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CHIN. PHYS. LETT. Vol.28, No.1 (2011)019301

Radioactivity Levels and Gamma-Ray Dose Rate in Soil Samples from Kohistan(Pakistan) Using Gamma-Ray Spectrometry

Hasan M. Khan1, M. Ismail1, K. Khan2, P. Akhter2

1National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistan2Health Physics Division, PINSTECH, P. O. Nilore, Islamabad, Pakistan

(Received 10 June 2010)

The analysis of naturally occurring radionuclides (226Ra, 232Th and40K) and an anthropogenic radionuclide137Csis carried out in some soil samples collected from Kohistan district of N.W.F.P. (Pakistan), using gamma-rayspectrometry. The gamma spectrometry is operated using a high purity Germanium (HPGe) detector coupledwith a computer based high resolution multi channel analyzer. The specific activity in soil ranges from 24.72to 78.48Bqkg1 for 226Ra, 21.73 to 75.28Bqkg1 for 232Th, 7.06 to 14.9Bqkg1 for 137Cs and 298.46 to570.77 Bqkg1 for 40K with the mean values of 42.11, 43.27, 9.5 and 418.27 Bqkg1, respectively. The radiumequivalent activity in all the soil samples is lower than the safe limit set in the OECD report (370 Bqkg1).Man-made radionuclide 137Cs is also present in detectable amount in all soil samples. Presence of 137Cs indicatesthat the samples in this remote area also receive some fallout from nuclear accident in Chernobyl power plantin 1986. The internal and external hazard indices have the mean values of 0.48 and 0.37 respectively. Absorbeddose rates and effective dose equivalents are also determined for the samples. The concentration of radionuclidesfound in the soil samples during the present study is nominal and does not pose any potential health hazard tothe general public.

PACS: 93. 85. Np, 95. 85. Pw DOI: 10.1088/0256-307X/28/1/019301

Human beings are exposed to ionizing radiationfrom natural sources throughout their lifetime. There-fore, knowledge of radionuclide distribution and radi-ation levels in the environment is important for assess-ing the effects of radiation exposure due to both ter-restrial and cosmogenic sources. The planet Earth is a

radioactive planet and natural radiations are presentin every human environment; Earths material, wa-ter, air, foods, and even human body contains natu-rally occurring radioactive materials.[1] Radionuclidespresent in the environment are normally found in lowconcentration. These may be naturally occurring (e.g.uranium-238, thorium-232, and their daughter prod-ucts and potassium-40 or those produced by cosmicrays interaction in the biosphere) or man made sources(e.g. originating from nuclear fallouts or emanatingfrom nuclear facilities).[2]

Naturally occurring radionuclides are not uni-formly distributed and the knowledge of their distri-bution in soil and rocks play an important role in ra-diation protection and measurement.[3] Natural envi-ronmental radioactivity and the associated externalexposure due to gamma radiation depend primarilyon the geological and geographical conditions, and ap-pear at different levels in the soils of each region in theworld.[4]

The experimental demonstration of fission by OttoFrisch in 1939, during World War II and its conclusionat Hiroshima and Nagasaki in August 1945 generatedpotential risks of large-scale environmental contam-

ination by artificial radioactivity.[5] Since then, sev-eral hundreds of various nuclear weapons have beentested, mainly by the USA and the former USSR andby some other countries, such as Britain, France andChina. However, the subject of radioactive contami-nation has gained considerable public importance be-

cause of Chernobyl accident. It was the worst nu-clear accident and released a large quantity of radioac-tive contamination throughout the world. 137Cs and90Sr are two of the fission products released into theatmosphere as a result of nuclear tests carried outsince 1945. Due to their long half-lives, these tworadionuclides are considered to be the major contrib-utors to the overall collective dose from artificial ra-diation. 137Cs is an abundant fission product thatdecays slowly (half-life 30.17 years) and is distributedglobally through the upper atmosphere. 137Cs is pri-marily released by ground level nuclear weapons tests.However it is also emitted due to routine activities ofnuclear power generation stations, fuel reprocessingplants and waste disposal sites.[6]

Several studies have been reported on natural andartificial radionuclides in the soil and other environ-mental samples as well as in food samples in differentparts of the world. However, very few data have beenreported in literature on the radiological conditionsand the background level of different radionuclidesin the environmental and food samples of Pakistan.Some studies have reported base line environment ra-dioactivity data in a few areas of Pakistan,[3,712] how-

To whom correspondence should be addressed. Email: [email protected] 2011Chinese Physical Societyand IOP Publishing Ltd

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ever, no data for Kohistan area of Pakistan was avail-able. Therefore, the primary aim of this work was tomeasure the radioactivity levels in soil samples fromdistrict Kohistan (N.W.F.P., Pakistan) for health riskassessment.

The word Kohistan is a Persian word, literallymeans Country of the Hills. Kohistan is a sparselypopulated district of the North West Frontier Provinceof Pakistan. This area lies between the longitude of7250 E to 7357 E and the latitude of3455 N to3548 N. The soil samples were collected from var-ious places of Kohistan district (N.W.F.P.) of Pak-istan. The sampling was carried out on equidistantbasis at longitude and latitude, each line of grid cov-ered the distance about 20 km to 30 km. The sam-pling sites were chosen to be relatively flat, open andundisturbed. The samples were taken from upper 5 cmlayer of land. Each sample was taken with a coringtool within area of 1m2. Five cores were taken foreach sample, one in the middle and four cores fromthe corner of 1m2 area. These samples were thenmixed to make a single sample and were collectedin labeled plastic bags. In the laboratory, after re-moving the roots and stones, soil samples were ovendried at100 5C for 24 h, so that the sample weightwas constant and then they were ground and sievedthrough 12 mm mesh sieve. We made 200 g soil sam-ples stored in air tight cylindrical plastic containersfor at least four weeks before counting, so that secu-lar equilibrium can be attained between 226Ra and itsshort lived progeny.[13]

Table 1. Radionuclides found in soil samples collected fromKohistan.

Sample Ra-226 Th-232 Cs-137 K-40No. Bqkg1 Bqkg1 Bqkg1 Bqkg1

S-1 24.72 0.95 25.77 1.62 11.36 0.64 407.38 15.66S-2 31.16 0.99 21.73 1.59 14.90 0.66 352.92 15.47S-3 35.38 1.01 32.59 1.68 9.27 0.64 363.81 15.50S-4 78.48 1.24 73.76 1.96 12.14 0.65 548.98 16.14S-5 35.61 1.01 41.94 1.75 7.84 0.63 385.60 15.58S-6 65.72 1.78 74.52 1.97 9.45 0.64 570.77 16.22S-7 27.76 0.97 29.05 1.65 7.95 0.63 374.70 15.54S-8 30.22 0.98 35.62 1.70 7.06 0.62 331.13 15.39

S-9 24.72 0.95 22.48 1.60 7.88 0.63 298.46 15.27S-10 67.36 1.18 75.28 1.97 7.06 0.62 548.98 16.14Min 24.72 0.95 21.73 1.59 7.06 0.62 298.46 15.27Max 78.48 1.24 75.28 1.97 14.90 0.66 570.77 16.22

Average 42.11 1.11 43.27 1.72 9.49 0.64 418.27 15.69

To estimate the activity levels of different radionu-clides in the soil samples, a high purity germanium(HPGe) detector based gamma-ray spectrometer wasused. Analysis of samples was carried out in the En-vironmental Monitoring Laboratory of Health PhysicsDivision (HPD), Pakistan Institute of Nuclear Sci-ences and Technology (PINSTECH), Nilore, Islam-

abad. At HPD, a closed-end coaxial high purity ger-manium (HPGe) detector (Model GC 3020 Canberra)with 8192 channels, coupled with computer-based

MCA acquisition board (Accuspec-A, Canberra) wasused. The relative efficiency of the detector was30% and its resolution (full width at half maximum(FWHM)) for full energy peak of 1332.5 keV gammaray of Co-60 was 2 keV and peak to Compton ratio was

54 : 1. The results were analyzed by using Windowsbased Genie-2000 software (Canberra). The HPGesystem was calibrated with a standard reference ma-terial, Soil 327, from IAEA (International Atomic En-ergy Agency) for gamma ray activity determinationin soil samples. The counting time for calibration,background measurement and measurement of activ-ity during this work was 65000 s (18.055 h). The ac-tivity concentration of radionuclide was determined byapplying the equation:[14]

AC(Bq/sample) = (CSCB)/tE P,

where CSis cps for samples, CB is cps for background,Eand P are detection efficiency and emission prob-ability of-ray,t is counting time.

The specific activities of natural radionuclidespresent in the soil samples of the selected areas ofKohistan district measured by direct gamma ray spec-trometry are given in Table1. The results for naturalradionuclides are reported in Bqkg1 on dry weightbasis. For 226Ra, the specific activity ranged from24.72 0.95 to 78.48 1.24 Bqkg1 with an averageof42.11 1.11 Bqkg1. For 232Th, the specific activ-ity ranged from 21.73 1.59 to 75.28 1.67 Bqkg1

with an average of43.271.72 Bqkg1

and for40

K, itranged from298.46 15.27to 570.77 16.22 Bqkg1

with an average of418.27 15.69 Bqkg1.Therefore, as far as the radioactivity from natural

radionuclides is concerned, the results clearly indicatethat40K is the only major radionuclide detected in sig-nificant amount in all soil samples. All other naturallyoccurring radionuclides (226Ra and 232Th) determinedwere in nominal concentrations. The specific activi-ties of these radionuclides in the soil were not uniformand even but varied from soil to soil and location-to-location depending upon the geological and topo-

graphic characters of the area under study.Cesium-137 is an anthropogenic radionuclide andamong the class of artificial isotopes released as fissionproducts, it is the most prominent isotope detected byits gamma radiation on Earths surface. The specificactivities of137Cs found in the soil samples of selectedareas are summarized in Table 1. These values var-ied from 7.06 0.62 to 14.90 0.66 Bqkg1 with anaverage value of9.49 0.64 Bqkg1. The presence of137Cs in all the soil samples is very important and itclearly indicates that the areas under study have re-ceived some fallout. It is very difficult to point out the

source of this contamination. However, it may be dueto Chernobyl accident and minor component, may bedue to deposition from different atmospheric nuclear

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weapons tests in the past.The concentration and distribution of 40K, 226Ra,

and232Th in soil is not uniform through out the world,so to represent the specific activities of 226Ra, 232Thand 40K by a single quantity, taking into account

the radiation hazard associated with them, a com-mon index, radium equivalent activity (Raeq), is of-ten calculated.[15] Radium equivalent activity can bedefined as

Raeq = ARa+10

7ATh+

10

130AK,

whereARa, ATh and AK are the specific activities of226Ra,232Th and 40K, respectively. It may be notedthat238U has been replaced by a decay product 226Ra.Defining Raeq by the above equation, it has been as-sumed that 10 Bqkg1 of 226Ra, 7Bqkg1 of 232Thand 130Bqkg1 of 40K produce the same gamma

dose. The Raeq is related with external gammaray dose and internal gamma ray dose due to radonand its daughters. The maximum Raeq should beless than 370Bqkg1 for safe use as recommendedby the Organization for Economic Cooperation andDevelopment.[16] Radium equivalent activity calcu-lated for different soil samples is given in Table 2.

Table 2. Radium equivalent activity (Raeq), external hazardindex (Hex) and internal hazard index (Hin) calculated for soilsamples.

Samples No. Raeq Activity (Bqkg1) Hex HinS-1 92.86 0.25 0.32S-2 89.34 0.24 0.33S-3 109.92 0.30 0.39S-4 226.09 0.61 0.82S-5 125.18 0.34 0.44S-6 216.08 0.58 0.76S-7 98.09 0.26 0.34S-8 106.58 0.29 0.37S-9 79.79 0.22 0.28

S-10 217.13 0.59 0.77Minimum 79.79 0.22 0.28Maximum 226.09 0.61 0.82

Average 136.11 0.37 0.48

In addition, external and internal hazard indiceswere also calculated for the selected area.[15] The ex-

ternal hazard index Hex can be defined asHex = ARa/370 + ATh/259 + AK/4810.

It must be less than unity for radiation hazard tobe negligible, i.e. the radiation exposure due to ra-dioactive substances from any material to be limitedto 1.5 mGy/year. For the maximum value ofHex tobe less than unity, the maximum value of Raeq mustbe < 370 Bqkg1.

Internal hazard index Hin is given by[17]

Hin = ARa/185 + ATh/259 + Ak/4810.

Here Hin should be less than one for safe use forthe soil samples. The values calculated for inter-nal and external hazard indices calculated for the

soil samples are shown in Table 2. These resultsshow that the value of radium equivalent activityranges from 79.79 to 226.09 Bqkg1 with a meanvalue of 136.11 Bqkg1, which is less than the safelimit (370 Bqkg1) recommended by the Organiza-

tion for Economic Cooperation and Development.

[16]

The internal hazard index ranges from 0.28 to 0.82with a mean value of 0.48, while external hazard indexranges from 0.22 to 0.61 with an average of 0.37. Thehighest values of internal and external hazard indiceshave been found to be less than one, as shown in Table2. Therefore, based on these results of radium equiva-lent activity, internal and external hazard indices, onecan conclude that there is no health hazard from thesoil of district Kohistan as far as gamma radioactivityis concerned. The values of all radiation parametersstudied are well within the permissible limits.

The absorbed dose rate in air from radioactivity insoil is also determined. Absorbed dose rate in air isthe dose that is received in the open air from the ra-diation emitted from the radionuclides present in thesoil. It was calculated at a height of 1 m above thesurface of the ground using a computer programme(INGRE) based on the volume integral method usingthe conversion factors as follows:[18]

Doutdoor = (4.27CRa+ 6.62CTh+ 0.43CK)

101 nGy h1,

where Doutdoor is the outdoor absorbed dose rate in

air, CRa, CTh and CK are the specific activities of226Ra, 232Th and 40K, respectively.

The indoor contribution is assumed to be 1.2 timeshigher than the outdoor absorbed dose and is givenas[19]

Dindoor = 1.2Doutdoor.

The annual effective dose equivalent, Deff, from out-door terrestrial gamma radiation is given as[19]

D(eff)outdoor = (Doutdoor(nGy h1) 0.7(Sv Gy1)

8760(h y1) 0.3) 106,

where 0.3 is the occupancy factor for Pakistan, 8760is the number of hours in one year, 0.7 SvGy1 is thequotient of effective dose equivalent rate to absorbeddose rate in air and 106 is the conversion factor be-tween nano and milli.

For indoor exposure, the annual effective doseequivalent is given as[19]

D(eff)indoor= (Dindoor(nGy h1) 0.7(Sv Gy1)

8760(h y1) 0.7) 106,

where 0.7 is the occupancy factor for Pakistan and

0.7 SvGy

1

is the conversion factor between effectivedose equivalent rate and gamma dose rate in the in-door.

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Table 3. Total radioactivity, outdoor and indoor absorbed dose rates in air and annual effective dose equivalents in soil samples.

Sample Total activity (Bqkg1) Doutdoor Dindoor D(eff)outdoor D(eff)indoorNo. (226Ra+232Th+40K) nGyh1 nGyh1 mSvy1 mSvy1

S-1 457.87 45.13 54.16 0.08 0.23S-2 405.8 42.86 51.44 0.08 0.22S-3 431.78 52.32 62.79 0.10 0.27

S-4 701.22 105.95 127.14 0.20 0.55S-5 463.15 59.55 71.46 0.11 0.31S-6 711.01 101.94 122.33 0.19 0.52S-7 431.51 47.20 56.64 0.09 0.24S-8 396.87 50.72 60.87 0.09 0.26S-9 345.66 38.27 45.93 0.07 0.20

S-10 691.62 102.20 122.64 0.19 0.53Minimum 345.66 38.27 45.53 0.07 0.20

Maximum 711.01 105.9 5 127.14 0.20 0.55Average 503.65 64.61 77.54 0.12 0.33

These factors for the soil samples studied in thepresent study have been calculated, and the resultsare summarized in Table 3. Total specific activityfrom natural terrestrial radionuclides (40K, 226Ra, and232Th) is given in Table3and total terrestrial activityranges from 345.66 to 711.01 Bqkg1 with an averageof 503.65 Bqkg1.

The outdoor and indoor absorbed doses in air dueto these terrestrial radionuclides are also calculated,as shown in Table3. The mean value of outdoor ab-sorbed dose rate in air is 64.61nGyh1 with a rangeof 38.27 nGyh1 to 105.95 nGyh1. However, in-door absorbed dose rate ranges from 45.53 nGyh1 to127.14 nGyh1 with a mean value of 77.54 nGyh1.The large variations in the dose rate, as shown in Table3, are due to large variations in the specific activityof different primordial radionuclides in the soil sam-ples, which vary from soil to soil. The annual effec-tive dose equivalent from outdoor terrestrial gamma-radiation ranges from 0.07 mSvy1 to 0.20mSvy1

with a mean value of 0.12 mSvy1. For indoor expo-sure, the annual effective dose equivalent has a rangefrom 0.20 mSvy1 to 0.55 mSvy1 with a mean valueof 0.33 mSvy1.

The total (outdoor plus indoor) annual effectivedose equivalent from terrestrial radiation is found tobe 0.45mSvy1, of which 0.12mSvy1 comes from

outdoor and 0.33 mSvy1 from indoor exposure. Thecorresponding world average value is 0.41 mSvy1 ofwhich 0.07 mSvy1 is from outdoor and 0.34 mSvy1

from indoor exposure.[18]

The soil samples analyzed in the present studyshow all four types of radioactive contaminants (40K,137Cs, 226Ra and 232Th). Among all the radionuclide,only 40K is found in appreciable amount in all thesamples, and other radionuclides (226Ra, 232Th and137Cs) are in nominal concentration. 137Cs is an ar-tificially produced radionuclide and was also detectedin the soil samples. The origin of this artificially pro-

duced radioactive contaminant in soil is expected to befrom Chernobyl accident fallout or from nuclear tests

conducted by different countries in the past. The val-ues of radium equivalent activity and hazard indicesare found to be lower than the safe limit. The valuesof absorbed dose rates and effective dose equivalentsshow that the radioactivity of radionuclides found inthe surveyed area is nominal and does not pose any po-tential health hazard to the general public. However,these data may provide a general background level forthe area studied and may also serve as a guidelinefor future measurement and assessment of radionu-clides in the case of any radiological emergency andwill play a major role in radioactivity mapping of theentire country.

References

[1] Hutchison S G and Hutchison F I 1997 J. Chem. Edu. 74501

[2] Khan H M et al 1997 J. Nucl. Sci. 34209[3] Khan H M et al 1994 J. Chem. Soci. Pak. 16183[4] UNSCEAR 2000 Sources and Effects of Ionizing Radiation

(New York: United Nations Scientific Committee on theEffect of Atomic Radiation)

[5] Vandecasteele C M 2004 J. Environ. Radioactivity7217[6] Rowan D J et al 1998 J. Environ. Radioactivity. 4015[7] Khan H M et al 1995 J. Nucl. Sci. 32249[8] Tahir et al 2006 Radiat Prot. Dosim. 118345[9] Khan H M et al 2010 Water Air Soil Pollut. 213353

[10] Khan K et al 2003 International Conference on Isotopic

and Nuclear Analytical Techniques for Health and Envi-ronment (Vienna, Austria 1013 June 2003) IAEA-CN-103/005P

[11] Khan K et al 2010 Chin. Phys. Lett. 27039301[12] Chaudhry Z S et al 2002 J. Radioanal. Nucl. Chem. 253

497[13] IAEA 1989 Measurement of Radionuclide in Food and En-

vironment (Vienna: International Atomic Energy Agency)p 27

[14] Fredj A B et al 2005 Radiat Prot. Dosim. 117419[15] Beretka J and Mathew P J 1995 Health Phys. 4887[16] OECD 1979Exposure to radiation from natural radioactiv-

ity in building materials(Paris: Organization for EconomicCooperation and Development Nuclear Energy Agency)

[17] Gallyas M and Torok I 1984 Radiat Prot. Dosim. 769

[18] UNSCEAR 1988 Sources and Effects of Ionizing Radiation(New York: United Nations Scientific Committee on theEffect of Atomic Radiation)

[19] Hamid B N et al 2002 Radiat Prot. Dosim. 98227

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