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New approaches to retrospective dosimetry using cementitious building materials
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Radiation Measurements 37 (2003) 323 – 327 www.elsevier.com/locate/radmeas
New approaches to retrospective dosimetry using cementitious building materials H.Y. G$oksua;∗ , I.K. Baili*b , V.B. Mikhailikb b Luminescence
a GSF-Institut f ur Strahlenschutz, D-85764 Neuherberg, Germany Dosimetry Laboratory, University of Durham, South Road, Durham DH1 3LE, UK
Received 11 July 2002; received in revised form 6 November 2002; accepted 13 November 2002
Abstract The potential of cementitious building materials for use in retrospective dosimetry has been investigated by examination of three types of Portland cement, 4ne grain polymineral samples extracted from concrete and quartz inclusions extracted from ‘ready-mix’ mortar and samples of mortar applied to the external walls of buildings. The results of X-ray and optically stimulated luminescence measurements with samples of hydrated cement suggest that the emission associated with its presence in mortar, render and concrete is likely to be too weak for absorbed dose determinations of less than a few Gy. Measurements of the TL signal from quartz grains extracted from mortars indicate that a TL peak located at ∼170◦ C (5◦ s−1 ) and measured with coarse grains of diameter ¿ 350 m provide the most promising approach for dose assessments above 100 mGy. c 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction The methodology for the application of retrospective luminescence dosimetry using quartz from 4red building materials such as brick is now well established (Baili* et al., 2000), and it has been demonstrated that it is feasible to routinely determine an absorbed dose due to gamma radiation from fallout of ∼20 mGy in typical bricks that are less than 50 years old. Interest is now turning to the use of cementitious building materials such as mortar and concrete to enlarge the scope of application, and is one of the new challenges of retrospective dosimetry. The main problem in using building materials of this type is that, although cement is heat-treated during manufacture, the building materials in which they are incorporated also contain luminescent minerals in the form of aggregates (e.g. sand, crushed rock) that are neither heated during manufacture nor thoroughly optically bleached and may therefore lack full zeroing of the luminescence signal of interest. BHtter-Jensen et al. (2000) found that dose levels of a few
∗ Corresponding author Tel.: +49-89-3187-2765; fax: +49-893187-3363. E-mail address: [email protected] (H.Y. G$oksu).
Gy could be determined by applying an OSL single-grain technique with quartz extracted from mortar and a probabilistic dose distribution analysis. However, for general application to dose reconstruction for populations and individuals the capability to determine much lower levels of dose is needed, 100 mGy being an appropriate target, and in addition to single grain measurements, other approaches need to be investigated. Previously published results relevant to the study of cementitious materials also include work by Placido (1980), who used the partial resetting of the geological TL signal as a means of testing for 4re damage, and more recently Kitis et al. (1993) performed dosimetry using the TL of barites extracted from concrete shielding in a high-energy physics radiation. In the latter study TL glow peaks at 155 and 190◦ C were used for dose determinations since, due to their relatively short lifetimes, the geological TL in this temperature range was weak compared with that arising from arti4cial radiation. However, levels of accrued dose after 34 years of operation of the facility were very high, being in the region of 1 kGy. In this paper we describe an assessment of two approaches to the use of luminescent minerals in cementitious materials, one investigating the potential of luminescent minerals in cement which is 4red during manufacture to temperatures in excess of 1200◦ C, and the other exploring a similar approach
c 2003 Elsevier Science Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00005-2
H.Y. G oksu et al. / Radiation Measurements 37 (2003) 323 – 327
to that used by Kitis et al. (1993) using TL peaks associated with relatively shallow traps, but in our case with quartz. 2. Experimental details Three di*erent types of luminescence measurement were performed in the study: TL, OSL and X-ray stimulated luminescence (referred here to as XL). The TL and OSL measurements were performed using semi-automated TL-OSL readers (BHtter-Jensen, 1997; RisH-TLDA-10 or 12); TL glow curves were recorded using either Schott BG-12 or BG-38 optical 4lters in the detection system and OSL decay curves were obtained by using the standard broad band green-blue stimulation (450 –570 nm) and Hoya U-340 optical 4lters in the detection system. Absorbed doses were administered using 90 Sr= 90 Y beta sources calibrated against a standard photon source at the Secondary Standard Dosimetry Laboratory (SSDL) facilities in GSF (G$oksu et al., 1995). Measurements of XL were performed by irradiating samples located in a cryostat at room temperature with x-rays (Phillips PW1130 generator; 40 kV, 20 mA); the emission spectra of the XL were recorded using a monochromator (Oriel Corp., f/4.0; holographic grating) and photomultiplier detector (EMI 9635QA). Powder X-ray di*raction measurements were also performed with samples of cement using a Siemens D5000 powder di*ractometer. 3. Samples 3.1. Cement Three commonly available types of Portland cement (PC) were tested in both anhydrous and hydrated form: standard PC, sulfate-resisting PC and PC with a fuel-ash additive. The hydrated samples were prepared by mixing equal parts of cement and water, allowing it to harden for 3 days; subsequently thin (∼2 mm) slices were cut using a diamond saw. Fine grain cement fractions were also extracted from concrete by a combination of mechanical crushing, sieving and liquid settling procedures similar to those developed for 4ne-grain dating; the settling was performed in a 0.01N solution of sodium oxalate (Na2 C2 O4 ). 3.2. Quartz inclusions extracted from mortars Quartz grains were extracted from: (i) Mortar A, a ready-mix mortar commonly available in Germany, type HASI-MG-II, containing cement, chalk and coarsely ground rock, and collected from a construction site where it had been exposed to daylight for several days; (ii) Render B, recently prepared for repairing a rendered wall and (iii) Render C, a sample of 40 year-old render taken from the external wall of a building. Render in the context of this discussion is the term used to describe a mortar applied to the external wall of a building that when set provides a protective layer. Quartz inclusions (100 –350 m) were
200
Absorbed dose (Gy)
324
150 100 50 0 150
200 250 Temperature (˚C)
300
Fig. 1. Comparison of absorbed dose determined at 5◦ C intervals, obtained using quartz grains (140 –200 m) extracted from Mortar A: exposed for 8 months to daylight (closed circles) and exposed to light for a few days awaiting use on a construction site (open circles).
extracted from each type of sample by crushing, sieving, washing in 10% HCl to eliminate carbonates, and then etching in concentrated HF (40%) for 1 h; all luminescence measurements were performed with multi-grain aliquots. 4. Procedures and results 4.1. Mortar and renders Mortar A (ready-mix) collected from the construction site was further exposed to light for various periods up to 8 months. After each selected period of exposure a fraction was removed and, using quartz grains extracted from it, a TL regenerative procedure was applied to determine the apparent absorbed dose. The absorbed dose determined as a function of glow curve temperature is shown in Fig. 1 where the results for samples tested (140 –200 m grains) immediately and after 8 months of further light exposure to daylight are compared. For glow curve temperatures above 200◦ C it can be seen that the absorbed dose evaluated is in excess of several tens of Gy. It is also to be noted that at the time of preparation the grains tested were incompletely bleached since further exposure to light resulted in a reduction in the TL signal. Similar measurements were also performed with polymineral 4ne-grain (¡ 4 and 4 –10 m) and coarser (10 –40 m and 40 –90 m) fractions extracted from the same aliquots of Mortar A (Fig. 2). The 4ner grains exhibit relatively weak natural TL and have a weak response to laboratory irradiation. The absorbed dose determined using the 10 –90 m fraction, although approximately an order of magnitude lower than those obtained with quartz inclusions, nonetheless exceeded several Gy. Hence, for the mortar type tested, the results indicate that quartz and polymineral 4ne-grain fractions extracted from the mortar are unlikely to have been suOciently bleached for use in retrospective measurements in the required dose range. This may be partly due to the coating of quartz grains with 4ne particulates (e.g. cement) that strongly attenuates the
H.Y. G oksu et al. / Radiation Measurements 37 (2003) 323 – 327
80
0-4 µm 4-10 µm 10-40 µm 40-90 µm
Natural TL
4000
OSL intensity (a.u.)
Integrated TL (a.u.)
6000
325
2000 0
a 60
b 40
c
20 0
0.0
0.5 1.0 1.5 Absorbed dose (Gy)
2.0
Fig. 2. The radiation dose response and natural TL signal of polymineral grains obtained from Mortar A. Note: the 4ner grains have relatively weak natural TL and are not sensitive to administrated radiation dose.
0
b
200
c 100
OSL intensity (a.u.)
X-luminescence intensity (a.u.)
300
OSL intensity (a.u.)
1
a
80
100
Fig. 4. OSL dose response curve for three types of Portland cement (PC): (a) ordinary PC, (b) sulphate-resisting PC, (c) PC with fuel ash additive.
500
400
40 60 Dose (Gy)
20
0.8 0.6 0.4 0.2 100
0.1
0
10
150 200 250 o Temperature (C )
20 30 Time (sec)
40
50
Fig. 5. OSL decay curve from a cement sample (ordinary PC) after administration of a 90 Gy laboratory dose. Inset: variation of integrated OSL intensity with pulse annealing temperature.
d 0 300
350
400
450
500
Wavelength (nm) Fig. 3. X-ray induced luminescence spectra of hydrated Portland cement (PC) samples (a, b, c) and 4ne grain fraction extracted from concrete (d) measured at room temperature (U = 40 kV, I = 20 mA) (a) ordinary PC, (b) sulphate-resisting PC, (c) PC with fuel ash additive.
penetration of sunlight during preparation of the material prior to use. 4.2. Cement Both anhydrous and hydrated cements exhibited XL in the violet (Fig. 3a–c, peak position 370 –400 nm), and the intensities of the ordinary PC and sulfate-resisting PC are both weak, while that containing fuel-ash additive is even weaker. The spectra suggest that the hydration reaction does not a*ect the luminescence band shape and position, and that any change in the chemical bonds initiated by this process has no signi4cant inQuence on the immediate environment
of the recombination centers. The XL spectrum of the cement fraction extracted from a concrete slab (Fig. 3d) has a wide emission band with a maximum at longer wavelengths (∼410 nm) compared with that for hydrated cement. Because complex chemical changes are known to take place in hydrated cement on heating, thermal treatment was avoided and only optical stimulation was performed. Using the same experimental con4guration as that used with quartz, both unhydrated and hydrated samples were found to produce an OSL signal, albeit weak. The dose response curves (Fig. 4) were linear within the dose range investigated (to 90 Gy) and, as expected, these con4rms a negligible OSL signal accumulated prior to testing in the laboratory. These results indicate that an absorbed dose of the order of 1 Gy can be resolved, the level being restricted by low sensitivity. The shape of the OSL decay curve (Fig. 5) suggests that the kinetics are complex; on the basis of an estimated thermal activation energy of 0:9 eV obtained by pulse annealing measurements (inset to Fig. 5) the thermal stability is likely to be limited for long-term dosimetry. Apart from the question of thermal stability yet to be fully investigated, these characteristics fall short of the performance required for use at ∼100 mGy.
326
H.Y. G oksu et al. / Radiation Measurements 37 (2003) 323 – 327
400 300 200
3
100
2
0
1
10
20
30
40 o 2θ
50
60
TL intensity (a.u.)
Intensity (a.u.)
1000
CaCO3 (Na,K)AlSi3O8 SiO2
500
600 400 200 0 100
70
Fig. 6. XRD patterns of hydrated cement (1) and 4ne grain samples extracted from concrete foundations (2) and a concrete slab (3). The dotted lines indicate positions of di*raction peaks of hydrated cement.
(geological TL) 530 mGy 1160 mGy 2320 mGy
800
150
200
250
300
Temperature (˚C)
Fig. 7. The comparison of natural TL glow curve of the quartz grains ¿ 350–650 m with those obtained following laboratory irradiation and preheating at 100◦ C. Note: The 170◦ C TL peak can be seen in freshly irradiated samples (Mortar A).
4.3. Mineral aggregates in cementitious materials
4.4. Quartz inclusions The TL glow curve measured immediately following laboratory irradiation with quartz inclusions extracted from Mortar A and Render B contained peaks at ∼110◦ C and ∼170◦ C (heating rate, 5◦ C s−1 ) and a large geological signal extending beyond 200◦ C; pre-heating (heat to 100◦ C
2000
Integrated TL (a.u)
When cementitious materials are produced by blending cement with coarser aggregates, (e.g. sand in the case of mortar) they undergo a reaction with water, creating hydrates. During the hardening reaction, cement particles become tightly bonded with the aggregates to form a strong agglomerate. The similarity of the chemical properties of the cement derivatives and aggregates make separation by means of a simple chemical procedure problematic (Lee, 1970). Also, the use of separation techniques relying on differences in the physical properties of the constituents (e.g., size, density and shape) requires mechanical fragmentation of the cementitious aggregate that inevitably results a mixture of the constituents. Powder XRD patterns (Fig. 6, curves 2 and 3) obtained for two concrete samples reQect the mixed mineral composition of cementitious samples, dominated by peaks associated with calcium carbonate, feldspar, and quartz. The pattern obtained for samples of hydrated PC, on the other hand, reQect the formation of complex calcium silicates and aluminates (de la Torte et al., 2001) during the 4ring process and indicate no quartz remains (Fig. 6, curve 1). Although the OSL discussed above is unlikely to be associated with quartz, the spectral range of luminescence, as indicated by the XL spectra, unfortunately overlaps with quartz OSL (300 –400 nm). The application of acid treatments, sedimentation and centrifuging in di*erent solutions would increase the proportion of the cement-related fraction, but the selective measurement of luminescence associated with cement appears to be problematic within the scope of the experiments discussed here.
1000
A
B
0 0
500
1000
1500
2000
Absorbed dose (mGy)
Fig. 8. Evaluation of absorbed dose for Render C irradiated at SSDL with a calibration dose of 1:45 Gy using a regenerative multi-aliquot procedure and measurement of the 170◦ C TL peak. A indicates the reconstructed calibration (1500 ± 310 mGy); B indicates the background dose (170 ± 30 mGy).
at 2◦ C s−1 ) allowed the 170◦ C peak to be distinguished from the 110◦ C peak (see Fig. 7). The intensity of the 170◦ C TL peak was found to be greatest for quartz grains of diameter larger than ∼350 m. The 4rst glow curve measured (due to the geological dose) contains a rising TL signal between 150◦ C and 200◦ C, and this ‘background’ signal was equivalent to an absorbed dose of 100 ± 30 mGy and 120 ± 40 mGy in samples A and B, respectively. A (blind) test was performed with a block of render (Render C); part of the block was removed and separately irradiated with a gamma source at the GSF-SSDL, where the dose administered was 1450 ± 44 mGy. A multiple aliquot regenerative procedure (Baili* et al., 2000) was applied to determine the values of absorbed dose for quartz extracted from the ‘natural’ (DBG ) and gamma irradiated blocks (D + DBG ), estimates for which were 170 ± 30 mGy and 1500 ± 310 mGy, respectively, as indicated in the growth characteristic shown in Fig. 8. The recovered value of D was 1330 ± 310 mGy and this is in reasonable agreement within the experimental error limits of the administered dose.
H.Y. G oksu et al. / Radiation Measurements 37 (2003) 323 – 327
5. Discussion and conclusions On the basis of the standard types of cement examined in this study, the OSL sensitivity of hydrated cement appears to be insuOcient to enable it to be used for dosimetry at the 100 mGy level. Also, when incorporated within a cementitious material further problems arise concerning its separation and puri4cation from the abundant mineral aggregates such as quartz and feldspar that possess signi4cantly greater luminescence sensitivity and, unlike cement, have not been thermally treated before incorporation in the building material. However, the 4ne grain fraction extracted from cementitious material also contains identi4able amounts of calcium carbonate that is formed during the hydration process (Lee, 1970). This luminescent component is potentially of interest and is currently under investigation, although it should be noted that calcium carbonates in cementitious materials are known to undergo time-dependent changes during the lifetime of a structure in which they are incorporated. The results of the examination of coarse quartz grains extracted from the ‘ready-mix’ mortar suggest that the exposure of the material to light either during factory preparation or on site is unlikely to result in a degree of zeroing of the TL signal by optical bleaching in the temperature range 200 –300◦ C of the glow curve. However, an ‘intermediate’ TL peak located at 170◦ C(5◦ C s−1 ) is a more promising candidate for dosimetry since the background dose associated with the residual geological TL intensity for ‘virgin’ material is ∼100 mGy for the samples tested. This peak is similar to that identi4ed by Petrov and Baili* (1997) in synthetic quartz and located at 150 ± 5◦ C (1◦ C s−1 ) within a complex of three peaks that had been generally associated with a singular 210◦ C TL peak. Based on the values of E and s determined by the fractional glow curve technique (E = 1:32 ± 0:03 eV; s = 7:3 × 1014 s−1 ) the lifetime for this peak at an time-averaged temperature of 15◦ C is predicted to be 5:6+13 −4 years, and this accounts for the favourably low ‘background’ dose. Since the lifetime is clearly a crucial parameter for retrospective dosimetry, both in terms of the level of background dose which governed by thermal fading and the survival of the trapped charge following arti4cial irradiation (the subject of a retrospective investigation), a more accurate determination of the mean lifetime is required. In summary, we conclude that in building materials such as mortar, render and concrete the luminescence signal due to the (4red) cement fraction is likely to be too weak for absorbed dose determinations of less than a few Gy. Using multiple grain aliquots, the geological TL signal from quartz extracted from the unheated building materials tested does not appear to be adequately reduced by optical bleaching during the production process. In three types of mortar studied, extracted quartz inclusions larger than 350 m provided
327
a more promising material for investigation, based on the measurements of a TL peak located at ∼170◦ C (5◦ s−1 ). The thermal stability of this peak, which will determine the chronological range within which retrospective measurements can be made, and the reduction and more accurate assessment of the relict geological TL within the range of the 170◦ C TL peak are both the subject of further investigation. Acknowledgements The work described in this paper forms part of a collaborative project with: CIEMAT, Madrid, The University of Durham, GSF Neuherberg, Risoe National Laboratory, Denmark, and the University of Helsinki under contract FIGD-CT-2000-00094 with the European Commission. Work at GSF was also partially supported by Bundesministers f$ur Umwelt, Naturschutz und Reaktorsicherheit (BMU) under a contract with Bundesamt f$ur StrahlenschutzStSch 4225. The work at Durham was also partly supported by the University of Durham and the donation of cement samples by Blue Circle (Weardale) is gratefully acknowledged.
References Baili*, I.K., Botter-Jensen, L., Correcher, V., Delgado, A., G$oksu, H.Y., Jungner, H., Petrov, S., 2000. Absorbed dose evaluations in retrospective dosimetry: methodological developments using quartz. Rad. Meas. 32, 609–613. BHtter-Jensen, L., 1997. Luminescence techniques: Instrumentation and method. Rad. Meas. 27, 749–768. BHtter-Jensen, L., Solongo, S., Murray, A.S., Banerjee, D., Jungner, H., 2000. Using the OSL single—aliquot regenerative—dose protocol with quartz extracted from building materials in retrospective dosimetry. Rad. Meas. 32, 841–845. de la Torte, A.G., Cabeza, A., Calvente, A., Bruque, S., Aranda, M.A.G., 2001. Full phase analysis of Portland clinker by penetrating synchrotron powder di*raction. Anal. Chem. 73, 151–156. G$oksu, H.Y., Baili*, I.K., BHtter-Jensen, L., H$utt, G., 1995. Interlaboratory beta source calibration using TL and OSL on natural quartz. Rad. Meas. 24, 479–483. Kitis, G., Charalambous, S., Tuyn, J.W., 1993. Thermoluminescence response of heavy concrete used in the shielding of the CERN 600 MeV synchrocyclotron. Rad. Protect. Dos. 47, 589–593. Lee, F.M., 1970. The Chemistry of Cement and Concrete. Edward Arnold, London. Petrov, S.A., Baili*, I.K., 1997. Determination of trap depth associated with TL peaks in synthetic quartz (350 –550 K). Rad. Meas. 27, 185–191. Placido, F., 1980. Thermoluminescence test for 4re-damaged concrete. Mag. Concrete Res. 32, 112–116.
Radiation Measurements 37 (2003) 323 – 327 www.elsevier.com/locate/radmeas
New approaches to retrospective dosimetry using cementitious building materials H.Y. G$oksua;∗ , I.K. Baili*b , V.B. Mikhailikb b Luminescence
a GSF-Institut f ur Strahlenschutz, D-85764 Neuherberg, Germany Dosimetry Laboratory, University of Durham, South Road, Durham DH1 3LE, UK
Received 11 July 2002; received in revised form 6 November 2002; accepted 13 November 2002
Abstract The potential of cementitious building materials for use in retrospective dosimetry has been investigated by examination of three types of Portland cement, 4ne grain polymineral samples extracted from concrete and quartz inclusions extracted from ‘ready-mix’ mortar and samples of mortar applied to the external walls of buildings. The results of X-ray and optically stimulated luminescence measurements with samples of hydrated cement suggest that the emission associated with its presence in mortar, render and concrete is likely to be too weak for absorbed dose determinations of less than a few Gy. Measurements of the TL signal from quartz grains extracted from mortars indicate that a TL peak located at ∼170◦ C (5◦ s−1 ) and measured with coarse grains of diameter ¿ 350 m provide the most promising approach for dose assessments above 100 mGy. c 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction The methodology for the application of retrospective luminescence dosimetry using quartz from 4red building materials such as brick is now well established (Baili* et al., 2000), and it has been demonstrated that it is feasible to routinely determine an absorbed dose due to gamma radiation from fallout of ∼20 mGy in typical bricks that are less than 50 years old. Interest is now turning to the use of cementitious building materials such as mortar and concrete to enlarge the scope of application, and is one of the new challenges of retrospective dosimetry. The main problem in using building materials of this type is that, although cement is heat-treated during manufacture, the building materials in which they are incorporated also contain luminescent minerals in the form of aggregates (e.g. sand, crushed rock) that are neither heated during manufacture nor thoroughly optically bleached and may therefore lack full zeroing of the luminescence signal of interest. BHtter-Jensen et al. (2000) found that dose levels of a few
∗ Corresponding author Tel.: +49-89-3187-2765; fax: +49-893187-3363. E-mail address: [email protected] (H.Y. G$oksu).
Gy could be determined by applying an OSL single-grain technique with quartz extracted from mortar and a probabilistic dose distribution analysis. However, for general application to dose reconstruction for populations and individuals the capability to determine much lower levels of dose is needed, 100 mGy being an appropriate target, and in addition to single grain measurements, other approaches need to be investigated. Previously published results relevant to the study of cementitious materials also include work by Placido (1980), who used the partial resetting of the geological TL signal as a means of testing for 4re damage, and more recently Kitis et al. (1993) performed dosimetry using the TL of barites extracted from concrete shielding in a high-energy physics radiation. In the latter study TL glow peaks at 155 and 190◦ C were used for dose determinations since, due to their relatively short lifetimes, the geological TL in this temperature range was weak compared with that arising from arti4cial radiation. However, levels of accrued dose after 34 years of operation of the facility were very high, being in the region of 1 kGy. In this paper we describe an assessment of two approaches to the use of luminescent minerals in cementitious materials, one investigating the potential of luminescent minerals in cement which is 4red during manufacture to temperatures in excess of 1200◦ C, and the other exploring a similar approach
c 2003 Elsevier Science Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00005-2
H.Y. G oksu et al. / Radiation Measurements 37 (2003) 323 – 327
to that used by Kitis et al. (1993) using TL peaks associated with relatively shallow traps, but in our case with quartz. 2. Experimental details Three di*erent types of luminescence measurement were performed in the study: TL, OSL and X-ray stimulated luminescence (referred here to as XL). The TL and OSL measurements were performed using semi-automated TL-OSL readers (BHtter-Jensen, 1997; RisH-TLDA-10 or 12); TL glow curves were recorded using either Schott BG-12 or BG-38 optical 4lters in the detection system and OSL decay curves were obtained by using the standard broad band green-blue stimulation (450 –570 nm) and Hoya U-340 optical 4lters in the detection system. Absorbed doses were administered using 90 Sr= 90 Y beta sources calibrated against a standard photon source at the Secondary Standard Dosimetry Laboratory (SSDL) facilities in GSF (G$oksu et al., 1995). Measurements of XL were performed by irradiating samples located in a cryostat at room temperature with x-rays (Phillips PW1130 generator; 40 kV, 20 mA); the emission spectra of the XL were recorded using a monochromator (Oriel Corp., f/4.0; holographic grating) and photomultiplier detector (EMI 9635QA). Powder X-ray di*raction measurements were also performed with samples of cement using a Siemens D5000 powder di*ractometer. 3. Samples 3.1. Cement Three commonly available types of Portland cement (PC) were tested in both anhydrous and hydrated form: standard PC, sulfate-resisting PC and PC with a fuel-ash additive. The hydrated samples were prepared by mixing equal parts of cement and water, allowing it to harden for 3 days; subsequently thin (∼2 mm) slices were cut using a diamond saw. Fine grain cement fractions were also extracted from concrete by a combination of mechanical crushing, sieving and liquid settling procedures similar to those developed for 4ne-grain dating; the settling was performed in a 0.01N solution of sodium oxalate (Na2 C2 O4 ). 3.2. Quartz inclusions extracted from mortars Quartz grains were extracted from: (i) Mortar A, a ready-mix mortar commonly available in Germany, type HASI-MG-II, containing cement, chalk and coarsely ground rock, and collected from a construction site where it had been exposed to daylight for several days; (ii) Render B, recently prepared for repairing a rendered wall and (iii) Render C, a sample of 40 year-old render taken from the external wall of a building. Render in the context of this discussion is the term used to describe a mortar applied to the external wall of a building that when set provides a protective layer. Quartz inclusions (100 –350 m) were
200
Absorbed dose (Gy)
324
150 100 50 0 150
200 250 Temperature (˚C)
300
Fig. 1. Comparison of absorbed dose determined at 5◦ C intervals, obtained using quartz grains (140 –200 m) extracted from Mortar A: exposed for 8 months to daylight (closed circles) and exposed to light for a few days awaiting use on a construction site (open circles).
extracted from each type of sample by crushing, sieving, washing in 10% HCl to eliminate carbonates, and then etching in concentrated HF (40%) for 1 h; all luminescence measurements were performed with multi-grain aliquots. 4. Procedures and results 4.1. Mortar and renders Mortar A (ready-mix) collected from the construction site was further exposed to light for various periods up to 8 months. After each selected period of exposure a fraction was removed and, using quartz grains extracted from it, a TL regenerative procedure was applied to determine the apparent absorbed dose. The absorbed dose determined as a function of glow curve temperature is shown in Fig. 1 where the results for samples tested (140 –200 m grains) immediately and after 8 months of further light exposure to daylight are compared. For glow curve temperatures above 200◦ C it can be seen that the absorbed dose evaluated is in excess of several tens of Gy. It is also to be noted that at the time of preparation the grains tested were incompletely bleached since further exposure to light resulted in a reduction in the TL signal. Similar measurements were also performed with polymineral 4ne-grain (¡ 4 and 4 –10 m) and coarser (10 –40 m and 40 –90 m) fractions extracted from the same aliquots of Mortar A (Fig. 2). The 4ner grains exhibit relatively weak natural TL and have a weak response to laboratory irradiation. The absorbed dose determined using the 10 –90 m fraction, although approximately an order of magnitude lower than those obtained with quartz inclusions, nonetheless exceeded several Gy. Hence, for the mortar type tested, the results indicate that quartz and polymineral 4ne-grain fractions extracted from the mortar are unlikely to have been suOciently bleached for use in retrospective measurements in the required dose range. This may be partly due to the coating of quartz grains with 4ne particulates (e.g. cement) that strongly attenuates the
H.Y. G oksu et al. / Radiation Measurements 37 (2003) 323 – 327
80
0-4 µm 4-10 µm 10-40 µm 40-90 µm
Natural TL
4000
OSL intensity (a.u.)
Integrated TL (a.u.)
6000
325
2000 0
a 60
b 40
c
20 0
0.0
0.5 1.0 1.5 Absorbed dose (Gy)
2.0
Fig. 2. The radiation dose response and natural TL signal of polymineral grains obtained from Mortar A. Note: the 4ner grains have relatively weak natural TL and are not sensitive to administrated radiation dose.
0
b
200
c 100
OSL intensity (a.u.)
X-luminescence intensity (a.u.)
300
OSL intensity (a.u.)
1
a
80
100
Fig. 4. OSL dose response curve for three types of Portland cement (PC): (a) ordinary PC, (b) sulphate-resisting PC, (c) PC with fuel ash additive.
500
400
40 60 Dose (Gy)
20
0.8 0.6 0.4 0.2 100
0.1
0
10
150 200 250 o Temperature (C )
20 30 Time (sec)
40
50
Fig. 5. OSL decay curve from a cement sample (ordinary PC) after administration of a 90 Gy laboratory dose. Inset: variation of integrated OSL intensity with pulse annealing temperature.
d 0 300
350
400
450
500
Wavelength (nm) Fig. 3. X-ray induced luminescence spectra of hydrated Portland cement (PC) samples (a, b, c) and 4ne grain fraction extracted from concrete (d) measured at room temperature (U = 40 kV, I = 20 mA) (a) ordinary PC, (b) sulphate-resisting PC, (c) PC with fuel ash additive.
penetration of sunlight during preparation of the material prior to use. 4.2. Cement Both anhydrous and hydrated cements exhibited XL in the violet (Fig. 3a–c, peak position 370 –400 nm), and the intensities of the ordinary PC and sulfate-resisting PC are both weak, while that containing fuel-ash additive is even weaker. The spectra suggest that the hydration reaction does not a*ect the luminescence band shape and position, and that any change in the chemical bonds initiated by this process has no signi4cant inQuence on the immediate environment
of the recombination centers. The XL spectrum of the cement fraction extracted from a concrete slab (Fig. 3d) has a wide emission band with a maximum at longer wavelengths (∼410 nm) compared with that for hydrated cement. Because complex chemical changes are known to take place in hydrated cement on heating, thermal treatment was avoided and only optical stimulation was performed. Using the same experimental con4guration as that used with quartz, both unhydrated and hydrated samples were found to produce an OSL signal, albeit weak. The dose response curves (Fig. 4) were linear within the dose range investigated (to 90 Gy) and, as expected, these con4rms a negligible OSL signal accumulated prior to testing in the laboratory. These results indicate that an absorbed dose of the order of 1 Gy can be resolved, the level being restricted by low sensitivity. The shape of the OSL decay curve (Fig. 5) suggests that the kinetics are complex; on the basis of an estimated thermal activation energy of 0:9 eV obtained by pulse annealing measurements (inset to Fig. 5) the thermal stability is likely to be limited for long-term dosimetry. Apart from the question of thermal stability yet to be fully investigated, these characteristics fall short of the performance required for use at ∼100 mGy.
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400 300 200
3
100
2
0
1
10
20
30
40 o 2θ
50
60
TL intensity (a.u.)
Intensity (a.u.)
1000
CaCO3 (Na,K)AlSi3O8 SiO2
500
600 400 200 0 100
70
Fig. 6. XRD patterns of hydrated cement (1) and 4ne grain samples extracted from concrete foundations (2) and a concrete slab (3). The dotted lines indicate positions of di*raction peaks of hydrated cement.
(geological TL) 530 mGy 1160 mGy 2320 mGy
800
150
200
250
300
Temperature (˚C)
Fig. 7. The comparison of natural TL glow curve of the quartz grains ¿ 350–650 m with those obtained following laboratory irradiation and preheating at 100◦ C. Note: The 170◦ C TL peak can be seen in freshly irradiated samples (Mortar A).
4.3. Mineral aggregates in cementitious materials
4.4. Quartz inclusions The TL glow curve measured immediately following laboratory irradiation with quartz inclusions extracted from Mortar A and Render B contained peaks at ∼110◦ C and ∼170◦ C (heating rate, 5◦ C s−1 ) and a large geological signal extending beyond 200◦ C; pre-heating (heat to 100◦ C
2000
Integrated TL (a.u)
When cementitious materials are produced by blending cement with coarser aggregates, (e.g. sand in the case of mortar) they undergo a reaction with water, creating hydrates. During the hardening reaction, cement particles become tightly bonded with the aggregates to form a strong agglomerate. The similarity of the chemical properties of the cement derivatives and aggregates make separation by means of a simple chemical procedure problematic (Lee, 1970). Also, the use of separation techniques relying on differences in the physical properties of the constituents (e.g., size, density and shape) requires mechanical fragmentation of the cementitious aggregate that inevitably results a mixture of the constituents. Powder XRD patterns (Fig. 6, curves 2 and 3) obtained for two concrete samples reQect the mixed mineral composition of cementitious samples, dominated by peaks associated with calcium carbonate, feldspar, and quartz. The pattern obtained for samples of hydrated PC, on the other hand, reQect the formation of complex calcium silicates and aluminates (de la Torte et al., 2001) during the 4ring process and indicate no quartz remains (Fig. 6, curve 1). Although the OSL discussed above is unlikely to be associated with quartz, the spectral range of luminescence, as indicated by the XL spectra, unfortunately overlaps with quartz OSL (300 –400 nm). The application of acid treatments, sedimentation and centrifuging in di*erent solutions would increase the proportion of the cement-related fraction, but the selective measurement of luminescence associated with cement appears to be problematic within the scope of the experiments discussed here.
1000
A
B
0 0
500
1000
1500
2000
Absorbed dose (mGy)
Fig. 8. Evaluation of absorbed dose for Render C irradiated at SSDL with a calibration dose of 1:45 Gy using a regenerative multi-aliquot procedure and measurement of the 170◦ C TL peak. A indicates the reconstructed calibration (1500 ± 310 mGy); B indicates the background dose (170 ± 30 mGy).
at 2◦ C s−1 ) allowed the 170◦ C peak to be distinguished from the 110◦ C peak (see Fig. 7). The intensity of the 170◦ C TL peak was found to be greatest for quartz grains of diameter larger than ∼350 m. The 4rst glow curve measured (due to the geological dose) contains a rising TL signal between 150◦ C and 200◦ C, and this ‘background’ signal was equivalent to an absorbed dose of 100 ± 30 mGy and 120 ± 40 mGy in samples A and B, respectively. A (blind) test was performed with a block of render (Render C); part of the block was removed and separately irradiated with a gamma source at the GSF-SSDL, where the dose administered was 1450 ± 44 mGy. A multiple aliquot regenerative procedure (Baili* et al., 2000) was applied to determine the values of absorbed dose for quartz extracted from the ‘natural’ (DBG ) and gamma irradiated blocks (D + DBG ), estimates for which were 170 ± 30 mGy and 1500 ± 310 mGy, respectively, as indicated in the growth characteristic shown in Fig. 8. The recovered value of D was 1330 ± 310 mGy and this is in reasonable agreement within the experimental error limits of the administered dose.
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5. Discussion and conclusions On the basis of the standard types of cement examined in this study, the OSL sensitivity of hydrated cement appears to be insuOcient to enable it to be used for dosimetry at the 100 mGy level. Also, when incorporated within a cementitious material further problems arise concerning its separation and puri4cation from the abundant mineral aggregates such as quartz and feldspar that possess signi4cantly greater luminescence sensitivity and, unlike cement, have not been thermally treated before incorporation in the building material. However, the 4ne grain fraction extracted from cementitious material also contains identi4able amounts of calcium carbonate that is formed during the hydration process (Lee, 1970). This luminescent component is potentially of interest and is currently under investigation, although it should be noted that calcium carbonates in cementitious materials are known to undergo time-dependent changes during the lifetime of a structure in which they are incorporated. The results of the examination of coarse quartz grains extracted from the ‘ready-mix’ mortar suggest that the exposure of the material to light either during factory preparation or on site is unlikely to result in a degree of zeroing of the TL signal by optical bleaching in the temperature range 200 –300◦ C of the glow curve. However, an ‘intermediate’ TL peak located at 170◦ C(5◦ C s−1 ) is a more promising candidate for dosimetry since the background dose associated with the residual geological TL intensity for ‘virgin’ material is ∼100 mGy for the samples tested. This peak is similar to that identi4ed by Petrov and Baili* (1997) in synthetic quartz and located at 150 ± 5◦ C (1◦ C s−1 ) within a complex of three peaks that had been generally associated with a singular 210◦ C TL peak. Based on the values of E and s determined by the fractional glow curve technique (E = 1:32 ± 0:03 eV; s = 7:3 × 1014 s−1 ) the lifetime for this peak at an time-averaged temperature of 15◦ C is predicted to be 5:6+13 −4 years, and this accounts for the favourably low ‘background’ dose. Since the lifetime is clearly a crucial parameter for retrospective dosimetry, both in terms of the level of background dose which governed by thermal fading and the survival of the trapped charge following arti4cial irradiation (the subject of a retrospective investigation), a more accurate determination of the mean lifetime is required. In summary, we conclude that in building materials such as mortar, render and concrete the luminescence signal due to the (4red) cement fraction is likely to be too weak for absorbed dose determinations of less than a few Gy. Using multiple grain aliquots, the geological TL signal from quartz extracted from the unheated building materials tested does not appear to be adequately reduced by optical bleaching during the production process. In three types of mortar studied, extracted quartz inclusions larger than 350 m provided
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a more promising material for investigation, based on the measurements of a TL peak located at ∼170◦ C (5◦ s−1 ). The thermal stability of this peak, which will determine the chronological range within which retrospective measurements can be made, and the reduction and more accurate assessment of the relict geological TL within the range of the 170◦ C TL peak are both the subject of further investigation. Acknowledgements The work described in this paper forms part of a collaborative project with: CIEMAT, Madrid, The University of Durham, GSF Neuherberg, Risoe National Laboratory, Denmark, and the University of Helsinki under contract FIGD-CT-2000-00094 with the European Commission. Work at GSF was also partially supported by Bundesministers f$ur Umwelt, Naturschutz und Reaktorsicherheit (BMU) under a contract with Bundesamt f$ur StrahlenschutzStSch 4225. The work at Durham was also partly supported by the University of Durham and the donation of cement samples by Blue Circle (Weardale) is gratefully acknowledged.
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