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Alpha efficiency under TL and OSL – A subtraction technique using OSL and TL to detect artificial irradiation
Radiation Measurements 45 (2010) 649–652
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Alpha efficiency under TL and OSL – A subtraction technique using OSL and TL to detect artificial irradiation A.J.C. Zink*, S. Dabis, E. Porto, J. Castaing Laboratoire du Centre de Recherche et de Restauration des Muse´es de France, C2RMF, MCC, CNRS, Palais du Louvre, Porte des lions, 14 quai F. Mitterrand, 75001 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2009 Received in revised form 17 December 2009 Accepted 6 January 2010
With the development of thermoluminescence (TL) and optically stimulated luminescence (OSL) to determine the authenticity of old ceramics, forgers use artificial irradiation by gamma ray to age modern productions. Besides fraudulent action, objects can be exposed to various sources of X-rays (e.g. radiography, security control at airports). For all these reasons, the determination of artificial irradiation is an important topic for dating art objects. The main technique to identify artificial irradiations is the subtraction technique. It is based on the fact that alpha efficiency varies according to the luminescence technique (fine grain, coarse grains, predose, OSL). Having observed a rather significant difference of alpha efficiency for TL and OSL, we propose a new subtraction technique using OSL and TL of fine grains. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Luminescence dating Alpha efficiency Artificial irradiation Polymineral fraction
1. Introduction The development of thermoluminescence (TL) and optically stimulated luminescence (OSL) for the determination of the authenticity of old ceramics started in the 1970s. This prompted forgers to use artificial irradiation to age modern productions. Objects can also be exposed inadvertently to various sources of X-rays (e.g. radiography, security control at airports). For all these reasons, the determination of artificial irradiation is an important topic for dating of art objects. The main technique to identify the nature of irradiations is the subtraction technique. It is based on the fact that alpha efficiency varies according to the luminescence technique (fine grain, coarse grains, predose, OSL). Indeed, alpha particles are produced by radionuclei that are present mostly in clays; they induce heavy damage in mineral structures along short penetration distances (20 mm) compared to other radiations (beta particles, X-rays, gamma rays) that induce only electronic damage on long distances (mm to cm). The ratio of the two kinds of damage is fixed for natural irradiation. On another hand, alpha irradiation cannot be artificially simulated in the core of ceramics. The subtraction technique was proposed initially by Fleming and Stoneham (1973) by using TL on fine grains and quartz inclusions. In art object dating, the samples are small and quartz inclusions are generally scarce. An alternative is based on the comparison between TL and predose for fine grain fraction (Aitken, 1985; Wang
* Corresponding author. E-mail address: [email protected] (A.J.C. Zink). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.017
et al., 2003). Having observed a rather significant difference of alpha efficiency for TL and OSL for Tanagra terracotta (Zink and Porto, 2005), we decided to investigate the alpha efficiency under TL and OSL for polymineral fine grains from various origins in space and time.
2. Measurements 2.1. Equipment A standard sample preparation technique was used (Zink and Porto, 2005). All measurements were made on the 4–13 mm polymineral fraction. Sixteen discs were prepared for thermoluminescence and four for OSL. The luminescence measurements were performed with a Risoe TL/OSL DA-15 equipped with an EMI 9235QA PMT and incorporated 90Sr/90Y source (6.71 Gy/min on the 1st January 2009). For optical stimulation, we used infrared laser diode (830 10 nm; 50% of 450 mW/cm2 full power – IR-OSL) or 21 pairs of blue diodes (470 30 nm; 50% of 19 mW/cm2 full power – BL-OSL) and the optical luminescence was detected through a 7.5 mm thick U-340 filter. The thermoluminescence (TL) was detected through a combination of 7–59 HA-3 Filters. The external irradiations were produced using a Daybreak 801 multi sample irradiator including beta 90Sr/90Y source (3.25 Gy/ min; 1/1/2009) and alpha 238Pu source (7.39 mm2/min; 1/1/ 2009). Irradiations were also performed with X-rays produced by Seifert Isovolt 420/10 (400 kV, 4 mA) and Seifert Isovolt 160/M1
650
A.J.C. Zink et al. / Radiation Measurements 45 (2010) 649–652
(40 kV, 4 mA) tubes with beryllium windows. The samples were located at 1.3 m from the window.
the predose method with quenching correction as described in Aitken (1985), with a test dose of 4 mGy. The equivalent beta dose to our alpha irradiation was obtained by the Eq. 6.2 of Aitken (1985).
2.2. Method to investigate the alpha efficiency 2.3. Subtraction techniques The alpha efficiency is the relative efficiency of alpha radiation to beta radiation. Instead of the k-value, we used the a-value as suggested by Aitken (1985). The a-value, based on the alpha track length instead of the energy, is independent of the energy spectrum. Hence, the a-values measured in the laboratory with an artificial source are equivalent to the a-values for natural irradiation. Beta and alpha doses are far from saturation, in the linear range of the dose response. For TL, we used the additive technique (Zimmerman, 1971). Multiple aliquots were irradiated at our alpha or beta external source and stored in darkness at 55 C during 1–3 months. After a preheat to 220 C, the disc was directly cooled to 60 C and then heated to 650 C for TL signal measurements and the remaining signal was annealed at 650 C during 2 min before monitoring the black body emission. Heat was applied at 5 C/s under nitrogen flow. All measurements were recorded by integrating one degree per channel. The palaeodose in equivalent beta dose was evaluated by extrapolation of the growth curve obtained by adding beta dose to natural dose. The results were corrected by the supralinearity effect. Similarly, the palaeodose in equivalent alpha was evaluated. In this case, the supralinearity was null (Aitken, 1985). The alpha efficiency (a-value) was given by the ratio of the palaeodose in equivalent alpha track length per unit volume (mm2) on the palaeodose in equivalent beta dose (Gy). For OSL, we used the single aliquot regeneration technique (SAR-OSL; Murray and Wintle, 2000). After irradiation, the disc was preheated 5 s to 275 C. Then, it was first stimulated by infrared light at 60 C during 100 s (IR-OSL) and then stimulated by blue light at 125 C during 100 s (post-IR BL-OSL or BL-OSL). All measurements were recorded by integrating 1 s per channel. After measurement of the palaeodose by the SAR protocol, the sample was irradiated by alpha particles and the intensity was plotted on the growth curve. We obtained by interpolation the equivalent beta dose to our alpha irradiation. For predose, after annealing, the sample was irradiated with the alpha source and then annealed at 190 C during 1 s. Then, we used
The age equation can be written as:
Age ¼ P=ða$Dalpha þ Dbeta þ DextÞ
(1)
where P is the palaeodose, a is the alpha efficiency, Dalpha, Dbeta and Dext are the annual doses of the respective radiations. The Eq. (1) can be rewritten as:
P ¼ Ageða$Dalpha þ DbetaÞ þ Age$Dext
(2)
The term a$Dalpha þ Dbeta corresponds to the internal annual dose. Assuming that a is function of the measurement techniques, we can plot the palaeodose as function of the internal annual dose for TL and OSL (Fig. 2). The slope corresponds to the age and the intercept with y-axis to the product of the age by the external dose rate. In the case of artificial irradiation, the palaeodose is independent of the internal dose, the slope must be zero. It is possible to directly quantify the annual alpha dose. To do this, Eq. (1) becomes:
PTL ¼ POSL þ ðaTL aOSLÞ$Dalpha$Age
(3)
P and a are the palaeodose and the alpha efficiency, respectively, of the techniques indicated. Their mean and standard uncertainties are obtained by measurements. The initial value of the age is estimated from archaeological assumptions. For the annual dose alpha, we believe a priori that it is between two extreme values such as 0 and 100 mGy/a. A Markov Chain–Monte-Carlo simulation will allow us to recalculate the age and the annual alpha dose using Eq. (3) and the measured values. Numerical implementation was realized using WinBUGS 1.4 (Lunn et al., 2000) with 5 chains each with 50,000 iterations (first 25,000 discarded). 3. a-Value distribution Fig. 1 shows histograms of a-values measured in our laboratory by predose, OSL and TL on polymineral samples from terracotta of
Fig. 1. a-Values distribution of polymineral samples from various origins in space and time for predose (21 measurements, values range between 0.020 and 0.079) BL-OSL (169 measurements, values range between 0.027 and 0.166) and TL (130 measurements, values range between 0.027 and 0.248).
A.J.C. Zink et al. / Radiation Measurements 45 (2010) 649–652
different geographical and historical origins. For the predose, the values range between 0.020 and 0.079. The mean value is 0.043 with a standard deviation of 0.017. For blue-OSL, the values range between 0.027 and 0.166. The mean value is 0.062 with a standard deviation of 0.021. It confirms our previous observations made on the Tanagra figurines (Zink and Porto, 2005). The a-value is low, close to the values for predose. For TL, the values range between 0.027 and 0.248. The mean value 0.128 agrees with our previous observations (Zink and Porto, 2005) as well as with usual values for polymineral fine grains (Aitken, 1985). But the results are spread with a standard deviation of 0.049. The large dispersion is not surprising since we measure polymineral samples of different origins. We do not expect a single true value, but a dispersion of values characteristic of each sample. We find, however, the general trend by which the effectiveness of alpha particles increases from the predose to OSL and TL. The low effectiveness of alpha irradiation in predose compared to TL is related to the limited number of shallow hole traps (Aitken, 1985). Yukihara et al. (2004) showed that high efficiencies in Al2O3:C is correlated with high dose responses and is specific to material/technique combination. 4. Subtraction technique Fig. 2 displays graphically the subtraction technique for an archaeological sample (Terracotta figurine, France, 16th c.). The palaeodose is plotted as a function of the internal dose for OSL (filled square) and TL (open square). The two points agree within two standard deviations with the theoretical data (dashed line). The slope of the trend created by the two experimental points corresponds to 560 90 years, close to the expected age of 400 years. The intercept with y-axis corresponds to an external annual dose of -0.7 1.9 mGy/a. Limited to only positive values of the external annual dose, the age becomes equal to 460 40 years. Numerical simulation, assuming a prior information on the age of 400 150 years, give us an annual alpha dose equal to 77 15 mGy/a for an age of 485 110 ans. This result is in good agreement with the annual alpha dose based on radioelements’ contents: 73.7 3.7 mGy/a. On the other hand, it is totally
651
Fig. 3. Palaeodose as function of the annual internal dose for an artefact with artificial irradiation. The annual internal dose rate parameters are annual alpha dose rate ¼ 25.5 1.3 Gy/ka and annual beta dose rate ¼ 2.1 0.1 Gy/ka. Filled square: OSL, palaeodose ¼ 0.36 0.02 Gy, annual internal dose rate ¼ 3.1 0.1 Gy/ka (alpha efficiency ¼ 0.040 0.006). Open square: TL, palaeodose ¼ 0.70 0.14 Gy, annual internal dose rate ¼ 6.6 0.2 Gy/ka (alpha efficiency ¼ 0.178 0.008). Solid line, straight line fitting the experimental data: age ¼ 97 40 years. Brick (quartz, iron oxide, lumps of clay) 2nd c. AD, France, annealed 550 C 1 h, X-ray irradiated 40 kV, 40 mA, 1.3 m, 6 min.
inconsistent with an absence of alpha radiation. We have a good assessment of the natural origin of the dose received by the sample, even if we see some variation in the measurement of age according to the statistical treatment. Fig. 3 shows the subtraction technique for samples after artificial irradiation. The samples are annealed at 550 C during 1 h to eliminate the archaeological signal, and then irradiated with a Xray source (40 kV, 4 mA, 1.3 m, 6 min) simulating an age of 100 years (Castaing et al., 2004). The palaeodose for OSL is smaller than for TL, as expected for natural irradiation. The slope of the trend corresponds to 97 40 years, close to the simulated age of 100 years. Numerical simulation, assuming a prior information on the age of 100 50 years, give us an annual alpha dose equal to 35 23 mGy/a for an age of 85 40 ans. This result is consistent with an absence of alpha irradiation. However, it is also in good agreement with the annual alpha dose based on radioelements’ contents: 25.5 1.3 mGy/a. Further development of statistical tools is needed to obtain good discrimination between natural and artificial radiation. 5. Conclusions
Fig. 2. Palaeodose as function of the annual internal dose for an archaeological artefact (terracotta figurine, ca. 1600 AD, France). The annual internal dose rate parameters are annual alpha dose rate ¼ 73.7 3.7 Gy/ka and annual beta dose rate ¼ 6.6 0.3 Gy/ka. Filled square: OSL, palaeodose ¼ 5.7 0.1 Gy, annual internal dose rate ¼ 11.2 0.3 Gy/ ka (alpha efficiency ¼ 0.062 0.002). Open square: TL, palaeodose ¼ 9.9 0.5 Gy, annual internal dose rate ¼ 18.8 0.7 Gy/ka (alpha efficiency ¼ 0.166 0.027). Dashed line, straight line corresponding to an age of 400 years and an external dose rate of 3.8 Gy/ka. Solid line, straight line fitting the experimental data: age ¼ 560 90 years, external dose rate ¼ 0.7 1.9 mGy/a.
The alpha efficiency shows a general trend, increasing from the predose to OSL and TL for polymineral samples. In fact, the alpha effectiveness is a function of the crystal and measurement methods, and is related to the beta high dose response. Further investigations are needed to characterize the alpha effectiveness of usual minerals. Where TL and OSL alpha efficiencies are sufficiently distinct, it is possible to use subtraction techniques to date the samples. Dates obtained by this method are in good agreement with expected dates although they vary according to the statistical treatment. A numerical approach of the subtraction technique allows to directly assess the alpha dose. Early results are encouraging. They, however, require further development of the method to reach a clear discrimination between natural and artificial irradiations. References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London, ISBN 0-12-0463814, 360 pp.
652
A.J.C. Zink et al. / Radiation Measurements 45 (2010) 649–652
Castaing, J., Zink, A., Borel, T., Porto, E., 2004. Influence de l’irradiation aux rayons X sur la luminescence et la datation des terres cuites. Techne´ 19, 130–133 (in French). Fleming, S.J., Stoneham, D., 1973. The substraction technique of thermoluminescence dating. Archaeometry 15 (2), 229–238. Lunn, D.J., Thomas, A., Best, N., Spiegelhalter, D., 2000. WinBUGS – a Bayesian modelling framework: concepts, structure, and extensibility. Statistics and Computing 10, 325–337. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73.
Wang, W., Leung, P.L., Xia, J., Zhou, Z., 2003. Distinguishing of artificial irradiation by a dose: a method of discriminating imitations of ancient pottery. Nuclear techniques 26 (1), 48–52 (in Chinese). Yukihara, E.G., Gaza, R., McKeever, S.W.S., Soares, C.G., 2004. Optically stimulated luminescence and thermoluminescence efficiencies for high-energy heavy charged particle irradiation in Al2O3:C. Radiation Measurements 38, 59–70. Zimmerman, D.W., 1971. Thermoluminescence dating using fine grains from pottery. Archaeometry 13, 29–52. Zink, A., Porto, E., 2005. Luminescence dating of the Tanagra terracottas of the Louvre collections. Geochronometria 24, 21–26.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Alpha efficiency under TL and OSL – A subtraction technique using OSL and TL to detect artificial irradiation A.J.C. Zink*, S. Dabis, E. Porto, J. Castaing Laboratoire du Centre de Recherche et de Restauration des Muse´es de France, C2RMF, MCC, CNRS, Palais du Louvre, Porte des lions, 14 quai F. Mitterrand, 75001 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2009 Received in revised form 17 December 2009 Accepted 6 January 2010
With the development of thermoluminescence (TL) and optically stimulated luminescence (OSL) to determine the authenticity of old ceramics, forgers use artificial irradiation by gamma ray to age modern productions. Besides fraudulent action, objects can be exposed to various sources of X-rays (e.g. radiography, security control at airports). For all these reasons, the determination of artificial irradiation is an important topic for dating art objects. The main technique to identify artificial irradiations is the subtraction technique. It is based on the fact that alpha efficiency varies according to the luminescence technique (fine grain, coarse grains, predose, OSL). Having observed a rather significant difference of alpha efficiency for TL and OSL, we propose a new subtraction technique using OSL and TL of fine grains. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Luminescence dating Alpha efficiency Artificial irradiation Polymineral fraction
1. Introduction The development of thermoluminescence (TL) and optically stimulated luminescence (OSL) for the determination of the authenticity of old ceramics started in the 1970s. This prompted forgers to use artificial irradiation to age modern productions. Objects can also be exposed inadvertently to various sources of X-rays (e.g. radiography, security control at airports). For all these reasons, the determination of artificial irradiation is an important topic for dating of art objects. The main technique to identify the nature of irradiations is the subtraction technique. It is based on the fact that alpha efficiency varies according to the luminescence technique (fine grain, coarse grains, predose, OSL). Indeed, alpha particles are produced by radionuclei that are present mostly in clays; they induce heavy damage in mineral structures along short penetration distances (20 mm) compared to other radiations (beta particles, X-rays, gamma rays) that induce only electronic damage on long distances (mm to cm). The ratio of the two kinds of damage is fixed for natural irradiation. On another hand, alpha irradiation cannot be artificially simulated in the core of ceramics. The subtraction technique was proposed initially by Fleming and Stoneham (1973) by using TL on fine grains and quartz inclusions. In art object dating, the samples are small and quartz inclusions are generally scarce. An alternative is based on the comparison between TL and predose for fine grain fraction (Aitken, 1985; Wang
* Corresponding author. E-mail address: [email protected] (A.J.C. Zink). 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.017
et al., 2003). Having observed a rather significant difference of alpha efficiency for TL and OSL for Tanagra terracotta (Zink and Porto, 2005), we decided to investigate the alpha efficiency under TL and OSL for polymineral fine grains from various origins in space and time.
2. Measurements 2.1. Equipment A standard sample preparation technique was used (Zink and Porto, 2005). All measurements were made on the 4–13 mm polymineral fraction. Sixteen discs were prepared for thermoluminescence and four for OSL. The luminescence measurements were performed with a Risoe TL/OSL DA-15 equipped with an EMI 9235QA PMT and incorporated 90Sr/90Y source (6.71 Gy/min on the 1st January 2009). For optical stimulation, we used infrared laser diode (830 10 nm; 50% of 450 mW/cm2 full power – IR-OSL) or 21 pairs of blue diodes (470 30 nm; 50% of 19 mW/cm2 full power – BL-OSL) and the optical luminescence was detected through a 7.5 mm thick U-340 filter. The thermoluminescence (TL) was detected through a combination of 7–59 HA-3 Filters. The external irradiations were produced using a Daybreak 801 multi sample irradiator including beta 90Sr/90Y source (3.25 Gy/ min; 1/1/2009) and alpha 238Pu source (7.39 mm2/min; 1/1/ 2009). Irradiations were also performed with X-rays produced by Seifert Isovolt 420/10 (400 kV, 4 mA) and Seifert Isovolt 160/M1
650
A.J.C. Zink et al. / Radiation Measurements 45 (2010) 649–652
(40 kV, 4 mA) tubes with beryllium windows. The samples were located at 1.3 m from the window.
the predose method with quenching correction as described in Aitken (1985), with a test dose of 4 mGy. The equivalent beta dose to our alpha irradiation was obtained by the Eq. 6.2 of Aitken (1985).
2.2. Method to investigate the alpha efficiency 2.3. Subtraction techniques The alpha efficiency is the relative efficiency of alpha radiation to beta radiation. Instead of the k-value, we used the a-value as suggested by Aitken (1985). The a-value, based on the alpha track length instead of the energy, is independent of the energy spectrum. Hence, the a-values measured in the laboratory with an artificial source are equivalent to the a-values for natural irradiation. Beta and alpha doses are far from saturation, in the linear range of the dose response. For TL, we used the additive technique (Zimmerman, 1971). Multiple aliquots were irradiated at our alpha or beta external source and stored in darkness at 55 C during 1–3 months. After a preheat to 220 C, the disc was directly cooled to 60 C and then heated to 650 C for TL signal measurements and the remaining signal was annealed at 650 C during 2 min before monitoring the black body emission. Heat was applied at 5 C/s under nitrogen flow. All measurements were recorded by integrating one degree per channel. The palaeodose in equivalent beta dose was evaluated by extrapolation of the growth curve obtained by adding beta dose to natural dose. The results were corrected by the supralinearity effect. Similarly, the palaeodose in equivalent alpha was evaluated. In this case, the supralinearity was null (Aitken, 1985). The alpha efficiency (a-value) was given by the ratio of the palaeodose in equivalent alpha track length per unit volume (mm2) on the palaeodose in equivalent beta dose (Gy). For OSL, we used the single aliquot regeneration technique (SAR-OSL; Murray and Wintle, 2000). After irradiation, the disc was preheated 5 s to 275 C. Then, it was first stimulated by infrared light at 60 C during 100 s (IR-OSL) and then stimulated by blue light at 125 C during 100 s (post-IR BL-OSL or BL-OSL). All measurements were recorded by integrating 1 s per channel. After measurement of the palaeodose by the SAR protocol, the sample was irradiated by alpha particles and the intensity was plotted on the growth curve. We obtained by interpolation the equivalent beta dose to our alpha irradiation. For predose, after annealing, the sample was irradiated with the alpha source and then annealed at 190 C during 1 s. Then, we used
The age equation can be written as:
Age ¼ P=ða$Dalpha þ Dbeta þ DextÞ
(1)
where P is the palaeodose, a is the alpha efficiency, Dalpha, Dbeta and Dext are the annual doses of the respective radiations. The Eq. (1) can be rewritten as:
P ¼ Ageða$Dalpha þ DbetaÞ þ Age$Dext
(2)
The term a$Dalpha þ Dbeta corresponds to the internal annual dose. Assuming that a is function of the measurement techniques, we can plot the palaeodose as function of the internal annual dose for TL and OSL (Fig. 2). The slope corresponds to the age and the intercept with y-axis to the product of the age by the external dose rate. In the case of artificial irradiation, the palaeodose is independent of the internal dose, the slope must be zero. It is possible to directly quantify the annual alpha dose. To do this, Eq. (1) becomes:
PTL ¼ POSL þ ðaTL aOSLÞ$Dalpha$Age
(3)
P and a are the palaeodose and the alpha efficiency, respectively, of the techniques indicated. Their mean and standard uncertainties are obtained by measurements. The initial value of the age is estimated from archaeological assumptions. For the annual dose alpha, we believe a priori that it is between two extreme values such as 0 and 100 mGy/a. A Markov Chain–Monte-Carlo simulation will allow us to recalculate the age and the annual alpha dose using Eq. (3) and the measured values. Numerical implementation was realized using WinBUGS 1.4 (Lunn et al., 2000) with 5 chains each with 50,000 iterations (first 25,000 discarded). 3. a-Value distribution Fig. 1 shows histograms of a-values measured in our laboratory by predose, OSL and TL on polymineral samples from terracotta of
Fig. 1. a-Values distribution of polymineral samples from various origins in space and time for predose (21 measurements, values range between 0.020 and 0.079) BL-OSL (169 measurements, values range between 0.027 and 0.166) and TL (130 measurements, values range between 0.027 and 0.248).
A.J.C. Zink et al. / Radiation Measurements 45 (2010) 649–652
different geographical and historical origins. For the predose, the values range between 0.020 and 0.079. The mean value is 0.043 with a standard deviation of 0.017. For blue-OSL, the values range between 0.027 and 0.166. The mean value is 0.062 with a standard deviation of 0.021. It confirms our previous observations made on the Tanagra figurines (Zink and Porto, 2005). The a-value is low, close to the values for predose. For TL, the values range between 0.027 and 0.248. The mean value 0.128 agrees with our previous observations (Zink and Porto, 2005) as well as with usual values for polymineral fine grains (Aitken, 1985). But the results are spread with a standard deviation of 0.049. The large dispersion is not surprising since we measure polymineral samples of different origins. We do not expect a single true value, but a dispersion of values characteristic of each sample. We find, however, the general trend by which the effectiveness of alpha particles increases from the predose to OSL and TL. The low effectiveness of alpha irradiation in predose compared to TL is related to the limited number of shallow hole traps (Aitken, 1985). Yukihara et al. (2004) showed that high efficiencies in Al2O3:C is correlated with high dose responses and is specific to material/technique combination. 4. Subtraction technique Fig. 2 displays graphically the subtraction technique for an archaeological sample (Terracotta figurine, France, 16th c.). The palaeodose is plotted as a function of the internal dose for OSL (filled square) and TL (open square). The two points agree within two standard deviations with the theoretical data (dashed line). The slope of the trend created by the two experimental points corresponds to 560 90 years, close to the expected age of 400 years. The intercept with y-axis corresponds to an external annual dose of -0.7 1.9 mGy/a. Limited to only positive values of the external annual dose, the age becomes equal to 460 40 years. Numerical simulation, assuming a prior information on the age of 400 150 years, give us an annual alpha dose equal to 77 15 mGy/a for an age of 485 110 ans. This result is in good agreement with the annual alpha dose based on radioelements’ contents: 73.7 3.7 mGy/a. On the other hand, it is totally
651
Fig. 3. Palaeodose as function of the annual internal dose for an artefact with artificial irradiation. The annual internal dose rate parameters are annual alpha dose rate ¼ 25.5 1.3 Gy/ka and annual beta dose rate ¼ 2.1 0.1 Gy/ka. Filled square: OSL, palaeodose ¼ 0.36 0.02 Gy, annual internal dose rate ¼ 3.1 0.1 Gy/ka (alpha efficiency ¼ 0.040 0.006). Open square: TL, palaeodose ¼ 0.70 0.14 Gy, annual internal dose rate ¼ 6.6 0.2 Gy/ka (alpha efficiency ¼ 0.178 0.008). Solid line, straight line fitting the experimental data: age ¼ 97 40 years. Brick (quartz, iron oxide, lumps of clay) 2nd c. AD, France, annealed 550 C 1 h, X-ray irradiated 40 kV, 40 mA, 1.3 m, 6 min.
inconsistent with an absence of alpha radiation. We have a good assessment of the natural origin of the dose received by the sample, even if we see some variation in the measurement of age according to the statistical treatment. Fig. 3 shows the subtraction technique for samples after artificial irradiation. The samples are annealed at 550 C during 1 h to eliminate the archaeological signal, and then irradiated with a Xray source (40 kV, 4 mA, 1.3 m, 6 min) simulating an age of 100 years (Castaing et al., 2004). The palaeodose for OSL is smaller than for TL, as expected for natural irradiation. The slope of the trend corresponds to 97 40 years, close to the simulated age of 100 years. Numerical simulation, assuming a prior information on the age of 100 50 years, give us an annual alpha dose equal to 35 23 mGy/a for an age of 85 40 ans. This result is consistent with an absence of alpha irradiation. However, it is also in good agreement with the annual alpha dose based on radioelements’ contents: 25.5 1.3 mGy/a. Further development of statistical tools is needed to obtain good discrimination between natural and artificial radiation. 5. Conclusions
Fig. 2. Palaeodose as function of the annual internal dose for an archaeological artefact (terracotta figurine, ca. 1600 AD, France). The annual internal dose rate parameters are annual alpha dose rate ¼ 73.7 3.7 Gy/ka and annual beta dose rate ¼ 6.6 0.3 Gy/ka. Filled square: OSL, palaeodose ¼ 5.7 0.1 Gy, annual internal dose rate ¼ 11.2 0.3 Gy/ ka (alpha efficiency ¼ 0.062 0.002). Open square: TL, palaeodose ¼ 9.9 0.5 Gy, annual internal dose rate ¼ 18.8 0.7 Gy/ka (alpha efficiency ¼ 0.166 0.027). Dashed line, straight line corresponding to an age of 400 years and an external dose rate of 3.8 Gy/ka. Solid line, straight line fitting the experimental data: age ¼ 560 90 years, external dose rate ¼ 0.7 1.9 mGy/a.
The alpha efficiency shows a general trend, increasing from the predose to OSL and TL for polymineral samples. In fact, the alpha effectiveness is a function of the crystal and measurement methods, and is related to the beta high dose response. Further investigations are needed to characterize the alpha effectiveness of usual minerals. Where TL and OSL alpha efficiencies are sufficiently distinct, it is possible to use subtraction techniques to date the samples. Dates obtained by this method are in good agreement with expected dates although they vary according to the statistical treatment. A numerical approach of the subtraction technique allows to directly assess the alpha dose. Early results are encouraging. They, however, require further development of the method to reach a clear discrimination between natural and artificial irradiations. References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London, ISBN 0-12-0463814, 360 pp.
652
A.J.C. Zink et al. / Radiation Measurements 45 (2010) 649–652
Castaing, J., Zink, A., Borel, T., Porto, E., 2004. Influence de l’irradiation aux rayons X sur la luminescence et la datation des terres cuites. Techne´ 19, 130–133 (in French). Fleming, S.J., Stoneham, D., 1973. The substraction technique of thermoluminescence dating. Archaeometry 15 (2), 229–238. Lunn, D.J., Thomas, A., Best, N., Spiegelhalter, D., 2000. WinBUGS – a Bayesian modelling framework: concepts, structure, and extensibility. Statistics and Computing 10, 325–337. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73.
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