Measurement of terrestrial gamma radiation dose and evaluation of annual effective dose in Shimoga District of Karnataka State, India :DR Rangaswamy, E Srinivasa, MC Srilatha, Jadiyappa Sannappa, Radiation Protection and Environment

ORIGINAL ARTICLE
Year : 2015 | Volume : 38 | Issue : 4 | Page : 154--159

Measurement of terrestrial gamma radiation dose and evaluation of annual effective dose in Shimoga District of Karnataka State, India

DR Rangaswamy¹, E Srinivasa², MC Srilatha³, Jadiyappa Sannappa¹,
¹ Department of Studies and Research in Physics, Kuvempu University, Shankaraghatta, Karnataka, India
² Department of Physics, IDSG Government College, Chikmagalur, Karnataka, India
³ Department of Physics, Government First Grade College, Malleswaram, Bengaluru, Karnataka, India

Correspondence Address:
Jadiyappa Sannappa
Department of Studies and Research in Physics, Kuvempu University, Jnanasahyadri, Shankaraghatta - 577 451, Bengaluru, Karnataka
India

Abstract

Human beings are continuously exposed to the radiations coming from terrestrial and extraterrestrial sources and inside their own bodies. This study presents the results of indoor and outdoor ambient gamma dose rates in and around granite regions of Shimoga District, and these measurements were carried out by using environmental radiation Dosimeter ER-709 which is a portable detector. By the measured average absorbed dose rates, annual effective dose (AED) has been calculated by a standard method. Results showed that the indoor and outdoor absorbed dose rates in air of Shimoga district ranged between 114.05 ± 2.11 to 332.6 ± 3.99 nGy/h and 87 ± 1.7 to 276.66 ± 4.76 nGy/h. The indoor and outdoor AED ranged between 0.559 to 1.631 mSv/year with an average value of 0.872 mSv/year and 0.106 to 0.339 mSv/year with an average value of 0.235 mSv/year, respectively. The calculated indoor and outdoor AEDs were found to be higher than the world average.

How to cite this article:
Rangaswamy D R, Srinivasa E, Srilatha M C, Sannappa J. Measurement of terrestrial gamma radiation dose and evaluation of annual effective dose in Shimoga District of Karnataka State, India.Radiat Prot Environ 2015;38:154-159

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Rangaswamy D R, Srinivasa E, Srilatha M C, Sannappa J. Measurement of terrestrial gamma radiation dose and evaluation of annual effective dose in Shimoga District of Karnataka State, India. Radiat Prot Environ [serial online] 2015 [cited 2023 Jun 2 ];38:154-159
Available from: https://www.rpe.org.in/text.asp?2015/38/4/154/176152

Full Text

Introduction

Exposure from natural background radiation to human beings is natural, continuous, and inescapable feature of life on earth. There are two main contributors to natural radiation exposures; first one is high-energy cosmic ray particles incident on the earth's atmosphere. The interactions of cosmic ray particles with the nuclei of atmospheric constituents can create a cascade of interactions and secondary reaction products as number of radioactive nuclei such as 3 H,7 Be, and 14 C.[1],[2] The radiation dose from cosmic rays increases with latitude and altitude so that polar and mountain dwellers as well as aircrew and frequent air travelers receive higher doses of cosmic radiation.[3] The other main contributors are the terrestrial radioactive materials which originate from the formation of the earth and are present everywhere in the earth's crust and in the human body itself. Excluding exposure from direct cosmic rays and cosmogenic radionuclide from extraterrestrial sources, natural exposures arise mainly from the primordial radionuclides such as 238 U,226 Ra,232 Th, and 40 K, which are spread widely and are present in almost all geological materials in the earth's environment.[4] As a result of rock weathering, the radionuclides are carried to the soils, streams, and rivers by rain. Natural environmental radioactivity and the associated external exposure due to gamma radiation depend primarily on the ecological and geographical conditions.[2] The levels of radioactive nuclides in rock and soil vary with the geological locations; therefore, it is important to measure the dose rates at different geological areas.[5] Natural sources contribute about 80% exposure to the world's collective radiation exposure of the world's population.[2],[6] In this context, environmental radioactivity measurements are necessary for determining the background radiation level due to natural radioactive sources of terrestrial and cosmic origins.[7],[8],[9] Knowledge on terrestrial gamma radiation and radioactivity is immense importance and interest in health physics. The presence of naturally occurring radionuclides in the environment may result in an external and internal dose received by a population exposed to them directly and via the ingestion/inhalation pathways. The assessment of the radiological impact on a population, as a result of the radiation emitted by these radionuclides, is important since they contribute to the collective dose of the population.[10] The aim of the present study is to measure the levels of environmental terrestrial gamma radiation level in and around granite regions of Shimoga district and to determine the annual effective radiation doses to which people are exposed from indoor and outdoor ambient terrestrial gamma radiation.

Geology of the study area

The exposure of human beings to ionizing radiation from natural sources is a continuing and inescapable feature of life on earth. The radiation levels and activity of primordial radionuclides are not uniform. The present study is carried out to know the radiation levels from terrestrial radionuclides, to provide vital radiological baseline information, and to measure the dose rates at different geological areas. For the above reason, an attempt has made first time in the study area, and it concentrate in and around granite region of Shimoga district.

[Figure 1] shows the geological map of Shimoga district with different study locations. Shimoga district is a part of the Malnad region of Karnataka and is also known as the “Gateway to Malnad.” Shimoga district is situated between 130 271 to 140 391 north latitude and 740 381 to 750 451 east longitudes. Shimoga district covers an area of 8477.84 sq. km with a population of –1.756 million and lies in the western part of Karnataka state. The district forms part of Western Ghats areas (Sahyadri Hill ranges), which can be demarcated into two zones: The densely forested high hilly Malnad in the west and sparsely forested tablelands Semi-Malnad in the east. The study area comprises rock formations belonging to Archaean to lower Proterozoic and Recent age. Numerous quartz and pegmatite veins occur as intrusives in the older schistose rocks (Amphibolites) and granitic gneiss rocks. Laterites occur over the schists and granitic gneiss with an approximate thickness of few centimeters to 40.00 m which covers the major part of Sorab taluk.[11] The major soil forms found in Shimoga district are Brown clay loamy soil, Red soil, Sandy soil, Red sandy soil, Yellowish loamy soil, Lateritic soil, and Mixed soil. The major minerals found in the district are limestone, white quartz, kaolin, kyanite, and manganese.[11]{Figure 1}

Materials and Methods

Environmental radiation dosimeter (ER-709 radiation survey meter) with halogen quenched gamma radiation detector type GM132 is used to measure natural background radiation dose rate. The instrument was calibrated at the Radiation Standard and Calibration Lab, Nucleonix Systems (P) Ltd, using 137 Cs as a standard source. The instrument is calibrated to read exposure rate in two ranges with measuring sensitivity of 0.1 μRh –1 and 1 μRh –1 and exposure with measuring sensitivity of 1 μR and accuracy ±10% with Cs-137. The energy response within ±20% ranges from 60 KeV to 1.33 MeV. The ER709 manufactured by NUCLEONIX SYSTEMS PVT LTD, Hyderabad, India, is exclusively designed to serve as low-level survey meter in indoor and outdoor atmosphere. It is an ideal choice for environmental radiation monitoring and also for geological prospecting of radioactive minerals. The terrestrial gamma dose rates were measured at the distance of approximately one meter above the ground at the inside and outside of the different types of buildings, soil, and in some quarries. For each location, eight measurements were done with 4 min interval, and these measurements were then averaged to single value and used these values to calculate effective dose. Data obtained for the external exposure rate in µRh −1 were converted into absorbed dose rate (nGy/h) using the conversion factor μRh −1 = 8.7 nGy/h, which stems from the definition of Roentgen.[2],[12]

For calculating AED, we have used dose conversion factor of 0.7 Sv/Gy, and the occupancy factor (OF) for indoor and outdoor was 0.8 and 0.2, respectively. OF for indoor and outdoor situations was calculated based on interviews with peoples of the study area. People of the study area spent 5 to 6 h in outdoor and 18 to 19 h in indoor environment. This OF changes for women of the area who spent slightly more time in indoor environment as compared to men (OF = 5/24 for outdoor, 19/25 for indoor environment). The AED for the external terrestrial radiation was calculated as described elsewhere [8],[13] using formula:

AED (mSv/year) = D × T × OF × CC (1)

Where D is absorbed dose rate; T is time in hour for 1 year (8760 h); OF is 0.8 and 0.2 for indoor and outdoor exposure, and CC is the conversion coefficient; in the UNSCEAR 1993 report, the Committee used 0.7 Sv/Gy for the conversion coefficient from absorbed dose in air to effective dose received by adults, respectively.

Results and Discussion

The average absorbed dose rates from indoor and outdoor terrestrial gamma radiation at all the selected location of Shimoga district are summarized in [Table 1]. [Figure 2] gives indoor gamma absorbed dose rates; the values were found in the range of 114.05 ± 2.11 to 332.6 ± 3.99 nGy/h with an average value of 192.42 nGy/h. This value is two times higher than the world average of 84 nGy/h, respectively.[2] Similarly, [Figure 3] gives outdoor gamma absorbed dose rates; the values were found in the range of 87 ± 1.72 to 323.64 ± 16.6 nGy.h with an average value of 177.81 nGy/h. This value is three times higher than the world average of 59 nGy/h, respectively.{Table 1}{Figure 2}{Figure 3}

Except some locations, indoor absorbed dose rates were higher than the outdoor atmosphere. Higher levels of absorbed dose rate in indoor atmosphere are mainly depends on the use of rocks and building materials for the construction of buildings which contain higher concentrations of natural radionuclides such as 226 Ra,232 Th, and 40 K.[14],[15] The use of gneissic granites, soil, and other decorative stones for the construction of walls and floor and due to poor ventilation conditions inside the buildings enhances the radon concentration and also radon daughter concentration; this contributes to the elevated gamma absorbed dose. The higher outdoor terrestrial gamma absorbed dose rate was observed at granite rock regions and in some granite quarries and some crushing plants; this is due to the fact that during activity of quarrying, drilling, and crushing plant operations large quantity of fines were generated, and it is dumped at some quarries and some crushing plant regions. The sizes of fines are very small; thereby, it increases the radioactivity.[16] The outdoor variations of gamma dose rates from place to place may be attributed due to change in weathering conditions. As under changing weathering conditions, fluctuations in radon progeny concentration in air take place due to rainfall, soil moisture, and snow cover.[2] In general, the highest mean terrestrial gamma absorbed radiation dose rate value based on geological background was found in areas with acid or basic intrusive geological features. These areas are extensively intruded by granitic rocks. The granite is relatively rich in radioactive minerals.[2] The terrestrial radionuclides present in the parent rocks may be the main contributors for the observed slightly higher background radiation level in this geological area. However, some measurements in areas with intrusive rocks of the granitic type also have doses rates below 200 nGy/h. This could be due to weathering mechanisms such as rainfall and flooding, which may have transferred the uranium and thorium from granitic soil.[17] The lower terrestrial gamma absorbed radiation dose rate was observed at some locations such as Hale Sigebagi, Marashetty halli, Kanakatte, and Meerapura; this due to the fact that these areas are attributed by laterites and schist; these rocks contains lower activity of primordial radionuclides. The mean outdoor terrestrial gamma dose rate in Shimoga district, India, and the world are presented in [Table 2].{Table 2}

Measured absorbed gamma dose rates were used to calculate the AED received by peoples of surveyed area. For calculating AED, we have used dose conversion factor of 0.7 Sv/Gy and the OF for indoor and outdoor was 0.8 and 0.2, respectively. In the UNSCEAR 1993[6] report, the Committee used 0.7 Sv/Gy for the conversion coefficient from absorbed dose in air to effective dose received by adults. Estimated values of AED for indoor exposures range from 0.559 to 1.631 mSv/year with an average value of 0.872 mSv/year. For outdoor exposure, AED ranges from 0.106 to 0.339 mSv/year with an average value of 0.235 mSv/year. The average outdoors AED received in habited areas of Shimoga district is 0.235 mSv/year; this is higher than the world outdoor AED average value of 0.07 mSv/year.[2] This is not expected to contribute significant additional hazard from the radiological health point of view. The annual dose limit for members of the public according to is 1 mSv/year, and this limit is not applicable to doses received from natural resources.[22]

Conclusions

The present study has measured the indoor and outdoor terrestrial gamma radiation dose rates in and around granite regions of Shimoga district. The average indoor terrestrial gamma radiation dose rate in the study area was found to be 192.42 nGy/h which is two times higher than the world average value, and the average outdoor terrestrial gamma radiation dose rate in the study area was found to be 177.81 nGy/h which is three times higher than the world average value. The higher values of terrestrial gamma radiation dose rate are observed at granite regions, granite quarries, and also in crushing plant location. An estimated value of AED for indoor exposures ranges from 0.559 to 1.631 mSv/year with an average value of 0.872 mSv/year. For outdoor exposure, AED ranges from 0.106 to 0.339 mSv/year with an average value of 0.235 mSv/year.

Financial support and sponsorship

Major Research Project (F. NO. 41-876/2012 [SR]) to carry out this research work.

Conflicts of interest

There are no conflicts of interest.

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