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Radioactivity Levels and Radiation Hazard Assessment in Soil Samples of Peshawar University.

Byline: Hasan Mahmood Khan Muhammad Ismail Muhammad Abid Zia and Khalid Khan

Summary: Presence of natural radionuclides (226Ra 232Th and 40K) and man-made radionuclide 137Cs has been investigated in soil samples collected from Peshawar University campus Khyber Pakhtunkhwa Pakistan using gamma-ray spectrometry. A High Purity Germanium (HPGe) detector was used for gamma spectrometry. The detector was connected with a computer based high resolution multi channel analyzer. The specific activities in soil ranged from 30.2 to 49.1 Bq/kg for 226Ra 54.8 to 77.3 Bq/kg for 232Th 9.8 to 46.6 Bq/kg for 137Cs and 373.6 to 1071 Bq/kg for 40K with the average values of 40.9 63.8 22.5 and 858.3 Bq/kg respectively. 137Cs an artificial radionuclide was also found in measurable concentration in all soil samples of University campus. The detection of 137Cs showed that the soil of University campus also gained some fallout from Chernobyl power plant nuclear accident in 1986. The radium equivalent activity was found lower than 370 Bq/kg (OECD safe limit) in all the soil samples.

The radium equivalent activity external and internal hazard indices have average values of 198.01 Bq/kg 0.54 and 0.64 respectively. The mean values of indoor and outdoor absorbed dose rate in air were found 109.36 nGy.h-1 and 91.13 nGy.h-1 respectively. In the present analysis 40K was the significant radionuclide detected in the University campus soil. The concentration of radionuclides found in the soil samples of University of Peshawar during the present investigations were nominal and do not produce any harmful health effects to the local residents.

Key words: Gamma emitting radionuclide soil radioactivity Raeq activity absorbed dose rates hazard indices.

Introduction Human beings are receiving radiation doses continuously from natural sources throughout their lifetime. Therefore information about radiation levels and radionuclide distribution in the environment is consider important for assessing the effects of radiation exposure due to both terrestrial and cosmogenic sources [1]. Human being may receive radioactivity and hazardous effects due to ionizing radiation by a number of ways. Ionizing radiation can cause harmful effects both internally and externally. The major way through which radioactive isotopes can affect human being is environmental segments and food chain [2]. The sources of these radionuclides may be man-made or natural. The man-made or artificial radioactivity sources include the uses of radionuclides in industries medicine in consumer products and nuclear weapon tests. The natural radioactivity background originates from thorium and uranium series from 40K and from the cosmic radiation interaction with matter [1 3].

At present more than 2500 radionuclides of different elements are known. Approximately 80 of these are found in nature while the rest are produced artificially as the direct product of different types of nuclear reactions or produced indirectly as the radioactive progenies of these products. Natural sources include various geological formations (such as rocks soils sediments) water air and vegetation contribute nearly 96% of the total radiation dose however about 4% is from man-made origin [4]. The radionuclides occur naturally in the environment are not uniformly distributed. The information about radionuclides distribution in rocks and soil plays a significant role in radiation protection and determination [5]. Natural environmental radioactivi- ty levels and related external exposure owing to - radiations occur at different levels in the soils of each area in the world depending upon primarily on the geological and geographical conditions of the earth [3].

Soil is a major source of continuous exposure of radiation for human. Soil acts as a medium for radionuclides transfer to the biological systems and therefore it is consider as the basic indicator of radiological contamination in the environment. Furthermore measurement of natural radioactivity from soil is generally essential for the purposes of radiation impact assessment radiation exploration and protection and for establishment of baseline data for future radiation investigations [6].

Artificial radioactivity produced high risks of large-scale environmental contamination during World War II after nuclear bombardment at Hiroshima and Nagasaki in August 1945 [7]. Later on mainly USA and former USSR as well as some other countries such as Britain France and China carried out several hundred nuclear weapons tests. Since the atomic bombing of Hiroshima and Nagasaki in Japan up to the recent nuclear tests by India and Pakistan more than 2400 nuclear weapon experiments have taken place world-wide. The total yield of all these tests was 530 megatons. In addition large amount of nuclear material for military purposes have been produced [8].

Due to the atmospheric nuclear weapons testing in the 1950s - 1960s the current annual doses due to radionuclide fall-out were estimated to be = 0.01mSv depending on regional rainfall compared with peak values (greater than 0.1 mSv) in the 1960s. The doses contributions from ingested 137Cs 14C and 90Sr were also included to these doses [9]. However the issue of radioactive pollution achieved extensive public importance because of Chernobyl accident. This accident released a huge quantity of radioactive contamination all over the world. Since 1945 two of the major fission products (137Cs and 90Sr) were discharged into the atmosphere as a result of various nuclear tests. These two radionuclides have very long half-lives and considered main contributors to the whole collective dose from artificial radioactivity. 137Cs (half-life 30.17 years) is a rich artificial product that decays slowly and is distributed widely through the upper atmosphere [10].

The entire annual worldwide effective dose per caput is 2.4 mSv due to natural radiation sources. The individual doses typical range is counted to be 1- 10 mSv. In any large population nearly10% would have annual effective doses higher than 3 mSv about 25% of the population would have annual effective doses smaller than 1 mSv and about 65% would be supposed to have annual effective doses between 1 and 3 mSv [11].

The present study of the soil samples for University of Peshawar Campus was performed to investigate the natural and artificial radioactivity in this area. This will complement our previous work on the radioactivity of soil and other items of the area giving the background radioactivity that can be useful in any radiological emergency [12-16]. This will also help to address the concerns in inhabitants of University Campus that disposal of waste from Institute of Radiotherapy and Nuclear Medicine (IRNUM) located at the University Campus may increase the radioactivity levels in the soils of Peshawar University Campus.

Results and Discussions

The specific activities of radionuclides (226Ra 232Th and 40K) investigated in the soil samples of Peshawar University area were determined using gamma ray spectrometry. The specific activities of radionuclides reported in Bq/kg (dry weight basis) are given in Table-1. The specific activity for 226Ra determined ranged from 30.2 to 49.1 Bq/kg with an average value of 40.9 Bq/kg. For 232Th the specific activity determined was from 54.8 to 77.3 Bq/kg with an average value of 63.8 Bq/kg. The specific activity ranged from 373.6 to 1071 Bq/kg with an average value of 858.3 Bq/kg for 40K in the present investigation. These results clearly indicate that major radionuclide found in higher amount in all soil samples was 40K. The -emitters (226 Ra and 232Th) were present in all soil samples however their activity was found somewhat higher than world average.

In the soil the specific activities of natural - emitters changed from location to location and soil to soil and were not uniform throughout the soil crust .137Cs is an artificial radionuclide that has entered in the environment. It is an anthropogenic and major artificial radionuclide that is released as fission products and has been detected on Earth's surface. Table-1 also shows the specific activities of 137Cs determined in the soil samples of Peshawar University area. The specific activities for 137Cs varied from 9.81 to 46.6 Bq/kg. The average value calculated for 137Cs was 22.5 Bq/kg. The 137Cs contamination in all soil samples is of great importance because it shows that the areas under study have gained some fallout. It is very complex to find out the source of this contamination. However it may be mainly due to Chernobyl accident occurred in 1986 and smaller part may be owing to deposition from various atmospheric nuclear weapons tests conducted in the past.

The average concentration of Cs-137 is comparable to average values found earlier in the soil samples of nearby area (Peshawar 19.6 Bq/kg and Nowshera 18.4 Bq/kg) [17].

The activity concentration of -emitters radionuclides (226Ra 232Th and 40K) in soil is not equally distributed all over the world. Therefore to express the specific activities of these -emitters

Table-1: Radionuclides determined in soil samples collected from Peshawar University.

Sample###Ra-226###Th-232###K-40###Cs-137

###Code###Bq/kg###Bq/kg###Bq/kg###Bq/kg

###S-1###34.9###60.1###1014###27.9

###S-2###49.1###68###1071###46.6

###S-3

###S-4

###30.2

###46.2

###67.7

###77.3

###817.2

###856.9

###....

###14

###S-5###39.5###58.1###968.9###22.4

###S-6###37.9###54.8###373.6###15.1

###S-7###46.3###61.1###814.4###9.81

###S-8###43.2###62.9###950.7###21.7

Minimum###30.2###54.8###373.6###9.81

Maximum###49.1###77.3###1071###46.6

Average###40.###63.8###858.3###22.5

radionuclides by a single value taking into account the radiation hazard related with them a common index radium equivalent activity (Raeq) is usually determined [18]. Raeq can be calculated as follow: Equation

where ARa ATh and AK are the specific activities of 226Ra 232Th and 40K respectively. It may be cleared that 226Ra (a decay product of 238U series) has replaced the 238U. Expressing Raeq by the above formula it has been considered that 10 Bq/kg of 226Ra 7 Bq/kg of 232Th and 130 Bq/kg of 40K generate similar gamma dose. The Raeq is associated with internal and external gamma ray dose due to radon and its daughters. According to OECD (Organization for Economic Cooperation and Development) for safe use the maximum Raeq should be less than 370 Bq/kg [19]. Raeq calculated for different soil samples investigated in the present study are given in Table-2.

Table-2: Radium equivalent activity (Raeq) internal hazard index (Hin) and external hazard index (Hex) calculated for Peshawar University soil samples.

Samples No###Raeq Activity (Bq/kg)###Hex###Hin

###S-1###198.76###0.54###0.63

###S-2###228.63###0.62###0.75

###S-3###189.78###0.51###0.59

###S-4###222.54###0.60###0.73

###S-5###197.03###0.53###0.64

###S-6###144.92###0.39###0.49

###S-7###196.23###0.53###0.66

###S-8

###Minimum

###206.19

###144.92

###0.56

###0.39

###0.67

###0.49

###Maximum###228.63###0.62###0.73

###Average###198.01###0.54###0.64

In addition internal and external hazard indices were also investigated for the Peshawar University area [18]. The Hex (external hazard index) can be calculated by the following equation:Equation

For radiation hazard to be negligible the Hex must be less than unity i.e. exposure to radiation due to any radioactive material to be limited to 1.5 mGy/year. For the maximum value of external hazard index to be less than 1 the maximum value of Raeq must be smaller than 370 Bq/kg.Equation

For safe use Hin should be less than unity for soil samples. The values for Hin and Hex determined for soil samples of Peshawar University are also given in Table-2.

Table-2 showed that the value of Raeq ranged from 144.92 to 228.63 Bq/kg with a mean value of 198.01 Bq/kg. The Raeq found in the present study is smaller than the safe limit (370 Bq/kg) as recommended by OECD [19]. The Hex and Hin ranged from 0.39 to 0.62 and 0.49 to 0.73 with a mean value of 0.54 and 0.64 respectively. Table-2 showed that the highest values have been found to be less than one for external and internal hazard indices. Therefore one can concluded on the basis of these results of Raeq activity Hex and Hin that there is no harmful effect to human health from radiation exposure of the soil of Peshawar University. The results showed for all the radiation parameters investigated for the soil of Peshawar University that all the values are well within the permissible limits. Table-3 showed a comparison of Raeq found in the present investigation with the reported values in literature for some other countries of the world and other areas of Pakistan.

The average Raeq value determined for the soil of Peshawar University was found somewhat higher than world average value. However it is comparable to the average values found for the other countries of the world and other areas of Pakistan.

For the radioactivity in soil of Peshawar University the absorbed dose rate in air was also calculated. Absorbed dose rate in air can be defined as the dose received in open air from the radiation emitted by the radionuclides present in the soil. It can be calculated using the following equation [21].Equation

where Doutdoor is the outdoor absorbed dose rate in air CRa CTh and CK are the specific activities of 226Ra 232Th and 40K respectively.

Table-3: Comparison of Raeq with some other countries of the world and some other areas of Pakistan.

Location###Minimum###Maximum###Average###Reference

###( Bq/kg)###( Bq/kg)###( Bq/kg)

###Charsadda###135###213###170.8###[12]

###(Pakistan)###

###Punjab

###(Pakistan)

###75###200###141###[23]

Jhangar Valley###102.4###327.7###195.3###[ 15 ]

###(Pakistan)###

###Kohistan###79.7###226.1###136.1###[ 10]

###(Pakistan)###

###Islamabad###128.3###206.4###166.2###[24]

###(Pakistan)

North Western###41.3###146.9###68.2###[25]

###areas of

###Pakistan

###Brazil###-###-###226###[26]

###India###131.05###242.04###185.37###[27]

###India###127.8###288.8###171.7###[28]

###Egypt

###Peshawar

###30.1

###144.9

###112.7

###228.6

###53.7

###198

###[29]

###Present

University Campus###study

###World###-###-###129.7###[21]

The total annual effective dose equivalents (outdoor + indoor) from terrestrial radiation was found to be 0.64 mSv y-1 of which 0.17 mSv y-1 came from outdoor and 0.47 mSv y-1 from indoor exposure. The corresponding world average value is 0.41 mSv y-1 of which major contribution is from indoor exposure (0.34 mSv.y-1) and minor contribution is from outdoor exposure (0.07 mSv y-1) [21]. Table-6 shows the comparison of present results with the corresponding world average values given in UNSCEAR Reports of 1988 and 2000 [3 21].

Experimental

The soil sample collected from the campus area were taken from top 5 cm layer of land and the soil from three different place were combined to make a single sample. The different sampling sites for soil samples selected were undisturbed open and relatively flat. Coring tool was used to take a sub- sample within 1 m2 area. For each sub-sample five cores were taken four cores from the corners and one core in the middle of 1 m2 area. To prepare a single sample these sub-samples were then mixed and were collected in labeled plastic bags. In the laboratory stones and roots were removed from the soil samples and then oven dried at 100 5 C for 24 hours so that the sample weight became constant. After drying the soil samples were grounded and sieved through 1-2 mm mesh sieve. To attained secular equilibrium between Ra and its short lived progeny 200 gram of soil samples were kept in air tight cylindrical plastic containers for at least four weeks before counting [22].

Table-4: Total radioactivity indoor and outdoor annual effective dose equivalents in soil samples and outdoor and indoor absorbed dose rates in air.

###Total activity

###Sample###Doutdoor###Dindoor###D (eff)outdoor###D (eff)indoor

###(Bq kg -1)

###No.###nGy h-1###nGy h-1###mSv y-1###mSv y-1

###226

###( Ra+232Th+40K)

###S-1###1109###100.74###120.89###0.18###0.52

###S-2###1188.1###101.12###121.34###0.19###0.52

###S-3###915.1###94.56###113.47###0.17###0.49

###S-4###980.4###112.56###135.08###0.21###0.58

###S-5###1066.5###71.39###85.67###0.13###0.37

###S-6###466.3###87.48###104.98###0.16###0.45

###S-7###921.8###101.10###121.32###0.18###0.52

###S-8###1056.8###60.09###72.10###0.11###0.31

###Minimum###466.3###60.09###72.10###0.11###0.31

###Maximum###1188.1###112.56###135.08###0.21###0.58

###Mean###963###91.13###109.36###0.17###0.47

Table-5: Comparison of outdoor absorbed dose rates in air of Peshawar University samples with other countries of the world [3].

###Location###Minimum (nGy h-1)###Maximum (nGy h-1)###Average (nGy h-1)

###China###2###340###62

###Hong Kong###51###120###87

###India###20###1100###56

###Egypt###20###133###32

###Japan###21###77###53

###Korea###18###200###79

###Belgium###13###180###43

###Denmark###35###70###52

###Switzerland###15###120###45

###Poland###18###97###45

###Spain###40###120###76

###Present study

###60###113###91

Table-6: Comparison of present results with the corresponding world average values as given in UNSCEAR 1988 and UNSCEAR 2000 [21 3].

###Present results (average)###Ratio of the

###Radiological parameters###World average

###Present average/ World average

###40

###K Bq/kg###858###420###2.04

###226

###Ra Bq/kg###41###35###1.17

###232

###Th Bq/kg###64###30###2.13

###Radium equivalent activity (Raeq)###198.01###129.7###1.53

###Internal hazard index (Hin)###0.64###0.5###1.28

###External hazard index (Hex)###0.54###0.5###1.08

###Outdoor Dose nGy h-1###91.13###61.9###1.47

###Indoor Dose nGy h-1###109.36###74.3###1.47

###Outdoor effective dose mSv y-1###0.17###0.1###1.7

###Indoor effective dose mSv y-1###0.47###0.3###1.57

###Total annual effective dose equivalents mSv y-1###0.64###0.41###1.56

A HPGe (high purity germanium) detector based gamma-ray spectrometer was used to measure the activity concentrations levels of various radionuclides in the soil samples of Peshawar University. Analysis of samples was performed in the Environmental Monitoring Laboratory of HPD (Health Physics Division) PINSTECH (Pakistan Institute of Nuclear Sciences and Technology) Nilore Islamabad. At HPD a closed-end coaxial high purity germanium (HPGe) detector (Model GC 3020 Canberra) with 8192 channels coupled with computer-based MCA acquisition board (Accuspec- A Canberra) was used. The detector resolution (full width at half maximum (FWHM)) for full energy peak of 1332.5keV gamma ray of Co-60 was 2 keV and peak to Compton ratio was 54:1. The detector relative efficiency was 30%. The results were investigated by using Windows based Genie-2000 software (Canberra). The HPGe system was calibrated with a standard reference material

Soil 375 from International Atomic Energy Agency (IAEA) for gamma ray activity determination in soil samples. The counting time for activity measurement calibration and background measurement during this work was 65000 seconds. The activity concentrations of radionuclides were investigated using the following equation [16]:Equation

where CS is counts per second (cps) for sample CB is cps for background E and P are detection efficiency and emission probability of -ray and t is counting time. The typical energy spectrum of the studied radionuclides in the soil samples is shown in Fig. 1. The single peak at 1460.8 keV (10.67) was used for analysis of 40K; however the determination of 226Ra and 232Th depended on the peaks of the decay products in equilibrium with their parent nuclides. For the analysis of 226Ra the gamma line of 214Pb at 351.9 keV was used for its specific activity. Similarly the determination of 232Th through its daughter radionuclide 228Ac was made assuming that it was in radioactive secular equilibrium with 232Th due to its short half-life of 6.15 h.

Conclusions

In the present study all four types of gamma emitter (40K 137Cs 226Ra and 232Th) were detected in the soil samples of Peshawar University. 40K was present in significant amount among all the radionuclides detected in all soil samples collected from Peshawar University. The radionuclides 226Ra and 232Th detected in soil samples were found in somewhat higher concentration than world average. 137Cs is an artificially generated radionuclide and was also present in all the soil samples. The amount of 137Cs determined in the soil samples of University Campus is comparable to that detected in soil samples of Peshawar and Nowshera area so it is concluded that there is no increase in the amount of 137Cs in soil of University Campus due to the nuclear facilities used in the IRNUM. The external and internal hazard indices were obtained to be less than acceptable limit of unity and Raeq activity was found smaller than 370 Bq/kg

Which reveals that the activity concentrations of radionuclides investigated in the selected area were nominal and do not cause harmful effect to the human health of the area. However this data may provide a general background level for the area studied and may also provide a guideline for future radioactivity investigation and assessment of radionuclides in the case of any radiological accident.

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Publication:Journal of the Chemical Society of Pakistan
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