J Radioanal Nucl Chem (2014) 300:61–65 DOI 10.1007/s10967-014-2947-y
Natural radioactivity studies of Bidar soil samples using gamma spectrometry T. Rajeshwari • S. Rajesh • B. R. Kerur S. Anilkumar • Narayani Krishnan • Amar D. Pant
•
Received: 27 December 2013 / Published online: 22 January 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract The activity of the terrestrial primordial radionuclides 226Ra, 232Th and 40K are measured for Bidar soil samples. The collected soil samples are analyzed using HPGe detector based on high resolution gamma spectrometric system. The activity of the three radionuclides 226Ra, 232 Th and 40K were found to be in the range of BDL–47.68, 7.65–59.08 and BDL–260.65 Bq kg-1 respectively. The mean gamma absorbed dose rate in air above 1 m from ground is estimated to be 34.47 nGy h-1. Annual effective dose equivalent and the radium equivalent activity were within the limits in the present study and it is found that the activity of the radionuclides are comparable with the worldwide literature values. Also the external hazard indices for the soil samples of Bidar district were within the limit of unity. Keywords Natural radioactivity Gamma spectrometry Absorbed dose rate Annual effective dose
Introduction Radiation is present everywhere and man is knowingly or unknowingly being continuously exposed to radiations present in environment (natural). The natural radiations surrounding the life on earth can either be terrestrial or extraterrestrial (cosmic) origin. The ionizing radiations a, b T. Rajeshwari S. Rajesh B. R. Kerur (&) Department of Physics, Gulbarga University, Gulbarga 585 106, Karnataka, India e-mail:
[email protected] S. Anilkumar N. Krishnan A. D. Pant Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
and c emitted from various terrestrial materials (soil, rock, sand etc.) coming from naturally occurring radionuclides such as uranium, thorium etc., their isotopes, their decay products and some singly occurring radionuclides such as 40 K, 87Rb contribute to the terrestrial radiations [1]. It is well known from earlier studies that the gamma radiations emitted from 226Ra, 232Th and 40K present in soil, rock, sand and other environmental materials contribute significantly to the collective dose received by organisms are due to ionizing radiations [2–4]. Also, the radiations can enter the human environment from some artificially prepared radionuclides such as 137Cs, 90Sr etc. resulting from the fallout of nuclear weapons during war, nuclear testing and other activities involving nuclear materials. The distribution of natural radionuclides is not uniform throughout the earth. It mainly depends on the geological formation, rock composition, chemical composition of the soil, geographical and climatic conditions of the region [5]. Accordingly, the extent of gamma activity of the radionuclides in the environment varies with region. The human beings are continuously exposed to the gamma radiations. The gamma radiations through external sources (both terrestrial and extraterrestrial) irradiate the body causing external hazard. The radiations can effect internally through the ingestion and inhalation of radioactive materials into the body. The ionizing radiations from soil are significant and contribute to the natural background radiation level. As the activity of 226 Ra, 232Th and 40K vary with region the dose received also will be varied. High background radiation areas have been found in some of the parts of the world as is evident from earlier studies where it was found to be 10–100 times the natural level due to high levels of radioactivity in soils, rocks and hot springs or due to high levels of indoor radon and its decay products [6–9]. Continuous exposure to radiation, even of low level, may bring out effects with
123
62
time. Estimates of total radiation dose to the world population have shown that about 96 % is from natural sources while 4 % is from artificial sources [10]. Since the natural radionuclides are not uniformly distributed the knowledge of the natural radioactivity in soil is important and essential for determination of population exposure to radiation. In this connection worldwide interest in the studies on measurements of natural radioactivity has increased and extensive work has been done in many countries. In the present study the radioactivity measurements have been carried out for Bidar soil samples. Bidar is a district of Karnataka covering between 17°350 –18°250 north latitudes and 76°420 –77°390 east longitudes with Bidar city lying at 17°550 1200 N and 77°310 1100 E. The entire district forms a part of the Deccan Plateau and is made up mostly of solidified lava [11]. Alluvial deposit is normally found along the banks of the Manjra river and its main tributaries. The top layers of the Deccan trap in parts of Bidar and Humnabad taluk are altered to reddish vesicular laterite, forming and extensive undulating plateau. The main minerals found in the area are bauxite, kaolin and red ochre. A deposit of highly siliceous bauxite clay has been located about 3 km south of Basavakalyan. The district is subdivided into five taluks or subdistricts namely Bidar, Humnabad, Basavakalyan, Bhalki, Aurad. Two types of soils founds in the district are lateritic red soil and black cotton soil. Aurad and Bhalki taluks have mainly black cotton soil. Bidar and Humnabad taluks have mainly lateritic red soil. Basavakalyan taluk has both types of soils. The activity of 226Ra, 232Th and 40K for Bidar district soil samples are measured using gamma spectrometry. The activity thus measured is used for determining the gamma absorbed dose and hazard indices. The data thus obtained will provide future researchers a baseline data for further environmental radioactivity research of the region. Also it helps in comparison and radiological mapping of the region.
Materials and methods Sample collection and preparation A total of 20 soil samples were collected from various locations of Bidar district. The ASTM standard procedure was followed for soil sample collection and preparation where surface soil over an area 50 cm 9 50 cm and 5 cm depth was mixed thoroughly and about 2–3 kg of each sample was collected. The collected samples were brought to the lab and each sample was cleaned by removing the dead leaves, roots or any other organic matter form the soil. The samples were then crushed to fine powder and sieved using 200 mesh size sieve. The fine powder of the sample
123
J Radioanal Nucl Chem (2014) 300:61–65 Table 1 Prominent gamma energies used for estimation of activity of radionuclides Parent nuclide
Gamma emitting nuclide
Gamma ray energy (keV)
Branching intensity of gamma decay (%)
226
214
Pb
352
35.10
214
Bi
Ra
232
Th
40
K
609.3
44.60
1,764
15.10
212
Pb
238.6
43.50
228
Ac
911
26.60
208
Tl
583.2
30.58
2,614.5
35.70
1,460
10.67
was placed in a hot air oven for drying at 110 °C for 24 h to ensure that moisture is completely removed. The dried pulverized fine powder of the sample was transferred to a 250 ml cylindrical plastic container. The containers were filled fully, sealed with an adhesive to prevent the escape of 222 Rn and 220Rn gas from the samples and kept for a period of 4–5 weeks to attain radioactive equilibrium between 226 Ra and 232Th and their daughter products. These samples were labeled on the basis of the origin they were collected and weighed using an electronic balance. After a month the samples were analyzed for gamma activity measurements of natural radionuclides using gamma ray spectrometry. Gamma ray spectrometry A high resolution HPGe detector based gamma spectrometric system was adopted for the sample counting. The detector is a co-axial p-type high purity germanium detector having a relative efficiency of 50 % and resolution of 2 keV for 1.332 MeV c-ray line of 60Co. The output of the detector was analyzed using a PC based 8k multichannel analyzer system (GAMMAFAST 5016). The detector was surrounded by 300 thick lead shield on all sides to reduce the background radiations originating from the walls and cosmic rays [12]. Efficiency calibration for the system was carried out using the IAEA standard (IAEA, RGU-1) uranium ore in the geometry available for sample counting. Efficiency values are plotted against energy for particular geometry and are fitted by least squares method to an empirical relation that takes care of the nature of efficiency curve for the HPGe detector. Each sample was counted for a longer period of 50,000 s and the obtained spectra were analyzed for the radionuclides activity. Prior to the sample counting background counts was recorded and subtracted to get the net count rate for each sample. A gamma transition of 1,460 keV for 40K was used for
J Radioanal Nucl Chem (2014) 300:61–65 Table 2 The activity of SL no.
226
Ra,
Sample name
232
Th and
63 40
K in Bq kg-1 and dose related parameters
Activity Bq kg-1 226
232
40
Ra
Th
K
Absorbed dose rate nGy h-1
Annual effective dose lSv year-1
Raeq Bq kg-1
Hex
1
SL01
BDL
7.65 ± 2.77
77.48 ± 2.77
10.16
12.46
21.90
0.06
2
SL02
17.85 ± 2.75
13.77 ± 3.04
BDL
17.80
21.83
39.82
0.11
3 4
SL03 SL04
14.79 ± 2.90 30.99 ± 3.13
12.26 ± 2.35 37.30 ± 3.01
43.55 ± 18.45 168.67 ± 0.29
16.05 43.88
19.68 53.81
35.68 97.32
0.10 0.26
5
SL05
30.73 ± 2.67
38.69 ± 2.50
172.64 ± 18.04
44.76
54.89
99.35
0.27
6
SL06
37.79 ± 3.02
46.24 ± 2.39
168.84 ± 19.31
52.43
64.30
116.91
0.31
7
SL07
21.78 ± 3.11
15.05 ± 2.10
94.36 ± 21.98
23.09
28.32
50.57
0.14
8
SL08
29.58 ± 4.09
27.68 ± 2.55
151.55 ± 18.49
36.70
45.01
80.83
0.22
9
SL09
8.27 ± 1.35
18.14 ± 1.94
126.17 ± 20.96
20.04
24.58
43.92
0.12
10
SL10
17.42 ± 2.45
21.44 ± 2.03
102.01 ± 18.40
25.25
30.97
55.93
0.15
11
SL11
7.53 ± 2.12
10.45 ± 1.51
77.62 ± 16.99
13.03
15.98
28.45
0.08
12
SL12
34.27 ± 3.15
40.26 ± 2.31
260.65 ± 21.24
51.02
62.57
111.91
0.30
13
SL13
32.56 ± 2.59
42.28 ± 2.47
248.73 ± 20.07
50.95
62.49
112.17
0.30
14
SL14
47.68 ± 3.38
59.08 ± 3.24
251.53 ± 24.65
68.20
83.64
151.53
0.41
15
SL15
32.60 ± 3.46
28.16 ± 2.82
216.47 ± 23.54
41.10
50.40
89.54
0.24
16
SL16
24.37 ± 3.66
28.29 ± 3.25
128.58 ± 25.41
33.71
41.34
74.72
0.20
17
SL17
15.33 ± 2.73
17.59 ± 2.67
140.15 ± 20.98
23.55
28.88
51.28
0.14
18 19
SL18 SL19
40.94 ± 2.96 34.67 ± 2.77
48.13 ± 2.63 38.69 ± 2.31
206.96 ± 21.29 107.76 ± 18.99
56.61 23.39
69.43 28.69
125.70 98.29
0.34 0.26
20
SL20
37.11 ± 2.44
28.91 ± 1.61
73.11 ± 13.16
37.65
46.17
84.08
0.23
Mean
26.06
29.0
142.32
34.47
42.27
78.50
0.21
MDA
0.93
0.58
10.60
activity measurement of potassium. The gamma transitions of 352 keV from 214Pb, 609.3 and 1,764 keV from 214Bi were used for 226Ra activity measurement. Similarly, 238.6 keV from 212Pb, 911 keV from 228Ac, 583.2 keV from 208Tl were used for thorium activity. The prominent gamma energies used for activity measurements, the emitting nuclide and their corresponding branching intensity are presented in Table 1. Activity of 226Ra, 232Th and 40 K were calculated from the intensity of several c-rays emitted by a nuclide, their branching ratios, sample mass using the relation Activity ðBqÞ Net Area under the photopeak ðcps) 100 100 ¼ : Efficiency ð%Þ BR ð%Þ ð1Þ The activity thus found for each gamma energy was averaged to obtain the 226Ra, 232Th and 40K activity.
Results and discussion Different soil samples collected from various regions of Bidar district are studied for activity measurement using
HPGe detector based gamma spectrometric system. The average values of activity of 226Ra, 232Th and 40K with their standard deviation for the soil samples collected from Bidar are presented in Table 2. The activity of 226Ra, 232Th and 40K in the collected soil samples were found to be in the range BDL–47.68, 7.65–59.08 and BDL– 260.65 Bq kg-1. It can be observed from the obtained results that the activity of the radionuclides are lower. Also the thorium activity is found to be higher than the radium activity as the uranium the parent radionuclide of radium is more susceptible to solubility whereas thorium is less soluble hence adsorbed to soil [13]. The measured activities of the radionuclides from the studied samples were found to be within the range of the world average values provided by [7]. A correlation plot between the radium and thorium activity demonstrated a good linear correlation with a coefficient of 0.9194 as observed in the Fig. 1. The positive correlation predicts that the samples collected in this region are geochemically coherent. A comparative study of the present data with other investigation was performed where it was found that the activity of 226Ra and 232 Th in present study is within the world average value of activity of 11–64 and 16–110 Bq kg-1 respectively. The activity values obtained for the soil samples obtained for
123
64
J Radioanal Nucl Chem (2014) 300:61–65
232
assuming that about 80 % of the time is spent indoors and 20 % outdoors. Annual effective dose lSv year1 ¼ D nGy h1 8760 h year1 0:7 0:2 ð3Þ
226
Th vs. Ra activity for soil samples R = 0.9194
60
-1
Th Activity (Bqkg )
50
40
The mean value of annual effective dose was estimated as 50.40 lSv year-1. The values of the radiological parameters obtained for samples study area were found within acceptable or permissible limits of 1 mSv year-1 for general public [14]. To represent the activity levels of 226Ra, 232Th and 40K by a single quantity, a common radiological index called radium equivalent activity has been introduced. It can be calculated by the relation [15]
30
232
20
10
0 0
10
20 226
30
40
50
Raeq ¼ CRa þ A CTh þ B CK
-1
Ra Activity (Bqkg )
ð4Þ
Bidar (present study)
26.06
29.0
142.32
Shahapur [16]
18.94
38.86
546.3
Punjab (India) [17]
56.74
87.42
143.04
where CRa, CTh and CK are the activity concentrations of Ra, 232Th and 40K in Bq kg-1 respectively and A, B are constants 1.43, 0.077 respectively. It is observed from the present study that the radium equivalent activity for the soils of study region is well below the recommended limit of 370 Bq kg-1. The value of Raeq greater than 370 Bq kg-1 shows a higher gamma dose rate. The external hazard index (Hex) estimated by the following equation derived by Beretka and Mathew [15] yielded the indices well within the limit of 1 for all the samples.
Istanbul, Turkey [18] Ewekoro cement factory (southwest Nigeria) [19]
21.0 7.78
37.0 8.99
342 17.63
Hex ¼
Jhangar valley (Pakistan) [20]
56.21
58.53
851.94
Fig. 1 Correlation between
232
Th and
226
Ra for Bidar soil samples
Table 3 Comparison of the mean activity of 226Ra, 232Th, and 40K (Bq kg-1) recorded in literature for different region soil samples Location (soil samples)
Activity in (Bq kg-1) 226
Ra
232
Th
40
K
Algeria [21]
47
33
329
Yemen [22]
48.2
41.7
939.1
World average [7]
16–110
11–64
140–850
this region were compared with the values from the samples of other regions and found to be low and within the permissible limits. A comparison of the present study results with literature study is shown in Table 3. The outdoor absorbed gamma dose rate in air due to terrestrial c rays at 1 m above the earth’s surface for the samples under study are calculated from the activity of 226 Ra, 232Th and 40K [7] D ¼ ð0:604 CTh þ 0:462 CRa þ 0:0417 CK Þ nGy h1
ð2Þ
where CRa, CTh and CK are the activity (Bq kg-1) of 226Ra, Th and 40K respectively. The mean gamma absorbed dose rate was estimated to be 34.47 nGy h-1. The annual effective dose equivalent received by a member is also calculated using a conversion factor of 0.7 SvG year-1 232
123
226
CRa CTh CK þ þ 370 259 4810
ð5Þ
The external hazard indices for the soil samples of the study region are shown in Table 2 and found to be within the limit of 1. The value of the hazard indices should be less than unity in order to keep the radiation hazard to be insignificant. The maximum value of Hex equal to unity corresponds to the upper limit of radium equivalent activity Raeq (370 Bq kg-1).
Conclusion The c-ray spectrometric analysis using a high resolution HPGe detector was employed for estimation of gamma activity of radionuclides 226Ra, 232Th and 40K. The average values of the activity concentrations of 226Ra, 232Th and 40 K for the Bidar district soil samples were found to be normal and well comparable with the values for other activity values obtained in literature. The non uniformity of the activity found in the soils may be attributed due to the varied geological formation and geochemical composition of the region. The gamma absorbed dose rate and the annual effective dose equivalent (outdoors) was also found
J Radioanal Nucl Chem (2014) 300:61–65
to be within the corresponding dose criterion of ICRP limits of 1 mSv year-1 for the public. Also the radiological parameters radium equivalent activity and external hazard index were also estimated to be within the permissible limits. The present study reveals that the activity values obtained are the natural background radiations and well comparable with the national and international values. The data produced in the present work can be used as baseline radiological data for future investigations and programs. Acknowledgments The authors express their deep sense of gratitude to Board of Research in Nuclear Science (BRNS) for providing the financial support to carry out this work. The authors are also thankful to Dr. D N Sharma, Director, HS&E Group, BARC, Mumbai for his continuous guidance and encouragement for the work.
References 1. Ramola RC, Gusain GS, Badoni M, Prasad Y, Prasad G, Ramachandran TV (2008) J Radiol Prot 28:379–385 2. El-Arabi AM (2007) Radiat Meas 42:94–100 3. Mehra R, Kumar S, Sonkawade R, Singh NP, Badhan K (2010) Environ Earth Sci 59:1159–1164. doi:10.1007/s12665-009-0108-3 4. Kerur BR, Rajeshwari T, Anilkumar S, Narayani K, Rekha AK (2012) J Radioanal Nucl Chem 294:191–196. doi:10.1007/ s10967-011-1599-4 5. Tzortzis M, Tsertos H (2004) J Environ Radioact 77(3):325–338 6. Nambi KSV, Subba Ramu MC, Eappen KP, Ramachandran TV, Murleedharan TS, Shaik AN (1994) Bull Radiat Prot 17:34–35
65 7. UNSCEAR, United Nations Scientific Committee of the Effect of Atomic Radiation (2000) Sources and effects of ionizing radiations. United Nations, New York 8. Baranwal VC, Sharma SP, Sengupta D, Sandilya MK, Bhaumik BK, Guin R, Saha SK (2006) Radiat Meas 41:602–610 9. Birajalaxmi D (2010) BARC newsletter, vol 313. pp. 28–37 10. Chougankar MP, Eappen KP, Ramachandran TV, Shetty PG, Mayya YS, Sadasivan S, Venkat Raj V (2004) J Environ Radioact 71(3):275–297 11. Radhakrishna BP, Vaidyanadhan R (1997) Geology of Karnataka, 2nd edn. Geological Society of India, Bangalore, pp 123–126 12. Kumar A, Narayani Krishnan S, Sharma DN, Abani MC (2001) Radiat Prot Environ 24:1–2 13. Tsai T-L, Lin C-C, Wang T-W, Chu T-C (2008) J Radiol Prot 28:347–360 14. Recommendations of the International Commission on Radiological Protection (1990) Publication 60. Pergamon Press, Oxford 15. Beretka J, Mathew PJ (1985) Health Phys 48(1):87–95. doi:10. 1097/00004032-198501000-00007 16. Kerur BR, Rajeshwari T, Nagabhushana NM, Anilkumar S, Narayani K, Rekha AK, Hanumaiah B (2011) Radiat Prot Environ 34(1):55–59 17. Singh S, Rani A, Mahajan RK (2005) Radiat Meas 39:431 18. Karahan G, Bayulken A (2000) J Environ Radioact 47:213–221 19. Gbadebo AM, Amos AJ (2010) Asian J Appl Sci 3(2):135–144 20. Khan HM, Chaudhry ZS, Ismail M, Khan K (2010) Water Air Soil Pollut 213:353–362 21. Boukhenfouf W, Boucenna A (2011) J Environ Radioact. doi:10. 1016/j.jenvrad.2011.01.006 22. Harb S, El-Hadi El-Kamel A, El-Basst Abbady A, Saleh II, ElMageed AIA (2012) J Med Phys 37(1):54–60. doi:10.4103/09716203.92721
123
