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Radiological responses of different types of Egyptian Mediterranean coastal sediments
ARTICLE IN PRESS Radiation Physics and Chemistry 79 (2010) 831–838
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Radiological responses of different types of Egyptian Mediterranean coastal sediments A. El-Gamal a,n, M. Rashad b, Z. Ghatass c a
Department of Oceanography, Coastal Research Institute, National Water Research Center, 15 Elpharaana St., Elshallalat, Postal code 21514, Alexandria, Egypt Land and Water Technologies Department, Arid Land Cultivation and Development Research Institute, Mubarak City for Scientific Research, Burg El-Arab, Alexandria, Egypt c Department of Environmental Studies, Institute of Graduate Studies and Research, Alexandria University, Alexandria, Egypt b
a r t i c l e in fo
abstract
Article history: Received 4 December 2009 Accepted 4 March 2010
The aim of this study was to identify gamma self-absorption correction factors for different types of Egyptian Mediterranean coastal sediments. Self-absorption corrections based on direct transmission through different thicknesses of the most dominant sediment species have been tested against point sources with gamma-ray energies of 241Am, 137Cs and 60Co with 2% uncertainties. Black sand samples from the Rashid branch of the Nile River quantitatively absorbed the low energy of 241Am through a thickness of 5 cm. In decreasing order of gamma energy self-absorption of 241Am, the samples under investigation ranked black sand, Matrouh sand, Sidi Gaber sand, shells, Salloum sand, and clay. Empirical self-absorption correction formulas were also deduced. Chemical analyses such as pH, CaCO3, total dissolved solids, Ca2 + , Mg2 + , CO23 , HCO3 and total Fe2 + have been carried out for the sediments. The relationships between self absorption corrections and the other chemical parameters of the sediments were also examined. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Egyptian coastal sediments Self-absorption Direct transmission Gamma-ray Rashid black sand
1. Introduction The Egyptian Mediterranean coast contains a wide variety of sediments. Coastal sediments are mainly composed of two principal types: carbonate sands and quartz-dominant sands (Hilmy, 1951). The western region from Mersa Matrouh to Alexandria (Agami) is mainly composed of pure oolitic carbonate (Hilmy, 1951; El-Sabrouti et al., 1981; Anwar et al., 1984). The oolitic grains constitute an average of 78% and 89% of the bottom and beach sediments, respectively. Further westward, sand at Salloum showed an average oolite content in the nearshore of 58% (Anwar et al., 1981). Beach sand from Dekhiela (Alexandria) to Rashid primarily consists of quartz grains with common shell fragments (Hilmy, 1951; El-Wakeel and El-Sayed, 1978). The Nile River has been identified as the major source of quartz-rich sediments and heavy minerals on the Nile Delta beaches (Frihy, 1994). As a result of the current erosion affecting beaches along the Nile delta (UNDP/UNESCO, 1978; Lotfy and Frihy, 1993; Frihy, 1994), the
n Corresponding author. Current address: Department of Oceanography, Florida State University, Tallahassee, Fl 32306, USA (valid till 10 April 2010). Tel.: + 850 644 6705; fax: + 850 644 2581; Permanent address: Department of Oceanography, Coastal Research Institute, National Water Research Center, Alexandria, Egypt (valid after 11 April 2010). Tel.: + 203 4844614/5/6; Mobile: +2010 5296027; fax: + 203 4844614. E-mail address: [email protected] (A. El-Gamal).
0969-806X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2010.03.004
eroded shorelines are characterized by high-density minerals, whereas the accreted shorelines are characterized by low-density minerals (Frihy et al., 1997). Black sand deposits are extensive along the beaches of Rashid on the Egyptian Mediterranean coast (El-Fishawi and Badr, 1991; El-Askary and Badr, 1996; El-Gamal et al., 2004; Saleh et al., 2004). These black sands contain some important minerals such as zircon and monazite (El-Khatib and Abou El-Khier, 1988), both of which contain uranium and thorium in their chemical structures. High background radioactivity values at Rashid have been detected as a result of these black sands (El-Naggar, 1990; El-Gamal et al., 2004). Gamma-ray spectrometry is a well-established qualitative and quantitative method of measuring the radioactive components of environmental samples (Galloway, 1991a). However, an accurate determination of sedimentary gamma-emitting radionuclide activities in the sediments cannot be achieved without taking sample self-absorption into account (McMahon et al., 2004). When self-absorption corrections cannot be made for a particular sediment sample, it is routine for researchers to simply calibrate to standards with similar composition and density to the samples. Gamma-ray self-absorption is always prevalent, but is only a problem when differences in self-absorption exist between the sample and calibration standard (Cable et al., 1994). The extent of self-absorption in sediment samples depends on a number of factors including sample composition, density, and size, as well as gamma-ray energy (McMahon et al., 2004; Galloway, 1991a).
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The attenuation of gamma rays passing through a length x in a sample matrix of linear attenuation coefficient m can be expressed by the self-absorption equation (McMahon et al., 2004) I ¼ Io emx
ð1Þ
where Io is the emission intensity (neglecting absorption) from the source, I the intensity of the emergent rays from the source (or attenuated beam), m (cm 1) the linear attenuation coefficient for the sample matrix, and x (cm) the mean path length of a gamma-ray for particular energy. The value of m can be calculated from the product of the density and the mass attenuation coefficient (m/r), which can be obtained through knowledge of the sample’s elemental composition (Miller, 2008). Transmission (T) can be calculated by dividing the attenuated radiation by the unattenuated radiation and can be expressed as (Miller, 2008) T¼
I ¼ emx Io
ð2Þ
For most environmental samples, the values of m and x could be estimated if not directly known. One method of determining the average value for mx for a specific sample type and geometry uses the transmission method proposed by Cutshall et al. (1983). This method allows the photopeak efficiency determined using a spiked aqueous calibration standard to be corrected for the degree of absorption of the ‘real’ sample (Miller, 2008). A more convenient method is to prepare a series of gamma-absorption curves. This approach involves making a series of direct transmission measurements of gamma self-absorption in similar samples of varying densities as proposed by different authors such as Galloway (1991a) and McMahon et al. (2004). The self-absorption correction factor is determined by comparing the transmission rates of point sources placed on top of the sample (I) and on an empty sample container (Io). The correction factor (F) for a gamma-ray of known energy (E) can be estimated using transmission (T) from Eq. (2) by FðEÞ ¼
ðT1Þ lnðTÞ
Fig. 1. Map of Egypt.
sediment samples were collected from the upper 10 cm using a plastic cup and then placed in plastic bags. Sediments were dried, crushed, and homogenized prior to analysis. The dried sediment samples were transferred to polyethylene containers of similar diameter with depths of 1.2, 2.5 and 5 cm. The container diameters are less than the diameter of the detector.
2.2. Gamma spectrometry system ð3Þ
This procedure is repeated for the standard medium of the detector calibration standards. By comparing the correction factor of the sample (F(E)sample) with the correction factor of the standard medium (F(E)standard), the correction factor for the efficiency at a particular energy can be estimated. This method only represents an approximation as the equations hold true only for a collimated beam of gamma-rays, which is not the case with the close geometries employed for most low-level gamma analyses (McMahon et al., 2004). The aim of this work is to study the radiological responses of different types of Egyptian Mediterranean coastal sediments. In particular, the self-absorption correction will be evaluated to validate the measurement of these sediments via gamma-ray spectrometry. A secondary goal of this work is to study the relationship between the self-absorption correction values and the energy of the radioactive standard, sediment weight, sediment thickness, and differences in sediment composition.
2. Materials and methods 2.1. Sample collection and preparation Samples were collected from the Egyptian Mediterranean coastal area (Fig. 1) to represent the most common types of sediments throughout the coast. The sampling sites and their geographical coordinates are listed in Table 1. The coastal
The gamma-ray measurements were performed using a high resolution, low background PC multichannel spectrometer, with a HPGe Aptec detector. The HPGe detector (model CS 20-A 31 CL) is a closed-end coaxial type with a sensitive volume of 108 cm3 and a diameter of 5.4 cm encased in a lead shielding of 10 cm thickness. The relative efficiency is 24.5% for 1332.5 keV of 60Co relative to 3 3 Inch NaI, with a 25 cm source-detector distance. Recorded spectra were processed on PCMCA/Super PC software.
2.3. Direct transmission method Sealed 241Am, 137Cs, and 60Co were used as point sources of gamma radiation. The same sources were used by McMahon et al. (2004) to determine gamma self-absorption corrections of environmental samples. These standard sources were capsulated in 1 mm plastic (density 1.23 g/cm3) by Amersham International Plc. To calculate self-absorption correction factors using the direct transmission method (Cutshall et al., 1983), a point source was placed on top of each sample container and counted until adequate counting statistics were achieved for the photopeaks of interest. The source was then placed in the same position on an identical but empty container and the measured count rates for both were calculated. Using Eqs. (1–3), self-absorption correction factors were obtained for each photopeak energy and sample type (McMahon et al., 2004).
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833
Table 1 Main features and geographical information of sediment samples. Sample Code
Sampling Site
Geographical Location
Main Features
Representing Area
A B C
311 33\ 40\\ N 251 09\ 44\\ E 311 21\ 38\\ N 271 15\ 31\\ E 311 15\ 39\\ N 291 57\ 20\\ E
Yellow sand White sand, carbonate oolite Coarse sand
El-Salloum From Mersa Matrouh to Agami (Alexandria) Alexandria to Rashid and Sinai coast
D
El-Salloum Matrouh Sidi Gaber (Alexandria) Rashid
311 28\ 12\\ N 301 21\ 45\\ E
Coastal sediments affecting from Nile River sedimentation
E F
Rashid El-Gamil (Port Said)
311 28\ 10\\ N 301 21\ 43\\ E 311 18\ 40\\ N 321 10\ 50\\ E
Black sand, sandy size heavy minerals as monazite and zirconium, dense quartz grains Clay Broken shells, calcium carbonate
Nile River Estuaries From El-Gamil to Sinai coast
80 Rashid black sand
Alexandria sand
Weight %
60
40
20
0 80 Matrouh carbonate white sand
Salloum yellow sand
Weight %
60 40 20 0 -1
0
1
2 3 4 Grain size (phi)
5
6 -1
0
1
2 3 4 Grain size (phi)
5
6
Fig. 2. Grain size histograms of four different types of the Egyptian Mediterranean coastal sediments.
2.4. Grain size analysis Grain size analysis was carried out using a W.S. Tyler Shaker, Model RX-29-10, and Type Rotap according to the procedure described by Lindholm (1987). The sieves used were USA Standard Testing Sieve (Stainless steel sieves) A.S.T.M.E-11 specifications, Tyler Equivalent, made by W.S. Tyler. The meshes are 4, 2, 1, 0.5, 0.25, 0.125, and 0.063 mm. These correspond to the sedimentological phi (F) scale (which is the negative logarithm with base 2 of the particle diameter in millimeters) as classes 2, 1, 0, 1, 2, 3 and 4 F, respectively.
with the same geometry and the same measurement conditions as for the samples. The uncertainty in the calibration of the photopeak area was within 72%. 2.6. Sediments chemical composition CaCO3%, pH, total dissolved solids (TDS), Ca2 + , Mg2 + , CO23 , HCO3 and total Fe2 + were measured in the different types of sediment. pH was measured in 1:2.5 soil:water suspension. TDS, Ca2 + , Mg2 + , CO23 and HCO3 were measured in a 1:5 extract as expressed as mg/kg. Fe was measured in the digested sample as total concentration in mg/kg.
2.5. Quality control Three types of calibrations were carried out for the gamma spectrometer, including energy, resolution, and efficiency calibrations. Efficiency calibration was performed using Amersham 152Eu standard sealed source. Precision and accuracy were evaluated as published earlier (El-Gamal et al., 2007) using parallel measurements of IAEA intercomparison sediment samples IAEA-300 and IAEA-315 (IAEA, 1996). In order to determine background activities due to natural radionuclides in the environment, an empty polyethylene container was analyzed
3. Results and discussion 3.1. Coastal sediment characteristics 3.1.1. Grain size classification Grain size analysis is an important aspect for its fundamental descriptive measure of sediments (Lindholm, 1987). It is commonly related to other properties which have major economic implications. Fig. 2 shows the grain size histograms of the
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Table 2 Chemical composition of the sediment samples under investigation. pH
A B C D E F
8.68 8.65 8.27 8.52 8.65 8.33
CaCO3%
TDS (mg/kg)
Ca2 + (mg/kg)
Mg2 + (mg/kg)
CO23 (mg/kg)
HCO3 (mg/kg)
Total Fe2 + (mg/kg)
13.00 12.88 4.16 1.30 1.06 94.88
2944 2720 2650 2624 1857 3741
240 168 152 160 143 482
24 28.8 18 21.6 22 120
24 18 12 15 14 32
122 91.5 73.20 76.25 88 137
58.7 66.0 122 2723 74.8 9.2
100
100
80
80
Self absorption %
Self absorption %
Sample ID
60 40 20
Matrouh
Salloum
B Sand
Clay Shells
S Gaber
Salloum
B Sand
Clay
S Gaber
60 40 20 0
0 0
1
2 3 Thickness (cm)
4
0
5
Fig. 3. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 241Am (59.54 keV) with 2% uncertainties.
1
2 3 Thickness (cm)
100
80
Salloum Clay S Gaber
Self absorption %
Matrouh B Sand Shells
60 40 20
4
5
Fig. 5. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 60Co (1173.24 keV) with 2% uncertainties.
100 Self absorption %
Matrouh
Matrouh B Sand S Gaber
80
Salloum Clay
60 40 20 0
0 0
1
2 3 Thickness (cm)
4
5
0
1
2 3 Thickness (cm)
4
5
Fig. 4. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 137Cs (661.62 keV) with 2% uncertainties.
Fig. 6. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 60Co (1332.50 keV) with 2% uncertainties.
four different types of sands under investigation. The grain size distribution was drawn based on phi (F) scale. Grain size analysis of El-Salloum yellow sand documents discrete size groups extending from gravel to fine sand. This sand is characterized as bimodal sediments predominantly in the fine sand size class. Alexandria, Matrouh, and Rashid sands are all characterized as unimodal sediments in the medium sand class size.
Matrouh sediments) have relatively higher amounts of CaCO3, TDS, Ca2 + , Mg2 + , CO23 and HCO3 than the other sands. Sima and Dovlete (1997) reported that the sample matrix can greatly affect the measurement efficiency for samples.
3.1.2. Sediment chemical composition Analyses of pH, CaCO3, TDS, Ca2 + , Mg2 + , CO23 , HCO3 and total 2+ were carried out for all sediment samples as shown in Fe Table 2. Black sand was enriched in total Fe2 + (2723 mg/kg) relative to the other sediment samples. The broken shells are dominated by calcium carbonate (94.88%) and have relatively high amounts of TDS, Ca2 + , Mg2 + , CO23 and HCO3 and relatively low total Fe2 + concentrations. The western sands (El-Salloum and
3.2. Self-absorption determination 3.2.1. Determination of the fraction of self-absorption Figs. 3–6 show gamma self-absorption percentage of the Egyptian Mediterranean coastal sediments under investigation based on the different radioactive point sources of 241Am, 137Cs and 60Co. 241Am emits low energy gamma radiation (59.54 keV). Fig. 3 shows the different responses of each sediment type and thickness against the low energy of 241Am. The intensity of gamma photons decreases with increasing sediment thickness. Significant self-absorption of low energy gamma-rays was
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Attenuation coefficient
1.00
0.80
Matrouh
Salloum
B Sand
Clay
Shells
S Gaber
Table 3 Self-absorption correction factor values F(E) for 1.2, 2.5 and 5 cm thicknesses of different types of the Egyptian Mediterranean sediments against energies of 241Am, 137Cs and 60Co. Energy (keV)
0.60
241
Am 59.54
0.40 137
Cs 661.62
0.20 0
200
400
600 800 Energy (keV)
1000
1200
835
1400
Fig. 7. Gamma attenuation coefficients of different sediment types collected from the Egyptian Mediterranean coast against different energies of 241Am (59.54 keV), 137 Cs (661.62 keV), and 60Co (1173.24 & 1332.50 keV).
detected, consistent with the finding of McMahon et al. (2004). Black sand appears to be a perfect absorber for low energy gamma radiation, absorbing 100% of the gamma energy emitted from 241 Am through the 5 cm thickness. These results are also in agreement with the findings of Cable et al. (1994) that gamma-ray attenuation is greatest for low energy radionuclides. In decreasing order of gamma-ray absorption through 5 cm sediments at 59.54 keV gamma energy were black sand, Matrouh sand, Sidi Gaber sand, shells, Salloum sand, and clay with self-absorption efficiencies of 100%, 95.4%, 91%, 90.3%, 85%, and 80.3%, respectively. Fig. 4 shows the gamma-ray absorption responses against the different types of sediments resulting from exposure of higher gamma energy at 661.62 keV of 137Cs. This figure shows overall lower self-absorption efficiency than for the case of lower energy gamma radiation of 241Am. The highest self-absorption efficiency was the black sand which absorbed about 56.5% of 137Cs gamma emission while the lowest one was clay at 38.5% absorption. The sediment samples ranked in the order of decreasing selfabsorption efficiency through 5 cm thickness by black sand, Matrouh sand, shells, Sidi Gaber sand, Salloum sand, and clay, a similar order as with the 241Am source. Figs. 5 and 6 show the self-absorption responses of the different types of sediments to the highest tested gamma point source of 60Co (1173.24 and 1332.5 keV). Black sand and Matrouh sand were identified as the highest absorbing sediments at both gamma energies of 60Co. Sidi Gaber sand, shells, Salloum sand and clay yielded the lowest self-absorption efficiencies of 41.1%, 36.5% and 31.6% at 1173.24 keV and 35.3%, 29.3% and 24.3% at 1332.5 keV, respectively. An inverse relationship is observed between the percentage of absorbed gamma-rays and the corresponding energy of radiation. The higher absorption is corresponds with lower gamma energy, while low sediment self-absorption correlated with higher gamma energy. Attenuation coefficients were calculated from the self-absorption variation of different sediment thicknesses for each gamma source. Variations were observed among the different sediment samples with different gamma energy. The attenuation coefficient varies from a maximum attenuation coefficient of 1 for 5 cm thickness in black sand to 0.35 for clays. Fig. 7 shows the wide variation in the attenuation coefficients for the different types of sediments at the low gamma energy. Alternatively, a narrow variation was observed among the lower self-absorption coefficient values from the relatively high gamma energies. Self-absorption correction factors were calculated according to Eq. (3) and the values are listed in Table 3 for different thicknesses of sediments (1.2, 2.5 and 5 cm) and for each point source gamma
60
Co 1173.24
60
Co 1332.50
Thickness (cm)
Matrouh Salloum
1.2 2.5 5 1.2 2.5 5 1.2 2.5 5 1.2 2.5 5
0.678 0.501 0.310 0.910 0.838 0.699 0.911 0.851 0.747 0.923 0.871 0.789
0.774 0.667 0.448 0.934 0.858 0.763 0.916 0.874 0.807 0.927 0.885 0.854
B Sand
Clay
Shells
S Gaber
0.497 0.235 0.084 0.897 0.802 0.679 0.913 0.848 0.747 0.918 0.857 0.773
0.805 0.687 0.494 0.944 0.878 0.792 0.854 0.889 0.832 0.932 0.903 0.873
0.690 0.539 0.388 0.918 0.851 0.742 0.903 0.890 0.804 0.905 0.895 0.845
0.715 0.591 0.379 0.924 0.844 0.748 0.923 0.858 0.777 0.932 0.873 0.811
radiation energy of 59.54, 661.62, 1173.78 and 1332.51 keV. Using different sediment thicknesses in this calculation was recommended by Galloway (1991a, b). A relatively higher selfabsorption correction was observed for the 1.2 cm thickness than the other thicknesses. The self-absorption values for different sediment thicknesses from 241Am were relatively lower than the other gamma energies associated with the other point sources, consistent with the findings of McMahon et al. (2004). The variation of self-absorption corrections across a wide range of sediment compositions was first reported by Cable et al. (1994). The dependence of self-absorption corrections on the weight of the sediment sample was also studied for these Egyptian Mediterranean coastal sediments (Fig. 8). Based on the natural logarithmic relationship between the absorption correction of specific gamma energy and sediment weight, mathematical models were formulated. These models can be used to predict the self-absorption correction in environmental samples. The models show the regression line of 241Am with the wide variation of self-absorption corrections ranging from 0.08 to 0.81. In contrast, the minimum self-absorption correction for the three energies of 137Cs and 60Co was 0.68 and the maximum was 0.94. This indicates that the self-absorption correction is vital for low gamma energy counting. The relationship between selfabsorption correction and sediment density is supported by Cable et al. (1994). 3.3. Statistical analyses Statistical analyses have been carried out to test the correlation and significant differences of the sediments under investigation and to classify them into groups according to their determined character. Correlation analysis, multivariate analysis of variance, and cluster analysis have been executed using the international STATGRAPHICS Plus for windows 4.0 software. 3.3.1. Correlation analysis Table 4 shows the correlation coefficients among the chemical characters of the sediments under investigation and the selfabsorption corrections of sediment thicknesses against 241Am, 137 Cs and 60Co point gamma sources. High negative correlations ( 4 0.85) were detected between total Fe2 + with self-absorption corrections through 1.2, 2.5 and 5 cm thicknesses of sediments against exposure to the low energy emitter 241Am with correlation coefficients of 0.89, 0.90 and 0.90, respectively. The correlation coefficient values were diminished to be 0.71, 0.84 and 0.68 between total Fe2 + with self-absorption
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1 y = -0.0781Ln(x) + 1.2834
Correction factor F(E)
0.8 y = -0.0815Ln(x) + 1.2918 y = -0.1554Ln(x) + 1.6781
0.6 241-Am 137-Cs 60-Co 1 60-Co 2 Log. (137-Cs) Log. (60-Co 1) Log. (60-Co 2) Log. (241-Am)
0.4
0.2
y = -0.3539Ln(x) + 2.4753
0 0
100
200
300 400 Weight (g)
500
600
700
Fig. 8. Gamma ray self-absorption correction factors against weight of different types of the Egyptian Mediterranean coastal sediments.
Dendrogram
Table 4 Correlation coefficients among the chemical characters of the sediments under investigation and the self-absorption corrections of 1.2, 2.5 and 5 cm sediment thicknesses against 241Am, 137Cs and 60Co gamma sources.
0.89 0.90 0.90 0.71 0.84 0.68 0.20 0.56 0.55 0.19 0.69 0.64
0.32 0.34 0.41 0.23 0.38 0.28 0.01 0.65 0.46 0.62 0.56 0.54
CO3
Mg
Ca
TDS
CaCO3%
pH
30 0.21 0.15 0.12 0.23 0.20 0.08 0.37 0.00 0.11 0.33 0.10 0.17
corrections against exposure to 137Cs (661.62 keV) and more as 0.2, 0.56 and 0.55 against 60Co (1173.24 keV) through 1.2, 2.5 and 5 cm thicknesses of sediments, respectively. This could indicate that there is inverse relationship between the tested energy for self-absorption correction factors with the total Fe2 + contents of the sediments. Also, high negative correlations were detected between Mg and CaCO3% with a self-absorption correction through 1.2 cm sediments against the exposure of 1332.50 keV from 60Co. Alternatively, high positive correlations were detected between the values of chemical characters of the sediments such as CaCO3%, TDS, Ca, Mg, CO3 and HCO3. 3.3.2. Multifactor analysis of variance Multifactor analysis of variance constructs various tests to determine which factors have a statistically significant effect on the sediments data, taking into consideration their type and character variations. It also tests for significant interactions among the factors. The F-tests in the analysis of variance test identify the significant factors of correlation. Analysis of variance separates the variability of sediment data into contributions from various factors (sediment type and character). Since Type III sum of squares has been chosen, the contribution of each factor is measured after removing the effects of all other factors. The P-value tests the statistical significance
20 10 0 Shells
0.03 0.05 0.16 0.08 0.14 0.04 0.07 0.53 0.23 0.86 0.41 0.26
B Sand
0.20 0.15 0.09 0.34 0.19 0.25 0.57 0.13 0.12 0.82 0.01 0.07
Clay
0.05 0.07 0.18 0.04 0.15 0.08 0.11 0.56 0.29 0.84 0.42 0.32
Salloum
0.01 0.00 0.12 0.10 0.12 0.03 0.00 0.54 0.23 0.87 0.42 0.25
S Gaber
0.11 0.13 0.21 0.00 0.18 0.07 0.13 0.53 0.28 0.78 0.40 0.35
Matrouh
Am_1.2 Am_2.5 241 Am_5 137 Cs_1.2 137 Cs_2.5 137 Cs_5 60 Co1_1.2 60 Co1_2.5 60 Co1_5 60 Co2_1.2 60 Co2_2.5 60 Co2_5
241
HCO3
40
Distance
241
Total Fe
Nearest Neighbor Method,Squared Euclidean
Fig. 9. Dendrogram illustrates groups of the nearest neighbor areas among the 6 different types of sediments collected from the Egyptian Mediterranean coast according to their characters.
of each of the factors. Since one P-value is less than 0.05, its factor has a statistically significant effect on data at the 95.0% confidence level. In general, there are no statistically significant differences among the sediment types while statistically significant differences were detected among the sediments according to their character variations. 3.3.3. Fisher’s least significant difference (LSD) analysis In order to determine which of the determined characters are significantly different from which others, a Fisher’s least significant difference (LSD) procedure was applied. This analysis indicated that TDS is significantly different from all other characteristics at the 95.0% confidence level. Also, total Fe2 + has no significant differences with Ca content but is significantly different from the other parameters. Ca is recognized as a common factor with no significant differences between it and the other parameters. 3.3.4. Cluster analysis Cluster analyses were carried out from two points of view. The first was carried out among the different types of sediments under investigation. This procedure was created 1 cluster from the
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Dendrogram Nearest Neighbor Method,Squared Euclidean 10
Distance
8 6 4 2
CaCO3% Mg Ca CO3 HCO3 TDS Co1_2.5 Co1_5 Co2_5 Co2_2.5 Cs_1.2 Cs_5 Cs_2.5 Am_1.2 Am_2.5 Am_5 Co1_1.2 Co2_1.2 pH Total Fe
0
Fig. 10. Dendrogram illustrates groups of the nearest neighbor areas among all the different sediment chemical characteristics and their self-absorption corrections of the 6 types of sediments under investigation.
6 observations supplied from the characters of the 6 different types of sediments collected from the Egyptian Mediterranean coast. The two groups closest together were combined as shown in Fig. 9. This analysis revealed that Matrouh and Sidi Gaber sands are more similar to each other in their characteristics than the other sediments. Shells are recognized as the most dissimilar sediment type than the others. The other test was carried out among all the different sediment chemical characteristics and their self-absorption corrections of the 6 types of sediments under investigation. The analysis recognized Ca, Mg, and CaCO3% were combined into similar groups and CO3 and HCO3 were also grouped in one subgroup due to their similar characteristics as shown in Fig. 10. From the selfabsorption correction values of the sediment types from each point source of 241Am, 137Cs and 60Co, this analysis grouped all together except thickness 1.2 cm against 60Co (both energies). The analysis classifies total Fe2 + as the most deviating character, with pH as the next most variant character. This could indicate that the matrix effect is important in the analysis of environmental samples by gamma spectrometry techniques. Neglecting these effects may result in high uncertainties, especially at low energies, as confirmed by Sima and Dovlete (1997).
4. Conclusion Direct transmission served as an excellent method to evaluate gamma self-absorption corrections of environmental samples. The investigation of gamma self-absorption of sediments revealed a diversity of responses at different emitted energies. A relatively high self-absorption was observed at the low energy of 241Am (59.54 keV). Rashid black sand was identified as a perfect absorber of gamma radiation especially at a thickness of 5 cm at low gamma energy. At 5 cm sediment thickness and low gamma energy, the decreasing order of sediment self-absorption was black sand, Matrouh sand, Sidi Gaber sand, shells, Salloum sand, and clay. The ability of the sediments to absorb gamma radiation decreased with increasing gamma energy value. The lowest absorption was observed with the higher energy of 1332.5 keV (60Co). Black sand was found to absorb through 5 cm thickness 100%, 56.5%, 45.9% and 41.7% the gamma energies of 59.54, 661.62, 1173.24 and 1332.5 keV, respectively.
837
Gamma self-attenuation coefficients of the investigated sediments were calculated. An inverse relationship was observed for attenuation coefficient values with energy. At lower energies, attenuation coefficient variations among sediments were higher than the narrow variations observed at higher energies. Evaluating the dependency of self-absorption correction factors against the weight of the sediment samples revealed that the absorption correction is vital for lower gamma energy counting. The regression lines and models found that the low gamma energy emission associated with 241Am exhibited a wide range of self-absorption correction values (0.08–0.81) as compared to the other tested energies (0.68–0.94). High negative correlations were detected between various coastal sediments self-absorption corrections and their measured chemical characteristics. Specifically, these correlations were found between total Fe2 + with self-absorption corrections of 1.2, 2.5 and 5 cm sediment thicknesses against exposure to the low energy emitter 241Am and between Mg and CaCO3% with selfabsorption correction of 1.2 cm sediments against the exposure of 1332.50 keV from 60Co. Multivariate analysis of variance indicated that there are no statistically significant differences among all sediment types while statistically significant differences were observed within the sediments according to their characteristic variations. The source of this significant difference was mainly from the values of TDS and total Fe2 + . Cluster analysis classifies shells as the most different type of sediments followed by black sand and the others.
References Anwar, Y.M., El Askary, M.A., Nasr, S.M., 1981. Petrography and origin of the oolitic carbonate sediments of Arab’s Bay, western part of the continental shelf of ¨ Geologie und Palaontologie ¨ Egypt. Neues Jahrbuch Fur – Monatshefte, 65–75. Anwar, Y.M., El Askary, M.A., Nasr, S.M., 1984. Arab’s Bay oolitic carbonate sediments: bathymetric, granulometric and chemical studies. Neues Jahrbuch ¨ Geologie und Palaontologie ¨ Fur – Monatshefte, 10, 594–610. Cable, J.E., Burnett, W.C., Moreland, S.C., Westmoreland, J.B., 1994. Empirical assessment of gamma-ray self absorption in environmental sample analyses. Radioact. Radiochem. 12 (3), 30–41. Cutshall, N.H., Larsen, I.L., Olsen, C.R., 1983. Direct analysis of 210Pb in sediment samples: self-absorption corrections. Nucl. Instrum. Methods B 206, 309–312. El-Askary, M., Badr, A.A., 1996. Foreshore sediments of the Nile Delta promon¨ tories: a correlation study. N. Jb. Geol. Palaont. Mh. 8, 461–472. El-Fishawi, N.M., Badr, A.A., 1991. Mineralogical characteristics of the western Nile Delta coast sediments. Acta Mineral.—Petrogr., Szeged. XXXII, 65–75. El-Gamal, A., Saleh, I., Nasr, S., Naim, M., 2004. Radiological assessment of the Egyptian Mediterranean coast. In: International Conference on Isotopes in Environmental Studies—Aquatic Forum 2004, Monte-Carlo, Monaco 25–29 October 2004, IAEA-CN-118/31P, pp. 396–397. El-Gamal, A., Nasr, S., Eltaher, A., 2007. Study of the spatial distribution of natural radioactivity in the upper Egypt Nile sediments. Radiat. Meas. 42 (3), 457–465. El-Khatib, A.M., Abou El-Khier, A.A., 1988. Regional study of black radioactivity. Isotopenproxis 24, 333–336. El-Naggar, A.M., 1990. Environmental radioactivity; sources and effects on man. In: El-Raey, M. (Ed.), Regional Symposium on Environmental Studies (UNARC). Alexandria University. El-Sabrouti, M.A., Saad, M.A., Shatta, M.H., 1981. Continental shelf sediments in Abu Hashaifa Bay, Mediterranean Sea, Egypt. Jed. J. Mar. Res. 1, 21–34. El-Wakeel, S.K., El-Sayed, M.K.h, 1978. The texture, mineralogy and chemistry of bottom sediments and beach sands from the Alexandria region, Egypt. Mar. Geol. 27, 137–160. Frihy, O.E., 1994. Discrimination of accreted and eroded coasts using heavy mineral compositions of the Nile Delta beach sands. Egypt, Water Sci., the 15th Issue, April 1994. Frihy, O.E., Lotfy, M.F., Komar, P.D., 1997. Spatial variations in heavy minerals and patterns of sediments sorting along the Nile Delta, Egypt. Sediment. Geol. 97 (1–2), 33–41. Galloway, R.B., 1991a. A simple method of correcting for variation of sample thickness in the determination of the activity of environmental samples by gamma spectrometry. Meas. Sci. Technol. 2, 941–945. Galloway, R.B., 1991b. Corrections for sample self absorption in activity determination by gamma spectrometry. Nucl. Instrum. Methods A 300, 367–373. Hilmy, M.E., 1951. Beach sands of the Mediterranean coast of Egypt. Sediment. Pet. 21 (2), 109–120.
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IAEA, 1996. IAEA-315, Report on the intercomparison run, Radionuclides in Marine Sediments 1992–1996, IAEA/AL/65, IAEA/MEL/61. Intercomparison Runs Reference Materials, AQCS, Analytical Quality Control Services, Marine Environment Laboratory (MEL) IAEA. Lindholm, R.C., 1987. A practical approach to sedimentology. London Allen & Unwin. Lotfy, M.F., Frihy, O.E., 1993. Sediment balance along the nearshore zone of the Nile Delta coast, Egypt. J. Coastal. Res. 9, 654–662. McMahon, C.A., Fegan, M.F., Wong, J., Long, S.C., Ryan, T.P., Colgan, P.A., 2004. Determination of self-absorption corrections for gamma analysis of environmental samples: comparing gamma-absorption curves and spiked matrixmatched samples. Appl. Radiat. Isot. 60, 571–577.
Miller, M., 2008. Advanced fuel cycle safeguards, NRC seminar June 10, 2008, Los Alamos, National Laboratory. /http://www.gnep.energy.gov/pdfs/NRCSeminar Safeguards.pdfS. Saleh, I., El-Gamal, A., Nasr, S., Naim, M., 2004. Spatial and temporal variations of uranium and thorium series along the Egyptian Mediterranean coast. In: International Conference on Isotopes in Environmental Studies—Aquatic Forum 2004, Monte-Carlo, Monaco 25–29 October 2004, IAEA-CN-118/113 P, pp. 550–551. Sima, O., Dovlete, C., 1997. Matrix effects in the activity measurements of environmental samples—implementation of specific corrections in a gamma-ray spectrometry analysis program. Appl. Radiat. Isot. 48 (1), 59–69. UNDP/UNESCO, 1978. Coastal protection studies. Final Technical Report, 1.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Radiological responses of different types of Egyptian Mediterranean coastal sediments A. El-Gamal a,n, M. Rashad b, Z. Ghatass c a
Department of Oceanography, Coastal Research Institute, National Water Research Center, 15 Elpharaana St., Elshallalat, Postal code 21514, Alexandria, Egypt Land and Water Technologies Department, Arid Land Cultivation and Development Research Institute, Mubarak City for Scientific Research, Burg El-Arab, Alexandria, Egypt c Department of Environmental Studies, Institute of Graduate Studies and Research, Alexandria University, Alexandria, Egypt b
a r t i c l e in fo
abstract
Article history: Received 4 December 2009 Accepted 4 March 2010
The aim of this study was to identify gamma self-absorption correction factors for different types of Egyptian Mediterranean coastal sediments. Self-absorption corrections based on direct transmission through different thicknesses of the most dominant sediment species have been tested against point sources with gamma-ray energies of 241Am, 137Cs and 60Co with 2% uncertainties. Black sand samples from the Rashid branch of the Nile River quantitatively absorbed the low energy of 241Am through a thickness of 5 cm. In decreasing order of gamma energy self-absorption of 241Am, the samples under investigation ranked black sand, Matrouh sand, Sidi Gaber sand, shells, Salloum sand, and clay. Empirical self-absorption correction formulas were also deduced. Chemical analyses such as pH, CaCO3, total dissolved solids, Ca2 + , Mg2 + , CO23 , HCO3 and total Fe2 + have been carried out for the sediments. The relationships between self absorption corrections and the other chemical parameters of the sediments were also examined. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Egyptian coastal sediments Self-absorption Direct transmission Gamma-ray Rashid black sand
1. Introduction The Egyptian Mediterranean coast contains a wide variety of sediments. Coastal sediments are mainly composed of two principal types: carbonate sands and quartz-dominant sands (Hilmy, 1951). The western region from Mersa Matrouh to Alexandria (Agami) is mainly composed of pure oolitic carbonate (Hilmy, 1951; El-Sabrouti et al., 1981; Anwar et al., 1984). The oolitic grains constitute an average of 78% and 89% of the bottom and beach sediments, respectively. Further westward, sand at Salloum showed an average oolite content in the nearshore of 58% (Anwar et al., 1981). Beach sand from Dekhiela (Alexandria) to Rashid primarily consists of quartz grains with common shell fragments (Hilmy, 1951; El-Wakeel and El-Sayed, 1978). The Nile River has been identified as the major source of quartz-rich sediments and heavy minerals on the Nile Delta beaches (Frihy, 1994). As a result of the current erosion affecting beaches along the Nile delta (UNDP/UNESCO, 1978; Lotfy and Frihy, 1993; Frihy, 1994), the
n Corresponding author. Current address: Department of Oceanography, Florida State University, Tallahassee, Fl 32306, USA (valid till 10 April 2010). Tel.: + 850 644 6705; fax: + 850 644 2581; Permanent address: Department of Oceanography, Coastal Research Institute, National Water Research Center, Alexandria, Egypt (valid after 11 April 2010). Tel.: + 203 4844614/5/6; Mobile: +2010 5296027; fax: + 203 4844614. E-mail address: [email protected] (A. El-Gamal).
0969-806X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2010.03.004
eroded shorelines are characterized by high-density minerals, whereas the accreted shorelines are characterized by low-density minerals (Frihy et al., 1997). Black sand deposits are extensive along the beaches of Rashid on the Egyptian Mediterranean coast (El-Fishawi and Badr, 1991; El-Askary and Badr, 1996; El-Gamal et al., 2004; Saleh et al., 2004). These black sands contain some important minerals such as zircon and monazite (El-Khatib and Abou El-Khier, 1988), both of which contain uranium and thorium in their chemical structures. High background radioactivity values at Rashid have been detected as a result of these black sands (El-Naggar, 1990; El-Gamal et al., 2004). Gamma-ray spectrometry is a well-established qualitative and quantitative method of measuring the radioactive components of environmental samples (Galloway, 1991a). However, an accurate determination of sedimentary gamma-emitting radionuclide activities in the sediments cannot be achieved without taking sample self-absorption into account (McMahon et al., 2004). When self-absorption corrections cannot be made for a particular sediment sample, it is routine for researchers to simply calibrate to standards with similar composition and density to the samples. Gamma-ray self-absorption is always prevalent, but is only a problem when differences in self-absorption exist between the sample and calibration standard (Cable et al., 1994). The extent of self-absorption in sediment samples depends on a number of factors including sample composition, density, and size, as well as gamma-ray energy (McMahon et al., 2004; Galloway, 1991a).
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The attenuation of gamma rays passing through a length x in a sample matrix of linear attenuation coefficient m can be expressed by the self-absorption equation (McMahon et al., 2004) I ¼ Io emx
ð1Þ
where Io is the emission intensity (neglecting absorption) from the source, I the intensity of the emergent rays from the source (or attenuated beam), m (cm 1) the linear attenuation coefficient for the sample matrix, and x (cm) the mean path length of a gamma-ray for particular energy. The value of m can be calculated from the product of the density and the mass attenuation coefficient (m/r), which can be obtained through knowledge of the sample’s elemental composition (Miller, 2008). Transmission (T) can be calculated by dividing the attenuated radiation by the unattenuated radiation and can be expressed as (Miller, 2008) T¼
I ¼ emx Io
ð2Þ
For most environmental samples, the values of m and x could be estimated if not directly known. One method of determining the average value for mx for a specific sample type and geometry uses the transmission method proposed by Cutshall et al. (1983). This method allows the photopeak efficiency determined using a spiked aqueous calibration standard to be corrected for the degree of absorption of the ‘real’ sample (Miller, 2008). A more convenient method is to prepare a series of gamma-absorption curves. This approach involves making a series of direct transmission measurements of gamma self-absorption in similar samples of varying densities as proposed by different authors such as Galloway (1991a) and McMahon et al. (2004). The self-absorption correction factor is determined by comparing the transmission rates of point sources placed on top of the sample (I) and on an empty sample container (Io). The correction factor (F) for a gamma-ray of known energy (E) can be estimated using transmission (T) from Eq. (2) by FðEÞ ¼
ðT1Þ lnðTÞ
Fig. 1. Map of Egypt.
sediment samples were collected from the upper 10 cm using a plastic cup and then placed in plastic bags. Sediments were dried, crushed, and homogenized prior to analysis. The dried sediment samples were transferred to polyethylene containers of similar diameter with depths of 1.2, 2.5 and 5 cm. The container diameters are less than the diameter of the detector.
2.2. Gamma spectrometry system ð3Þ
This procedure is repeated for the standard medium of the detector calibration standards. By comparing the correction factor of the sample (F(E)sample) with the correction factor of the standard medium (F(E)standard), the correction factor for the efficiency at a particular energy can be estimated. This method only represents an approximation as the equations hold true only for a collimated beam of gamma-rays, which is not the case with the close geometries employed for most low-level gamma analyses (McMahon et al., 2004). The aim of this work is to study the radiological responses of different types of Egyptian Mediterranean coastal sediments. In particular, the self-absorption correction will be evaluated to validate the measurement of these sediments via gamma-ray spectrometry. A secondary goal of this work is to study the relationship between the self-absorption correction values and the energy of the radioactive standard, sediment weight, sediment thickness, and differences in sediment composition.
2. Materials and methods 2.1. Sample collection and preparation Samples were collected from the Egyptian Mediterranean coastal area (Fig. 1) to represent the most common types of sediments throughout the coast. The sampling sites and their geographical coordinates are listed in Table 1. The coastal
The gamma-ray measurements were performed using a high resolution, low background PC multichannel spectrometer, with a HPGe Aptec detector. The HPGe detector (model CS 20-A 31 CL) is a closed-end coaxial type with a sensitive volume of 108 cm3 and a diameter of 5.4 cm encased in a lead shielding of 10 cm thickness. The relative efficiency is 24.5% for 1332.5 keV of 60Co relative to 3 3 Inch NaI, with a 25 cm source-detector distance. Recorded spectra were processed on PCMCA/Super PC software.
2.3. Direct transmission method Sealed 241Am, 137Cs, and 60Co were used as point sources of gamma radiation. The same sources were used by McMahon et al. (2004) to determine gamma self-absorption corrections of environmental samples. These standard sources were capsulated in 1 mm plastic (density 1.23 g/cm3) by Amersham International Plc. To calculate self-absorption correction factors using the direct transmission method (Cutshall et al., 1983), a point source was placed on top of each sample container and counted until adequate counting statistics were achieved for the photopeaks of interest. The source was then placed in the same position on an identical but empty container and the measured count rates for both were calculated. Using Eqs. (1–3), self-absorption correction factors were obtained for each photopeak energy and sample type (McMahon et al., 2004).
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Table 1 Main features and geographical information of sediment samples. Sample Code
Sampling Site
Geographical Location
Main Features
Representing Area
A B C
311 33\ 40\\ N 251 09\ 44\\ E 311 21\ 38\\ N 271 15\ 31\\ E 311 15\ 39\\ N 291 57\ 20\\ E
Yellow sand White sand, carbonate oolite Coarse sand
El-Salloum From Mersa Matrouh to Agami (Alexandria) Alexandria to Rashid and Sinai coast
D
El-Salloum Matrouh Sidi Gaber (Alexandria) Rashid
311 28\ 12\\ N 301 21\ 45\\ E
Coastal sediments affecting from Nile River sedimentation
E F
Rashid El-Gamil (Port Said)
311 28\ 10\\ N 301 21\ 43\\ E 311 18\ 40\\ N 321 10\ 50\\ E
Black sand, sandy size heavy minerals as monazite and zirconium, dense quartz grains Clay Broken shells, calcium carbonate
Nile River Estuaries From El-Gamil to Sinai coast
80 Rashid black sand
Alexandria sand
Weight %
60
40
20
0 80 Matrouh carbonate white sand
Salloum yellow sand
Weight %
60 40 20 0 -1
0
1
2 3 4 Grain size (phi)
5
6 -1
0
1
2 3 4 Grain size (phi)
5
6
Fig. 2. Grain size histograms of four different types of the Egyptian Mediterranean coastal sediments.
2.4. Grain size analysis Grain size analysis was carried out using a W.S. Tyler Shaker, Model RX-29-10, and Type Rotap according to the procedure described by Lindholm (1987). The sieves used were USA Standard Testing Sieve (Stainless steel sieves) A.S.T.M.E-11 specifications, Tyler Equivalent, made by W.S. Tyler. The meshes are 4, 2, 1, 0.5, 0.25, 0.125, and 0.063 mm. These correspond to the sedimentological phi (F) scale (which is the negative logarithm with base 2 of the particle diameter in millimeters) as classes 2, 1, 0, 1, 2, 3 and 4 F, respectively.
with the same geometry and the same measurement conditions as for the samples. The uncertainty in the calibration of the photopeak area was within 72%. 2.6. Sediments chemical composition CaCO3%, pH, total dissolved solids (TDS), Ca2 + , Mg2 + , CO23 , HCO3 and total Fe2 + were measured in the different types of sediment. pH was measured in 1:2.5 soil:water suspension. TDS, Ca2 + , Mg2 + , CO23 and HCO3 were measured in a 1:5 extract as expressed as mg/kg. Fe was measured in the digested sample as total concentration in mg/kg.
2.5. Quality control Three types of calibrations were carried out for the gamma spectrometer, including energy, resolution, and efficiency calibrations. Efficiency calibration was performed using Amersham 152Eu standard sealed source. Precision and accuracy were evaluated as published earlier (El-Gamal et al., 2007) using parallel measurements of IAEA intercomparison sediment samples IAEA-300 and IAEA-315 (IAEA, 1996). In order to determine background activities due to natural radionuclides in the environment, an empty polyethylene container was analyzed
3. Results and discussion 3.1. Coastal sediment characteristics 3.1.1. Grain size classification Grain size analysis is an important aspect for its fundamental descriptive measure of sediments (Lindholm, 1987). It is commonly related to other properties which have major economic implications. Fig. 2 shows the grain size histograms of the
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Table 2 Chemical composition of the sediment samples under investigation. pH
A B C D E F
8.68 8.65 8.27 8.52 8.65 8.33
CaCO3%
TDS (mg/kg)
Ca2 + (mg/kg)
Mg2 + (mg/kg)
CO23 (mg/kg)
HCO3 (mg/kg)
Total Fe2 + (mg/kg)
13.00 12.88 4.16 1.30 1.06 94.88
2944 2720 2650 2624 1857 3741
240 168 152 160 143 482
24 28.8 18 21.6 22 120
24 18 12 15 14 32
122 91.5 73.20 76.25 88 137
58.7 66.0 122 2723 74.8 9.2
100
100
80
80
Self absorption %
Self absorption %
Sample ID
60 40 20
Matrouh
Salloum
B Sand
Clay Shells
S Gaber
Salloum
B Sand
Clay
S Gaber
60 40 20 0
0 0
1
2 3 Thickness (cm)
4
0
5
Fig. 3. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 241Am (59.54 keV) with 2% uncertainties.
1
2 3 Thickness (cm)
100
80
Salloum Clay S Gaber
Self absorption %
Matrouh B Sand Shells
60 40 20
4
5
Fig. 5. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 60Co (1173.24 keV) with 2% uncertainties.
100 Self absorption %
Matrouh
Matrouh B Sand S Gaber
80
Salloum Clay
60 40 20 0
0 0
1
2 3 Thickness (cm)
4
5
0
1
2 3 Thickness (cm)
4
5
Fig. 4. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 137Cs (661.62 keV) with 2% uncertainties.
Fig. 6. Gamma self-absorbed fraction percentage of 1.2, 2.5 and 5 cm thicknesses of different environmental coastal sediments collected from the Egyptian Mediterranean coast against 60Co (1332.50 keV) with 2% uncertainties.
four different types of sands under investigation. The grain size distribution was drawn based on phi (F) scale. Grain size analysis of El-Salloum yellow sand documents discrete size groups extending from gravel to fine sand. This sand is characterized as bimodal sediments predominantly in the fine sand size class. Alexandria, Matrouh, and Rashid sands are all characterized as unimodal sediments in the medium sand class size.
Matrouh sediments) have relatively higher amounts of CaCO3, TDS, Ca2 + , Mg2 + , CO23 and HCO3 than the other sands. Sima and Dovlete (1997) reported that the sample matrix can greatly affect the measurement efficiency for samples.
3.1.2. Sediment chemical composition Analyses of pH, CaCO3, TDS, Ca2 + , Mg2 + , CO23 , HCO3 and total 2+ were carried out for all sediment samples as shown in Fe Table 2. Black sand was enriched in total Fe2 + (2723 mg/kg) relative to the other sediment samples. The broken shells are dominated by calcium carbonate (94.88%) and have relatively high amounts of TDS, Ca2 + , Mg2 + , CO23 and HCO3 and relatively low total Fe2 + concentrations. The western sands (El-Salloum and
3.2. Self-absorption determination 3.2.1. Determination of the fraction of self-absorption Figs. 3–6 show gamma self-absorption percentage of the Egyptian Mediterranean coastal sediments under investigation based on the different radioactive point sources of 241Am, 137Cs and 60Co. 241Am emits low energy gamma radiation (59.54 keV). Fig. 3 shows the different responses of each sediment type and thickness against the low energy of 241Am. The intensity of gamma photons decreases with increasing sediment thickness. Significant self-absorption of low energy gamma-rays was
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Attenuation coefficient
1.00
0.80
Matrouh
Salloum
B Sand
Clay
Shells
S Gaber
Table 3 Self-absorption correction factor values F(E) for 1.2, 2.5 and 5 cm thicknesses of different types of the Egyptian Mediterranean sediments against energies of 241Am, 137Cs and 60Co. Energy (keV)
0.60
241
Am 59.54
0.40 137
Cs 661.62
0.20 0
200
400
600 800 Energy (keV)
1000
1200
835
1400
Fig. 7. Gamma attenuation coefficients of different sediment types collected from the Egyptian Mediterranean coast against different energies of 241Am (59.54 keV), 137 Cs (661.62 keV), and 60Co (1173.24 & 1332.50 keV).
detected, consistent with the finding of McMahon et al. (2004). Black sand appears to be a perfect absorber for low energy gamma radiation, absorbing 100% of the gamma energy emitted from 241 Am through the 5 cm thickness. These results are also in agreement with the findings of Cable et al. (1994) that gamma-ray attenuation is greatest for low energy radionuclides. In decreasing order of gamma-ray absorption through 5 cm sediments at 59.54 keV gamma energy were black sand, Matrouh sand, Sidi Gaber sand, shells, Salloum sand, and clay with self-absorption efficiencies of 100%, 95.4%, 91%, 90.3%, 85%, and 80.3%, respectively. Fig. 4 shows the gamma-ray absorption responses against the different types of sediments resulting from exposure of higher gamma energy at 661.62 keV of 137Cs. This figure shows overall lower self-absorption efficiency than for the case of lower energy gamma radiation of 241Am. The highest self-absorption efficiency was the black sand which absorbed about 56.5% of 137Cs gamma emission while the lowest one was clay at 38.5% absorption. The sediment samples ranked in the order of decreasing selfabsorption efficiency through 5 cm thickness by black sand, Matrouh sand, shells, Sidi Gaber sand, Salloum sand, and clay, a similar order as with the 241Am source. Figs. 5 and 6 show the self-absorption responses of the different types of sediments to the highest tested gamma point source of 60Co (1173.24 and 1332.5 keV). Black sand and Matrouh sand were identified as the highest absorbing sediments at both gamma energies of 60Co. Sidi Gaber sand, shells, Salloum sand and clay yielded the lowest self-absorption efficiencies of 41.1%, 36.5% and 31.6% at 1173.24 keV and 35.3%, 29.3% and 24.3% at 1332.5 keV, respectively. An inverse relationship is observed between the percentage of absorbed gamma-rays and the corresponding energy of radiation. The higher absorption is corresponds with lower gamma energy, while low sediment self-absorption correlated with higher gamma energy. Attenuation coefficients were calculated from the self-absorption variation of different sediment thicknesses for each gamma source. Variations were observed among the different sediment samples with different gamma energy. The attenuation coefficient varies from a maximum attenuation coefficient of 1 for 5 cm thickness in black sand to 0.35 for clays. Fig. 7 shows the wide variation in the attenuation coefficients for the different types of sediments at the low gamma energy. Alternatively, a narrow variation was observed among the lower self-absorption coefficient values from the relatively high gamma energies. Self-absorption correction factors were calculated according to Eq. (3) and the values are listed in Table 3 for different thicknesses of sediments (1.2, 2.5 and 5 cm) and for each point source gamma
60
Co 1173.24
60
Co 1332.50
Thickness (cm)
Matrouh Salloum
1.2 2.5 5 1.2 2.5 5 1.2 2.5 5 1.2 2.5 5
0.678 0.501 0.310 0.910 0.838 0.699 0.911 0.851 0.747 0.923 0.871 0.789
0.774 0.667 0.448 0.934 0.858 0.763 0.916 0.874 0.807 0.927 0.885 0.854
B Sand
Clay
Shells
S Gaber
0.497 0.235 0.084 0.897 0.802 0.679 0.913 0.848 0.747 0.918 0.857 0.773
0.805 0.687 0.494 0.944 0.878 0.792 0.854 0.889 0.832 0.932 0.903 0.873
0.690 0.539 0.388 0.918 0.851 0.742 0.903 0.890 0.804 0.905 0.895 0.845
0.715 0.591 0.379 0.924 0.844 0.748 0.923 0.858 0.777 0.932 0.873 0.811
radiation energy of 59.54, 661.62, 1173.78 and 1332.51 keV. Using different sediment thicknesses in this calculation was recommended by Galloway (1991a, b). A relatively higher selfabsorption correction was observed for the 1.2 cm thickness than the other thicknesses. The self-absorption values for different sediment thicknesses from 241Am were relatively lower than the other gamma energies associated with the other point sources, consistent with the findings of McMahon et al. (2004). The variation of self-absorption corrections across a wide range of sediment compositions was first reported by Cable et al. (1994). The dependence of self-absorption corrections on the weight of the sediment sample was also studied for these Egyptian Mediterranean coastal sediments (Fig. 8). Based on the natural logarithmic relationship between the absorption correction of specific gamma energy and sediment weight, mathematical models were formulated. These models can be used to predict the self-absorption correction in environmental samples. The models show the regression line of 241Am with the wide variation of self-absorption corrections ranging from 0.08 to 0.81. In contrast, the minimum self-absorption correction for the three energies of 137Cs and 60Co was 0.68 and the maximum was 0.94. This indicates that the self-absorption correction is vital for low gamma energy counting. The relationship between selfabsorption correction and sediment density is supported by Cable et al. (1994). 3.3. Statistical analyses Statistical analyses have been carried out to test the correlation and significant differences of the sediments under investigation and to classify them into groups according to their determined character. Correlation analysis, multivariate analysis of variance, and cluster analysis have been executed using the international STATGRAPHICS Plus for windows 4.0 software. 3.3.1. Correlation analysis Table 4 shows the correlation coefficients among the chemical characters of the sediments under investigation and the selfabsorption corrections of sediment thicknesses against 241Am, 137 Cs and 60Co point gamma sources. High negative correlations ( 4 0.85) were detected between total Fe2 + with self-absorption corrections through 1.2, 2.5 and 5 cm thicknesses of sediments against exposure to the low energy emitter 241Am with correlation coefficients of 0.89, 0.90 and 0.90, respectively. The correlation coefficient values were diminished to be 0.71, 0.84 and 0.68 between total Fe2 + with self-absorption
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1 y = -0.0781Ln(x) + 1.2834
Correction factor F(E)
0.8 y = -0.0815Ln(x) + 1.2918 y = -0.1554Ln(x) + 1.6781
0.6 241-Am 137-Cs 60-Co 1 60-Co 2 Log. (137-Cs) Log. (60-Co 1) Log. (60-Co 2) Log. (241-Am)
0.4
0.2
y = -0.3539Ln(x) + 2.4753
0 0
100
200
300 400 Weight (g)
500
600
700
Fig. 8. Gamma ray self-absorption correction factors against weight of different types of the Egyptian Mediterranean coastal sediments.
Dendrogram
Table 4 Correlation coefficients among the chemical characters of the sediments under investigation and the self-absorption corrections of 1.2, 2.5 and 5 cm sediment thicknesses against 241Am, 137Cs and 60Co gamma sources.
0.89 0.90 0.90 0.71 0.84 0.68 0.20 0.56 0.55 0.19 0.69 0.64
0.32 0.34 0.41 0.23 0.38 0.28 0.01 0.65 0.46 0.62 0.56 0.54
CO3
Mg
Ca
TDS
CaCO3%
pH
30 0.21 0.15 0.12 0.23 0.20 0.08 0.37 0.00 0.11 0.33 0.10 0.17
corrections against exposure to 137Cs (661.62 keV) and more as 0.2, 0.56 and 0.55 against 60Co (1173.24 keV) through 1.2, 2.5 and 5 cm thicknesses of sediments, respectively. This could indicate that there is inverse relationship between the tested energy for self-absorption correction factors with the total Fe2 + contents of the sediments. Also, high negative correlations were detected between Mg and CaCO3% with a self-absorption correction through 1.2 cm sediments against the exposure of 1332.50 keV from 60Co. Alternatively, high positive correlations were detected between the values of chemical characters of the sediments such as CaCO3%, TDS, Ca, Mg, CO3 and HCO3. 3.3.2. Multifactor analysis of variance Multifactor analysis of variance constructs various tests to determine which factors have a statistically significant effect on the sediments data, taking into consideration their type and character variations. It also tests for significant interactions among the factors. The F-tests in the analysis of variance test identify the significant factors of correlation. Analysis of variance separates the variability of sediment data into contributions from various factors (sediment type and character). Since Type III sum of squares has been chosen, the contribution of each factor is measured after removing the effects of all other factors. The P-value tests the statistical significance
20 10 0 Shells
0.03 0.05 0.16 0.08 0.14 0.04 0.07 0.53 0.23 0.86 0.41 0.26
B Sand
0.20 0.15 0.09 0.34 0.19 0.25 0.57 0.13 0.12 0.82 0.01 0.07
Clay
0.05 0.07 0.18 0.04 0.15 0.08 0.11 0.56 0.29 0.84 0.42 0.32
Salloum
0.01 0.00 0.12 0.10 0.12 0.03 0.00 0.54 0.23 0.87 0.42 0.25
S Gaber
0.11 0.13 0.21 0.00 0.18 0.07 0.13 0.53 0.28 0.78 0.40 0.35
Matrouh
Am_1.2 Am_2.5 241 Am_5 137 Cs_1.2 137 Cs_2.5 137 Cs_5 60 Co1_1.2 60 Co1_2.5 60 Co1_5 60 Co2_1.2 60 Co2_2.5 60 Co2_5
241
HCO3
40
Distance
241
Total Fe
Nearest Neighbor Method,Squared Euclidean
Fig. 9. Dendrogram illustrates groups of the nearest neighbor areas among the 6 different types of sediments collected from the Egyptian Mediterranean coast according to their characters.
of each of the factors. Since one P-value is less than 0.05, its factor has a statistically significant effect on data at the 95.0% confidence level. In general, there are no statistically significant differences among the sediment types while statistically significant differences were detected among the sediments according to their character variations. 3.3.3. Fisher’s least significant difference (LSD) analysis In order to determine which of the determined characters are significantly different from which others, a Fisher’s least significant difference (LSD) procedure was applied. This analysis indicated that TDS is significantly different from all other characteristics at the 95.0% confidence level. Also, total Fe2 + has no significant differences with Ca content but is significantly different from the other parameters. Ca is recognized as a common factor with no significant differences between it and the other parameters. 3.3.4. Cluster analysis Cluster analyses were carried out from two points of view. The first was carried out among the different types of sediments under investigation. This procedure was created 1 cluster from the
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Dendrogram Nearest Neighbor Method,Squared Euclidean 10
Distance
8 6 4 2
CaCO3% Mg Ca CO3 HCO3 TDS Co1_2.5 Co1_5 Co2_5 Co2_2.5 Cs_1.2 Cs_5 Cs_2.5 Am_1.2 Am_2.5 Am_5 Co1_1.2 Co2_1.2 pH Total Fe
0
Fig. 10. Dendrogram illustrates groups of the nearest neighbor areas among all the different sediment chemical characteristics and their self-absorption corrections of the 6 types of sediments under investigation.
6 observations supplied from the characters of the 6 different types of sediments collected from the Egyptian Mediterranean coast. The two groups closest together were combined as shown in Fig. 9. This analysis revealed that Matrouh and Sidi Gaber sands are more similar to each other in their characteristics than the other sediments. Shells are recognized as the most dissimilar sediment type than the others. The other test was carried out among all the different sediment chemical characteristics and their self-absorption corrections of the 6 types of sediments under investigation. The analysis recognized Ca, Mg, and CaCO3% were combined into similar groups and CO3 and HCO3 were also grouped in one subgroup due to their similar characteristics as shown in Fig. 10. From the selfabsorption correction values of the sediment types from each point source of 241Am, 137Cs and 60Co, this analysis grouped all together except thickness 1.2 cm against 60Co (both energies). The analysis classifies total Fe2 + as the most deviating character, with pH as the next most variant character. This could indicate that the matrix effect is important in the analysis of environmental samples by gamma spectrometry techniques. Neglecting these effects may result in high uncertainties, especially at low energies, as confirmed by Sima and Dovlete (1997).
4. Conclusion Direct transmission served as an excellent method to evaluate gamma self-absorption corrections of environmental samples. The investigation of gamma self-absorption of sediments revealed a diversity of responses at different emitted energies. A relatively high self-absorption was observed at the low energy of 241Am (59.54 keV). Rashid black sand was identified as a perfect absorber of gamma radiation especially at a thickness of 5 cm at low gamma energy. At 5 cm sediment thickness and low gamma energy, the decreasing order of sediment self-absorption was black sand, Matrouh sand, Sidi Gaber sand, shells, Salloum sand, and clay. The ability of the sediments to absorb gamma radiation decreased with increasing gamma energy value. The lowest absorption was observed with the higher energy of 1332.5 keV (60Co). Black sand was found to absorb through 5 cm thickness 100%, 56.5%, 45.9% and 41.7% the gamma energies of 59.54, 661.62, 1173.24 and 1332.5 keV, respectively.
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Gamma self-attenuation coefficients of the investigated sediments were calculated. An inverse relationship was observed for attenuation coefficient values with energy. At lower energies, attenuation coefficient variations among sediments were higher than the narrow variations observed at higher energies. Evaluating the dependency of self-absorption correction factors against the weight of the sediment samples revealed that the absorption correction is vital for lower gamma energy counting. The regression lines and models found that the low gamma energy emission associated with 241Am exhibited a wide range of self-absorption correction values (0.08–0.81) as compared to the other tested energies (0.68–0.94). High negative correlations were detected between various coastal sediments self-absorption corrections and their measured chemical characteristics. Specifically, these correlations were found between total Fe2 + with self-absorption corrections of 1.2, 2.5 and 5 cm sediment thicknesses against exposure to the low energy emitter 241Am and between Mg and CaCO3% with selfabsorption correction of 1.2 cm sediments against the exposure of 1332.50 keV from 60Co. Multivariate analysis of variance indicated that there are no statistically significant differences among all sediment types while statistically significant differences were observed within the sediments according to their characteristic variations. The source of this significant difference was mainly from the values of TDS and total Fe2 + . Cluster analysis classifies shells as the most different type of sediments followed by black sand and the others.
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