Indian Journal of Pure & Applied Physics Vol. 48, July 2010, pp. 482-485
Estimation of radioactivity in some sand and soil samples Monika Guptaa*, R P Chauhana, Ajay Gargb, Sushil Kumarc & R G Sonkawaded a
Department of Physics, National Institute of Technology, Kurukshetra (Haryana) b
Department of Physics, Arya PG College , Panipat (Haryana)
c
Department of Physics, Choudhary Devi Lal University, Sirsa (Haryana) d
Inter University Accelerator Centre, New Delhi 110 067
*E-mails:
[email protected],
[email protected] Received 22 April 2010; accepted 2 June 2010 Natural radioactivity is composed of the cosmogenic and primordial radionuclides. It is common in the rocks and soil that make up our planet, in water and oceans, and in our building materials and homes. Natural radioactivity in sand and soils comes from 238U and 232Th series and natural 40K. Radon is formed from the decay of radium which in turn is formed from uranium. The gaseous radioactive isotope of radon from natural sources has a significant share in the total quantum of natural sources exposure to human beings. Gamma radiation from 238U, 232Th and 40K represents the main external source of irradiation of the human body. In the present study, the activity for 238U, 232Th and 40K is found to vary from 45±1.2 to 97± 4.9 Bq/kg, 63 ± 2.0 to 132 ± 3.2 Bq/kg and 492 ± 5.9 to 1110 ± 10.5 Bq/kg, respectively in the soil samples while the variations have been observed from 63 ± 3.8 to 65 ± 3.7 Bq/kg, 86 ± 2.5 to 96 ± 2.6 Bq/kg and 751± 7.7 to 824 ± 8.2 Bq/kg, respectively in the sand samples. Keywords: Uranium, Thorium, Potassium, Sand, Soil
1 Introduction Natural radioactivity is common in the rocks and soil that make up our planet, in water and oceans, and in our building materials. We inhale and ingest radionuclides every day in our lives and radioactive material has been ubiquitous on earth since its creation. The inhalation and ingestion of these radionuclides above the permissible level become a health hazard. Therefore, concern of the monitoring of these radionuclides in the environment is increasing at all levels, due to their harmful effects. Uranium is the ultimate source of radium and radon. Radon isotopes are the decay products of radium in uranium decay series. As an inert gas, radon can diffuse through the soil and enter the atmosphere. Radon exposure is associated with the risk of leukemia and certain other cancers, such as malenoma and cancers of kidney and prostate1. If uranium rich material lies close to the surface of earth there can be high radium exposure hazards2,3. In the present paper, the activity concentrations of 238U, 232Th and 40K, Radium equivalents, annual effective doses, external and internal indices in the soil and sand samples have been calculated. The results are important from radiation protection point of view. The aim of the present work is to explore the possibility of uranium
exploration and health risk assessments due to uranium and radium in the study area. 2 Experimental Details Using HPGe detector of high-resolution gamma spectrometry system, the activity in the samples was measured. The detector is a co-axial n-type highpurity germanium detector (Make EG & G, ORTEC, Oak Ridge, USA). The detector has a resolution of 2.0 keV at 1332 keV and relative efficiency of 20%. The output of the detector is analyzed using a 4 K ADC system connected to PC, the spectrum is analyzed using the locally developed software “CANDLE (Collection and Analysis of Nuclear Data using Linux nEtwork)”. The detector is shielded using 4 inch lead on all sides to reduce the background level of the system4,5. The efficiency calibration for the system is carried out using secondary standard source of uranium ore in geometry available for the sample counting. Efficiency values are plotted against energy for particular geometry and fitted by least squares method to an empirical relation that takes care of the nature of efficiency curve for the HPGe detector. After collection, samples were crushed into fine powder by using mortar and pestle. Fine quality of the sample was obtained using a scientific sieve of
GUPTA et al.: ESTIMATION OF RADIOACTIVITY IN SAND AND SOIL SAMPLES
150 µm mesh size. Before measurement, the samples are dried in an oven at about 110°C for 24 h. Each sample is packed and sealed in an airtight PVC container and kept for about 4-week period to allow radioactive equilibrium among the radon (222Rn), thoron (220Rn) and their short lived decay products. An average of 300-400 g of sample in powder form was taken for each material. The samples were counted for a period of 72,000 s and the spectra are analyzed for the photopeak of uranium, thorium daughter products and 40K. The net count rates under the most prominent photo peaks of radium and thorium daughter peaks are calculated by subtracting the respective count rate from the background spectrum obtained for the same counting time. Then, the activity of the radionuclides is calculated from the background subtracted area of prominent gamma ray peaks. Gamma transitions of 1461 keV for K40, 186 keV for Ra226, 295 and 352 keV for Pb214, 609, 1120 and 1764 keV for Bi214, 338, 463, 911 and 968 keV for Ac228, 727 keV for Bi212, 238keV for Pb212 were used for the laboratory measurement of activity concentration.
97±4.9 Bq/kg, 63±2.0 Bq/kg to 132±3.2 Bq/kg and 492±5.9 Bq/kg to 1110±10.5 Bq/kg, respectively in the soil samples studied in the present work. Similarly, the activity for U238, Th232 and K40 is found to vary from 63±3.8 Bq/kg to 65±3.7 Bq/kg, 86±2.5 Bq/kg to 96±2.6 Bq/kg and 751±7.7 Bq/kg to 824±8.2 Bq/kg, respectively in the sand samples studied in the present work. All the samples analyzed in the present work satisfy the safety criterion, i.e. the annual effective dose is less than 1 mSv, the recommended safety limit of general public6. Hence, these samples do not pose any health hazard for the occupants. 3.1 Estimation of radium equivalent activity (Raeq)
Radium Equivalent Activity (Raeq) is defined to compare the activity concentration of samples containing different amounts of U238, Th232 and K40 . It is calculated through the following expression7: Raeq=CU+1.43×CTh+0.077×CK
CPS × 100 × 100 B.I . × Eff
±
CPSerror × 100 × 100 B.I . × Eff
…(2)
where CU, CTh and CK are the concentrations (Bqkg−1) of U238, Th232 and K40, respectively in the samples. Table 2 presents the range of radium equivalent from minimum to maximum was 172 Bq/kg to
3 Results and Discussion The concentrations of uranium, thorium and potassium were calculated using Eq. (1): Activity(Bq) =
483
Table 1 — Activity concentration of uranium, thorium and potassium for soil and sand samples Samples
…(1)
where CPS is the net count rate per second, BI is the branching intensity and Eff is the efficiency of the detector. Table 1 presents the average concentration of the radionuclides, U238, Th232 and K40, as well as the corresponding statistical error in the samples under investigation. The activity for U238, Th232 and K40 is found to vary from 45±1.2 Bq/kg to
Soil
Sand
Sample codes
U-238 (Bq/kg)
Th-232 (Bq/kg)
K-40 (Bq/kg)
JK MK DK JGY S1 S2 CS1 CS2 CS3 CS4
63±1.4 71±3.4 45±1.2 61±3.9 97± 4.9 59±1.3 63±3.9 65±3.7 63±3.8 63±3.8
82±2.3 84±3.1 63±2.0 72±2.5 132 ±3.2 93±2.6 96±2.6 86±2.5 87±2.5 89±2.5
707±7.9 735±8.2 492±5.9 539±6.4 1110±10.5 754±7.8 824±8.2 751±7.7 775±7.9 758±7.8
Table 2 — Radium equivalent, absorbed dose, dose equivalents and external internal indices for soil and sand samples Samples
Sample codes
Raeq (Bq/kg)
Absorbed dose D (nGyh−1)
Indoor annual Outdoor annual effective dose, (mSv) effective dose, (mSv)
External index Internal index (Iγ) (Iα)
Soil
JK MK DK JGY S1 S2
234 247 172 204 372 251
108 114 79 94 171 115
0.53 0.56 0.39 0.46 0.84 0.57
0.13 0.14 0.11 0.12 0.21 0.14
0.85 0.90 0.63 0.74 1.35 0.92
0.32 0.35 0.22 0.30 0.49 0.30
Sand
CS1 CS2 CS3 CS4
263 246 247 248
121 113 114 114
0.60 0.56 0.56 0.56
0.15 0.14 0.14 0.14
0.96 0.90 0.90 0.91
0.32 0.32 0.32 0.31
484
INDIAN J PURE & APPL PHYS, VOL 48, JULY 2010
372 Bq/kg in the soil samples and 246 Bq/kg to 263 Bq/kg in the sand samples. In the present study, the samples were found to have Raeq values smaller that the upper recommended value8 of 370 Bqkg−1 except in S1 (soil). The sample S1 has been collected from district Panchkula (Haryana) which is in the vicinity of Shivalik range of Himalayas.
and hence, these samples do not pose any health hazard problems. 3.3 Estimation of indices
External index: The gamma index (Iγ) is defined in order to examine the applicability of using materials in construction. For a typical material, it is given by the following expression10:
3.2 Estimation of absorbed and effective dose
The measured activity of U238, Th232 and K40 were converted into doses (nGyh−1Bq−1kg−1) by applying the factors 0.462, 0.604 and 0.0417 for uranium, thorium and potassium, respectively9. These factors were used to calculate the total absorbed gamma dose rate in air at 1 m above the ground level using Eq. (3): Absorbed dose D (nGyh−1)=0.462CU+0.604CTh +0.0417CK …(3) where CU, CTh and CK are the activity (Bq kg−1) of uranium, thorium and potassium in the samples. To estimate annual effective doses, account must be taken of (a) the conversion coefficient from absorbed dose in air to effective dose and (b) the indoor occupancy factor. Annual estimated average effective dose equivalent received by a member is calculated using a conversion factor of 0.7 SvGy−1, which is used to convert the absorbed rate to annual effective dose with an outdoor occupancy of 20% and 80% for indoors3. The annual effective doses are determined as follows: Indoor annual effective dose (mSv) =(Absorbed dose)nGyh−1×8760 h×0.8×0.7 SvGy−1 …(4)
Iγ =
Cx
∑A x
≤1
…(6)
x
where Cx (Bq kg−1) is the measured activity of each nuclide in the building material, Ax (Bq kg−1) is the activity concentration of each nuclide in the material, and it is assumed to produce the same gamma dose rate, i.e. 300, 200 and 3000 Bq kg−1 for U238, Th232 and K40, respectively10. Based on the dose criterion of 1 mSvy−1, Table 2 indicates that Iγ is less than unity in all the materials investigated except in S1 (soil), regardless of the ways and the amounts in which the material is used. Internal index: Several α indices have been proposed to assess the exposure level due to radon inhalation originating from building materials10,11. The internal index is defined as follows: Iα =
CU 2 0 0 B q k g −1
…(7)
where CU (Bqkg−1) is the activity concentration of U238. The recommended exemption level and the recommended upper level for U238 suggested by the International Commission on Radiological 12 −1 Protection are 100 and 200 Bq kg , respectively. As
Outdoor annual effective dose (mSv) =(Absorbed dose)nGyh−1×8760 h×0.2×0.7 SvGy−1 …(5) Using Eqs (3)-(5), the absorbed and annual effective dose rates from the samples were calculated as shown in Table 2. The minimum and maximum values of absorbed dose, indoor annual effective dose and outdoor annual effective dose were found to vary from 79 to 171 nGyh−1, 0.39 to 0.84 mSv and 0.11 to 0.21 mSv in the soil samples studied in the present work. Similarly, the minimum and maximum values of absorbed dose, indoor annual effective dose and outdoor annual effective dose were found to vary from 113 to 121 nGyh−1, 0.56 to 0.60 mSv and 0.14 to 0.15 mSv, respectively in the sand samples studied in the present work. The samples were found to satisfy the safety criteria for radiation safety point of view
Fig. 1 — Variation of U238 Concentration (Bq/kg) with Raeq activity (Bq/kg)
GUPTA et al.: ESTIMATION OF RADIOACTIVITY IN SAND AND SOIL SAMPLES
presented in Table 2, the U238 activities in the samples investigated were lower than 200 Bqkg−1, so the alpha indices in materials did not exceed the recommended upper level. The graph for the correlation between uranium concentration and radium equivalent activity is shown in Fig. 1. 4 Conclusions The natural radioactivity in the samples was determined using gamma- ray spectrometry. Different types of samples including soil and sand were analysed for their U238, Th232 and K40 contents using a high-resolution gamma-ray spectrometry system. Some main conclusions are drawn from the present study. The samples satisfy the universal standards limiting the radioactivity within the safe limits of 1000, 1000 and 4000 Bq kg−1 for U238, Th232 and K40, respectively. The dose equivalents found in all the samples also lie within the safe limit of 1 mSv yr−1. The values of uranium concentrations are not significant from exploration point of view. Most of the samples were found to have Raeq values less than the upper recommended value of 370 Bqkg−1. The results reveal that the area is safe as far as the health hazard effects of uranium and radium are concerned. A strong correlation coefficient (= 0.885) is observed between uranium concentration and radium equivalent activity. Acknowledgement The experimental facilities provided by Health Physics Division of IUAC, New Delhi, for gamma
spectroscopy of acknowledged.
the
samples
485 are
thankfully
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