|Year : 2017 | Volume
| Issue : 3 | Page : 110-115
Analysis and statistical treatment of 238U series isotopic ratios using gamma-ray spectrometry in phosphate samples
Kh A Allam1, Z Ahmed2, Samy El-Sharkawy3, Ahmed Salman3
1 Department of Nuclear Law, ian Nuclear and Radiological Regulatory Authority, Nasr City, Cairo, Egypt
2 Department of Safeguard, ian Nuclear and Radiological Regulatory Authority, Nasr City, Cairo, Egypt
3 Department of Physics, Faculty of Science, Sohag University, Sohag, Egypt
|Date of Submission||17-Nov-2017|
|Date of Decision||23-Nov-2017|
|Date of Acceptance||25-Nov-2017|
|Date of Web Publication||16-Feb-2018|
Kh A Allam
Department of Nuclear Law, Egyptian Nuclear and Radiological Regulatory Authority, 3 Ahmed El Zommer, Nassr City, Cairo
Source of Support: None, Conflict of Interest: None
The propose of this work is to identify the isotopic ratios and their variations for uranium-238 series isotopes in some collected phosphate samples and experimental examination of 186 keV gamma line separation method and also to study the effects on these values to investigate and interpret the distribution of radionuclides and check the secular equilibrium in uranium decay series. These results will contribute to a better understanding of the behavior of radionuclides during the development of the phosphate industry in the El-Sebaiya area. The isotopic activity ratios of 238U/235U, 238U/226Ra, 226Ra/214Bi, 226Ra/214Pb, and 214Pb/214Bi have been calculated, and the phosphate sample was analyzed using a hyper-pure germanium detector. The analysis of the spectra was done using GammaVision analysis software. The samples were collected from El-Sebaiya locality, Aswan, Egypt.
Keywords: Activity ratios, gamma-ray spectrometry, natural radionuclides, phosphate samples, U-238 and daughters
|How to cite this article:|
Allam KA, Ahmed Z, El-Sharkawy S, Salman A. Analysis and statistical treatment of 238U series isotopic ratios using gamma-ray spectrometry in phosphate samples. Radiat Prot Environ 2017;40:110-5
|How to cite this URL:|
Allam KA, Ahmed Z, El-Sharkawy S, Salman A. Analysis and statistical treatment of 238U series isotopic ratios using gamma-ray spectrometry in phosphate samples. Radiat Prot Environ [serial online] 2017 [cited 2018 May 27];40:110-5. Available from: http://www.rpe.org.in/text.asp?2017/40/3/110/225588
| Introduction|| |
Phosphate ore is a complex material, which contains calcium, phosphate, fluoride, carbonate, and other elements or groups bound together in crystal lattice. It is an important natural source for fertilizers and also widely used in chemical industries. In addition, it commonly contains relatively high concentrations of useful elements such as uranium, fluorine, potassium, and vanadium. Those elements are associated with organic-rich mudstones and potential hydrogen source rocks. The major form of phosphate ore is sedimentary phosphate which represented 85% of the worldwide production. It tends to have high concentrations of uranium isotopes ranged from 50 to 200 mg/kg, while the activity concentrations of 232 Th and 40 K are much lower than those of 238 U and comparable to those normally observed in soil. In general,238 U is found in radioactive equilibrium with its decay products such as 226 Ra. The degree of the secular equilibrium is expressed as the activity ratio (AR) that when the system is closed, it should be unity. Thus, the presence of the secular equilibrium provides no migration of radionuclides from or into the system during the equilibrium is established. The variations of ARs for 238 U series depend mainly on natural chemical or physical process and anthropogenic contamination.
Uranium has three natural isotopes such as 238 U (99.275% abundance),235 U (0.720% abundance), and 234 U (0.0054% abundance). The half-life of the parent in the U-decay series is long compared to the half-lives of the daughters, so the daughter radionuclides have an enough time to reach the same activity as the parent that is called a secular equilibrium.
In this work, the isotopic ARs of 238 U/235 U,238 U/226 Ra,226 Ra/214 Bi,226 Ra/214 Pb, and 214 Pb/214 Bi have been measured and calculated using gamma spectroscopy analysis in the phosphate samples. The measurements were done using hyper-pure germanium (HpGe) detectors and also the variation behavior of these values has been studied and investigated.
| Materials and Methods|| |
Nineteen representative samples of phosphate ore were collected from El-Sebaiya area, Aswan, Egypt. The samples were thoroughly crushed and pulverized to powder. The powder was sieved through 0.2 mm mesh, which is the optimum size enriched in heavy minerals. Containers with cylindrical shape have been used to contain the samples. Container material was polypropylene with density 0.946 g/cm 3 and the container inner diameter 6.5 cm, thickness wall 1.0 cm, the container height 6.0 cm, and tare weight 22.0 g.
The containers were tightly sealed for 4 weeks to avoid the escape of 222 Rn gas to ensure secular equilibrium between 226 Ra and their respective progenies.
The samples were analyzed for their gamma emitters using a spectrometer based on HpGe detector for 43,200 s. The HpGe detector (EG and G Ortec Model GEM100P4) with a 100% relative efficiency and a 2.1 keV full width at half-maximum at the 1.33 MeV gamma transition of 60 Co. The Ortec software GammaVision (Model A66-B32, version 6.00) was used for data analysis.
The specific activity concentration A (Bq/kg) of each detected photopeak of energy E is based on the following equation:
Where C (E, n) is the net photopeak count of gamma ray transition with energy E of radionuclide (n), M is the mass of the sample (kg), η is the efficiency of the gamma lines (abundance corrected), and t is the counting time (s).
The average activity concentration of 214 Pb was calculated based on the energy transitions of 295.1 keV (19.2%) and 351.93 keV (37.1%) and 214 Bi activity determined from the 609.3 keV (46.1%), 1120.28 (15.12%), and 1764.5 keV (15.4%) emission gamma lines. The specific activity of 238 U activity was determined indirectly from the energy transitions of its daughter 234m Pa based on 1001.03 keV (0.837%).
The most intensive gamma-ray emitted by 226 Ra is 186.21 keV (3.59%) which overlaps with the 185.72 keV (57. 2%) peak of 235 U. The peaks from the two gamma rays cannot be separated correctly using the conventional analysis, so the activity for one isotope cannot be directly determined by gamma analysis.
Ebaid et al., 2005, have suggested a reliable empirical equation to separate between counting rate contributions from both 235 U and 226 Ra to the 186 keV energy regions. The equation is based on an assumption that 226 Ra is in equilibrium with 238 U in the natural samples. It has showed that in the total count rate of the 186 keV peak consists of 58.3% of 226 Ra and 41.7% of 235 U at radioactivity equilibrium. Then
CRT (186) = CRRa (186.21) + CRU (185.72)(2)
CRU (185.72) = 0.417 × CRT (186)
CRRa (186.21) = 0.583 × CRT (186)(3)
Where CRT is the total count rate (counts. sec –1) in the 186 keV energy peak, CRU is the count rate due to 185.72 keV of 235 U, and CRRa is the count rate due to 186.21 keV of 226 Ra. Furthermore, these calculations are useful in estimating the 226 Ra in samples with no need for the secular equilibrium to happen between 226 Ra and its respective progenies.
| Results and Discussion|| |
The specific activities of 238 U,226 Ra,235 U,222 Rn daughters 214 Pb and 214 Bi were calculated in Bq/kg for all samples and the values have been illustrated in [Table 1].
|Table 1: The specific activity in (Bq/kg) of 238U, 235U, 226Ra, 214Bi, and 214Pb in the measured samples|
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The specific activity of 238 U is ranged from 731.94 ± 106.86 to 905.73 ± 195.00 Bq/kg with an average of 796.14 ± 112.44 Bq/kg. For 235 U, it is ranged from 32.68 ± 1.52 to 43.17 ± 3.14 Bq/kg with an average of 35.714 ± 1.86 Bq/kg, while it is ranged from 746.76 ± 34.13 to 986.41 ± 66.58 Bq/kg with an average of 816.17 ± 34.72 Bq/kg for 226 Ra. For the 222 Rn daughters,214 Bi, it is ranged from 503.43 ± 4.89 to 687.11 ± 10.10 Bq/kg with an average of 567.31 ± 5.37 Bq/kg and 214 Pb from 574.84 ± 5.51 to 780.73 ± 10.32 with an average of 646.40 ± 5.83 Bq/kg.
The correlation between the calculated 238 U (228m Pa) activity concentration and 226 Ra activity concentration by Ebaid method  has been determined by calculating the slope of the line in the graph of 238 U activity concentration versus 226 Ra activity concentration plotted in [Figure 1].
The linear fitting of the graph shows a strong correlation (R2 = 0.997), and the slope of the line has a value (0.973 ± 0.0122) that provides an average value of 238 U (228m Pa)/226 Ra AR of 0.973. This value shows that 238 U is in equilibrium with 226 Ra and the capability of Ebaid method. The presence of the secular equilibrium provides no migration of 226 Ra from or into the system and 238 U was not leached from the phosphate ore surface and transferred to the deeper sections where it accumulates during the equilibrium is established. The present study of 238 U (228m Pa)/226 Ra provides further evidences that the uranium content in the measured samples is of natural origin and an evidence of no contamination of depleted uranium [Table 2].
The 238 U/235 U AR can be to indicate and measure the level of the radioactive contamination by unnatural uranium in the environment. Hence, by plotting the activity concentrations of 238 U isotope against the activity concentrations of 235 U isotope and using the linear fitting, the graph shows a strong correlation (R2 = 0.997) and the slope of the line, 22.23 ± 0.279 provides an average value of the 238 U (228m Pa)/235 U AR as shown in [Figure 2]. Therefore, in this phosphate ore, there is no evidence of contamination of unnatural uranium due to any source, where it is well known that naturally occurring uranium has a constant 238 U/235 U AR of 21.7, that is clear that the natural origin of uranium in this phosphate ore.
The little deviation from the natural ratio (21.7) may be due to the self-absorption effects, especially in 185.72 keV gamma line and the low probability of gamma lines intensity (low yield value) of 234m Pa (1001.03 keV [0.837%]).
The activity concentration ratios of (238 U/average [214 Bi and 214 Pb]),226 Ra/214 Bi,226 Ra/214 Pb, and (226 Ra/average [214 Bi and 214 Pb]) have been determined by calculating the slope of the line in the graphs as shown in [Figure 3], [Figure 4], [Figure 5], [Figure 6]. The linear fitting of the graphs show a strong correlations (R2 = 0.996, 0.999, 0.999, and 0.999, respectively) and the slopes of the lines have the values (1.30 ± 0.0198, 1.44 ± 0.012, 1.26 ± 0.011, and 1.34 ± 0.011, respectively) that provide an average values of (238 U/average [214 Bi and 214 Pb]),226 Ra/214 Bi,226 Ra/214 Pb, and (226 Ra/average [214 Bi and 214 Pb]) ARs of 1.30, 1.44, 1.26, and 1.34, respectively. Although these values show that the ARs are larger than the unity, they do not indicate that there is a migration of the daughter radionuclides between 238 U and 214 Bi where 238 U is in equilibrium with 226 Ra as shown above mentioned. The deviation from the equilibrium value (the unity value) can be interpreted due to the emanation of radon from the sealed sample, the collection of the gaseous radon on the surface of the samples, and the self-absorption effects, especially in 295.1 keV and 351.93 keV gamma lines. Furthermore, the high average values of 226 Ra/214 Bi,226 Ra/214 Pb, and (226 Ra/average [214 Bi and 214 Pb]) ARs indicate a higher bias for the 226 Ra activity. This higher bias may be due in part to elevated 235 U, as well as greater susceptibility of the 226 Ra line to Compton interference.
214 Pb activity values were compared to 214 Bi activity values. Because 214 Bi the first daughter of the short-lived 214 Pb (~27 min), so the two radionuclides should have essentially identical activities. To check this, activities of the two radionuclides for each of the samples were plotted against each other. The plot of 214 Pb versus 214 Bi in [Figure 7] shows a very good agreement with a slope of 1.14 ± 0.003 and a correlation coefficient of R2 = 0.999, which is to be expected.
| Conclusions|| |
The results obtained from this work validate that the HpGe gamma-ray spectrometry measurements can be used to quantify the radioactivity of the uranium radioisotopes and to derive reliable estimation of their isotopic ARs in samples containing measurable uranium concentrations. This technique is highly recommended to perform safeguard analyses for all types of environmental samples.
This work shows that the phosphate ore samples in the El-Sebaiya have normal levels of 235 U,238 U,226 Ra,214 Bi, and 214 Pb activity concentrations and the deduced values of the isotopic ARs reveal the natural origin of uranium and the radionuclides in the uranium decay series are in secular equilibrium. Furthermore, the results indicate that the 235 U can be measured directly if corrected to subtract the contribution from 226 Ra.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2]