|Year : 2015 | Volume
| Issue : 1 | Page : 11-13
Measurement of natural radioactivity in rock samples using gamma ray spectrometry
Department of Applied Physics, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India
|Date of Web Publication||14-Aug-2015|
Department of Applied Physics, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh - 202 002, Uttar Pradesh
Source of Support: Nil., Conflict of Interest: None
Natural radioactivity due to 226Ra, 232Th and 40K in the rock samples collected from Dhanbad city of Jharkhand, India was measured using gamma-ray spectrometry. The concentrations of 226Ra, 232Th and 40K in the studied rock samples range from below the detection limit to 3.08 Bq/kg, 5.35-37.39 Bq/kg and 168.8-416.9 Bq/kg, respectively. The concentrations of these radionuclides are compared with the recommended values. To evaluate the radiological impact of the natural radioactivity, the radium equivalent activity, the annual effective dose rate, the values of both external and internal indices and the gamma index were estimated and compared with the internationally acceptable values.
Keywords: Annual effective dose, gamma ray spectrometer, rock samples, radium equivalent activity
|How to cite this article:|
Zubair M. Measurement of natural radioactivity in rock samples using gamma ray spectrometry. Radiat Prot Environ 2015;38:11-3
|How to cite this URL:|
Zubair M. Measurement of natural radioactivity in rock samples using gamma ray spectrometry. Radiat Prot Environ [serial online] 2015 [cited 2018 Dec 14];38:11-3. Available from: http://www.rpe.org.in/text.asp?2015/38/1/11/162820
| Introduction|| |
Studies and assessment of natural environmental radiation are of high importance. The advanced growth of the nuclear industry and ever- increasing use of radiation and radioactive isotopes makes it necessary to evaluate the background natural radiation in order to detect any increase in the levels due to the human activities involving radionuclides. At the same time, since natural radiation is the main source of human exposure, studies of the dose from this source and its effects on health improve the understanding of radiation damage. For these purposes, nationwide surveys have been made in many countries to determine the concentration of the natural terrestrial radionuclides.,
| Materials and Methods|| |
In the present study, the rock samples are collected from the market, these rock samples is used for dwelling construction. The samples have been crushed into a fine powder and sieved through a 100 µm mesh sieve. About 500 g of the samples were sealed in a Can of 500 ml capacity to allow uniform distribution of 220Rn and 222Rn and their decay products. These sample containers were stored for a period of one month before gamma spectrometric analysis, so as to allow the establishment of secular equilibrium between 226Ra, 232Th and their decay products. The radionuclides: 226Ra, 232Th and 40K were measured in soil samples using a gamma ray spectrometer with NaI(Tl) scintillation detector based gamma spectrometer at Hemwati Nandan Bahuguna Garhwal University, Srinagar (Garhwal). This technique has been discussed by Ramola et al. The prepared rock samples were placed in shielded gamma ray spectrometry unit for a counting time of 3 h. The measurement of natural radionuclides in rock samples was carried out using a NaI(Tl) gamma radiation detector of size 63 mm × 63 mm with a multichannel analyzer. The activity of 40K was evaluated from the 1460 keV photo peak; the activity of 226Ra from the 1764 keV gamma line of 214Bi; and 232Th from the 2610 keV gamma line of 208Tl. This spectral analysis was performed with the aid of computer software SPTR-ATC (AT-1315). The peak energies of the gamma spectra were measured in reference to the 661 keV photopeak of 137Cs. The activity concentrations of these rock samples were calculated from the intensity of each line in the spectrum, taking into account the mass, the geometry of the samples, the counting time and the efficiency of the detector. The detection limits of different radionuclides are presented in [Table 1].
|Table 1: Minimum detection limits for a 500 g sample and counting time of 10800 s|
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It is assumed that all the decay products of 226Ra and 232Th are in radioactive equilibrium with their precursors. The Raeq is calculated according to the following equation:
where, CRa, CTh and CK are the specific activities Bq/kg of 226Ra, 232Th and 40K, respectively. This equation is created on the valuation that 370 Bq/kg of 226Ra, 259 Bq/kg of 232Th or 4810 Bq/kg of 40K produce the same gamma ray dose rate.
The absorbed dose rates nGy/h in outdoor air from terrestrial gamma radiation at 1 m above the ground were calculated using the following equation by the UNSCEAR.
The annual effective dose expected to be received by the general public due to the radioactivity in rock was calculated using a conversion factor of 0.7 SvGy−1, to convert the absorbed dose rate to the effective dose with indoor and outdoor occupancy factor of 0.8 and 0.2, respectively. The annual effective doses are determined as follows:
The internal and external hazard indexes are calculated by the following expressions:
| Results|| |
Gamma ray spectrometry has been used to determine the radioactivity concentrations of 226Ra, 232Th and 40K in the rock samples of Dhanbad city, Jharkhand (India) are shown in [Table 2]. Equations (1-6) have been used. High value of 226Ra was found in sample number 1 and high value of 232Th was found in sample number 5, whereas high value of 40K was found in sample number 4. The values of thorium (sample number 5) and potassium (sample number 4) was found quite higher than the reported values of UNSCEAR (8), which are 35 Bq/kg and 400 Bq/kg, respectively. The radium equivalent activity, indoor effective dose and outdoor effective dose are found to vary from 39.8 to 67.8 Bq/kg, 0.11 to 0.15 mSvy−1 and 0.027 to 0.04 mSvy−1, respectively. These values are below the permissible limits. The values of external, internal hazard index and Gamma index are varying from 0.11 to 0.18, 0.047 to 0.19 and 0.17 to 0.25, respectively and are found less than unity. On the basis of the radium equivalent activity, the representative level index, the indoor absorbed dose rate and the annual effective dose, all the rock samples do not pose any significant source of radiation hazard and the use of the rock samples in construction of dwellings is considered to be safe for inhabitants.
|Table 2: Measured values of 226Ra, 232Th and 40K activity (Bq/kg) radium equivalent activity (Bq/kg), indoor and outdoor effective dose rate (mSv), external and internal hazard index and gamma index in rock samples|
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| Conclusions|| |
The results obtained have shown that the radium equivalent activity due to natural radioactivity of 51.34 Bq/kg in the rocks in the two states is very low compared to the world average of 370 Bq/kg. This indicates that the level of the radioactivity concentrations in rocks around these areas is low being about 14% of the world average and about 14% of the value obtained for Ondo and Ekiti.
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[Table 1], [Table 2]