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ORIGINAL ARTICLE
Year : 2012  |  Volume : 35  |  Issue : 1  |  Page : 14-16  

A comparative study of 232 Th and 238 U activity estimation in soil samples by gamma spectrometry and neutron activation analysis technique


Radiation Safety System Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India

Date of Web Publication6-May-2013

Correspondence Address:
Rekha Anilkumar
RSSD, BARC, Trombay, Mumbai- 400 085
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.111404

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  Abstract 

Neutron activation analysis (NAA) is a well-established analytical technique. It has many advantages as compared to the other commonly used techniques. NAA can be performed in a variety of ways depending on the element, its activity level in the sample, interference from the sample matrix and other elements, etc., This technique is used to get high analytical sensitivity and low detection limits (ppm to ppb). The high sensitivity is due to the irradiation at high neutron flux available from the research reactors and the activity measurement is done using high resolution HPGe detectors. In this paper, the activity estimation of soil samples using neutron activation and direct gamma spectrometry methods are compared. Even though the weights of samples considered and samples preparation methods are different for these two methods, the estimated activity values are comparable.

Keywords: 232 Th and 238 U, HPGe, neutron activation analysis


How to cite this article:
Anilkumar R, Anilkumar S, Narayani K, Babu D, Sharma D N. A comparative study of 232 Th and 238 U activity estimation in soil samples by gamma spectrometry and neutron activation analysis technique. Radiat Prot Environ 2012;35:14-6

How to cite this URL:
Anilkumar R, Anilkumar S, Narayani K, Babu D, Sharma D N. A comparative study of 232 Th and 238 U activity estimation in soil samples by gamma spectrometry and neutron activation analysis technique. Radiat Prot Environ [serial online] 2012 [cited 2021 Apr 13];35:14-6. Available from: https://www.rpe.org.in/text.asp?2012/35/1/14/111404


  Introduction Top


Radioactivity in the environment is mainly due to the naturally occurring radionuclides like uranium, thorium with their daughter products and potassium-40. Even though gamma spectrometry is the most commonly used nondestructive method for the quantification of these naturally occurring radionuclides, Neutron Activation Analysis (NAA), a well-established analytical technique, can also be used particularly for low specific active nuclides like 232 Th and 238 U. [1] But the NAA technique is a time consuming process and needs proper standards, proper sample preparation, and thermal neutron irradiation facility. In this paper, the performance of both the analytical methods were studied based on the estimation of 232 Th and 238 U in soil samples. 232 Th and 238 U activity estimated in various samples using gamma ray spectrometry and NAA technique are compared.


  Materials and Methods Top


NAA is one of the popular nuclear analytical techniques and has extensive applications in science and technology fields for macro, micro, and trace element analysis. NAA method is based on the nuclear reaction between neutrons and target nuclei. Depending upon the requirement, NAA can be carried out either nondestructively or by performing pre- or postirradiation chemical separations. This is a useful method for the simultaneous determination of elements in various types of samples. In NAA, samples are activated by thermal neutrons. During irradiation, the naturally occurring stable isotopes of most elements transformed into radioactive isotopes by neutron capture. These activated nuclei decays by emitting gamma rays with specific energies. These energies can be measured using an HPGe detector from the characteristic gamma ray spectrum of each sample. This technique also can be extended to low specific active nuclides like 232 Th and 238 U, making it difficult for estimation in

Trace levels in environmental and biological samples. The estimation of radioactivity of these nuclides by gamma ray spectrometry is based on the activities of gamma active daughter products. For the present study, we have selected soils samples where these nuclides are present in trace levels and are in equilibrium with their daughter products.

For the NAA analysis of 232 Th and 238 U, elemental standards of 10 μg uranium and 25μg thorium were coirradiated along with six soil samples. The mass of soil samples considered for the activity estimation are about 300 mg. The soil samples were properly weighed and double sealed before the irradiation at A8 position of APSARA reactor with a flux of 7.5 × 10 11 n/s/cm for 7 hours. After irradiation, the samples were allowed for a 24 hours cooling to decay the short lived radionuclides. Later these samples were counted by 50% HPGe detector at a distance of 10 cm. The gamma energies considered for the activity estimation of 238 U and 232 Th are 228 and 277 keV of 239 Np and 312 and 340.8 keV of 233 Pa, respectively.

For the estimation of 238 U and 232 Th by gamma spectrometry methods, the soil samples were analyzed using a precalibrated high resolution gamma spectrometry system. The detector used is a p-type coaxial HPGe (EURISYS measures) with 50% relative efficiency and the resolution obtained is 1.9 keV at 1332 keV of 60 Co peak. Each sample was dried, powdered, packed in 250 ml plastic container (7 × 7.5 cm), weighed and stored for a month to attain radioactive equilibrium between 226 Ra and its daughter products. The efficiency calibration was done using an IAEA standard U-ore (RGU-1) in the geometry similar to that of the samples. Each sample was counted for a period of 50,000 seconds. Since the 238 U and 232 Th are in equilibrium with daughter products in the sample, prominent gamma energy lines from daughter nuclides are considered for the activity estimation. The gamma energy lines used for the 232 Th activity estimation are 583 keV of 208 Tl and 911 keV of 228 Ac. Similarly, 352 keV of 214 Pb and 609 keV of 214 Bi are used for the activity estimation of 238 U. A typical gamma ray spectrum of soil sample is shown in [Figure 1].
Figure 1: Typical gamma ray spectrum of soil sample

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In case of the neutron activated soil samples, the mass of 238 U and 232 Th in samples are estimated by comparing the spectral data from that of standard sources of uranium and thorium coirradiated with samples. The activities of 232 Th and 238 U are calculated using the specific activities of 238 U and 232 Th. In case of gamma spectrometry measurements, the activity estimation of 232 Th and 238 U was done using the count rate and branching intensities of the corresponding gamma energy lines of uranium and thorium daughters. The results of the activity analysis for the soil samples by direct gamma spectrometry and NAA technique are summarized in [Table 1].
Table 1: 238U and 232Th activities estimated using gamma spectrometry and NAA

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In both cases, the concentration of uranium and thorium were reported in Bq/kg. The source uncertainties in these measurements are (i) the weights of samples; (ii) in the detection efficiency; (iii) heterogeneity in the samples; and (iv) sample counting. The uncertainties quoted along with the activity values for gamma spectrometry measurements, given in [Table 1] are only due to counting and efficiency. The uncertainty values are evaluated for NAA measurement for all samples based on counting statistics, uncertainty in efficiency, and mass of the samples.

A correlation study was done with the radioactivity values estimated by direct gamma spectrometry and NAA technique for uranium and thorium. The correlation curve with activity values estimated by both methods for uranium and thorium are shown in [Figure 2] and [Figure 3], respectively.
Figure 2: Activity correlation curve for uranium

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Figure 3: Activity correlation curve for thorium

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The activity data obtained from our present study for soil samples by direct gamma spectrometry and NAA technique in [Table 1] are compared with similar data obtained from the literature in different places in India and are shown in [Table 2].
Table 2: Comparison of activity concentrations of 238U, 232Th, and 40K in surface soil samples of different places in India

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  Results Top


For the direct gamma spectrometry analysis, the samples were prepared as per the standard procedure and sealed in a 250 ml container geometry. These samples are counted in a well shielded HPGe-based gamma ray spectrometry system for a counting time of 50,000 seconds. The activity of 238 U and 232 Th isotopes were estimated based on the assumption that all daughter products are in equilibrium with the parent in the soil samples considered. Whereas for the NAA, about 300 mg of each sample, after irradiation were subjected to gamma spectrometry at a distance of 10 cm. The 238 U and 232 Th activities (in Bq/kg) in samples were estimated from the activities of 237 Np and 233 Pa formed after irradiation by comparison with the standards, uranium and thorium coirradiated with the sample. In the present study we carried out measurement using environmental samples where uranium and thorium are present in the trace levels (ppm). Also the quantity of the sample taken for NAA measurement is approximately 300 mg and assumed as representative of the sample volume (250 ml) considered for gamma spectrometry measurements. There may be significant amount of errors arising from heterogeneity of the samples. Because of the reactor availability problem, we could not carry out repeated measurements and the data presented were from single measurement. The data presented is from the preliminary study and further detailed systematic studies will be carried out to improve the correlation.


  Acknowledgment Top


Authors express their sincere thanks to Dr. A. K. Ghosh, Director, HS and E Group BARC for his continuous encouragement in this work.[6]

 
  References Top

1.DAE-BRNS discussion meet on current trends and future perspectives of neutron activation analysis-CFNAA, 2006, Mumbai, India.  Back to cited text no. 1
    
2.Kannan V, Rajan MP, Iyengar MA, Ramesh R. Distribution of natural and anthropogenic radionuclides in soil and beach sand samples of Kalppakkam (India) using hyper pure germanium (HPGe) gamma ray spectrometry. Appl Radiat Isot 2002;57:109-19.  Back to cited text no. 2
    
3.Vijayan V, Behera SN. Study of natural radioactivity in soils of Bhubaneshwar. Eighth National Symposium on Environment. Kalppakkam, India: Indira Gandhi Centre for Atomic Research; 1999. p. 146-7.  Back to cited text no. 3
    
4.Verma PC, Gurg RP, Sundaram M, Sharma LN. Natural Radioactivity in Rawatbhata and Narora Soils. 7 th National Sym. On Environment, Dhanbad, India: Indian School of Mines; 1998. p. 132-4.  Back to cited text no. 4
    
5.Mishra UC, Sadasivan. Natural Radioactivity levels in Indian Soils. J Sci Ind Res 1971;30:59-62.  Back to cited text no. 5
    
6.UNSCEAR-Report 1988: Sources and effects and Risks of Ionizing Radiations. New York: United Nations; 1988.  Back to cited text no. 6
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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