ORIGINAL ARTICLE
Year : 2022 | Volume
: 45 | Issue : 3 | Page : 121--126
Equilibrium status of naturally occurring 226Ra with radon daughters and estimation of 238U activity from 235U using natural radioactivity ratio of 238U/235U in soil around Rawatbhata, Rajasthan
Ajay Kumar Gocher1, Mohit Sisodia1, SN Tiwari1, IV Saradhi2, A Vinod Kumar2,
1 Environment Survey Laboratory, Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Rawatbhata, Rajasthan, India
2 Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
Correspondence Address:
Ajay Kumar Gocher
Environment Survey Laboratory, Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, PO: Bhabhanagar, Rawatbhata - 323 307, Rajasthan
India
Abstract
Equilibrium status among 226Ra and radon daughters was studied in soil samples around Rawatbhata, Rajasthan. It was found that the increase in radioactivity of 214Bi (radon daughter) due to complete radon buildup in the sealed container is within 15% which establishes that 226Ra remains in equilibrium with radon daughter in soil of Rawatbhata. Radioactivity of 238U was estimated from the radioactivity of 235U using natural uranium isotopic radioactivity ratio (238U/235U) of 21.5. 238U radioactivity in soil was found in the range 25.8–49.5 Bq/kg with an average of 36.7 Bq/kg which is comparable to the global average of 35 Bq/kg.
How to cite this article:
Gocher AK, Sisodia M, Tiwari S N, Saradhi I V, Kumar A V. Equilibrium status of naturally occurring 226Ra with radon daughters and estimation of 238U activity from 235U using natural radioactivity ratio of 238U/235U in soil around Rawatbhata, Rajasthan.Radiat Prot Environ 2022;45:121-126
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How to cite this URL:
Gocher AK, Sisodia M, Tiwari S N, Saradhi I V, Kumar A V. Equilibrium status of naturally occurring 226Ra with radon daughters and estimation of 238U activity from 235U using natural radioactivity ratio of 238U/235U in soil around Rawatbhata, Rajasthan. Radiat Prot Environ [serial online] 2022 [cited 2023 May 30 ];45:121-126
Available from: https://www.rpe.org.in/text.asp?2022/45/3/121/377236 |
Full Text
Introduction
Natural soil contains isotopes of uranium (238U) series, thorium (232Th) series, and actinium (235U) series along with other primordial radionuclides such as 40K. Uranium series radioisotopes include mainly 238U, 226Ra, and 222Rn along with its daughters. Radon (222Rn) gas is normally trapped within a solid; therefore, in undisturbed soil of natural origin, all the uranium series radioisotopes usually remain in secular equilibrium with parent radioisotope.[1] Radon gas can escape from soil due to any disturbance in soil resulting loss of secular equilibrium among radon and its daughters.[1] Secular equilibrium may also be disturbed due to sampling and physical processing of soil samples, hence soil samples are kept in sealed containers to avoid any radon loss and to ensure secular equilibrium among radon and its daughters for radioactivity estimation of 226Ra from radon daughters by gamma-spectrometry. One-month time is required to attain secular equilibrium between radon daughters and 226Ra in the sealed container.[1]
Uranium isotopes are present naturally in the soil with the abundance of 238U (99.28%), 234U (0.0057%), and 235U (0.72%) of total uranium. The isotopic radioactivity ratio of 238U/235U remains constant in natural soil with its value of 21.5.[2] In an undisturbed area, where 238U is in the secular equilibrium with 226Ra, 238U activity is usually estimated assuming secular equilibrium among its daughter radionuclides. Various gamma-energies of radon (222Rn) daughters routinely used for uranium estimation are 295.2 keV (19.3%) and 351.9 keV (37.6%) of 214Pb and 609.3 keV (46.1%) and 1764.5 keV (15.4%) of 214Bi due to better yield and no interfering energies.[3]
In a disturbed area, where 238U is not in the secular equilibrium with 226Ra, gamma-energies 63.29 keV of 234Th and 1001.03 keV of 234mPa are the most suitable gamma-energies to determine 238U by gamma-ray spectrometry.[4] However, these two direct gamma-energies are not much suitable for detection by P-type co-axial HPGe detector with aluminum (Al) window because (1) 63.29 keV, being a low gamma-energy, is attenuated significantly due to by thick dead layer and Al window of the HPGe detector[5] and (2) low-abundance and high-energy peak of 234mPa (0.83%, 1001.1 keV) produces results with large uncertainty due to very low counts which makes it inappropriate for the analysis of soil samples in which uranium activity typically amounts to <100 Bq/kg.[2] In the absence of proper gamma-energy at 63.29 keV of 234Th and 1001.1 keV of 234mPa, 238U activity can also be estimated from 235U (185.7 keV, 57.2%) activity and 238U/235U isotopic radioactivity ratio of 21.5.
The present work is aimed to (1) study the equilibrium status among naturally occurring 226Ra and radon daughters in soil samples around Rawatbhata, Rajasthan, and (2) determine the radioactivity of 238U from the radioactivity of 235U using 238U/235U radioactivity ratio.
Materials and Methods
Soil samples from an undisturbed area were collected from 12 locations as shown in [Figure 1] around Rawatbhata, Rajasthan site, as per the standard procedure.[6] The sampling locations along with their latitude and longitude are given in [Table 1].{Figure 1}{Table 1}
Top soil up to the 2 cm was discarded to remove an unwanted part and the remaining soil after 2 cm depth was collected. The samples were dried in hot air oven at 100°C, sieved through 2 mm mesh size. Physically processed soil samples were mixed thoroughly by shaking to disturb equilibrium condition, if present, between 226Ra and radon daughters and sealed in precalibrated Marinelli beaker geometry. These soil samples were counted immediately after sealing and counted after 1 month using a P-type co-axial HPGe gamma-ray detector (Eurisys make, 110% R.E., Al window, resolution: 2.2 keV at 1332.2 keV peak of 60Co) coupled with Genie 2000 software of Canberra make. The detector is shielded by 5.5 cm thick lead to reduce background gamma-radiation. The detection efficiencies of the HPGe detector for different gamma-energies were determined using standard reference materials RGU-1 and RGTh-1 supplied by IAEA.[7] Sample spectrums were acquired for 80,000 s for each sample. The net count rate of respective peaks was obtained after subtracting the corresponding background count rates and was used for estimation of activity concentrations of the radionuclides of interest. Radioactivity in the soil sample is calculated using the standard formula;
[INLINE:1]
The following methodologies were used for estimation of 238U in soil sample for comparison and validation of results:
First method (assuming nonexistence of secular equilibrium among 238U and 226Ra in soil sample):
In the absence of proper direct gamma-line of 234Th at 63.29 keV and 234mPa at 1001.1 keV, 238U activity in the soil sample of natural origin was estimated indirectly by multiplying 235U activity with 238U/235U isotopic activity ratio of 21.5. 235U activity was estimated from direct gamma-energy at 185.7 keV (57.2%). However, at 185.7 keV energy, 226Ra (186.2 keV, 3.6%) also has contribution in count which is unresolved by detector; therefore, 235U activity was calculated by subtracting 226Ra (186.2 keV, 3.6%) counts from peak at 185.7 keV.[8] 226Ra activity was calculated from radon daughter of 214Bi (609.3 keV, 46.1%) after attaining secular equilibrium among 226Ra, radon, and radon daughters. The formula for calculation of 235U activity is given below:
235U (Bq/kg) =
[INLINE:2]
Counts attributed to 226Ra at 185.7 keV were calculated as:
[INLINE:3]
The activity of 238U was calculated as.
[INLINE:4]
This method for estimation of 238U activity is valid where secular equilibrium among 238U and 226Ra does not exist in soil.
Second method (assuming secular equilibrium exists among 238U and 226Ra in soil samples):
If 226Ra is in radioactive equilibrium with its parent 238U and 238U/235U isotopic radioactivity ratio is the expected natural value of 21.5, the counts in the 186.2 keV peak can be mathematically apportioned between 226Ra and 235U using specific activity of 235U and 238U and the gamma-ray emission probabilities as mentioned in [Table 2].[1]{Table 2}
The gamma-ray spectrum analysis is performed assuming that the entire 186.2 keV peak is due to 226Ra which is assumed as apparent activity of 226Ra. We can then use the following correction factors to correct the 226Ra value and calculate the activity of 235U using eq. 5 and 6:[1]
[INLINE:5]
[INLINE:6]
In the second method, since activity of 226Ra was estimated from 226Ra peak at 186.2 keV by immediate counting after sealing of soil sample in container, therefore 1-month equilibration period to attain secular equilibrium among 226Ra, radon, and radon daughters is not required to estimate the activity of 238U from radon daughter. Activity of 238U will be equal to activity of 226Ra in secular equilibrium condition.
Results and Discussion
Equilibrium status among 226Ra and radon daughters in soil
214Bi (a radon daughter) activity represents activity of 226Ra in secular equilibrium condition. 214Bi activity ratio is calculated from the activity of 214Bi in the sample after attaining secular equilibrium to the activity of 214Bi in the same sample after immediate counting. Here, 214Bi activity ratio of unity indicates that there is no increase in the activity of 214Bi due to radon buildup which means that 226Ra was already in secular equilibrium with radon daughters in soil sample. 214Bi activity ratio was estimated in the soil samples, and the results are shown in [Table 3].{Table 3}
It was evident from [Table 3] that the radioactivity ratio of 214Bi varied from 1.00 to 1.24 with an average value of 1.11 which shows that even after disturbance of equilibrium between radon daughter and 226Ra by physical processing (digging during sample collection, drying at 100°C and sieving), majority of samples showed an increase in activity of 214Bi due to radon buildup within 15% relative to activity of 214Bi counted immediately. Two measurements of the same sample obtained from two methods shown in [Table 3] were also compared using Student's t-test.[9] The t-statistic for all samples was evaluated at significance level of 0.05, i.e., at 95% confidence level for which ttable value is 1.96. The test was performed by calculating a value tcal using eq. (7) and comparing it with a true probability value ttable.
[INLINE:7]
Where M1 and M2 are the mean results of 214Bi activity and σM1 and σM2 are the corresponding standard errors of the means obtained from the two methods. It was observed from [Table 3] that tcal value for measurement is less than ttable value (ttable = 1.96) which proves that the differences between the 214Bi activity results of the two methods were not significant which indicates that the 214Bi activity results from the two methods are same.[9] The 226Ra (214Bi) activity results obtained from the two methods are plotted in [Figure 2] for comparison and correlation between the two results. The correlation coefficient (R2 value) of 226Ra activity obtained from the two methods was 0.91 which depicted good agreement between the results.{Figure 2}
This study demonstrated that 226Ra and radon daughter remains in secular equilibrium in the soil of Rawatbhata, Rajasthan area, even after disturbance of secular equilibrium by physical processing of soil sample. Since 226Ra was found in secular equilibrium with radon daughter in the soil, therefore 226Ra activity can be estimated from radon daughter by immediate counting without waiting of 1-month equilibrium period among 226Ra and radon daughter.
238U activity estimation in soil
238U activity has been estimated by two methods for comparison and validation of results. The first method can be used for the soil of disturbed area where secular equilibrium between 238U and 226Ra is not sure; therefore, 238U activity cannot be estimated from radon daughters in secular equilibrium with 226Ra. In such case, activity of 238U is calculated from activity of 235U using natural uranium isotopic radioactivity ratio (238U/235U) of 21.5. The 235U and 238U radioactivities in soil were found in the range of 1.2–2.3 Bq/kg and 25.8–49.5 Bq/kg, respectively. The estimated average radioactivity of 238U was 36.7 Bq/kg which is comparable to the global average of 35 Bq/kg.[10]
The second method of measurement of 238U activity is based on assumption that since soil samples were collected from an undisturbed area, therefore it can be assumed that (i) 226Ra is in secular equilibrium with 238U and (ii) 238U/235U isotopic radioactivity ratio has a natural value of 21.5. If these two conditions are not true, then this method will give misleading results for 238U activity. It was observed from result in [Table 4] that 226Ra activity estimated directly from 214Bi at 609.3 keV (after attain of secular equilibrium) is almost same as estimated directly at 186.2 keV by the second method in which it was assumed that 226Ra is in secular equilibrium with 238U. This indirectly establishes that 226Ra was in secular equilibrium with 238U in soil sample and 238U/233U activity ratio has natural value of 21.5; therefore, 238U activity can be estimated here from 226Ra activity for soil samples. By using the second method, 235U and 238U radioactivities in soil are found in the range 1.3–2.2 Bq/kg and 25.4–47.2 Bq/kg, respectively. The estimated average radioactivity of 238U was 36.0 Bq/kg which is again comparable to the global average of 35 Bq/kg.[10]{Table 4}
238U activity estimated by the above two methods was also compared using Student's t-test. The t-statistic for all samples was evaluated at significance level of 0.05, i.e., at 95% confidence level for which ttable value is 1.96. It was observed from [Table 4] that tcal value for each measurement is less than ttable value (ttable = 1.96) which showed that the differences between the 238U activity results obtained from two methods were not significant means both the methods were producing comparable results of 238U activity in soil.
The 238U activity results obtained from the two methods are plotted in [Figure 3] for comparison and correlation between the two results. The correlation coefficient (R2 value) of 238U activity obtained from the two methods was 0.92 which depicted good agreement between the results.{Figure 3}
238U/235U and 238U/226Ra radioactivity ratio
The radioactivity ratio of 238U/235U and 238U/226Ra was estimated and the obtained values are shown in [Table 5].{Table 5}
It was evident from [Table 5] that the average radioactivity ratio of 238U/235U in soil samples is 21.83 which was found very close to the natural isotopic radioactivity ratio of 238U/235U of 21.5. Furthermore, the average radioactivity ratio of 238U/226Ra was found very close to unity which demonstrated that 238U and 226Ra were found in equilibrium in the soil of Rawatbhata, Rajasthan site.
Conclusions
An attempt was made to determine the equilibrium status among 226Ra and radon daughters in the soil and also to find out 238U activity from 235U activity in soil. It was found that 226Ra and radon daughters remain in secular equilibrium in the soil around Rawatbhata, Rajasthan. It was also found in the study that 238U and 226Ra remain in secular equilibrium condition in the soil of around Rawatbhata, Rajasthan. 238U radioactivity in soil was found in the range 25.8–49.5 Bq/kg with an average of 36.7 Bq/kg which is comparable to global average value of 35 Bq/kg. The first method of 238U radioactivity estimation from 235U activity is useful for the soil of disturbed area where secular equilibrium between 238U and 226Ra is not sure.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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