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Luminescence dating of Somero sacristy, SW Finland using the 210°C TL peak of quartz
Quaternary Science Reviews 20 (2001) 773}777
Luminescence dating of Somero sacristy, SW Finland using the 2103C TL peak of quartz夽 G. HuK tt q, H.Y. GoK ksu, I. Jaek *, M. Hiekkanen Institute of Geology, Tallinn Technical University, Estonia ave. 7, 10143 Tallinn, Estonia GSF Forshungszentrum, ISS Ingolstadter Landstr.1, D-85764 Neuherberg, Germany National Board of Antiquities, P.O. Box 187, FIN-00171 Helsinki, Finland
Abstract The 2103C thermoluminescence (TL) peak of quartz, extracted from brick fragments of Somero sacristy (Finland), which were partly elements of construction, were used for the measurement of archaeological dose for the purpose of dating. The annual gamma dose-rate with cosmic component and beta dose rate of bricks were measured using two types of TLD detectors (AL O }C) inserted in situ and in laboratory, respectively. The improvements in the precision of the TL measurements, components of annual dose assessment are described, and the results of TL age determination are discussed and compared with the archaeologically expected age. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction There have been considerable improvements in accumulated radiation dose measurements using the 2103C peak of quartz in recent years due to the attempts made to reconstruct radiation doses in the areas a!ected with radioactive fall outs (Higashimura et al., 1963; Haskell, 1993; HuK tt et al., 1993; Stoneham et al., 1993; GoK ksu et al., 1996; Baili!, 1997, 1999; Bougrov, 1998; Baili! et al., 2000). The reproducibility of the TL measurements has been reduced to 2}4% on the dose levels in the order of 100 mGy, due to the available automatic equipments (Markey et al., 1996), improved understanding of 2103C peak of quartz (Petrov and Baili!, 1997; Baili! and Petrov, 1999) and the possibility to use in situ highly sensitive dosimeters for annual dose measurements and beta dose rate measurements (GoK ksu et al., 1999). The 2103C peak is usually not used for archaeological studies. The reason is the relatively short lifetime of the corresponding traps, found not monoenergetic, which is estimated to range from 750 to 2000 years at 203C (Petrov and Baili!, 1997). Using computer glow curve deconvolution, Delgado (private communication) separated the peak at 2103C and estimated the mean lifetime of
夽
Paper published in December 2000. Deceased. * Corresponding author. Tel.: #372-6720078. q
charge carriers on the associated trap as 450}500 years at 153C. Considering the lower average annual ambient temperature (#4.53C) in this region of Finland (Atlas of Finland, Climate, 1987), we were encouraged to use the 2103C peak for the age determination of 500-year-old bricks. At this temperature, the expected lifetime of the charge carriers following our rough assessments would be about a factor of 10}15 higher than the above-mentioned values.
2. Short history of Somero sacristy (SW Finland) Except for the As land Islands, stone churches became economically feasible in Finland only in the 1420 s, and continued to be until the 1550 s, when the economic basis for church building in stone ceased to exist. All of the church projects planned between the 1420 s and 1480 s were carried out successfully, while those between the 1480 s and 1550 s were mostly left un"nished because of the late medieval economical and political crisis of Sweden of which Finland was a part. The sacristy of Somero was built as a "rst step towards a stone church, which never was accomplished because of the reasons above. According to the architectural and technical elements as well as historical evidence, the sacristy was built between the years 1480 and 1560, most likely in the 1490 s or around the 1500 s. As there was no wood
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 1 7 - 2
774
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
needed for dendrochronological dating, an attempt was made to de"ne the age of Somero sacristy by luminescence methods using bricks as a study object.
3. Sampling and sample preparations The exterior part of the sacristy is not original, but inside the building some parts of the gable and vault are well preserved. The original walls of the lower part of the building are made from grey stone, but the vault and the north gable are partly from bricks. The sampling took place in the attic of the building. The brick fragments were carefully removed from the north gable at di!erent heights from the vault and from the vault itself. The colour of bricks from di!erent sampling places was di!erent. So, the brick from the vault (sample No. 2) was greyish, while the bricks from the gable (sample No. 1,4) were orange and (sample No. 3) dark orange. This was apparently caused by the di!erent processes of manufacture. All brick fragments were treated in the same way to extract quartz: E E E E
removal of outer 3 mm layer, crushing by press and agate pestle, removal of "ne grains, step-by-step etching by HF of di!erent concentration in ultrasonic bath for 3 min: 5%, 10%, 20% and for 40 min}40%, E washing in distilled water and drying at room temperature after every etching procedure and E sieving to separate quartz grains in the span 125}200 lm. All procedures were performed in dim red light.
4. Equipment The thermoluminescence measurements using peak at 2103C and 3103C were carried out in an automated TL-OSL readers (RISO TLD-DA 12 and TL-DA 10) using two di!erent types of "lters (BG-12 (Schott) or U-340 (Hoya). Thermoluminescence measurements were performed by heating the samples to 4003C at a rate of 23C/s and a peak maximum was obtained around 2003C. The irradiation was carried out in situ with Sr- beta source, which was calibrated with respect to Secondary Standard Dosimetry Laboratory (SSDL) facilities in GSF for 140}200 lm grain size of quartz. The pre-dose technique, additive dose version, (Fleming, 1973) was also applied. The pre-dose measurements and gamma annual dose rates using AL O were per formed by TL-OSL reader designed in Tallinn.
5. Methodology, results and discussion The principle of age determination using the luminescence techniques can be found in various books (e.g. Aitken, 1985, 1990). Furthermore, in recent years several laboratory protocols, which were developed for the evaluation of accumulated archaeological doses, are summarized in review papers by Wintle (1997). The age determination using quartz or feldspars incorporated in to bricks is based on the following equation: ¹"D /D , (1) where ¹ is the age of a brick in years, D the archae ological (accumulated) dose due to natural radiation measured by luminescence techniques in (Gy), D the annual dose rate, measured in (Gy/yr). D "D #D #D #D , (2) a b c
where D is the contribution of the a-radiation of the a uranium and thorium in the brick (mGy/yr), D the b internal beta-particle dose rate due to the uranium, thorium and potassium content in the brick (mGy/yr), D the c dose rate due to c-radiation of uranium, thorium and potassium in the brick and in the environment (mGy/yr) and D the dose rate due to cosmic rays (mGy/yr). 6. The accumulated absorbed dose measurement using TL of quartz, extracted from bricks (Dac ) In this study regeneration and additive dose techniques for TL peaks at 2103C and 3103C as well as pre-dose sensitization of 1103C TL peak were used for the archaeological dose reconstruction. The "rst step of this study is to con"rm experimentally the fact that the bricks were heated to a su$ciently high temperature during their manufacture to bleach the geologically accumulated radiation dose. This is achieved applying the plateau test on samples, heated to 4003C (Figs. 1 and 2). It is observed that Tl glow curves consist of two peaks with maximums at 2103C and 3003C. The presence of plateau in the high-temperature region assured that the samples had been heated at su$ciently high temperature during their manufacture. The archaeological doses are separately obtained for 2103C and 3103C peaks by the additive dose-technique integrating the lightsum in the span of 210}2403C and 310}3403C. As is seen from Fig. 3, the archaeological dose for 2103C D "2.39$0.58 mGy is slightly higher than D "1.76$0.4 mGy. Due to the fact, that in the addi tive dose method the archaeological dose is found by the extrapolation of the growth curves, the obtained value has a large uncertainty. Since the archaeological doses measurements using both peaks are found to be the same
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
Fig. 1. TL glow curves of quartz: (1) Natural TL, (2) Natural #0.63 Gy, (3) Natural #2.53 Gy.
Fig. 2. Plateau test using additive dose glow curves for quartz extracted from sample 3. The preheat: 1403C for 50 s.
775
avoid the sensitization, have been established six aliquots of quartz are heated to 3003C. The archaeological dose is then calculated for each aliquot individually increasing the regeneration doses. The four bricks, which were used in these measurements, di!er both in appearance and TL sensitivity. The sample-to-sample variation for samples 1 and 4 was much smaller than for samples 2 and 3. Therefore, the archaeological doses of samples 1}4 could be assessed within 2}4% accuracy, while in the case of samples 2}3 the accuracy was 10%. The latter samples contained some bright grains, which were detected even under di!erent types of "lter. The values of archaeological doses obtained by the regeneration technique, which provides smaller error, were used for the age calculation. These results are presented in Table 1. Our attempts to apply the pre-dose technique (Fleming, 1973) were not fruitful, due to the problem of dose}response saturation: the intensity of the TL signal, measured for the natural samples was close to the saturation level. Following Aitken (1985), in the case of geological, unheated samples the thermoactivated curve (TAC) is almost #at and the ratio of initial (S ) and sensibilized (S ) sensitivities of the TL signal to test dose is rather low. In our case, S }S was about 2000 (a bit less for the greyish sample No. 2). So, this high value of the sensitivity change can also be used as a con"rmation of su$cient heating of bricks.
7. Assessment of annual dose (Da ) 7.1. Gamma and cosmic dose measurements
Fig. 3. Dose}response curves for 2103C (empty) and for 3103C (full). The accumulated doses, obtained using extrapolation of dose growth curves for both peaks, were within the experimental errors. Because the sample to sample variation is about 10%, additive dose usually gives a higher experimental error. Therefore, the "nal analysis is based only on 2103C peak using multi aliquots regeneration.
within the experimental errors, the peak 2103C can be used for the assessment of archaeological dose. Therefore, the regeneration technique using peak at 2103C was applied to reduce the uncertainty of the measurements. Once the measurement parameters: preheat temperature, and optimum temperature of measurements to
From Eq. (1) it follows that the calculation of age requires the assessment of annual dose rate due to the alpha-, beta-, gamma- and cosmic components of the natural radiation dose rate in the samples. In this study we have used etched quartz grains (140}200 lm) and, therefore, the alpha component could be ignored. The gamma#cosmic components of the dose rate were measured in situ (Aitken, 1985; HuK tt et al., 1982) using TLD Al O : C detectors (Akselrod et al., 1990; B+tter-Jensen et al., 1997). In usual practice, TLD detectors must be left in situ for 1 yr to take into account the absorbed dose variation due to seasonal changes. The water content of the samples and their environment may introduce variations in the order of $30% on annual dose calculations. Since the sampling took place inside the building, these variations were assumed to be minimum. Therefore, the duration of the exposure of the dosimeters was kept shorter than a year (4 months). The TLD detectors were annealed to 5003C and placed in an aluminum container (length 13 mm, thickness
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
776
Table 1 The weighted value of age was calculated following the formula: x"[(1/*x )Hx #(1/*x )Hx #2.]/[(1/*x )#(1/*x )#2]#1/[(1/*x )#(1/*x )#2.] . Sample no. Locations
D (mGy/yr)
D b (mGy/yr)
D c> (mGy/yr)
Age (years)
C C C C
3.54$0.15 1.94$0.31 2.4$0.28 2.8$0.05
4.51$0.10 3.18$0.12 3.97$0.49 3.11$0.05
1.54$0.01 1.36$0.01 1.41$0.01 1.25$0.01
586$44 430$25 450$45 640$40
1 2 3 4
Lower part of gable Vault Upper part of gable Lower part of gable
2.5 mm, diameter 10 mm). Each container, "lled with 3 TLD detectors was inserted in the sampling place for 4 months. All procedures were performed in a dark room. After 4 months, the detectors were collected and measured in the laboratory. The procedure of measurements includes a pre-heat at 1353C with a heating rate of 53C/s. The informative signal was the result of integrating in the span of 1003C around TL maximum at 2173C. Every detector was calibrated by c-source Cs. The TL signal intensities were intercalibrated to radiation dose using a secondary standard Co c-source (GSF, MuK nchen). The detectors were calibrated in the same containers, in which they were kept in situ. 7.2. b-dose measurement The D dose in the bricks was measured using thin b layer a-AL O -C detectors (Akselrod et al., 1996; GoK ksu et al., 1999). Luminescence emission was detected using a combination of blue transmitting Corning C 7}59 and Chance-Pilkington heat absorption (HA3) "lters. Thermoluminescence measurements were performed by heating of the sample to 4003C at a rate of 23C/s. The in-built radiation source was removed during the measurements. The dosimeters were annealed (4003C at a rate of 23C/s) "ve times before they were used for routine measurements. The TL signal intensities in the region of the Tl glow curve peak at 2003C were calibrated to the radiation dose using a secondary standard Sr/Y b-source (dose rate: 87.3 lGy/min). Plastic sample holders (diameter 25 mm, height 8 mm) originally manufactured for sample placements for the GM counter system were developed at Ris+ (B+tterJensen and Meidahl, 1988). Two grams of powder were weighed and placed into sample holders. For most of the samples this amount completely "lls the volume of the sample holder so that the sample is always `beta thicka. The open surface of the sample holder was covered with a mylar sheet (density 1.0 mg/cm) and "xed with an external plastic ring. The samples were inserted into a lead-shield (wall thickness &5 cm) using a plastic
Age Weighted (years)
Age Mean (years)
Age Predicted (years)
486$19
526$30
520-440
sample tray that can carry "ve sample holders. The sample-to-sample distance (measured from centre to centre) was 5 cm so that cross talk between the samples is minimised. The thin-layer TL detectors were exposed to beta radiation from the samples for several weeks. The background c-radiation inside the shield was measured using dosimeters placed underneath quartz powders and subtracted from the individual measurements. The results of the annual dose measurements are also presented in Table 1.
8. Conclusion It has been shown in this study that mean and weighed mean values of age, average of four bricks from the Somero sacristy (Finland), dated using the 2103C TL peak, are in good agreement with the archaeologically expected age (see Table 1). In spite of the good precision of individual TL age determinations, the discrepancy of TL ages obtained between two groups of bricks could not be explained within the precision of the measurements. The di!erence may be due to the fact that the bricks were manufactured in di!erent ways and they have di!erent porosities, i.e. the water absorption coe$cient is di!erent. However, this could not be checked since the procedure requires a larger amount of samples: at least half of the bricks would be needed to get reliable results. Furthermore, it is normally assumed that quartz grains, extracted from bricks, have no internal radioactivity. However, there has been some evidence that quartz may also contain radioactive impurities, or radioactive elements may di!use into grains during the manufacture of the bricks. This would also add some fraction of alpha dose in the grains. Since all the bricks used in this study had di!erent TL properties it can be supposed that manufacture of bricks had some variations. More correct measurements could be done if large amount of samples would be available. However, the results have indicated that the 2103C peak could be used in recent archaeology in climatic regions with ambient mean annual temperature at #4}53C.
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
Acknowledgements The authors are very thankful to T. Kankainen (Geological Survey of Finland) for the help during the sampling and fruitful scienti"c discussions. Galina HuK tt and Ivar Jaek appreciate partial "nancial support of the Estonian Science Foundation (Grants 2720 and 2721). References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1990. Science-based Dating in Archaeology. Longman Archaeology Series, London and New York. Akselrod, M.S., Kortov, V.S., Kravetsky, D.J., Gotlib, V.I., 1990. Highly sensitive thermoluminescent anion defect a-Al O : single crystal detector. Radiation Protection Dosimetry 32, 15}20. Akselrod, M.S., McKeever, S.W.S., Moscovitch, M., Em"etsoglon, D., Durham, J.S., Soares, C.G., 1996. A thin layer Al O : C beta TL detector. Radiation Protection Dosimetry 66, 105}110. Atlas of Finland, 1987. Folio 131. Climate. 5th Edition. National Board of Survey, Geographical Society of Finland, Helsinki, 32 pp. Baili!, I.K., 1999. The development of retrospective luminescence dosimetry for dose reconstruction in areas downwind of Chernobyl. Radiation Protection Dosimetry 84, 411}419. Baili!, I.K., 1997. Retrospective dosimetry with ceramics. Radiation Measurements 27, 923}947. Baili!, I.K., B+tter-Jensen, L., Correher, V., Delgado, A., GoK ksu, H.Y., Jungner, H., Petrov, S., 2000. Adsorbed dose evaluations in retrospective dosimetry } Methodological developments using quartz. Radiation Measurements 32, 609}613. Baili!, I.K., Petrov, S.A., 1999. The 2103C peak in quartz for retrospective dosimetry Radiation. Protection Dosimetry 48 (1-4), 551}554. B+tter-Jensen, L., Meidahl, V., 1988. Assessment of beta dose rate using a GM multicounter system. Nuclear Tracks and Radiation Measurements 14, 187}191.
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B+tter-Jensen, L., Agersnap, W., Larsen, N., Markey, B.G., McKeever, S.W.S., 1997. Al O : C as sensitive OSL dosimeter for rapid assess ment of environmental photon dose rates. Radiation Measurements 27, 295}298. Bougrov, N.G., GoK ksu, H.Y., Haskell, E., Degteva, M.O., Meckbach, R., Jakob, P., 1998. Issues in the reconstruction of environmental doses on the basis of thermoluminescence measurements in the Techa riverside. Health Physics 75, 574}583. Fleming, S.J., 1973. The predose technique: a new thermoluminescence dating method. Archaeometry 15, 13}30. GoK ksu, H.Y., Bulur, E., Wahl, W., 1999. Beta dosimetry using thin layer a-Al O : C TL detectors. Radiation Protection Dosimetry. (Part 1) 4, 451}457. GoK ksu, H.Y., Heide, L.M., Bougrow, N.G., Dalheimer, A., Meckbach, R., Jacob, P., 1996. Depth dose distribution in bricks determined by thermoluminescence and by Monte Carlo calculations for dose reconstruction. Applied Radiation and Isotopes 47, 433}440. Haskell, E.H., 1993. Accident dosimetry using environmental materials. Radiation Protection Dosimetry 47, 297}303. Higashimura, T., Ichikawa, Y., Sidei, T., 1963. Dosimetry of atomic bomb radiation in Hiroshima by thermoluminescence of roof tiles. Science 139, 32}44. HuK tt, G., Brodski, L., Baili!, I.K., GoK ksu, Y., Haskell, F., Jungner, H., Stoneham, D., 1993. TL accident dosimetry using environmental material collected from regions downwind of Chernobyl: a preliminary evaluation. Radiation Protection Dosimetry 47, 307}311. HuK tt, G., Smirnov, A., Punning, J-M., 1982. Problems in the measuring of annual dose. PACT 6, 27}33. Markey, B.G., B+tter- Jensen, L., Poolton, N.R.J., 1996. A new sensitive system for TL and OSL measurements. Radiation Protection Dosimetry 65 (1-4), 413}418. Petrov, S.A., Baili!, I.K., 1997. Determination of trap depths associated with TL in Synthetic quartz (359K}550K). Radiation Measurements 27, 185}191. Stoneham, D., Baili!, I., Brodski, L., GoK ksu, Y., Haskell, E., HuK tt, G., Jungner, H., Nagotomo, T., 1993. TL accident dosimetry measurements on samples from the town of Pripyat. Nuclear Tracks and Radiation Measurements 21, 195}200. Wintle, A.G., 1997. Luminescence Dating: Laboratory procedures and protocols. Radiation Measurements 27, 769}817.
Luminescence dating of Somero sacristy, SW Finland using the 2103C TL peak of quartz夽 G. HuK tt q, H.Y. GoK ksu, I. Jaek *, M. Hiekkanen Institute of Geology, Tallinn Technical University, Estonia ave. 7, 10143 Tallinn, Estonia GSF Forshungszentrum, ISS Ingolstadter Landstr.1, D-85764 Neuherberg, Germany National Board of Antiquities, P.O. Box 187, FIN-00171 Helsinki, Finland
Abstract The 2103C thermoluminescence (TL) peak of quartz, extracted from brick fragments of Somero sacristy (Finland), which were partly elements of construction, were used for the measurement of archaeological dose for the purpose of dating. The annual gamma dose-rate with cosmic component and beta dose rate of bricks were measured using two types of TLD detectors (AL O }C) inserted in situ and in laboratory, respectively. The improvements in the precision of the TL measurements, components of annual dose assessment are described, and the results of TL age determination are discussed and compared with the archaeologically expected age. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction There have been considerable improvements in accumulated radiation dose measurements using the 2103C peak of quartz in recent years due to the attempts made to reconstruct radiation doses in the areas a!ected with radioactive fall outs (Higashimura et al., 1963; Haskell, 1993; HuK tt et al., 1993; Stoneham et al., 1993; GoK ksu et al., 1996; Baili!, 1997, 1999; Bougrov, 1998; Baili! et al., 2000). The reproducibility of the TL measurements has been reduced to 2}4% on the dose levels in the order of 100 mGy, due to the available automatic equipments (Markey et al., 1996), improved understanding of 2103C peak of quartz (Petrov and Baili!, 1997; Baili! and Petrov, 1999) and the possibility to use in situ highly sensitive dosimeters for annual dose measurements and beta dose rate measurements (GoK ksu et al., 1999). The 2103C peak is usually not used for archaeological studies. The reason is the relatively short lifetime of the corresponding traps, found not monoenergetic, which is estimated to range from 750 to 2000 years at 203C (Petrov and Baili!, 1997). Using computer glow curve deconvolution, Delgado (private communication) separated the peak at 2103C and estimated the mean lifetime of
夽
Paper published in December 2000. Deceased. * Corresponding author. Tel.: #372-6720078. q
charge carriers on the associated trap as 450}500 years at 153C. Considering the lower average annual ambient temperature (#4.53C) in this region of Finland (Atlas of Finland, Climate, 1987), we were encouraged to use the 2103C peak for the age determination of 500-year-old bricks. At this temperature, the expected lifetime of the charge carriers following our rough assessments would be about a factor of 10}15 higher than the above-mentioned values.
2. Short history of Somero sacristy (SW Finland) Except for the As land Islands, stone churches became economically feasible in Finland only in the 1420 s, and continued to be until the 1550 s, when the economic basis for church building in stone ceased to exist. All of the church projects planned between the 1420 s and 1480 s were carried out successfully, while those between the 1480 s and 1550 s were mostly left un"nished because of the late medieval economical and political crisis of Sweden of which Finland was a part. The sacristy of Somero was built as a "rst step towards a stone church, which never was accomplished because of the reasons above. According to the architectural and technical elements as well as historical evidence, the sacristy was built between the years 1480 and 1560, most likely in the 1490 s or around the 1500 s. As there was no wood
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 1 7 - 2
774
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
needed for dendrochronological dating, an attempt was made to de"ne the age of Somero sacristy by luminescence methods using bricks as a study object.
3. Sampling and sample preparations The exterior part of the sacristy is not original, but inside the building some parts of the gable and vault are well preserved. The original walls of the lower part of the building are made from grey stone, but the vault and the north gable are partly from bricks. The sampling took place in the attic of the building. The brick fragments were carefully removed from the north gable at di!erent heights from the vault and from the vault itself. The colour of bricks from di!erent sampling places was di!erent. So, the brick from the vault (sample No. 2) was greyish, while the bricks from the gable (sample No. 1,4) were orange and (sample No. 3) dark orange. This was apparently caused by the di!erent processes of manufacture. All brick fragments were treated in the same way to extract quartz: E E E E
removal of outer 3 mm layer, crushing by press and agate pestle, removal of "ne grains, step-by-step etching by HF of di!erent concentration in ultrasonic bath for 3 min: 5%, 10%, 20% and for 40 min}40%, E washing in distilled water and drying at room temperature after every etching procedure and E sieving to separate quartz grains in the span 125}200 lm. All procedures were performed in dim red light.
4. Equipment The thermoluminescence measurements using peak at 2103C and 3103C were carried out in an automated TL-OSL readers (RISO TLD-DA 12 and TL-DA 10) using two di!erent types of "lters (BG-12 (Schott) or U-340 (Hoya). Thermoluminescence measurements were performed by heating the samples to 4003C at a rate of 23C/s and a peak maximum was obtained around 2003C. The irradiation was carried out in situ with Sr- beta source, which was calibrated with respect to Secondary Standard Dosimetry Laboratory (SSDL) facilities in GSF for 140}200 lm grain size of quartz. The pre-dose technique, additive dose version, (Fleming, 1973) was also applied. The pre-dose measurements and gamma annual dose rates using AL O were per formed by TL-OSL reader designed in Tallinn.
5. Methodology, results and discussion The principle of age determination using the luminescence techniques can be found in various books (e.g. Aitken, 1985, 1990). Furthermore, in recent years several laboratory protocols, which were developed for the evaluation of accumulated archaeological doses, are summarized in review papers by Wintle (1997). The age determination using quartz or feldspars incorporated in to bricks is based on the following equation: ¹"D /D , (1) where ¹ is the age of a brick in years, D the archae ological (accumulated) dose due to natural radiation measured by luminescence techniques in (Gy), D the annual dose rate, measured in (Gy/yr). D "D #D #D #D , (2) a b c
where D is the contribution of the a-radiation of the a uranium and thorium in the brick (mGy/yr), D the b internal beta-particle dose rate due to the uranium, thorium and potassium content in the brick (mGy/yr), D the c dose rate due to c-radiation of uranium, thorium and potassium in the brick and in the environment (mGy/yr) and D the dose rate due to cosmic rays (mGy/yr). 6. The accumulated absorbed dose measurement using TL of quartz, extracted from bricks (Dac ) In this study regeneration and additive dose techniques for TL peaks at 2103C and 3103C as well as pre-dose sensitization of 1103C TL peak were used for the archaeological dose reconstruction. The "rst step of this study is to con"rm experimentally the fact that the bricks were heated to a su$ciently high temperature during their manufacture to bleach the geologically accumulated radiation dose. This is achieved applying the plateau test on samples, heated to 4003C (Figs. 1 and 2). It is observed that Tl glow curves consist of two peaks with maximums at 2103C and 3003C. The presence of plateau in the high-temperature region assured that the samples had been heated at su$ciently high temperature during their manufacture. The archaeological doses are separately obtained for 2103C and 3103C peaks by the additive dose-technique integrating the lightsum in the span of 210}2403C and 310}3403C. As is seen from Fig. 3, the archaeological dose for 2103C D "2.39$0.58 mGy is slightly higher than D "1.76$0.4 mGy. Due to the fact, that in the addi tive dose method the archaeological dose is found by the extrapolation of the growth curves, the obtained value has a large uncertainty. Since the archaeological doses measurements using both peaks are found to be the same
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
Fig. 1. TL glow curves of quartz: (1) Natural TL, (2) Natural #0.63 Gy, (3) Natural #2.53 Gy.
Fig. 2. Plateau test using additive dose glow curves for quartz extracted from sample 3. The preheat: 1403C for 50 s.
775
avoid the sensitization, have been established six aliquots of quartz are heated to 3003C. The archaeological dose is then calculated for each aliquot individually increasing the regeneration doses. The four bricks, which were used in these measurements, di!er both in appearance and TL sensitivity. The sample-to-sample variation for samples 1 and 4 was much smaller than for samples 2 and 3. Therefore, the archaeological doses of samples 1}4 could be assessed within 2}4% accuracy, while in the case of samples 2}3 the accuracy was 10%. The latter samples contained some bright grains, which were detected even under di!erent types of "lter. The values of archaeological doses obtained by the regeneration technique, which provides smaller error, were used for the age calculation. These results are presented in Table 1. Our attempts to apply the pre-dose technique (Fleming, 1973) were not fruitful, due to the problem of dose}response saturation: the intensity of the TL signal, measured for the natural samples was close to the saturation level. Following Aitken (1985), in the case of geological, unheated samples the thermoactivated curve (TAC) is almost #at and the ratio of initial (S ) and sensibilized (S ) sensitivities of the TL signal to test dose is rather low. In our case, S }S was about 2000 (a bit less for the greyish sample No. 2). So, this high value of the sensitivity change can also be used as a con"rmation of su$cient heating of bricks.
7. Assessment of annual dose (Da ) 7.1. Gamma and cosmic dose measurements
Fig. 3. Dose}response curves for 2103C (empty) and for 3103C (full). The accumulated doses, obtained using extrapolation of dose growth curves for both peaks, were within the experimental errors. Because the sample to sample variation is about 10%, additive dose usually gives a higher experimental error. Therefore, the "nal analysis is based only on 2103C peak using multi aliquots regeneration.
within the experimental errors, the peak 2103C can be used for the assessment of archaeological dose. Therefore, the regeneration technique using peak at 2103C was applied to reduce the uncertainty of the measurements. Once the measurement parameters: preheat temperature, and optimum temperature of measurements to
From Eq. (1) it follows that the calculation of age requires the assessment of annual dose rate due to the alpha-, beta-, gamma- and cosmic components of the natural radiation dose rate in the samples. In this study we have used etched quartz grains (140}200 lm) and, therefore, the alpha component could be ignored. The gamma#cosmic components of the dose rate were measured in situ (Aitken, 1985; HuK tt et al., 1982) using TLD Al O : C detectors (Akselrod et al., 1990; B+tter-Jensen et al., 1997). In usual practice, TLD detectors must be left in situ for 1 yr to take into account the absorbed dose variation due to seasonal changes. The water content of the samples and their environment may introduce variations in the order of $30% on annual dose calculations. Since the sampling took place inside the building, these variations were assumed to be minimum. Therefore, the duration of the exposure of the dosimeters was kept shorter than a year (4 months). The TLD detectors were annealed to 5003C and placed in an aluminum container (length 13 mm, thickness
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
776
Table 1 The weighted value of age was calculated following the formula: x"[(1/*x )Hx #(1/*x )Hx #2.]/[(1/*x )#(1/*x )#2]#1/[(1/*x )#(1/*x )#2.] . Sample no. Locations
D (mGy/yr)
D b (mGy/yr)
D c> (mGy/yr)
Age (years)
C C C C
3.54$0.15 1.94$0.31 2.4$0.28 2.8$0.05
4.51$0.10 3.18$0.12 3.97$0.49 3.11$0.05
1.54$0.01 1.36$0.01 1.41$0.01 1.25$0.01
586$44 430$25 450$45 640$40
1 2 3 4
Lower part of gable Vault Upper part of gable Lower part of gable
2.5 mm, diameter 10 mm). Each container, "lled with 3 TLD detectors was inserted in the sampling place for 4 months. All procedures were performed in a dark room. After 4 months, the detectors were collected and measured in the laboratory. The procedure of measurements includes a pre-heat at 1353C with a heating rate of 53C/s. The informative signal was the result of integrating in the span of 1003C around TL maximum at 2173C. Every detector was calibrated by c-source Cs. The TL signal intensities were intercalibrated to radiation dose using a secondary standard Co c-source (GSF, MuK nchen). The detectors were calibrated in the same containers, in which they were kept in situ. 7.2. b-dose measurement The D dose in the bricks was measured using thin b layer a-AL O -C detectors (Akselrod et al., 1996; GoK ksu et al., 1999). Luminescence emission was detected using a combination of blue transmitting Corning C 7}59 and Chance-Pilkington heat absorption (HA3) "lters. Thermoluminescence measurements were performed by heating of the sample to 4003C at a rate of 23C/s. The in-built radiation source was removed during the measurements. The dosimeters were annealed (4003C at a rate of 23C/s) "ve times before they were used for routine measurements. The TL signal intensities in the region of the Tl glow curve peak at 2003C were calibrated to the radiation dose using a secondary standard Sr/Y b-source (dose rate: 87.3 lGy/min). Plastic sample holders (diameter 25 mm, height 8 mm) originally manufactured for sample placements for the GM counter system were developed at Ris+ (B+tterJensen and Meidahl, 1988). Two grams of powder were weighed and placed into sample holders. For most of the samples this amount completely "lls the volume of the sample holder so that the sample is always `beta thicka. The open surface of the sample holder was covered with a mylar sheet (density 1.0 mg/cm) and "xed with an external plastic ring. The samples were inserted into a lead-shield (wall thickness &5 cm) using a plastic
Age Weighted (years)
Age Mean (years)
Age Predicted (years)
486$19
526$30
520-440
sample tray that can carry "ve sample holders. The sample-to-sample distance (measured from centre to centre) was 5 cm so that cross talk between the samples is minimised. The thin-layer TL detectors were exposed to beta radiation from the samples for several weeks. The background c-radiation inside the shield was measured using dosimeters placed underneath quartz powders and subtracted from the individual measurements. The results of the annual dose measurements are also presented in Table 1.
8. Conclusion It has been shown in this study that mean and weighed mean values of age, average of four bricks from the Somero sacristy (Finland), dated using the 2103C TL peak, are in good agreement with the archaeologically expected age (see Table 1). In spite of the good precision of individual TL age determinations, the discrepancy of TL ages obtained between two groups of bricks could not be explained within the precision of the measurements. The di!erence may be due to the fact that the bricks were manufactured in di!erent ways and they have di!erent porosities, i.e. the water absorption coe$cient is di!erent. However, this could not be checked since the procedure requires a larger amount of samples: at least half of the bricks would be needed to get reliable results. Furthermore, it is normally assumed that quartz grains, extracted from bricks, have no internal radioactivity. However, there has been some evidence that quartz may also contain radioactive impurities, or radioactive elements may di!use into grains during the manufacture of the bricks. This would also add some fraction of alpha dose in the grains. Since all the bricks used in this study had di!erent TL properties it can be supposed that manufacture of bricks had some variations. More correct measurements could be done if large amount of samples would be available. However, the results have indicated that the 2103C peak could be used in recent archaeology in climatic regions with ambient mean annual temperature at #4}53C.
G. Hu( tt et al. / Quaternary Science Reviews 20 (2001) 773}777
Acknowledgements The authors are very thankful to T. Kankainen (Geological Survey of Finland) for the help during the sampling and fruitful scienti"c discussions. Galina HuK tt and Ivar Jaek appreciate partial "nancial support of the Estonian Science Foundation (Grants 2720 and 2721). References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1990. Science-based Dating in Archaeology. Longman Archaeology Series, London and New York. Akselrod, M.S., Kortov, V.S., Kravetsky, D.J., Gotlib, V.I., 1990. Highly sensitive thermoluminescent anion defect a-Al O : single crystal detector. Radiation Protection Dosimetry 32, 15}20. Akselrod, M.S., McKeever, S.W.S., Moscovitch, M., Em"etsoglon, D., Durham, J.S., Soares, C.G., 1996. A thin layer Al O : C beta TL detector. Radiation Protection Dosimetry 66, 105}110. Atlas of Finland, 1987. Folio 131. Climate. 5th Edition. National Board of Survey, Geographical Society of Finland, Helsinki, 32 pp. Baili!, I.K., 1999. The development of retrospective luminescence dosimetry for dose reconstruction in areas downwind of Chernobyl. Radiation Protection Dosimetry 84, 411}419. Baili!, I.K., 1997. Retrospective dosimetry with ceramics. Radiation Measurements 27, 923}947. Baili!, I.K., B+tter-Jensen, L., Correher, V., Delgado, A., GoK ksu, H.Y., Jungner, H., Petrov, S., 2000. Adsorbed dose evaluations in retrospective dosimetry } Methodological developments using quartz. Radiation Measurements 32, 609}613. Baili!, I.K., Petrov, S.A., 1999. The 2103C peak in quartz for retrospective dosimetry Radiation. Protection Dosimetry 48 (1-4), 551}554. B+tter-Jensen, L., Meidahl, V., 1988. Assessment of beta dose rate using a GM multicounter system. Nuclear Tracks and Radiation Measurements 14, 187}191.
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