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ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 2  |  Page : 90-94  

Choosing an appropriate method for measurement of 232Th in environmental samples


1 Radiation safety Systems Division, Bhabha Atomic Research Centre, Trombay; Homi Bhabha National Institute, TSH Complex, Anushakti Nagar, Mumbai, India
2 Radiation safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India

Date of Submission02-May-2017
Date of Decision16-May-2017
Date of Acceptance26-May-2017
Date of Web Publication13-Jul-2017

Correspondence Address:
R S Sathyapria
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Homi Bhabha National Institute, Trombay, Mumbai - 400 085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_20_17

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  Abstract 

The article discusses the anomalies in using gamma spectrometry for the determination of 232Th in environmental samples against other methods. 232Th in environmental samples (soil, sediment, etc.,) is measured using several techniques, the most commonly used are gamma spectrometry, neutron activation analysis (NAA), and inductively coupled plasma-mass spectrometry. However, gamma spectrometry measurement is based on the assumption that 232Th is in secular equilibrium with its progenies, which is often not true, particularly in the case of vegetation and biological samples. Food samples collected from a high background radiation area in Tamil Nadu, India, were analyzed for 232Th by gamma spectrometry and NAA. This paper gives details of measurements and results obtained from both NAA and gamma spectrometry. 232Th activity concentration determined using gamma spectrometry was an order or two higher than those measured using NAA indicating an extreme overestimation of 232Th by gamma spectrometry technique.

Keywords: Gamma spectrometry, instrumental neutron activation analysis, 232Th


How to cite this article:
Sathyapria R S, Rao D D, Prabhath R K. Choosing an appropriate method for measurement of 232Th in environmental samples. Radiat Prot Environ 2017;40:90-4

How to cite this URL:
Sathyapria R S, Rao D D, Prabhath R K. Choosing an appropriate method for measurement of 232Th in environmental samples. Radiat Prot Environ [serial online] 2017 [cited 2019 Sep 21];40:90-4. Available from: http://www.rpe.org.in/text.asp?2017/40/2/90/210576


  Introduction Top


Thorium is a naturally occurring primordial actinide presents in earth's crust and occurs entirely as 232Th. Long-lived 232Th isotope is the parent radionuclide of a natural decay series with a half-life of 1.39 × 1010y. Quantitative determination of thorium isotopes in materials of natural or commercial origin is an important task that finds it application in geochemistry, radiobiology, radiochronology, radioecology, and radiation safety. Measurement of 232Th concentration in environmental and biological samples in radiation safety point of view is required to: (i) estimate the individual radioactive dose received by the workers or the public using excreta samples (urine and feces), (ii) estimate the radiation dose due to the consumption of food produced in the farms near the nuclear facilities or a mining and milling site, (iii) establish compliance with radiation protection standards, especially, at sites where mining, milling, and processing of uranium and thorium-bearing minerals carried out, and (iv) baseline studies to evaluate the impact caused by non-nuclear activities such as assessment of the radiological impact of phosphate fertilizer used in agriculture.[1] There are different radiometric methods such as alpha and gamma spectrometry, neutron activation analysis (NAA) and liquid scintillation spectrometry, spectrophotometric methods such as visible spectrophotometer, inductively coupled plasma-atomic emission spectrometry (ICP-AES), and electrochemical methods using adsorptive stripping voltammetry and mass spectrometric such as thermal ionization mass spectrometry and inductively coupled plasma-mass spectrometry (ICP-MS) capable of determining thorium in variety of samples.[2],[3],[4] The choice of method depends on sample matrix, the levels of thorium in the sample, and application. Each method has its own pros and cons. Mass spectrometric and alpha spectrometric methods are used for determining isotopic composition. Whereas, NAA, voltammetry, and spectrophotometric techniques are used to determine the total thorium content in the samples.232Th being the parent in the thorium decay series when the daughter radionuclides are in equilibrium with the parent 232Th, gamma spectrometry is a suitable method for analysis as this method is nondestructive and convenient being less complex. For radionuclide incorporation studies, the use of analytical methods that can quantitatively determine trace level thorium in samples such as urine, feces, diet, and drinking water, ICP-MS and NAA are two well-established methods that can precisely measure the thorium isotope (232Th) at levels of a few ppb. Alpha spectrometry is a reliable method for detecting short-lived isotopes 228Th and 230Th down to activities of 0.3 mBq (10−17 g for 228Th and 10−13 g for 230Th). In NAA,232Th is determined through the (n, γ) reaction and subsequent beta decay of the short-lived 233Th (t½= 22.3 min) product to 233Pa (t½=27.0 days).232Th has large (n, γ) thermal neutron cross section (σth-737 b) and 233Pa is determined by measuring the 300 (6.2%), 312 (36%), or 340 (4.2%) keV gamma rays.

232Th decays to stable 208Pb by series of alpha and beta decay. Its radioactive series contains ten daughter radionuclides and is as follows:



It can be seen from the half-lives of the progenies that the daughter nuclides can be in secular equilibrium with 232Th under undisturbed conditions for over a few decades, particularly for first two members. The decay of the daughters is mostly accompanied by emission of γ-rays, and hence gamma spectrometry is long being used by many researchers for estimation of 232Th using strong 228Ac lines at 338.4, 911.1, and 968.9 keV,212Pb line at 238.6 keV,212Bi at 728.3 keV, and 208Tl at 583.1 keV and 2.62MeV.[5],[6],[7] In most mineral samples, equilibrium between 232Th,228Ra, and 228Ac can be assumed, whereas in biological samples, assuming an equilibrium can lead to erroneous results in 232Th estimation. For environmental and dietary matrices, the transfer rates from soil to vegetation for thorium and radium isotopes are also quite different as radium being more mobile. This adds a further degree of disequilibrium between parents and daughters. Even in sediment samples, Olley et. al. have observed that 228Ra concentration was in excess compared to 232Th concentration, implying a disequilibrium as a result of radium redistribution due to short-term radium mobility at a site.[8] Many groups still continue to determine the 232Th concentration in vegetation and diet samples using gamma spectrometry. Hence, an effort was made to confirm and substantiate the inappropriate use of gamma spectrometry for measuring 232Th in vegetation and animal origin food samples. In this work, the presence of 232Th in the samples was quantified by the γ-ray line of 911.2 keV of 228Ac and using NAA, the results were compared.


  Materials and Methods Top


A hyper-pure germanium (HPGe) detection system including multichannel pulse height analyzer was used. The HPGe detector of this system is a 50% relative efficiency P-type coaxial detector with full width half maximum of 1.9 keV at 1332.5 keV of 60Co. The detector is well shielded in a graded cylindrical lead shield. A stand-alone electronic module with a high voltage supply, amplifier, and the multichannel analyzer was coupled to the detector. The spectrum was analyzed using suitable software compatible with the detector system.

Sample collection and preparation

Samples from major food groups, namely, green leafy vegetable, vegetable, roots and tubers, fish, meat, and milk were collected from southern parts of Tamil Nadu, India. The samples were washed with running water and distilled water to remove the adhering dust particle to minimize extraneous contaminants. The edible parts were separated, air dried, weighed, and again dried in a hot air oven at a temperature of 70°C–80°C for constant weight. The dried samples were then freeze-dried and homogenized and taken up for analysis.

Determination of 232Th concentration

Neutron activation analysis

About 100 mg aliquots each of dried samples and NIST standard reference material (NIST SRM 1570a, Spinach leaves, used as reference standard) were packed in a high-purity Al foil of 25 μm thickness and irradiated in tray rod facility of DHRUVA reactor for 3 days at a neutron flux of 5 × 1013n.cm −2/s and were allowed to cool for 30 days and repacked into a clean paper. Samples were assayed by measuring 311.6 keV γ-ray by high-resolution γ-ray spectrometry. Peak areas under the full energy peaks were evaluated by peak fit method using the Gamma Vision software. Relative method was used for thorium concentration calculation in the samples using the known thorium concentration in SRM orchard leaves. The limit of detection for 50,000 s counting was 0.8 ng/g (3.2 μBq/kg) dry weight. The relative method and quality assurance details are found elsewhere.[9]

Gamma spectrometry

The dried samples were sealed in 125 mL geometry polypropylene bottles and were counted in a well-shielded HPGe detector after 30 days from the date of sealing.232Th was measured from the daughter product 228Ac using energy of 911.2 keV.[5],[6],[7] The samples were counted for 86,400 s and from the area under the peak, the activity concentration (Bq/kg) in the samples were obtained using the following equation.



Where C is the γ-ray count rate under the peak (number per se cond), η is the detector efficiency at specific gamma ray, Pγ is the absolute transition probability of gamma decay, and Ms is the mass of the sample (kg). The minimum detectable activity was 0.36 Bq/kg fresh weight using 100 g of sample.


  Results and Discussion Top


Results obtained with both gamma spectrometry and NAA in various food materials were compared and summarized in [Table 1]. Using gamma spectrometry,232Th concentration ranged between BDL and 11400 mBq/kg, whereas 232Th concentration determined using NAA varied between 1.4 and 86.4 mBq/kg. The results obtained using NAA are comparable with results obtained using ICP-MS, alpha spectrometry, or NAA by other researchers but much lower compared to results reported using gamma spectrometry. Activity concentration ranging from 44 to 92 mBq/kg measured using NAA was reported in plant materials collected from Malaysia.[10] In a study conducted by another group of researchers, to estimate the ingestion annual intake of 232Th by population residing in Northeastern parts of India, the 232Th concentration in ranged from 26.5–48.8, 5.3–16.3, 4.5–81.4, and 1.6–56.9 mBq/kg leafy vegetables, cereals, roots and tubers, and in other vegetables, respectively.[11]232Th was measured using ICP-MS in food samples from Brazil, and the radionuclide activity concentrations ranged from <0.003 to 393 mBq/kg.[12] Choi et al. have used alpha spectrometry for determining 232Th activity in food components.[3] The concentration of 232Th determined by Choi et al. (0.08–65.2 mBq/kg) was in the range observed in the study using NAA, whereas the observed concentrations using gamma spectrometry was an order or two higher than that of NAA values. Similar trend was observed in values reported by other workers who have estimated 232Th using gamma spectrometry. Ademola and Ehiedu have reported a concentration range of 31900–96700 mBq/kg in fish using gamma spectrometry.[13] Activity concentration range of 180–3220 mBq/kg and 520–2640 mBq/kg for fruit and vegetable, respectively, from Turkey was reported.[14] Activity concentration in water samples from regions Malaysia was found to range from 0.55–8.64 Bq/L.[15] Activity concentrations for 232Th ranging from 8 to 21 Bq/L with a mean value of 12 ± 1.7 Bq/L were reported by another group from Nigeria using gamma spectrometry.[16]
Table 1: Comparison of 232Th concentration in food groups using neutron activation analysis and gamma spectrometry

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When 232Th is in secular equilibrium with its daughters, activity of the daughter will have the same parent activity concentration, i.e., 232Th=228Ac. However, this equilibrium is often disturbed due to chemical and physical separation processes. The daughter product 228Ra is more mobile compared to that of parent radionuclide 232Th, resulting in greater uptake of radium isotope by the plants. The concentration of natural radionuclides in food varies greatly from one area to another and depends on the soil type, its chemical characteristics, the physical and chemical forms in the soil, and the radionuclide accumulation by different plant parts. Thorium is known to be relatively immobile in soils and plant-to-soil concentration ratios are typically an order of magnitude lower than those observed for radium. The transfer factor of radionuclide, defined as the ratio of the dry weight concentration in the plants to the dry weight concentration in a specified soil layer, varied between 0.005 (kg/kg dry weight) for fodder crops and fruit to 0.057 (kg/kg dry weight) for pasture, whereas in case of radium, the transfer factor was found to range from 0.06 to 0.6 (kg/kg dry weight).[17] Hence, there is a greater uptake of radium than its parent nuclide thorium from the soil causing disequilibrium in the 232Th decay chain.[18],[19],[20],[21] The daughter product 228Ra has a longer half-life of 5.75 years, which is a long time enough for selective migration and absorption by the plants resulting in separation of 228Ra from its parent, and once the radioactive series has been interrupted, the equilibrium is not achieved again in a short time.[22] Hence, by measuring the 228Ac gamma line or the gamma lines of 212Pb,212Bi, or 208Tl, one is measuring the activity of 228Ra and not of 232Th. Higher concentration of 232Th is estimated using gamma spectrometry due to selective uptake of 228Ra and also in the growth of 228Ra due to decay of 232Th radionuclide within the sample. Using gamma spectrometry for determination of 232Th in vegetation samples will result in overestimation of 232Th concentration in the samples and it at the most represents activity of 228Ra. Another major disadvantage of using gamma spectrometry is the poor detection limit compared to NAA. To achieve better detection limit using gamma spectrometry, large sample mass is required, whereas small sample size of 100–200 mg is sufficient for NAA.


  Conclusions Top


Different quantification methods can be used for 232Th analysis in various matrices. For the measurement of 232Th in soil or ore samples where the equilibrium between parent and daughter is not disturbed, high-resolution gamma spectrometry is often the most suitable counting technique. Whereas in vegetation and biological samples, where disequilibrium often persists due to varying uptake parameters of the parent and daughter, mass spectrometric, alpha spectrometry, and NAA are preferred methods. Since 228Ra is more readily available to plants and animals from soil than 232Th,228Ra contents mainly come from the intake of 228Ra itself rather than from radioactivity from 232Th. The comparison of results obtained from gamma spectrometry and NAA for dietary items revealed that 232Th is about two orders higher for the former technique vis-à-vis with the latter technique. Hence gamma spectrometry technique which is based on measurement of daughter products such as 228Ac,212Pb,212Bi, or 208Tl of 232Th should not be used to measure 232Th in vegetation or dietary items. When the concentration level is very low, as in case of vegetation and biological samples, NAA and mass spectrometry are considerably superior methods for detection of traces of primordial 232Th.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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