Gamma spectrometric analysis of ore samples from Sandur of Bellary, Karnataka, India

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1 J Radioanal Nucl Chem DOI /s Gamma spectrometric analysis of ore samples from Sandur of Bellary, Karnataka, India B. R. Kerur T. Rajeshwari S. Anilkumar K. Narayani A. K. Rekha Received: 17 December 2011 Ó Akadémiai Kiadó, Budapest, Hungary 2012 Abstract In the present paper the gamma spectrometric technique has been used to study the natural radioactivity levels present in the iron ore samples collected from the Sandur of Karnataka, India. This region, a member of Dharwar craton, known as Sandur schist belt is rich in Manganese and Hematite ore (Fe). The entire belt is characterized by numerous bands of banded magnetite and hematite quartzite. The ore samples were collected and prepared as per the ASTM standard procedures. The gamma spectrometric measurements were performed using a HPGe detector (Eurosys) based high-resolution 8k multi channel analyzer. The detector has a relative efficiency of 50% and resolution of 2.0 kev for MeV gamma energy of 60 Co. The output of the detector was analyzed using a PC based 8k channel analyzer system. The mean activity concentrations of the 238 U, 232 Th and 40 K in the ore samples were found to be 14.59, and Bq/kg, respectively. The mean absorbed dose rate and the annual dose equivalent were estimated to be ngy h -1 and lsv y -1, respectively, which is lower than the world recommended average of 1 msv y -1. The present work concludes that the natural radioactivity levels in ore samples of this region is within the natural limits and pose no health hazard to the public. Keywords Natural radioactivity Background radiation Radioactive equilibrium Gamma Spectrometry HPGe detector B. R. Kerur (&) T. Rajeshwari Department of Physics, Gulbarga University, Gulbarga, India kerurbrk@yahoo.com S. Anilkumar K. Narayani A. K. Rekha Radiation Safety Systems Division, BARC, Trombay, Mumbai, India Introduction Natural radioactivity measurement and assessment is very important aspect from both health and awareness point of view. Natural radioactivity is present everywhere on earth. Human beings and the other living organisms are continuously being exposed to these radiations knowingly or unknowingly. The source of natural radiation can be either extraterrestrial (cosmic) or terrestrial. It is well known from earlier studies that the important source of radiations are the a, b and c, the ionizing radiations, emitted from the naturally occurring radionuclides such as 238 U, 232 Th decay series and singly occurring 87 Rb, 40 K etc. [1, 2]. The environment where we live contains these gamma-emitting radionuclides which are found in soils, rocks, ores, vegetables, water and in building materials and this is a significant contributor to the collective dose rate to human beings [3]. The environmental radioactivity and associated external exposure due to gamma radiation from environmental materials like ores, soil, rock, water etc., depend mainly on the geological formations and rock compositions of the region [1, 4]. High natural background radiation area is found primarily in Brazil, India, China and Ramsar in Iran [5 8]. It is well known that the highest background radiation areas in India are the coastal regions of Kerala, Karnataka, Tamilnadu and Orissa. The present article highlights the study performed over the ore samples collected from the Sandur mines of Bellary district. Bellary, a district of North East Karnataka lies at latitude N and longitude E. The study region Sandur lies at latitude N and E. Sandur and surrounding regions are rich source of high grade hemetite (Fe) and manganese (Mn) ore and is geologically known as Sandur Schist belt [9, 10]. The geological map of the study region is shown in Fig. 1. Due to the abundance of Fe and

2 B. R. Kerur et al. Fig. 1 Geological map of the study region showing the Sandur Schist belt Mn ore a large number of mining activities have emerged and are continuously going on. Mining operations in such Fe and Mn rich regions changes the levels of natural radioactivity in the environment. The change in the environmental radioactivity contributes to the collective radiation dose to the general population. These Fe and Mn mining activities have disturbed the ecology and environmental balance of the region [11, 12]. Hence, our interest lies in assessing the radionuclides and estimating their concentration in the environmental materials (ores) of the region. For the purpose, ore samples collected from Sandur mines have been studied for the three radionuclide contents ( 238 U, 232 Th and 40 K) by low background gamma spectrometric technique using high purity germanium (HPGe) detector. The study will provide a reference data on natural background radiation concentration for the region, as there is lack of similar studies in the area. The study also estimates the external gamma dose rate due to natural radioactivity, which is relevant to public awareness and provides information relevant to radiological protection to the people. Experimental methods Sample collection and preparation A total of sixteen ore samples were collected from the Iron and Manganese ore mining areas of Sandur region. The details of the samples are given in Table 1. The samples were placed in a hot air oven for drying at 110 C for 24 h to ensure that the moisture was completely removed. All samples were pulverized to a fine powder and sieved through a 200-mesh screen to separate the unevenly crushed particles. Each pulverized sieved sample was then transferred to a 250 ml cylindrical plastic container. Each sample was coded according to the origin and location of the sampling site. The containers were filled fully, sealed with an adhesive, named, weighed and stored for a period of 4 5 weeks to attain secular equilibrium between 226 Ra and 222 Rn and its daughter products before subjecting them to gamma spectrometric analysis. Gamma ray spectrometry and calibration The ore samples in the present work were analyzed for the activity of 232 Th, 238 U and 40 K by using a high-resolution gamma spectrometry system. The gamma spectrometric system comprised of a HPGe detector, amplifier and PC based MCA. The detector is a coaxial 190 cc P-type high purity germanium detector (manufactured by Eurisys Mesures, France). The detector has a resolution of 2 kev for MeV gamma ray energy for 60 Co. The detector has a relative efficiency of 50% compared to NaI (Tl) detector. The output of the detector was analyzed using a PC based 8k multichannel analyzer system. The detector was surrounded throughout by 3 00 thick lead shield to reduce the surrounding background natural radiation [13]. The efficiency calibration of the system was carried out using IAEA standard source of uranium ore (RGU-1) in

3 Gamma spectrometric analysis of ore samples Table 1 Table showing the details of the ore samples Sl. no. Sample Type of the sample Collection depth (above MSL) Mining area 1 MN Shale (red) 945 m Donimalai mines 2 MN Shale (red) 981 m 3 MN Limonite ore 993 m 4 MN Lateritic ore 993 m 5 MN Fe ore 1,005 m 6 MN Shale (white) 1,005 m 7 MN Shale (red) 1,005 m 8 MN Blue dust 1,005 m 9 MN Mixed (ore? shale) 1,005 m 10 MN Limonite ore 900 ft Deogiri mines, 11 MN Ore waste (low grade Fe? soil) 900 ft Sandur 12 MN White clay Surface 13 MN Fe ore 900 ft 14 MN050693A Fe ore 900 ft 15 MN Mn ore 900 ft 16 MN050694A Mn ore 900 ft geometry available for sample counting. The source was counted for sufficiently long time for good statistics. Photopeak areas of the most prominent and well separated photopeaks were calculated. For efficiency calibration ten prominent gamma energies from 226 Ra, 214 Pb and 214 Bi are used [14]. From these photo-peak areas the respective photo-peak efficiencies for different energies were calculated using the relation g E ð% Þ ¼ Area under the photopeak (cps) ; Activity (Bq:ÞBIð%Þ where BI is the branching intensity for the particular energy E (kev). The efficiency (%) for each gamma energy E (kev) was calculated and fitted using a semi-empirical mathematical function of the form LnðEffÞ ¼A þ BLnðEÞþCðLnðEÞÞ 2 where A, B and C are fitted coefficients whose values are A = , B = , C = Using the above function the efficiency for different energies can be calculated. Activity measurement Each sample was measured for an accumulating period of 50,000 s (13.88 h). Assuming the daughter products of U and Th were in equilibrium, the 238 U activity of a sample was estimated from the gamma emissions of 214 Pb (352 kev) and 214 Bi (609.3, 1764 kev). The gamma ray energies from 212 Pb (238.6 kev), 228 Ac (911 kev) and 208 Tl (583.2 kev) were used to estimate the concentration of 232 Th. The activity concentration of 40 K was measured directly using its own gamma ray (1,460 kev). The activity concentrations were calculated from the intensity of each gamma line, taking into account the mass of the sample, the branching ratio of the c-decay, the time of counting and the efficiency of the detector. Activity concentrations, calculated from the intensity of several c-rays emitted by a nuclide using the relation Activity ðbq=kgþ Net area under the photopeak (cps) ¼ ; Efficiencyð%ÞBIð%Þm where BI is the intensity of gamma-line in a radionuclide and m is the mass of the sample in kg. Results The obtained results of the activity concentration of the ore samples are shown in Table 2. The error values of the activity concentrations represent the standard deviation derived on the basis of counting statistics. The activity concentrations of 238 U ranged from below detection limit (BDL) to 42 Bq kg -1, with an average of Bq kg -1, 232 Th ranged from BDL to 48 Bq kg -1, with an average of Bq kg -1 and 40 K ranged from BDL to 97 Bq kg -1, with an average of Bq kg -1, respectively. The minimum detectable activity of the radionuclides 232 Th, 238 U and 40 K is 1.43 Bq kg -1, 1.06 Bq kg -1 and Bq kg -1, respectively. The estimated values of concentrations of the radionuclides in ore samples from the Sandur mines were found within the range of worldwide

4 B. R. Kerur et al. values [6]. Hence, these radioactivity levels do not pose any radiation risk for the people working in the mines. Correlations were studied between 238 U and 232 Th for ore samples shown in Fig. 2. Uranium and thorium activity in the samples demonstrated a positive correlation with a coefficient of showing their geochemical coherence. Dose assessment The absorbed dose rate due to gamma radiation is a considerable parameter for the risk and hazards. The outdoor air absorbed gamma dose rates due to terrestrial gamma rays at 1 m above the earth s surface for the samples under study were calculated from the concentrations of 232 Th, 238 U and 40 K with conversion factors of 0.604, and , respectively [6] D ¼ ð0:604c Th þ 0:462C U þ 0:0417C K ÞnGy h 1 ; where C Th, C U and C K are the average activity concentrations (Bq/kg) of 232 Th, 238 U and 40 K respectively. The estimated average absorbed dose rates for all the samples from the various sampling stations are shown in Table 3. The annual effective dose was estimated using a conversion factor of 0.7 Sv Gy -1 to convert the absorbed dose rate to annual effective dose with an outdoor and indoor occupancy of 20 and 80% respectively, as given by UNSCEAR [6]. Annual effective dose ðlsv y 1 Þ ¼ D ngy h h y 1 0:7 0: Concentration of Th-232 in Bq/kg The calculated results are presented in Table 3. The outdoor absorbed dose rates due to gamma radiation was found to be in the range of ngy h -1 with a mean of ngy h -1 for the ore samples, whereas the mean annual effective dose was estimated to be lsv y -1. Hazard indices Graph of Th-232 Vs. U-238 for Mines ore samples. R= Concentration of U-238 in Bq/kg Fig. 2 The correlation between 232 Th and 238 U concentrations for the ore samples In order to assess the gamma radiation hazards associated with these samples, a commonly used radiation hazard index, the Radium equivalent activity or concentration (Ra eq ) was also estimated for all the samples which are a Table 2 The activity concentrations of 232 Th, 238 U and 40 K determined in ore samples of Sandur mines of Bellary district Sl. no. Sample Weight (g) 232 Th (Bq/kg) 238 U (Bq/kg) 40 K (Bq/kg) 1 MN ± ± ± MN ± ± ± MN ± ± 2.89 BDL 4 MN ± ± ± MN ± ± 1.06 BDL 6 MN ± ± 1.86 BDL 7 MN ± 0.98 BDL BDL 8 MN BDL BDL BDL 9 MN ± ± 1.08 BDL 10 MN ± ± 2.20 BDL 11 MN ± ± ± MN BDL BDL BDL 13 MN ± ± 1.44 BDL 14 MN050693A ± ± 1.73 BDL 15 MN ± ± ± MN050694A ± ± ± Mean Minimum detectable activity (MDA)

5 Gamma spectrometric analysis of ore samples Table 3 The mean gamma ray absorbed dose rate and dose related parameters for the mines ore samples Sl. no Sample Absorbed dose rate (ngy h -1 ) Effective dose equivalent (lsv y -1 ) Annual dose equivalent (lsv y -1 ) Dose equivalent indoor (lsv y -1 ) Ra eq (Bq/kg) H ex H in 1 MN ± MN ± MN ± MN ± MN ± MN ± MN ± MN \2.04 \12.51 \2.50 \10.01 \4.381 \0.01 \ MN ± MN ± MN ± MN \2.04 \12.51 \2.50 \10.01 \4.381 \0.01 \ MN ± MN050693A ± MN ± MN050694A ± Mean MDA weighted sum of activities of the three radionuclides. It is defined as, Ra eq ¼ C U þ AC Th þ BC K ; where C U, C Th and C K are the activity concentrations of 238 U, 232 Th and 40 KinBqkg -1, respectively, and A, B are constants determined as 1.43, 0.077, respectively. It is assumed that 370 Bq/kg of 238 U, 259 Bq/kg of 232 Th and 4,810 Bq/kg of 40 K produce the same gamma radiation dose rate. The present study estimated the mean radium equivalent as Bq kg -1. The results obtained are shown in Table 3. The mean Ra eq for the mines ore samples was estimated to be Bq kg -1, which is very much less than the recommended value of 370 Bq kg -1. The value of Ra eq greater than 370 Bq kg -1 poses a higher gamma dose rate. Radium equivalent is related to the external gamma dose and internal dose due to radon and its daughter products present in the environment, and has the largest contribution towards the average effective dose received by human beings. The radiation hazard in terms of external and internal radiation hazard indices were calculated using the relations given by [15] H ex ¼ C U 370 þ C Th 259 þ C K 4810 H in ¼ C U 185 þ C Th 259 þ C K 4810 where C U, C Th and C K are the activity concentrations of 238 U, 232 Th and 40 K in Bq kg -1, respectively. For insignificant radiation hazard the indices should be less than unity or 1 [16]. The data from the present study is well in accordance with the condition of unity and pose that the ores of the study area is safe from radiation hazards. The radiation hazard indices for all the ore samples are given in the Table 3. The variations in the hazard indices might be due to the distribution of the radionuclides in the ores of the region but the indices remained less than unity. Conclusion The ore samples collected from the Sandur mines, Bellary were studied for the concentrations of the three primordial radionuclides using gamma ray spectrometry. The c-ray spectrometric analysis showed that, as the concentrations of the radionuclides are within the worldwide range. Also, the health hazard indices for all the samples were within the limits. The present data can be referred as a reference data in future measurements and assessments. The present data provides the natural radioactivity in ore produced by the mines of the study area. Still a lot of work is being carried in this region for better results. Acknowledgments The authors are grateful to Board of Research in Nuclear Science (BRNS) for providing the financial support to carry out this work. The Author (RT) acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi, for awarding her Senior Research Fellowship to carry out the work successfully. The authors are also thankful to Dr. D N Sharma, Head, RSSD, BARC, Mumbai for his continuous guidance and encouragement for the work.

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