Radiological assessment of Abu-Tartur phosphate, Western Desert Egypt

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Radiological assessment of Abu-Tartur phosphate, Western Desert Egypt Article in Radiation Protection Dosimetry · March 2008 DOI: 10.1093/rpd/ncm502 · Source: PubMed

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Radiation Protection Dosimetry (2008), Vol. 130, No. 2, pp. 228–235 Advance Access publication 5 February 2008

doi:10.1093/rpd/ncm502

RADIOLOGICAL ASSESSMENT OF ABU-TARTUR PHOSPHATE, WESTERN DESERT EGYPT M. A. M. Uosif * and A. El-Taher Department of Physics, Faculty of Science, Al-Azhar University, Assuit Branch, Egypt

Received September 15 2007, revised December 16 2007, accepted December 26 2007 The contents of natural radionuclides (226Ra, 232Th and 40K) were measured in sedimentary phosphate rock samples (Abu-Tartur phosphate, Western Desert Egypt) by using gamma spectrometry (NaI (Tl) 3”3 3”). Phosphate and environmental samples were collected from Abu-Tartur phosphate mine and the surrounding region. The results are discussed and compared with the levels in phosphate rocks from different countries. The activities of 226Ra, 232Th series and 40K are between (14.9 + 0.8 and 302.4 + 15.2), (2.6 + 1.0 and 154.9 + 7.8) and (10.0 + 0.5 and 368.4 + 18.4) Bq kg21, respectively. The Abu-Tartur phosphate deposit was found to have lower activity than many others exploited phosphate sedimentary deposits, with its average total annual dose being only 114.6 mSv y21. This value is about 11.46% of the 1.0 mSv y21 recommended by the International Commission on Radiological Protection (ICRP-60, 1990) as the maximum annual dose to members of the public.

INTRODUCTION The world is naturally radioactive, and around 90% of human radiation exposure arises from natural sources such as cosmic radiation, exposure to radon gas and terrestrial radiation. There are many studies that have been undertaken on exposure to many of the forms of natural radiation. The earth contains numerous radioactive elements; their origin, for part of them, dates back to the formation of our world, while others are continuously produced through nuclear reactions in the universe. Among the former elements, the most abundant are potassium-40 and the radioisotopes of the natural series of uranium, actinium and thorium including the parent nuclei 235 U, 238U and 232Th and the decay products from the successive alpha or beta decays, whereas the most abundant of the cosmogenic origin nuclei are 14 C, 10Be and 26Al(1). Uranium and its decay products are found in phosphate rocks of sedimentary origin(2). Now, these phosphates are largely used for the production of phosphoric acid and fertilizers. Their radioactivities result in health problems from radiation at the level of the industrial processes for the preparation of fertilizers as well as for the fertilizers themselves at the origin of radioactivity dispersion in the geo- and biospheres(3). In this paper, an analytical method is presented using gamma spectrometry to measure uranium, thorium and potassium series for phosphate and environmental samples collected from Abu-Tartor phosphate plateau. Phosphate rock is the starting

*Corresponding author: [email protected]

raw material for all phosphate products. A typical concentration of 238U in sedimentary phosphate deposits is 1500 Bq kg21(4). Investigators have reported a wide variation in the concentrations of uranium and radium in phosphate rocks from various parts of the world. For uranium, a range from 3 to 400 ppm, corresponding to 37 –4900 Bq 238 U kg21 (1 ppm U ¼ 12.23 Bq 238U kg21) and for 226 Ra, a range from 100 to 10 000 Bq kg21 is reported(5). It is within this context that the present study has been undertaken aimed at the determination of the radioactivities of naturally occurring nuclides in phosphate. MATERIALS AND METHODS The study area The study area is deep in the desert, 50 km west of El-Kharga city, capital of New Valley Govemorate, Egypt. There lies the Abu-Tartur plateau. The plateau is situated in the Southwestern sector of Egypt in the Western desert at 300 km west of Assiut city at River Nile, 650 km south of Cairo city and 700 km west of Safaga port at the Red Sea coast. Abu-Tartur plateau forms a part of the rugged stretch that separates Dakhla and Kharga Oases in the Western Desert of Egypt (6). The plateau has an area of about 1200 km2. The geological section in this area includes (from bottom to top) the Nubian formation, Duwi ( phosphate) formation, Dakhla formation and Kurkur formation(7). The area of study lies in the central Western Desert of Egypt, located between latitudes 258150 – 58450 and longitudes 298300 –308100 , shown in Figure 1. Generally, the area is a semi-circular

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RADIOLOGICAL ASSESSMENT OF ABU-TARTUR PHOSPHATE

The samples were crushed, homogenized and sieved through a 200 mm, which is the optimum size enriched in heavy minerals. Weighed samples were placed in polyethylene beaker, of 350-cm3 volumes each. The beakers were completely sealed for 4 weeks to reach secular equilibrium where the rate of decay of the progency becomes equal to that of the parent (radium and thorium)(8,9). This step is necessary to ensure that radon gas conﬁned within the volume and the progency will also remain in the sample. Instrumentation and calibration

Figure 1. Location of different samples in Abu-Tartur area.

plateau protruding out of the main scarp bounding the Kharga– Dakhla depressions. Its long axis is about 65 kms with an ENE direction, whereas its short axis is about 33 kms with NNW direction. The highest point on the plateau surface attains an elevation of 540 m a.s.l, whereas the lowest point at the foot of the scarp is 250 m a.s.l. Sampling and sample preparation The geologic sequence in the study area(7) is as follows from top to base: † † † †

Kurkur formation (Paleocene): organic reefal limestone. Dakhla formation (Maastrichtian): 200– 135 m of alternation of shale and mudstone. Duwi formation (Maastrichtian): 20 –30 m of shales with two phosphatic horizons, lower and upper horizons separated with shale horizon. Nubia formation (Pre-Maastrichtian): 100– 300 m of variegated shales and cross-bedded sandstones.

Fifteen samples were collected from three locations in the phosphatic horizons (Duwi formation). Five samples were collected from the subsurface phosphate horizon. These samples are 6, 7, 13, 14 and 15. Five samples were collected from the exposed lower phosphate horizon. These samples are 1, 2, 3, 8 and 10. Three samples were collected from the exposed upper phosphate horizon. These samples are 9, 11 and 12. Finally, two samples were collected from production line, and these samples are 4 and 5. Each sample was dried in an oven at about 1108C to ensure that moisture was completely removed.

Activity measurements were performed by gamma ray spectrometer, employing a scintillation detector 3” 3”. It is hermetically sealed assembly, which includes a NaI(Tl) crystal, coupled to PC-MCA Canberra Accuspes. To reduce gamma ray background, a cylindrical lead shield (100 mm thick) with a ﬁxed bottom and movable cover shielded the detector. The lead shield contained an inner concentric cylinder of copper (0.3 mm thick) in order to absorb X rays generated in the lead. In order to determine the background distribution in the environment around the detector, an empty sealed beaker was counted in the same manner and in the same geometry as the samples. The measurement time of activity or background was 43 200 s. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. A dedicated software program Genie 2000(10) from Canberra has carried out the online analysis of each measured gamma ray spectrum. The efﬁciency calibration curve was made using different energy peaks covering the range up to 2000 keV. Measurements were performed with calibrated source samples, which contain a known activity of one or more gamma ray emitters of the radionuclides 60Co (1173.2 and 1332.5 keV), 133 Ba (356.1 keV), 137Cs (661.9 keV) and 226Ra (1764.49 keV). With certiﬁed accuracies of 2% supplied by PTB Braunschweig, Germany, Equation (1) is used for calculating the absolute efﬁciency Eff ¼

100 Np Ig TOC ABOC

ð1Þ

where Np the net peak area at Eg, Ig the intensity of emitted gamma ray, TOC the time of counting and ABOC the activity of the standard source at beginning of counting (BOC). ABOC was calculated by equation (2) ABOC ¼ ADOC expðl ðBOC DOCÞÞ

ð2Þ

where ADOC is the activity of the standard source at date of calibration (DOC), and l is the decay constant.

229

M. A. M. UOSIF AND A. EL-TAHER

Daily efﬁciency and energy calibrations for each sample measurement were carried out to maintain the quality of the measurements. The absolute efﬁciency of the detector was calculated at the speciﬁc energy of the standard sources for the same geometry of the samples. But, the gamma spectra of the samples have different gamma energies. So, some ﬁtting function is needed to calculate the absolute efﬁciency for any considered gamma energy. A function(11) is used, for this purpose, for calculating the absolute efﬁciency at any gamma energy of interest in the energy range below 2000 keV, which is in the following form:

h ¼ a b expðc Egd Þ

ð3Þ

where Eg represents energy in MeV, where a, b, c and d are coefﬁcient data. By equation (3), the absolute efﬁciency, h, can be determined at any speciﬁc energy Eg, if the energies and the coefﬁcient data are known. From the experimental efﬁciency curves, the coefﬁcient data were determined, by using the curve-ﬁtting program Curve Expert 1.34(12). Uncertainty of efﬁciency The combined standard uncertainty of absolute efﬁciency u(EFF) consists of u(Np), u(Ig), u(TOC) and u(ABOC). So, uðEFFÞ 2 uðNp Þ 2 uðIg Þ 2 uðTOCÞ 2 ¼ þ þ EFF Np Ig TOC 2 uðABOC Þ þ ABOC ð4Þ Because u(TOC) ,, TOC, u(TOC) was neglected. The value of u(ABOC) was calculated by equation (5) uðABOC Þ 2 uðADOC Þ 2 ¼ þðBOC DOCÞ2 ABOC ADOC u2 ðlÞ ð5Þ (10)

u(Np) was obtained from the code Genie 2000 , whereas u(l) and u(Ig) were taken from the compilation of Reus and Westmeier(13). The calibration standards used had a certiﬁed accuracy of 2%. By measurements for many times, it could be veriﬁed with a total uncertainty of the full-energy-peak efﬁciency of 5%. RESULTS AND DISCUSSION Calculations of count rates for each detected photopeak and radiological concentrations (activity per mass unit or speciﬁc activity) of detected

radionuclides depend on the establishment of secular equilibrium in the samples. The 232Th concentration was determined from the average concentrations of 212 Pb(238.6 keV) and 228Ac(911.1 keV) in the samples, and that of 226Ra was determined from the average concentrations of the 214Pb(351.9 keV) and 214 Bi(609.3 and 1764.5 keV) decay products(14). The activity concentration in Bq kg21(A) in the environmental samples was obtained as follows: A¼

Np ehm

ð6Þ

where Np is the (cps) sample –(cps) B.G, e the abundance of the gamma line in a radionuclide, h the measured efﬁciency for each gamma line observed for the same number of channels either for the sample or the calibration source and m the mass of the sample in kilograms. The uncertainty of activity u(A) was calculated by the following equation: s ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ uðNp Þ 2 uðhÞ 2 uðmÞ 2 þ þ ð7Þ uðAÞ ¼ A ðNp Þ ðhÞ ðmÞ From the equation, it can be found that, there are many sources of uncertainties of the activity and some may result in considerable uncertainties. The following sources of uncertainties were considered: Uncertainty of the determination of net peak areas The uncertainty of each single net-peak area is determined by the spectrum-evaluation code. It takes into account the Poisson uncertainties of the counts in the individual channels as well as the uncertainty of the background determination. Sometimes, a peak cannot be attributed unambiguously to a single nuclide. If it seems that the contributions of other nuclides to a peak are very small, no correction was applied. Due to this procedure, it is thought that a maximum inaccuracy of 2% was assumed due to contributions of other nuclides but it must be pointed out that in the average, this uncertainty should be smaller(15). By repeated measurements, it could be veriﬁed that the total uncertainty of the efﬁciency calibration was 5%. The activities of 226Ra, 232Th series and 40K in Bq kg21 in the area under investigation determined for each of the measured samples together with their total uncertainties are presented in Table 1. Radium and its decay products in phosphate deposits had activity concentrations ranging from 14.9–302.35 Bq kg21. The highest activity levels were measured in surface samples (lower bed), whereas the lowest values were met with upper bed samples. The obtained results, in Table 1, show that the values of the measured speciﬁc gamma ray activities

230

RADIOLOGICAL ASSESSMENT OF ABU-TARTUR PHOSPHATE Table 1. Activity concentrations of the radioelements (in Bq kg21) found in studied samples. Sample location Subsurface samples

Surface samples upper bed

Surface samples lower bed

Production line

Sample name 6 7 13 14 15 9 11 12 2 1 3 8 10 4 5

226

Ra A (Bq kg21)

32 + 3.2 44.3 + 4.1 122.6 + 6.5 169.4 + 8.9 163.1 + 8.5 43.9 + 4.2 14.9 + 0.8 40.4 + 2.7 135.6 + 7.4 302.4 + 15.2 234.2 + 12.3 112.4 + 6.1 82.8 + 4.6 149.2 + 8.0 39.0 + 3.6

232

Th A (Bq kg21)

3.0 + 0.7 3.6 + 0.9 111.7 + 5.6 84.8+ 4.3 88.9 + 4.5 2.6 + 1.0 5.70+ 0.3 51.7 + 4.6 74.7 + 3.9 154.9 + 7.8 106.8 + 5.4 71.2 + 3.6 73.4 + 3.7 77.4 + 3.9 3.44 + 0.2

40

K A (Bq kg21)

27.9 + 1.4 13.3 + 0.7 165.2 + 8.3 129.8 + 6.5 125.0 + 6.3 10.0 + 0.5 103.0 + 5.1 97.9+ 4. 9 114.1 + 5.7 368.4 + 18.4 154.5 + 7.7 174.6 + 8.7 154.8 + 7.7 125.1 + 6.3 12.3 + 0.6

Figure 2. Activity concentrations of the radioelements (in Bq kg21) found in studied samples.

(Bq kg21) in different samples as follows. For 226 Ra, the activity concentrations are ranged from (14.9 + 0.8) to (302.4 + 15.2) Bq kg21 for upper and lower bed samples, respectively, whereas the activity concentration values of 232Th are between (2.6 + 1.0) and (154.9 + 7.8) Bq kg21 for subsurface and lower bed samples, respectively. The 40 K activity concentrations ranged between (10.0 + 0.5) and (368.4 + 18.4) Bq kg21 for upper bed and lower bed samples, respectively. All measured results are in the range of typical concentrations cited in UNSCEAR, 1993. Figure 2 shows the results of Table 1 in graphical form, clearly indicating the high- and low-activity samples. Figure 3 shows the good correlation between thorium and radium in samples under investigation for the major

samples (correlation coefﬁcient ¼ 0.8196). Figure 4 also shows a good correlation between the concentrations of the two radioactive isotopes (40K, 232Th) (correlation coefﬁcient ¼ 0.7645). Derivation of the radiation hazard indices The distribution of 226Ra, 232Th and 40K in soil is not uniform. Uniformity with respect to exposure to radiation has been deﬁned in terms of radium equivalent activity (Raeq) in Bq kg21 to compare the speciﬁc activity of materials containing different amounts of 226Ra, 232Th and 40K. It is calculated through the following relation(15).

231

Raeq ¼ ARa þ 1:43ATh þ 0:077AK

ð8Þ

M. A. M. UOSIF AND A. EL-TAHER

Where ARa, ATh and AK are the activities of 226Ra, 232 Th and 40K, respectively, in Bq kg21. The radium equivalent activities (Raeq) have been calculated on the estimation that 370 Bq kg21 (10 pCi21) 226Ra, 259 Bq kg21 (7 pCi g21) 232Th or 4810 Bq kg21 (130 pCi g21) 40K produce the same gamma ray dose rate(16). The gamma radiation doses (in outdoor air Do, nGy h21) for the population living in the areas under investigation is due to radionuclides, which can be estimated by employing a half-inﬁnite source of a homogenous distribution and by considering only the contribution from the natural radionuclides in the samples. The convenient formula(15) is given as: Do ¼ 0:427CRa þ 0:662CTh þ 0:043CK ;

CRa CTh CK þ þ 370 259 4810

ð10Þ

Where CRa, CTh and CK are the concentration in (Bq Kg21) of radium, thorium and potassium.

Figure 3. The correlation between 226Ra and concentration in marble samples.

Annual effective dose rate ¼ DTF

ð11Þ

ð9Þ

While the external hazard index (Hex): is given by the following equation: Hex ¼

Finally, in order to estimate the annual effective doses, one has to take into account the conversion coefﬁcient from absorbed dose in air to effective dose and the indoor occupancy factor. In the recent (17) reports, a value of 0.7 Sv Gy21 was used for the conversion coefﬁcient from absorbed dose in air to effective dose received by adults, and 0.8 for the indoor occupancy factor, implying that 20% of time is spent outdoors, on average, around the world. The annual effective dose rate outdoors in units of mSv y21, is calculated by the following formula(18).

232

Th

where D is the calculated dose rate (in nGy h21), T the outdoor occupancy time (0.2 24 h 365.25 d 1753 h y21), and F is the conversion factor (0.7 1026 Sv Gy21). The experimental results of (Raqe, Do and Hex) are presented in Table 2. The radium equivalent activity (Raeq) values for subsurface samples ranged between (38.3– 299.7) Bq kg21, whereas those values for surface samples (upper bed), are lower than subsurface samples ranged between (30.2–121.2) Bq kg21. For the surface samples lower bed and production line the values are ranged between (198.5–549.7) Bq kg21 and (44.8–268.6) Bq kg21, respectively. The measured gamma radiation doses (in outdoor air Do) in nGy h21 received by the workers ranged between (16.9 –134), (14.5 –55.9), (90.6 – 247.5) and (19.5 –120.3) nGy h21 for subsurface, surface (upper bed), surface (lower bed) and production line samples, respectively. From those values, one can observe that the highest dose comes from surface (lower bed) samples, whereas the lowest one comes from (upper bed) samples. By looking in Figure 5, one can observe the highest and lowest values of

Figure 4. The correlation between 40K and 232Th concentration in marble samples.

232

RADIOLOGICAL ASSESSMENT OF ABU-TARTUR PHOSPHATE

equivalent radium (Bq kg21) and the dose rate (nGy h21) The environmental pathways(6) of natural radionuclides from phosphate rocks are given in Figure 6. The untreated ground rock phosphate has been used in many parts of the world as plant fertilizer(19) at a rate ranging from 300 to 600 kg ha21 (104 m2). Studies in some African countries show that the additional external radiation exposure for the public, when using ground rock phosphate from Tanzania, Sudan and Egypt (El-Mahamid and Wadi El-Mashash)(19,20) phosphate rock was negligible. Abu-Tartur has a lower radioactivity content than those other ground rock phosphates (Table 3). From the comparison of the results with the others in Table 3, it is clear that the average total annual dose from phosphate rocks Abu-Tartur (based on the samples of this study) is 114.6 mSv y21. This value is about 11.46% of the 1.0 mSv y21 recommended by the International Table 2. Equivalent radium (Bq kg21), the dose rate (nGy h21), external hazard indices Hex and annual effective dose rate of the studied samples. Sample location

Subsurface samples Surface Samples upper bed Surface samples lower bed Production line

Sample name

Raeq (Bq kg21)

Dose rate (nGy h21)

Hex

6 7 13 14 15 9 11 12 1 2 3 8 10 4 5

38.3 50.4 293.9 299.7 299.0 48.3 30.2 121.2 549.7 250.5 397.7 226.4 198.5 268.6 44.8

16.9 21.9 133.4 134.0 133.9 20.9 14.5 55.7 247.5 112.3 177.3 102.6 90.6 120.3 19.5

0.01 0.01 0.13 0.11 0.12 0.01 0.01 0.06 0.21 0.10 0.15 0.09 0.09 0.10 0.01

Annual efﬁciency dose (mSv y21) 20.7 26.9 163.7 164.4 164.3 25.6 17.8 68.3 303.7 137.8 217.6 125.9 111.2 147.6 23.9

Figure 5. Values of radium equivalent (Bq kg21) and the dose rate (nGy h21) different samples.

Commission on Radiological Protection as the maximum annual dose to members of the public. Basic approaches to radiation protection are consistent all over the world. The ICRP-60 (1990)(21) recommends that any exposure above the natural background radiation should be kept as low as reasonably achievable—ALARA—but below the individual dose limits, which for radiation workers averaged over 5 years is 100 mSv and for members of the general public is 1 mSv y21. These dose limits have been established on the prudent approach by assuming that there is no threshold dose below which there would be no effect. This means that any additional dose will cause a proportional increase in the chance of a health effect. In general, as shown in Table 3, the Egyptian phosphate rocks have a lower-activity content than do many phosphate rocks in other countries. The higher the activity content in phosphate rocks, the higher are the radiological impacts and the radiation doses through mining, processing, phosphate product manufacturing and using phosphate products or by-products. CONCLUSIONS Selected samples in sedimentary phosphate rock samples (Abu-Tartur phosphate, Western Desert Egypt) were analysed by natural activity measurements to detect the presence of radioactive elements. From the analysis of radiations from 226Ra, 232Th and 40K isotopes, the samples were found to contain U, Th and K in concentrations are between (14.9 + 0.8 and 302.4 + 15.2), (2.6 + 1.0 and 154.9 + 7.8) and (10.0 + 0.5 and 368.4 + 18.4) Bq kg21, respectively. The average annual dose rate from Abu-Tartur phosphate rocks is 114.6 mSv y21 for the samples under investigation. It is far below the worldwide-

Figure 6. Environmental pathways of natural radionuclides from phosphate rocks(6).

233

M. A. M. UOSIF AND A. EL-TAHER Table 3. Activity concentration of

Country Egypt (Abu-Tartur) Egypt (El-Mahamid) Egypt (W. El-Mashash) Egypt (El-Sibaiya) Egypt (El-Quseir) Egypt (Abu-Zaabal) Finland Pakistan (Hazara) Sudan (Uro) Sudan (Kurun) Tanzania (Arusha) USA (Western) USA (Florida) Morocco USSR (Kola) Jordan Tunisia Algeria

226

Ra

117.6 567 666 538 358 214 10 440 4131 393 5022 1000 1600 1600 30 1044 821 619

226

Ra,

232

Th

65 217.3 329.4 2.5 38 37 10 50 7.5 69 717 20 20 20 80 2 29 64

232

Th,

K (Bq kg21) and radiation hazard parameters in phosphate rocks from different countries.

40

40

K

126 217.3 329.4 N.F N.F 19 110 207 62.3 141.3 286 N.F N.F 10 40 8 32 22

Raeq (Bq kg21)

Annual dose (mSv y21)

Dosea (%)

References

219.5 921 1182 574 412 568 36 527.4 4147 414 6069 1029 1629.6 1629.4 147 1047 865 712

128 418 538 251 181 249 16 234 1806 182 2683 449 710 710 69 456 378 314

0.64 2.1 2.7 1.3 0.9 1.2 0.05 1.17 9 0.9 13.4 2.2 3.5 3.6 0.3 2.3 1.9 1.6

This work (19) (19) (20) (20) (22) (23) (23) (24) (24) (19) (25) (25) (25) (25) (26) (26) (26)

N.F denote to data not found in this reference. Calculated external gamma radiation dose received by the workers of the phosphate mines, the world allowed dose of 20 mSv y21(27) for workers. a

allowed dose of 1.0 m Svy21 (ICRP-60 1990) as the maximum annual dose to members of the public. 5.

ACKNOWLEDGEMENTS This work was carried out using the nuclear analytical facilities at Physics Department, Faculty of Sciences, Al-Azhar University, Assiut, Egypt. The authors wish to thank Prof. Dr Ahmed Oraby, Head of Geology Department, Faculty of Sciences, Al-Azhar University, Assiut, Egypt and the staff of the Geology Department for assistance collecting the samples and Dr M. El-Amin (NRIAG, Helwan, Egypt) for his assistance during this work.

6.

7. 8.

9.

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Radiological assessment of Abu-Tartur phosphate, Western Desert Egypt

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