Measuring apparent dose rate factors using beta and gamma rays, and alpha efficiency for precise thermoluminescence dating of calcite

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1 Journal of Mineralogical and Petrological Sciences, Volume 112, page , 2017 Measuring apparent dose rate factors using beta and gamma rays, and alpha efficiency for precise thermoluminescence dating of calcite Manabu OGATA *, Noriko HASEBE *, Naoki FUJII ** and Minoru YAMAKAWA ** * Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa , Japan ** Radioactive Waste Management Funding and Research Center, Tokyo , Japan In addition to the conventional 14 C and Th/U dating methods, thermoluminescence (TL) dating has been applied to calcite, but has been less popular partly because the luminescence responses for different types of radiation are unclear. To report more reliable TL ages for calcite, the fundamental characteristics of its response to radiation exposure were investigated and related to chemical composition. Relative TL factors for calcite after beta and gamma irradiation normalized with quartz, hereafter termed the beta and gamma factors, were measured as and , respectively. These lower values than for quartz may be caused by differences in common substitution elements in calcite ( 20 Ca, 25 Mn, and 26 Fe) versus quartz ( 3 Li, 11 Na, 13 Al, and 14 Si), and the interaction between mediums with different atomic numbers and radiation energies. The beta factor is higher than the gamma factor for some samples. These samples show relatively higher concentrations in lighter elements (up to Ba); thus, the concentration of minor elements may cause differing behavior between beta and gamma rays. The gamma factor may depend on Mn concentration; however, the elements most affecting the beta factor remain unknown. The accumulated dose from alpha rays is affected by sample thickness because of the spatial energy density around the center of the alpha track and luminescence detection range. Thus, for accurate alpha efficiency measurements, evaluation of the effective alpha ray range and luminescence detection thickness is important. The alpha efficiency against the gamma factor, known as the k value, increases with Mn concentration. Previous studies have suggested that the alpha efficiency is lower than beta and gamma efficiency because the ionization density produced by alpha particles is so great that the thermoluminescence traps in the tracks central core become saturated. This leads to a much greater proportion of the ionized electrons being wasted compared with beta and gamma radiation. Thus, we concluded that luminescence traps increase with increasing Mn concentrations. Keywords: Thermoluminescence dating, Calcite, Dose rate factor, Luminescence efficiency INTRODUCTION doi: /jmps M. Ogata, oga835@gmail.com Corresponding author Calcite (CaCO 3 ) is a ubiquitous mineral found in many geological formations. Different formations and fossils containing calcite, such as shells, corals and stalactites, have been used to reconstruct global environmental change (Wefer and Berger, 1991; Suzuki et al., 1999; Kano, 2012), while the age of archaeological artifacts made of limestone or marble provides chronological markers (Roque et al., 2001). Calcite veins found along fault planes are used to understand the nature and timing of fluid migration through fault zones (Watanabe et al., 2008). Thus, precise calcite dating is a key interest in geoscience and archaeological research. 14 C and Th/U disequilibrium dating have been applied to calcite (Plagnes et al., 2003), however, the limit of applicable ages from these dating methods are ( 14 C) and (Th/U) years. Furthermore, Th/U dating is difficult to apply to samples contaminated with detrital thorium (Debenham and Aitken, 1984). Dielectric minerals that interact with ionizing radiation emit thermoluminescence (TL) when heated. The emitted luminescence intensity is proportional to the accumulated radiation dose applied to the mineral by environmental radioisotopes. Therefore, TL can be applied to the dating of archaeological artifacts, volcanic products, and sediments (Aitken, 1985). The advantages of TL dating are (1) a dating limit of 10 6 years, which is longer

2 Measuring apparent dose rate factor for luminescence dating of calcite 337 than the 14 C and Th/U dating methods, (2) radioelements are not required in the target mineral, so it is applicable to a variety of minerals, and (3) age information is reset by heating and thus can record thermal events. TL emitted from calcite has been used to date many types of events, for example, the formation of materials, e.g., calcite veins (Franklin et al., 1988) and stalagmites (Debenham and Aitken, 1984), or the final heating event of a material, e.g., paleolithic artifacts (Roque et al., 2001). To estimate the accumulated dose, an artificial dose is used to produce a response curve, or calibration curve, for luminescence emission. An artificial radiation source, beta or X ray, is equipped with a luminescence reader (Yawata et al., 2007; Bøtter Jensen et al., 2003) and the dose rate is calibrated using quartz to calculate laboratory irradiation doses (Kadereit and Kreutzer, 2013). The absorbed dose rate of electromagnetic waves, e.g., gamma and X rays, may be different for calcite because its density and constituent elements, including impurities, differ from quartz. Pernicka and Wagner (1979) reported that the relationship between beta and gamma ray luminescence efficiencies is linear for quartz, but, to our knowledge, there is no such data for calcite. The TL efficiency of alpha particle irradiation, hereafter called alpha efficiency, is less than beta and gamma irradiation because of the spatial energy density around the center of the alpha track, thus the k value or a value (Zimmerman, 1972; Aitken, 1985) is introduced. The k value is the ratio between TL sensitivities to alpha radiation and beta or gamma radiation, while the a value is related to the equivalent dose by an alpha source divided by source strength. In previous studies, the obtained k and a values for calcite TL were and , respectively (Wintle, 1978; Debenham and Aitken, 1984; Theocaris et al., 1997; Roque et al., 2001). The absorbed dose rate of calcite with beta or gamma rays may be different from quartz, and the alpha efficiency of calcite may vary between samples. In this study, we measured the alpha efficiency and the ratios of equivalent dose by beta and gamma radiation of calcite to quartz using external artificial radiation (X ray) calibrated with quartz, hereafter termed the beta and gamma factors, respectively, using natural samples. Furthermore, we evaluated the relationships between the irradiation results and chemical composition, i.e., the concentration of impurities in calcite. Sample characterization MATERIALS Four natural calcite samples, DH1A, DH1B, DR10 3, Figure 1. Sample XRD patterns. All samples were identified as calcite. and ST01, were analyzed in this study. DH1A, DH1B, and DR10 3 are Cambrian carbonates from Mongolia. ST01 is a calcite vein that precipitated along a crack in the olivine basalt (breccia) from Luzon, the Philippines. All samples were identified as calcite using X ray diffraction (XRD, Ultima IV, Rigaku Corp.; Fig. 1). The chemical compositions of the samples were measured by laser ablation inductively coupled plasma mass spectrometry (LA ICP MS, MicroLas GeoLas Q plus 193 nm ArF excimer laser system and 7500s, Agilent) on a pressed powder pellet (Ito et al., 2009). The LA ICP MS sample signal was correlated to a reference material, synthetic glass SRM 610, National Institute of Standards and Technology (NIST) (Pearce et al., 1997), with a known chemical composition after normalizing an ablated volume using the internal standard ( 42 Ca) to calculate elemental concentrations. The detailed experimental procedure is described in Ito et al. (2009). Figure 2 shows the sample chemical compositions. Sample preparation The samples were crushed with a mortar and pestle, and immersed in acetic acid (0.5%) for 1 min (Wintle, 1975). The treated samples were then annealed at 450 C for 30 min to eliminate naturally accumulated TL. The samples were sieved and <125 µm grains were selected for beta and gamma factor measurements. For alpha irradiation, 1 8 µm grains were extracted by stokes settling. TL MEASUREMENTS Beta, gamma, and alpha doses were introduced to calcite samples, and then measured using the single aliquot regenerative dose (SAR) method (Murray and Wintle,

3 338 M. Ogata, N. Hasebe, N. Fujii and M. Yamakawa Figure 2. Chemical compositions of samples. 2000) with X rays as an external artificial source. The beta and gamma factors, and alpha efficiency were calculated as measured dose/given dose. Experimental configuration TL measurements were performed with a luminescence analyzer (MOSL 22, MEDEC) with an X ray source. The X ray dose rate was calibrated to 0.10 Gy/s for quartz using a known quartz dose (Yawata et al., 2007; Ganzawa and Ike, 2011). Red TL (RTL, nm) was detected using a photomultiplier tube (R649S, Hamamatsu, nm, peak 420 nm) with two filters (R60, HOYA + IRC 65L, KENKO). The RTL signals were obtained by heating the sample from 100 to 450 C at a rate of 1 C/s. Samples were preheated at 180 C for 120 s to eliminate low temperature unstable peaks and to selectively measure the thermally stable high temperature peaks (around 240 C). Integrated TL intensities around the TL peak temperature (±10 C) were used to calculate the equivalent dose. The background intensity was corrected with a blank measurement, which was conducted after each luminescence measurement cycle. Recycling tests were performed for all measurements, and recycling ratios were for most samples. The beta ray source was a disc type 90 Sr ( 90 Sr to 90 Y, 0.55 MeV; 90 Yto 90 Zr, 2.3 MeV, 10 kbq, Japan Radioisotope Association). The gamma ray source was a lift up type 60 Co (1.2 MeV and 1.3 MeV) located at Kyoto University Research Reactor Institute (KURRI). The alpha ray source was a disc type 241 Am (5.5 MeV, 3 MBq, Japan Radioisotope Association). Dose rate determination for each radiation source for quartz Beta source. Two beta ray sources (B1 and B2) were used to calibrate dose rates with quartz. Quartz grains were distributed on a 6 mm diameter silver disc in the dark. The sample thickness was 1 mm, and the distance between the sample and radiation source was 1 mm. Irradiation by B1 and B2 was performed for 90 h and 1,154 h, respectively, and the dose amount was estimated by the quartz luminescence via the SAR method. The B1 and B2 dose rates were ± and ± Gy/hour, respectively.

4 Measuring apparent dose rate factor for luminescence dating of calcite 339 Table 1. Calculated beta and gamma factors 1) The error due to beta irradiation was calculated from source strength. 2) The error in a given gamma ray dose was calculated from three measurements on each aliquot. 3) The error in measured gamma ray dose was calculated from three measurements on each aliquot. Gamma source. The dose rate for gamma source has been published by KURRI, and is Gy/h for water at the experiment site; thus, we estimated the dose rate during our irradiation experiment through measurements of quartz, which was irradiated together with the calcite samples. The samples were wrapped in 20 µm thick aluminum foil to prevent exposure to light. The application of gamma radiation to samples can be influenced by distance from the gamma ray source; therefore, to estimate the relationship between the distance from the gamma ray source and dose rate, calcite samples were sandwiched between two quartz samples. There was no shielding except for the sample containers, consisting of paper and aluminum foil, between the gamma source and samples. Samples were irradiated for 14.3 min and applied doses were estimated from the quartz luminescence using the SAR method with X rays. Three aliquots of each quartz sample were measured to determine given doses. The dose rates of gamma ray for quartz were Gy/h. Alpha source. Quartz has a variety of absorbed dose rates for alpha particles (Aitken, 1985), thus the alpha ray source strength was estimated using an alpha track detector (Joshirao et al., 2013). The radiation source, with an 8 mm diameter, and polycarbonate CR 39, a widely used track detector (Kanasaki et al., 2012), were placed 1 mm apart. This is the same placement as used in the sample irradiation procedure. After irradiation for 63 s at atmospheric conditions, CR 39 was etched with 6 M NaOH for 1 h at 70 C (Kanasaki et al., 2012), and the number of produced tracks on CR 39 were counted under the microscope (Eclipse 80i, Nikon). Tracks were evenly distributed. Five 100 µm 2 screens were observed and the average track density was calculated as 55 ± 4 per 100 µm 2 without the screens having the maximum or minimum counted tracks. For every 1 mm of atmospheric air that alpha particles traveling through, they lose ~ 0.1 MeV of energy due to the stopping power of the air. The dose rate of the alpha ray source (D α ) was calculated based on the number of alpha particles reaching the sample disc (N) using the following equation: D A ¼ NE A 1: ; where D α is in J/h, N is the number of alpha tracks/h, and E α is the energy of the alpha radiation source in MeV. The irradiating dose rate from the alpha source is J/h. Measurement of the applied radiation dose to calcite Beta ray. The calcite samples were irradiated on a silver disc in the dark for h (Table 1). The sample thickness was 1 mm, and the distance between the sample and radiation source was 1 mm. This maintained the same geometry as the experiment to determine the source strength with quartz. One aliquot of each calcite sample was analyzed. Irradiation, i.e., given, doses normalized for quartz were calculated from the irradiation duration, and were 19.5 ± 0.2 Gy, 25.5 ± 0.3 Gy, 22.6 ± 0.2 Gy, and 32.8 ± 0.3 Gy for DR10 3, DH1A, DH1B, and ST01, respectively (Table 1). The doses absorbed to the calcite samples were measured using the SAR method with X rays. The apparent equivalent, i.e., measured, doses of DR10 3, DH1A, DH1B, and ST01 were 5.27 ± 0.05 Gy, 4.87 ± 0.05 Gy, 7.28 ± 0.07 Gy, and 8.68 ± 0.09 Gy, respectively (Table 1). Thus, the ratios of the apparent measured doses to the given dose, the beta factors (Table 1), were 0.27 ± (DR10 3), 0.19 ± (DH1A), 0.32 ± (DH1B), and 0.26 ± (ST01). Gamma ray. Calcite samples were irradiated for 14.3 min together with quartz. The doses given to the calcite samples were estimated by placing proportional samples between two quartz samples whose doses were determined from the luminescence measurements. The

5 340 M. Ogata, N. Hasebe, N. Fujii and M. Yamakawa Table 2. Alpha ray dose, measured dose and alpha efficiency of DR10 3 with different thicknesses 1) The error due to alpha irradiation was calculated from the error in alpha track density. 2) Alpha ray doses for thicknesses <11.8 µm were corrected for sample thickness and effective range for alpha radiation. 3) The measured dose is the dose absorbed to the calcite samples measured using the SAR method with X rays. 4) The measured dose (J) was calculated with the sample weight. 5) Measured doses (J) from samples thicker than 17.6 µm were corrected to fit the sample weight at a thickness of 17.6 µm. normalized (to quartz) irradiation doses to DR10 3, DH1A, DH1B, and ST01 were 163 ± 8 Gy, 161 ± 8 Gy, 159 ± 8 Gy, and 156 ± 8 Gy, respectively. Three aliquots of each calcite sample were then measured to determine the measured dose. These were analyzed using the SAR method and were 53.1 ± 2.4 Gy, 27.3 ± 0.7 Gy, 28.2 ± 1.6 Gy, and 24.7 ± 0.7 Gy for DR10 3, DH1A, DH1B, and ST01, respectively (Table 1). Thus, the ratios of apparent measured to given doses, i.e., the gamma factors, were 0.33 ± 0.02 (DR10 3), 0.17 ± 0.01 (DH1A), 0.18 ± 0.01 (DH1B) and 0.16 ± 0.01 (ST01). Alpha ray. The accumulated dose using alpha rays, the measured dose, may be affected by the sample thickness due to the spatial energy density around the center of the alpha track. The effective X ray range is ~ 30 cm and highly uniform irradiation is possible with X rays. Therefore, we assume that X rays affect the whole sample regardless of thickness. However, a sample thickness that allows luminescence to reach the detector may differ from the effective X ray range. To evaluate the effective luminescence detection limit, we performed an experiment using samples with different thicknesses. The effective range for alpha rays was also estimated using samples of different thicknesses. The samples were prepared by depositing fine grains (1 8 µm) suspended in acetone on silver discs (Zimmerman, 1972; Debenham and Aitken, 1984). The sample thicknesses were calculated using the disc diameter, sample weight, and calcite density (Table 2). The calcite samples were irradiated with alpha radiation on a silver disc in the dark. The distance between the sample and radiation source was 1 mm, which was the same as in the experiment to determine the alpha source strength. Samples were irradiated for 2 h, corresponding to a given dose of J; the measured dose to calcite was estimated using the SAR method with X rays. The experiments to estimate the effective ranges of luminescence detection and alpha rays were performed on aliquots of DR10 3 by analyzing the µm thicknesses. The equivalent doses of DR10 3 were Gy (Table 2). For other samples, one aliquot of each was prepared with µm thicknesses to determine alpha efficiency. The measured doses of DH1A, DH1B, and ST01 were 46.1 ± 3.4 Gy, 36.6 ± 2.7 Gy, and 66.3 ± 4.8 Gy, respectively (Table 3). RESULTS AND DISCUSSION Beta and gamma factors The ratios of measured to given doses, where the given dose is the quartz value, are the beta and gamma factors, which were and , respectively (Table 1). These values indicate that the apparent equivalent dose by beta and gamma radiation of calcite is lower than quartz using X rays calibrated with quartz. This may be caused by differences in the substitution elements common in calcite, e.g., Ca, Mn, and Fe, and quartz, e.g., Li, Na, Al, and Si. Electromagnetic waves, e.g., gamma and X rays, interact with a medium in three ways, via the (1) photoelectric effect, (2) Compton effect, and (3) pair production. The ratio of these contributions depends on the radiation energy and atomic number of the medium. The atomic numbers of common substitution elements in calcite, e.g., 20 Ca, 25 Mn, and 26 Fe, are heavier than quartz, e.g., 3 Li, 11 Na, 13 Al, and 14 Si. Thus, the absorbed dose rate of electromagnetic waves for calcite is different from quartz because the ratios reflect contributions of the three

6 Measuring apparent dose rate factor for luminescence dating of calcite 341 Table 3. Sample thickness, measured dose, alpha efficiency, and k values 1) The errors in alpha irradiation measurements were calculated from the error in alpha track density. 2) Alpha ray doses were ± J for all samples. 3) k values were calculated using the equation (alpha efficiency for calcite against quartz/gamma factor). 4) The k value of DR10 3 is an average of results for different thicknesses. 5) The measured dose (J) of ST01 was corrected to fit the sample weight at a thickness of 17.6 µm. Figure 3. Plot of beta factor versus gamma factor. The dashed line shows a 1:1 correlation. interactions between electromagnetic waves and calcite or quartz (Davisson and Evans, 1952). In this study, we used tungsten target X rays (0.059 MeV, where the low energy component is eliminated with an Al absorber, Yawata et al., 2007) as artificial radiation and 60 Co (1.173 MeV and MeV) as gamma radiation. The difference in radiation energy between X rays and gamma rays also may affect the beta and gamma factors. The beta and gamma factors are generally consistent for DR10 3 and DH1A (Fig. 3). However, DH1A and ST01 have higher beta than gamma factors. These two samples have higher concentrations of lighter elements, through Ba (Fig. 2). The minor element concentrations may produce different behaviors between gamma and beta rays. Calcite TL properties have been studied in earlier research. Medlin (1959) suggested that impurities in calcite (e.g., Mn 2+ ) act as a TL activator. Medlin (1963) found that the orange emission from calcite reflects the transition from the lowest excited 4 G(T 1g ) state to the 6 S ground state in Mn ions. Medlin (1959) suggested that Figure 4. Beta and gamma factors as a function of (a) Mn and (b) Fe concentrations. Small insets in the upper right corner of (a) and (b) are enlarged views. the quenching effect of Fe 3+ affects the intensity of the glow peak in calcite TL. When beta and gamma factors are plotted against concentrations of Mn or Fe (Fig. 4), ST01 shows higher concentrations in Mn and Fe than the other three samples. Figure 4a shows an inverse correlation between gamma factors and Mn concentrations; however, no clear trend is found between beta radiation and Mn concentrations. Concentrations of Fe in DH1A, DH1B, and DR10 3 are similar, but their beta and gam-

7 342 M. Ogata, N. Hasebe, N. Fujii and M. Yamakawa ma factors shows variance outside of the error range. These results suggest that the gamma factor may depend on Mn concentration. The major element affecting the beta factor is unclear. Lighter elements, through Ba, may affect the beta factor, so we may have to consider that a combination of several elements is needed to fully describe the relationship between chemical composition and beta and gamma factors. In this study, we measured natural calcite samples with complex chemical compositions, which precludes us from determining the relationship between individual elemental concentrations and beta and gamma factors. Further analysis is necessary to evaluate and quantify the relationship between multiple impurities, their concentrations, and the beta and gamma factors. This could be accomplished using synthetic calcite where the chemistry is controlled. Alpha efficiency Figure 5. Sample luminescence intensities (10 Gy by X ray) as a function of sample thickness. The error in luminescence intensity was calculated by the intensities induced by test doses over seven cycles. The error in sample thickness was established with repetition and linearity of the balance. Figure 6. Luminescence intensity of samples irradiated with alpha rays for 2 h plotted as a function of sample thickness. See Figure 5 for error estimates. The effective range of luminescence detection and alpha rays was measured using DR10 3 (Table 2). For alpha rays, the relationship between lost energy and passing distance through a given material is not constant (Zimmerman, 1972); therefore, the effective range of alpha rays is important for determining accurate k values. First, the sample thickness that allowed luminescence to reach the detector was examined. Luminescence intensity was measured after applying 10 Gy of X rays to different sample thicknesses (Fig. 5). Saturation was reached at 17.6 µm, which implies luminescence from the surface to a depth of 17.6 µm contributes to intensity. Next, luminescence intensity after alpha ray irradiation for 2 h was examined (Fig. 6). It reached a maximum at 11.8 µm, which sets the effective range of alpha rays at ~ 12 µm. Luminescence intensities when thicknesses are >11.8 µm are lower than at 11.8 µm. Considering the error, luminescence intensities over a thickness of 11.8 µm, except for 52.9 µm, are similar to that at 11.8 µm. The 52.9 µm thick sample emitted low luminescence intensity, but this may be an accident. Considering the density differences between calcite (2.71 g/cm 3 ) and quartz (2.65 g/cm 3 ), our results indicate that the effective range of alpha rays for calcite is shorter than quartz, 14 µm, in agreement with Okumura et al. (2008). The measured doses show two different values, a constant value at ~ 41 Gy for samples <11.8 µm, and ~ 31 Gy for samples >17.6 µm (Fig. 7). The growth of alpha ray induced TL with increasing thickness is equal to X ray until 11.8 µm, resulting in a constant measured dose. We might overlook the possible increase in the dose due to the Bragg peak (Fig. 7), because the alpha experiment accompanies significant errors from the alpha decay in the source disc and weighing machine precision. When the sample thickness is >11.8 µm, the alpha ray induced TL is constant while X ray induced TL increases, resulting in a reduction in the measured dose. The stable measured dose for sample thicknesses >17.6 µm may be caused by constant luminescence emission, and is unrelated to thickness because sample thicknesses are over the luminescence detection limit. Alpha efficiencies were calculated after considering the effective range of alpha ray and luminescence detection limit (Fig. 8 and Table 2). Alpha ray doses, i.e., given doses, for samples with thicknesses <11.8 µm were corrected for the effective range of alpha rays and sample thickness, assuming a homogeneous distribution of luminescence sites within a sample. For thicknesses >17.6

8 Measuring apparent dose rate factor for luminescence dating of calcite 343 Figure 7. A plot of the equivalent dose from alpha ray, i.e., the measured dose, against sample thickness. See Figure 5 for error estimates. Figure 9. The alpha efficiency as a function of (a) Mn and (b) Fe concentrations. Small insets in the lower right corner of (a) and (b) are enlarged views. Figure 8. Relationship between alpha efficiency and sample thickness. See Figure 5 for error estimates. The shadowed area indicates the mean value and 2SD error. µm, measured doses were corrected for sample weight at a thickness of 17.6 µm. The alpha efficiencies of samples after correction were similar regardless of sample thickness ( ). The gamma factor is 0.33 for DR10 3, resulting in k values of This is consistent with earlier studies that found lower k values of (Theocaris et al., 1997; Roque et al., 2001). Accurate k values may be possible to estimate for any sample thickness when corrected for the alpha ray effective range and luminescence detection limit. However, for alpha rays, the relationship between lost energy and distance should not be assumed to be constant over the entire sample thickness. Thus, sample thicknesses over the effective range of alpha rays, and below the luminescence detection limit, are preferable for measuring alpha efficiency. The alpha efficiencies of DH1A, DH1B and ST01 were analyzed for samples with thicknesses of 12.5 µm (DH1A, DH1B) and 25.0 µm (ST01, Table 3). The resultant alpha efficiencies were ± 0.002, ± 0.002, and ± for DH1A, DH1B, and ST01, respectively. The k values were calculated using the gamma factor, and were 0.17 ± 0.02, 0.13 ± 0.01, and 0.36 ± 0.04 for DH1A, DH1B, and ST01, respectively. This result is consistent with previous studies, with k values of (Theocaris et al., 1997; Roque et al., 2001). The alpha efficiencies and k values are also plotted against concentrations of Mn and Fe (Figs. 9 and 10), and alpha efficiencies are correlated with Mn and Fe concentrations (Fig. 9). Figure 10a shows the correlation between k values and Mn concentrations. Concentrations of Fe in DH1A, DH1B, and DR10 3 are similar, but their k values vary significantly (Fig. 10b). These results suggest that the k values for calcite may depend on Mn concentrations. Zimmerman (1972) found that higher beta saturation doses are correlated with higher k values. These results support the interpretation that alpha efficiency is lower than beta and gamma efficiency because the ionization density produced by alpha particles is so great that the TL traps, lying in the tracks central core, become saturated. This saturation results in a much greater proportion of the ionized electrons being wasted compared to in beta and gamma radiation. Our results indicate that higher k values correspond to samples with higher

9 344 M. Ogata, N. Hasebe, N. Fujii and M. Yamakawa The equivalent dose by beta and gamma radiation of calcite is lower than quartz using X rays calibrated with quartz. This relationship may be caused by differences in the common substitution elements of calcite and quartz, and the interactions between mediums with different atomic numbers and various radiation energies. Beta factors are higher than gamma factors in some samples. These samples show high concentrations in lighter elements, through Ba, suggesting that the concentration of minor elements may cause the differing behavior for beta versus gamma rays. The gamma factor may depend on Mn concentrations. However, no primary element appears to affect the beta factor. Further analysis is needed to quantitatively evaluate the relationship between multiple impurities, their concentrations, and the beta and gamma factors. The accumulated dose from alpha rays is influenced by the sample thickness because of spatial energy density around the center of the alpha track and the detection range of luminescence. Thus, for accurate k value measurements, an evaluation of the effective range of alpha rays and the luminescence detection limit is important. The k values for calcite may depend on Mn concentrations, and we suggest that luminescence traps, i.e., the saturation doses, increase with increasing Mn concentration. The beta and gamma factors and k values for calcite may depend on Mn and other elemental concentrations. For accurate TL dating of calcite, an important component is determining the beta and gamma factors and k values or a values. ACKNOWLEDGMENTS Figure 10. The k value as a function of (a) Mn and (b) Fe concentrations. Small insets in the lower right corner of (a) and (b) are enlarged views. Mn concentrations. Therefore, we conclude that luminescence traps (saturation dose) increase with increasing Mn concentration. CONCLUSIONS We would like to express our gratitude to Prof. Shoji Arai and Dr. Akihiro Tamura for their help with the LA ICP MS analyses. We thank Dr. Yukihiro Nakano and Dr. Masaaki Sakamoto for helping with the Co 60 gamma ray irradiation at the Kyoto University Research Reactor. The authors are grateful to Prof. Akihiko Yokoyama for valuable advice regarding the alpha irradiation experiment. This research was part of a project spanning to develop an integrated natural analogue program, and was funded by the Agency for Natural Resource and Energy (Ministry of Economy, Trade and Industry, Japan). REFERENCES Aitken, M.J. (1985) Thermoluminescence Dating. Academic Press, London. Bøtter Jensen, L., Andersen, C.E., Duller, G.A.T. and Murray, A.S. (2003) Developments in radiation, stimulation and observation facilities in luminescence measurements. Radiation Measurements, 37, Davisson, C.M. and Evans, R.D. (1952) Gamma Ray Absorption Coefficients. Reviews of Modern Physics, 24, Debenham, N.C. and Aitken, M.J. (1984) Thermoluminescence dating of stalagmitic calcite. Archaeometry, 26, Franklin, A.D., Hornyak, W.F. and Tschirgi, A.A. (1988) Thermoluminescence dating of tertiary period calcite. Quaternary Science Reviews, 7, Ganzawa, Y. and Ike, M. (2011) SAR RTL dating of single grains of volcanic quartz from the late Pleistocene Toya Caldera. Quaternary Geochronology, 6, Ito, K., Hasebe, N., Sumita, R., Arai, S., Yamamoto, M., Kashiwaya, K. and Ganzawa, Y. (2009) LA ICP MS analysis of pressed powder pellets to luminescence geochronology. Chemical Geology, 262, Joshirao, P.M., Shin, J.W., Vyas, C.K., Kulkarni, A.D., Kim, H., Kim, T., Hong, S. and Manchanda, V.K. (2013) Development of optical monitor of alpha radiations based on CR 39. Applied Radiation and Isotopes, 81, Kadereit, A. and Kreutzer, S. (2013) Risø calibration quartz A challenge for β source calibration. An applied study with relevance for luminescence dating. Measurement, 46, Kanasaki, M., Fukuda, Y., Sakaki, H., Nishiuchi, M., Kondo, K., Kurashima, S., Kamiya, T., Hattori, A., Oda, K. and Yama-

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