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Depth-dose distribution in bricks determined by thermoluminescence and by Monte-Carlo calculation for external γ-dose reconstruction
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Appl. Radiat. Isot. Vol. 47, No. 4, pp. 433-440, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/96 $15.00 + 0.00
Depth-dose Distribution in Bricks Determined by Thermoluminescence and by Monte-Carlo Calculation for External 7-Dose Reconstruction H. Y. G O K S U 1, L. M. H E I D E 2, N. G. B O U G R O V 3, A. R. D A L H E I M E R 2, R. M E C K B A C H 1 a n d P. J A C O B l ~GSF-Forschungszentrum, Institut ffir Strahlenschutz, Oberschleil3heim, D-85758, Germany zBfS--Institut fiir Strahlenhygiene, OberschleiBheim, D-85762, Germany 3Ural Research Centre for Radiation Medicine, Chelyabinsk 454076, Russian Federation (Received 21 August 1995; in revised form 18 October 1995) The analysis of depth~zlose distributions in bricks sampled from walls in areas with nuclear waste or accident contamination has the potential of providing information on the energy and source configuration of the v-radiation that had been incident on the brick. In this study, a brick from a mill facing a shallow water reservoir of the contaminated Techa river in the South Ural region is investigated. Thermoluminescence (TL) methods were used to measure the accumulated dose at several depths in the brick. The accidental external v-dose is obtained by subtracting the natural radiation background dose from the total accumulated dose. In the first segment of the brick, at a depth of about 1.5 cm, the accident dose was found to be roughly 3.5 Gy. Monte-Carlo simulations of the photon transport from the reservoir bed contaminated with I37Cswere calculated for different depths in the brick. The calculations were made assuming different attenuating water levels. It is found that the depth~lose distribution determined by measurements corresponds to a water level between 20 and 50 cm. The results indicate that TL measurements combined with Monte-Carlo modelling calculations are highly promising for external v-dose reconstruction applications.
Introduction An intensive nuclear programme conducted during the past 50 yr in South Ural area caused heavy occupational exposures at the " M a y a k " facilities (industrial complex in the South Ural, including nuclear and non-nuclear activities) as well as prolonged exposure to the population that inhabited the region along the Techa river. The exposures of the population were mainly due to: --releases of radioactive wastes into the river Techa during normal operation of the nuclear power and reprocessing facilities at Mayak since 1948; ----explosion of a radioactive waste tank in 1957, the so-called Kyshtym accident; and --resuspension of radioactive bank sediments of Lake Karachay, becoming airborne during a long dry period in 1967. The above-mentioned releases and accidents in the South Ural region, as well as the Chernobyl accident, have led to the exposure of a large number of people AR147/4--D
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to significant internal and external radiation doses over extended periods of time, presenting major challenges to practical and regulatory aspects of radiation protection. The health risk of this population cannot be assessed by simply using the results of studies of atomic bomb survivors, whose exposure times were very short. Therefore, the type and magnitude of health risk to these populations needs to be reassessed. Experimental methods and model calculations are needed for the reconstruction of the radiation doses and dose equivalents of the affected populations, in order to provide the basic data for epidemiological investigations from which health risks can be determined. Thermoluminescence (TL) methods have been used to assess the external 7-dose due to exposures in Hiroshima and Nagasaki (Maruyama et al., 1987; Haskell et al., 1987), in the areas of the Nevada Test site (Haskell et al., 1994) and in the town of Pripyat near the Chernobyl event (Stoneham et al., 1993; H/itt et al., 1993). In the present study the major sources of external
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irradiation of the population of the Techa riverside villages (see Figure 1) were assumed to be mainly from '~7Cs which is known to be trapped in the reservoir clay minerals (Syers et al., 1972; Rhodes, 1957 and Ritchie et al., 1972). The residents of these villages were also exposed to internal irradiation, because the river represents for them the main source of water supply for drinking and other domestic uses. Relatively high external radiation doses were received by the inhabitants of the village Metlino, located 7 km below the site of the release of radioactive waste. These inhabitants were evacuated between 1953 and 1956 (Akleyev and Lyubchansky, 1994; Degteva et al., 1994). The TL method is applied for the assessment of depth distributions of accumulated doses in bricks of buildings in the contaminated regions. The knowledge of the depth-dose distribution has the potential of providing information on the time-averaged energy distribution of the incident radiation and on the configuration of the radiation sources. MonteCarlo simulations of photon transport have been performed to calculate depth-dose distributions in bricks according to the dependence on photon energy and source configuration (Meckbach et al., 1995). In the present study, a fired brick sampled from a mill in the village of Metlino (see Figure 1) was investigated for the purpose of assessing the potential of TL measurements combined with Monte-Carlo calculations to relate measured depth-dose distributions to the time-averaged radiation energy and source configuration. The depth-dose distribution in the brick was measured by TL and calculated by Monte-Carlo simulations for the assumed source configuration and energy of the radiation that had been incident on the brick.
Materials and Methods Method o f dose assessment Minerals when heated emit light following exposure to ionising radiation known as thermoluminescence. When a recently fired material is exposed to a transient ionising radiation field it acquires an excess dose over and above that which
can be accounted for from natural sources. The total accumulated dose received by a building brick can thus be assessed using minerals like quartz and feldspar incorporated in it. The external 7-dose component of the accidental dose DAce can be estimated by use of the following equation:
DAcc= DTL - - DAge DAcc = DTL -- [A(R~ + Rt~ + R:. + C)]
(1)
where: DTL = total accumulated dose as measured by TL (Gy), Dago = total accumulated dose due to the age of the sample, A = age of the building in years, R~ = internal effective c~-particle dose rate due to uranium and thorium content of the brick (Gy/yr), Rp --- internal fl-particle dose rate due to uranium, thorium and potassium of the brick (Gy/yr), R~.-- external 7-ray dose rate from natural radionuclides, at the sample position (Gy/yr), C = dose rate due to cosmic rays (Gy/yr). As can be seen from Eq. 1, the total accumulated dose DTL evaluated by using TL measurements from a brick corresponds to the sum of doses arising both due to the age of the sample (naturally occurring radionuclides within the brick and surrounding medium irradiate the brick continuously) and from the accident. Therefore, it is important to know the age, i.e. the time of production of the brick as well as the natural radionuclides inside the brick and the environmental material. Description of the sample The sample used in this study was collected from a mill in the village Metlino which is 7 km from the site of release of radioactive waste. The investigated brick No. 16 (density, p = 1.8 g/cm 3) was located in the outside wall of the mill of Metlino, about 1 m above the shallow reservoir water surface level, as shown in Fig. 2. The depth of the water close to the wall of the mill is reported to vary between 0.2 and 1 m (Bougrov, 1994). A fragment of the brick was used for TL measurements.
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Fig. 2. Position of brick No. 16 sampled from the wall of the Metlino mill.
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The Metlino mill is located inside the restricted zone of the Mayak facility. Therefore it is difficult to f i n d information about its age and construction history. However, the name of Metlino mill is mentioned among the companies, mills and plants including the name of the owner in official documents dating back to 1899 (Vershova, 1899). Bougrov observed in 1994 that there is another architecturally similar mill at the village of Muslyumovo known to be 150yr old. Considering the architectural similarities, we assume that both Metlino's and Muslyumovo's mills have nearly the same age of about 150 yr. Furthermore, a brick from an internal wall with a thickness of 1 m is used for age determination by the TL method and found to be 122 _+ 30, +45 yr with standard and overall error, respectively (Aitken, 1976). The details of the age determination will be published elsewhere. This latter brick is chosen as a background sample for age determination measurements due to its extensive shielding from the external 7-accident exposures. The accumulated dose due to the age of the brick is calculated for both 100 and 150 yr in order to assess the possible uncertainty due to natural background. Sample preparation for T L measurements
There are several luminescence techniques available to evaluate the total accumulated dose DTL in bricks. In this study the preliminary measurements indicate that the accumulated dose is large enough to use the so-called 'TL fine-grain method' described by Zimmerrnann (1971). All sample preparations described below were carried out in dim laboratory red light. The outer 5 mm from all surfaces of the fragment of the brick were removed with a water-cooled diamond blade. A slice was cut into 6 segments of 2 cm thickness each (Figure 3). The segments were washed with diluted acetic acid and distilled water and dried at 60°C overnight. The segments were individually crushed and sieved. 700 mg of the grains <45 pm were used for preparation of fine-grain samples. The samples were put inside two reagent tubes filled with acetone. After immersion in an ultra-sound bath for 2 min, the
suspension was allowed to stand for 2 min, during which grains larger than about 10 #m were deposited on the bottom of the glass. Grains smaller than 10 #m were decanted into a second tube and allowed to stand for a further 20 min. The acetone suspension was discarded and the residue resuspended in about 25 mL of acetone. Aliquots of 1 mL of the suspension were pipetted into 24 flat-bottomed tubes containing stainless steel disks. The 24 fine-grain deposited disks obtained in this way were dried at 60°C overnight for TL measurements. T L measurements and T L dose assessment
TL glow curves were measured using an automatic reader (TL-DA9, RISO) with a heating rate of 5°C/s in a nitrogen flow of 4 L/min. The heat-absorbing filter was used together with a Blue (Chance Pilkington) 7/59 filter. TL dose evaluation was made by using the additive-dose method. Additive doses were given using a 9°Sr r-ray source which was calibrated with respect to a 6°Co v-ray source at the Secondary Standard Dosimetry Laboratory (SSDL) in GSF. The procedure of calibration is described elsewhere (G6ksu et al., 1995). The 9°Sr source which was attached to the automatic TL reader had a dose rate of 5.6 mGy/s at the sample irradiation position using stainless steel disks. The samples were stabilized at 100°C for 100 s after irradiation and before TL measurements. The TL glow curves were rather broad with a TL peak maximum at 225°C (Figure 4). The stability of the signal was tested by the so-called "plateau-test" (Aitken, 1985). The onset of the plateau obtained from the ratio between the "natural" and "artificial" additive glow curves was indicative that the stable region of the glow curve had been reached. For the plateau test, first and second growth TL curves were analyzed by the 'TL- Plus' programme developed at GSF (Waibel, 1990). The plateau was observed between 250 and 350°C. The deviation in the plateau was found to be less than 10%. The TL dose response was plotted by integrating the TL glow curves in the plateau region. The accumulated dose obtained from the additive dose
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curves required correction due to the initial supralineraity of the dose response. This was usually obtained from the second glow growth curves of the drained samples. The intercept of the dose vs TL glow area was added to the intercept of the first growth curves. The correction was obtained from the second growth curves so-called intercept correction (for the justification and validity of the method see the arguments provided in Aitken, 1985) was found not to be constant through the depth of the 13.5 cm long brick. The effect could be explained due to the temperature gradient during the production of the brick block. The total accumulated doses DrL in brick with intercept correction from the second glow for
different depths in the brick are listed in the second column of Table 2. Assessment o f dose rate due to natural radionuclides
The measured total accumulated dose Drr, as described above, contains not only the accidental dose (Dace) but age dose (DAgo) which is the accumulated dose due to natural radioactive nuclides inside the brick as well as the external ~,-ray dose from the environment and cosmic rays [see Equation 1]. This is especially important for bricks, like the one used in this study, which comes from a building older than 100 yr.
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Table 1. The components of natural background dose rates (ct, fl, Yand cosmicrays) and total dose for 100 and 150 yr old fragments of brick No. 16 from the wall of the Metlino mill Brick Dose rate Dose rate Dose rate Dose rate Age dose age R~ R# R~. C DA~ (yr) (mGy/yr) (mGy/yr) (mGy/yr) (mGy/yr) (Gy) 100 1.08 1.89 1.05 0.28 0.43 _+0.02 150 1.08 1.89 1.05 0.28 0.65 + 0.02
Dose rates from natural:nuclides were calculated according to conversion tables (Bell, 1979; Nambi and Aitken, 1986) from the measured uranium, thorium and potassium of the brick, as described below. The individual components of the natural background are summarized in Table 1 for a brick of a 100 and 150 yr old building. Internal effective c~-particle dose rate R~. The uranium and thorium content of the brick was measured using a 4.5 cm diameter ZnS screen with the thick-sample ~-counting method, calibrated by using the U.S. Geological Standard (BCR-1). The uranium and thorium contents Were found to be 2.18 and 9.54ppm, respectively. The internal effective o-particle dose rate was calculated using the so-called a-value system developed by Bowman and Huntley (1984), which takes into account the efficiency of the s-particles independent of their energy for producing TL. The irradiation was performed with six plaque sources with a nominal 24~Am activity of 6.66 GBq each and calibrated individually in a vacuum. The a-value was found to be 0.103, yielding an internal effective s-particle dose rate R~ of 1.08 mGy/yr. In this calculation no correction was applied for the water content of the brick. Internal [3-particle dose rate R#. The Berthold LB770 fl-counter was used to measure the fl-particle dose rate due to the uranium, thorium and potassium content of the brick. A slice of the brick was crushed and two aliquots of 12 g, representing an infinite matrix, were counted three times. The thick-source fl-counting method was used for measurements and assessment of fl-dose rate (Sanderson, 1988). The internal fl-dose rate R# of the brick was found to be 1.89 mGy/yr. External ray dose rate R:. The Ural region is known to have a slightly higher natural 7-ray dose rate than the other parts of the country due to naturally occurring uranium and thorium in the environmental soil (Bougrov, 1994). The value of 1.05mGy/yr was used for the annual y-dose equivalent rate determined from a measured dose rate of 0.12 #Sv/h. Dose rate due to cosmic rays C. The contribution of cosmic radiation is usually small. At ground level it has been estimated to be 0.28 mGy/yr (Prescott and Stephan, 1982).
the code SAM-CE (Lichtenstein et al., 1979). It was assumed that the accumulated accidental dose was primarily due to the retention of ~7Cs in the clay at the bottom of the water reservoir. The solubility of -137Cs and the contamination of the water is not considered in this model. A homogeneous distribution of isotropic sources extending along the bottom of the reservoir to a distance of 20 m from the wall was defined as a source of the 661.6 keV photons which are emitted by this radionuclide. Separate calculations were made for the water levels of 50 and 20 cm. Photon fluences were determined in scoring regions corresponding to the layers at different depths in the brick, indicated in Figure 5, used for the experimental determination of the depth~ctose dependence. Doses in brick for the respective layers were calculated from the photon fluences using mass energy-absorption coefficients for photon interactions in bricks. A more detailed description of this type of Monte-Carlo calculation is given elsewhere (Meckbach et al., 1995).
Results and Discussion In this study, TL properties of a brick from the Metlino mill in the South Ural region were studied for the purpose of accidental dose assessment. It was important in this work to calculate and assess the dose rate due to natural background, since the brick used in this study stems from an old mill which was over 100 yr old. Furthermore, the accident occurred more than four decades ago. The accumulated doses DTL and extended accidental doses DAce due to contamination are listed in Table 2 for different depths in the brick. The natural background doses
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The dependence of accumulated dose on depth in the brick from the Metlino mill was calculated by Monte-Carlo simulation of photon transport using
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Depth-dose distribution in bricks Table 2. The accidental dose DA~obtained by subtracting the natural age dose of the I00 and 150 yr old brick No. 16 from the wall of the Metlino mill, respectively Depth in Dose DAce Dose DA~ brick Dose DTL (100 yr) (150 yr) (cm) (Gy) (Gy) (Gy) 0.7-2.7 4.04 3.62 + 0.12 3.40 + 0.12 2.8-4.8 3.02 2,60 _ 0.11 2.38 _ 0.11 4.9-6.9 2.11 1.69 _+0.14 1.47 +_0.14 7.0-9.0 1.67 1.25 _+0.06 1.03 _+0.05 9.1-11.I 1.22 0.80 + 0.06 0.58 _+_0.04 11.2-13.5 0.99 0.57 +_0.06 0.35 _ 0.04
a c c u m u l a t e d due to the possible ages of the brick f r a g m e n t (100 a n d 150yr, respectively) are given in T a b l e 1. T h e variations o f accidental dose t h r o u g h the brick were o b t a i n e d b y s u b t r a c t i n g the n a t u r a l age dose f r o m the total a c c u m u l a t e d dose. T h e results of M o n t e - C a r l o calculations were normalized to the m e a s u r e d T L values in the first segment o f the brick which is the average a b s o r b e d dose in a 2 cm layer. T h e m e a s u r e d a c c u m u l a t e d accidental dose vs d e p t h in the brick is s h o w n together with the results o b t a i n e d by the M o n t e - C a r l o calculations in Fig. 6. T h e calculated results c o r r e s p o n d to a c o n t a m i nation, for example, in the clays o f the reservoir bed with 137Cs for a water levels o f 20 a n d 50 cm. I n order to allow for a better c o m p a r i s o n , they were normalized to the accidental dose m e a s u r e d in the first layer o f the brick. W i t h this n o r m a l i z a t i o n , the source strength c o r r e s p o n d i n g to the height o f the 50 cm water level is a factor o f 11 larger t h a n the one for a d e p t h o f 20 cm. T h e calculated results are e n d o w e d with statistical errors which range between less t h a n 5 % for the first layers a n d less t h a n 10% for the deepest layer in the brick. One c a n see t h a t b o t h
439
the m e a s u r e d a n d the calculated d e p t h - d o s e dependences are basically exponential. The dependence o f the calculated d e p t h ~ l o s e distribution o n the source configuration is quite p r o n o u n c e d ; for the deepest layer in the brick, the dose calculated for a c o n t a m i n a t i o n at 20 cm below water level is m o r e t h a n a factor o f two larger t h a n the dose o b t a i n e d for a c o n t a m i n a t i o n at 50 cm. F o r the whole range o f depths in the brick which were considered, the m e a s u r e d accidental doses lie between the ones calculated for the two depths u n d e r the water level. O n the o t h e r h a n d , the measured d e p t h - d o s e distribution would deviate strongly f r o m the one resulting from a h o m o g e n e o u s ~37Cs c o n t a m i n a t i o n on the g r o u n d a r o u n d the mill ( M e c k b a c h et al., 1995) which would lead to a relative dose m o r e t h a n a factor o f three larger at the deepest layer in the brick.
Conclusion T h e T L m e t h o d using n a t u r a l materials for external dose assessment has long been used. However, in this study it is the first time a n actual sample was used to o b t a i n d e p t h - d o s e distribution in a m o r t a r e d brick sample. The d e p t h - d o s e profile o b t a i n e d experimentally c o m p a r e d with the M o n t e Carlo calculations ,yields i n f o r m a t i o n a b o u t the past distribution o f a specific radioactive source in the e n v i r o n m e n t . This i n f o r m a t i o n would provide d a t a to reconstruct the external doses to the local population, necessary for epidemiological investigations. The m e t h o d developed here will be used in c o l l a b o r a t i o n with the U r a l Research Centre for the assessment o f external doses as a c o n t r i b u t i o n to the total doses to the p o p u l a t i o n along the Techa river.
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Acknowledgements--Part of the work discussed in this paper is supported by Commission of the European Communities under Contract F13 PC 92 00 40 and COSU-CT 93 0051 within the Radiation Protection Research Action.
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Fig. 6. Result of Monte-Carlo calculations compared with experimental accidental dose. Dependence of accumulated accidental dose on the depth in the brick, for an age of 150 yr as obtained by TL measurements (O). For comparison, the results of Monte-Carlo calculations, normalized to the measured value in the first brick layer, are shown for a homogeneous contamination of the reservoir bed with J37Cs at a depth of 50 cm below the water level (A) and of 20 cm ([3). The dotted lines are included to guide the eye and indicate the width of the brick layers.
Aitken M.J. (1976) Thermoluminescence age evaluation and assessment of error limits revised system. Archaeornetry 18, 233. Aitken M.J. (1985) Thermoluminescence Dating--Studies in Archaeological Science. Academic Press, London. Akleyev A.V. and Lyubchansky E. R. (1994) Environmental and medical effects of nuclear weapon production in the Southern Urals. The Sci. Total Environ. 142, 1. Bell W. T. (1979) Thermoluminescence dating: radiation dose rate data. Archaeometry 21, 243. Bowman S.G.E and Huntley D.J. (1984) A new proposal for the expression of alpha efficiency in TL dating. Ancient TL 2, 6. Bougrov N.G. (1994) Private communication. Degteva M.O., Kozheurov V.P. and Vorobiova M.I. (1994) General approach to dose reconstruction in the
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population exposed as a result of the release of radioactive wastes into the Techa river. The Sci. Total Environ. 142, 49. G6ksu H.Y., Bailiff I., Botter-Jensen L., Brodski L., Hiitt G. and Stoneham D. (1995) Interlaboratory source calibration using TL and OSL of natural quartz. Radiat. Meas. In press. Haskell E.H,, Kaipa P.L. and Wrenn M.E. (1987) Results of University of Utah: TL analysis of bricks and tiles at Hiroshima and Nagasaki. In DS86-Dosimetry System 1986, Vol. 2, p. 153. The Radiation Effects Research Foundation. Haskell E.H., Bailiff I.K., Keuner G.H., Kaipa P.L. and Wrenn M.E. (1994) Thermoluminesence measurements of ),-ray doses attributed to fallout from the Nevada Test Site using building bricks and natural dosemeters. Hlth Phys. 66, 380. Htitt G., Brodski L., Bailiff I.K., G6ksu H.Y., Haskell E.H., Jungner H. and Stoneham D. (1993) Accident dosimetry using environmental materials collected from regions downwind of Chernobyl: a preliminary evaluation. Radiat. Protect. Dosim. 47, 307. Lichtenstein H., Cohen H., Steinberg E., Troubetzkoy and Beer M. (1979) The SAM-CE Monte Carlo System for Radiationtransport and Criticality Calculations in Complex Configurations, Revision 7: a computer code manual. Mathematical Applications Group Inc., Westchester Plaza, Elmsford, NY. Maruyama T., Kumamoto Y. and Noda Y. (1987) Reassessment of ? doses from the atomic bombs in Hiroshima and Nagasaki. Radiat. Res. 113, 1. Meckbach R., Bailiff I.K., G6ksu H.Y., Jacob P. a n d Stoneham D. (1995) Calculation and measurement of depth dose distribution in bricks. Int. Solid State
Dosimetry Conf., to be published in Radiat. Protect. Dosim . Nambi K.S.V. and Aitken M.J. (1986) Annual dose conversion factors for TL and ESR dating. Archaeometrv 28, 202. Prescott J.R. and Stephan L.G. (1982) Contribution of cosmic radiation to environmental dose. The J. Eur. Study Group Phys. Chem. Biol. Mathl Techn. Appl. Archaeol. (PACT) 6, 17. Rhodes D.W. (1957) Effect of pH on the uptake of radioactive isotopes from solutin by a soil. Soil Sci. Soc. Am. Proc. 21, 389. Ritchie J.C., McHenry J.R., Gill A.C. and Hawks P.H. (1972) Fallout 137-Cs in reservoir sediments. Hlth Phys. 22, 96. Sanderson D.C.W. (1988) Thick sources beta counting (TSBC): a rapid method for measuring beta dose rates. Nucl. Tracks Radiat. Meas. 14, 203. Stoneham D., Bailiff I.K., Brodski L., G6ksu H.Y., Haskel E.H., Hiitt G., Jungner H. and Nagatomo T. (1993) Accident dosimetry measurements on samples from the town of Pripyat. Nucl. Tracks Radiat. Meas. 21, 195. Syers J.K. Mokma D.L., Jacson M.L., Dolcater D.L. and Rex R.W. (1972) Mineralogical composition and cesium-137 retention, properties of continental aerosolic dust. Soil Science 113, 116. Waibel A. (1990) TL Plus Laboratory Manual (unpublished). Vershova A.L. (1899) Urals Commercial Addresses and Calendar on 1899, Regions: Ufa, Orenburg, Viatica and Tobol. Publishing House of Regional Government (in Russian). Zimmermann J. (1971) Thermoluminescent dating using fine grains from pottery. Archaeometry 13, 29.
Pergamon 0969-8043(95)00312-6
Appl. Radiat. Isot. Vol. 47, No. 4, pp. 433-440, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/96 $15.00 + 0.00
Depth-dose Distribution in Bricks Determined by Thermoluminescence and by Monte-Carlo Calculation for External 7-Dose Reconstruction H. Y. G O K S U 1, L. M. H E I D E 2, N. G. B O U G R O V 3, A. R. D A L H E I M E R 2, R. M E C K B A C H 1 a n d P. J A C O B l ~GSF-Forschungszentrum, Institut ffir Strahlenschutz, Oberschleil3heim, D-85758, Germany zBfS--Institut fiir Strahlenhygiene, OberschleiBheim, D-85762, Germany 3Ural Research Centre for Radiation Medicine, Chelyabinsk 454076, Russian Federation (Received 21 August 1995; in revised form 18 October 1995) The analysis of depth~zlose distributions in bricks sampled from walls in areas with nuclear waste or accident contamination has the potential of providing information on the energy and source configuration of the v-radiation that had been incident on the brick. In this study, a brick from a mill facing a shallow water reservoir of the contaminated Techa river in the South Ural region is investigated. Thermoluminescence (TL) methods were used to measure the accumulated dose at several depths in the brick. The accidental external v-dose is obtained by subtracting the natural radiation background dose from the total accumulated dose. In the first segment of the brick, at a depth of about 1.5 cm, the accident dose was found to be roughly 3.5 Gy. Monte-Carlo simulations of the photon transport from the reservoir bed contaminated with I37Cswere calculated for different depths in the brick. The calculations were made assuming different attenuating water levels. It is found that the depth~lose distribution determined by measurements corresponds to a water level between 20 and 50 cm. The results indicate that TL measurements combined with Monte-Carlo modelling calculations are highly promising for external v-dose reconstruction applications.
Introduction An intensive nuclear programme conducted during the past 50 yr in South Ural area caused heavy occupational exposures at the " M a y a k " facilities (industrial complex in the South Ural, including nuclear and non-nuclear activities) as well as prolonged exposure to the population that inhabited the region along the Techa river. The exposures of the population were mainly due to: --releases of radioactive wastes into the river Techa during normal operation of the nuclear power and reprocessing facilities at Mayak since 1948; ----explosion of a radioactive waste tank in 1957, the so-called Kyshtym accident; and --resuspension of radioactive bank sediments of Lake Karachay, becoming airborne during a long dry period in 1967. The above-mentioned releases and accidents in the South Ural region, as well as the Chernobyl accident, have led to the exposure of a large number of people AR147/4--D
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to significant internal and external radiation doses over extended periods of time, presenting major challenges to practical and regulatory aspects of radiation protection. The health risk of this population cannot be assessed by simply using the results of studies of atomic bomb survivors, whose exposure times were very short. Therefore, the type and magnitude of health risk to these populations needs to be reassessed. Experimental methods and model calculations are needed for the reconstruction of the radiation doses and dose equivalents of the affected populations, in order to provide the basic data for epidemiological investigations from which health risks can be determined. Thermoluminescence (TL) methods have been used to assess the external 7-dose due to exposures in Hiroshima and Nagasaki (Maruyama et al., 1987; Haskell et al., 1987), in the areas of the Nevada Test site (Haskell et al., 1994) and in the town of Pripyat near the Chernobyl event (Stoneham et al., 1993; H/itt et al., 1993). In the present study the major sources of external
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irradiation of the population of the Techa riverside villages (see Figure 1) were assumed to be mainly from '~7Cs which is known to be trapped in the reservoir clay minerals (Syers et al., 1972; Rhodes, 1957 and Ritchie et al., 1972). The residents of these villages were also exposed to internal irradiation, because the river represents for them the main source of water supply for drinking and other domestic uses. Relatively high external radiation doses were received by the inhabitants of the village Metlino, located 7 km below the site of the release of radioactive waste. These inhabitants were evacuated between 1953 and 1956 (Akleyev and Lyubchansky, 1994; Degteva et al., 1994). The TL method is applied for the assessment of depth distributions of accumulated doses in bricks of buildings in the contaminated regions. The knowledge of the depth-dose distribution has the potential of providing information on the time-averaged energy distribution of the incident radiation and on the configuration of the radiation sources. MonteCarlo simulations of photon transport have been performed to calculate depth-dose distributions in bricks according to the dependence on photon energy and source configuration (Meckbach et al., 1995). In the present study, a fired brick sampled from a mill in the village of Metlino (see Figure 1) was investigated for the purpose of assessing the potential of TL measurements combined with Monte-Carlo calculations to relate measured depth-dose distributions to the time-averaged radiation energy and source configuration. The depth-dose distribution in the brick was measured by TL and calculated by Monte-Carlo simulations for the assumed source configuration and energy of the radiation that had been incident on the brick.
Materials and Methods Method o f dose assessment Minerals when heated emit light following exposure to ionising radiation known as thermoluminescence. When a recently fired material is exposed to a transient ionising radiation field it acquires an excess dose over and above that which
can be accounted for from natural sources. The total accumulated dose received by a building brick can thus be assessed using minerals like quartz and feldspar incorporated in it. The external 7-dose component of the accidental dose DAce can be estimated by use of the following equation:
DAcc= DTL - - DAge DAcc = DTL -- [A(R~ + Rt~ + R:. + C)]
(1)
where: DTL = total accumulated dose as measured by TL (Gy), Dago = total accumulated dose due to the age of the sample, A = age of the building in years, R~ = internal effective c~-particle dose rate due to uranium and thorium content of the brick (Gy/yr), Rp --- internal fl-particle dose rate due to uranium, thorium and potassium of the brick (Gy/yr), R~.-- external 7-ray dose rate from natural radionuclides, at the sample position (Gy/yr), C = dose rate due to cosmic rays (Gy/yr). As can be seen from Eq. 1, the total accumulated dose DTL evaluated by using TL measurements from a brick corresponds to the sum of doses arising both due to the age of the sample (naturally occurring radionuclides within the brick and surrounding medium irradiate the brick continuously) and from the accident. Therefore, it is important to know the age, i.e. the time of production of the brick as well as the natural radionuclides inside the brick and the environmental material. Description of the sample The sample used in this study was collected from a mill in the village Metlino which is 7 km from the site of release of radioactive waste. The investigated brick No. 16 (density, p = 1.8 g/cm 3) was located in the outside wall of the mill of Metlino, about 1 m above the shallow reservoir water surface level, as shown in Fig. 2. The depth of the water close to the wall of the mill is reported to vary between 0.2 and 1 m (Bougrov, 1994). A fragment of the brick was used for TL measurements.
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Fig. 2. Position of brick No. 16 sampled from the wall of the Metlino mill.
435
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The Metlino mill is located inside the restricted zone of the Mayak facility. Therefore it is difficult to f i n d information about its age and construction history. However, the name of Metlino mill is mentioned among the companies, mills and plants including the name of the owner in official documents dating back to 1899 (Vershova, 1899). Bougrov observed in 1994 that there is another architecturally similar mill at the village of Muslyumovo known to be 150yr old. Considering the architectural similarities, we assume that both Metlino's and Muslyumovo's mills have nearly the same age of about 150 yr. Furthermore, a brick from an internal wall with a thickness of 1 m is used for age determination by the TL method and found to be 122 _+ 30, +45 yr with standard and overall error, respectively (Aitken, 1976). The details of the age determination will be published elsewhere. This latter brick is chosen as a background sample for age determination measurements due to its extensive shielding from the external 7-accident exposures. The accumulated dose due to the age of the brick is calculated for both 100 and 150 yr in order to assess the possible uncertainty due to natural background. Sample preparation for T L measurements
There are several luminescence techniques available to evaluate the total accumulated dose DTL in bricks. In this study the preliminary measurements indicate that the accumulated dose is large enough to use the so-called 'TL fine-grain method' described by Zimmerrnann (1971). All sample preparations described below were carried out in dim laboratory red light. The outer 5 mm from all surfaces of the fragment of the brick were removed with a water-cooled diamond blade. A slice was cut into 6 segments of 2 cm thickness each (Figure 3). The segments were washed with diluted acetic acid and distilled water and dried at 60°C overnight. The segments were individually crushed and sieved. 700 mg of the grains <45 pm were used for preparation of fine-grain samples. The samples were put inside two reagent tubes filled with acetone. After immersion in an ultra-sound bath for 2 min, the
suspension was allowed to stand for 2 min, during which grains larger than about 10 #m were deposited on the bottom of the glass. Grains smaller than 10 #m were decanted into a second tube and allowed to stand for a further 20 min. The acetone suspension was discarded and the residue resuspended in about 25 mL of acetone. Aliquots of 1 mL of the suspension were pipetted into 24 flat-bottomed tubes containing stainless steel disks. The 24 fine-grain deposited disks obtained in this way were dried at 60°C overnight for TL measurements. T L measurements and T L dose assessment
TL glow curves were measured using an automatic reader (TL-DA9, RISO) with a heating rate of 5°C/s in a nitrogen flow of 4 L/min. The heat-absorbing filter was used together with a Blue (Chance Pilkington) 7/59 filter. TL dose evaluation was made by using the additive-dose method. Additive doses were given using a 9°Sr r-ray source which was calibrated with respect to a 6°Co v-ray source at the Secondary Standard Dosimetry Laboratory (SSDL) in GSF. The procedure of calibration is described elsewhere (G6ksu et al., 1995). The 9°Sr source which was attached to the automatic TL reader had a dose rate of 5.6 mGy/s at the sample irradiation position using stainless steel disks. The samples were stabilized at 100°C for 100 s after irradiation and before TL measurements. The TL glow curves were rather broad with a TL peak maximum at 225°C (Figure 4). The stability of the signal was tested by the so-called "plateau-test" (Aitken, 1985). The onset of the plateau obtained from the ratio between the "natural" and "artificial" additive glow curves was indicative that the stable region of the glow curve had been reached. For the plateau test, first and second growth TL curves were analyzed by the 'TL- Plus' programme developed at GSF (Waibel, 1990). The plateau was observed between 250 and 350°C. The deviation in the plateau was found to be less than 10%. The TL dose response was plotted by integrating the TL glow curves in the plateau region. The accumulated dose obtained from the additive dose
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curves required correction due to the initial supralineraity of the dose response. This was usually obtained from the second glow growth curves of the drained samples. The intercept of the dose vs TL glow area was added to the intercept of the first growth curves. The correction was obtained from the second growth curves so-called intercept correction (for the justification and validity of the method see the arguments provided in Aitken, 1985) was found not to be constant through the depth of the 13.5 cm long brick. The effect could be explained due to the temperature gradient during the production of the brick block. The total accumulated doses DrL in brick with intercept correction from the second glow for
different depths in the brick are listed in the second column of Table 2. Assessment o f dose rate due to natural radionuclides
The measured total accumulated dose Drr, as described above, contains not only the accidental dose (Dace) but age dose (DAgo) which is the accumulated dose due to natural radioactive nuclides inside the brick as well as the external ~,-ray dose from the environment and cosmic rays [see Equation 1]. This is especially important for bricks, like the one used in this study, which comes from a building older than 100 yr.
H. Y. Grksu et al.
438
Table 1. The components of natural background dose rates (ct, fl, Yand cosmicrays) and total dose for 100 and 150 yr old fragments of brick No. 16 from the wall of the Metlino mill Brick Dose rate Dose rate Dose rate Dose rate Age dose age R~ R# R~. C DA~ (yr) (mGy/yr) (mGy/yr) (mGy/yr) (mGy/yr) (Gy) 100 1.08 1.89 1.05 0.28 0.43 _+0.02 150 1.08 1.89 1.05 0.28 0.65 + 0.02
Dose rates from natural:nuclides were calculated according to conversion tables (Bell, 1979; Nambi and Aitken, 1986) from the measured uranium, thorium and potassium of the brick, as described below. The individual components of the natural background are summarized in Table 1 for a brick of a 100 and 150 yr old building. Internal effective c~-particle dose rate R~. The uranium and thorium content of the brick was measured using a 4.5 cm diameter ZnS screen with the thick-sample ~-counting method, calibrated by using the U.S. Geological Standard (BCR-1). The uranium and thorium contents Were found to be 2.18 and 9.54ppm, respectively. The internal effective o-particle dose rate was calculated using the so-called a-value system developed by Bowman and Huntley (1984), which takes into account the efficiency of the s-particles independent of their energy for producing TL. The irradiation was performed with six plaque sources with a nominal 24~Am activity of 6.66 GBq each and calibrated individually in a vacuum. The a-value was found to be 0.103, yielding an internal effective s-particle dose rate R~ of 1.08 mGy/yr. In this calculation no correction was applied for the water content of the brick. Internal [3-particle dose rate R#. The Berthold LB770 fl-counter was used to measure the fl-particle dose rate due to the uranium, thorium and potassium content of the brick. A slice of the brick was crushed and two aliquots of 12 g, representing an infinite matrix, were counted three times. The thick-source fl-counting method was used for measurements and assessment of fl-dose rate (Sanderson, 1988). The internal fl-dose rate R# of the brick was found to be 1.89 mGy/yr. External ray dose rate R:. The Ural region is known to have a slightly higher natural 7-ray dose rate than the other parts of the country due to naturally occurring uranium and thorium in the environmental soil (Bougrov, 1994). The value of 1.05mGy/yr was used for the annual y-dose equivalent rate determined from a measured dose rate of 0.12 #Sv/h. Dose rate due to cosmic rays C. The contribution of cosmic radiation is usually small. At ground level it has been estimated to be 0.28 mGy/yr (Prescott and Stephan, 1982).
the code SAM-CE (Lichtenstein et al., 1979). It was assumed that the accumulated accidental dose was primarily due to the retention of ~7Cs in the clay at the bottom of the water reservoir. The solubility of -137Cs and the contamination of the water is not considered in this model. A homogeneous distribution of isotropic sources extending along the bottom of the reservoir to a distance of 20 m from the wall was defined as a source of the 661.6 keV photons which are emitted by this radionuclide. Separate calculations were made for the water levels of 50 and 20 cm. Photon fluences were determined in scoring regions corresponding to the layers at different depths in the brick, indicated in Figure 5, used for the experimental determination of the depth~ctose dependence. Doses in brick for the respective layers were calculated from the photon fluences using mass energy-absorption coefficients for photon interactions in bricks. A more detailed description of this type of Monte-Carlo calculation is given elsewhere (Meckbach et al., 1995).
Results and Discussion In this study, TL properties of a brick from the Metlino mill in the South Ural region were studied for the purpose of accidental dose assessment. It was important in this work to calculate and assess the dose rate due to natural background, since the brick used in this study stems from an old mill which was over 100 yr old. Furthermore, the accident occurred more than four decades ago. The accumulated doses DTL and extended accidental doses DAce due to contamination are listed in Table 2 for different depths in the brick. The natural background doses
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The dependence of accumulated dose on depth in the brick from the Metlino mill was calculated by Monte-Carlo simulation of photon transport using
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Depth-dose distribution in bricks Table 2. The accidental dose DA~obtained by subtracting the natural age dose of the I00 and 150 yr old brick No. 16 from the wall of the Metlino mill, respectively Depth in Dose DAce Dose DA~ brick Dose DTL (100 yr) (150 yr) (cm) (Gy) (Gy) (Gy) 0.7-2.7 4.04 3.62 + 0.12 3.40 + 0.12 2.8-4.8 3.02 2,60 _ 0.11 2.38 _ 0.11 4.9-6.9 2.11 1.69 _+0.14 1.47 +_0.14 7.0-9.0 1.67 1.25 _+0.06 1.03 _+0.05 9.1-11.I 1.22 0.80 + 0.06 0.58 _+_0.04 11.2-13.5 0.99 0.57 +_0.06 0.35 _ 0.04
a c c u m u l a t e d due to the possible ages of the brick f r a g m e n t (100 a n d 150yr, respectively) are given in T a b l e 1. T h e variations o f accidental dose t h r o u g h the brick were o b t a i n e d b y s u b t r a c t i n g the n a t u r a l age dose f r o m the total a c c u m u l a t e d dose. T h e results of M o n t e - C a r l o calculations were normalized to the m e a s u r e d T L values in the first segment o f the brick which is the average a b s o r b e d dose in a 2 cm layer. T h e m e a s u r e d a c c u m u l a t e d accidental dose vs d e p t h in the brick is s h o w n together with the results o b t a i n e d by the M o n t e - C a r l o calculations in Fig. 6. T h e calculated results c o r r e s p o n d to a c o n t a m i nation, for example, in the clays o f the reservoir bed with 137Cs for a water levels o f 20 a n d 50 cm. I n order to allow for a better c o m p a r i s o n , they were normalized to the accidental dose m e a s u r e d in the first layer o f the brick. W i t h this n o r m a l i z a t i o n , the source strength c o r r e s p o n d i n g to the height o f the 50 cm water level is a factor o f 11 larger t h a n the one for a d e p t h o f 20 cm. T h e calculated results are e n d o w e d with statistical errors which range between less t h a n 5 % for the first layers a n d less t h a n 10% for the deepest layer in the brick. One c a n see t h a t b o t h
439
the m e a s u r e d a n d the calculated d e p t h - d o s e dependences are basically exponential. The dependence o f the calculated d e p t h ~ l o s e distribution o n the source configuration is quite p r o n o u n c e d ; for the deepest layer in the brick, the dose calculated for a c o n t a m i n a t i o n at 20 cm below water level is m o r e t h a n a factor o f two larger t h a n the dose o b t a i n e d for a c o n t a m i n a t i o n at 50 cm. F o r the whole range o f depths in the brick which were considered, the m e a s u r e d accidental doses lie between the ones calculated for the two depths u n d e r the water level. O n the o t h e r h a n d , the measured d e p t h - d o s e distribution would deviate strongly f r o m the one resulting from a h o m o g e n e o u s ~37Cs c o n t a m i n a t i o n on the g r o u n d a r o u n d the mill ( M e c k b a c h et al., 1995) which would lead to a relative dose m o r e t h a n a factor o f three larger at the deepest layer in the brick.
Conclusion T h e T L m e t h o d using n a t u r a l materials for external dose assessment has long been used. However, in this study it is the first time a n actual sample was used to o b t a i n d e p t h - d o s e distribution in a m o r t a r e d brick sample. The d e p t h - d o s e profile o b t a i n e d experimentally c o m p a r e d with the M o n t e Carlo calculations ,yields i n f o r m a t i o n a b o u t the past distribution o f a specific radioactive source in the e n v i r o n m e n t . This i n f o r m a t i o n would provide d a t a to reconstruct the external doses to the local population, necessary for epidemiological investigations. The m e t h o d developed here will be used in c o l l a b o r a t i o n with the U r a l Research Centre for the assessment o f external doses as a c o n t r i b u t i o n to the total doses to the p o p u l a t i o n along the Techa river.
10.0
Acknowledgements--Part of the work discussed in this paper is supported by Commission of the European Communities under Contract F13 PC 92 00 40 and COSU-CT 93 0051 within the Radiation Protection Research Action.
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Fig. 6. Result of Monte-Carlo calculations compared with experimental accidental dose. Dependence of accumulated accidental dose on the depth in the brick, for an age of 150 yr as obtained by TL measurements (O). For comparison, the results of Monte-Carlo calculations, normalized to the measured value in the first brick layer, are shown for a homogeneous contamination of the reservoir bed with J37Cs at a depth of 50 cm below the water level (A) and of 20 cm ([3). The dotted lines are included to guide the eye and indicate the width of the brick layers.
Aitken M.J. (1976) Thermoluminescence age evaluation and assessment of error limits revised system. Archaeornetry 18, 233. Aitken M.J. (1985) Thermoluminescence Dating--Studies in Archaeological Science. Academic Press, London. Akleyev A.V. and Lyubchansky E. R. (1994) Environmental and medical effects of nuclear weapon production in the Southern Urals. The Sci. Total Environ. 142, 1. Bell W. T. (1979) Thermoluminescence dating: radiation dose rate data. Archaeometry 21, 243. Bowman S.G.E and Huntley D.J. (1984) A new proposal for the expression of alpha efficiency in TL dating. Ancient TL 2, 6. Bougrov N.G. (1994) Private communication. Degteva M.O., Kozheurov V.P. and Vorobiova M.I. (1994) General approach to dose reconstruction in the
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population exposed as a result of the release of radioactive wastes into the Techa river. The Sci. Total Environ. 142, 49. G6ksu H.Y., Bailiff I., Botter-Jensen L., Brodski L., Hiitt G. and Stoneham D. (1995) Interlaboratory source calibration using TL and OSL of natural quartz. Radiat. Meas. In press. Haskell E.H,, Kaipa P.L. and Wrenn M.E. (1987) Results of University of Utah: TL analysis of bricks and tiles at Hiroshima and Nagasaki. In DS86-Dosimetry System 1986, Vol. 2, p. 153. The Radiation Effects Research Foundation. Haskell E.H., Bailiff I.K., Keuner G.H., Kaipa P.L. and Wrenn M.E. (1994) Thermoluminesence measurements of ),-ray doses attributed to fallout from the Nevada Test Site using building bricks and natural dosemeters. Hlth Phys. 66, 380. Htitt G., Brodski L., Bailiff I.K., G6ksu H.Y., Haskell E.H., Jungner H. and Stoneham D. (1993) Accident dosimetry using environmental materials collected from regions downwind of Chernobyl: a preliminary evaluation. Radiat. Protect. Dosim. 47, 307. Lichtenstein H., Cohen H., Steinberg E., Troubetzkoy and Beer M. (1979) The SAM-CE Monte Carlo System for Radiationtransport and Criticality Calculations in Complex Configurations, Revision 7: a computer code manual. Mathematical Applications Group Inc., Westchester Plaza, Elmsford, NY. Maruyama T., Kumamoto Y. and Noda Y. (1987) Reassessment of ? doses from the atomic bombs in Hiroshima and Nagasaki. Radiat. Res. 113, 1. Meckbach R., Bailiff I.K., G6ksu H.Y., Jacob P. a n d Stoneham D. (1995) Calculation and measurement of depth dose distribution in bricks. Int. Solid State
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