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The use of ceramics for retrospective dosimetry in the Chernobyl exclusion zone
Pergamon
Radiation Measurements,Vol. 24, No. 4, pp. 507 51 I, 1995 Copyright c 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4487/95 $9.50 + 0.00 1350-4487(95)00020-8
THE USE OF CERAMICS FOR RETROSPECTIVE DOSIMETRY IN THE CHERNOBYL EXCLUSION ZONE I. K. BAILIFF Luminescence Dosimetry Laboratory, Environmental Research Centre, University of Durham. South Road, Durham DHI 3LE, U.K.
(Receired 27 September 1994; revised 6 February 1995~ in .final jorm 8 February 1995) Abstract--The use of luminescence techniques with ceramic materials is playing an increasingly important role in retrospective dosimetry. Thermoluminescence measurements with ceramics at Hiroshima and Nagasaki and in areas downwind of the Nevada Test Site have shown that dose estimates may be obtained which are of value in comparisons with the results of modelling calculations. The Chernobyl accident has provided a recent example where retrospective dosimetry is urgently required in order to advance epidemiological studies of the population. This paper examines some of the approaches which are being used with luminescence techniques to provide dose estimates for samples from Pripyat and how they can be used to contribute to the wider problem of dose reconstruction in the 30 km Exclusion Zone.
1. I N T R O D U C T I O N Dose reassessment studies at Hiroshima and Nagasaki (Maruyama et al., 1987) and in areas downwind of the Nevada Test Site (Haskell et al., 1994) have demonstrated the value of using thermoluminescence (TL) techniques with ceramic materials for retrospective measurement of gamma close many years after the event. The ceramic materials used in these studies were obtained from buildings which were either constructed of ceramic (brick) or had tiles fastened to the outer surface, The extensive period of investigation required for each of the above dosimetry applications underlines the variability of the properties of the luminescent minerals within ceramics. The properties of such minerals (typically quartz and feldspar) often vary significantly with thermal history; consequently building and domestic ceramics do not necessarily possess the same dose response characteristics. N o n e the less the techniques developed so far have provided a relatively wide working range, from the higher dose levels encountered at Hiroshima ( > 3 0 G y ) to the much lower levels present in the samples from southern U t a h ( < 100 mGy) of which the mean dose attributable to fall-out was ~ 4 0 mGy. Since the Chernobyl accident the need to apply a range of techniques for dose reconstruction has become increasingly evident because of the complexity and scale of the fall-out and the sporadic nature of monitoring procedures, particularly in rural regions. The application of luminescence techniques to ceramic materials from Pripyat was chosen because of its proximity to the power plant and the extensive scientific work which has been undertaken nr 24J4--N
in the vicinity of the town. Preliminary evaluations (Hfitt et al., 1993; Vischnevekii et al., 1993) have indicated the suitability of a range of ceramic samples from the Exclusion Zone for dose evaluation. In further work discussed here more detailed investigations of the luminescence properties of the ceramics have been performed, including the use of recently developed measurement procedures based on optical stimulation (Godfrey-Smith and Haskell, 1993). The techniques employed with ceramics for accident dosimetry have been reviewed by Haskell (1993a, b, 1994).
2. EXPERIMENTAL METHODS In common with TL dosimetry phosphors, minerals present in ceramics such as quartz and feldspar can be used to register the administration of absorbed dose and, at some later time, to yield a quantitative measure of that dose. This property is based on the accumulation of trapped charge carriers at defect sites within the crystal lattice, which is a function of dose. The transient gamma dose arising from exposure due to an accident, Dx, determined by luminescence techniques is given by
D x = D L - A(D: + b~ + I9 + l) ),
(1)
where D L = accrued dose determined by luminescence measurements (TL or OSL); A = sample age; /),, b/~,/)~.,/)~ = effective annual alpha, beta, gamma and cosmic dose, respectively. There are several luminescence techniques which may be used to evaluate DE. They are differentiated by the mineral composition of the sample extracted 507
508
I. K. B A I L I F F
for measurement, the grain size and the procedures employed to determine the accrued dose. The techniques were originally developed for archaeological dating applications (see Aitken, 1985, for a detailed description) and have been adapted for contemporary dosimetry application. Two techniques mentioned in this paper are the fine-grain and pre-dose techniques. The former is based on the extraction of ~ 2-10 p m grains from the ceramic and the measurement of TL in a temperature region (e.g. 250-400°C) where the half-lives of the traps associated with the TL are well in excess of 1000y. The pre-dose technique (McKeever, 1991; Bailiff, 1994)employs a sensitization effect in the 100°C TL peak of quartz and is usually, but not exclusively, used with quartz inclusions (90-150 pm). It is the most sensitive TL technique and has also been applied to porcelain, where the sample is prepared as cut slices rather than crushed and sized material. All have in common the determination of the growth of luminescence signal with additional laboratory dose, referred to as a growth characteristic. During the measurement of TL the release of the trapped charge carriers and consequent emission of luminescence is achieved using heat; however, for OSL measurements, the release of trapped charge is achieved by stimulation using light within particular wavelength regions. Under constant illumination of a sample and using suitable optical filters in the detection system to reject the stimulating light, an OSL decay curve is obtained. The wavelengths used for stimulation differ according to the mineral(s) within the sample; in the case of quartz, green light (e.g. 514 nm, Ar ion laser) is used for stimulation and for feldspars it has been found that infra-red wavelengths are also suitable, enabling simpler light sources to be used. When infra-red wavelengths are used the emission is referred to as infra-red stimulated luminescence (IRSL). Procedures employing optical stimulation are a recent introduction to the field and as such are under development. For samples receiving little or no transient fall-out dose, De is substantially due to natural radiation dose. The annual dose in equation (1) arises from the decay of uranium, thorium and potassium within the sample and the surrounding medium. For typical brick structures the accrued dose due to natural sources of radiation is expected to be in the region of 2.5-3.5 mGy/a with contributions of approximately 60%, 25% and 5% for beta, gamma and cosmic components, respectively (using inclusions, assumed to be of negligible internal radioactivity and where the alpha dose contribution is negligible). For such a structure, the accrued dose due to natural sources of radiation is expected to be roughly 30 mGy per decade--this forms the baseline above which further transient dose contributions are made. In areas of low fall-out or in heavily shielded locations compared with the naturally accrued dose where the additional transient dose is small, the uncertainty associated
with Dx is strongly influenced by the uncertainty associated with the determination of the age and annual dose rate. Thus, for heavily shielded interior locations, both high luminescence sensitivity and accurate annual dose evaluations are sought. 3. SURVEY With the exception of a small number of buildings, reinforced concrete has been used for the construction of all multiple storey apartment blocks in Pripyat. Within Zone 1 (the most heavily contaminated region; see H/itt et al., 1993) there are several substantial brick buildings with ceramic tile outer cladding. For such buildings a mortar layer (1-2 cm thick) underlying the tiles ( ~ l cm thick) represents a potentially important section of the absorbing medium but currently presents difficulties for dose evaluation. In addition to high fired terracotta floor, wall and skirting tiles, porcelain fittings provide a ubiquitous source of samples, present in most rooms and available in abundance within bathrooms. An important advantage provided by porcelain in exterior locations is the glazed surface of porcelain which inhibits the accumulation of fall-out contamination. A number of evacuated settlements within the Exclusion Zone surveyed for potential samples present a more diverse range of dwellings in age (up to ~ 5 0 y ) and style. Older traditional houses are constructed of timber frame and board with plastered interiors and have a substantial brick oven with chimney stack. One or two courses of external bricks are often used as part of the foundations between the timber building and an underlying concrete plinth; however their porosity and location render them susceptible to the ingress of contamination. As in Pripyat, the use of porcelain fittings and components is commonplace. Of the more recent housing stock concrete bricks are frequently used. In the absence of previous testing of ceramics from this region, the mineral composition and luminescence characteristics were largely unknown. Consequently, the application of luminescence techniques to sites within the Exclusion Zone required that a survey of potential samples was completed before more detailed work commenced. The survey evaluation for Pripyat (H/itt et al., 1993) included: (1) The mineralogical composition of samples and the (natural) radioisotope content of the ceramic matrix. (2) The intensity of TL signals due to the dose accrued since manufacture and the TL growth characteristic following laboratory irradiation; a preliminary evaluation of the transient fallout dose where the sample characteristics are satisfactory. (3) Identification of buildings with ceramics available from interior and exterior locations to determine the degree of shielding.
CERAMICS FOR RETROSPECTIVE DOSIMETRY The ceramics, as expected, contained differing relative proportions of quartz and feldspar, depending on the nature of the manufacturing process; the survival of feldspar diminishes in the case of highfired ceramics. Both minerals have TL peaks above 250°C with mean-lives sufficient for dosimetry over time-scales extending to millennia, although their behaviour over the shorter time-scales demands scrutiny in view of the short-term fading effects in certain feldspars. The supralinear growth of TL peaks of fired quartz at low doses in combination with weak emission due to thermal quenching (Wintle, 1975a) in the region 250-400~'C often renders the corresponding region of the quartz glow curve unsuitable for dose measurements below 1 Gy. Other luminescent minerals such as calcite (found in brick) and mullite (found in tiles and porcelain) were also present. The accrued dose averaged for five interior samples within Zone 1 of 200_+ 100mGy confirmed dose levels in Pripyat approaching natural background (average 60 + 20 mGy; SD 12)--requiring the use of the pre-dose technique. The accrued dose averaged for external tile (1 cm thickness; surface layer to depth of 2-3 mm removed) at three locations within the same area were ~ 2 Gy; comparisons with that obtained for the underlying brick (depth 2 - 4 cm) indicate a reduction by a factor of ~3. If it is assumed that the dose evaluated by TL is primarily attributable to gamma radiation, this reduction is comparable to that expected for photons (isotropic field) of less than 500 keV obtained by Monte Carlo calculation (Meckbach, personal communication). The determination of dose as a function of depth in ceramic (referred to as a dose-depth curve) also reflects the time-averaged incident energy spectrum. The detailed investigation of dose as a function of depth, where suitable sample permits, requires a considerable experimental effort using standard sample extraction techniques. New approaches to reduce this effort are being investigated using recently developed techniques based on the measurement of optically stimulated luminescence (OSL). As part of a test experiment intended for a subsequent laboratory intercomparison, a stack of contemporary ceramic tiles were irradiated with 6°Co and 1 3 7 C s photon beams. Powder samples were rapidly obtained from each tile using a tungsten carbide drill and prepared for measurement using the fine-grain technique. The accrued dose was evaluated by measurement of infra-red stimulated luminescence using an additive dose procedure. The results (shown for caesium irradiation in Fig. 1) indicate promising agreement with dose-depth profiles computed by Monte Carlo methods. The possibility of performing OSL measurements with cores using scanning apparatus is also being investigated and offers a potentially powerful tool of rapid surveying of dose-depth characteristics (Bmter-Jensen et al., 1995). Dose-depth measurements based on the drilling
509
procedure discussed above have also been used with brick samples taken from the outer and inner walls projecting from the roof of a 4-storey building in Zone 1 and located immediately under heavy concrete capping. Powder X-ray diffraction analysis had revealed the presence of a complex mixture of minerals including quartz, diopside, feldspars (albite, orthoclase and anorthite) and calcite. TL and IRSL (880 nm stimulation) measurements had been unsuccessful because of extremely weak emission when recorded with the standard detection windows (TL: UV/blue pass; IRSL: broad band). Subsequent spectral measurement of the TL revealed the major part of the emission to be orange/red (Fig. 2). Using appropriate optical filters a marked gain in sensitivity was obtained, providing a minimum resolvable dose in the region of 10 mGy. The equivalent dose for the sample tested was 100 mGy. On the basis of available spectral data for such minerals, calcite and sodiumrich feldspars are likely to be the major contributors to the emission; the weak IRSL indicates that the TL emissions from feldspars are a minor contributor. Carbonates, while providing high sensitivity, require careful preparation and an oxygen-free atmosphere during glow curve measurement in order to suppress the effects of spurious luminescence (Wintle, 1975b). An understanding of the nature of the spectral emission is also of relevance to measurements with porcelain. Previous work (Stoneham, 1985) using the pre-dose technique had demonstrated dose measurement capability in the region of 50 mGy; application to samples from the Exclusion Zone (Stoneham, 1995) has confirmed similar sensitivity. Porcelain is
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Fig. 1. Measured absorbed dose vs depth in ceramic stack of tiles measured using IRSL for a sampled volume as discussed in the main text. Each stack was given a known incident photon dose from either a Cs or Co source at the GSF Neuherberg SSDL facility. The relative kerma in terracotta due to parallel irradiation with Cs and Co photon beams was determined using Monte Carlo calculations (SAM-CE code). The average kerma is calculated for a volume element (1 cm thick x 2 cm × 2 cm) at various depths located: (1) along the central axis of the tile stack (upper open triangles); and (2) along an axis parallel to the central axis located such that the volume element is located at the (same) corner of each tile (lower open triangles). Each tile was 12 cm2. Calculations by R. Meckbach, GSF.
510
I. K. BAILIFF
%
Fig. 2. TL emission spectrum for powder sample (2-10 #m) extracted from brick; the peak of the main glow curve occurs at 300°C. An additional beta dose of 100Gy had been administered to the sample before measurement. Sample temperature in °C may be obtained by dividing the values on time/temp axis by 4. The spectrum was obtained using a high sensitivity spectrometer described previously (Bailiff et al., 1977) and has been corrected for instrument response.
used as cut slices and thus the bulk luminescence detected is from components of a complex mixture of crystalline and glassy phases of the constituent minerals (quartz, mullite and aluminosilicates). The pre-dose technique employs the pre-dose effect, the term used to describe the sensitization (McKeever, 1985) of the ~ 100°C TL peak in the quartz glow curve. The current interpretation of the original pre-dose model (Zimmerman, 1971) for this effect is underpinned by the enhancement of the 360nm emission (McKeever, 1991), which is the dominant emission band of the ~100°C TL peak. However, measurement of the ~ 100°C TL peak emission spec-
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Fig. 3. Emission spectra of the ~ 100°C TL peak obtained with a porcelain slice (interior light fitting). The emission
spectrum was measured during the equivalent determinations performed for the pre-dose technique of: (1) the initial sensitivity SO (lower curve); and (ii) the activated sensitivity Ss (upper curve) following thermal activation to ~550°C. A test dose of ~2Gy was administered. Spectrometer details as described in caption to Fig. 2.
trum for a sample of porcelain shows (Fig. 3) that, in addition to the 360 nm band, a substantial part of the emission is at longer wavelengths (~420 nm band) and that both bands take part in the sensitization process. Consequently substantial gain in the TL signal may be obtained by broadening the transmission range of the filters used in the detection system to pass both bands. On the other hand such results also indicate that for the porcelain samples of the type tested, the interpretation of the pre-dose model for quartz should not be simply assumed to apply to porcelain and that further examination is required. The OSL characteristics of porcelain have also been investigated (Godfrey-Smith and Haskell, 1993; Poolton et al., 1995); although the current minimum resolvable dose appears to be several times higher than for TL, OSL procedures may offer simplified measurement procedures. 4. DEPLOYMENT OF LUMINESCENCE RESULTS One of the primary requirements of the application of solid-state dosimetry techniques (luminescence and also EPR of tooth enamel) is to aid dose reconstruction for epidemiological studies of the population in contaminated areas (as evacuees or residents). The dose evaluated by luminescence represents an integrated value since manufacture for a specific sampled volume of ceramic extracted from a standing structure or a fixture within the interior. For an urban environment evacuated at an early stage compared with the duration since the incident, this has important implications for the use of the method (in Pripyat) since the estimation of dose to evacuees is not feasible. Although ceramics will have registered any dose delivered prior to the commencement of monitoring, the method does not (using samples from fixed structures) allow differentiation of the contribution from that received afterwards without the use of estimates obtained by modelling calculations (based on monitoring data). On the other hand, a key advantage provided by luminescence is the flexibility to provide integrated dose estimates from a range of locations which is unrivalled by other techniques in retrospective analysis. The effectiveness of the building structures in providing shielding for inhabitants from radiation is a central aspect of retrospective dosimetry. Using the capability to determine absorbed dose at exterior and interior locations and to obtain dose profiles for ceramic walls, the method is being used in Pripyat to provide dose estimates for comparison with modelling calculations based on the use of Monte Carlo techniques (Meckbach et al., 1988; Meckbach and Jacob, 1988). This comparison will rely heavily on the extent of monitoring data during the period in which the majority of the dose has been delivered. The aim of current work is to test
CERAMICS
FOR RETROSPECTIVE
materials from buildings for which modelling can be performed; this will provide c o m p a r i s o n s of shielding factors and integrated dose. D e p e n d i n g on the o u t c o m e of such comparisons, the future role for luminescence dose evaluations will be to serve as b e n c h m a r k d e t e r m i n a t i o n s for use in modelling in p o p u l a t e d c o n t a m i n a t e d areas which lack extensive m o n i t o r i n g data. Since m a n y rural dwellings are constructed of wood, experimental checks of effective shielding factors will also m a k e a valuable contrib u t i o n to the retrospective dose assessment. F o r application of the m e t h o d in such areas, the search for i m p r o v e m e n t s in our u n d e r s t a n d i n g of the luminescence characteristics of minerals within ceramics will be a central element of the work. Acknowledgements--Part of the work discussed in this paper is supported by the Commission of the European Communities under contract F13PCT920040 within the Radiation Protection Research Action. The author is pleased to acknowledge discussions with Dr H. G. Menzel of DGXII, CEC, and project partners. Dr R. Meckbach kindly supplied unpublished data concerning Monte Carlo calculations.
REFERENCES Aitken M. J. (1985) Thermoluminescence Dating. Academic Press, London. Bailiff I. K., Morris D. A. and Aitken M. J. (1977) A rapid-scanning interference spectrometer: application to low level thermoluminescence emission. J. Phys. E: Sci. lnstrum. 10, 1156-1160. Bailiff I. K. (1994) The pre-dose technique. Radiat. Meas. 23, 471 479. Botter-Jensen L., Jungner H. and Poolton N. R. J. (1995) A continuous OSL scanning method for analysis of radiation depth dose profiles in bricks. Radiat. Meas. 24, 525 529. Godfrey-Smith D. 1. and Haskell E. H. (1993) Application of optically stimulated luminescence to the dosimetry of recent radiation events involving low total absorbed doses. Health Phys. 65, 396 404. Haskell E. El. (1993a) Accident dosimetry using environmental materials: the role of thermoluminescence. Nucl. Tracks Radiat. Meas. 21, 87-93. Haskcll E. H. (1993b) Retrospective accident dosimetry using environmental materials. Radiat. Prot. Dosim. 47, 297 303.
DOSIMETRY
511
Haskell E. H. (1994) Some recent developments in accident dosimetry using TL of environmental materials. Radiat. Prot. Dosim. 47, 491. Haskell E. H., Bailiff I. K., Kenner G. H., Kaipa P. L. and Wrenn M. E. (1994) Thermoluminescence measurements of gamma-ray doses attributable to fall-out from the Nevada Test Site using building bricks as natural dosemeters. Health Phys. 66, 380 391. H/itt G., Brodski L., Bailiff I. K., G6ksu Y., Haskell E. H., Jungner H. and Stoneham D. (1993) TL accident dosimetry using environmental material collected from regions downwind of Chernobyl: a preliminary evaluation. Radiat. Prot. Dosim. 47, 307 311. Poolton N. R. J., Botter-Jensen L. and Jungner H. (1995) An optically stimulated luminescence study of porcelain related to radiation dosimetry. Radiat. Meas. 24, 543-549. Maruyama T., Yoshikazu K., Yoneta I., Tsuneto N., Masaharu H., Haskell E. H. and Kaipa P. (1987) Thermoluminescence Measurements of Gamma Rays. In U.S. Japan Joint Reassessment of Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Final Report Vol. I, The Radiation Effects Research Foundation (W. Roesch, Ed.), pp. 143 184. McKeever S. W. S. (1985) Thermoluminescence ~/ Solids. Cambridge University Press, Cambridge. McKeever S. W. S. (1991) Mechanisms of thermoluminescence production: some problems and a few answers? Nucl. Tracks Radiat. Meas. 18, 5 12. Meckbach R., Jacob P. and Paretzke H. G. (1988) Gamma exposures due to radionuclides deposited in urban environments. Part I: Kerma rates from contaminated urban surfaces. Radiat. Prot. Dosim. 25, 167 179. Meekbach R. and Jacob P. (1988) Gamma exposures due to radio nuclides deposited in urban environments. Part II: Location factors for different deposition patterns. Radiat. Prot. Dosim. 25, 181 190. Stoneham D. (1985) The use of porcelain as a low-dose background dosimeter. Nucl. Tracks 10, 509 512. Stoneham D. (1995) Accident dosimetry using porcelain. Radiat. Meas. 24, 499 505. Vischnevekii I. N., Drozd I. P., Koval G. N., Fominych V. I., Baran N. P., Bartchuk V. 1., Bugal A. A., Maksimenko M. and Baryachtar V. G. (1993) The use of quartz inclusion thermoluminescence for the retrospective dosimetry of the Chernobyl area. Radiat. Prot. Dosim. 47, 305 306. Wintle A. G. (1975a) Thermal quenching of thermoluminescence in quartz. Geophys. J. R. Astr. So('. 41, 107 113. Wintle A. G. (1975b) Effects of sample preparation on the thermoluminescence characteristics of calcite. Mod. Geol. 5, 165 167. Zimmerman J. (1971) The radiation-induced increase of thermoluminescence sensitivity of fired quartz. J. Phys. C: Solid State Phys. 4, 3277 329l.
Radiation Measurements,Vol. 24, No. 4, pp. 507 51 I, 1995 Copyright c 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4487/95 $9.50 + 0.00 1350-4487(95)00020-8
THE USE OF CERAMICS FOR RETROSPECTIVE DOSIMETRY IN THE CHERNOBYL EXCLUSION ZONE I. K. BAILIFF Luminescence Dosimetry Laboratory, Environmental Research Centre, University of Durham. South Road, Durham DHI 3LE, U.K.
(Receired 27 September 1994; revised 6 February 1995~ in .final jorm 8 February 1995) Abstract--The use of luminescence techniques with ceramic materials is playing an increasingly important role in retrospective dosimetry. Thermoluminescence measurements with ceramics at Hiroshima and Nagasaki and in areas downwind of the Nevada Test Site have shown that dose estimates may be obtained which are of value in comparisons with the results of modelling calculations. The Chernobyl accident has provided a recent example where retrospective dosimetry is urgently required in order to advance epidemiological studies of the population. This paper examines some of the approaches which are being used with luminescence techniques to provide dose estimates for samples from Pripyat and how they can be used to contribute to the wider problem of dose reconstruction in the 30 km Exclusion Zone.
1. I N T R O D U C T I O N Dose reassessment studies at Hiroshima and Nagasaki (Maruyama et al., 1987) and in areas downwind of the Nevada Test Site (Haskell et al., 1994) have demonstrated the value of using thermoluminescence (TL) techniques with ceramic materials for retrospective measurement of gamma close many years after the event. The ceramic materials used in these studies were obtained from buildings which were either constructed of ceramic (brick) or had tiles fastened to the outer surface, The extensive period of investigation required for each of the above dosimetry applications underlines the variability of the properties of the luminescent minerals within ceramics. The properties of such minerals (typically quartz and feldspar) often vary significantly with thermal history; consequently building and domestic ceramics do not necessarily possess the same dose response characteristics. N o n e the less the techniques developed so far have provided a relatively wide working range, from the higher dose levels encountered at Hiroshima ( > 3 0 G y ) to the much lower levels present in the samples from southern U t a h ( < 100 mGy) of which the mean dose attributable to fall-out was ~ 4 0 mGy. Since the Chernobyl accident the need to apply a range of techniques for dose reconstruction has become increasingly evident because of the complexity and scale of the fall-out and the sporadic nature of monitoring procedures, particularly in rural regions. The application of luminescence techniques to ceramic materials from Pripyat was chosen because of its proximity to the power plant and the extensive scientific work which has been undertaken nr 24J4--N
in the vicinity of the town. Preliminary evaluations (Hfitt et al., 1993; Vischnevekii et al., 1993) have indicated the suitability of a range of ceramic samples from the Exclusion Zone for dose evaluation. In further work discussed here more detailed investigations of the luminescence properties of the ceramics have been performed, including the use of recently developed measurement procedures based on optical stimulation (Godfrey-Smith and Haskell, 1993). The techniques employed with ceramics for accident dosimetry have been reviewed by Haskell (1993a, b, 1994).
2. EXPERIMENTAL METHODS In common with TL dosimetry phosphors, minerals present in ceramics such as quartz and feldspar can be used to register the administration of absorbed dose and, at some later time, to yield a quantitative measure of that dose. This property is based on the accumulation of trapped charge carriers at defect sites within the crystal lattice, which is a function of dose. The transient gamma dose arising from exposure due to an accident, Dx, determined by luminescence techniques is given by
D x = D L - A(D: + b~ + I9 + l) ),
(1)
where D L = accrued dose determined by luminescence measurements (TL or OSL); A = sample age; /),, b/~,/)~.,/)~ = effective annual alpha, beta, gamma and cosmic dose, respectively. There are several luminescence techniques which may be used to evaluate DE. They are differentiated by the mineral composition of the sample extracted 507
508
I. K. B A I L I F F
for measurement, the grain size and the procedures employed to determine the accrued dose. The techniques were originally developed for archaeological dating applications (see Aitken, 1985, for a detailed description) and have been adapted for contemporary dosimetry application. Two techniques mentioned in this paper are the fine-grain and pre-dose techniques. The former is based on the extraction of ~ 2-10 p m grains from the ceramic and the measurement of TL in a temperature region (e.g. 250-400°C) where the half-lives of the traps associated with the TL are well in excess of 1000y. The pre-dose technique (McKeever, 1991; Bailiff, 1994)employs a sensitization effect in the 100°C TL peak of quartz and is usually, but not exclusively, used with quartz inclusions (90-150 pm). It is the most sensitive TL technique and has also been applied to porcelain, where the sample is prepared as cut slices rather than crushed and sized material. All have in common the determination of the growth of luminescence signal with additional laboratory dose, referred to as a growth characteristic. During the measurement of TL the release of the trapped charge carriers and consequent emission of luminescence is achieved using heat; however, for OSL measurements, the release of trapped charge is achieved by stimulation using light within particular wavelength regions. Under constant illumination of a sample and using suitable optical filters in the detection system to reject the stimulating light, an OSL decay curve is obtained. The wavelengths used for stimulation differ according to the mineral(s) within the sample; in the case of quartz, green light (e.g. 514 nm, Ar ion laser) is used for stimulation and for feldspars it has been found that infra-red wavelengths are also suitable, enabling simpler light sources to be used. When infra-red wavelengths are used the emission is referred to as infra-red stimulated luminescence (IRSL). Procedures employing optical stimulation are a recent introduction to the field and as such are under development. For samples receiving little or no transient fall-out dose, De is substantially due to natural radiation dose. The annual dose in equation (1) arises from the decay of uranium, thorium and potassium within the sample and the surrounding medium. For typical brick structures the accrued dose due to natural sources of radiation is expected to be in the region of 2.5-3.5 mGy/a with contributions of approximately 60%, 25% and 5% for beta, gamma and cosmic components, respectively (using inclusions, assumed to be of negligible internal radioactivity and where the alpha dose contribution is negligible). For such a structure, the accrued dose due to natural sources of radiation is expected to be roughly 30 mGy per decade--this forms the baseline above which further transient dose contributions are made. In areas of low fall-out or in heavily shielded locations compared with the naturally accrued dose where the additional transient dose is small, the uncertainty associated
with Dx is strongly influenced by the uncertainty associated with the determination of the age and annual dose rate. Thus, for heavily shielded interior locations, both high luminescence sensitivity and accurate annual dose evaluations are sought. 3. SURVEY With the exception of a small number of buildings, reinforced concrete has been used for the construction of all multiple storey apartment blocks in Pripyat. Within Zone 1 (the most heavily contaminated region; see H/itt et al., 1993) there are several substantial brick buildings with ceramic tile outer cladding. For such buildings a mortar layer (1-2 cm thick) underlying the tiles ( ~ l cm thick) represents a potentially important section of the absorbing medium but currently presents difficulties for dose evaluation. In addition to high fired terracotta floor, wall and skirting tiles, porcelain fittings provide a ubiquitous source of samples, present in most rooms and available in abundance within bathrooms. An important advantage provided by porcelain in exterior locations is the glazed surface of porcelain which inhibits the accumulation of fall-out contamination. A number of evacuated settlements within the Exclusion Zone surveyed for potential samples present a more diverse range of dwellings in age (up to ~ 5 0 y ) and style. Older traditional houses are constructed of timber frame and board with plastered interiors and have a substantial brick oven with chimney stack. One or two courses of external bricks are often used as part of the foundations between the timber building and an underlying concrete plinth; however their porosity and location render them susceptible to the ingress of contamination. As in Pripyat, the use of porcelain fittings and components is commonplace. Of the more recent housing stock concrete bricks are frequently used. In the absence of previous testing of ceramics from this region, the mineral composition and luminescence characteristics were largely unknown. Consequently, the application of luminescence techniques to sites within the Exclusion Zone required that a survey of potential samples was completed before more detailed work commenced. The survey evaluation for Pripyat (H/itt et al., 1993) included: (1) The mineralogical composition of samples and the (natural) radioisotope content of the ceramic matrix. (2) The intensity of TL signals due to the dose accrued since manufacture and the TL growth characteristic following laboratory irradiation; a preliminary evaluation of the transient fallout dose where the sample characteristics are satisfactory. (3) Identification of buildings with ceramics available from interior and exterior locations to determine the degree of shielding.
CERAMICS FOR RETROSPECTIVE DOSIMETRY The ceramics, as expected, contained differing relative proportions of quartz and feldspar, depending on the nature of the manufacturing process; the survival of feldspar diminishes in the case of highfired ceramics. Both minerals have TL peaks above 250°C with mean-lives sufficient for dosimetry over time-scales extending to millennia, although their behaviour over the shorter time-scales demands scrutiny in view of the short-term fading effects in certain feldspars. The supralinear growth of TL peaks of fired quartz at low doses in combination with weak emission due to thermal quenching (Wintle, 1975a) in the region 250-400~'C often renders the corresponding region of the quartz glow curve unsuitable for dose measurements below 1 Gy. Other luminescent minerals such as calcite (found in brick) and mullite (found in tiles and porcelain) were also present. The accrued dose averaged for five interior samples within Zone 1 of 200_+ 100mGy confirmed dose levels in Pripyat approaching natural background (average 60 + 20 mGy; SD 12)--requiring the use of the pre-dose technique. The accrued dose averaged for external tile (1 cm thickness; surface layer to depth of 2-3 mm removed) at three locations within the same area were ~ 2 Gy; comparisons with that obtained for the underlying brick (depth 2 - 4 cm) indicate a reduction by a factor of ~3. If it is assumed that the dose evaluated by TL is primarily attributable to gamma radiation, this reduction is comparable to that expected for photons (isotropic field) of less than 500 keV obtained by Monte Carlo calculation (Meckbach, personal communication). The determination of dose as a function of depth in ceramic (referred to as a dose-depth curve) also reflects the time-averaged incident energy spectrum. The detailed investigation of dose as a function of depth, where suitable sample permits, requires a considerable experimental effort using standard sample extraction techniques. New approaches to reduce this effort are being investigated using recently developed techniques based on the measurement of optically stimulated luminescence (OSL). As part of a test experiment intended for a subsequent laboratory intercomparison, a stack of contemporary ceramic tiles were irradiated with 6°Co and 1 3 7 C s photon beams. Powder samples were rapidly obtained from each tile using a tungsten carbide drill and prepared for measurement using the fine-grain technique. The accrued dose was evaluated by measurement of infra-red stimulated luminescence using an additive dose procedure. The results (shown for caesium irradiation in Fig. 1) indicate promising agreement with dose-depth profiles computed by Monte Carlo methods. The possibility of performing OSL measurements with cores using scanning apparatus is also being investigated and offers a potentially powerful tool of rapid surveying of dose-depth characteristics (Bmter-Jensen et al., 1995). Dose-depth measurements based on the drilling
509
procedure discussed above have also been used with brick samples taken from the outer and inner walls projecting from the roof of a 4-storey building in Zone 1 and located immediately under heavy concrete capping. Powder X-ray diffraction analysis had revealed the presence of a complex mixture of minerals including quartz, diopside, feldspars (albite, orthoclase and anorthite) and calcite. TL and IRSL (880 nm stimulation) measurements had been unsuccessful because of extremely weak emission when recorded with the standard detection windows (TL: UV/blue pass; IRSL: broad band). Subsequent spectral measurement of the TL revealed the major part of the emission to be orange/red (Fig. 2). Using appropriate optical filters a marked gain in sensitivity was obtained, providing a minimum resolvable dose in the region of 10 mGy. The equivalent dose for the sample tested was 100 mGy. On the basis of available spectral data for such minerals, calcite and sodiumrich feldspars are likely to be the major contributors to the emission; the weak IRSL indicates that the TL emissions from feldspars are a minor contributor. Carbonates, while providing high sensitivity, require careful preparation and an oxygen-free atmosphere during glow curve measurement in order to suppress the effects of spurious luminescence (Wintle, 1975b). An understanding of the nature of the spectral emission is also of relevance to measurements with porcelain. Previous work (Stoneham, 1985) using the pre-dose technique had demonstrated dose measurement capability in the region of 50 mGy; application to samples from the Exclusion Zone (Stoneham, 1995) has confirmed similar sensitivity. Porcelain is
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Fig. 1. Measured absorbed dose vs depth in ceramic stack of tiles measured using IRSL for a sampled volume as discussed in the main text. Each stack was given a known incident photon dose from either a Cs or Co source at the GSF Neuherberg SSDL facility. The relative kerma in terracotta due to parallel irradiation with Cs and Co photon beams was determined using Monte Carlo calculations (SAM-CE code). The average kerma is calculated for a volume element (1 cm thick x 2 cm × 2 cm) at various depths located: (1) along the central axis of the tile stack (upper open triangles); and (2) along an axis parallel to the central axis located such that the volume element is located at the (same) corner of each tile (lower open triangles). Each tile was 12 cm2. Calculations by R. Meckbach, GSF.
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Fig. 2. TL emission spectrum for powder sample (2-10 #m) extracted from brick; the peak of the main glow curve occurs at 300°C. An additional beta dose of 100Gy had been administered to the sample before measurement. Sample temperature in °C may be obtained by dividing the values on time/temp axis by 4. The spectrum was obtained using a high sensitivity spectrometer described previously (Bailiff et al., 1977) and has been corrected for instrument response.
used as cut slices and thus the bulk luminescence detected is from components of a complex mixture of crystalline and glassy phases of the constituent minerals (quartz, mullite and aluminosilicates). The pre-dose technique employs the pre-dose effect, the term used to describe the sensitization (McKeever, 1985) of the ~ 100°C TL peak in the quartz glow curve. The current interpretation of the original pre-dose model (Zimmerman, 1971) for this effect is underpinned by the enhancement of the 360nm emission (McKeever, 1991), which is the dominant emission band of the ~100°C TL peak. However, measurement of the ~ 100°C TL peak emission spec-
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spectrum was measured during the equivalent determinations performed for the pre-dose technique of: (1) the initial sensitivity SO (lower curve); and (ii) the activated sensitivity Ss (upper curve) following thermal activation to ~550°C. A test dose of ~2Gy was administered. Spectrometer details as described in caption to Fig. 2.
trum for a sample of porcelain shows (Fig. 3) that, in addition to the 360 nm band, a substantial part of the emission is at longer wavelengths (~420 nm band) and that both bands take part in the sensitization process. Consequently substantial gain in the TL signal may be obtained by broadening the transmission range of the filters used in the detection system to pass both bands. On the other hand such results also indicate that for the porcelain samples of the type tested, the interpretation of the pre-dose model for quartz should not be simply assumed to apply to porcelain and that further examination is required. The OSL characteristics of porcelain have also been investigated (Godfrey-Smith and Haskell, 1993; Poolton et al., 1995); although the current minimum resolvable dose appears to be several times higher than for TL, OSL procedures may offer simplified measurement procedures. 4. DEPLOYMENT OF LUMINESCENCE RESULTS One of the primary requirements of the application of solid-state dosimetry techniques (luminescence and also EPR of tooth enamel) is to aid dose reconstruction for epidemiological studies of the population in contaminated areas (as evacuees or residents). The dose evaluated by luminescence represents an integrated value since manufacture for a specific sampled volume of ceramic extracted from a standing structure or a fixture within the interior. For an urban environment evacuated at an early stage compared with the duration since the incident, this has important implications for the use of the method (in Pripyat) since the estimation of dose to evacuees is not feasible. Although ceramics will have registered any dose delivered prior to the commencement of monitoring, the method does not (using samples from fixed structures) allow differentiation of the contribution from that received afterwards without the use of estimates obtained by modelling calculations (based on monitoring data). On the other hand, a key advantage provided by luminescence is the flexibility to provide integrated dose estimates from a range of locations which is unrivalled by other techniques in retrospective analysis. The effectiveness of the building structures in providing shielding for inhabitants from radiation is a central aspect of retrospective dosimetry. Using the capability to determine absorbed dose at exterior and interior locations and to obtain dose profiles for ceramic walls, the method is being used in Pripyat to provide dose estimates for comparison with modelling calculations based on the use of Monte Carlo techniques (Meckbach et al., 1988; Meckbach and Jacob, 1988). This comparison will rely heavily on the extent of monitoring data during the period in which the majority of the dose has been delivered. The aim of current work is to test
CERAMICS
FOR RETROSPECTIVE
materials from buildings for which modelling can be performed; this will provide c o m p a r i s o n s of shielding factors and integrated dose. D e p e n d i n g on the o u t c o m e of such comparisons, the future role for luminescence dose evaluations will be to serve as b e n c h m a r k d e t e r m i n a t i o n s for use in modelling in p o p u l a t e d c o n t a m i n a t e d areas which lack extensive m o n i t o r i n g data. Since m a n y rural dwellings are constructed of wood, experimental checks of effective shielding factors will also m a k e a valuable contrib u t i o n to the retrospective dose assessment. F o r application of the m e t h o d in such areas, the search for i m p r o v e m e n t s in our u n d e r s t a n d i n g of the luminescence characteristics of minerals within ceramics will be a central element of the work. Acknowledgements--Part of the work discussed in this paper is supported by the Commission of the European Communities under contract F13PCT920040 within the Radiation Protection Research Action. The author is pleased to acknowledge discussions with Dr H. G. Menzel of DGXII, CEC, and project partners. Dr R. Meckbach kindly supplied unpublished data concerning Monte Carlo calculations.
REFERENCES Aitken M. J. (1985) Thermoluminescence Dating. Academic Press, London. Bailiff I. K., Morris D. A. and Aitken M. J. (1977) A rapid-scanning interference spectrometer: application to low level thermoluminescence emission. J. Phys. E: Sci. lnstrum. 10, 1156-1160. Bailiff I. K. (1994) The pre-dose technique. Radiat. Meas. 23, 471 479. Botter-Jensen L., Jungner H. and Poolton N. R. J. (1995) A continuous OSL scanning method for analysis of radiation depth dose profiles in bricks. Radiat. Meas. 24, 525 529. Godfrey-Smith D. 1. and Haskell E. H. (1993) Application of optically stimulated luminescence to the dosimetry of recent radiation events involving low total absorbed doses. Health Phys. 65, 396 404. Haskell E. El. (1993a) Accident dosimetry using environmental materials: the role of thermoluminescence. Nucl. Tracks Radiat. Meas. 21, 87-93. Haskcll E. H. (1993b) Retrospective accident dosimetry using environmental materials. Radiat. Prot. Dosim. 47, 297 303.
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Haskell E. H. (1994) Some recent developments in accident dosimetry using TL of environmental materials. Radiat. Prot. Dosim. 47, 491. Haskell E. H., Bailiff I. K., Kenner G. H., Kaipa P. L. and Wrenn M. E. (1994) Thermoluminescence measurements of gamma-ray doses attributable to fall-out from the Nevada Test Site using building bricks as natural dosemeters. Health Phys. 66, 380 391. H/itt G., Brodski L., Bailiff I. K., G6ksu Y., Haskell E. H., Jungner H. and Stoneham D. (1993) TL accident dosimetry using environmental material collected from regions downwind of Chernobyl: a preliminary evaluation. Radiat. Prot. Dosim. 47, 307 311. Poolton N. R. J., Botter-Jensen L. and Jungner H. (1995) An optically stimulated luminescence study of porcelain related to radiation dosimetry. Radiat. Meas. 24, 543-549. Maruyama T., Yoshikazu K., Yoneta I., Tsuneto N., Masaharu H., Haskell E. H. and Kaipa P. (1987) Thermoluminescence Measurements of Gamma Rays. In U.S. Japan Joint Reassessment of Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Final Report Vol. I, The Radiation Effects Research Foundation (W. Roesch, Ed.), pp. 143 184. McKeever S. W. S. (1985) Thermoluminescence ~/ Solids. Cambridge University Press, Cambridge. McKeever S. W. S. (1991) Mechanisms of thermoluminescence production: some problems and a few answers? Nucl. Tracks Radiat. Meas. 18, 5 12. Meckbach R., Jacob P. and Paretzke H. G. (1988) Gamma exposures due to radionuclides deposited in urban environments. Part I: Kerma rates from contaminated urban surfaces. Radiat. Prot. Dosim. 25, 167 179. Meekbach R. and Jacob P. (1988) Gamma exposures due to radio nuclides deposited in urban environments. Part II: Location factors for different deposition patterns. Radiat. Prot. Dosim. 25, 181 190. Stoneham D. (1985) The use of porcelain as a low-dose background dosimeter. Nucl. Tracks 10, 509 512. Stoneham D. (1995) Accident dosimetry using porcelain. Radiat. Meas. 24, 499 505. Vischnevekii I. N., Drozd I. P., Koval G. N., Fominych V. I., Baran N. P., Bartchuk V. 1., Bugal A. A., Maksimenko M. and Baryachtar V. G. (1993) The use of quartz inclusion thermoluminescence for the retrospective dosimetry of the Chernobyl area. Radiat. Prot. Dosim. 47, 305 306. Wintle A. G. (1975a) Thermal quenching of thermoluminescence in quartz. Geophys. J. R. Astr. So('. 41, 107 113. Wintle A. G. (1975b) Effects of sample preparation on the thermoluminescence characteristics of calcite. Mod. Geol. 5, 165 167. Zimmerman J. (1971) The radiation-induced increase of thermoluminescence sensitivity of fired quartz. J. Phys. C: Solid State Phys. 4, 3277 329l.