Evaluation of natural radioactivity and radiological hazards caused by different marbles of India

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 3, November 200, pp Evaluation of natural radioactivity and radiological hazards caused by different marbles of India V Ramasamy a, V Ponnusamy a, J Hemalatha a, V Meenakshisundaram b & V Gajendiran b a Department of Physics, Annamalai University, Annamalainagar b Health and Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam * srsaranram@rediffmail.com Received 2 January 200; revised 2 August 200; accepted 12 September 200 The samples used in this study are of various coloured varieties of marbles collected from marble dealers. The specific activity concentration of 238 U, 232 Th and 0 K has been determined by gamma ray spectrometry. The materials showed concentrations of 238 U, 232 Th and 0 K which were found to be dramatically variable depending on mineral content and type of formation. Using FTIR and thin section analyses, quartz, feldspar, calcite and mafic minerals present in the samples have been determined quantitatively. From our experimental data, it can be seen that the geochemical parameters such as SiO 2 or mafic minerals or CaCO 3 may be considered to be an appropriate index to select the marbles of low radiological risk. The average specific activities of 0 K are found to be higher than 232 Th and 238 U. The ratio of Th/U was calculated and correlated. Assessment of radiological hazards was made by calculating radium equivalent activities, external and internal hazard indices which were found to vary from to., 0.08 to 0.1 and 0.10 to 0.17 Bq/kg, respectively. The observed values are lower than the recommended limits. Keywords: Natural radioactivity, Marble, Radiological hazards, Gamma ray spectrometry IPC Code:G01T 1 Introduction The knowledge of radioactivity present in building materials enables one to assess any possibility of radiological hazard to human by the use of such materials. This knowledge is essential for the development of standards and guidelines for the use and management of these materials. The most important naturally occurring radionuclides present in the soils and rocks are 238 U, 232 Th and 0 K 1. Like other construction materials, the natural radioactivity in marble may give rise to external and internal exposure. The external radiation exposure is caused by gamma radiation from primordial nuclides, however, the internal radiation exposure mainly affecting respiratory tract, is due to short lived daughter products of radon which are exhaled from construction materials from into room in air 2. In buildings, marbles are used commonly as floor laying material. Marble is one of the type of metamorphic rock, is found to occur on the earth's surface. The colours of the marble depend upon the mineral composition and also the metamorphism. The aim of this study is to determine the concentration of the natural radioactivity and assess the radiological hazards to human from commercially available different coloured marbles in India. 2 Materials and Methods 2.1 Sample collection and preparation Different coloured marble (twelve) samples were collected from marble dealers. In each coloured marble, four to five specimens were collected. The collected samples were powdered to obtain more or less the same grain size of 120 μ and packed fully in a 20 ml plastic container and the net weight was determined before counting. These containers were sealed hermetically to ensure that all daughter products of 238 U and 232 Th, in particular, gaseous radon isotopes did not escape. A time of 30 days was allowed after packing to attain secular equilibrium between 226 Ra and its short-lived daughter products Experimental technique To estimate the activity levels of 238 U, 232 Th and 0 K in marble samples, a gamma ray spectrometer in the laboratory of Health and Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, was made use of in the present investigations. NaI(Tl)

2 816 INDIAN J PURE & APPL PHYS, VOL 3, NOVEMBER 200 crystal detector of size 3" 3" along with a 8 K multichannel analyzer was used to record the gamma spectra. Standard sources of natural uranium ( Bq), natural thorium ( Bq) and KCl (181.9 Bq) with a standard 20 ml container obtained from International Atomic Energy Agency (IAEA) were used for calibrating the gamma ray spectrometer. For the counting time of 20,000 s for each sample, the minimum detectable activity (MDA) limits were 13.2 Bq/kg for 0 K,. Bq/kg for 238 U and 1 Bq/kg for 232 Th. The activities of 238 U, 232 Th and 0 K were calculated by using well known method. The values are presented in Table 1 and shown in Fig. 1. The mineralogical compositions and quantification of major minerals were determined using FTIR (The Nicolet Avatar 360 series with range from 000 to 00 cm -1 accuracy of ±0.01 cm -1 and resolution of ±1 cm -1 ) spectrophotometer and the presence of minor minerals (mafic) were quantified using thin section analysis. FTIR spectrophotometer was calibrated for its accuracy with the spectra of a standard polystyrene film at room temperature. Wet hand grinding was done by taking to 10 mg of the sample in an agate mortar followed by addition 10 to 1 drops of ethanol. The specimens ground to a 3 μ were mixed with KBr in the ratio of 1:20 and pellets with 1 mm in thickness and 13 mm in diameter were prepared. For each specimen, five pellets were prepared and the spectra were taken. That best one with well resolved and maximum number of peaks was taken for analysis. The quantitative estimation through infrared technique was achieved using Bouguer - Beer law. For estimation of quartz, feldspar and calcite in all samples, nearly pure form of the same have been obtained from State Geology Department, Chennai. Their IR spectra for different concentrations were recorded. The characteristic peaks of 780 cm -1 for quartz, 6 cm -1 for feldspar and 21 cm -1 for calcite were used for quantitative analysis. The absorbance at different concentration for these characteristic peaks was observed and the calibration curves were drawn. Making use of these curves, the amount of quartz, feldspar and calcite in different marble samples was estimated (Table 2). To estimate the mafic minerals, thin section analysis was carried out by the use of petrological microscope (Ernst Leitz Wetzlar, Germany, magnification 10X). A thin section represents a part of rock specimen which contains the mineral characteristics of the rock. Four to five thin sections were prepared from each specimen and that particular section from each specimen containing a maximum number of minerals was selected for the study. The petrological features and optical characteristics of mineral like colour, cleavage, form, birefringence, interference properties, twinning and extinction qualities of the individual mineral were observed. Minerals (mafic) were identified with their diagnostic optical features. The quantitative estimation of the mafic (minor) mineral content was determined using modal analysis (Table 2). Table 1 Specific activities, calculated absorbed dose rate, annual effective dose equivalent, Th/U, Ra equivalent and hazard indices of various marbles Commercial name of the marbles (variety) Number of samples analysed Specific Activity (Bq/kg) Th U K Calculated absorbed dose rate (ngy/h) Annual effective dose equivalent (μsv/y) Th/U (Bq/kg) Ra eq (Bq/kg) H ext H in Adanga (white) Moreward (greenish white) Tomato (pink) Sawver (grey) Indo Italian (white) Reymonds (thick grey) Pakistan Onyx (light green) Pista (white with black dots) Udaypur (light pink) Green Marble -1 (dark) Majoli (green with white patches) Green Marble -2 (dark green with black and white patches) Min Max Mean

3 RAMASAMY et al.: NATURAL RADIOACTIVITY AND RADIOLOGICAL HAZARD 817 Fig.1 Radioactivity concentration of U, Th and K for different samples Table 2 Quantitative estimation of various minerals through FTIR and thin section analysis for various marbles Commercial name of the marbles (variety) Adanga (white) Moreward (greenish white) Tomato (pink) Sawver (grey) Indo Italian (white) Reymonds (thick grey) Pakistan Onyx (light green) Pista (white with black dots) Udaypur (light pink) Green Marble -1 (dark) Majoli (green with white patches) Green Marble -2 (dark green with black and white patches) Quartz FTIR analysis Feldspar Total Calcite Magnetite Thin section analysis Haematite Iron oxides Total Results and Discussion From Table 1, it is observed that the activity concentration of 232 Th, 238 U and 0 K in all samples varied differently. This variation may be attributed to the mineral content of the marbles. It is well-known fact that the metamorphic rocks like marbles are composed of felsic minerals (quartz and feldspar) and CaCO 3 in major amounts and mafic minerals (magnetite, haematite, iron oxides) in minor amounts. Felsic minerals are leucocratic (light coloured) and mafic minerals are malanocratic (dark coloured) in nature.

4 818 INDIAN J PURE & APPL PHYS, VOL 3, NOVEMBER 200 The observed absorption frequencies from FTIR spectra for all the samples show the presence of quartz (SiO 2 ), feldspar, calcite (CaCO 3 ), magnetite and haematite. The percentage of major minerals (quartz, feldspar and calcite) was determined by making the calibration graph of the respective pure minerals (IR quantitative method) and minor minerals (mafic: magnetite, haematite and iron oxides) were quantified using thin section analysis which are given elsewhere. The values are reported in Table 2. The absorbed dose rate was calculated from the activity values obtained from the gamma ray spectra using conversion factors given by Beck (Table 1) varying from 1.7 to ngy/h with a mean of 21.8 ngy/h which is lower than global average value ( ngy/h). A graphical representation of these values with percentage of felsic minerals, calcite and mafic minerals is shown in Fig. 2. From this, it is observed that there is a positive correlation between calculated absorbed dose rate with percentage of felsic and mafic minerals and negative correlation with percentage of calcite content. From Table 1, it is observed that the green marbles (sample 12) are having maximum radioactivity while Adanga white marbles (sample 1) exhibit a minimum. The world average concentrations of 238 U, 232 Th and 0 K are 3, 30 and 00 Bq/kg respectively 6. Comparing these values with the present study, the activity concentrations of these radionuclides are found to be lower than the world average, suggesting the dependence of calcium content in these samples (Fig. 2). The presence of calcium content reduces the uranium level and increases the thorium level 7. From Table 1, it is also observed that the activity concentration of thorium is found to be higher than uranium, which is evident from the fact that thorium is 1. times higher than that of uranium in earth's crust 7. The annual effective dose equivalent was estimated 8 (Table 1) using ICRP and UNSCEAR dose conversion factors 9 of an indoor occupancy of 0.8 (=7000 hy -1 ), a dose rate at one metre height above the floor and a SvGy -1 conversion factor for adults. This dose was determined as a product of calculated absorbed dose, conversion factor ( SvGy -1 ) and occupancy factor (0.8) and the values are presented in Table 1. The values are found to be varying from to μsv/y with a mean of 26.3 μsv/y. This is lower when compared to the world average 70 μsv/y and is still very lower than the Srilankan average 10 of 0 msv/y (building materials). The estimated average annual effective dose rate from terrestrial gamma rays for 13 countries were given 9 in UNSCEAR, ranging from 3 msv/y (Canada) to 0.6 msv/y (Sweden), which are again very higher than the present values. Earlier investigations of the radioactivity values ( 238 U, 232 Th and 0 K) of granitic samples 11 to those of marbles show that 238 U and 232 Th are 16 times lower and 0 K is times lower. According to Fernando and Arnaldo 12, low radioactivity yields low volumetric heat generations, which will not affect the health. The comparison is quite important in the sense that both marbles and granites are commonly used building materials. Fig. 2 Correlation between calculated absorbed dose rate versus percentage of various minerals (felsic, mafic and calcite minerals)

5 RAMASAMY et al.: NATURAL RADIOACTIVITY AND RADIOLOGICAL HAZARD 819 Rizzo et al. 13, suggested that geological knowledge is an important factor to study the evolution process for indicating a safety index in the building materials. They investigated the 232 Th and 0 K contents of eight different marbles ( to 6% of Si) collected from Italy varying from 0.90 to 3.6 Bq/kg for 232 Th and 16 to 20 Bq/kg for 0 K which were very much lower when compared to the present values. Further, they concluded that the low radioactivity in the Italian marbles was due to the minimum content of Si. The mineralogy of the present samples through FTIR studies reveals 1 to 19% of Si content. Fortunately, the radioactivity of 232 Th and 0 K was observed to be high. Based on the lines of Rizzo et al. 13, the radioactivity in the present samples positively correlates with the Si content of the samples. Hence, Si may be considered to be an index to recognize the magmatic differentiation and can be used to select materials of low radiological impact 1. If the values of other type of metamorphic rock (Ca rich) like anorthosites (Th/U = 3.3 Bq/kg) observed by Atal et al. 1 are compared with those of present samples, the value of Th/U in the present samples (1.7 Bq/kg) is much lower (Table 1). They reasoned out that the higher values of Th/U in anorthosites were due to more felsic minerals (quartz-sio 2, anorthite feldspar-caal 2 Si 3 O 8 ) and less amount of mafic minerals. Because of more SiO 2, the mobilization of (U, Th), might have taken place during metamorphism. Uranium associated with felsic minerals gets more readily mobilized than with mafic minerals. This allows a general upward concentration of radio elements in the crust 16. In the present study, the felsic and mafic mineral content of the marbles are less when compared with the above and hence the lower ratio of Th/U. 3.1 Radiological hazards A common index to evaluate the relative radiological risk due to external γ-ray irradiation is defined in terms of radium equivalent activity (Ra eq ) as given by the following equation 17 : Ra eq = C U + AC Th + BC K Bq/kg (1) where C U, C Th and C K are the activity concentrations from 238 U, 232 Th and 0 K in Bq/kg respectively and A and B are constants. The assumptions used in Eq. (1) are that 1 Bq/kg of U, Bq/kg of Th and 13 Bq/kg of K produce an equal gamma ray dose. For the safe utilization of materials, the annual limit on the external gamma ray dose is 1. msv, which corresponds to the value of 370 Bq/kg for radium equivalent 17. Radium equivalent indices for all the samples were calculated and given in Table 1. The obtained values were found low compared to world wide recommended limit of 370 Bq/kg. The Ra eq values reported by Amrani and Tahtat 18 for eight Algerian marbles ranging from 73 to 78 Bq/kg and are found to be higher than with those determined in this study. Rizzo et al 8., investigated the radiological hazards about some marbles along with other building materials from Sicily. They found that Ra eq of the marbles varied from 128 to 203 Bq/kg which are high compared to present values. They have also suggested that the variation in values are due to the Si and total alkali (TA) contents. The other quantities indicating radiological hazards are external (H ex ) and internal (H in ) hazard indices and defined by the following equations 17 :. Cu Cth Ck H ex = Cu Cth Ck H in = To limit the annual external gamma ray dose from building materials to 1. msv and to keep Ra eq less than 370 Bq/kg, an external hazard index (H ex ) should be always less than one. The internal exposure to radon (Rn) and its decay product is controlled by internal hazard index (H in ) and for safe use this index must be less than unity. The calculated values of these indices (H ex and H in ) from our measurements are given in Table 1. The maximum values of H ex and H in are and respectively. These values are much lower than the recommended limit. Conclusion It can be concluded from the overall results in this study that different variety of marbles used for building construction is of low radioactivity. The Ra eq activity obtained for the marbles in this study was below the criterian limit of radiation dose. Therefore, according 19 to NEA-OECD, use of these marbles in construction of dwellings is considered to be quite safe for inhabitants. The obtained results in this study will also show a baseline data for proper assessment of radioactivity exposure to the dwellers.

6 820 INDIAN J PURE & APPL PHYS, VOL 3, NOVEMBER 200 References 1 Khan H M, Khan H M, Atta M A & Ian F, J Chem Soc Pak, 16 (199) Mustonen R, Health Phys, 6 (198) IAEA Technical Report series 29. International Atomic Energy Agency, Vienna, Austria (1989). Beck H L, The Natural Radiation Environment II (NRE-II), USDOE CONF PL, NTIS, Springfield, Virginia (1972) p Dheenathayalu M, Arumugam M & Ramasamy V, Acta Ciencia Indica XXVII P, No.3 (2001) United Nations Scientific Committee on the Effects of Atomic Radiation Sources effects and risks of ionizing radiation, New York, United Nations, (2000). 7 Aswathanarayana U, Principles of Nuclear Geology (Ozonian Press Pvt. Ltd., New Delhi) (198) p ICRP, Protection against radon at home and at work. (Oxford: Pergamon Press; ICRP publication 6 Annals of the ICRP) (1993) 23:1. 9 UNSCEAR, Source and Effects of ionizing radiation. Report to General Assembly, with Scientific annexes (New York, United Nations) (1993) E.9.IX.2UN. 10 Hewamanna R, Sumithrarachchi C S, Mahawatte P, Nanayakkara H L C & Ratnayake H C, Appl Rad Isot, (2001) Ramasamy V, Dheenathayalu M, Meenakshisundaram V & Ponnusamy V, Current Science, 83(9) (2002) Fernando Brenha Ribeiro & Arnaldo Roque, Appl Rad Isot, (2001) Rizzo S, Brai M, Basile S, Ballia S & Hauser S, Appl Rad Isot, (2001) Bellia S, Brai M, Hauser S, Puccio P & Rizzo S, Chem Ecol, 12 (1996) Atal B S, Bhalla N S, Lall Y, Mhadevan T M & Udas G R, Archaean Geochemistry, (Eds.: Windley B F & Naqui S M, Elsevier Publication) (1978) p Rogers J J W & Adams J A S, Chapter on Thorium (90) and Uranium (92), in Handbook of Geochemisty (Springer - Verlag, Berlin) (11) (1969) p Beretka J & Mathew P J, Health Phys, 8 (198) Amrani D & Tahtat M, Appl Rad Isot, (2001) NEA-OECD, Nuclear Energy Agency. Exposure to radiation from natural radioactivity in building materials. Report by NEA group of Experts, OECD, Paris, France (1979).