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Assessment Of Radiological Consequence Of Radioactivity In Monazite Beach Sand In South West Coastal Region In Southern India
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 16 (2019) 784–791
www.materialstoday.com/proceedings
ICAMMAS17
Assessment Of Radiological Consequence Of Radioactivity In Monazite Beach Sand In South West Coastal Region In Southern India S.R. Soniyaa*, S. Monicaa, A.K Visnu Prasada, P.J. Jojoa aCenter for Advanced Research in Physical Sciences, Department of Physics,Fatima Mata National College (Autonomous), Kollam, Kerala 691001, India
Abstract The natural radionuclide ( 226Ra, 232Th and 40K) contents of sand samples at various locations in Sakthikulangara area in the south west coastal region of Kerala state have been determined using NaI(Tl) based gamma spectrometric measurement method and radiation hazard indices in the study area are evaluated. The specific activities of the radionuclides were found to vary from below detectable level to 32.38 BqKg-1, 34.77 BqKg-1 to 229.30 BqKg-1 and 20.76 BqKg-1 to 88.39 BqKg-1, respectively for 226 Ra, 232Th and 40K. The radium equivalent activity (Raeq) was quite less than the recommended limit of 370BqKg-1. The radiological parameters such as internal and external hazard indices, absorbed dose rate, indoor and outdoor annual effective dose, alpha index and representative level index were also determined from the estimated value of the specific activity of 226Ra, 232 Th and40K. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences Keywords: Gamma ray spectrometer; Specific Activity; Radiometric measurements;
1.
Introduction
Kerala state in the south west part of India, is separated from the rest of the peninsula by natural geographic boundaries. The 'Western Ghats' with their rich primeval forests having a high degree of rainfall, form the eastern boundary and extend from the north to Kanyakumari in the south. The entire western border is caressed by the Arabian Sea. Between these natural boundaries the narrow strip of land extending from Kasargode in the north to Parasala in the south lies. Costal tracts in the Kollam district of the state possesses one of the world class deposits of minerals sand. The heavy mineral sand deposits in Kollam contain an assemblage of Ilmenite, Rutile, Leucoxene, Monazite, Zircon and Sillimanite. Beach sand in the region has rich Monazite deposits along with other rare earth metals. It arises generally in minor isolated rock crystal. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, with density about 4.6 to 5.7 gcm-3. Monazite is an important ore for thorium lanthanum, and cerium.
* Corresponding author. Tel.: 9645444766 ; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
785
Monazite is radioactive mainly due to the occurrence of naturally occurring radioactive nuclides such as the members of
232
Th and
238
U series and 40K which are the major source of external gamma radiation along with soil,
rock and materials obtained from the earth crusts, such as building materials and its existence in geological formation [1]. Out of these, 40K contributes 13.8%, 232Th 14% and 226Ra 55.8% to the world wide average terrestrial gamma dose rate of 60 nGy h-1. In the 238U series, the decay chain segment starting from radium (226Ra) is the most 226
important in radiological isotope and therefore reference is often made to
Ra instead of
238
U. The radionuclides
are not uniformly distributed in soil and therefore, environmental radioactivity and associated external gamma exposure depends strongly on geological and geographical conditions, and appear at different levels in the soils of each region in the world [2]. Long term exposure to radioactivity and inhalation of radionuclides such as uranium and radium could cause many health effects such as chronic lung diseases, acute leucopoenia, anemia and necrosis of the mouth. Thorium exposure can cause lung, pancreas, hepatic bone, kidney cancers and leukemia[3] and these radionuclides originates predominantly from upper 30cm layer of soil present on earth[4]. The worldwide average annual effective dose for natural sources is 2.4mSv of which 1.1mSv is due to basic background radiation and 1.3 mSv is due to exposure to radon [5]. The worldwide average concentrations of respectively. The presence of
226
Ra,
232
Th and
226
Ra,
232
Th and 40K are 35, 30 and 400 BqKg-1,
40
K elements in the sub-soil leads us to identify the origin and
abundance of their daughter elements and for assessing dose exposure. To evaluate the radiation hazards, it is very important to estimate the natural radioactivity level in soil [6]. The objective of this work was to measure the radioactive nuclide distribution in beach sand of Sakthikulangara in the south west coastal region in Kollam. The present work utilizes gamma spectrometric methods for determination of activity concentrations of radionuclides in sand samples. 2.
Methodology
Samples were collected from Sakthikulangara region for the measurement of natural radiation from surface sand. A total of 12 samples of sands collected in a systematic way from the region using standard protocol. These samples were crushed to get fine powder and moisture content was completely removed by heating at 110oC in an oven. The sample is then homogenized and sealed in impermeable airtight can with capacity of 305cm3 for more than 30 days to reach secular equilibrium were the rate of decay of the daughter becomes equal to that of the parent radionuclides. All samples were analyzed using a gamma spectrometer with NaI(Tl) detector. The samples were counted for 10000seconds. The spectrum was stored in a PC based multichannel analyzer. Radiometric measurements were carried out for the determination of radionuclides present in the samples of sand. 2.1 Determination of natural radioactivity The activity concentrations of the radionuclides 226Ra, 232Th and 40K for the samples were determined using the equation [7].
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Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
Activity ( Bq ) =
cps×100×100 BI ×E ff
Where cps is the net count per second; BI is the branching intensity and Eff is the efficiency of the detector. 2.2 Radium equivalent activity (Raeq) Radium equivalent activity is an index that has been introduced to evaluate the specific activities of 226Ra, 232
Th and 40K by a single quantity [8]. It is generally defined as
Ra eq = C Ra +1.43CTh + 0.077C k Where CRa, CTh and Ck are activities of
(2) 226
Ra,
232
Th and
40
-1
activity is defined on the assumption that 10 BqKg of
-1
K respectively in BqKg . The radium equivalent
226
Ra, 7 BqKg-1of
232
Th and 130 BqKg-1of 40K produce
the same gamma ray dose rates [9]. The maximum value of radium equivalent must be less than 370 BqKg-1 for the safe limit according to the report of United Nations Scientific Committee on the effects of atomic radiation, 2000. 2.3 Estimation of absorbed dose rate The absorbed dose rates (D) in air at above the ground surface for the uniform distribution of radionuclides 226
( Ra, 232Th and 40K) was calculated using the following equation [10],
D(nGyhr -1 ) = 0.461C Ra + 0.623C Th + 0.041C k
(3)
Where the numerical values 0.462, 0.621 and 0.417 are dose conversion factors and is used for converting activity concentrations of
226
Ra, 232Th and 40K into doses.
2.4 External and internal hazard index The value of external hazard index should be less than or equal to unity for the safe limit, which corresponds to the upper limit of Raeq 370BqKg-1for limiting the dose to 1.5mGyy-1. External hazard index can be calculated using the equation [11],
H ex =
CRa CTh CK + + ≤1 370 259 4810
Internal exposure to
(4)
222
Rn and its radioactive progeny is controlled by the internal hazard index (Hin) and is
obtained by the equation [12], for the safe use of a material in the construction of dwellings internal hazard index should be less than unity.
Hin =
CRa CTh C + + K ≤1 185 259 4810
(5)
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
787
2.5 Annual effective dose rate The annual effective dose rate is determined by considering the conversion coefficient from absorbed dose in air to effective dose 0.7 SvGy-1 and the indoor occupancy factor of 0.8 and the outdoor occupancy factor of 0.2 proposed by UNSCEAR 2000. The annual effective dose was calculated from the equation [13],
(
Indoor mSvy −1
)
(
)
(
)
= D nGyh −1 × 8760 hy −1 × 0.8 × 0.7 SvGy −1 × 10−6
Outdoor ( mSvy −1 ) = D ( nGyh −1 ) × 8760 hy −1 × 0.2 × 0.7 ( SvGy −1 ) × 10 −6
(6) (7)
-1
Where D is the absorbed dose rate in nGyh . 2.6 Radioactivity level index The radioactivity level index is used to represent the γ radiation hazards associated with the natural radio nuclide, and the upper limit of the index is unity. The representative level of Iγ was obtained by the equation [14],
Iγ =
CRa CTh CK + + 150 100 1500
(8)
2.7 Alpha index The index is used for the assessment of internal hazard due to the radon inhalation originating from building materials and is defined by the equation [15],
Iα=
(9)
CRa 200
Where CRa is the activity concentration of radium in and its recommended limit is 200Bq/Kg. Hence for the safe use of building materials the value of Iαchosen to be less than unity. 3.
Result and discussion
In the selected sand samples, the activity concentrations are assessed .The level -1 232
up to 32.38 BqKg .
226
Th varies from 34.77 BqKg to a maximum of 229.30 BqKg-1 . and 40K varies from 20.76
BqKg-1 to a maximum level of 88.39 BqKg-1 and it is confirm that The values of recommended world average values for 232
Ra from below detectable level
-1
226
Ra and 40K are lower than the
226
Ra, and 40K, which are 35, 400 BqKg-1 respectively while the value of
Th is higher than the worldwide average values of 30 Bqkg-1..the analysis of radionuclide content
40
K shows rich
232
226
Ra ,
232
Th,
Th content in sand in comparison with the other two due to the presence rich monazite content,
which is an important ore for thorium. Thus the result indicate that sand sample is radioactive due the presences of thorium and less commonly radium formed by decay of uranium. The variation of the activity concentration in the studied samples may be due to the variation in mineral content. The concentrations of the radionuclides present in the collected samples are summarized in the table 1. Non-uniform distribution of natural radionuclides in the sand of Sakthikulangara is obtained. Therefore, a common radiological index has been introduce to evaluate the actual
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Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
radioactivity level of
226
Ra,
232
Th and40K in the samples and the radiation hazards associated with the estimated
value of radionuclides. From the table it is clear that Raeq values of the sand samples ranges from 6.39BqKg-1 to 40.47BqKg-1is less than the maximum admissible value of 370 BqKg-1. Thus, all the sample collected for this study shows that it would not pose a significant radiological hazard for the living environment. Table 1: Activity concentrations of 226Ra, 232Th, 40K and Radium equivalent activity 226
Ra(BqKg-1)
Sand
232
Th(BqKg-1)
40
K(BqKg-1)
Raeq(BqKg-1)
Sample 1
4.95
123.20
25.54
17.13
Sample 2
BDL
229.30
88.39
14.59
Sample 3
1.56
34.77
20.76
6.39
Sample 4
4.97
75.83
61.87
11.89
Sample 5
28.99
99.38
19.44
40.47
Sample 6
2.18
46.97
5.21
8.50
Sample 7
7.53
89.28
18.35
19.14
Sample 8
9.30
208.45
76.27
22.93
Sample 9
11.51
47.39
25.78
17.99
Sample 10
6.48
97.95
65.74
15.41
Sample 11
32.38
59.24
58.01
39.61
Sample 12
17.49
94.79
62.65
26.13
Activity Concentration (Bq/Kg)
250.00 200.00 150.00 226-Radium(Bq/Kg) 100.00
232-Thorium(Bq/Kg)
50.00
40-Potassium(Bq/Kg)
0.00 1
2
3
4
5
6
7
8
9
10
11
Sample Number Fig.1.Bar graph showing the concentration of radionuclides in different sample.
12
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
789
Fig.1 shows the bar graph of radium, thorium and potassium concentration in the selected samples respectively. From the figure it is clear that the activity concentration of 232Th is the highest and that of
226
Ra is the least for the
selected sample. Table 2 presents the radiological risk factors such as absorbed dose rate, indoor and outdoor annual effective dose rate, internal and external hazard index, radioactivity level index (gamma index), alpha index. From the table it is observed that the absorbed dose rate calculated from the activity concentration of
226
Ra,
232
Th and40K ranges from
22.38to 142.39 nGyh-1. The corresponding indoor and outdoor annual effective doses have an average value of 0.11mSvy-1and 0.70mSvy-1respectively.It can see that the total values of each sample are less than the corresponding worldwide values of 1 mSvy-1.The estimated value of gamma index is listed in table 2, which shows an average value of 1.11, greater than unity. This confirms that the samples selected for the study exhibit some lower to higher gamma radiation level. The observed values of alpha index are less than unity showing that the selected samples are safe from the point of viewof environmental radiation hazards.
Table 2 : Absorbed dose rate, External and internal hazard index, Representative level index, alpha index and annual effective dose rate. Annual Effective Dose Rate (mSvy-1)
Sand
D(nGyh-1)
Hex
Hin
Iγ
Iα
Sample 1
78.86
0.49
0.51
1.28
0.02
Indoor 0.39
Outdoor 0.10
Sample 2
142.39
0.90
0.90
2.35
0.00
0.70
0.17
Sample 3
22.38
0.14
0.15
0.37
0.01
0.11
0.03
Sample 4
49.49
0.32
0.33
0.83
0.02
0.24
0.06
Sample 5
75.58
0.47
0.54
1.20
0.14
0.37
0.09
Sample 6
30.25
0.19
0.19
0.49
0.01
0.15
0.04
Sample 7 Sample 8
59.06 133.81
0.37 0.85
0.39 0.87
0.96 2.20
0.04 0.05
0.29 0.66
0.07 0.16
Sample 9
35.14
0.22
0.25
0.57
0.06
0.17
0.04
Sample 10
63.92
0.41
0.43
1.07
0.03
0.31
0.08
Sample 11
52.63
0.33
0.42
0.85
0.16
0.26
0.06
Sample 12
67.24
0.43
0.47
1.11
0.09
0.33
0.08
790
4.
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
Conclusion
The natural radioactivity concentration of
226
Ra,
232
Th and40K is measured in Sakthikulangara south west coastal
region using Gamma ray spectrometry. The values of values for
226
Ra, and
226
Ra and 40K are lower than the recommended world average
40
K, which are 35, 400 BqKg-1 respectively while the value of
232
Th is higher than the
worldwide average values of 30 Bqkg-1. It is inferred that for all sand sample sanalyzed, Raeq value is well within the permissible limit of 370 BqKg-1. Values of external and internal hazard indices for all samples investigated are below unity .The estimated value of gamma index shows that for some samples the index is greater than unity indicating the existence of higher level of gamma radiation. The annual gamma dose rates for the studied samples are within the recommended safety limit. This study is important for radiological mapping of the study area in future. References 1.
Ramassay, V., Dheenathayalu, M., Ravisankar R., Ponnusamy, V., ‘ Natural radioactivity measurements in Beach-roack samples of South – East Coast of Tamilnadu, India ’, Radiat. Prot. Dosimetry, Vol. 111, No. 2, 2004, pp. 229.
2.
UNSCEAR, 2000. United Nations Scientific Committee on the effects of atomic radiation: exposure due to natural radiation sources. United nations, New York.
3.
Taskin, H., Karavas, M., Ay, p., Topuzoglu, A et al.,(2009) ‘ Radionuclide concentration in soil and life time cancer risk due to the gamma radioactivity in Kirklareli, Turkey’, Journal of Environmental Radioactivity 100, pp. 52
4.
Chikasawa, K., Ishii, T., Ugiyama, H.,(2001) ‘Terrestrial gamma radiation in Kochi Prefecture, Japan.’, J Health Science 47: pp. 361-372
5.
UNSCEAR, Report to the general assembly, Source (1988). “Effects and risks of ionizing radiation”, (New York : UN)
6.
Organization for Economic Cooperation and Development, 1979. Exposure to radiation from the natural radioactivity distribution in Building materials. Report by a group of experts of the OECD Nuclear Energy Agency, OECD, Paris, France.
7.
Rati, V., Mahur, A., Sonkawade, R et al.,(2010) ‘Evaluation and analysis of 226Ra,232Th, 40K and radon exhalation rate in various grey cements’, Indian Journel of pure and applied Physics, 48, pp.473.
8.
PankajBala.,RohitMehra., Ramola, R.C.,(2014) ‘Distribution of natural radioactivity in soil samples and radiological hazards in buiding material of Una, Himachal Pradesh’,142 pp.11-15.
9.
Krisiuk, E.M., Tarasov, S.I., Shamov, V.P et al., (1971) ‘A study of radioactivity in building materials’, Research institute for Radiation Hygiene, Leningrad.
10. Thabayneh, K.M., Jazzar, M.M.,(2013) ‘Radioactivity levels in plant sampls in tuikarem district, Palestein and its impact on human health’ , Radiation Protection Dosimetry, Vol.153, No. 4, pp.467
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11. Beretka, J., Mathew P.J (1985). “Natural radioactivity of Australian building materials, industrial wastes and by products”. Health Physics 48 pp: 87-95 12. Krieger ,V.R.,(1981). ‘Radioactivity of construction materials, BetonwerkFertigteil Tech’, Vol. 47 pp. 468-473 13. Parminder Singh., Prabhjot Singh., Bikramjit Singh Bajwa et al., (2016) ‘Radionuclide contents and their correlation with radon-thoron exhalation in soil samples from mineralized zone of Himachal pradesh, India’, J Radiational Nucl Chem. 14. Khalil M Dabayneh.(2008). ‘Natural radioactivity in different commercial ceramic samples used in Palastein buildings as construction materials’. Vol (3) pp. 49-58 15. Directorate- general environment, Nuclear safety and civil protection. Luxembourg Belgium (2000). European Commission Radiological protection principles concerning the natural radioactivity of building materials. Radiation protection 112.
ScienceDirect Materials Today: Proceedings 16 (2019) 784–791
www.materialstoday.com/proceedings
ICAMMAS17
Assessment Of Radiological Consequence Of Radioactivity In Monazite Beach Sand In South West Coastal Region In Southern India S.R. Soniyaa*, S. Monicaa, A.K Visnu Prasada, P.J. Jojoa aCenter for Advanced Research in Physical Sciences, Department of Physics,Fatima Mata National College (Autonomous), Kollam, Kerala 691001, India
Abstract The natural radionuclide ( 226Ra, 232Th and 40K) contents of sand samples at various locations in Sakthikulangara area in the south west coastal region of Kerala state have been determined using NaI(Tl) based gamma spectrometric measurement method and radiation hazard indices in the study area are evaluated. The specific activities of the radionuclides were found to vary from below detectable level to 32.38 BqKg-1, 34.77 BqKg-1 to 229.30 BqKg-1 and 20.76 BqKg-1 to 88.39 BqKg-1, respectively for 226 Ra, 232Th and 40K. The radium equivalent activity (Raeq) was quite less than the recommended limit of 370BqKg-1. The radiological parameters such as internal and external hazard indices, absorbed dose rate, indoor and outdoor annual effective dose, alpha index and representative level index were also determined from the estimated value of the specific activity of 226Ra, 232 Th and40K. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences Keywords: Gamma ray spectrometer; Specific Activity; Radiometric measurements;
1.
Introduction
Kerala state in the south west part of India, is separated from the rest of the peninsula by natural geographic boundaries. The 'Western Ghats' with their rich primeval forests having a high degree of rainfall, form the eastern boundary and extend from the north to Kanyakumari in the south. The entire western border is caressed by the Arabian Sea. Between these natural boundaries the narrow strip of land extending from Kasargode in the north to Parasala in the south lies. Costal tracts in the Kollam district of the state possesses one of the world class deposits of minerals sand. The heavy mineral sand deposits in Kollam contain an assemblage of Ilmenite, Rutile, Leucoxene, Monazite, Zircon and Sillimanite. Beach sand in the region has rich Monazite deposits along with other rare earth metals. It arises generally in minor isolated rock crystal. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, with density about 4.6 to 5.7 gcm-3. Monazite is an important ore for thorium lanthanum, and cerium.
* Corresponding author. Tel.: 9645444766 ; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials, Manufacturing and Applied Sciences
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
785
Monazite is radioactive mainly due to the occurrence of naturally occurring radioactive nuclides such as the members of
232
Th and
238
U series and 40K which are the major source of external gamma radiation along with soil,
rock and materials obtained from the earth crusts, such as building materials and its existence in geological formation [1]. Out of these, 40K contributes 13.8%, 232Th 14% and 226Ra 55.8% to the world wide average terrestrial gamma dose rate of 60 nGy h-1. In the 238U series, the decay chain segment starting from radium (226Ra) is the most 226
important in radiological isotope and therefore reference is often made to
Ra instead of
238
U. The radionuclides
are not uniformly distributed in soil and therefore, environmental radioactivity and associated external gamma exposure depends strongly on geological and geographical conditions, and appear at different levels in the soils of each region in the world [2]. Long term exposure to radioactivity and inhalation of radionuclides such as uranium and radium could cause many health effects such as chronic lung diseases, acute leucopoenia, anemia and necrosis of the mouth. Thorium exposure can cause lung, pancreas, hepatic bone, kidney cancers and leukemia[3] and these radionuclides originates predominantly from upper 30cm layer of soil present on earth[4]. The worldwide average annual effective dose for natural sources is 2.4mSv of which 1.1mSv is due to basic background radiation and 1.3 mSv is due to exposure to radon [5]. The worldwide average concentrations of respectively. The presence of
226
Ra,
232
Th and
226
Ra,
232
Th and 40K are 35, 30 and 400 BqKg-1,
40
K elements in the sub-soil leads us to identify the origin and
abundance of their daughter elements and for assessing dose exposure. To evaluate the radiation hazards, it is very important to estimate the natural radioactivity level in soil [6]. The objective of this work was to measure the radioactive nuclide distribution in beach sand of Sakthikulangara in the south west coastal region in Kollam. The present work utilizes gamma spectrometric methods for determination of activity concentrations of radionuclides in sand samples. 2.
Methodology
Samples were collected from Sakthikulangara region for the measurement of natural radiation from surface sand. A total of 12 samples of sands collected in a systematic way from the region using standard protocol. These samples were crushed to get fine powder and moisture content was completely removed by heating at 110oC in an oven. The sample is then homogenized and sealed in impermeable airtight can with capacity of 305cm3 for more than 30 days to reach secular equilibrium were the rate of decay of the daughter becomes equal to that of the parent radionuclides. All samples were analyzed using a gamma spectrometer with NaI(Tl) detector. The samples were counted for 10000seconds. The spectrum was stored in a PC based multichannel analyzer. Radiometric measurements were carried out for the determination of radionuclides present in the samples of sand. 2.1 Determination of natural radioactivity The activity concentrations of the radionuclides 226Ra, 232Th and 40K for the samples were determined using the equation [7].
786
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
Activity ( Bq ) =
cps×100×100 BI ×E ff
Where cps is the net count per second; BI is the branching intensity and Eff is the efficiency of the detector. 2.2 Radium equivalent activity (Raeq) Radium equivalent activity is an index that has been introduced to evaluate the specific activities of 226Ra, 232
Th and 40K by a single quantity [8]. It is generally defined as
Ra eq = C Ra +1.43CTh + 0.077C k Where CRa, CTh and Ck are activities of
(2) 226
Ra,
232
Th and
40
-1
activity is defined on the assumption that 10 BqKg of
-1
K respectively in BqKg . The radium equivalent
226
Ra, 7 BqKg-1of
232
Th and 130 BqKg-1of 40K produce
the same gamma ray dose rates [9]. The maximum value of radium equivalent must be less than 370 BqKg-1 for the safe limit according to the report of United Nations Scientific Committee on the effects of atomic radiation, 2000. 2.3 Estimation of absorbed dose rate The absorbed dose rates (D) in air at above the ground surface for the uniform distribution of radionuclides 226
( Ra, 232Th and 40K) was calculated using the following equation [10],
D(nGyhr -1 ) = 0.461C Ra + 0.623C Th + 0.041C k
(3)
Where the numerical values 0.462, 0.621 and 0.417 are dose conversion factors and is used for converting activity concentrations of
226
Ra, 232Th and 40K into doses.
2.4 External and internal hazard index The value of external hazard index should be less than or equal to unity for the safe limit, which corresponds to the upper limit of Raeq 370BqKg-1for limiting the dose to 1.5mGyy-1. External hazard index can be calculated using the equation [11],
H ex =
CRa CTh CK + + ≤1 370 259 4810
Internal exposure to
(4)
222
Rn and its radioactive progeny is controlled by the internal hazard index (Hin) and is
obtained by the equation [12], for the safe use of a material in the construction of dwellings internal hazard index should be less than unity.
Hin =
CRa CTh C + + K ≤1 185 259 4810
(5)
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
787
2.5 Annual effective dose rate The annual effective dose rate is determined by considering the conversion coefficient from absorbed dose in air to effective dose 0.7 SvGy-1 and the indoor occupancy factor of 0.8 and the outdoor occupancy factor of 0.2 proposed by UNSCEAR 2000. The annual effective dose was calculated from the equation [13],
(
Indoor mSvy −1
)
(
)
(
)
= D nGyh −1 × 8760 hy −1 × 0.8 × 0.7 SvGy −1 × 10−6
Outdoor ( mSvy −1 ) = D ( nGyh −1 ) × 8760 hy −1 × 0.2 × 0.7 ( SvGy −1 ) × 10 −6
(6) (7)
-1
Where D is the absorbed dose rate in nGyh . 2.6 Radioactivity level index The radioactivity level index is used to represent the γ radiation hazards associated with the natural radio nuclide, and the upper limit of the index is unity. The representative level of Iγ was obtained by the equation [14],
Iγ =
CRa CTh CK + + 150 100 1500
(8)
2.7 Alpha index The index is used for the assessment of internal hazard due to the radon inhalation originating from building materials and is defined by the equation [15],
Iα=
(9)
CRa 200
Where CRa is the activity concentration of radium in and its recommended limit is 200Bq/Kg. Hence for the safe use of building materials the value of Iαchosen to be less than unity. 3.
Result and discussion
In the selected sand samples, the activity concentrations are assessed .The level -1 232
up to 32.38 BqKg .
226
Th varies from 34.77 BqKg to a maximum of 229.30 BqKg-1 . and 40K varies from 20.76
BqKg-1 to a maximum level of 88.39 BqKg-1 and it is confirm that The values of recommended world average values for 232
Ra from below detectable level
-1
226
Ra and 40K are lower than the
226
Ra, and 40K, which are 35, 400 BqKg-1 respectively while the value of
Th is higher than the worldwide average values of 30 Bqkg-1..the analysis of radionuclide content
40
K shows rich
232
226
Ra ,
232
Th,
Th content in sand in comparison with the other two due to the presence rich monazite content,
which is an important ore for thorium. Thus the result indicate that sand sample is radioactive due the presences of thorium and less commonly radium formed by decay of uranium. The variation of the activity concentration in the studied samples may be due to the variation in mineral content. The concentrations of the radionuclides present in the collected samples are summarized in the table 1. Non-uniform distribution of natural radionuclides in the sand of Sakthikulangara is obtained. Therefore, a common radiological index has been introduce to evaluate the actual
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radioactivity level of
226
Ra,
232
Th and40K in the samples and the radiation hazards associated with the estimated
value of radionuclides. From the table it is clear that Raeq values of the sand samples ranges from 6.39BqKg-1 to 40.47BqKg-1is less than the maximum admissible value of 370 BqKg-1. Thus, all the sample collected for this study shows that it would not pose a significant radiological hazard for the living environment. Table 1: Activity concentrations of 226Ra, 232Th, 40K and Radium equivalent activity 226
Ra(BqKg-1)
Sand
232
Th(BqKg-1)
40
K(BqKg-1)
Raeq(BqKg-1)
Sample 1
4.95
123.20
25.54
17.13
Sample 2
BDL
229.30
88.39
14.59
Sample 3
1.56
34.77
20.76
6.39
Sample 4
4.97
75.83
61.87
11.89
Sample 5
28.99
99.38
19.44
40.47
Sample 6
2.18
46.97
5.21
8.50
Sample 7
7.53
89.28
18.35
19.14
Sample 8
9.30
208.45
76.27
22.93
Sample 9
11.51
47.39
25.78
17.99
Sample 10
6.48
97.95
65.74
15.41
Sample 11
32.38
59.24
58.01
39.61
Sample 12
17.49
94.79
62.65
26.13
Activity Concentration (Bq/Kg)
250.00 200.00 150.00 226-Radium(Bq/Kg) 100.00
232-Thorium(Bq/Kg)
50.00
40-Potassium(Bq/Kg)
0.00 1
2
3
4
5
6
7
8
9
10
11
Sample Number Fig.1.Bar graph showing the concentration of radionuclides in different sample.
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Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
789
Fig.1 shows the bar graph of radium, thorium and potassium concentration in the selected samples respectively. From the figure it is clear that the activity concentration of 232Th is the highest and that of
226
Ra is the least for the
selected sample. Table 2 presents the radiological risk factors such as absorbed dose rate, indoor and outdoor annual effective dose rate, internal and external hazard index, radioactivity level index (gamma index), alpha index. From the table it is observed that the absorbed dose rate calculated from the activity concentration of
226
Ra,
232
Th and40K ranges from
22.38to 142.39 nGyh-1. The corresponding indoor and outdoor annual effective doses have an average value of 0.11mSvy-1and 0.70mSvy-1respectively.It can see that the total values of each sample are less than the corresponding worldwide values of 1 mSvy-1.The estimated value of gamma index is listed in table 2, which shows an average value of 1.11, greater than unity. This confirms that the samples selected for the study exhibit some lower to higher gamma radiation level. The observed values of alpha index are less than unity showing that the selected samples are safe from the point of viewof environmental radiation hazards.
Table 2 : Absorbed dose rate, External and internal hazard index, Representative level index, alpha index and annual effective dose rate. Annual Effective Dose Rate (mSvy-1)
Sand
D(nGyh-1)
Hex
Hin
Iγ
Iα
Sample 1
78.86
0.49
0.51
1.28
0.02
Indoor 0.39
Outdoor 0.10
Sample 2
142.39
0.90
0.90
2.35
0.00
0.70
0.17
Sample 3
22.38
0.14
0.15
0.37
0.01
0.11
0.03
Sample 4
49.49
0.32
0.33
0.83
0.02
0.24
0.06
Sample 5
75.58
0.47
0.54
1.20
0.14
0.37
0.09
Sample 6
30.25
0.19
0.19
0.49
0.01
0.15
0.04
Sample 7 Sample 8
59.06 133.81
0.37 0.85
0.39 0.87
0.96 2.20
0.04 0.05
0.29 0.66
0.07 0.16
Sample 9
35.14
0.22
0.25
0.57
0.06
0.17
0.04
Sample 10
63.92
0.41
0.43
1.07
0.03
0.31
0.08
Sample 11
52.63
0.33
0.42
0.85
0.16
0.26
0.06
Sample 12
67.24
0.43
0.47
1.11
0.09
0.33
0.08
790
4.
Soniya S R et al. / Materials Today: Proceedings 16 (2019) 784–791
Conclusion
The natural radioactivity concentration of
226
Ra,
232
Th and40K is measured in Sakthikulangara south west coastal
region using Gamma ray spectrometry. The values of values for
226
Ra, and
226
Ra and 40K are lower than the recommended world average
40
K, which are 35, 400 BqKg-1 respectively while the value of
232
Th is higher than the
worldwide average values of 30 Bqkg-1. It is inferred that for all sand sample sanalyzed, Raeq value is well within the permissible limit of 370 BqKg-1. Values of external and internal hazard indices for all samples investigated are below unity .The estimated value of gamma index shows that for some samples the index is greater than unity indicating the existence of higher level of gamma radiation. The annual gamma dose rates for the studied samples are within the recommended safety limit. This study is important for radiological mapping of the study area in future. References 1.
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