|Year : 2011 | Volume
| Issue : 4 | Page : 253-256
Performance of LaCl 3 detector for environmental radioactivity measurements
K Narayani, S Anilkumar, AK Rekha, D. A. R. Babu
Radiation Safety Systems Division, Bhabha Atomic Research Centre (BARC), Mumbai, India
|Date of Web Publication||17-Jan-2013|
Radiation Safety Systems Division, Bhabha Atomic Research Centre (BARC), Mumbai
Source of Support: None, Conflict of Interest: None
Assessment of natural radioactivity in the environment using gamma spectrometry is common method. NaI (Tl) detectors and HPGe detectors are used for gamma spectrometry in general. The recent arrival of Lanthanum Halide detectors have better resolution compared to NaI (Tl) detectors. In the present work, the performance of Lanthanum Chloride (LaCl 3 ) detector for the natural radioactivity estimation is studied and compared with NaI (Tl) and HPGe detectors.
Keywords: Gamma spectrometry, lanthanum chloride, least square method, natural radioactivity
|How to cite this article:|
Narayani K, Anilkumar S, Rekha A K, Babu D. Performance of LaCl 3 detector for environmental radioactivity measurements. Radiat Prot Environ 2011;34:253-6
|How to cite this URL:|
Narayani K, Anilkumar S, Rekha A K, Babu D. Performance of LaCl 3 detector for environmental radioactivity measurements. Radiat Prot Environ [serial online] 2011 [cited 2020 Jun 6];34:253-6. Available from: http://www.rpe.org.in/text.asp?2011/34/4/253/106182
| 1. Introduction|| |
Environmental radioactivity estimation is very important for the assessment of radiological impact of various nuclear facilities to the environment. Radioactivity in the environment is mainly due to the naturally occurring radionuclides like uranium, thorium with their daughter products and potassium. Gamma ray spectrometry is the commonly used analytical technique for the measurement of natural radioactivity in various environmental matrices. Scintillation based NaI (Tl) detectors and HPGe detectors are used for this purpose. Recently Lanthanum Halide detectors (LaCl 3 and LaBr 3 ) are commercially available which are having better resolution than NaI (Tl) and can be operated at room temperature. , In the present work Lanthanum Chloride (LaCl 3 ) detector is used for studying its performance for the natural radioactivity measurements.
| 2. Materials and Methods|| |
38 × 38 mm LaCl 3 (0.9Ce) (Make SCIONIX, Holland) scintillator detector having resolution of 4.0% at 662 keV was used. The detector was shielded by 3" lead on all sides. 138 La is a naturally occurring radioisotope of La with 0.09% abundance and has the decay scheme  shown in [Figure 1]. In 66.4% of its decays, 138 La undergoes electron capture (EC) to produce excited 138 Ba, which in turn decays by emission of a 1436 keV gamma. A necessary byproduct of electron capture is refilling of the electron shell which results in emission of coincident barium X-rays in the 35 keV region. The remaining decays, 33.6%, proceed by beta emission to 138 Ce, which decays by emitting a 789 keV gamma in coincidence with the beta having end point energy of 255 keV.
Because of the inherent radioactivity in the detector the background spectrum is normally dominated by components of 138 La decay. [Figure 2] shows the background spectrum acquired from LaCl 3 detector for 50000 sec. In the spectrum, the low energy (0 to 255 keV) is due to beta continuum. From 255 to 750 keV the spectrum displays the Compton continua from the 789 and 1436 keV gamma rays. Next is 789 keV gamma line but since it is in coincidence with the beta, it is smeared to high energy in a gamma plus beta continuum ending a little above 1 MeV. Then we see the 1436 keV gamma, but displaced to higher energy by approximately 37 keV to 1473 keV due to coincident capture of X-rays resulting when the Ba K level fills following K-electron capture. Similarly, the hump near 1441 keV on the low energy side of the 1473 line is due to the 1436 keV gamma plus 5 keV due to coincident capture of X-rays when the Ba L-level fills following L-electron capture. Above 1750 keV, the presence of low level alpha contaminants is revealed. These peaks have been shown to result from 227 Ac contamination. Thus the background spectrum obtained from LaCl 3 detector is complex spectrum and background components due to natural radioactivity is not visible. In order to have comparison, background spectrum using 3" × 3" NaI (Tl) detector is shown in [Figure 3] along with background spectrum of LaCl 3 detector.
As a common practice for the analysis of natural uranium, thorium and potassium using scintillation detectors, high energy gamma lines from the daughter products of uranium and thorium are used assuming that uranium and thorium are in equilibrium with their daughter products. 1764 keV gamma line from 214 Bi is taken for uranium and 2614 keV gamma line from 208 Tl is taken for thorium. 1460 keV gamma from 40 K is taken for potassium. Because of the intrinsic radioactivity components in the background spectrum of LaCl 3 , it is difficult to analyze peaks due to high energy photons from uranium and thorium daughters and from potassium. For the analysis of spectra from uranium, thorium and potassium using La detector, we have adapted the response function based method known as Least squares method which is commonly used for the analysis of NaI (Tl) scintillation detector spectra. , This method takes into account the data from each and every channel. The principle of least squares requires the sum of the square of the random errors for the individual channel be a minimum. When a radioactive sample is counted, the net counts after background subtraction in any channel is the function of the activity of each nuclide present in the sample. The channel efficiencies or Compton fraction for each of the individual nuclides in the particular channel is determined by counting standards of each nuclide for specific geometry. The unknown concentrations of the different nuclides contributing counts to the sample spectrum can be analyzed using a computer program.
PVC cylindrical box (7.5 cm × 7.0 cm) geometry is used for the measurement of samples as well as standards. Standard reference materials of uranium, thorium (RGU, RGTh) and KCl are selected as standards for radioactivity estimation. Background spectrum was collected for 50,000 sec. Each standard spectrum was acquired for 10000 sec. Using the background and standard spectra, the Compton fractions were calculated for all standards with the help of the computer program. Various environmental samples like soil, granite and rock were counted for 50000 sec. each. After background subtraction and using the Compton fractions, the peak area in the spectrum due to uranium, thorium and potassium in each sample was estimated with the help of the least squares computer program. Using the corresponding efficiency, the activity of uranium, thorium and potassium in the samples were estimated.
To validate the performance of the LaCl 3 detector, the same samples in the same geometry were analyzed by NaI (Tl) detector and HPGe detector. 3" × 3" NaI (Tl) detector and P type HPGe detector with 50% Relative Efficiency were used for the purpose. In both the detectors, the samples were counted for 50,000 sec. each. In the case of NaI (Tl) detector, sample spectra were analyzed with the help of Least square method. The same reference standards were used for efficiency calibration. [Figure 4] shows the soil spectrum obtained from LaCl 3 detector as well as NaI (Tl) detector. In the case of LaCl 3 detector, higher energy peaks due to uranium and thorium are not visible. At the same time for spectra obtained from NaI detector, the higher energy peaks due to uranium and thorium are clearly visible. In the lower energy region, for LaCl 3 spectrum, the peaks due to uranium and thorium are masked by beta continuum in the detector background. By using the least squares method for LaCl 3 spectrum analysis, natural radioactivity measurements were carried out in environmental samples.
In the case of HPGe detector efficiency calibration was done with RGU for the same geometry. The gamma energies from daughter products of uranium and thorium were used for the activity estimation. 352 keV from Pb-214, 609 keV and 1764 keV from Bi-214 were taken as the index for uranium. 238 keV from Pb-212,583 keV from Tl-208 and 911 keV from Ac-228 were taken as the index for thorium and 1460 keV was taken for potassium. The activity estimated was tabulated. [Table 1] shows the natural activity estimated in the samples by using the three different detectors and spectrum analysis methods. The activity estimated using LaCl 3 detector is in close agreement with the activity estimated by NaI (Tl) detector and HPGe detector. However the uncertainty in activity calculated from LaCl 3 spectra is more compared to the uncertainty obtained from the other two detectors. This may be due to low efficiency and high MDA (Minimum Detectable Activity) for LaCl 3 detector. MDA is generated from the background for the particular energy region. Since LaCl 3 detector is having very high self-background, the MDA is also very high. [Table 2] shows the MDA calculated for uranium, thorium and potassium using the three detectors.
| 3. Conclusion|| |
In the present work the performance of LaCl 3 detector was studied for its use in the estimation of natural radioactivity in environmental samples. Using LaCl 3 detector natural activity in various soil and granite samples was estimated. The results were compared with the results obtained from NaI (Tl) detector and HPGe detector. Though the results are matching the uncertainty was very high. This is due to low efficiency and high MDA because of which the application of these detectors for low level activity estimation is limited. These detectors can be used for the applications where the activity levels are high. But low level radioactivity levels can also be estimated with moderate accuracy by careful analysis of the spectra using least squares method. Normal spectrum analysis based on peak region will not give the correct results.
| 4. Acknowledgement|| |
Authors are thankful to Dr. A. K. Ghosh, Director, HS and E Group and Dr. D. N. Sharma, Associate Director, HS and E Group and Head, RSSD for their interest and support in this work.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]