Evaluation and analysis of 226 Ra, 232 Th, 40 K and radon exhalation rate in various grey cements

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 48, July 2010, pp Evaluation and analysis of 226 Ra, 232 Th, 40 K and radon exhalation rate in various grey cements Rati Varshney a, A K Mahur c, R G Sonkawade b *, M A Suhail a, A Azam a & R Prasad c a Department of Applied Physics, Z H College of Engineering & Technology Aligarh Muslim University, Aligarh , India b Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi , India c Vivekananda College of Technology and Management, Aligarh , India * sonkawade@gmail.com Received 22 April 2010; accepted 31 May 2010 The natural terrestrial gamma radiation dose rate is an important contribution to the average dose rate received by the world s population. Natural radio nuclides in building material are sources of external and internal radiation exposure in dwellings. The natural radioactivity and radon exhalation rate of the commonly used building construction material (grey cements) have been studied. Positive correlation was found between radium concentration and radon concentration. The external radiation exposure caused by gamma emitting radionuclide is determined from obtained values of 226 Ra, 232 Th and 40 K, using high purity Germanium Detector. The absorbed dose is found to be higher in the AMB cement. This study presents the results of estimation of 226 Ra, 232 Th and 40 K concentration, gamma absorbed dose rates and radon exhalation rates of grey cements collected from different manufacturers. Keywords: Radon exhalation rate, Gamma ray spectroscopy, Radium equivalent activity, Gamma absorbed dose rate 1 Introduction Natural radioactivity is widely spread in the earth s environment and it exists in various geological formations e.g. soils, rocks, plants, water and air 1,2. Naturally occurring radioactive materials (NORM), became the focus of regulatory interest with the publication of International Atomic Energy Agency. This part of our study deals with the determination of grey cement exhalation rate by using solid state nuclear track detector and natural radioactivity measurement using HPGe detector. The decay chains of uranium and thorium present in the earth based building materials cause the emanation of radioactive gases as 222 Rn and 220 Th. Out of these, 70% of the radon in the living rooms originates from the applied building materials and also due to poor ventilation Short lived decay products of radon pose most of the health hazards by inhalation and may be the cause of lung cancer, skin cancer and kidney diseases. Common building material used for constructions of houses (i.e. cement) which are considered as major source of these gases in indoor environment, have been studied for radon exhalation rate. Natural radioactivity arises mainly from the primordial radionuclides, such as 226 Ra, 232 Th and 40 K series and their decay products, which are present in traces in all over the ground. The radiological implication of these radionuclides is due to the gamma ray exposure of the body and irradiation of lung tissue from inhalation of radon and its daughters. Therefore, the assessment of gamma radiation dose from natural sources is of particular importance as natural radiation is the largest contributor as the external dose to the world population 4. External gamma dose estimation due to the terrestrial sources is essential as these doses vary depending upon the concentrations of the natural radio nuclides, 226 Ra, 232 Th, their daughter products and 40 K, present in the soils and rocks, which further depend upon the local geology of each region in the world 5. 2 Experimental Details 2.1 Radon concentration and radon exhalation rate For this study, cement samples were collected from different cement manufacturers. Radon concentration and exhalation rate measurements were done for the cement samples using the Can technique 6,7. For the measurement of exhalation rate, 100 g of samples were taken. Fine quality of samples were obtained by using scientific sieve of 150 µm mesh size and then dried in oven at 110 C for twenty four hours so that

2 474 INDIAN J PURE & APPL PHYS, VOL 48, JULY 2010 moisture can be removed from samples then packed and sealed in impermeable air tight polyvinyl chloride (PVC) container to prevent the escape of radiogenic gases ( 222 Rn) radon and ( 220 Rn) thoron. LR-115 type II non peliculable foils are used in this study, foils are exposed for a stipulated period of 100 days, same were retrieved and chemically etched in 2.5 N of NaOH (sodium hydroxide) solution at 60 C temperature for about one and half hour for developing the tracks registered in the films. Developed tracks were then recorded by microscope at 400 X. 2.2 Activity concentration of radium, thorium and potassium The gamma spectrometric measurement was carried out using HPGe gamma ray spectrometric system. The detector is a co-axial n-type high purity germanium detector (EG & G, ORTEC, Oak Ridge, USA). The resolution of detector is 2.0 kev at 1332 kev and relative efficiency is of 20%. The detector is coupled with 4K ADC system connected to PC, the spectrum is analyzed using the software CANDLE (Collection and Analysis of Nuclear Data using Linux network) locally developed by Inter- University Accelerator Centre, New Delhi. Four inch lead bricks are used as a shielding on all sides of detector to reduce the background level of the system 8. IAEA standard reference materials RGU-1 (uranium ore), RGTh-1 (thorium ore) and RGK-1 potassium (K 2 SO 4 ) are used in the same geometry for the efficiency calibration of the system, spectra is calibrated with known sources of radioactivity such as Eu-152. Efficiency values are plotted against energy for particular geometry. For the present study, samples are packed and sealed in an airtight polyvinyl chloride container and kept for about four week period to allow radioactive equilibrium among the radon ( 222 Rn), thoron ( 220 Rn) and their short lived decay products. In powder form, sample of 250 g was weighted for each material. The samples were counted for a period of 72,000 seconds and the spectra are analyzed for the peak of radium, thorium daughter products and potassium. The activity of each sample was determined using the total net counts under the selected photo peak after subtracting appropriate background counts and applying appropriate factors for photo peak efficiency, branching intensity of radionuclides and weight of the samples. Gamma transitions of 1461 kev for 40 K, 186 kev for 226 Ra, 295 and 352 kev for 214 Pb, 609, 1120 and 1764 kev for 214 Bi, 338, 463, 911 and 968 kev for 228 Ac, 727 kev for 212 Bi, 238 kev for 212 Pb are used for the measurement of activity concentration. 3 Results and Discussion 3.1 Radon activity and radon exhalation rate The activity of radon was obtained by track density from the etched detectors using calibration factor tr cm 2 d 1 /Bq m 3 obtained from the experiment 9. Exhalation rate of radon is calculated by using the equations 10,11 : E E s m = = CV λ / A t [ T + 1/ λ( e λ 1)] CV λ / M t [ T + 1/ λ( e λ 1)] (Bq m 2 h 1 ) (Bq Kg 1 h 1 ) (1) (2) where E s is the radon surface exhalation rate (Bqm 2 h 1 ); E m the mass exhalation rate (Bq Kg 1 h 1 ); C the integrated radon exposure as measured by LR-115 detector (Bqm 3 h); V the effective volume of Can; λ the decay constant for radon (h 1 ); T the exposure time (h); A the area covered by Can (m 2 ) and M is mass of sample in Can Using Eqs (1) and (2) radon surface and mass exhalation rates are found. Surface and mass exhalation rates vary from 29.0±3.2 to 122.5± 7.5 mbq m 2 h 1 and 1.1±0.2 to 4.7±0.3 mbq Kg 1 h 1, respectively. The activity of radon varies from 80.7±9.0 to 340.9±20.8 Bq m 3. Radon surface and mass exhalation rates found for MCM cement are lower and for A1C cement it is found higher and similar kind of trend was found with radon concentration. Table 1 presents the values of radon concentration, radon surface and mass exhalation rates. A strong positive correlation was observed between radium concentration and radon concentration (correlation coefficient = 0.84), which may be due to the radium content 12 and porosity 13 in the samples. Graph for the correlation is shown in Fig Radium, thorium and potassium in various samples The concentrations of radium, thorium and potassium were calculated using the following equation: CPS CPSerror Activity(Bq) = ± B. I. Eff B. I. Eff...(3)

3 VARSHNEY et al.: RADON EXHALATION RATE IN VARIOUS GREY CEMENTS 475 Table 1 Radon activity and radon exhalation rate in different cement samples samples Track density (tr.cm 2 d 1 ) Radon activity (Bq m 3 ) Surface Mass exhalation exhalation rates rates (mbq m 2 h 1 ) (mbq Kg 1 h 1 ) UTC 14.8± ± ± ±0.3 J1P 14± ± ± ±0.3 MCM 4.5± ±9.0 29± ±0.2 BST 8.8± ± ± ±0.2 PRM 8.1± ± ± ±0.2 A1C 19± ± ± ±0.3 SUT 8.9± ± ± ±0.2 T1F 10.5± ± ± ±0.2 J1K 9.6± ± ± ±0.2 BNG 13.6± ± ± ±0.2 AMB 13.8± ± ± ±0.2 Samples Table 2 Activity concentration of 226 Ra, 232 Th and 40 K 226 Ra (Bq Kg 1 ) 232 Th (Bq Kg 1 ) 40 K (Bq Kg 1 ) UTC 60.7± ± ±3.5 J1P 71.2± ± ±3.5 MCM 28.7± ± ±4.6 BST 45.4± ± ±3.8 PRM 36.6± ± ±3.9 A1C 46.2± ± ±4.1 SUT 61.4± ± ±5.0 T1F 64.9± ± ±4.7 J1K 42.4± ± ±4.5 BNG 52.0± ± ±3.6 AMB 64.35±1.6 80± ±4.7 Table 3 Absorbed dose rate and effective dose for cement samples samples Ra eq (Bq Kg 1 ) Absorbed dose D(nGyh 1 ) Indoor annual effective dose (msv) Outdoor annual effective dose (msv) Health hazard index (H ex ) Fig. 1 Linear regression of the concentration of radium and radon. where CPS is counts per second; B.I. the branching intensity and E ff is the efficiency of the detector. Table 2 presents the average concentration of radionuclides, 226 Ra, 232 Th and 40 K. The concentration for 226 Ra, 232 Th and 40 K varies from 28.7±0.9 to 71.2 ±4.8 Bq kg 1, 28.3 ±1.0 to 80±1.1 Bq kg 1, and 255.5±3.5 to 408 ± 3.6 Bq kg 1, respectively in cement samples used in this study. The activity found for 226 Ra is the lowest for MCM and maximum for the J1P cement, 232 Th concentration found for MCM cement is lower and AMB cement is higher, 40 K concentration found in BNG cement is higher and in UTC cement it is lower. These samples do not pose any health risk for the people who spend their most of the time in buildings, since all the reported values are well below the permissible limits, stipulated as per the guidelines of Atomic Energy Regulatory Board of India. UTC J1P MCM BST PRM A1C SUT T1F J1K BNG AMB Estimation of dose rate A quantity, while considering the radiation risk to human and other, is absorbed dose rate. Assessment of gamma radiation hazard to human associated with the cement used for building material can be done by calculating the different radiation hazard indices 14. Table 3 presents the outdoor air absorbed dose rate D(nGy h 1 ) due to terrestrial gamma rays at one meter above the ground level can be estimated by the concentrations of 226 Ra, 232 Th and 40 K by applying the factors 0.462, and for radium, thorium and potassium, respectively (ngyh ) (0.462 Ra Th T ) D = C + C + C...(4) where C Ra, C Th and C K are the activity concentrations (Bq kg 1 ) of radium, thorium and potassium in the samples. To estimate annual effective dose rates, the

4 476 INDIAN J PURE & APPL PHYS, VOL 48, JULY 2010 conversion coefficient from absorbed dose in air to effective dose 0.7 Sv Gy 1 and the indoor occupancy factor of 0.2 proposed by UNSCEAR, The annual effective doses are determined as follows: Indoor(mSvy 1 )= Absorbed dose)ngyh hy SvGy (5) Outdoor(mSvy 1 ) = Absorbed dose)ngyh hy SvGy (6) Using Eqs (4)-(6), the absorbed and annual effective dose rates from the samples were calculated as given in Table 3. The values of absorbed dose and indoor dose equivalents were found to vary from 45.9 to 95.7 ngy h 1 and 0.22 to 0.46 msv y 1. As the outdoor annual effective dose varies from 0.05 to 0.83 msv y 1. The lowest dose was found in MCM cement while maximum dose was found in AMB cement. All these samples satisfy the permissible limits for radiation safety point of view. The calculated results for indoor and outdoor annual effective dose due to natural radioactivity of studied samples are lower than the average world recommended level of 1 msv y External hazard index Consideration of external radiation exposure is usually associated with gamma radiation emitted by radio nuclides of concern. Ra eq is related to the external gamma dose and internal dose due to radon and its daughters. The values of H ex must be lower than unity to keep the radiation hazard insignificant 16,17. H ex = C Ra /370+C Th /259+C K /4810 < 1...(7) The external hazard index is obtained from Ra eq expression. The values of external hazard index have been found to be lower for MCM cement i.e and higher 0.55 for AMB cement, which are under permissible limits Conclusions Gamma ray spectrometry has been used to determine the radioactivity concentrations 226 Ra, 232 Th and 40 K in the studied cement samples of various Indian cement suppliers. High values of radium concentration and radon exhalation found in J1P and A1C cement samples, respectively, whereas lowest for MCM cement. The radon exhalation rates measured by the Passive technique showed a good linear correlation with radium content. The values of external hazard index in the studied samples are less than the recommended safe levels. If external hazard index exceeds unity, we conclude that external doses to exposed individuals will exceed the acceptable levels. The external hazard index for all samples under investigation is lower than the acceptable range. The result shows radon concentration, mass and surface exhalation rates for A1C cement which is around four times higher than MCM cement. The presence of natural radioactivity level in cement samples have been found to be well below the permissible limits 1000, 1000 and 4000 Bq kg -1 for 226 Ra, 232 Th and 40 K, respectively. The absorbed dose and annual effective dose varies from to ngy h 1 and 0.22 to 0.46 msv y 1, respectively. The outdoor annual effective dose varies from 0.05 to 0.83 msv y 1. The absorbed and effective dose values are well below the permissible limits. Acknowledgement One of the authors (RV) is thankful to Dr Amit Roy, Director, Inter-University Accelerator Centre, New Delhi, for providing the facilities required in this research work. Author is thankful to the vendors of Aligarh for providing various types of grey cement for this study. References 1 Malance A, Gaidolfi L, Pessina V & Dallara G, J Environ Radioact, 30 (1996) Abdo Aly, Hassan A A, Huwait M H & M R A, Fourth Rad Phys Conf, 15-19, November, Alexandria, Egypt, (1999) Stoop P, Glastra P, Hiemstra Y, De Vries L, Lembrechts J & Bilthoven, The Netherland RIVM Report (1998) 4 UNSCEAR, United Nations Scientific Committee of the Effect of Atomic Radiation, United Nations. New York (1988). 5 Sohrabi M, Appl Rad & Isotop, 49 (1998) Abu-jarad F, Nucl Tracks Radiat Meas, 15(1-4) (1988) Chauhan R P, Kant K, Mahesh K & Chakarvarti S K, Indian J Pure & Appl Phys, 39 (2001) Sonkawade R G, Kant K, Muralithar S, Kumar R & Ramola R C, Atmosp Environ, 42 (2008) Singh A K, Jojo P J, Khan A J, Prasad R & Ramchandran T V, Radiat Prot Environ, 20 (1997) Fleischer R L, Morgo-campero A, Tech and Principles Geophys Res, 83 (1978) Khan A J, Prasad R & Tyagi R K, Nucl Tracks Radiat Meas, 20 (1992) Ramachandran, T V & Subba Ramu M C, Oncology, 13 (1989) Folkerts K H, Keller G & Muth R, Rad Prot Dosim, 9 (1984) 27.

5 VARSHNEY et al.: RADON EXHALATION RATE IN VARIOUS GREY CEMENTS El-Aydarous A, Global J Environ Res, 1(2) (2007) UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York, (2000). 16 Yu K N, Guan Z J, Stoks M J & Young E C, J Environ Radioact,17 (1992) Akkurt I, Mavi B, Akyildirim H & Gungoglu K, Int J Phys Sci, 4 (7) (2009) European Commission on Radiation Protection. Europ Commission, Brussels Report No.112 (1999). 19 Sonkawade R G, Ramola, R C, Kant K, Kanjilal D K, Dhiaryawan M P & Gupta P, Proc. 27 th IARP Conference, Rad Prot Environ, Vol 28 (1-4) (2005) 156.