Natural radioactivity in the newly discovered high background radiation area on the eastern coast of Orissa, India

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Radiation Measurements 38 (2004) 153 – 165 www.elsevier.com/locate/radmeas

Natural radioactivity in the newly discovered high background radiation area on the eastern coast of Orissa, India A.K. Mohantya;∗ , D. Senguptaa , S.K. Dasb , V. Vijayanc , S.K. Sahab a Department

of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal 721 302, India Division, Variable Energy Cyclotron Centre, BARC, Kolkata, West Bengal 700 064, India c Institute of Physics, Sachivalaya Marg, Bhubaneswar, Orissa 751 005, India

b Radiochemistry

Received 20 June 2003; accepted 14 August 2003

Abstract High levels of natural radiation areas occur in some parts of coastal tracts of India. The newly discovered Erasama beach placer deposit, part of the eastern coast of Orissa State, India is a high natural background radiation area, due to the presence of radiogenic heavy minerals. The average activity concentrations of radioactive elements such as 232 Th, 238 U and 40 K, measured by gamma-ray spectrometry using an HPGe detector, and found to be 2825±50, 350±20 and 180±25 Bq kg−1 , respectively, for the bulk sand samples. The absorbed gamma dose rates in air due to the naturally occurring radionuclides varied from 650 to 3150 nGy h−1 with a mean value of 1925 ± 718 nGy h−1 . The annual external e>ective dose rates for the region varyed from 0.78 to 3:86 mSv yr −1 with a mean value of 2:36 ± 0:88 mSv yr −1 . The external gamma dose rate level of Erasama coastal region is similar to the other monazite sand bearing high background radiation areas of southern and southwestern coastal regions of India. The major contributors to the enhanced level of radiation are monazites and to a lesser extent in zircons. c 2003 Elsevier Ltd. All rights reserved.  Keywords: Natural radioactivity; High background radiation area; Gamma dose rate; Monazite; HPGe detector; Orissa

1. Introduction Naturally occurring radionuclides of terrestrial origin are present in the earth’s crust since its origin. These primordial radionuclides have suBciently longer half-lives, that they survived since their creation and decaying to attain the stable state and produces ionizing radiation in various degrees. In most places on the earth, the natural radioactivity varies only within narrow limits, but in some places there are wide deviations from normal levels because of abnormally high levels of radioactive minerals. The distribution of naturally occurring radionuclides mainly uranium (238 U), thorium (232 Th), potassium (40 K) and other radioactive elements, depends on the distribution of rocks from which they originate and the processes which concentrate them. Exposure to ion∗ Corresponding author. Tel.: +91-3222-283380; fax: +91-3222-255303. E-mail addresses: [email protected] (A.K. Mohanty), [email protected] (D. Sengupta).

c 2003 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter
doi:10.1016/j.radmeas.2003.08.003

1izing radiation from natural sources is a continuous and unavoidable feature of life on earth. The major sources responsible for this exposure are due to the presence of naturally occurring radionuclides in the earth’s crust (UNSCEAR, 1993). Cosmic radiation can also contribute signiGcantly in areas at high altitudes (NCRP, 1987; UNSCEAR, 1993; Bennett, 1997). The aim is, therefore, to measure the radiation exposure, know the distribution of source-rock materials containing elevated levels of radionuclides, and to understand the physical and geochemical processes that concentrate the radionuclides. The higher concentrations of radionuclides in the earth’s crust such as 232 Th, 238 U and 40 K, which occur in minerals, such as monazites and zircons. There are few regions in the world, which are known for high background radiation areas (HBRAs), are due to the local geological controls and geochemical e>ects and cause enhanced levels of terrestrial radiation (UNSCEAR, 1993, 2000). Very HBRAs are found at Guarapari, coastal region of Espirito Santo and the Morro Do Forro in Minas Gerais in Brazil (Cullen, 1977; Penna Franca, 1977;

154

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

Bennett, 1997; Paschoa, 2000); Yangjiang, in China (Wei et al., 1993; Wei and Sugahara, 2000); southwest coast of India (Sunta et al., 1982; Sunta, 1993; Mishra, 1993; Paul et al., 1998); Ramsar and Mahallat in Iran (Sohrabi, 1993; Ghiassi-nejad et al., 2002); in the United States and Canada (NCRP, 1987), and in some other counties (UNSCEAR, 2000). The sources of these high background radiations are the monazite sand deposits in the Grst three cases, while the radium in soil/water and the radon in air are responsible for the radiation at Ramsar in Iran. In India, there are quite a few monazite sand bearing placer deposits causing natural HBRAs along its long coastline (UNSCEAR, 2000). Ullal in Karnataka (Radhakrishna et al., 1993), Kalpakkam (Kannan et al., 2002) in Tamilnadu, coastal parts of Tamilnadu and Kerala state and the southwestern coast of India are known for HBRAs (Mishra, 1993; Sunta, 1993). Some of these areas have been under study for many years in order to determine the risks and e>ects of long-term, low-level and natural radiation exposure (Sohrabi, 1998). Radiological investigations have been carried out in the southern coast of Orissa to measure the radiation dose rates and to trace the nature of minerals causing enhanced level of natural radiation (Nambi et al., 1994; Mohanty et al., 2004). However, there are no reports on the radionuclide enrichment and gamma dose rates in air in the eastern part of coastal Orissa. The present work investigates the activity concentration of radioactive elements such as 232 Th, 238 U and 40 K, in beach sand samples (for the bulk sands, individual fractions, heavy and light fraction minerals), and the associated radiation dose levels in air of the region. Also, attempt has been made to understand the possible geological factors responsible for the enrichment of radiogenic heavy minerals in such region.

2. Experimental methods 2.1. Sample collection and preparation techniques The sand samples containing heavy minerals were collected from the Erasama beach placer deposit by the grab sampling method at an interval of ∼ 1 km (Fig. 1). About 1 kg of sand samples was collected from each location. In the laboratory, the samples were cleaned with warm water, dried and subjected to heavy mineral separation by gravimetric method using bromoform (tribromo methane,  = 2:89). The light fraction mineral sands are essentially quartz, potash feldspar, and some mica. The total heavy mineral (THM) population by weight varies between 45 and 60 wt%. The THM concentrate indicates above did not reLect the average grade of the entire beach placer deposit, which is considerably lower. The heavy mineral concentrates were then separated into di>erent individual mineral fractions with a hand magnet and Frantz isodynamic separator (Rosenblum, 1958). In the latter, the setting was used for the separation of minerals with a forward and side slope of 15◦ and 25◦ , respectively. Sample splitting was conducted meticulously to ensure that the mineral fractions are statistically representative. The following current settings were used for the separation of heavy fraction minerals (i) magnetite, 0 –0:1 A, (ii) ilmenite, 0.1–0:35 A, (iii) garnet, 0.35 –0:45 A and (iv) monazite, 0.40 –0:8 A. However, due to overlapping of magnetic susceptibilities of sillimanite, rutile and zircons, it was not possible to separate the pure fractions of these minerals by using Frantz isodynamic separator, high force magnetic separator and lift roll magnetic susceptibility separator. So these three minerals fractions, produced in this manner, seem to be not entirely homogeneous, and were measured for the radiometric analysis.

1.1. Physiographic setting

2.2. Microscopic study

The study area, Erasama beach (named after the local administrative area), a coastal strip (Lat. 86◦ 25 –86◦ 33 N, Long. 20◦ 1 –20◦ 11 38 E) is a part of the eastern coast of Orissa state, India (Fig. 1). The area extended over a coastal length of 24 km and average width more than 1 km, trending almost NE–SW, bounded by the Bay of Bengal in the southeast and coastal alluviums of Pleistocene age in the northwest side. The area is almost like a Lat terrain with a gentle slope towards the Bay of Bengal and characterized by recent alluvium and coastal deltaic plain sediments. The major drainage pattern of the area is Mahanadi River and its tributaries (Devi River) and they join the Bay of Bengal at a number of places at eastern coast of Orissa. The streams are meandering to braided in nature. Also the area is characterized by a wide number of geomorphologic features such as beach, berm and dune in various dimensions. There are a few numbers of easterly Lowing streams cut across the coastal belt and Low to the sea through numerous lagoons and backwaters.

Microscopic study reveals that the heavy mineral assemblage consists of ilmenite, garnet, sillimanite, rutile, zircon, monazite, and magnetite were the dominant heavy minerals. Also minor amounts of kyanite, hornblende, diopside, sphene, tourmaline, epidote, amphibole and pyroxene were found, while the light sand minerals consist of quartz, feldspars and micas. It is also noticed that the concentrations of magnetite, zircon and garnet were signiGcant. Ilmenites were mostly subrounded and show the textural patterns of seriate, granular, myrmeckititic and emulsion. Leucoxene and anatase occurs as patches along margin, fracture and within ilmenite due to alteration. Subrounded grains of rutiles are found in small amounts. Monazite grains were subrounded to rounded and show some pitted marks on the surface due to action of chemical leaching. Zircon grains were elliptical to subhedral. Some zircon grains show zoning and some metamict varieties were also found. Garnet grains were coarser in size, angular to subangular in shape and bu> red to brown red in color. Sillimanite grains show

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

155

Fig. 1. Map showing the sample locations in the Erasama beach placer deposit.

prismatic forms with smooth edges. These heavy mineral sands were also further identiGed using X-ray di>raction method (XRD, Phillips PW-1710 di>ractometer, Co K). 2.3. Radiometric analysis The gamma-ray spectrometric analysis was carried out at the Radiochemistry Division, Variable Energy Cy-

clotron Centre, BARC, Kolkata, using a coaxial HPGe detector (EG & G, ORTEC) with a 15% relative eBciency. The detector was placed in a 10 cm shield of lead bricks to reduce the background radiation originating from building materials and cosmic rays. The detector was cooled to liquid-nitrogen temperatures and coupled to a PC-based 4k multi-channel analyzer and an ADC (ORTEC model 92X-spectrum master), with appropriate

156

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

software (Winmca-maestro) for the data acquisition and analysis. EBciency of the detector was determined with a 152 Eu liquid source (Amersham Company, UK) of known activity. 152 Eu liquid source have been widely used for calibration and eBciency determination due to their large range of energies (122, 244, 344, 411, 443, 779, 964, 1112 and 1408 keV) with emission probabilities of 3–29% (Firestone and Shirley, 1998; Grigorescu et al., 2002). The 152 Eu liquid source was thoroughly mixed with the normal silica sands, whose activity level was similar to the background radiation level. An ideal measuring geometry of cylindrical source (homogeneously distributed activity with constant volume and distance) was placed coaxially with the detector for the eBciency determination and the same procedure applied for the sample measurements. The samples were sealed and stored in a tight container to prevent the escape of radiogenic gases 222 Rn and 220 Rn, and to allow the attainment of radioactive equilibrium in the decay chain (Evans, 1969). After attainment of secular equilibrium between 232 Th, 238 U and their daughter products, the samples were subjected to gamma-ray spectrometric analysis. Natural radionuclides of relevance for this work are mainly gamma-ray emitting nuclei in the decay series of 232 Th and 238 U, and single occurring 40 K. While 40 K can be measured directly by its own gamma-rays, 232 Th and 238 U are not directly gamma-ray emitters, but it is possible to measure gamma-rays of their decay products. Decay products for 238 U (214 Pb: 295 and 352 keV; and 214 Bi: 609 keV) and 232 Th (228 Ac: 209, 338, and 911 keV; 212 Pb: 239 keV; 212 Bi: 727 keV; and 208 Tl: 583 keV) were used by assuming the decay series to be in secular equilibrium (Firestone and Shirley, 1998). Weighted averages of several decay products were used to estimate activity concentrations of 238 U and 232 Th. The natural abundance of 235 U is only 0.72% of the total uranium content and hence was not considered in the present study. The activity concentrations of 40 K were measured directly by its own gamma rays (1460:8 keV). However 228 Ac, a daughter nuclide of 232 Th, produces 1459:2 keV gamma rays, which interferes with 40 K (1460:8 keV). Thus, when the Th content was very high, it becomes diBcult to determine the 40 K content accurately. This problem is encountered in the analysis of monazite and zircon-rich samples. The activity concentrations were calculated from the intensity of each line taking into account the mass of the sample, the branching ratios of the gamma decay, the time of counting and the eBciencies of the detector. The relationship between the number of atoms of a certain species, N , and its activity, A, is deGned as N = T1=2 A=ln 2;

(1)

where T1=2 is the half-life of the radionuclides. Activity concentrations are given as Bq kg−1 . With Eq. (1) and Avogadros’s number, one can calculate that, 1 ppm of Th and U corresponds to 4.04 and 12:36 Bq kg−1 , respec-

tively; where as 1% of K2 O corresponds to 252 Bq kg−1 of K. Activity concentrations, calculated from the intensity of several gamma-rays emitted by a nuclide, are grouped together to produce a weighted average activity per nuclide. Self-absorptions in monazite samples are found to be very minor and also similar for all the samples. Errors arise due to a number of factors, like the volume of the samples, eBciency calibrations, peak area determination and random uncertainties associated with background and sample counts. All these errors were estimated to be in the order of 5 % for the 232 Th and 238 U decay series radionuclides. The error for 40 K is larger (more than 20 %), due to the contamination of its gamma-ray line (1460:8 keV) with that of 228 Ac (1459:2 keV). The energy resolution of the detector was 1:95 keV at 1332 keV of a 60 Co source. For the inter-laboratory comparison, a few samples were analyzed at the Nuclear Geophysics Division, Kernfysisch Versneller Insituut, Groningen, the Netherlands. The inter-comparison results were found within the limits of experimental error. The activity concentrations of bulk sand samples are shown in Table 1 and those for various mineral fractions are shown in Table 2. A gamma-ray spectrum of monazite sand samples recorded with the HPGe detector is presented as an example, showing the gamma lines from various daughter radionuclides of 232 Th, 238 U and 40 K series, forms well-deGned peaks (Fig. 2). 40

2.4. Absorbed gamma dose rate measurement The absorbed gamma dose rates in air at 1m above the ground surface for the uniform distribution of radionuclides (232 Th, 238 U and 40 K) were computed on the basis of guidelines provided by UNSCEAR (1993, 2000). We assumed that the contributions from other radionuclides, such as 137 Cs, 235 U, 87 Rb, 90 Sr, 138 La, 147 Sm and 176 Lu to the total dose rates were insigniGcant. The conversion factors used to compute absorbed gamma dose rate (D) in air per unit activity concentration in (1 Bq kg−1 ) sand corresponds to 0:621 nGy h−1 for 232 Th, 0:462 nGy h−1 for 238 U, and 0:0417 nGy h−1 for 40 K. D = [0:621CTh + 0:462CU + 0:0417CK ] nGy h−1 ;

(2)

where, CTh , CU and CK are the average activity concentrations of 232 Th, 238 U and 40 K in Bq kg−1 , respectively. To estimate the annual e>ective dose rates, the conversion coeBcient from absorbed dose in air to e>ective dose (0:7 Sv Gy−1 ) and outdoor occupancy factor (0.2) proposed by UNSCEAR (2000) were used. The e>ective dose rate in units of mSv yr −1 was calculated by the following formula: E>ective dose rate (mSv yr −1 ) =Dose rate (nGy h−1 ) × 8760 h ×0:2 × 0:7 Sv Gy−1 × 10−6 :

(3)

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165 Table 1 Activity concentrations of

232 Th, 238 U

and −1

40 K

157

in bulk sand samples and absorbed dose rates in air Dose rate (nGy h−1 )

ER1 ER2 ER3 ER4 ER5 ER6 ER7 ER8 ER9 ER10 ER11 ER12 ER13 ER14 ER15 ER16 ER17 ER18 ER19 ER20 ER21 ER22 ER23 ER24

1600 ± 50 2600 ± 60 2000 ± 50 3000 ± 70 2900 ± 50 900 ± 30 1100 ± 30 4500 ± 80 2900 ± 40 3500 ± 50 1750 ± 40 4600 ± 70 4700 ± 80 2050 ± 40 1800 ± 30 1900 ± 40 3400 ± 40 4000 ± 70 2400 ± 40 3100 ± 50 2300 ± 40 3600 ± 50 3900 ± 50 3300 ± 40

200 ± 15 400 ± 20 250 ± 10 450 ± 20 450 ± 20 150 ± 10 200 ± 15 500 ± 25 400 ± 20 500 ± 25 300 ± 15 500 ± 30 500 ± 30 300 ± 20 250 ± 15 200 ± 10 350 ± 20 450 ± 25 250 ± 15 400 ± 20 250 ± 20 400 ± 25 500 ± 30 450 ± 20

200 ± 30 150 ± 20 180 ± 25 200 ± 25 120 ± 25 200 ± 30 200 ± 25 100 ± 15 250 ± 30 180 ± 25 250 ± 30 150 ± 20 120 ± 20 230 ± 15 300 ± 20 250 ± 40 180 ± 30 170 ± 20 200 ± 20 200 ± 25 180 ± 20 120 ± 15 100 ± 20 150 ± 25

1094 ± 40 1806 ± 50 1365 ± 35 2079 ± 50 1991 ± 40 637 ± 25 784 ± 25 3030 ± 60 1996 ± 35 2412 ± 45 1236 ± 35 3094 ± 60 3155 ± 65 1421 ± 35 1246 ± 30 1283 ± 30 2280 ± 35 2700 ± 55 1614 ± 30 2118 ± 40 1551 ± 35 2425 ± 45 2657 ± 45 2263 ± 35

Mean Std. dev. Median Skewness Kurtosis Frequency distribution

2825 ± 50 1079 2900 0.09 −0:80 Normal

350 ± 20 115 400 −0:24 −1:36 Normal

180 ± 25 51 180 0.25 −0:14 Normal

1925 ± 40 718 1994 0.06 −0:84 Normal

)

The results obtained for the gamma radiation dose rate in air are presented in Table 1 and the comparative results for similar high background radiation areas of India are shown in Table 3. 2.5. Chemical analysis Chemical analyses of monazite and ilmenite samples were also carried out for the provenance study and also to trace their origin. In the present work, the chemical analysis was carried out using proton-induced X-ray emission (PIXE) technique (Johansson and Campbell, 1988). The advantages of PIXE technique are for its non-destructive, rapid quantitative estimation and high sensitivity for simultaneous multi-elemental analysis in geological samples (Halden et al., 1995). For chemical analyses of samples by PIXE method, small amounts (∼ 2 g) of monazite and ilmenite sand samples were crushed, Gnely powdered and thoroughly mixed with

40 K

(Bq kg−1 )

232 Th

(Bq kg

238 U

(Bq kg−1 )

Sample no.

the binding material (such as pure graphite powder and cellulose nitrate powder) in the ratio of 1:1 by weight. These solid mixtures were thoroughly ground, homogenized and pressed into pellets. The pellets were made of 13 mm diameter and 1:5 mm thickness. Pellets made of elemental standards (SPEX International standards) were also prepared following the same procedure. The PIXE analysis was carried out using a 3 MV Tandem Pelletron accelerator (9SDH-2, NEC, USA) at the Institute of Physics, Bhubaneswar, India. This accelerator has been routinely used for carrying out material analysis using the techniques of PIXE, RBS/Channeling, NRA, micro-PIXE, etc. (Rout et al., 2001; Mohanty et al., 2003). The proton beam was collimated to a diameter of 4 mm on the target under a vacuum condition (10−6 Torr) inside a PIXE chamber. The multiple-target holder was placed in the plane normal to the proton beam direction. The target holder was mounted on an insulated stando> and is surrounded by a cylindrical electron suppressor held at negative potential with re-

158 Table 2 Activity concentration of

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

232 Th, 238 U

and

40 K

in di>erent individual fraction mineral sands (Bq kg−1 )

40 K

(Bq kg−1 )

Sample I.D.

232 Th

Heavy sands

HS1 HS2 HS3 Average

7250 ± 150 6800 ± 150 7000 ± 200 7000 ± 170

750 ± 50 800 ± 50 700 ± 50 750 ± 50

100 ± 20 100 ± 20 100 ± 20 100 ± 20

Light sands

LS1 LS2 LS3 Average

150 ± 10 150 ± 10 150 ± 10 150 ± 10

150 ± 10 120 ± 10 120 ± 10 130 ± 10

250 ± 30 250 ± 30 250 ± 30 250 ± 30

Monazite

MZ1 MZ2 MZ3 Average

325000 ± 1700 312600 ± 2000 305000 ± 1800 314200 ± 1850

22000 ± 200 22500 ± 250 21500 ± 300 22000 ± 250

Zircon

ZR1 ZR2 ZR3 Average

1700 ± 120 1800 ± 100 1800 ± 100 1750 ± 100

3500 ± 100 3500 ± 100 3450 ± 100 3500 ± 100

Ilmenite

IL1 IL2 IL3 Average

150 ± 10 150 ± 10 150 ± 10 150 ± 10

50 ± 10 50 ± 10 50 ± 10 50 ± 10

Rutile

RT1 RT2 RT3 Average

200 ± 15 300 ± 10 250 ± 10 250 ± 10

300 ± 30 300 ± 20 300 ± 20 300 ± 25

Garnet

GT1 GT2 GT3 Average

270 ± 10 250 ± 10 250 ± 10 250 ± 10

150 ± 25 150 ± 30 150 ± 25 150 ± 25

Illuminate

SL1 SL2 SL3 Average

320 ± 30 300 ± 25 300 ± 25 350 ± 25

250 ± 20 220 ± 20 250 ± 20 250 ± 20

1000

238 U

(Bq kg−1 )

Mineral sand

212

Pb 208

Counts

800

228

Tl

Ac

600

40 214

400

Pb

214

Pb 208 228

200 0 100

300

K

228

Tl

Ac

500

214

Ac

Bi

228 212

Ac

Bi

700 900 Energy (keV)

1100

1300

1500

Fig. 2. A Gamma-ray spectrum of monazite sand sample recorded with the HPGe detector, where the gamma-lines of various radionuclides forms well deGned peaks.

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

159

Table 3 Comparison of radiation dose rate of Erasama coast, Orissa with similar HBRAs in di>erent parts of India Location in India

Characteristics of area

Absorbed dose rate in air (nGy h−1 )

Reference

Kerala coast Kalpakkam (Tamilnadu) Ullal (Karnataka) Ayirmamthengu (Kerala) Neendakara (Kerala) Tamilnadu coast Kudiraimozhi (Tamilnadu) Bhimilipatanam (Andhra Pradesh) Chhatrapur (Southern Orissa) Erasama (Eastern Orissa)

Monazite sands Monazite sands

200 – 4000 3500

Sunta et al. (1982) Kannan et al. (2002)

Monazite sands

2100

Radhakrishna et al. (1993)

Monazite sands

200 –1400

Sunta (1993)

Monazite sands

200 –3000

Sunta (1993)

Monazite sands Monazite sands

200 – 4000 200 –900

Sunta (1993) Paul et al. (1998)

Monazite sands

200 –3000

Paul et al. (1998)

Monazite sands

375 –5000

Mohanty et al. (2004)

Monazite sands

650 –3150

Present study

spect to the target. Integrated charge on the thick sample was measured using a current integrator, which was connected to the target holder. The targets were bombarded with 3 MeV proton beams with the beam current in the range of 3–5 nA. A 30 mm2 Si (Li) detector (ORTEC EG & G) with beryllium window thickness of 12
m was placed at ±90◦ with respect to the beam to record the characteristics X-rays, which were coming from the sample. The energy resolution of the detector found to be 170 eV full-width at half-maximum (FWHM) at 5:9 keV. A thick Glter (105
m aluminized mylar absorber) was kept in front of the detector to suppress the bremsstralung and the dominant low-energy X-ray peaks. The spectra were recorded by using a Canberra multi-channel analyzer with associated electronics. The system was calibrated using 55 Fe and 241 Am radioisotope standard X-ray sources, for the identiGcation of the X-ray peaks. The data were analyzed with the GUPIX programme (Campbell et al., 2000). Detection limit (95% conGdence) were typically 20 –50 ppm for Th and U. The results for ThO2 and UO3 in monazite samples are shown in Table 4 and those for TiO2 and FeO in ilmenite samples are shown in Table 5.

3. Results The activity concentrations of 232 Th, 238 U and 40 K, together with their average values for the bulk sand samples are shown in Table 1. The activity concentrations in bulk sand samples for 232 Th ranged from 900 to 4700 Bq kg−1 with an average of 2825 ± 50 Bq kg−1 , 238 U ranged from 150 to 500 Bq kg−1 with an average of 350 ± 20 Bq kg−1

Table 4 PIXE analyses of monazite sands for the ThO2 and UO3 (results are expressed in wt%) Sample no.

ThO2

UO3

Ratio (Th/U)

MZ1 MZ2 MZ3 MZ4 MZ5 MZ6 MZ7 MZ8 MZ9 MZ10 Average

8.75 9.67 9.20 8.38 9.32 8.44 8.10 8.60 8.45 9.11 8:77 ± 0:47

0.20 0.26 0.23 0.26 0.26 0.23 0.25 0.24 0.28 0.19 0:24 ± 0:03

46.20 39.27 42.24 34.03 36.63 38.75 34.21 37.84 31.87 50.63 38:59

Table 5 PIXE analyses of the ilmenite sands for TiO2 and FeO (results are expressed in wt%) Sample no.

TiO2

FeO

IL1 IL2 IL3 IL4 IL5 IL6 IL7 IL8 IL9 IL10 Average

58.08 56.46 56.44 56.66 55.79 55.39 56.88 54.79 55.91 55.24 55:24 ± 0:94

41.97 41.51 41.38 41.96 42.93 41.12 41.37 41.52 42.46 42.28 41:86 ± 0:58

160

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

238

-1

U (Bq kg )

600 R 2 = 0.84

500 400 300 200 100 0 0

1000

2000 232

3000

4000

5000

-1

Th (Bq kg )

Fig. 3. Correlation between activity concentrations of

232 Th

and

238 U

in bulk sand samples.

400

40

-1

k (Bq kg )

2

R = 0.44

300 200 100 0 0

1000

2000 232

3000

Fig. 4. Correlation between activity concentrations of

and 40 K ranged from 100 to 250 Bq kg−1 with an average of 180 ± 25 Bq kg−1 , respectively. In the samples of heavy and the light fraction mineral sands, the average activity concentrations were 7000 ± 170 and 150 ± 10 Bq kg−1 for 232 Th, 750 ± 50 and 130 ± 10 Bq kg−1 for 238 U and 100 ± 20, 253 ± 30 Bq kg−1 for 40 K, respectively. The average activity concentrations of 232 Th and 238 U in monazite sands were 314; 200 ± 1850 and 22; 000 ± 250 Bq kg−1 , respectively. The average activity concentrations of 232 Th and 238 U in zircon sands were 1750 ± 100 and 3500 ± 100 Bq kg−1 , respectively. The average activity concentrations of ilmenites and rutiles were found to be 150 ± 10, 250±10 Bq kg−1 for 232 Th, and 50±10, 300±25 Bq kg−1 , for 238 U, respectively. The activity concentrations of garnets and sillimanites were 250 ± 10, 350 ± 25 Bq kg−1 for 232 Th and 150±10, 250±20 Bq kg−1 for 238 U, respectively (Table 2). Trace amount of Th and U were found in ilmenite and rutile sands may be due to the alteration characteristics or incorporation of trace elements from surrounding environment during the formation of the magma (Frost et al., 1983). Also higher activity concentrations found in garnets were due to overlapping of magnetic susceptibility with the monazite grains during the separation process by Frantz isodynamic separator. The slight enrichment of 232 Th and 238 U activity in rutile and sillimanite samples may be due to the possible contamination of zircon sands from overlapping of magnetic susceptibility during the separation process.

4000

5000

-1

Th (Bq kg ) 232 Th

and

40 K

in bulk sand samples.

The ratio of activity concentrations (232 Th/238 U) in bulk sand samples varied from 5.83 to 9.71 with a mean (±SD) value of 7:85 ± 1:30. A correlation exists between 232 Th and 238 U in bulk sand samples (R2 = 0:84), which shows that the sample population is dominated by Th and U bearing minerals (Fig. 3). This clearly indicates the presence of signiGcant amounts of monazite and zircon sands in beach sand samples. A weak correlation exists between 232 Th and 40 K in bulk sand samples (R2 = 0:44), which indicates 40 K concentrations may not be related to the presence of 232 Th bearing mineral sands (Fig. 4). Table 1 shows the respective mean value, skewness, kurtosis coeBcients and the type of theoretical frequency distribution that best Gts each empirical distribution. It is observed that the values of skewness and kurtosis coeBcients for 232 Th, 238 U and 40 K activities and also in absorbed dose rates are closer to the null value, indicating the existence of normal distribution and the activity concentration is practically symmetrical, as shown in Fig. 5(a – d). From the activity concentrations of 232 Th, 238 U and 40 K of bulk sand samples, the gamma-absorbed dose rates in air were computed and the results are presented in Table 1. The absorbed dose rates varied from 650 to 3150 nGy h−1 with a mean (±SD) value of 1925 ± 718 nGy h−1 . The annual external e>ective dose rates varied from 0.78 to 3.86 with a mean (±SD) value of 2:36 ± 0:88 mSv yr −1 . The absorbed dose rate in air of Ersama beach sand region is compared with other similar monazite sand bearing HBRAs, such as in southern and southwestern coastal areas of India (Table 3).

A.K. Mohanty et al. / Radiation Measurements 38 (2004) 153 – 165

161

5 5

4 4

3

Frequency

Frequency

3

2

2

1

1

0 100

0 0

2000

3000

Activity concent ration of

(a)

4000 232

5000

6000

-1

Th (Bq kg )

6

6

5

5

4

4

3

300

400

500

238

600

-1

U (Bq kg )

3

2

2

1

1

0

200

Activ ity concen tration of

(b)

Frequency

Frequency

1000

0 50

(c)

100

150

200

250 40

300

350

-1

Activ ity concen tration of K (Bq kg )

0

(d)

500

1000

1500

2000

2500

3000

3500

4000

-1

Absorbed dose rates (nGy h )

Fig. 5. (a) Frequency distribution of 232 Th activity in bulk sand sample. (b) Frequency distribution of 238 U activity in bulk sand samples. (c) Frequency distribution of 40 K activity in bulk sand samples. (d) Frequency distribution of absorbed dose rate (nGy h−1 ) within the beach placer deposit.

4. Discussion Study of natural radioactivity is of considerable importance with respect to such as in future exploitation of nuclear energy, nuclear waste management problems, investigations of origin and distribution of radionuclides, radiation exposure in natural high background areas, processing of radioactive materials, and also in radiological research programmes (Eisenbud and Gesell, 1997; MacKenzie, 2000; UNSCEAR, 1993, 2000). Also measurement of natural radioactivity has been widely used in search for new mineral deposits in coastal zones, o>shore areas and also for sea Loor mapping for nuclear mineral resources (Grosz, 1983; Noakes et al., 1989), evaluation of mineral sand deposits such as heavy mineral estimation, provenance investigation,

various applications in mineral sand industries for the quality and grade control during mineral separation processes (Macdonald et al., 1997; de Meijer et al., 1997) and also in monitoring of waste disposal in seabed (de Meijer et al., 2002). The absorbed dose rates due to the presence of 232 Th, 238 U and 40 K, in sand samples of Erasama beach placer deposit varied in the range 650 –3150 nGy h−1 with a mean (±SD) value of 1925±718 nGy h−1 , which is much higher than the world average value of 55 nGy h−1 (UNSCEAR, 1988). The presence of 232 Th in beach sand contributed a maximum of 91% (1750 nGy h−1 ) to the total absorbed dose rate in air followed by 238 U of 8.5% (165 nGy h−1 ) and the minimum contribution by 40 K of 0.5% (8 nGy h−1 ). The annual external e>ective dose rates varied from 0.78

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to 3:86 mSv yr −1 with a mean (±SD) value of 2:36 ± 0:88 mSv yr −1 , which is similar to the HBRA in southern coast of Orissa like Chhatrapur beach placer region (Mohanty et al., 2004). In normal background areas, the average annual external e>ective doses from terrestrial radionuclides are 0:46 mSv yr −1 (UNSCEAR, 1993; Bennett, 1997). On the basis of higher levels of natural radioactivity and gamma-absorbed dose rates in air, Erasama beach placer region, eastern part of coastal Orissa can be considered as a high natural background radiation area (UNSCEAR, 1993, 2000). The gamma-absorbed dose rates in air of the Erasama beach placer deposit have been compared with the monazite sand bearing similar HBRAs of coastal regions in di>erent parts of India. The monazite sand bearing beach placer deposits of southern and southwestern coastal regions of India like Ullal in Karnataka (Radhakrishna et al., 1993), Manavalakkurichy (Sunta, 1993), Kalpakkam in Tamilnadu (Kannan et al., 2002), coastal tracts of Tamilnadu and Andhra Pradesh (Paul et al., 1998), Chavara in Kerala (Sunta et al., 1982), other parts of southwest coast of India (UNSCEAR, 2000). HBRAs occur in di>erent parts of world such as, Guarapari and coastal regions of Espirito Santo, which are due to the presence of monazite sands, Morro Do Forro in Minas Gerais is due to volcanic intrusive, all are in Brazil (Cullen, 1977; Penna Franca, 1977; Pfei>er, et al., 1981; Paschoa, 2000), Yangjiang of China due to monazite sands (Wei et al., 1993; Wei and Sugahara, 2000), parts of southwest France due to presence of uranium minerals (Delpoux et al., 1997) and Nile delta in Egypt due to presence of monazite sands (El-Khatib and El-Kher, 1988). The very HBRAs of Ramsar are primarily due to the presence of very high amounts of 226 Ra and its decay products in hot spring waters brought to the surface by hot springs (Sohrabi, 1993; Ghiassi-nejad et al., 2002). Radiometric analysis of various fractions of heavy mineral sands shows that the monazite and zircon sands are highly radioactive as compared to other minerals in the heavy mineral suite. The PIXE analyses of monazite sands for Th and U are in good agreement with the HPGe analysis and the results are found within the experimental limit error. Monazite [(La,Ce,Nd,Th)PO4 ] is a natural phosphate mineral and one of the principal sources of rare-earth elements (REE) and thorium in the continental crust. The abundance of thorium typically about 10 wt% and that of U about 0:5 wt% are found in monazite crystals. Zircon typically contains 5 – 4000 ppm of U and 2–2000 ppm of Th (Deer et al., 1997). The actinides get incorporation into monazite crystal lattices may be due to the similar ionic radius and also due to their charge balance factors with other elements like Si and Ca during the formation of magma (Gramaccioli and Segalstad, 1978; Emden et al., 1997). The actinide incorporation is dominated by Th, as compared to U in monazite structure as observed in all monazite samples. The higher concentrations of Th have been commonly observed in all monazite samples, especially those of granitic origin (Murata et al., 1958),

although there exist several samples where U makes a significant contribution to the total actinide content (Gramaccioli and Segalstad, 1978; Demartin et al., 1991). Th and U substitution in monazite structure occurs via a combination of the two charge-balance-maintaining mechanisms (Th; U)4+ + Ca2+ = 2REE3+

(rare earth element);

(Th; U)4+ + Si4+ = REE3+ + P5+ : ThO2 concentrations in monazite sand grains can be geochemically correlated with monazites from di>erent geological provinces to identify the possible source rocks. The monazite sands from the Erasama beach placer deposit shows average content of ThO2 8:77 wt% and UO3 0:24 wt%. Such concentrations of ThO2 in monazite grains, may suggest a higher grade granulite facies metamorphic rocks are the principal source rocks such as khondalites, charnockites, granites, pegmatites, leptynites and granulites (Overstreet, 1967; Andreoli et al., 1994; Watt, 1995; Read et al., 2002). Ilmenites contain TiO2 (55.24 – 58:08 wt%) with an average of 56:24 ± 0:94 wt% and FeO (41.37–42:93 wt%) with an average of 41:86 ± 0:58 wt%. On the basis of TiO2 concentrations in ilmenites, early workers suggested the nature of source rocks, when TiO2 less than 50 wt%, considered as igneous source, while TiO2 greater than 50 wt%, considered as metamorphic source (Darby and Tsang, 1987; Basu and Molinaroli, 1989). The average concentrations of TiO2 56:24 wt% in ilmenite sand grains suggest higher grade metamorphic rocks such as granulite facies metamorphic rocks consists of khondalites, charnockites, metasediments, intrusive granites, leptynites, pegmatites and granulites. These rocks occur in nearby highland areas in the Mahanadi River drainage basin area. The geology of catchments area of Mahanadi River drainage basin is underlain by rocks of diverse age and origin, from the high lands of Eastern Ghats, Precambrian granites, gneisses and metasediments of Archaeans, Gondwana sedimentary rocks, deltaic sediments and coastal alluviums covering the central Indian terrain to the coast of Bay of Bengal, which are exposed in the hinterland areas (Rao et al., 2001). The Eastern Ghats group of rocks and Precambrians consist of granites, pegmatites, khondalites, charnockites, leptynites and other metasediments. These rocks consist of minor to trace amount of heavy minerals such as monazite, zircon, ilmenite, magnetite, garnet, rutile and sillimanite (Bea, 1996). Khondalite suites of rocks include garnetiferous quartz-sillimanite schists and gneisses, garnetiferous quartzites, leptynites, quartz-granulites, calc-silicate rocks and quartz-garnet-sillimanite-graphite schists (Nanda and Pati, 1989). Charnockite suite of rocks include tonalitic and granodioritic varieties in composition as well as pyroxene granulites (Park and Dash, 1984). Khondalites contain quartz, garnet, sillimanite and feldspar as major constituents and magnetite, ilmenite, rutile, sphene, apatite, zircon and monazite as minor constituents (Kamineni and Rao, 1988). The Mahanadi River also traverses

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through the coal bearing Gondwana rocks in the cental part of Orissa. Gondwana Group of rocks includes various types of sandstones, conglomerates, siltstones and undecomposed sediments, which consists of above minerals in a signiGcant concentration (Mohanty, 1996). The area enjoys a humid tropical monsoon climate with a hot season commencing from March upto mid-June. The southwest monsoon sets by middle of June and continues upto October. The basin gets 90% of the total rainfall from monsoon rains and also observed that more than 90% of the sediment discharge takes place during the monsoon season only. It has been observed that the Mahanadi River and its tributaries delivers 15:75 × 106 ton of sediments (dissolved and suspended) annually to the Bay of Bengal (Mahallik, 2000). Favorable geological, geomorphologic, humid tropical climate, chemical weathering and hydrodynamic conditions with well-developed drainage system have resulted in the formation of heavy mineral deposits of varied dimensions and concentrations along east coast beaches of India. The dimension of the beach placer deposits is controlled by the geomorphology of the coast, whereas the mineral assemblage is a reLection of the varied hinterland geology. The concentration depends on the hydrodynamic conditions like sediment inLux from the hinterland, wave energy and its velocity, long-shore current and wind speed, which controls the littoral transport, sorting and deposition of placer minerals in suitable locales (Singerland, 1977; Sallenger, 1979; Komar and Wang, 1984; Peterson et al., 1986; Li and Komar, 1992). Chemical weathering due to the humid tropical climate, Luvial and marine processes and the presence of suitable source rocks in the nearby hinterland areas are important factors responsible for the enrichment of huge volume of heavy mineral sand placer deposits in the beaches of Bay of Bengal.

the region. The sediments were possibly delivered through the Mahanadi River drainage systems to the coast of Bay of Bengal, which enriched in huge volume of mineral sand deposit.

5. Conclusions

Andreoli, M.A.G., Smith, C.B., Watkeys, M., Moore, J.M., Ashwal, L.D., Hart, R.J., 1994. The geology of the Steenkampskarl monazite deposit, South Africa: implications for REE–Th– Cu mineralization in charnockite–granulite terrains. Econom Geology 89, 994–1016. Basu, A., Molinaroli, E., 1989. Provenance characteristics of detrital opaque Fe–Ti oxide minerals. J. Sediment Petrol. 59, 922–934. Bea, F., 1996. Residence of REE, Y, Th and U in granites and crustal protoliths; implications for the chemistry of melts. J. Petrol. 37, 521–552. Bennett, B.G., 1997. Exposure to natural radiation worldwide. In: Proceedings of the Fourth International Conferene on High Levels of Natural Radiation: Radiation Doses and Health E>ects, 1996, Beijing, China. Elsevier, Tokyo, pp. 15 –23. Campbell, J.L., Hopman, L.T., Maxwell, J.A., Nejedly, Z., 2000. The Guelph PIXE software package III. Nucl. Instrum. Methods B 170, 193–204. Cullen, T.L., 1977. A Review of Brazilian investigations in areas of high natural radioactivity, Part I: radiometric and dosimetric studies. In: Cullen, T.L., Penna Franca, E. (Eds.), Proceedings of International Symposium on Areas of High

The absorbed gamma dose rates in air for the Erasama beach region, on the eastern coast of Orissa varied from 650 to 3150 nGy h−1 with a mean (±SD) value of 1925 ± 718 nGy h−1 and the annual external e>ective equivalent dose rate of 2:36 mSv yr −1 . Based on the higher levels of natural radioactivity and gamma-absorbed dose rates in air, this region can be considered as a high natural background radiation area (HBRA) and is comparable to other monazite sand bearing HBRAs in southern and southwestern coastal regions of India. The study indicates that the sources of enhanced level of natural radioactivity in the beach sands are found chieLy in monazite sands and to a lesser extent in zircons. In monazite crystal structure, Th concentration is more favored than U in the actinide content. The enrichment of radiogenic heavy minerals in the beach placer deposits, are chieLy controlled by the local geological conditions, geochemical nature of the source rocks, degree of weathering, favorable geomorphology and humid tropical climate of

Acknowledgements We are greatly indebted to our beloved friend late Shri Mihir Kumar Behera for his kind help and support provided during the Geldwork. Without his great support, it would not have possible to locate a new placer deposit in such a region. We are grateful to Prof. R.J. de Meijer of Nuclear Geophysics Division, Kernfysisch Versneller Instituut, Groningen, the Netherlands, for his numerous suggestions and help in radiometric analysis by HPGe detector, which considerably helped in the improvement of the present study. The support and encouragement from Pradipta Kumar Ram, during the sample collection is highly regarded. We are greatly acknowledge to Mr. Antaryami Sahoo, Directorate of Geology, Government of Orissa, Bhubaneswar and Mr. Rabindra Prasad, Dy. Manager, Mineral Separation Plant, IREL, Chhatrapur for their kind help, support and keen interest taken during the various stages of work. Also our sincere thanks to Tapash Ranjan Rautray, IOP, Bhubaneswar, for his kind help and keen interest taken during various stages of PIXE experimental work. This study is a joint collaborative research programme between the Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, the Radiochemistry Division, Variable Energy Cyclotron Centre, BARC, Kolkota and the Institute of Physics, Bhubaneswar. References

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