|Year : 2017 | Volume
| Issue : 1 | Page : 18-20
Study of equilibrium status among uranium and thorium series radionuclides in soil samples from Visakhapatnam, India
Pradyumna Lenka1, Anil Gupta1, SK Sahoo1, Aditi C Patra1, A Vinod Kumar2, PM Ravi1, Raj Mangal Tripathi1
1 Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Submission||14-Mar-2017|
|Date of Decision||14-Mar-2017|
|Date of Acceptance||20-Mar-2017|
|Date of Web Publication||24-Apr-2017|
Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
Surface soil samples from coastal region of Visakhapatnam were analyzed for uranium series and thorium series radionuclides by high-resolution gamma-ray spectrometry. Isotopic ratios of 226Ra/238U and 224Ra/232Th were calculated to investigate the radioactive equilibrium status in the samples. 226Ra/238U and 224Ra/232Th ratios were found to vary from 0.80–1.40 to 0.94–1.40 with mean values of 1.05 and 0.99, respectively. Results showed a prevalent radioactive equilibrium for both uranium and thorium series radionuclides in the surface soil of the study area.
Keywords: Gamma spectrometry, isotopic ratio, monazite, secular equilibrium
|How to cite this article:|
Lenka P, Gupta A, Sahoo S K, Patra AC, Kumar A V, Ravi P M, Tripathi RM. Study of equilibrium status among uranium and thorium series radionuclides in soil samples from Visakhapatnam, India. Radiat Prot Environ 2017;40:18-20
|How to cite this URL:|
Lenka P, Gupta A, Sahoo S K, Patra AC, Kumar A V, Ravi P M, Tripathi RM. Study of equilibrium status among uranium and thorium series radionuclides in soil samples from Visakhapatnam, India. Radiat Prot Environ [serial online] 2017 [cited 2018 Aug 18];40:18-20. Available from: http://www.rpe.org.in/text.asp?2017/40/1/18/205049
| Introduction|| |
Monazite has been reported as a common accessory mineral in the Eastern Ghats granulite terrain around Visakhapatnam. The granitic rocks that are predominant in the region are the kind of rocks, that serve as good geochemical hosts for uranium and thorium. A major portion of this region is occupied by Proterozoic rocks covering a vast time span from 3800 to 570 million years. The crust in this region shows an enrichment of rare earths with which U and Th are in variably associated in the foam of minerals such as monazite, samarskite, fergusonite, and allanite. Sandstones, limestones, quartzites, slates with poor amounts of U, Th, and K will also contribute to the low radiation dose. Furthermore, an association of uranium and thorium with monazite-rich sand and soil is well known and well documented., Elevated levels of thorium in soil samples than average values are evident in these monazite-rich areas. Soil radioactivity and corresponding external gamma radiation dose from Visakhapatnam region has been reported elsewhere. In the present study, 47 soil samples were collected from the area around the upcoming nuclear facility in the region. The samples were analyzed for natural radionuclides as a part of the baseline data generation of this region. Efforts were made to understand the various geochemical processes of the surface soil, based on the results found in the study.
Naturally, soil contains both uranium series, thorium series radionuclides along with other primordial radionuclides like 40 K and actinium series isotopes  in varying proportions depending on the geology, geochemistry, and environmental processes of the region leading to soil formation. Different radium isotopes (226 Ra,224 Ra, etc.) exist in soil as a result of decay of uranium and thorium series isotopes. Chemistry of U, Th and Ra are distinct from each other, and they have different solubility and leachability under a given geochemical condition of the soil. In secular equilibrium conditions, activity concentrations of 238 U and 226 Ra in uranium series and 224 Ra and 232 Th in thorium series remain same. Therefore, studying the equilibrium status of these isotopes can give an insight into the natural geochemical processes of the study area.
238 U and 232 Th concentrations are usually being estimated by employing high-resolution gamma-ray spectrometry, as the process is nondestructive, highly precise, multi-elemental, and less time-consuming technique. Characteristics gamma energies are used to identify and estimate the respective isotopes.238 U cannot be measured directly from gamma ray spectrometry as it possesses only a feeble gamma line 49.55 keV of very low emission probability (0.064%).238 U activity is usually estimated from gamma lines of radon daughters assuming secular equilibrium among its daughter radionuclides. Various energies routinely used for uranium estimation are 295.2 keV (19.3%) and 351.9 keV (37.6%) of 214 Pb and 609.3 keV (46.1%) and 1764.5 keV (15.4%) of 214 Bi due to their better yield and no interfering energies. This assumption holds good, only under undisturbed conditions, where there is a natural secular equilibrium between 238 U and 226 Ra. Under oxidizing conditions, where uranium mostly stays in hexavalent states is more susceptible to leaching than radium. In such case, direct gamma line 63.29 keV of 234 Th, is the most suitable gamma energy to determine 238 U by gamma ray spectrometry.
However thorium is highly insoluble of nature and therefore in case of disequilibrium between 224 Ra and thoron (220 Rn) daughters, thorium concentrations cannot be measured from gamma lines of thoron daughters. In this case, two prominent gamma energies of 228 Ac, the third member in the series, 338 keV (11.3%) and 911 keV (25.8%) are usually being used to estimate the parent thorium.
In the present study,238 U,226 Ra,232 Th, and 224 Ra were calculated from the gamma energies 63.29, 1764, 911, and 2614 keV, respectively, and the results were used to find out the equilibrium status of uranium series and thorium series radioisotopes and the data are presented here.
| Materials and Methods|| |
Forty-seven soil samples were collected from the area around the upcoming New BARC complex, Visakhapatnam. The samples were oven dried, sieved through 2 mm mesh size using a sieve shaker and then sealed in 250 ml cylindrical containers (7 cm diameter × 6.5 cm height). After a waiting period of 30 days to ensure the secular equilibrium among 226 Ra and its daughter products, the samples were analyzed using a p-type HPGe detector (40% R.E., Carbon Fibre Window, resolution: 1.7 keV at 1332 keV peak of 60 Co) coupled with analysis software. The detector is shielded by 7.5 cm thick lead to minimize background gamma radiation.
The absolute detection efficiencies of the HPGe detector for different gamma energy ranges were prior determined using IAEA standard reference materials RGU-1 and RGTh-1 placed in identical geometries to those used for analysis of samples. Sample spectrums were acquired for 60,000 s for each sample. The net count rate of respective peaks were obtained after subtracting the corresponding background count rates and were used to estimate the activity concentrations of the radionuclides of interest.
| Results and Discussion|| |
The concentrations of the isotopes 238 U,226 Ra,232 Th, and 224 Ra in the soil samples were estimated, and the data were interpreted to understand the equilibrium status of uranium series and thorium series radionuclides. The range and mean activity concentration levels in the samples were presented in [Table 1]. High values of thorium were observed at few locations which may be attributed to the prevalence of monazites in the region. The results are comparable with our previous studies around the region, where they reported similar ranges of activities for 238 U and 232 Th.
|Table 1: Range and mean activity levels and other statistical parameters for soil samples of Visakhapatnam region|
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In both the series,222 Rn and 220 Rn, daughters of 226 Ra and 224 Ra are noble gasses and escapes the soil matrix. Therefore, the ratios 226 Ra/238 U and 224 Ra/232 Th were used to study the equilibrium in uranium and thorium series, respectively. [Figure 1]a and [Figure 1]b shows a good correlation between 226 Ra and 238 U and 224 Ra and 232 Th, respectively.
|Figure 1: (a) Correlation between 226Ra and 238U in Visakhapatnam soil samples. (b) Correlation between 224Ra and 232Th in Visakhapatnam soil samples|
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The ratios 226 Ra/238 U for the samples, varied between 0.80 and 1.40 with a mean of 1.05 and in thorium series 224 Ra/232 Th ratio was found to be in the range of 0.94–1.40 with a mean of 0.99 (average of individual ratios). In both the cases, the mean ratios were found to be close to unity, depicting a prevalent equilibrium in both uranium and thorium series. [Figure 2]a and [Figure 2]b represent the frequency distributions of the 226 Ra/238 U and 224 Ra/232 Th ratios. It is clear from the figures that in case of thorium series, in most of the samples the equilibrium ratio 224 Ra/232 Th remains very close to unity, whereas in case of uranium series, the equilibrium ratio 226 Ra/238 U is spread in the range of 0.8–1.2.
|Figure 2: (a) Frequency distribution of isotopic ratios 226Ra/238U in Visakhapatnam soil. (b) Frequency distribution of isotopic ratios 224Ra/232Th in Visakhapatnam soil|
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| Conclusions|| |
HPGe gamma-ray spectrometry was used to analyze uranium and thorium series radionuclides in soil samples from the Visakhapatnam region of the eastern coast of India. The values of 238 U and 232 Th agree with previous measurements conducted in the area. The ratios 226 Ra/238 U and 224 Ra/232 Th were determined to study the equilibrium status of uranium and thorium series in the surface soils of the region. Mean values of both ratios were found close to unity which conveys the fact that, there is prevalent radioactive equilibrium among uranium and thorium series radionuclides in surface soil of the study area.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]