|Year : 2018 | Volume
| Issue : 1 | Page : 47-50
Radioactive disequilibrium in uranium series of core samples from Rasimalai and Pakkanadu areas of Tamil Nadu, India
V Madhavi Shankar1, Taru Bhattacharya1, B Ram Mohan Reddy1, P Sahoo1, Pravin Kumar Sharma2
1 Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Bengaluru, Karnataka, India
2 Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Hyderabad, Telangana, India
|Date of Submission||31-Jan-2018|
|Date of Decision||02-Mar-2018|
|Date of Acceptance||20-Mar-2018|
|Date of Web Publication||31-May-2018|
V Madhavi Shankar
Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, Bengaluru, Karnataka
Source of Support: None, Conflict of Interest: None
Uranium mineral in a geological environment may behave like an open system. Various factors such as mobility of radium, α-recoil of 234U causing migration of uranium radioisotopes, and radon emanation lead to disequilibrium in uranium decay series. A study has been carried out to find the disequilibrium status of subsurface samples of Rasimalai area of Vellore district and Pakkanadu area of Salem district, Tamil Nadu. This study has a significance in connection with exploration of atomic minerals. It was observed that samples of Rasimalai area show a disequilibrium in favor of parent uranium whereas samples from nearby Pakkanadu area are in equilibrium.
Keywords: Disequilibrium factor, gamma ray spectrometry, Ra(eU3O8), ThO2, U3O8
|How to cite this article:|
Shankar V M, Bhattacharya T, Reddy B R, Sahoo P, Sharma PK. Radioactive disequilibrium in uranium series of core samples from Rasimalai and Pakkanadu areas of Tamil Nadu, India. Radiat Prot Environ 2018;41:47-50
|How to cite this URL:|
Shankar V M, Bhattacharya T, Reddy B R, Sahoo P, Sharma PK. Radioactive disequilibrium in uranium series of core samples from Rasimalai and Pakkanadu areas of Tamil Nadu, India. Radiat Prot Environ [serial online] 2018 [cited 2021 Mar 1];41:47-50. Available from: https://www.rpe.org.in/text.asp?2018/41/1/47/233649
| Introduction|| |
The 238 U isotope of uranium is the parent of 14 daughter isotopes forming the uranium series. The first five isotopes of this series constitute the uranium group and sixth to the last isotope, i.e.,206 Pb, form the radium group. Isotopes in the uranium and radium group usually remain in equilibrium within the group, but disequilibrium commonly exists between the two groups., In an open system, the mobility of radium is significant due to radon gas produced as a daughter element in the radium series. Anomalies with excess radium are common around springs and seepages whereas uranium anomalies are usually found under reducing environments. The disequilibrium factor (DF) is defined as the ratio of U3O8 to Ra(eU3O8) where Ra(eU3O8) represents the U3O8 measured by radium group (214 Bi) concentration assuming that uranium series is in equilibrium and U3O8 is the true value of uranium measured using beta–gamma method. The disequilibrium is toward the parent uranium if the value of DF >1. This condition is favorable for uranium exploration. In contrast, if the value of DF <1, disequilibrium is toward the daughter radium and is an undesirable condition for uranium prospecting. It signifies partial removal of uranium from the system leading to a lowering of ore reserve estimates based on total gamma ray logging data.
| Materials and Methods|| |
Estimation of uranium
For a uranium-bearing sample in equilibrium, U3O8 content can be determined by measuring total gamma (γ) or total beta (β) activity. However, for a rock sample in general, it may not give the true value of U3O8 due to the disequilibrium of uranium series or due to the presence of thorium or both. In such cases, concentration of actual U3O8 can be determined using beta–gamma method  by simultaneous measurement of total beta and total gamma radiations using the equation as follows:
U3O8= (1 + C) Uβ− CUγ
Where Uβ is apparent uranium equivalent derived from the beta activity of the sample, Uγ is apparent uranium equivalent derived from the gamma activity of the sample, and C is the ratio of beta contributions from radium group (BR) to uranium group (BU) in an equilibrium uranium sample:
C = BR/BU = 0.57/0.43 = 1.30
In this method, measurement of beta and gamma radiations was carried out using an end-window beta tube and 13/4″ × 2″ NaI (Tl) crystal detector, respectively. Beta and gamma sensitivities were determined using equilibrium and disequilibrium uranium standards developed in-house by Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad, Telangana, India. The detection limit of the system is 100 ppm ±10% error for 50 g of sample and 2000 s measuring time.
The eU3O8 measured by the total gamma measurement includes thorium concentration also. Hence, for calculating the DF, Ra(eU3O8) was determined by gamma ray spectrometry.
Estimation of Ra(eU3O8), ThO2, and K
Estimation of Ra(eU3O8), ThO2, and K was carried out by gamma ray spectrometry using 5″ × 4″ NaI (Tl) crystal and a photomultiplier tube combination. The detector was coupled to a 1K multichannel analyzer. Software developed in-house was used to integrate the counts in different regions of interest (ROIs). These ROIs were centered around the peak energy values of 1.46 MeV (40 K), 1.76 MeV (214 Bi), and 2.62 MeV (208 Tl) for the estimation of K, RaeU3O8, and ThO2, respectively. The stripping and sensitivity factors were determined using the standard reference materials RGU-1, RGTh-1, and RGK supplied by the International Atomic Energy Agency.
| Results and Discussions|| |
Disequilibrium studies were carried out for the borehole samples of Rasimalai and Pakkanadu areas of Tamil Nadu. Radioactive samples with U3O8 and Ra(eU3O8) >0.010% were chosen for this study. U3O8 content >0.010% is considered as economically viable concentration for uranium exploration. Hence, these samples were treated as radioactive and were classified according to the rock type.
It has been reported that uranium mineralization in Rasimalai had occurred during late magmatic stage in fine-grained pink syenite dykes and subsequently during hydrothermal stage in quartz-rich veins. A total of 367 borehole samples of syenite rock from boreholes RSM-2 and RSM-12 were considered for the study. Of these, 295 samples had both Ra(eU3O8) and U3O8 concentrations >0.010%.
In Pakkanadu, 234 samples were taken for the study from boreholes PKD/1 and PKD-2. Of these, seventy samples had both U3O8 and Ra(eU3O8) concentrations >0.010%. Most of these samples were found in pyroxenite with carbonatite and pyroxenite with quartz veins. Very few radioactive samples were of fine-grained syenite. However, the number of samples of pyroxenite with quartz veins (7) and fine-grained syenite (4) was much less than samples of pyroxenite with carbonatite (59). Hence, disequilibrium studies were carried out for pyroxenite with carbonatite samples only.
The range of concentration of U3O8, Ra(eU3O8), and ThO2 obtained in both the areas is shown in [Table 1].
|Table 1: Concentration of uranium, radium, and thorium in samples collected from Rasimalai and Pakkanadu areas, Tamil Nadu|
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The DF for all the samples was calculated using the following relation:
DF = U3O8/Ra(eU3O8)
The variations of DF in the samples of the above-mentioned areas are shown in [Figure 1]a and [Figure 1]b.
|Figure 1: (a) Variation of disequilibrium factor for the samples from Rasimalai area, (b) variation of disequilibrium factor for samples from Pakkanadu area|
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Scatter diagrams were plotted for U3O8 versus Ra(eU3O8) values. The slope of these graphs represents the average DF. These scatter diagrams are shown in [Figure 2]a and [Figure 2]b, respectively.
|Figure 2: (a) U3O8 versus Ra(eU3O8) for syenite from Rasimalai area. (b) U3O8 versus Ra(eU3O8) for pyroxenite with carbonatite veins of Pakkanadu area|
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These figures show a good correlation between U3O8 and Ra(eU3O8) in samples from both the areas. Rasimalai syenite shows a positive intercept of 32 ppm, suggesting a prevailing concentration of U3O8. This also agrees with the syngenetic nature of uranium in this syenite. This shows that radium group present has developed from inherent uranium of the rock. The slope of >1 indicates disequilibrium in favor of uranium group (parent). This disequilibrium toward the parent could be due to two reasons:
- Due to some hydrogeochemical processes in this area, radium group could have leached out of the system
- Uranium could have remobilized and deposited at the present location, leading to enrichment of parent.
[Figure 2]b shows a positive intercept of 18 ppm suggesting a prevailing concentration of U3O8 in Pakkanadu samples too. However, these samples show a slope of 0.97, indicating near equilibrium in the samples.
| Conclusions|| |
Major activity due to uranium in Rasimalai area is hosted in pink syenite rocks, whereas in Pakkanadu area, radioactivity due to uranium is found mainly in pyroxenite with carbonatite veins. In this study, both the areas show a prevailing background of uranium. Rasimalai syenite shows disequilibrium in favor of parent uranium whereas pyroxenite with carbonatite vein samples of Pakkanadu do not show any disequilibrium in favor of either parent or daughter group. It also suggests that pyroxenite of Pakkanadu with carbonatite vein forms a relatively closed system compared to syenite of Rasimalai.
The authors would like to express their gratitude to Shri. L. K. Nanda, Director, and Shri. M. B. Verma, Additional Director (OP-I), Atomic Minerals Directorate for Exploration and Research (AMD), Hyderabad, Telangana, India, for providing encouragement and permission to present this paper. The authors would also like to thank Shri. A. K. Bhatt, Regional Director, AMD, Southern Region, Bengaluru, Karnataka, India, for his constant support in carrying out this work.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]