Radioactivity Levels and dose evaluation in Some Environmental Rock Samples from Taiz, Yemen

Radioactivity Levels and dose evaluation in Some Environmental Rock Samples from Taiz, Yemen M. Al-Abyad 1, S. U. El-Kameesy, S.A. El-Fiki, M. N. Dahesh,3 1 Physics department (Cyclotron Facility), Nuclear Research Centre, Atomic Energy Authority, Cairo 13759, Egypt Physics department, Faculty of science, Ain Shams University, Cairo, Egypt,3 Ministry of higher education and scientific research- Republic of Yemen Abstract The specific activities of 6 Ra, 3 Th and 40 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-ray fluorescence technique has been applied to detect the natural elements that may have industrial importance. The mean activity concentrations of 6 Ra, 3 Th and 40 K were found to be 65.58±1.38, 8.93±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 (H ex), 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-ray 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 38 U and thorium 3 Th series, in addition to potassium 40 K. 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 exposure of human and irradiation of lung tissue caused by the inhalation of radon and its daughters [,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]. Several 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. : 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 66 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

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-five rock samples of different types such as Granite, Diorite, Rhyolite and Basalt were collected from the study area as shown in Fig.. Each sample is crushed to small pieces and grinded. Then the samples were weighed and packed in cans (50 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-600 of efficiency 60% and energy resolution of.4 kev full width at half maximum (FWHM) for the 133.5 kev gamma ray line of 60 Co was used for activity measurements. The instrument was calibrated by using standard source of known activity of 6 Ra 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-ray 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 6 Ra was based on the energy transitions of 95.1 kev (19.%) and 35.0 kev (37.1%) of 14 Pb; 609.3 kev (46.1%), 110 kev (15.1%) and 1764.5 kev (15.9%) of 14 Bi. The 67 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

corresponding activity concentrations for 3 Th were calculated making use of the energy transitions 338.4 kev (1.4%) and 911. kev (5.9%) of 8 Ac; 583.19 kev (30.4%) and 416 kev (35.6%) of 08 Ti and 38.63 kev (43.3%) of 1 Pb. The activity concentration for 40 K was determined from the emissions at 1460.7 kev (10.67%) gamma line. The gamma transition with energies 9.8 kev (.39%) and 1001 kev (0.84%) were used to determine the activity concentration of 38 U while the gamma transition 46.5 kev (4.05%) was used to determine the activity concentration of 10 Pb. The activity concentration is based on the following equation: A= Np / (ε η m) (1) Where Np = 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 activity u (A) can be calculated by the following equation: u ( A) u ( N A N Gamma-ray radiation hazard indices p p ) u ( ) u ( m) m The natural radioactivity of building materials is usually determined from 6 Ra, 3 Th and 40 K contents. As the activity of 6 Ra or any of its daughters represents about 98.5% of that of 38 U, the contribution from 38 U could be replaced with any of them. The gamma radiation hazards due to the specific radionuclides were assessed by four different indices; radium equivalent (Raeq), external radiation hazard (Hex), 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 (3) Where ARa, ATh and AK are the specific activities of 6 Ra, 3 Th and 40 K in Bq kg -1 respectively. The Raeq 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/59 + AK/4810 1 (4) Hin = ARa/185 + ATh/59 + AK/4810 1 (5) Hex and Hin must not exceed the limit of unity for the radiation hazard to be negligible. Another hazard index called the representative level index (Iγ) was determined for rock and soil samples according to the following equation [18]: Iγ = ARa /300 + ATh /00 +AK /3000 (6) The representative level index should be less than unity. Absorbed dose rate (Gy h -1 ) one meter above the ground (ngy h -1 by Bq kg -1 ) due to natural radionuclides was determined via the following equation [19]: D = 0.46ARa + 0.604ATh + 0.0417AK (7) The annual effective dose rate outdoor in units of msv y -1 is calculated by the following formula [0]: AEDE (outdoor) (msv y - 1 ) = Dose rate (ngy h -1 ) 8760 h 0.7 Sv Gy -1 0. 10-6 (8) Where the 0.7 Sv Gy - 1 and 0. factors are the conversion factor from absorbed dose in air to effective dose a nd the outdoor occupancy factor respectively (UNSCEAR) [1]. Additionally, the excess lifetime cancer risk (ELCR) was calculated using the following equation [1]: ELCR = HR DL RF (9) Where HR is the annual effective dose equivalent, DL is the duration of life (70 years) and 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 []: AGDE (msv/y) = (3.09A Ra + 4.18A Th + 0.314A K) 10-3 (10) Assuming an equilibrium state, the activity of radon is calculated by the following equation: ARa = AD + ARn (11) Where ARa is the measured activity of 6 Ra, AD is the measured activity of daughter elements 14 Pb (or 14 Bi) and ARn is the estimated activity of Rn, which can be expressed through the introduction of the radon emanation factor F, which is defined as: () 68 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

F = A Rn / A R a = (A Ra A D) / A Ra (1) The mass exhalation rate or radon mass exhalation rate is product of the emanation coefficie nt and Rn production rate [3]. The mass exhalation rate ERn (Bq/kg.s) was determined by the following equation: ERn = ARn x ARa x λra (13) Where λra is the decay constant of Rn (.1 10-6 s -1 ). Results and discussion According to XRF results (table 1) Si in rocks is ranging from 51.94% to 6.66%. Also K contribution in the samples is ranging from 9.3% to 16.5%. Additionally, Al is ranging from 8.% to 10.76% Fe is ranging from 9.04% to 4.16% and Ti is ranging from 404 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 6 Ra, 3 Th and 40 K for all samples are presented in table. The values are given in Bq/kg on a dry weight basis. The activity concentration of 6 R is ranged from 7.54±1.01 to 34.49±3.13 Bq/kg with an average 65.58±1.83 Bq/kg. The mean value of 6 Ra is higher than the average international radioactivity levels, which is 33 Bq/kg [4, 5]. The activity concentration of 3 Th is ranged from 15.8±0.40 to 415.31±.47 Bq/kg with an average 8.93±0.93 Bq/kg. The mean value of 3 Th is higher than the average international radioactivity levels, which is 45 Bq/kg [4, 5]. The activity concentration of 40 K is ranged from 57.91±.61 to 1107.47±6.9 Bq/kg with an average 976.40±6.11 Bq/kg. The mean value of 40 K is higher than the average international radioactivity levels, which is 40 Bq/kg [4, 5]. Table (1): Trace elements concentration for some samples in the area under investigation. Element Sample 1 Sample Sample 3 Sample 4 Sample 5 Si 61.43% 58.90% 6.66% 51.94% 57.79% K 16.5% 14.38% 14.13% 9.3% 15.1% Al 9.66% 9.1% 9.41% 8.% 10.76% Fe 9.04% 14.48% 10.61% 4.16% 1.5% Zr 1.69% 5361ppm 795ppm 3.35% 1.04% Ti 4459ppm 7800ppm 404ppm 1.36% 1.18% Zn 1019ppm 160ppm 1183ppm 1604ppm 133ppm Rb 6016ppm 168ppm 316ppm 146ppm 1957ppm Nb 6759ppm 609ppm 001ppm 4350ppm 1415ppm Mn 497ppm 5553ppm 60ppm 1616ppm 1.01% Table : The activity concentrations of 6 Ra, 3 Th and 40 K in Bq kg -1. NO Activity (Bq/kg) Ra-6 Th-3 K-40 1 79.77±1.4 91.9±0.83 1094.74±5.4 59.79±.36 61.45±1.53 1101.85±11.37 3 84.74±1.76 96.10±1.19 107.5±7.38 4 34.49±3.13 415.31±.47 887.36±7.71 5 8.60±1.81 105.60±1.8 1008.99±7.57 6 68.49±1.1 91.89±0.86 1104.33±5.67 7 84.06±1.08 93.10±0.73 106.9±4.68 8 70.77±1.1 8.90±0.76 931.33±4.79 9 63.61±1.5 74.51±0.90 108.08±6.9 69 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

10 79.67±1.9 91.08±0.86 1005.16±5.4 11 50.66±1. 77.67±0.86 1004.46±5.85 1 48.±1.10 58.99±0.75 1096.15±4.7 13 37.5±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 43.1±1.4 1035.±10.41 16 77.49±1.35 85.58±1.0 993.78±5.86 17 61.56±1.00 74.73±0.68 988.36±4.6 18 64.66±1.90 8.09±0.81 1006.94±5.36 19 7.54±1.01 7.15±0.63 957.83±6.05 0 58.31±1.45 86.68±0.99 101.34±6.77 1 7.79±0.77 15.8±0.40 57.91±.61 61.35±1.0 81.1±0.78 109.35±5.13 3 59.86±1.43 69.75±0.97 1107.47±6.9 4 7.81±1.10 6.47±0.66 1007.95±6.64 5 40.90±0.94 39.10±0.59 996.6±5.00 Mean 65.58±1.38 8.93±0.93 976.40±6.11 Fig.4 : The activity concentrations of 6 Ra and 3 Th series and 40 K (Bq kg -1 ) of the samples. The experimental results of Raeq, Hex, Hin, Iγ, absorbed dose and annual effective dose equivalent outdoor are presented in table 3. The calculated Raeq 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 Hex, internal hazard index Hin 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 11.10 ngy h -1. According to the recent UNSCEAR Reports (008), the corresponding worldwide average value is 58 ngy h -1 [6]. This reveals that the mean absorbed dose rate in air is higher than that of worldwide average value [6]. Furthermore, the annual outdoor effective dose varied from 0.04 to 0.49 msv y -1, with an average 0.15 msv y -1. Hence, the obtained results 70 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

concerning the annual outdoor effective dose are higher than the world average, (UNSCEAR, 008) where the world average for rock is 0.07 msv y -1 [6]. 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 Ra eq H ex H in I γ Dose rate (ngy h -1 ) Annual effective dose rate(outdoor) (msv y -1 ) 1 94.61 0.80 1.01 1.09 137.64 0.17 3.51 0.63 0.79 0.87 110.69 0.14 3 301.8 0.81 1.04 1.11 140.04 0.17 4 896.71.4 3.06 3.15 396.18 0.49 5 311.30 0.84 1.06 1.14 144.0 0.18 6 84.93 0.77 0.95 1.06 133.19 0.16 7 99.04 0.81 1.03 1.10 139.39 0.17 8 61.03 0.70 0.90 0.96 11.60 0.15 9 49.3 0.67 0.85 0.93 117.6 0.14 10 87.31 0.78 0.99 1.06 133.74 0.16 11 39.07 0.65 0.78 0.89 11.0 0.14 1 16.98 0.59 0.7 0.8 103.57 0.13 13 165.75 0.45 0.55 0.61 77.10 0.09 14 185.1 0.50 0.61 0.71 89.39 0.11 15 188.44 0.51 0.64 0.7 90.95 0.11 16 76.39 0.75 0.96 1.0 18.93 0.16 17 44.53 0.66 0.83 0.91 114.79 0.14 18 59.58 0.70 0.88 0.96 11.44 0.15 19 140.1 0.38 0.45 0.55 69.06 0.08 0 60.91 0.70 0.86 0.97 11.88 0.15 1 70.7 0.19 0.6 0.6 33.15 0.04 56.79 0.69 0.86 0.95 10.6 0.15 3 44.88 0.66 0.8 0.9 115.97 0.14 4 143.7 0.39 0.46 0.56 70.87 0.09 5 173.53 0.47 0.58 0.66 84.06 0.10 Mean 59.35 0.70 0.88 0.96 11.10 0.15 Table 4 contains the value of annual gonadal dose equivalent (AGDE), excess lifetime cancer risk (ELCR), estimate activity of Rn (ARn), radon emanation factor (F) and mass exhalation rate (ERn). The annual gonadal dose equivalent varied from 0.3 to.74 msv y -1, with an average 0.86 msv y -1. Also, the excess lifetime cancer risk (ELCR) mean value is (0.5 10-3 ), which is higher than the world average (0.9 10-3 ) [4]. Additionally, the average values of radon emanation factor (F) and mass exhalation rate (ERn in Bq kg -1 s -1 ) are 0.85, 0.59 respectively. Table 5 contains the specific activity concentrations of 38 U and 10 Pb along with the ratios 6 Ra/ 38 U, 10 Pb/ 38 U and 10 Pb/ 6 Ra. The ratios 6 Ra/ 38 U, 10 Pb/ 38 U and 10 Pb/ 6 Ra are equal to 0.8, 0.81and 0.99 respectively, these values nearly equal to 1 indicating a state of equilibrium. 71 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

Table (4): Excess lifetime cancer risk (ELCR), annual gonadal dose equivalent (AGDE), radon emanation factor (F) and the mass exhalation rate (E Rn) in all samples. No AGDE(mSv/y) ELCR*10-3 ARn F ERn 1 0.97 0.59 479.43 0.85 0.56 0.79 0.48 39.35 0.84 0.7 3 0.99 0.60 485.40 0.85 0.58 4.74 1.70 1750.74 0.88 7.8 5 1.01 0.6 494.4 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 0.5 389.79 0.84 0.38 9 0.83 0.50 409.87 0.86 0.41 10 0.94 0.57 471.5 0.85 0.54 11 0.80 0.48 67.79 0.83 0.18 1 0.74 0.44 55.77 0.83 0.16 13 0.55 0.33 5.17 0.86 0.15 14 0.64 0.38 44.00 0.85 0.14 15 0.65 0.38 50.05 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 0.5 360.09 0.84 0.3 19 0.50 0.30 147.06 0.83 0.05 0 0.86 0.5 448.58 0.88 0.48 1 0.3 0.14 143.15 0.84 0.05 0.85 0.5 40.53 0.87 0.39 3 0.8 0.50 346.8 0.85 0.30 4 0.51 0.30 15.44 0.83 0.06 5 0.60 0.36 7.43 0.84 0.13 Mean 0.86 0.5 400.1 0.85 0.59 7 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

Table (5): The activity concentrations of 38 U and 10 Pb in Bq/kg and the activity ratios of 6 Ra/ 38 U, 10 Pb/ 38 U and 10 Pb/ 6 Ra. No Activity (Bq/kg) 6 Ra/ 38 U 10 Pb/ 38 U 10 Pb/ 6 Ra 38 U 10 Pb Conclusion 1 98.91 78.78 0.806 0.796 0.988 8.87 59.05 0.71 0.713 0.988 3 86.50 83.69 0.979 0.968 0.988 4 41.60 31.60 0.971 0.959 0.988 5 99.05 81.58 0.834 0.84 0.988 6 85.37 67.64 0.80 0.79 0.988 7 95.56 83.0 0.879 0.869 0.988 8 74.68 69.90 0.947 0.936 0.988 9 75.85 6.8 0.838 0.88 0.988 10 86.09 78.68 0.95 0.914 0.988 11 63.5 50.03 0.801 0.791 0.988 1 56.58 47.68 0.85 0.843 0.989 13 54.5 37.06 0.688 0.680 0.988 14 61.70 40.3 0.660 0.65 0.988 15 8.01 46.36 0.57 0.565 0.988 16 63.4 76.53 1.5 1.10 0.988 17 78.61 60.80 0.784 0.773 0.988 18 78.50 63.93 0.83 0.814 0.989 19 43.48 7.0 0.633 0.66 0.988 0 100.6 57.59 0.581 0.574 0.988 1 37.43 7.45 0.74 0.733 0.988 84.94 60.77 0.74 0.715 0.988 3 58.89 59.1 1.016 1.004 0.988 4 43.1 7.47 0.645 0.637 0.988 5 4.40 40.39 0.964 0.953 0.988 Mean 79.0 64.77 0.8 0.81 0.99 The present study has been devoted to evaluate the background radioactivity levels 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 6 Ra, 3 Th and 40 K in different collected samples are found to be 65.58 ±1.38, 8.93 ±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 = 11.10 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 0.5 10-3, 0.86 msv y -1, 0.85 and 0.59Bq kg -1 s -1 respectively. 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. 73 P a g e e d i t o r @ g p c p u b l i s h i n g. c o m

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