Applied Radiation and Isotopes 54 (2001) 365-369

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Technical note

Natural radioactivity and gamma dose from Sri Lankan clay bricks used in building construction

R. Hewamanna,, C.S. Sumithrarachchi, P. Mahawatte, H.L.C. Nanayakkara, H.C. Ratnayake aRadioisotope Centre, University of Colombo, Colombo 03, Sri Lanka *Corresponding author. Tel.: +94-1-502-525; fax: +94-1-583-810 bAtomic Energy Authority, 343, Olcott Mawatha, Colombo 11, Sri Lanka

Received 30 August 1999; received in revised form 4 November 1999; accepted 27 December 1999

Abstract

The specific radioactivity concentration of 226Ra, 232Th and 40K have been determined by gamma ray spectrometry with an HPGe detector in clay brick samples from kiln sites located in 17 towns. The average values of the measured activities are 35, 72, and 585 Bq kg-1, respectively, for the above radionuclides. The average estimated radium equivalent concentration is 183 Bq kg-1 and is comparable with reported values for many countries in the world. This value and the value obtained from the criteria formula suggest that the use of local clay bricks do not pose a radiological hazard. The calculated average absorbed dose rate in air within buildings was found to be 102 nGy h-1 while the population weighted indoor annual effective dose was 0.20 mSv. &copyright 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Specific radioactivity concentrations, Radiological hazard; Annual effective dose

1. Introduction

The most important source of external radiation exposure in buildings is caused by the gamma rays emitted from members of the uranium and thorium decay chains and 40K occurring naturally in building materials. Knowledge of radioactivity present in building materials enables one to assess any possible radiological hazard to man by the use of such materials. Nationwide surveys have been carried out to determine the specific activity of building materials in many countries. This interest reflects the magnitude of the total exposure of the public due to indoor radiation. Estimated world average absorbed dose rates in air within masonry dwellings using typical masonry is reported in UNSCEAR (1993) to be 80 nGy h-1 (20-190 nGy h-1).

238U and 232Th are present in the Earth's crust in parts per million levels (Evan, 1969). Potassium is also present in the Earth's crust and 0.0118% of the total amount of potassium is 40K. As such the measurement of radioactivity due to the presence of these nuclides in building materials is important for developing standards concerning the use of building materials. Some studies have given special attention to the radioactivity of the brick components of building structures (Tufail et al., 1991; Malanca et al., 1993). Very little is known about the natural radioactivity of Sri Lankan building materials, particularly that of bricks which form the major component of public and residential buildings in the country. It may also be noted that, in Sri Lanka, bricks are made mainly of clay originating from sediments deposited by the rivers. Raw materials and processed building products can vary in radionuclide content according to the origin and the geological conditions at the site of production. An environmental survey along the South West Coast and the West Coast of Sri Lanka, carried out by Hewamanna et al. (1988), revealed the presence of radioactive anomalies. The objectives of the present study was to determine the specific radioactivity concentrations of 226Ra, 232Th and 40K in brick samples collected from different areas, to assess any radiological hazard and to determine the indoor gamma radiation dose rate and the annual effective dose from bricks used in buildings. The results obtained in the present study are also compared with the corresponding results for clay bricks of different national origin given in the literature.

2. Materials and methods

2.1. Samples

Sampling was carried out from nine out of 25 districts in Sri Lanka. A minimum of two different towns from each district were selected for sample collection. Of the districts that fabricate clay bricks, only one was not included in this study due to the present security situation in the country. A total of 24 fired brick samples were collected from various kiln sites located in 17 different towns (Fig. 1).

The samples were crushed, sieved through meshes, homogenised and airdried. The prepared samples were transferred to uniform containers, weighed and sealed for 4 weeks so that a secular equilibrium between 238U and 232Th and their progeny could be reached.

2.2. Measurement of gamma ray activity in samples

The gamma ray spectra of the prepared samples were measured using a typical high resolution gamma spectrometer based on a coaxial type shielded HPGe detector, with a relative photopeak efficiency of 21% and energy resolution of 1.85 keV FWHM for the 1332 keV gamma ray line of 60Co. GANAAS software from the International Atomic Energy Agency (IAEA) was used to analyse the photopeaks.

The activity of 40K was measured directly via its 1461 (10.7%) keV peak of the gamma ray spectra. To determine the activity concentration of 226Ra, the gamma ray lines 295.1 (19.2%) and 351.9 (37.1%) keV from 214Pb and 609.3 (46.1%) and 1764.5 (15.9%) keV gamma rays from 214Bi were used. Activity concentration of 232Th was determined using the gamma ray lines 238.6 (43.6%) keV from 212Pb, 338.4 (12%), 911.1 (29%) and 968.9 (17.4%) keV from 228Ac, 583.1 (86%) and 2614 keV from 208Tl. The spectrometer was calibrated for energy and efficiency over the photon energy range 186-2700 keV using IAEA reference materials RGU-1 (U-ore), RGTH-1 (Th-ore) and RGK-1 (K2SO4) packed in a similar fashion to the samples in identical containers. The accumulation time for gamma ray spectra measurements ranged between 25,000 and 50,000 s depending upon the activity levels present in the samples.

3. Results and discussion

The activity concentration (in Bq kg-1 dry weight) of 226Ra, 232Th and 40K in the clay brick samples analysed in this study are given in Table 1. As listed in the table, the activity values obtained in this study ranged from 11 to 78, 6 to 131 and 111 to 1406 Bq kg-1 for 226Ra, 232Th and 40K, respectively. The average value of activity per unit mass for 226Ra is 35 Bq kg-1 and is less than the corresponding typical world average of 50 Bq kg-1 (NEA-OECD, 1979). However, the average values for 232Th (72 Bq kg-1) and 40K (585 Bq kg-1) are greater than the typical world average values of 50 and 500 Bq kg-1 for the said nuclides, respectively (NEA-OECD, 1979).

An index, Raeq, called the radium equivalent activity, is also presented in Table 1 to compare the specific activities of materials containing different amounts of radium, thorium and potassium. This is a widely used radiological hazard index and was calculated according to Beretka and Mathew (1985)

where C(Ra), C(Th) and C(K) are the activities of 226Ra, 232Th and 40K, respectively, in Bq kg-1.

This formula is based on the estimation that 1 Bq kg-1 of 226Ra, 0.7 Bq kg-1 232Th or 13 Bq kg-1 40K produce the same gamma dose rate. In Table 2, the mean specific activities of the three radionuclides under study and the corresponding Raeq index for clay bricks reported from 10 different countries are given for comparison. As seen from this table the average activity concentration of 226Ra (35 Bq kg-1) determined in this study is comparable with those from Australia, Netherlands, China and India. In other places, including Finland and Norway, higher values have been reported, while for Cairo a lower value is noted. For 232Th, our value (72 Bq kg-1) is higher than all other countries in the table except that of Australia (89 Bq kg-1) and Sweden (127 Bq kg-). The value for 40K (585 Bq kg-1) determined in this study can be compared only with that from Netherlands. Values from Cairo and India are lower and higher than values reported in other geographical locations. For bricks from Cairo values for all three nuclides are exceptionally low. Somlai et al. (1998) have recommended the use of construction materials with an average Raeq value of less than 370 Bq kg-1 for dwellings. The results of this study show that the average value for Raeq obtained for the local bricks is 183 Bq kg-1 and as such does not pose a radiological hazard when used for construction of buildings
.
Based on models suggested by Krisiuk et al. (1971) and Stranden (1976), a value of 1.5 mGy was obtained by Kreiger (1981) to evaluate annual external radiation dose inside dwellings constructed of building materials, with an Raeq value of 370 Bq kg-1. These authors later corrected their assumptions after taking into consideration a wall of finite thickness, and applying a weighting factor of 0.7 (Keller and Muth, 1990) due to the presence of windows and doors. This can be used as a criterion to limit the annual radiation dose from building materials based on the formula

where CRa, CTh and CK are the activities of 226Ra, 232Th and 40K in Bq kg-1, respectively, in building materials. Calculating the sum for the three quotients, the values for the brick samples in the present study ranged from 0.07 to 0.49 (Table 1). This indicates that gamma activity in the local bricks do not exceed the proposed criterion level. To calculate dose rates in air from the different combinations of the three radionuclides present in the clay bricks used in this study an activity utilisation index (A.U. Index) was constructed. The activity utilisation index is given by the expression:

where CK, CRa and CTh are actual values of the activities per unit mass of 40K, 226Ra and 232Th in the building material considered (Bq kg-1); fK, fRa and fTh are the fractional contributions to the dose rates in air from standard or typical concentrations of these radionuclides and wm is the fractional usage of the building materials in the building with the activity characteristic (UNSCEAR, 1993). AK ARa and ATh are typically 500, 50 and 50 Bq kg-1 for the three radionuclides respectively. For full utilisation of typical masonry, this index is unity by definition and is deemed to imply a dose rate of 80 nGy h-1 (UNSCEAR, 1993). As given in Table 1, the values obtained for this index in the present study ranged from 0.31 to 2.39 with an average of 1.27. The results of estimated absorbed dose rates in air within buildings based on this index, as given in Table 1, ranges from 25 to 192 nGy h-1 with an average value of 102 nGy h-1 considering full usage of the building materials. National averages of the dose rate in air from terrestrial gamma rays inside dwellings representing over a third of the world population as reported in UNSCEAR (1993) range from 20 to 190 nGy h-1.

The annual effective dose was estimated using ICRP (1991) and UNSCEAR (1993) dose conversion factors. An indoor occupancy factor of 0.8 (&approximate-sign 7000 h y-1), a dose rate at 1 m height above the floor, and a 0.7 Sv Gy-1 conversion factor (UNSCEAR, 1993) for adults have been used. The indoor annual effective dose was calculated as a product of the measured absorbed dose, the conversion factor and the occupancy, and is presented in Table 1. The values obtained range from 0.13 to 0.97 mSv with an average value of 0.51 mSv. The population weighted Sri Lankan average is 0.20 mSv. National estimates of average annual effective dose from terrestrial gamma rays for 13 countries is given in UNSCEAR (1993) and has a range from 0.23 mSv in Canada to 0.65 mSv in Sweden, with a population weighted world average of 0.45 mSv. It can be concluded from the overall results in this study that clay bricks used in Sri Lanka or building construction do not pose a radiological hazard.
