74.73±0.68
988.36±4.62
18
64.66±1.90
82.09±0.81
1006.94±5.36
19
27.54±1.01
27.15±0.63
957.83±6.05
20
58.31±1.45
86.68±0.99
1021.34±6.77
21
27.79±0.77
15.82±0.40
257.91±2.61
22
61.35±1.02
81.12±0.78
1029.35±5.13
23
59.86±1.43
69.75±0.97
1107.47±6.92
24
27.81±1.10
26.47±0.66
1007.95±6.64
25
40.90±0.94
39.10±0.59
996.26±5.00
Mean
65.58±1.38
82.93±0.93
976.40±6.11
Fig.4 : The activity concentrations of
226
Ra and
232
Th series and 40K (Bq kg-1) of the samples.
The experimental results of Ra eq , H ex, H in, Iγ, absorbed dose and annual effective dose equivalent outdoor are
presented in table 3. The calculated Ra eq activities of all samples (Table 3) are below the recommended value 370 Bq kg -1
except the value of sample 4 (896.71 Bq kg -1) which is higher than the recommended value (370 Bq kg -1) [18].
The mean values of external hazard index H ex, internal hazard index H in and gamma index Iγ for all samples under
investigation (Table 3) are 0.70, 0.88, and 0.96 Bq/kg, respectively, which are less than unity. The a bsorbed dose rate in
air 1.0 m above the ground was calculated (Table 3) and is ranged from 33.15 to 396.18 nGyh -1 with an average 121.10
nGy h -1 . According to the recent UNSCEAR Reports (2008), the corresponding worldwide average value is 58 nGy h -1 [26].
This reveals that the mean absorbed dose rate in air is higher than that of worldwide average value [26]. Furthermore, the
annual outdoor effective dose varied from 0.04 to 0.49 mSv y-1 , with an a verage 0.15 mSv y-1 . Hence, the obtained results
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concerning the annual outdoor effective dose are higher than the world average, (UNSCEAR, 2008) where the world
average for rock is 0.07 mSv y-1 [26].
Table (3): Radium equivalent Ra eq (Bq/kg), external hazard Hex, internal hazard Hin, gamma index Iγ , absorbed dose
and annual effective dose equivalent (outdoor) in all samples.
No
Raeq
Hex
Hin
Iγ
Annual
effective dose
rate(outdoor)
Dose rate
(nGy h-1)
(mSv y-1)
1
294.61
0.80
1.01
1.09
137.64
0.17
2
232.51
0.63
0.79
0.87
110.69
0.14
3
301.28
0.81
1.04
1.11
140.04
0.17
4
896.71
2.42
3.06
3.15
396.18
0.49
5
311.30
0.84
1.06
1.14
144.02
0.18
6
284.93
0.77
0.95
1.06
133.19
0.16
7
299.04
0.81
1.03
1.10
139.39
0.17
8
261.03
0.70
0.90
0.96
121.60
0.15
9
249.32
0.67
0.85
0.93
117.26
0.14
10
287.31
0.78
0.99
1.06
133.74
0.16
11
239.07
0.65
0.78
0.89
112.20
0.14
12
216.98
0.59
0.72
0.82
103.57
0.13
13
165.75
0.45
0.55
0.61
77.10
0.09
14
185.12
0.50
0.61
0.71
89.39
0.11
15
188.44
0.51
0.64
0.72
90.95
0.11
16
276.39
0.75
0.96
1.02
128.93
0.16
17
244.53
0.66
0.83
0.91
114.79
0.14
18
259.58
0.70
0.88
0.96
121.44
0.15
19
140.12
0.38
0.45
0.55
69.06
0.08
20
260.91
0.70
0.86
0.97
121.88
0.15
21
70.27
0.19
0.26
0.26
33.15
0.04
22
256.79
0.69
0.86
0.95
120.26
0.15
23
244.88
0.66
0.82
0.92
115.97
0.14
24
143.27
0.39
0.46
0.56
70.87
0.09
25
173.53
0.47
0.58
0.66
84.06
0.10
Mean
259.35
0.70
0.88
0.96
121.10
0.15
Table 4 contains the value of annual gonadal dose equivalent (AGDE), e xcess lifetime cancer risk (ELCR),
estimate activity of 222Rn (ARn), radon emanation factor (F) and mass exhalation rate (ERn). The annual gonadal dose
equivalent varied from 0.23 to 2.74 mSv y-1 , with an average 0.86 mSv y-1. Also, the excess lifetime cancer risk (ELCR)
-3
-3
mean value is (0.52×10 ), which is higher than the world average (0.29×10 ) [24]. Additionally, the average values of
-1 -1
radon emanation factor (F) and mass exhalation rate (E Rn in Bq kg s ) are 0.85, 0.59 respectively.
238
210
226
238
210
238
Table 5 contains the specific activity concentrations of U and Pb along with the ratios Ra/ U, Pb/ U
and 210Pb/ 226Ra. The ratios 226Ra/238U, 210Pb/238U and 210Pb/226Ra are equal to 0.82, 0.81and 0.99 respectively, these
values nearly equal to 1 indicating a state of equilibrium.
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Table (4): Excess lifetime cancer risk (ELCR), annual gonadal dose equivalent (AGDE), radon emanation factor (F)
and the mass exhalation rate (ERn) in all samples.
No
AGDE(mSv/y)
ELCR*10 -3
ARn
F
ERn
1
0.97
0.59
479.43
0.85
0.56
2
0.79
0.48
329.35
0.84
0.27
3
0.99
0.60
485.40
0.85
0.58
4
2.74
1.70
1750.74
0.88
7.28
5
1.01
0.62
494.42
0.86
0.57
6
0.94
0.57
396.47
0.84
0.39
7
0.98
0.60
461.43
0.84
0.53
8
0.86
0.52
389.79
0.84
0.38
9
0.83
0.50
409.87
0.86
0.41
10
0.94
0.57
471.25
0.85
0.54
11
0.80
0.48
267.79
0.83
0.18
12
0.74
0.44
255.77
0.83
0.16
13
0.55
0.33
252.17
0.86
0.15
14
0.64
0.38
244.00
0.85
0.14
15
0.65
0.38
250.05
0.84
0.15
16
0.91
0.55
455.40
0.85
0.51
17
0.81
0.49
381.65
0.86
0.36
18
0.86
0.52
360.09
0.84
0.32
19
0.50
0.30
147.06
0.83
0.05
20
0.86
0.52
448.58
0.88
0.48
21
0.23
0.14
143.15
0.84
0.05
22
0.85
0.52
402.53
0.87
0.39
23
0.82
0.50
346.82
0.85
0.30
24
0.51
0.30
152.44
0.83
0.06
25
0.60
0.36
227.43
0.84
0.13
Mean
0.86
0.52
400.12
0.85
0.59
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Table (5): The activity concentrations of
238
U and
210
Pb in Bq/kg and the activity ratios of
Pb/226 Ra.
210
226
Ra/ 238U,
210
Pb/ 238 U and
Activity (Bq/kg)
226
No
238
U
Ra/238 U
210
Pb/238 U
210
Pb/226 Ra
210
Pb
1
98.91
78.78
0.806
0.796
0.988
2
82.87
59.05
0.721
0.713
0.988
3
86.50
83.69
0.979
0.968
0.988
4
241.60
231.60
0.971
0.959
0.988
5
99.05
81.58
0.834
0.824
0.988
6
85.37
67.64
0.802
0.792
0.988
7
95.56
83.02
0.879
0.869
0.988
8
74.68
69.90
0.947
0.936
0.988
9
75.85
62.82
0.838
0.828
0.988
10
86.09
78.68
0.925
0.914
0.988
11
63.25
50.03
0.801
0.791
0.988
12
56.58
47.68
0.852
0.843
0.989
13
54.52
37.06
0.688
0.680
0.988
14
61.70
40.23
0.660
0.652
0.988
15
82.01
46.36
0.572
0.565
0.988
16
63.24
76.53
1.225
1.210
0.988
17
78.61
60.80
0.784
0.773
0.988
18
78.50
63.93
0.823
0.814
0.989
19
43.48
27.20
0.633
0.626
0.988
20
100.26
57.59
0.581
0.574
0.988
21
37.43
27.45
0.742
0.733
0.988
22
84.94
60.77
0.724
0.715
0.988
23
58.89
59.12
1.016
1.004
0.988
24
43.12
27.47
0.645
0.637
0.988
25
42.40
40.39
0.964
0.953
0.988
Mean
79.02
64.77
0.82
0.81
0.99
Conclusion
The present study has been devoted to e valuate the background radioactivity le vels of Taiz region in Yemen
through gamma ray spectroscopy technique making use of high resolution HPGe detector. The results of the mean activity
concentrations attributed to 226Ra, 232Th and 40K in different collected samples are found to be 65.58 ±1.38, 82.93 ±0.93
and 976.40 ±6.11 Bq kg -1. All these values exceed the recommended world limits. Consequently, the absorbed dose rate
was quite significant (mean value = 121.10 nGy h -1) having higher values than the world mean average value (58 nGy h -1).
Furthermore, the health hazard indices were calculated and discussed. From the calculation, it is obvious that the mean
value of the annual effective dose (0.15 mSv y-1) is nearly two order of magnitude higher than the world average value.
Also, the mean values of excess lifetime cancer risk (ELCR), annual gonadal dose equivalent (AGDE), emanation factor
(F) and mass exhalation rate of radon (ERn) are found to be 0.52×10 -3, 0.86 mSv y-1 , 0.85 and 0.59Bq kg -1 s -1respectively.
Additionally, the XRF analysis reveals that there are considerable amounts of Al, Fe, Zr, Ti, Zn, Rb, Nb and Mn in the
investigated area in Taiz region.
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ISSN: 2454-7042
Finally, the obtained results strongly give serious warning against exploiting rocks of the investigated area to be
used as building materials or constructing houses partially or completely inside the mountains located there. The impact of
indoor Radon concentration on inhabitants is under our consideration in a near work.
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