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Determination of 238U, 232Th and 40K activity concentrations in riverbank soil along the Chao Phraya river basin in Thailand
Journal of Environmental Radioactivity 138 (2014) 80e86
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Determination of 238U, 232Th and 40K activity concentrations in riverbank soil along the Chao Phraya river basin in Thailand T. Santawamaitre a, D. Malain a, H.A. Al-Sulaiti a, D.A. Bradley a, M.C. Matthews b, P.H. Regan a, c, * a b c
Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford GU2 7XH, UK Centre for Environmental Health Engineering, Department of Civil Engineering, University of Surrey, Guildford GU2 7XH, UK National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2014 Received in revised form 10 July 2014 Accepted 18 July 2014 Available online
The activity concentrations of 238U, 232Th and 40K in riverbank soil along the Chao Phraya river basin was determined through gamma-ray spectrometry measurements made using a hyper-pure germanium detector in a low background configuration. The ranges of activity concentrations of 238U, 232Th and 40K were found to be 13.9 4 76.8, 12.9 4 142.9 and 178.4 4 810.7 Bq kg1, respectively. The anthropogenic radionuclide, 137Cs, was not observed in statistically significant amounts above the background level in the current study. The absorbed gamma dose rate in air at 1 m above the ground surface, the outdoor annual effective dose equivalent, the values of the radium equivalent activity and the external hazard index associated with all the soil samples in the present work were evaluated. The results indicate that the radiation hazard from primordial radionuclides in all soil samples from the area studied in this current work is not significant. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Naturally occurring radioactive materials NORM Gamma-ray spectrometry
1. Introduction The most common radiation sources to which humans are exposed arise from radionuclides in the earth's surroundings and the interaction of cosmic rays on the earth's atmosphere (NCRP, 1975; UNSCEAR, 2000). This exposure to naturally occurring radiation accounts for up to 85% of annual exposure dose received by the world population. Apart from the exposure from direct cosmic rays and cosmogenic radionuclides, natural exposures arise mainly from the primordial nuclear species which are spread widely and are present in almost all geological materials in the earth's environment (NCRP, 1975; Wilson, 1994). The majority of naturally occurring radionuclides belongs to the decays in the 235,238 U and 232Th series and the single decay radionuclide, 40K (IAEA, 2003). The Chao Phraya river basin is made up of the main Chao Phraya River and its tributaries (Ping, Wang, Yom, Nan, Sakae Krung, Pa Sak and Tha Chin) cover a large area of the northern and central plain of Thailand (as shown in Fig. 1). The Chao Phraya River and its
* Corresponding author. Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford GU2 7XH, UK. E-mail address: [email protected] (P.H. Regan). http://dx.doi.org/10.1016/j.jenvrad.2014.07.017 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
tributaries are the principal river system in Thailand and are also the largest watershed, playing a significant role in the local irrigation. This river system supports agricultural activities which are widely spread along both sides of the rivers and the local vicinity (ONWRC, 2003). As a result of a large number of agricultural activities in this area, the high demand of fertiliser usage may result in contamination and an enhancement of the background activity level in particular areas (Almayahi et al., 2012). Fertilisers may be washed into the river and deposition of sediments might be expected to occur at riverbanks along the river (El-Gamal et al., 2007). The aim of this study is to evaluate the level of natural radioactivity in river sediments and riverbank surface soils collected along the Chao Phraya river basin in Thailand. The measurements of the activity concentrations of 238U, 232Th and 40K in soil samples were carried out by using high-resolution gamma-ray spectrometry with a hyper-pure germanium detector in a low background configuration. An assessment of the biological effects on humans due to the natural radioactivity arising from riverbank soils was determined in terms of the absorbed dose in air, the annual effective dose equivalent, the radium equivalent activity and the external hazard index. Preliminary analyses and results from this programme of work have been presented in conference proceedings (Santawamaitre et al., 2010, 2011). This paper presents the full, completed analysis of this study.
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Fig. 1. The map of Thailand showing the Chao Phraya river basin.
2. Material and methods 2.1. Sample collection and preparation In order to evaluate the levels of natural radioactivity, riverbank soil and sediment samples were collected at random positions from several different locations along the Chao Phraya River and its tributaries between June and July 2008. Fifty-one sampling locations were designated to be representative of the levels of natural radioactivity from the five different sampling areas, corresponding to the five main rivers in the Chao Phraya river basin, namely the Ping, Wang, Yom, Nan and Chao Phraya rivers. At each location, soil samples were taken from approximately 1 to 2 m away from the river edge. The samples were obtained at a typical depth of 5 cm from the top surface layer to produce approximately 2 kg wet weight per sample. After removing stones and vegetable matter, each soil sample was packed into its own secure water-tight bag to prevent cross contamination and shipped to the Environmental Radiation Laboratory at the University of Surrey, United Kingdom. On arrival at the laboratory at the University of Surrey, sample preparation was carried out by placing each soil sample in an oven for drying at a temperature of 60 C until a constant weight was reached, thus ensuring complete removal of any residual moisture. The dried samples were pulverised into a fine powder and passed through a standard 1 mm mesh size (ASTM No. 18). The homogenised samples were filled into 550 ml Marinelli beakers which were then hermetically sealed with the aid of PVC tape to prevent the escape of airborne 222Rn and 220Rn from the samples. All samples were weighed and stored for at least one month prior to measurement in order to attain radioactive secular equilibrium between 226Ra and 228Ac and their short-lived progeny (>7 half-lives of 222Rn).
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spectrometry system used consists of a coaxial hyper-pure germanium (HPGe) detector with passive shielding. The detector was enclosed with a 10 cm cylindrical lead shield to reduce the background radiation from various natural radiation sources and to isolate from other radiation sources used in nearby surroundings, and the lead shielding was graded with an inner layer of 0.1 cm thick copper to reduce the contribution from Pb X-ray fluorescence. The detector characterisation was performed in terms of energy and efficiency calibration using four different certified reference sources, (i) 152Eu, (ii) 226Ra, (iii) 232Th and (iv) a mixed radionuclide source (NG3), containing 57Co, 60Co, 85Sr, 88Y, 109Cd, 137Cs, 139Ce, 203 Hg and 241Am. The additional certification for the reference standards used in our Marinelli beakers has been included as has the use of RADWARE software and peak-fitting analysis (Radford, 1995 and references therein) in the analysis of the efficiency data. The source certificates were standardised by nuclitech GmbH via direct traceability to PTB and NIST gamma-ray standards using a high-purity germanium detector with a multi-channel analyser. The sources used were 152Eu (3.02 kBq on 1 Feb 2009) in a density of 1.6 g/cm3 with a relative uncertainty of 3%; 232Th (1.08 kBq) in a gel of density 1.1 g/cm3; 226Ra (3.10 kBq); Mixed radionuclide source (241Am, 109Cd, 57Co, 139Ce, 203Hg, 113Sn, 85Sr, 137Cs, 88Y and 60 Co) in density 1.6 g/cm3. The certification reference dates for each of these sources was 1 Feb 2009, with the relative uncertainty in the source quoted activities for k ¼ 2 was 5%. Each of these reference sources was contained in a homogenous gel matrix in a 500 ml Marinelli beaker in the same geometry as that of sample measurements using the same technique established in reference (Malain et al., 2012). With the same geometry and the same range in density between reference sources (1.1 g/cm3 for the 232Th and 226 Ra sources; 1.6 g/cm3 for the 152Eu and NG3 sources) and sand samples (which had typical measured densities of ~1.3e1.5 g/cm3), differences in the self-attenuation for gamma rays for the sources and samples was found to be a negligible contribution to the overall experimental uncertainties in the radiometric evaluations discussed in the current work. The measured absolute photopeak efficiency of the system was approximately 0.50% and 0.25% at 662 keV and 1332 keV, respectively (see Fig. 2). The measured full-energy peak efficiency data were fitted to a response function of the form suggested by Gray and Ahmad (Saradri and Macmahon, 2000)
i. h εg ¼ P1 þP2 ðln EÞþP3 ðln EÞ2 þP4 ðln EÞ3 þP5 ðln EÞ5 þP6 ðln EÞ7 E
2.2. Instrumentation and calibration The radioactivity concentration of radionuclides in soil samples was measured by using a high-resolution gamma-ray spectrometry system in a low background configuration. The gamma-ray
Fig. 2. Absolute full-energy peak efficiency as function of gamma-ray energy for the HPGe detector used in the current work using the 4 separate gel-matrix based Marinelli sources (see text for details).
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where P1eP6 are parameters of the fitting function and εg is the efficiency at energy E. The fitted curve for this full-energy peak absolute efficiency response function is shown in Fig. 2. 2.3. Activity concentration determination Each prepared soil sample was measured for 48 h using the coaxial hyper-pure germanium detector. The number of counts under the full-energy peak areas (corrected for background peak areas), the counting time, the absolute full-energy peak efficiency for the energy of interest and the gamma-ray emission probability corresponding to the peak energy were used for the calculation of the activity concentration of a particular radionuclide in the measured samples. One problem with the direct determination of the activity of 238U and 232Th is due to the low relative gamma-ray intensities following their decay. However, in a state of secular equilibrium, the activity of 238U and 232Th can be estimated through several intense gamma-ray lines of their daughter products in the decay chains. A range of gamma-ray transitions from a number of the decay chain members of the 238U and 232Th primordial decay chains, as listed in Table 1, was utilised to obtain consistent values for the activity concentrations of these nuclei and their subsequent decay daughters. The activity concentration values quoted assume secular equilibrium for the different isotopic activities in the decay chains. The activity concentrations of the singly decaying nuclei, 40 K and 137Cs were determined directly by measurement of the gamma-ray transitions at 1460.8 and 661.6 keV, respectively. The specific activity, in terms of the activity concentration, is defined as the activity per unit mass of the sample. The specific activity of individual radionuclide in soil samples is given by the following equation (Dovlete and Povinec, 2004; ASTM, 2005):
A¼
Cn εf Pg ts m
where A is the activity concentration of a particular nuclide in units of Bq kg1, Cn the net count (background subtracted) of the corresponding full-energy peak, εg the absolute full-energy peak detection efficiency, Pg the emission probability per decay corresponding to the specific gamma-ray energy, t the counting time in s and m the mass of soil sample in kg. 2.4. Dose calculation 2.4.1. Absorbed dose rate in air (D) To assess the degree of any associated radiological hazard, the exposure to radiation arising from radionuclides present in soil can be determined in terms of a number of different, but related parameters. A direct connection between radioactivity concentrations of natural radionuclides and their exposure is known as the absorbed dose rate (D) in the air at 1 m above the ground surface. The mean activity concentrations of 226Ra (238U), 232Th and 40K in units of Bq kg1 in the soil samples can be used to calculate the absorbed dose rate using the following relation (Beck, 1972; Chang et al., 2008):
D nGy h1 ¼ ð0:462ARa þ 0:604ATh þ 0:0417AK Þ where ARa, ATh and AK are the activity concentrations of 226Ra (238U), 232 Th and 40K, respectively. The dose coefficients in units of nGy h1 per Bq kg1 were taken from the UNSCEAR (2000) report. 2.4.2. Annual effective dose equivalent (AEDE) The absorbed dose rate in air at 1 m above the ground surface does not directly provide the radiological risk to which an
Table 1 Details of the location of soil samples from the 5 sampling areas measured in the current work. Sample No.
Sample Code
1 2 3 4 5 6
CMP1 CMP2 CMP3 TKP1 TKP2 KPP1
7
KPP2
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
NWP1 LPW1 LPW2 LPW3 LPW4 PHY3 PHY2 PHY4 PHY1 STY3 STY2 STY1 PLY1 PCY2 PCY1 NAN3 NAN2 NAN1 UDN3 UDN2 UDN1 PLN1 PCN2 PCN1 NWN1 NWN2 NWC1 NWC2 CNC1 CNC2 CNC3 SIC2 SIC1 ATC1 ATC2 AYC1 AYC2 AYC3 PTC1 NNC1 NNC3 UTS1
50
CNT1
51
APS1
Location River Ping
Wang
Yom
Nan
Chao Phraya
Sakae Krang Tha Chin Pa Sak
Position Province
Latitude ( N) 0
Longitude ( E)
00
19 5 36.4 18 480 6.100 18 170 26.700 17 90 32.400 16 550 39.200 16 270 6.200
98 560 24.500 99 00 11.200 98 380 51.800 99 40 25.100 99 40 43.000 99 310 20.300
Chiang Mai Chiang Mai Chiang Mai Tak Tak Kamphaeng Phet Kamphaeng Phet Nakhon Sawan Lampang Lampang Lampang Lampang Phrae Phrae Phrae Phrae Sukhothai Sukhothai Sukhothai Phitsanulok Phichit Phichit Nan Nan Nan Uttaradit Uttaradit Uttaradit Phitsanulok Phichit Phichit Nakhon Sawan Nakhon Sawan Nakhon Sawan Nakhon Sawan Chai Nat Chai Nat Chai Nat Sing Buri Sing Buri Ang Thong Ang Thong Ayutthaya Ayutthaya Ayutthaya Pathum Thani Nontaburi Nontaburi Uthai Thani
16 110 36.000
99 460 6.300
15 430 28.500 18 420 12.400 18 180 09.700 18 60 46.400 17 380 18.100 18 260 29.200 18 80 16.400 18 30 3.000 17 500 38.200 17 320 51.300 17 100 6.900 16 540 25.900 16 450 26.100 16 50 22.400 16 300 33.500 19 50 26.000 18 470 20.000 18 340 3.200 17 440 24.700 17 370 04.400 17 180 44.000 16 480 7.200 16 250 54.200 16 120 10.100 15 510 39.500 15 420 43.600 15 410 13.100 15 270 11.300 15 150 13.300 15 100 5.300 15 60 55.500 14 570 41.900 14 510 52.400 14 410 23.800 14 280 46.300 14 200 27.700 14 180 51.900 14 120 3.700 14 30 53.300 13 560 30.500 13 530 48.200 15 210 36.200
100 080 10.300 99 340 50.800 99 300 44.400 99 200 53.200 99 130 53.000 100 100 16.800 100 70 26.900 99 480 39.600 99 370 51.900 99 460 14.800 99 510 10.800 99 520 54.000 100 60 53.200 100 150 30.500 100 120 12.200 100 470 50.300 100 470 26.400 100 450 27.700 100 330 19.200 100 50 57.400 100 30 39.200 100 130 50.800 100 210 44.000 100 240 40.800 100 140 56.100 100 90 32.800 100 70 14.100 100 60 49.800 100 50 7.600 100 70 37.000 100 150 58.300 100 200 52.200 100 240 32.800 100 270 10.600 100 270 8.600 100 320 43.000 100 340 11.300 100 330 42.700 100 320 19.500 100 300 40.200 100 290 20.900 100 30 19.400
Chai Nat
15 110 40.000
100 30 24.100
Ayutthaya
14 230 45.500
100 340 55.600
individual is exposed (Jibiri et al., 2007). The absorbed dose can be considered in terms of the annual effective dose equivalent from outdoor terrestrial gamma radiation which is converted from the absorbed dose by taking into account two factors, namely the conversion coefficient between the absorbed dose in air to effective dose and the outdoor occupancy factor. The annual effective dose equivalent can thus be estimated using the following relation (UNSCEAR, 2000; Al-Kharouf et al., 2008; Turhan and Gundiz, 2008; Ajayi, 2009; Nada et al., 2009; Belivermis et al., 2010):
AEDE mSvy1 ¼D nGyh1 8760h0:20:7SvGy1 103
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Fig. 3. Background-subtracted gamma-ray spectrum associated with decays from radionuclides detected from a sample no. 34 obtained at location NWC1 (see Table 1).
The values of those parameters used in the UNSCEAR (2000) report are 0.7 Sv Gy1 for the conversion coefficient from absorbed dose in air to effective dose received by adults and 0.2 for the outdoor occupancy factor.
2.4.3. Radium equivalent activity (Raeq) Due to a non-uniform distribution of natural radionuclides in the soil samples, the activity level of 226Ra, 232Th and 40K in the samples can be evaluated by means of a common radiological index, named the radium equivalent activity (Raeq) (Beretka and Mathew, 1985). It is the most widely used index to assess the radiation hazards and can be calculated using the relation
Raeq Bq kg1 ¼ ARa þ 1:43ATh þ 0:077AK
same gamma-ray dose rate (Beretka and Mathew, 1985; Kurnaz et al., 2007; Al-Hamarneh and Awadallah, 2009; Belivermis et al., 2010). The permissible maximum value of the radium equivalent activity is 370 Bq kg1 (UNSCEAR, 2000) which corresponds to an effective dose of 1 mSv for the general public (Ajayi, 2009). 2.4.4. External hazard index (Hex) To limit the radiation exposure attributable to natural radionuclides in the samples to the permissible dose equivalent limit of 1 mSv y1, the external hazard index based on a criterion have been introduced using a model proposed by Krieger (1981) which is given by (Kurnaz et al., 2007; Al-Hamarneh and Awadallah, 2009)
Hex ¼
where ARa, ATh and AK are the activity concentration of 226Ra, 232Th and 40K in Bq kg1, respectively. This estimates that 370 Bq kg1 of 226 Ra, 259 Bq kg1 of 232Th and 4810 Bq kg1 of 40K produce the
ARa ATh AK þ þ 1 370 259 4810
In order for the radiation hazard to be deemed as ‘insignificant’, the value of external hazard index must not exceed unity. A value of
Table 2 Gamma-ray energies and their associated emission probability per decay (Fraires and Boswell, 1981; Baum et al., 2002) used in the current work for the activity concentration determination. Radionuclide
Energy (keV) 238
226 214
214
40 a
Ra Pb Bi
K
Emission Probability
Radionuclide
Energy (keV) 232
U Series 0.0617a 0.1842 (4) 0.3560 (7) 0.4549 (16) 0.1492 (3) 0.05834 (15) 0.1530 (3) 0.04924 (18)
186.2 295.2 351.9 609.3 1120.2 1238.1 1764.4 2204.2 1460.8
0.1066 (13) 226
228
Ac
212
Pb
212
Bi Tl
208
137
Cs
338.3 911.2 968.9 238.6 300.0 727.3 1620.5 583.1 2614.5 661.6
Emission Probability
Th Series 0.1127 0.258 0.158 0.436 0.0318 0.0674 0.0151 0.3055 0.3585 0.8510
(19) (4) (3) (3) (13) (12) (3) (17) (7) (20)
186.2 keV gamma-ray intensity from the decay of Ra is adjusted to correct for the contribution from the overlapping gamma-ray transition associated with the alpha decay of 235Ue231Th at 185.7 keV (Gilmore, 2008; Newman et al., 2008).
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Hex ¼ 1 corresponds to a radium equivalent activity 370 Bq kg1 (Dragovic et al., 2006; Nada et al., 2009). 3. Results and discussion 3.1. Soil radioactivity A representative measured gamma-ray spectrum associated with decays from the radionuclides detected from one sample (sample no. 34 obtained at location NWC1, see Table 1) is shown in Fig. 3. Most of the identified radionuclide present in the spectrum belong to the 238U and 232Th decay chains. The radionuclides whose decays are directly detected in the sample are 234Th, 226Ra, 214Pb and 214Bi from the 238U decay series and 228Ac, 212Pb, 212Bi and 208Tl from 232Th decay series. In addition to these radionuclides from the uranium and thorium series, the signature gamma-ray energy peak from 40K at 1460.8 keV was observed in all samples. The anthropogenic radionuclide, 137Cs at 661.6 keV which is a residue from the fallout of nuclear weapon testing and nuclear accidents was also observed in all samples, albeit at close to background levels. Assuming a state of secular equilibrium, a range of relatively intense gamma-ray transitions (see Table 2) was used and these could be combined to estimate the activity concentrations of 238U and 232Th in the samples. The activity concentration of 238U was determined using the gamma-ray transitions associated with decays of 226Ra (186.2 keV), 214Pb (295.2 and 351.9 keV) and 214Bi (609.3, 1120.2, 1238.1, 1764.4 and 2204.2 keV). The gamma-ray energy peaks associated with decays of 228Ac (338.3, 911.2 and 968.9 keV), 212Pb (238.6 and 300.0 keV), 212Bi (727.3 and 1620.5 keV) and 208Tl (583.1 and 2614.5 keV) were used to determine the activity concentration of 232Th. Each spectral line was used to calculate an activity concentration with the uncertainty associated with that line obtained assuming a k ¼ 1 standard deviation uncertainties associated with statistical counting uncertainties in the peak area, gamma-ray emission probability and full-energy peak detection efficiency. These were then combined for the specific decay chain to give a weighted mean and uncertainty on the weighted mean (k ¼ 1) for the full decay chain. The evaluated literature values for the gamma-ray emission probability and any reported spectral interference (i.e., known energy doublets such as 185.7 and 186.2 keV from the 235U and 226Ra decays respectively) were taken into account and the measured gammaray lines were each used to determine the activity concentration for that particular radioisotope. The activity concentrations of 137Cs and 40K were derived directly from the measured intensity of the single 661.6 and 1460.8 keV gamma-ray transitions, respectively. Fig. 4(a) and (b) shows the calculated activity concentrations for selected gamma-ray transitions from the decays of 226Ra, 214Pb and 214 Bi (from the 238U series) and 228Ac, 212Pb, 212Bi and 208Tl (from the 232Th series) of the sample no. 34, obtained at location NWC1 (see Table 1). The dotted lines in Fig. 4(a) and (b) represent the calculated weighted mean concentrations of 238U and 232Th from these combined data points. From Fig. 5, it can be seen that the highest activity concentrations of 238U, 232Th and 40K were found to be 76.8 ± 1.7, 142.9 ± 2.8 and 810.7 ± 26.7 Bq kg1, respectively, for soil sample no. 4 which was obtained at location TKP1. Conversely, the lowest activity concentrations of 238U, 232Th and 40K were found to be 13.9 ± 0.4, 12.9 ± 0.3 and 178.4 ± 6.1 Bq kg1 respectively, measured from soil sample no. 9 obtained at the location LPW1 (see Table 1). The arithmetic mean average activity concentrations of 238U, 232Th and 40 K for all soil samples in the current study are 29.2 ± 0.1, 30.4 ± 0.1 and 308.9 ± 1.6 Bq kg1, respectively. The activity concentration of 137 Cs was found to be below the minimum detectable limit, implying that no artificial radionuclide was observed in statistically
Fig. 4. (a) and (b) The weighted mean activity concentrations of 238U and 232Th respectively as measured for sample no. 34 (see Table 1). The weighted means were computed using individual data points for the calculated activity concentrations of daughter nuclides 226Ra, 214Pb and 214Bi (from the 238U chain) and 228Ac, 212Pb, 212Bi and 208Tl (from the 232Th chain). The dashed lines are the uncertainties in the calculated weighted mean values.
significant amounts in the current study. From Fig. 5, it is apparent that 40K exhibited the highest activity concentrations for all measured radionuclides in all of the soil samples measured. The results of the current study have been compared with the world mean activity concentrations in soil, as shown in Table 3. According to the UNSCEAR (2000) report, the worldwide activity concentrations of 238U, 232Th and 40K were reported to be in the range 17 4 60, 11 4 64 and 140 4 850 Bq kg1 with mean concentrations of 35, 30 and 400 Bq kg1, respectively. The obtained results show that the ranges of the activity concentrations of 238U and 232Th vary from 13.9 to 76.8 and 12.9 to 142.9 Bq kg1, respectively. The activity concentrations of 238U and 232Th are above the upper range of the worldwide values due to the high concentration values found in some soil samples from the Ping and Chao Phraya river areas. The means of the activity concentrations of 238 U and 232Th from samples in Ping and Chao Phraya river areas also show slightly higher values than the worldwide mean concentrations. However, the overall mean activity concentrations of 238 U and 232Th are comparable to the mean activity worldwide concentrations. The activity concentrations of 40K in all soil samples
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85
Table 4 The calculated, combined absorbed dose rate (D), annual effective dose equivalent (AEDE), radium equivalent activity (Raeq) and external hazard index (Hex) obtained from all the soil samples measured in the current work.
Min. Max. Mean ± S.D. Worldwide Mean Range
Fig. 5. The overall activity concentrations of 238U, 232Th and 40K in soil samples studied from five different sampling areas corresponding to the five main rivers in the Chao Phraya river basin, the Ping, Wang, Yom, Nan and Chao Phraya rivers. The uncertainties correspond to the uncertainties in the evaluated weighted mean values for the activity concentrations at these positions.
range from 178.4 to 810.7 Bq kg1 and fall within the worldwide range, with a weighted mean value for the activity concentration of 308.9 ± 1.6 Bq kg1, which is slightly lower than the worldwide average value. The results shown in Fig. 5 reveal that soil samples from the Ping River area contain the highest activity concentrations of 40K. The higher concentrations of 238U, 232Th and 40K in some soil samples may be influenced in part by a variation in geological structure and/or high utilisation of fertilisers in the areas studied (Al-Hamarneh and Awadallah, 2009; Santawamaitre et al., 2010). 3.2. Assessment of radiological hazard One of the main objectives of the radioactivity measurement in environmental sample is to estimate the radiation exposure dose and to assess the biological effects on humans. The assessment of radiological risk can be considered in various terms. In the current study four related quantities were deduced, these being: (i) the absorbed dose rate (D) in air at 1 m above the ground surface; (ii) the annual effective dose equivalent (AEDE) from outdoor terrestrial gamma radiation; (iii) the radium equivalent activity (Raeq); and (iv) the external hazard index (Hex). These radiological parameters can be calculated from the measured activity concentrations of three main primordial radionuclides in soil samples. The values of these radiological hazard parameters as deduced in the current work are listed in Table 4.
D (nGy h1)
AEDE (mSv y1)
Raeq (Bq kg1)
Hex
21.7 ± 0.4 155.7 ± 2.2 65 ± 27
26.6 ± 0.4 190.9 ± 2.7 79 ± 33
46.2 ± 0.7 343.7 ± 4.8 140 ± 60
0.120 ± 0.002 0.93 ± 0.01 0.38 ± 0.16
57 18e93
70 e
<370
<1
From Table 4, the estimated absorbed dose rates based on soil radioactivity range from 21.7 ± 0.4 to 155.7 ± 2.2 nGy h1 with a mean value and standard deviation of 64.5 ± 27.1 nGy h1. As can be seen in Fig. 6, 232Th is the main contributor to the absorbed dose rate in most of the soil samples measured in the current work. The relative averaged contributions of 238U, 232Th and 40K to the total absorbed dose averaged across all the soil samples were 30, 43 and 27%, respectively. Compared with the worldwide values, most of the results obtained in this current study (with the exception of the values associated with sample nos.1, 2, 4, 6 and 42 which are greater than the upper range of the worldwide range) fall within the expected range, with the average mean value of absorbed dose rate from all the samples being slightly higher than the worldwide mean value. The absorbed dose rate in air at 1 m above the ground surface does not directly provide the radiological risk to which an individual is exposed (Jibiri et al., 2007; Santawamaitre et al., 2010). The annual effective dose equivalent from outdoor terrestrial gamma radiation was estimated by taking into account the conversion coefficients from absorbed dose in air to effective dose and the outdoor occupancy factor. The effective dose for the different locations of soil samples in this study varied from 26.6 ± 0.4 to 190.9 ± 2.7 mSv y1, with the arithmetic mean value and standard deviation of 79 ± 33 mSv y1, which is comparable to the worldwide effective dose of 70 mSv y1 (UNSCEAR, 2000). The radiation hazard parameters in terms of the radium equivalent activity (Raeq) and the external hazard index (Hex) were also evaluated. The radium equivalent activity (Raeq) is a single quantity which compares the activity concentrations of 226Ra, 232Th and 40K in soil samples in order to obtain a total activity concentration. The results for the calculated Raeq from the current work are summarised in Table 4. The values of Raeq range from 46.2 to 343.7 Bq kg1 with an overall arithmetic mean and standard
238U
232Th
40K
180
Sampling area
Activity Concentration (Bq kg1) 238
Ping Wang Yom Nan Chao Phraya Total Worldwide Range Mean
232
U
39.71 18.50 30.73 25.56 36.99 29.19
± ± ± ± ± ±
17e60 35
0.34 0.24 0.24 0.19 0.22 0.10
40
Th
44.60 16.69 32.84 27.79 42.95 30.43
± ± ± ± ± ±
11e64 30
0.36 0.21 0.24 0.19 0.23 0.10
K
626.6 237.5 311.7 251.3 364.5 309.0
± ± ± ± ± ±
7.4 4.2 3.5 2.7 3.1 1.6
140e850 400
Absorbed dose (nGy h-1)
160
Table 3 Comparison between the average mean activity concentrations of 238U, 232Th and 40 K in 5 sampling areas measured in the current work and the mean value worldwide (UNSCEAR, 2000).
140 120 100 80 60 40 20 0 1
5
9
13
17
21
25
29
33
37
41
45
49
Sample number
Fig. 6. The calculated absorbed dose rate in air outdoors due to gamma radiation arising from 238U, 232Th and 40K for all the measured soil samples in the current work. See Table 1 for locations of each sample.
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deviation of 140 ± 60 Bq kg1. It can be seen that the Raeq values for all soil samples in the present work are lower than the accepted safety limit value of 370 Bq kg1 as recommended by the Organisation for Economic Cooperation and Development (OECD) (UNSCEAR, 1988; Belivermis et al., 2010). According to Beretka and Mathew, Turhan and Gundiz, the use of these soils as raw materials for building does not constitute a health hazard of radiation (Beretka and Mathew, 1985; Turhan and Gundiz, 2008). As listed in Table 3, the calculated values of the external hazard index for all soil samples studied vary from 0.120 ± 0.002 to 0.93 ± 0.01 and the average value was found to be 0.38 ± 0.16. The results show that the Hex values for all soil samples are below the limit of unity, meaning that the radiation dose is below the permissible limit of 1 mSv y1 recommended by ICRP (1991, 2007). It can be concluded that the radiological health risks to the people living in the areas studied in this current work is not significant. 4. Conclusion In this current study, the measurements showed that primordial radionuclides namely the 238U and 232Th decay chains and 40K are contained in all soil samples. The measured activity concentrations of 238U, 232Th and 40K across all of the soil samples varied from 13.9 to 76.8, 12.9 to 142.9 and from 178.4 to 810.7 Bq kg1 with the average mean values of 29, 30 and 309 Bq kg1, respectively. The obtained results of the activity concentrations of 238U and 232Th were found to be higher than the upper range of the worldwide values of 17 4 60, 11 4 64 Bq kg1 identified by UNSCEAR (2000) due to the high concentration values found in some soil samples from the Ping and Chao Phraya river areas. The activity concentration of 40K in the current study falls within the worldwide range of values which are 140 4 850 Bq kg1 reported by UNSCEAR (2000). The average mean activity concentrations of 238U, 232Th and 40K are comparable to the worldwide mean values. The results are influenced in part by a geological structure variation and/or high utilisation of fertilisers and thus may have resulted in the variations of the concentrations of 238U, 232Th and 40K found in some soil samples in the area studied. The artificial, radionuclide, 137 Cs, was not observed in statistically significant amounts above the background level in the current study. The results in this current study can be used as a baseline for the observation of any possible change in the future. The values of the radiation hazard parameters from this current study are not extremely high compared to the world averages and the recommended values and therefore are unlikely to cause additional radiological health risks to the people living in the areas studied. References Ajayi, O.S., 2009. Measurement of activity concentrations of 40K, 226Ra and 232Th for assessment of radiation hazards from soils of the southwestern region of Nigeria. Radiat. Environ. Biophys. 48, 323e332. Al-Hamarneh, I.F., Awadallah, M.I., 2009. Soil radioactivity levels and radiation hazard assessment in the highlands of Northern Jordan. Radiat. Meas. 44, 102e110. Al-Kharouf, S.J., Al-Hamarneh, I.F., Dababneh, M., 2008. Natural radioactivity, dose assessment and uranium uptake by agricultural crops at Khan Al-Zabeeb, Jordan. J. Environ. Radioact. 99 (7), 1192e1199. Almayahi, B.A., Tajuddin, A.A., Jaafar, M.S., 2012. Radiation hazard indices of soil and water samples in Northern Malaysian Peninsula. Appl. Radiat. Isot. 70, 2652e2660. ASTM, 2005. C1402-04 Standard Guide for High-resolution Gamma-ray Spectrometry of Soil Samples, Annual Book of ASTM Standard. In: Nuclear Energy (I), vol. 12.01. American Society for Testing and Materials.
Baum, M., Knox, H.D., Miller, T.R., 2002. Nuclides and Isotopes: Chart of the Nuclides, 16th ed. Lockheed Martin Co. Beck, H.L., 1972. The physics of environmental radiation fields. Natural radiation environment II, CONF-720805 P2. In: Proceedings of the Second International Symposium on the Natural Radiation Environment. Belivermis, M., Kikic, O., Cotuk, Y., Topcuoglu, S., 2010. The effect of physicochemical properties on gamma emitting natural radionuclide levels in the soil profile of Istanbul. Environ. Monit. Assess. 163, 15e26. Beretka, J., Mathew, P.J., 1985. Natural radioactivity of Australian building materials, industrial wastes and by-products. Health Phys. 48, 87e95. Chang, B.U., Koh, S.M., Kim, Y.J., Seo, J.S., Yoon, Y.Y., Row, J.W., Lee, D.M., 2008. Nationwide survey on the natural radionuclides in industrial raw minerals in South Korea. J. Environ. Radioact. 99, 455e460. Dovlete, C., Povinec, P.P., 2004. Quantification of Uncertainty in Gammaspectrometric Analysis of Environmental Samples. IAEA-TECDOC-1401. International Atomic Energy Agency, Austria, pp. 103e126. Dragovic, S., Jankovic, L., Onjia, A., 2006. Assessment of gamma dose rate from terrestrial exposure in Serbia and Montenegro. Radiat. Prot. Dosimetry 121 (3), 297e302. El-Gamal, A., Nasr, S., El-Taher, A., 2007. Soil Radioactivity levels and radiation hazard assessment in the highlands of northern Jordan. Radiat. Meas. 42, 457e465. Fraires, R.A., Boswell, G.G.J., 1981. Radioisotope Laboratory Techniques, fourth ed. Butterworth & Co (Publishers) Ltd, London. Gilmore, G.R., 2008. Practical Gamma-ray Spectrometry, second ed. John Wiley & Sons Ltd., Chichester. IAEA, 2003. Extent of Environmental Contamination by Naturally Occurring Radioactive Material (NORM) and Technological Options for Mitigation. Technical Reports Series No.419. International Atomic Energy Agency, Vienna. ICRP, 1991. International Commission on Radiological Protection. ICRP Publication 60. Pergamon Press, Inc, Elmsford, NY. ICRP, 2007. The 2007 recommendations of the International Commission on Radiological Protection. ICRP Publication 103 Ann. ICRP 37, 2e4. Jibiri, N.N., Farai, I.P., Alausa, S.K., 2007. Estimation of annual effective dose due to natural radioactive elements in ingestion of foodstuffs in tin mining area of JosPlateau, Nigeria. J. Environ. Radioact. 94, 31e40. Krieger, R., 1981. Radioactivity of construction materials. Betonwerk Fertigteil Tech. 47, 468e473. Kurnaz, A., Kucukomeroglu, B., Keser, R., Okumusoglu, N.T., Kprkmaz, F., Karahan, G., Cevik, U., 2007. Determination of radioactivity levels and hazards of soil and sediment samples in Firtina Valley (Rize, Turkey). Appl. Radiat. Isot. 65, 1281e1289. Malain, D., Regan, P.H., Bradley, D.A., Matthews, M., Al-Sulaiti, H., Santawamaitre, T., 2012. An evaluation of the natural radioactivity in the Andaman beach samples in Thailand after the 2004 Tsunami. Appl. Radiat. Isot. 70, 1467e1474. Nada, A., Maksoud, T.M.A., Hosnia, M.A., El-Nagar, T., Awad, S., 2009. Distribution of radionuclides in soil samples from a petrified wood forest in El-Qattamia, Cairo, Egypt. Appl. Radiat. Isot. 67, 643e649. NCRP, 1975. Natural Background Radiation in the United States. NCRP Report No.45. National Council on Radiation Protection and Measurements, Washington, DC. Newman, R.T., Lindsay, R., Maphoto, K.P., Mlwilo, N.A., Mohanty, A.K., Roux, D.G., Meijer, R.J., Hlastshwayo, I.N., 2008. Determination of soil, sand and ore primordial radionuclide concentrations by full-spectrum analyses of high-purity germanium detector spectra. Appl. Radiat. Isot. 66, 855e859. ONWRC, 2003. Chao Phraya river basin, Thailand. In: United Nations/World Water Assessment Programme (UN/WWAP), First UN World Water Development Report: Water for People, Water for Life. Office of Natural Water Resources Committee of Thailand. Radford, D.C., 1995. Background subtraction from in-beam HPGe coincidence data sets. Nucl. Instrum. Methods Phys. Res. A 361, 306. Santawamaitre, T., Regan, P.H., Bradley, D.A., Matthews, M., Malain, D., AlSulaiti, H.A., 2010. An evaluation of the level of naturally occurring radioactive material in soil samples along the Chao Phraya river basin. Nucl. Instrum. Methods Phys. Res. A 619, 453e456. Santawamaitre, T., Malain, D., Al-Sulaiti, H.A., Matthews, M., Bradley, D.A., Regan, P.H., 2011. Study of natural radioactivity in riverbank soils along the Chao Phraya River Basin in Thailand. Nucl. Instrum. Methods Phys. Res. A 652, 920e924. Saradri, D., Macmahon, T.D., 2000. Gamma-ray emission probabilities in the decay of 226Ra and its daughters. J. Radioanal. Nucl. Chem. 244 (2), 463e469. Turhan, S., Gundiz, L., 2008. Determination of specific activity of 226Ra, 232Th and 40 K for assessment of radiation hazards from Turkish pumice samples. J. Environ. Radioact. 99, 332e342. UNSCEAR, 1988. Sources and Effects of Ionizing Radiation. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York. UNSCEAR, 2000. Source and Effects of Ionizing Radiation. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York. Wilson, W.F., 1994. A Guide to Naturally Occurring Radioactive Material. Pennwell Books, Oklahoma.
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Determination of 238U, 232Th and 40K activity concentrations in riverbank soil along the Chao Phraya river basin in Thailand T. Santawamaitre a, D. Malain a, H.A. Al-Sulaiti a, D.A. Bradley a, M.C. Matthews b, P.H. Regan a, c, * a b c
Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford GU2 7XH, UK Centre for Environmental Health Engineering, Department of Civil Engineering, University of Surrey, Guildford GU2 7XH, UK National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2014 Received in revised form 10 July 2014 Accepted 18 July 2014 Available online
The activity concentrations of 238U, 232Th and 40K in riverbank soil along the Chao Phraya river basin was determined through gamma-ray spectrometry measurements made using a hyper-pure germanium detector in a low background configuration. The ranges of activity concentrations of 238U, 232Th and 40K were found to be 13.9 4 76.8, 12.9 4 142.9 and 178.4 4 810.7 Bq kg1, respectively. The anthropogenic radionuclide, 137Cs, was not observed in statistically significant amounts above the background level in the current study. The absorbed gamma dose rate in air at 1 m above the ground surface, the outdoor annual effective dose equivalent, the values of the radium equivalent activity and the external hazard index associated with all the soil samples in the present work were evaluated. The results indicate that the radiation hazard from primordial radionuclides in all soil samples from the area studied in this current work is not significant. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Naturally occurring radioactive materials NORM Gamma-ray spectrometry
1. Introduction The most common radiation sources to which humans are exposed arise from radionuclides in the earth's surroundings and the interaction of cosmic rays on the earth's atmosphere (NCRP, 1975; UNSCEAR, 2000). This exposure to naturally occurring radiation accounts for up to 85% of annual exposure dose received by the world population. Apart from the exposure from direct cosmic rays and cosmogenic radionuclides, natural exposures arise mainly from the primordial nuclear species which are spread widely and are present in almost all geological materials in the earth's environment (NCRP, 1975; Wilson, 1994). The majority of naturally occurring radionuclides belongs to the decays in the 235,238 U and 232Th series and the single decay radionuclide, 40K (IAEA, 2003). The Chao Phraya river basin is made up of the main Chao Phraya River and its tributaries (Ping, Wang, Yom, Nan, Sakae Krung, Pa Sak and Tha Chin) cover a large area of the northern and central plain of Thailand (as shown in Fig. 1). The Chao Phraya River and its
* Corresponding author. Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford GU2 7XH, UK. E-mail address: [email protected] (P.H. Regan). http://dx.doi.org/10.1016/j.jenvrad.2014.07.017 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
tributaries are the principal river system in Thailand and are also the largest watershed, playing a significant role in the local irrigation. This river system supports agricultural activities which are widely spread along both sides of the rivers and the local vicinity (ONWRC, 2003). As a result of a large number of agricultural activities in this area, the high demand of fertiliser usage may result in contamination and an enhancement of the background activity level in particular areas (Almayahi et al., 2012). Fertilisers may be washed into the river and deposition of sediments might be expected to occur at riverbanks along the river (El-Gamal et al., 2007). The aim of this study is to evaluate the level of natural radioactivity in river sediments and riverbank surface soils collected along the Chao Phraya river basin in Thailand. The measurements of the activity concentrations of 238U, 232Th and 40K in soil samples were carried out by using high-resolution gamma-ray spectrometry with a hyper-pure germanium detector in a low background configuration. An assessment of the biological effects on humans due to the natural radioactivity arising from riverbank soils was determined in terms of the absorbed dose in air, the annual effective dose equivalent, the radium equivalent activity and the external hazard index. Preliminary analyses and results from this programme of work have been presented in conference proceedings (Santawamaitre et al., 2010, 2011). This paper presents the full, completed analysis of this study.
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Fig. 1. The map of Thailand showing the Chao Phraya river basin.
2. Material and methods 2.1. Sample collection and preparation In order to evaluate the levels of natural radioactivity, riverbank soil and sediment samples were collected at random positions from several different locations along the Chao Phraya River and its tributaries between June and July 2008. Fifty-one sampling locations were designated to be representative of the levels of natural radioactivity from the five different sampling areas, corresponding to the five main rivers in the Chao Phraya river basin, namely the Ping, Wang, Yom, Nan and Chao Phraya rivers. At each location, soil samples were taken from approximately 1 to 2 m away from the river edge. The samples were obtained at a typical depth of 5 cm from the top surface layer to produce approximately 2 kg wet weight per sample. After removing stones and vegetable matter, each soil sample was packed into its own secure water-tight bag to prevent cross contamination and shipped to the Environmental Radiation Laboratory at the University of Surrey, United Kingdom. On arrival at the laboratory at the University of Surrey, sample preparation was carried out by placing each soil sample in an oven for drying at a temperature of 60 C until a constant weight was reached, thus ensuring complete removal of any residual moisture. The dried samples were pulverised into a fine powder and passed through a standard 1 mm mesh size (ASTM No. 18). The homogenised samples were filled into 550 ml Marinelli beakers which were then hermetically sealed with the aid of PVC tape to prevent the escape of airborne 222Rn and 220Rn from the samples. All samples were weighed and stored for at least one month prior to measurement in order to attain radioactive secular equilibrium between 226Ra and 228Ac and their short-lived progeny (>7 half-lives of 222Rn).
81
spectrometry system used consists of a coaxial hyper-pure germanium (HPGe) detector with passive shielding. The detector was enclosed with a 10 cm cylindrical lead shield to reduce the background radiation from various natural radiation sources and to isolate from other radiation sources used in nearby surroundings, and the lead shielding was graded with an inner layer of 0.1 cm thick copper to reduce the contribution from Pb X-ray fluorescence. The detector characterisation was performed in terms of energy and efficiency calibration using four different certified reference sources, (i) 152Eu, (ii) 226Ra, (iii) 232Th and (iv) a mixed radionuclide source (NG3), containing 57Co, 60Co, 85Sr, 88Y, 109Cd, 137Cs, 139Ce, 203 Hg and 241Am. The additional certification for the reference standards used in our Marinelli beakers has been included as has the use of RADWARE software and peak-fitting analysis (Radford, 1995 and references therein) in the analysis of the efficiency data. The source certificates were standardised by nuclitech GmbH via direct traceability to PTB and NIST gamma-ray standards using a high-purity germanium detector with a multi-channel analyser. The sources used were 152Eu (3.02 kBq on 1 Feb 2009) in a density of 1.6 g/cm3 with a relative uncertainty of 3%; 232Th (1.08 kBq) in a gel of density 1.1 g/cm3; 226Ra (3.10 kBq); Mixed radionuclide source (241Am, 109Cd, 57Co, 139Ce, 203Hg, 113Sn, 85Sr, 137Cs, 88Y and 60 Co) in density 1.6 g/cm3. The certification reference dates for each of these sources was 1 Feb 2009, with the relative uncertainty in the source quoted activities for k ¼ 2 was 5%. Each of these reference sources was contained in a homogenous gel matrix in a 500 ml Marinelli beaker in the same geometry as that of sample measurements using the same technique established in reference (Malain et al., 2012). With the same geometry and the same range in density between reference sources (1.1 g/cm3 for the 232Th and 226 Ra sources; 1.6 g/cm3 for the 152Eu and NG3 sources) and sand samples (which had typical measured densities of ~1.3e1.5 g/cm3), differences in the self-attenuation for gamma rays for the sources and samples was found to be a negligible contribution to the overall experimental uncertainties in the radiometric evaluations discussed in the current work. The measured absolute photopeak efficiency of the system was approximately 0.50% and 0.25% at 662 keV and 1332 keV, respectively (see Fig. 2). The measured full-energy peak efficiency data were fitted to a response function of the form suggested by Gray and Ahmad (Saradri and Macmahon, 2000)
i. h εg ¼ P1 þP2 ðln EÞþP3 ðln EÞ2 þP4 ðln EÞ3 þP5 ðln EÞ5 þP6 ðln EÞ7 E
2.2. Instrumentation and calibration The radioactivity concentration of radionuclides in soil samples was measured by using a high-resolution gamma-ray spectrometry system in a low background configuration. The gamma-ray
Fig. 2. Absolute full-energy peak efficiency as function of gamma-ray energy for the HPGe detector used in the current work using the 4 separate gel-matrix based Marinelli sources (see text for details).
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T. Santawamaitre et al. / Journal of Environmental Radioactivity 138 (2014) 80e86
where P1eP6 are parameters of the fitting function and εg is the efficiency at energy E. The fitted curve for this full-energy peak absolute efficiency response function is shown in Fig. 2. 2.3. Activity concentration determination Each prepared soil sample was measured for 48 h using the coaxial hyper-pure germanium detector. The number of counts under the full-energy peak areas (corrected for background peak areas), the counting time, the absolute full-energy peak efficiency for the energy of interest and the gamma-ray emission probability corresponding to the peak energy were used for the calculation of the activity concentration of a particular radionuclide in the measured samples. One problem with the direct determination of the activity of 238U and 232Th is due to the low relative gamma-ray intensities following their decay. However, in a state of secular equilibrium, the activity of 238U and 232Th can be estimated through several intense gamma-ray lines of their daughter products in the decay chains. A range of gamma-ray transitions from a number of the decay chain members of the 238U and 232Th primordial decay chains, as listed in Table 1, was utilised to obtain consistent values for the activity concentrations of these nuclei and their subsequent decay daughters. The activity concentration values quoted assume secular equilibrium for the different isotopic activities in the decay chains. The activity concentrations of the singly decaying nuclei, 40 K and 137Cs were determined directly by measurement of the gamma-ray transitions at 1460.8 and 661.6 keV, respectively. The specific activity, in terms of the activity concentration, is defined as the activity per unit mass of the sample. The specific activity of individual radionuclide in soil samples is given by the following equation (Dovlete and Povinec, 2004; ASTM, 2005):
A¼
Cn εf Pg ts m
where A is the activity concentration of a particular nuclide in units of Bq kg1, Cn the net count (background subtracted) of the corresponding full-energy peak, εg the absolute full-energy peak detection efficiency, Pg the emission probability per decay corresponding to the specific gamma-ray energy, t the counting time in s and m the mass of soil sample in kg. 2.4. Dose calculation 2.4.1. Absorbed dose rate in air (D) To assess the degree of any associated radiological hazard, the exposure to radiation arising from radionuclides present in soil can be determined in terms of a number of different, but related parameters. A direct connection between radioactivity concentrations of natural radionuclides and their exposure is known as the absorbed dose rate (D) in the air at 1 m above the ground surface. The mean activity concentrations of 226Ra (238U), 232Th and 40K in units of Bq kg1 in the soil samples can be used to calculate the absorbed dose rate using the following relation (Beck, 1972; Chang et al., 2008):
D nGy h1 ¼ ð0:462ARa þ 0:604ATh þ 0:0417AK Þ where ARa, ATh and AK are the activity concentrations of 226Ra (238U), 232 Th and 40K, respectively. The dose coefficients in units of nGy h1 per Bq kg1 were taken from the UNSCEAR (2000) report. 2.4.2. Annual effective dose equivalent (AEDE) The absorbed dose rate in air at 1 m above the ground surface does not directly provide the radiological risk to which an
Table 1 Details of the location of soil samples from the 5 sampling areas measured in the current work. Sample No.
Sample Code
1 2 3 4 5 6
CMP1 CMP2 CMP3 TKP1 TKP2 KPP1
7
KPP2
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
NWP1 LPW1 LPW2 LPW3 LPW4 PHY3 PHY2 PHY4 PHY1 STY3 STY2 STY1 PLY1 PCY2 PCY1 NAN3 NAN2 NAN1 UDN3 UDN2 UDN1 PLN1 PCN2 PCN1 NWN1 NWN2 NWC1 NWC2 CNC1 CNC2 CNC3 SIC2 SIC1 ATC1 ATC2 AYC1 AYC2 AYC3 PTC1 NNC1 NNC3 UTS1
50
CNT1
51
APS1
Location River Ping
Wang
Yom
Nan
Chao Phraya
Sakae Krang Tha Chin Pa Sak
Position Province
Latitude ( N) 0
Longitude ( E)
00
19 5 36.4 18 480 6.100 18 170 26.700 17 90 32.400 16 550 39.200 16 270 6.200
98 560 24.500 99 00 11.200 98 380 51.800 99 40 25.100 99 40 43.000 99 310 20.300
Chiang Mai Chiang Mai Chiang Mai Tak Tak Kamphaeng Phet Kamphaeng Phet Nakhon Sawan Lampang Lampang Lampang Lampang Phrae Phrae Phrae Phrae Sukhothai Sukhothai Sukhothai Phitsanulok Phichit Phichit Nan Nan Nan Uttaradit Uttaradit Uttaradit Phitsanulok Phichit Phichit Nakhon Sawan Nakhon Sawan Nakhon Sawan Nakhon Sawan Chai Nat Chai Nat Chai Nat Sing Buri Sing Buri Ang Thong Ang Thong Ayutthaya Ayutthaya Ayutthaya Pathum Thani Nontaburi Nontaburi Uthai Thani
16 110 36.000
99 460 6.300
15 430 28.500 18 420 12.400 18 180 09.700 18 60 46.400 17 380 18.100 18 260 29.200 18 80 16.400 18 30 3.000 17 500 38.200 17 320 51.300 17 100 6.900 16 540 25.900 16 450 26.100 16 50 22.400 16 300 33.500 19 50 26.000 18 470 20.000 18 340 3.200 17 440 24.700 17 370 04.400 17 180 44.000 16 480 7.200 16 250 54.200 16 120 10.100 15 510 39.500 15 420 43.600 15 410 13.100 15 270 11.300 15 150 13.300 15 100 5.300 15 60 55.500 14 570 41.900 14 510 52.400 14 410 23.800 14 280 46.300 14 200 27.700 14 180 51.900 14 120 3.700 14 30 53.300 13 560 30.500 13 530 48.200 15 210 36.200
100 080 10.300 99 340 50.800 99 300 44.400 99 200 53.200 99 130 53.000 100 100 16.800 100 70 26.900 99 480 39.600 99 370 51.900 99 460 14.800 99 510 10.800 99 520 54.000 100 60 53.200 100 150 30.500 100 120 12.200 100 470 50.300 100 470 26.400 100 450 27.700 100 330 19.200 100 50 57.400 100 30 39.200 100 130 50.800 100 210 44.000 100 240 40.800 100 140 56.100 100 90 32.800 100 70 14.100 100 60 49.800 100 50 7.600 100 70 37.000 100 150 58.300 100 200 52.200 100 240 32.800 100 270 10.600 100 270 8.600 100 320 43.000 100 340 11.300 100 330 42.700 100 320 19.500 100 300 40.200 100 290 20.900 100 30 19.400
Chai Nat
15 110 40.000
100 30 24.100
Ayutthaya
14 230 45.500
100 340 55.600
individual is exposed (Jibiri et al., 2007). The absorbed dose can be considered in terms of the annual effective dose equivalent from outdoor terrestrial gamma radiation which is converted from the absorbed dose by taking into account two factors, namely the conversion coefficient between the absorbed dose in air to effective dose and the outdoor occupancy factor. The annual effective dose equivalent can thus be estimated using the following relation (UNSCEAR, 2000; Al-Kharouf et al., 2008; Turhan and Gundiz, 2008; Ajayi, 2009; Nada et al., 2009; Belivermis et al., 2010):
AEDE mSvy1 ¼D nGyh1 8760h0:20:7SvGy1 103
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83
Fig. 3. Background-subtracted gamma-ray spectrum associated with decays from radionuclides detected from a sample no. 34 obtained at location NWC1 (see Table 1).
The values of those parameters used in the UNSCEAR (2000) report are 0.7 Sv Gy1 for the conversion coefficient from absorbed dose in air to effective dose received by adults and 0.2 for the outdoor occupancy factor.
2.4.3. Radium equivalent activity (Raeq) Due to a non-uniform distribution of natural radionuclides in the soil samples, the activity level of 226Ra, 232Th and 40K in the samples can be evaluated by means of a common radiological index, named the radium equivalent activity (Raeq) (Beretka and Mathew, 1985). It is the most widely used index to assess the radiation hazards and can be calculated using the relation
Raeq Bq kg1 ¼ ARa þ 1:43ATh þ 0:077AK
same gamma-ray dose rate (Beretka and Mathew, 1985; Kurnaz et al., 2007; Al-Hamarneh and Awadallah, 2009; Belivermis et al., 2010). The permissible maximum value of the radium equivalent activity is 370 Bq kg1 (UNSCEAR, 2000) which corresponds to an effective dose of 1 mSv for the general public (Ajayi, 2009). 2.4.4. External hazard index (Hex) To limit the radiation exposure attributable to natural radionuclides in the samples to the permissible dose equivalent limit of 1 mSv y1, the external hazard index based on a criterion have been introduced using a model proposed by Krieger (1981) which is given by (Kurnaz et al., 2007; Al-Hamarneh and Awadallah, 2009)
Hex ¼
where ARa, ATh and AK are the activity concentration of 226Ra, 232Th and 40K in Bq kg1, respectively. This estimates that 370 Bq kg1 of 226 Ra, 259 Bq kg1 of 232Th and 4810 Bq kg1 of 40K produce the
ARa ATh AK þ þ 1 370 259 4810
In order for the radiation hazard to be deemed as ‘insignificant’, the value of external hazard index must not exceed unity. A value of
Table 2 Gamma-ray energies and their associated emission probability per decay (Fraires and Boswell, 1981; Baum et al., 2002) used in the current work for the activity concentration determination. Radionuclide
Energy (keV) 238
226 214
214
40 a
Ra Pb Bi
K
Emission Probability
Radionuclide
Energy (keV) 232
U Series 0.0617a 0.1842 (4) 0.3560 (7) 0.4549 (16) 0.1492 (3) 0.05834 (15) 0.1530 (3) 0.04924 (18)
186.2 295.2 351.9 609.3 1120.2 1238.1 1764.4 2204.2 1460.8
0.1066 (13) 226
228
Ac
212
Pb
212
Bi Tl
208
137
Cs
338.3 911.2 968.9 238.6 300.0 727.3 1620.5 583.1 2614.5 661.6
Emission Probability
Th Series 0.1127 0.258 0.158 0.436 0.0318 0.0674 0.0151 0.3055 0.3585 0.8510
(19) (4) (3) (3) (13) (12) (3) (17) (7) (20)
186.2 keV gamma-ray intensity from the decay of Ra is adjusted to correct for the contribution from the overlapping gamma-ray transition associated with the alpha decay of 235Ue231Th at 185.7 keV (Gilmore, 2008; Newman et al., 2008).
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Hex ¼ 1 corresponds to a radium equivalent activity 370 Bq kg1 (Dragovic et al., 2006; Nada et al., 2009). 3. Results and discussion 3.1. Soil radioactivity A representative measured gamma-ray spectrum associated with decays from the radionuclides detected from one sample (sample no. 34 obtained at location NWC1, see Table 1) is shown in Fig. 3. Most of the identified radionuclide present in the spectrum belong to the 238U and 232Th decay chains. The radionuclides whose decays are directly detected in the sample are 234Th, 226Ra, 214Pb and 214Bi from the 238U decay series and 228Ac, 212Pb, 212Bi and 208Tl from 232Th decay series. In addition to these radionuclides from the uranium and thorium series, the signature gamma-ray energy peak from 40K at 1460.8 keV was observed in all samples. The anthropogenic radionuclide, 137Cs at 661.6 keV which is a residue from the fallout of nuclear weapon testing and nuclear accidents was also observed in all samples, albeit at close to background levels. Assuming a state of secular equilibrium, a range of relatively intense gamma-ray transitions (see Table 2) was used and these could be combined to estimate the activity concentrations of 238U and 232Th in the samples. The activity concentration of 238U was determined using the gamma-ray transitions associated with decays of 226Ra (186.2 keV), 214Pb (295.2 and 351.9 keV) and 214Bi (609.3, 1120.2, 1238.1, 1764.4 and 2204.2 keV). The gamma-ray energy peaks associated with decays of 228Ac (338.3, 911.2 and 968.9 keV), 212Pb (238.6 and 300.0 keV), 212Bi (727.3 and 1620.5 keV) and 208Tl (583.1 and 2614.5 keV) were used to determine the activity concentration of 232Th. Each spectral line was used to calculate an activity concentration with the uncertainty associated with that line obtained assuming a k ¼ 1 standard deviation uncertainties associated with statistical counting uncertainties in the peak area, gamma-ray emission probability and full-energy peak detection efficiency. These were then combined for the specific decay chain to give a weighted mean and uncertainty on the weighted mean (k ¼ 1) for the full decay chain. The evaluated literature values for the gamma-ray emission probability and any reported spectral interference (i.e., known energy doublets such as 185.7 and 186.2 keV from the 235U and 226Ra decays respectively) were taken into account and the measured gammaray lines were each used to determine the activity concentration for that particular radioisotope. The activity concentrations of 137Cs and 40K were derived directly from the measured intensity of the single 661.6 and 1460.8 keV gamma-ray transitions, respectively. Fig. 4(a) and (b) shows the calculated activity concentrations for selected gamma-ray transitions from the decays of 226Ra, 214Pb and 214 Bi (from the 238U series) and 228Ac, 212Pb, 212Bi and 208Tl (from the 232Th series) of the sample no. 34, obtained at location NWC1 (see Table 1). The dotted lines in Fig. 4(a) and (b) represent the calculated weighted mean concentrations of 238U and 232Th from these combined data points. From Fig. 5, it can be seen that the highest activity concentrations of 238U, 232Th and 40K were found to be 76.8 ± 1.7, 142.9 ± 2.8 and 810.7 ± 26.7 Bq kg1, respectively, for soil sample no. 4 which was obtained at location TKP1. Conversely, the lowest activity concentrations of 238U, 232Th and 40K were found to be 13.9 ± 0.4, 12.9 ± 0.3 and 178.4 ± 6.1 Bq kg1 respectively, measured from soil sample no. 9 obtained at the location LPW1 (see Table 1). The arithmetic mean average activity concentrations of 238U, 232Th and 40 K for all soil samples in the current study are 29.2 ± 0.1, 30.4 ± 0.1 and 308.9 ± 1.6 Bq kg1, respectively. The activity concentration of 137 Cs was found to be below the minimum detectable limit, implying that no artificial radionuclide was observed in statistically
Fig. 4. (a) and (b) The weighted mean activity concentrations of 238U and 232Th respectively as measured for sample no. 34 (see Table 1). The weighted means were computed using individual data points for the calculated activity concentrations of daughter nuclides 226Ra, 214Pb and 214Bi (from the 238U chain) and 228Ac, 212Pb, 212Bi and 208Tl (from the 232Th chain). The dashed lines are the uncertainties in the calculated weighted mean values.
significant amounts in the current study. From Fig. 5, it is apparent that 40K exhibited the highest activity concentrations for all measured radionuclides in all of the soil samples measured. The results of the current study have been compared with the world mean activity concentrations in soil, as shown in Table 3. According to the UNSCEAR (2000) report, the worldwide activity concentrations of 238U, 232Th and 40K were reported to be in the range 17 4 60, 11 4 64 and 140 4 850 Bq kg1 with mean concentrations of 35, 30 and 400 Bq kg1, respectively. The obtained results show that the ranges of the activity concentrations of 238U and 232Th vary from 13.9 to 76.8 and 12.9 to 142.9 Bq kg1, respectively. The activity concentrations of 238U and 232Th are above the upper range of the worldwide values due to the high concentration values found in some soil samples from the Ping and Chao Phraya river areas. The means of the activity concentrations of 238 U and 232Th from samples in Ping and Chao Phraya river areas also show slightly higher values than the worldwide mean concentrations. However, the overall mean activity concentrations of 238 U and 232Th are comparable to the mean activity worldwide concentrations. The activity concentrations of 40K in all soil samples
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85
Table 4 The calculated, combined absorbed dose rate (D), annual effective dose equivalent (AEDE), radium equivalent activity (Raeq) and external hazard index (Hex) obtained from all the soil samples measured in the current work.
Min. Max. Mean ± S.D. Worldwide Mean Range
Fig. 5. The overall activity concentrations of 238U, 232Th and 40K in soil samples studied from five different sampling areas corresponding to the five main rivers in the Chao Phraya river basin, the Ping, Wang, Yom, Nan and Chao Phraya rivers. The uncertainties correspond to the uncertainties in the evaluated weighted mean values for the activity concentrations at these positions.
range from 178.4 to 810.7 Bq kg1 and fall within the worldwide range, with a weighted mean value for the activity concentration of 308.9 ± 1.6 Bq kg1, which is slightly lower than the worldwide average value. The results shown in Fig. 5 reveal that soil samples from the Ping River area contain the highest activity concentrations of 40K. The higher concentrations of 238U, 232Th and 40K in some soil samples may be influenced in part by a variation in geological structure and/or high utilisation of fertilisers in the areas studied (Al-Hamarneh and Awadallah, 2009; Santawamaitre et al., 2010). 3.2. Assessment of radiological hazard One of the main objectives of the radioactivity measurement in environmental sample is to estimate the radiation exposure dose and to assess the biological effects on humans. The assessment of radiological risk can be considered in various terms. In the current study four related quantities were deduced, these being: (i) the absorbed dose rate (D) in air at 1 m above the ground surface; (ii) the annual effective dose equivalent (AEDE) from outdoor terrestrial gamma radiation; (iii) the radium equivalent activity (Raeq); and (iv) the external hazard index (Hex). These radiological parameters can be calculated from the measured activity concentrations of three main primordial radionuclides in soil samples. The values of these radiological hazard parameters as deduced in the current work are listed in Table 4.
D (nGy h1)
AEDE (mSv y1)
Raeq (Bq kg1)
Hex
21.7 ± 0.4 155.7 ± 2.2 65 ± 27
26.6 ± 0.4 190.9 ± 2.7 79 ± 33
46.2 ± 0.7 343.7 ± 4.8 140 ± 60
0.120 ± 0.002 0.93 ± 0.01 0.38 ± 0.16
57 18e93
70 e
<370
<1
From Table 4, the estimated absorbed dose rates based on soil radioactivity range from 21.7 ± 0.4 to 155.7 ± 2.2 nGy h1 with a mean value and standard deviation of 64.5 ± 27.1 nGy h1. As can be seen in Fig. 6, 232Th is the main contributor to the absorbed dose rate in most of the soil samples measured in the current work. The relative averaged contributions of 238U, 232Th and 40K to the total absorbed dose averaged across all the soil samples were 30, 43 and 27%, respectively. Compared with the worldwide values, most of the results obtained in this current study (with the exception of the values associated with sample nos.1, 2, 4, 6 and 42 which are greater than the upper range of the worldwide range) fall within the expected range, with the average mean value of absorbed dose rate from all the samples being slightly higher than the worldwide mean value. The absorbed dose rate in air at 1 m above the ground surface does not directly provide the radiological risk to which an individual is exposed (Jibiri et al., 2007; Santawamaitre et al., 2010). The annual effective dose equivalent from outdoor terrestrial gamma radiation was estimated by taking into account the conversion coefficients from absorbed dose in air to effective dose and the outdoor occupancy factor. The effective dose for the different locations of soil samples in this study varied from 26.6 ± 0.4 to 190.9 ± 2.7 mSv y1, with the arithmetic mean value and standard deviation of 79 ± 33 mSv y1, which is comparable to the worldwide effective dose of 70 mSv y1 (UNSCEAR, 2000). The radiation hazard parameters in terms of the radium equivalent activity (Raeq) and the external hazard index (Hex) were also evaluated. The radium equivalent activity (Raeq) is a single quantity which compares the activity concentrations of 226Ra, 232Th and 40K in soil samples in order to obtain a total activity concentration. The results for the calculated Raeq from the current work are summarised in Table 4. The values of Raeq range from 46.2 to 343.7 Bq kg1 with an overall arithmetic mean and standard
238U
232Th
40K
180
Sampling area
Activity Concentration (Bq kg1) 238
Ping Wang Yom Nan Chao Phraya Total Worldwide Range Mean
232
U
39.71 18.50 30.73 25.56 36.99 29.19
± ± ± ± ± ±
17e60 35
0.34 0.24 0.24 0.19 0.22 0.10
40
Th
44.60 16.69 32.84 27.79 42.95 30.43
± ± ± ± ± ±
11e64 30
0.36 0.21 0.24 0.19 0.23 0.10
K
626.6 237.5 311.7 251.3 364.5 309.0
± ± ± ± ± ±
7.4 4.2 3.5 2.7 3.1 1.6
140e850 400
Absorbed dose (nGy h-1)
160
Table 3 Comparison between the average mean activity concentrations of 238U, 232Th and 40 K in 5 sampling areas measured in the current work and the mean value worldwide (UNSCEAR, 2000).
140 120 100 80 60 40 20 0 1
5
9
13
17
21
25
29
33
37
41
45
49
Sample number
Fig. 6. The calculated absorbed dose rate in air outdoors due to gamma radiation arising from 238U, 232Th and 40K for all the measured soil samples in the current work. See Table 1 for locations of each sample.
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deviation of 140 ± 60 Bq kg1. It can be seen that the Raeq values for all soil samples in the present work are lower than the accepted safety limit value of 370 Bq kg1 as recommended by the Organisation for Economic Cooperation and Development (OECD) (UNSCEAR, 1988; Belivermis et al., 2010). According to Beretka and Mathew, Turhan and Gundiz, the use of these soils as raw materials for building does not constitute a health hazard of radiation (Beretka and Mathew, 1985; Turhan and Gundiz, 2008). As listed in Table 3, the calculated values of the external hazard index for all soil samples studied vary from 0.120 ± 0.002 to 0.93 ± 0.01 and the average value was found to be 0.38 ± 0.16. The results show that the Hex values for all soil samples are below the limit of unity, meaning that the radiation dose is below the permissible limit of 1 mSv y1 recommended by ICRP (1991, 2007). It can be concluded that the radiological health risks to the people living in the areas studied in this current work is not significant. 4. Conclusion In this current study, the measurements showed that primordial radionuclides namely the 238U and 232Th decay chains and 40K are contained in all soil samples. The measured activity concentrations of 238U, 232Th and 40K across all of the soil samples varied from 13.9 to 76.8, 12.9 to 142.9 and from 178.4 to 810.7 Bq kg1 with the average mean values of 29, 30 and 309 Bq kg1, respectively. The obtained results of the activity concentrations of 238U and 232Th were found to be higher than the upper range of the worldwide values of 17 4 60, 11 4 64 Bq kg1 identified by UNSCEAR (2000) due to the high concentration values found in some soil samples from the Ping and Chao Phraya river areas. The activity concentration of 40K in the current study falls within the worldwide range of values which are 140 4 850 Bq kg1 reported by UNSCEAR (2000). The average mean activity concentrations of 238U, 232Th and 40K are comparable to the worldwide mean values. The results are influenced in part by a geological structure variation and/or high utilisation of fertilisers and thus may have resulted in the variations of the concentrations of 238U, 232Th and 40K found in some soil samples in the area studied. The artificial, radionuclide, 137 Cs, was not observed in statistically significant amounts above the background level in the current study. The results in this current study can be used as a baseline for the observation of any possible change in the future. The values of the radiation hazard parameters from this current study are not extremely high compared to the world averages and the recommended values and therefore are unlikely to cause additional radiological health risks to the people living in the areas studied. References Ajayi, O.S., 2009. Measurement of activity concentrations of 40K, 226Ra and 232Th for assessment of radiation hazards from soils of the southwestern region of Nigeria. Radiat. Environ. Biophys. 48, 323e332. Al-Hamarneh, I.F., Awadallah, M.I., 2009. Soil radioactivity levels and radiation hazard assessment in the highlands of Northern Jordan. Radiat. Meas. 44, 102e110. Al-Kharouf, S.J., Al-Hamarneh, I.F., Dababneh, M., 2008. Natural radioactivity, dose assessment and uranium uptake by agricultural crops at Khan Al-Zabeeb, Jordan. J. Environ. Radioact. 99 (7), 1192e1199. Almayahi, B.A., Tajuddin, A.A., Jaafar, M.S., 2012. Radiation hazard indices of soil and water samples in Northern Malaysian Peninsula. Appl. Radiat. Isot. 70, 2652e2660. ASTM, 2005. C1402-04 Standard Guide for High-resolution Gamma-ray Spectrometry of Soil Samples, Annual Book of ASTM Standard. In: Nuclear Energy (I), vol. 12.01. American Society for Testing and Materials.
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