Radiocarbon dating of charred human bone remains preserved in urns excavated from medieval Buddhist cemetery in Japan

Nuclear Instruments and Methods in Physics Research B 268 (2010) 985–989

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Radiocarbon dating of charred human bone remains preserved in urns excavated from medieval Buddhist cemetery in Japan Toshio Nakamura a,*, Shinichi Sagawa b, Tetsuya Yamada b, Masaaki Kanehara c, Norio Tsuchimoto d, Masayo Minami a, Takayuki Omori e, Mitsuru Okuno f, Tomoko Ohta a a

Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan Gangoji Institute for Research of Cultural Properties, Nakain, Nara 630-8392, Japan School of Science Education, Nara University of Education, Takabatake, Nara 630-8528, Japan d Ichinomiya City Museum, Yamato, Ichinomiya 491-0922, Japan e Graduate School of Environmental Studies, Nagoya University, Chikusa, Nagoya 464-8602, Japan f Faculty of Science, Fukuoka University, Jonan, Fukuoka 814-0180, Japan b c

a r t i c l e

i n f o

Article history: Available online 26 November 2009 Keywords: Radiocarbon dating Cremation Charred bone Urn Urnfield Typological chronology

a b s t r a c t For a preliminary test of 14C dating of cremated human remains, we have collected charred bone and wood–charcoal fragments from cremated remains contained in cinerary urns that had been excavated from medieval Buddhist cemetery at the Hoenji temple in Aichi prefecture, central Japan. More than 230 urn vessels were discovered from the excavated area of ca. 14 m wide and 14 m long. The identification of charred bone or charcoal fragments among the remains was performed by observation of surface appearance, inspection of fine structures by a microscope, bubble formation during the HCl treatments in preparing target material for AMS 14C dating, carbon and nitrogen contents, d13C and d15N values of the fragments. All 14C ages obtained for the samples that were identified as charred bone remains were almost consistent with the archeological age estimated based on typological analysis of respective urns. On the other hand, some 14C ages for the remains identified as wood charcoal, which had been produced from firewood or a wooden coffin during the cremation, were not consistent with archeological estimation, shifting toward older 14C ages, most probably as the result of old wood effect. Ó 2009 Published by Elsevier B.V.

1. Introduction It is known from official records such as the historical document, Shokunihongi (chronicle of Japan, continued) that a Buddhistic priest, Dosho, was cremated for the first time in AD700 and successively the Emperor Jito was in AD703 in Japan. In the Nara period (AD710–794), cremation became popular among people who lived in the areas ruled by the central government at that time. However, the popularity of cremation had declined by the mid-Heian period (ca. 12th century), as suggested archeologically by a smaller number of excavated urnfields belonging to that period. In the medieval period, cremation became popular largely again, and has been utilized as burial rites to a fairly large extent until the early modern period. Human remains after inhumation are well preserved in some cases and 14C dating can be normally applied to human bone residues among them. On the other hand, the full human body includ* Corresponding author. Tel.: +81 52 789 3082; fax: +81 52 789 3092. E-mail address: [email protected] (T. Nakamura). 0168-583X/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.nimb.2009.10.080

ing bones is almost completely combusted to ashes when it is heated at more than 800 °C for several hours in cremation. Age estimation of the burials of the human remains after cremation, which is necessary for recent investigation of the cremation history in Japan, has been conducted based on typological analysis of urns, i.e., pottery vessels that contained the cremated remains. However, typological analysis does not exhibit any numerical/historical age of the samples. Its application is limited only to domestic pottery vessels, but not to foreign ones in case of chronological intercomparison studies. Thus application of 14C dating directly to cremated human remains has been earnestly desired. It is well established that 14C dating of cremated bones can be carried out using the small structural carbonate components contained within the hydroxyapatite (or bio-apatite) [1–3]. Instead of applying the rather complicated procedures of extracting the carbonate components [1,2], we have tried a more direct method of 14C dating for the cremated bones, by collecting directly charred bone remains and applying a normal alkali-acid-alkali treatments on them. We have collected charred remains, including both charred bone and wood–charcoal remains. During the sample

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preparation procedures for 14C dating, we separated carefully charred bone from charcoal remains, as will be described later, because the former is more appropriate material for the determination of cremation age than the latter. Pottery vessels used as ash containers do not normally possess any charred materials adhered to their surfaces, being different from Jomon and Yayoi pottery fragments in Japan that possess charcoal or soot remains on their surfaces that have been produced during cooking of food [4]. Therefore, we have searched for charred materials directly among the cremated remains for 14C dating, and discovered that some parts of heated bones or cremation pyres may sometimes contain charred residues, maybe due to insufficient oxygen supply during the cremation. Thus, we have collected some charred materials from the cremated remains contained in cinerary urns that had been excavated at the cemetery of the Hoenji temple in Ichinomiya-city, Aichi prefecture, central Japan, to estimate the calendar age of cremation in the medieval period by 14 C dating and to compare the obtained numerical ages with the age estimated by preceding typological analysis of pottery vessels.

2. Cremated human remains from medieval cemetery at Hoenji temple It is not well known when the Hoenji temple was established. However, the basis for stone monuments located at the graveyard in the precincts of the temple keeps a written historical record that suggests that the temple was reopened for the Pure Land (Jodo) sect of Buddhism in 1557. The Hoenji temple is located at Daiwa-cho, Ichinomiya-city, Aichi prefecture, central Japan, locating on a natural river terrace at 7.5 m above sea level. Medieval urnfields were discovered inside the grounds of the Hoenji temple in 1931. After half a century from the discovery, full-scale excavation surveys were conducted three times in 1982, 1991 and 1992 [5]. The sediments of the excavated graveyards are composed of four soil layers: surface soil layer (I), liver-brown colored layer (II), yellowish-brown sandy-silt layer (III), and yellowish-brown layer (IV), in descending order. From a detailed investigation of the structure of the layers I and II, it is concluded that the layer II covered the medieval graveyards completely leaving only the top parts of several grave stones in open air, by an event like a huge flood occurred in a short period, because the lower surface of the layer I is bumpy, while that of the layer II is rather flat. The excavated area of medieval graveyards at the Hoenji temple was ca.14 m wide and 14 m long. Structural remnants of the excavated urnfields are classified into four types that are composed of: (1) five sets of piled-up tombstones surrounded by a square boundary (ca. 3.5 m  3.5 m) made of stones; (2) five sets of piled-up tombstones without boundary stones; (3) two sets of grave composed of accumulated stones; and (4) four sets of grave with a stone monument without piled-up tombstones. Totally around 230 urn vessels, which have kept almost original shape or lost some parts of their original body, were excavated from the graveyards. The cremated human remains contained in individual urns have been collected and packed separately in respective plastic bags, and preserved together with urns and other excavated archeological remains at Ichinomiya City Museum in Aichi prefecture, Japan. We recently selected eight cinerary urns of Nos. 4P24, 4P33, 4P41, 4P42, 4P43, 4P44, 7P2 and 990426JA that contained blackcolored fragments among cremated remains more abundantly than others, according to an excavation survey report [5], which describes physical structure and decorations on the outer surface of the urns, chronology estimated by typological analysis of them, characteristics of the cremated remains, along with the details of the graveyards from which the urns were excavated. We carefully

picked up black-colored fragments from each plastic bag containing the cremated remains, as candidate samples for 14C analysis. 3. Experimental procedure 3.1. Selection of charred bone and wood charcoal samples from cremated remains Two different kinds of carbonaceous charred remains were selected from the cremated human remains that had been preserved in the medieval urns after cremation: (1) wood–charcoal fragments remained from firewood used as fuel or a wooden coffin during cremation; (2) charred remains from human bone. First, we checked an appearance of each charred fragment to select a lump possessing black portion inside. Since surfaces of all fragments were completely covered with ashes, we carefully separated pieces containing charred portion from the fragments that had turned into ashes by combustion. Some fragments were identified as wood charcoal possessing a clear annual ring structure. However, most of the selected charred fragments were not decisive whether they were produced from bones or wood, by only an optical inspection. We have applied some other checks to the collected samples, to distinguish charred bones or wood charcoal among them, as will be discussed latter. 3.2. Sample preparation and

14

C dating with AMS

Removal of carbonaceous contaminants by acid-alkali-acid treatments and CO2 extraction from charred remains, production of graphite from the CO2, and AMS 14C measurements, were performed at the Center for Chronological Research, Nagoya University [6,7]. In brief, the charred remains were treated several times with 1.2 M HCl for 2 h at 90 °C, to remove any possible carbonate contaminant. Next, the samples were treated with 1.2 M NaOH solution for 2 h at 80 °C several times until the solution remained clear after the 2-h treatment. The samples were treated again with 1.2 M HCl for 2 h at 90 °C, and rinsed with distilled water to completely remove any residual HCl. During the first HCl treatment, we watched the charred materials carefully to detect small bubbles coming out of the materials. It is most probable that the sample is originated from bone if bubble formation continues until the inorganic portion of the bone will dissolve with leaving solid charred portion. Contrarily, if the bubble formation stops in a short time until bone powder disappeared from the sample surface, the sample is regarded to have originated from charred wood. The charred remains were dried in an electric oven at 90 °C. About 4 mg of the samples were placed in Vycor tubes of 9-mm outside diameter, with ca. 500 mg of granular CuO, and then the tubes were connected to a vacuum line, evacuated completely and sealed to a tube length of ca. 300 mm. The Vycor tubes were heated to 900 °C for 3 h to convert the sample to CO2 completely. The CO2 produced of ca. 1.5 mg in carbon was purified cryogenically in a vacuum line and reduced to graphite on ca. 3 mg of Fe powder by hydrogen at 620 °C for 6 h [8]. The graphite materials were pressed into aluminum target holders for AMS 14C measurements. We used the HOx-II standard (NIST oxalic acid, SRM-4990C) as a 14C-concentration reference. The d13C values were measured by the AMS system with errors less than ±1‰, including the effects from both machine instability and graphite production [7], and were used for a mass-fractionation correction in calculating sample 14C concentrations [9]. Finally, the conventional 14C ages were calculated using the half-life of 5568 years, as summarized in Table 1, for the cremated remains collected from urns. The 14C errors cited here include statistical uncertainties based on 14C counting of

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T. Nakamura et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 985–989 Table 1 Amount of sample for combustion, CO2 yield rate, d13C,

14

C age, calibrated age, lab code #, and archeologically estimated chronology of urns. d13C (‰)a

14 C age (BP)

1r error

Calibrated age range using IntCal04 with 1r error (probability)

Lab code # (NUTA2-)

Archeological age estimated by typology of urns

982

36

11267

Early 13th C

24.3

936

36

11268

Early 13th C

3.56 (58.8)

24.6

876

36

11269

Early 13th C

6.10 6.00

3.57 (58.5) 3.42 (57.7)

23.1 22.4

1400 867

37 36

11270 11271

Early 13th C Early 13th C

Bone Wood

n.m. n.m.

6.90 (n.m.) 2.39 (n.m.)

22.7 23.6

731 562

27 26

13093 13085

Early 13th C Early 13th C

4P33-L3

Bone

6.23

3.40 (54.8%)

22.7

766

27

13086

Early 13th C

9

4P41-U1

Wood

6.16

3.29 (53.4%)

28.2

652

27

13087

Late 13th C

10

4P42-U1

Wood

6.12

3.69 (60.0%)

28.8

676

27

13088

Late 13th C

11

4P42-L1

Wood

6.04

3.74 (62.0%)

27.6

623

30

cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal cal

13089

Late 13th C

12 13 14 15 16

4P43-A 4P44L1 4P44L2 7P2-L1 990426JA

Bone Wood Bone Bone Bone

n.m. n.m. n.m. n.m. 6.24

0.04 (n.m.) 0.81 (n.m.) n.m. 0.09 (n.m.) 3.87 (62.1%)

n.m. 25.2 n.m. n.m. 23.9

n.m. 831 n.m. n.m. 792

No.

Sample #

Original material analyzed

Amount of pretreated material

CO2yield (mg) (yield rate; %)

1

4P24-1-1

Wood

6.06

3.42 (56.4)

24.7

2

4P24-1-2

Wood

6.01

3.55 (59.1)

3

4P24-2

Bone

6.06

4 5

4P24-3 4P24-5

Wood Bone

6 7

4P33-1 4P33-L2

8

AD1016–1048 (46.8%), AD1087–1123 (40.1%), AD1138–1150(13.0%) AD1036–1054 (19.6%), AD1077–1154 (80.4%) AD1053–1079 (22.6%), AD1153–1216 (77.4%) AD617–659 (100%) AD1056–1076(13.6%), AD1154–1219 (86.4%) AD1265–1284(100%) AD1324–1345 (48.1%), AD1393–1413 (51.9%) AD1228–1232 (8.0%), AD1240–1247 (14.1%), AD1251–1276 (77.9%) AD1288–1309 (44.1%), AD1361–1386 (55.9%) AD1281–1300 (64.5%), AD1368–1381 (35.5%) AD1298–1322 (38.9%), AD1348–1372(39.4%), AD1377–1392 (21.7%)

36

cal AD1184–1254 (100%)

13091

Early 13th C

26

cal AD1224–1261 (100%)

13092

Middle 13th C

n.m.: not measured, mainly because of sample shortage. a d13C values shown are measured with AMS system with 1r error of ca. ±1‰.

sample, standard and 14C-background targets, machine errors evaluated by the 14C reproducibility of repeated measurements on standard targets, and errors in 14C-background removal calculations. The obtained 14C age was calibrated to calendar age by using a calibration program Calib5.0.1 [10] and IntCal04 calibration data set [11], as given in Table 1. It is important to distinguish charred bone residues from charcoal remains produced from cremation pyres for the exact determination of cremation age. However, the amounts of charred materials recovered after the chemical treatments were so small for most of the samples, they were used for 14C dating preferentially. Ten charred remain samples out of 12 were successfully prepared for 14C dating. Since CO2 prepared for 14C dating were less than 0.1 mg in carbon for two samples (4P43, 7P2 in Table 2), we could not conduct 14C analysis to them. Only for the rather abundant sample 4P24, along with 14C dating, we have also checked fine structures on the surface of the sample fragments of ca. 3 mm thick tips by using both a stereoscopic microscope and a polarization microscope. In addition, we measured N and C contents with an element analyzer (NC2500, CE Instruments), and analyzed d15N and d13C for sample 4P24 by using an isotope ratio mass analyzer (Finnigan MAT-252) [12].

4. Results and discussion For the cremated remains from six out of eight urn vessels excavated from urnfields at the Hoenji temple, the amount of the pretreated materials used for combustion, CO2 yield, yield rate, d13C, 14 C ages, calibrated age ranges for 1r error, archeological chronology of urns estimated based on typology, are summarized in Table 1. The charred samples were finally judged whether they originated from bone or wood, as shown on the 3rd column in Table 1, based on careful tests on them in the following, (1) optical observation with a microscope; (2) existence of bubble formation during acid treatment; (3) C and N contents, d13C and d15N values (Table 2). Some samples were identified as wood charcoal by their specific d13C values, C and N contents. The charcoal samples showed 14C ages older or younger than those for bone charcoal, depending on the sample features. On the other hand, historical age obtained by calibration with IntCal04 of 14C dates of the charred bone residues was almost consistent with the age estimated by typological analysis of pottery vessel that contained the bone residues (Fig. 1). The meanings of the obtained results on the cremated remains from each urn are discussed below separately.

4.1. Urn No. 4P24 Table 2 C and N contents, d13C and d15N values for cremated remains from urn No. 4P24.

4P24-1-1 4P24-1-2 4P24-2 4P24-3 4P24-5

C content (%)

N content (%)

64.6 65.5 n.m. 64.5 62.0

0.341 0.362 n.m. 0.342 6.74

d13C (‰)

n.m.: not measured, mainly because of sample shortage.

24.7 24.3 24.6 23.1 22.4

d15N (‰) n.m. n.m. 13.0 n.m. 13.7

Calibrated ages for 4P24 samples (4P24-1-1, 4P24-1-2, 4P24-3) that were recognized as charred wood remained from cremation pyres or a wooden coffin scattered ranging from cal AD617 to cal AD1154 (Table 1). Although two 14C ages quite consistent with each other were obtained for samples 4P24-1-1 and 4P24-1-2 that were divided into two from a piece of wood charcoal, the dates for the charred wood are remarkably older compared to the dates for charred bone samples (4P24-2, 4P24-5, shown in Table 1). By

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microscopic analysis (Figs. 2a and 2b), three wood fragments (4P24-1-1, 4P24-1-2, 4P24-3) were identified as Japanese cypress (Chamaecyparis obtusa Endl.), which may grow quite large (>400 years old). The older 14C ages for these three charcoal fragments have resulted most probably from an old wood effect, being dependent on the way how the wood materials had been derived either from inner part of a log with growth age as many as ca. 400 years or from an old scrapped piece of lumber. On the other hand, calibrated ages for two charred bone samples are consistent with each other, giving calendar year of cal AD1050–1080 and cal AD1150– 1220, as a plausible age for cremation of the dead. The natural fluctuation of the 14C age values on the IntCal04 calibration curve in the range covered by 14C ages obtained for the cremated remains, as shown in Fig. 1, have forced the estimated calendar age range wider than 150 years and even split it into two or more periods in some cases. Archeological age estimation of urn No. 4P24 based on typological analysis concludes that the vessel (26.7 cm high) was produced at around early 13th century and the urn cover at middle 13th century [5]. The estimated age range of cremation of the dead based on 14 C dating of charred bone samples overlaps with that estimated by typological analysis of the urn vessel. However, the former is a bit older than the latter. It is possible that bone collagen of the dead can show a bit older 14C age than the year of death for a person, because of retention time in metabolism of organic matter in human bone. 4.2. Urns Nos. 4P41 and 4P42 Both urns of Nos. 4P41 and 4P42 were settled side by side in a pit of 30 cm deep and 60 cm diameter digging into the base soil layer, and covered by pebbles, in the graveyard. Contents of the urns were a mixture of cremated bones of a woman and a child, according to osteological analysis. Thus it is most likely that the contents of the two urns are of the same origin and should show similar 14C ages. A wood sample from 4P41 was dated as 652 ± 27 BP, and two wood samples from 4P42 were 676 ± 27 BP and 623 ± 30 BP. These results of 14C analysis are consistent with

Fig. 2a. Microscopic photo of wood charcoal (4P24-3), showing the straight grain of wood. Annual rings can be observed. Species of the wood is identified as Japanese cypress (Chamaecyparis obtusa Endl.).

the archeological estimation on the contents of the two urns. All three samples showed d13C values from 28‰ to 29‰, which are consistent with a value of C3 plant and much lower than a value of bone collagen.

4.3. Urn 4P33 Two bone and one wood fragments were selected from urn No. 4P33. This urn could have been produced in early 13th century as suggested by typological analysis. Since yields of carbonaceous materials after chemical pre-treatments looked so small that we used all of the residues after pre-treatments for sample combustion to CO2 for 4P33-1 and 4P33-L2, as shown in Table 1. Two bone residues showed 14C ages consistent with each other (731 ± 27 BP and 766 ± 27 BP), but a bit older than the archeological estimation. However, if the urn was reused a few 10 years after its production, then the 14C ages may suggest reasonable age of cremation. On the other hand, the 14C age for the wood fragment (562 ± 26 BP) is considerably younger than the archeological estimation. At the moment, we suspect that the wood fragment could be a contamination derived from upper layers.

4.4. Other urns Fig. 1. Comparison of 14C ages obtained for cremated human remains preserved in cinerary urns excavated at cemetery of the Hoenji temple in central Japan. Solid lines indicate 14C ages for burnt bone residues and dotted lines for wood–charcoal remains.

A wood and a bone samples were collected from runs of Nos. 4P44 and 990426JA and have shown 14C ages of 831 ± 36 BP and 792 ± 26 BP, respectively. Production ages of the urns are estimated typologically to be early 13th century and middle 13th cen-

T. Nakamura et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 985–989

989

perfect candidates for the age determination of cremation. The older 14C ages for three wood fragments (4P24-1-1, 4P24-1-2, 4P24-3) most probably resulted from old wood effect. The younger wood fragment (4P33-L2) may have mixed in the remains from the upper layers. (3) To identify both charred bone and wood-charcoal materials from the cremated remains, we analyzed C and N contents, d13C and d15N values for the remains collected from urn No. 4P24. The C and N contents for 4P24-5 were 62.0% and 6.74% in weight, respectively, and gave C/N ratio as 9.2, which is far higher than a typical value of 3.2 ± 0.5 [13] for bone collagen. This shift in C/N ratio to the higher value of 9.2, but the ratio is still much lower compared with that of wood material, may be the result of heating bone material at around 800 °C in cremation; nitrogen escapes more preferentially than carbon during the heating process that produced charred bone remains. In addition, d15N values for charred bone materials (Table 2) were higher compared with typical values for plants (0–7‰), and consistent with those for human bone collagen [14]. d13C values obtained for charred bone samples ( 24.6‰ to 22.4‰) were somewhat lower than typical values for bone collagen of 23.5‰ to 20.5‰ [14] for human such as a Japanese Buddhist who preferentially ate terrestrial-plant resources (Table 1).

Acknowledgements

Fig. 2b. Microscopic photo of charred bone (4P24-2) after cremation.

tury, respectively, and are consistent with the calendar age ranges calibrated from respective 14C ages (Table 1). 5. Summary We have selected both charred bone and wood–charcoal fragments from cremated human remains contained in cinerary urns that were excavated from the Hoenji temple, Ichinomiya-city, Aichi prefecture, Japan. AMS 14C dating was applied to both the charred bone and charcoal samples, and the calendar age ranges calibrated from the obtained 14C ages were compared with archeological estimation based on typological analysis of the urns, i.e., pottery vessels for the cremated remains. The results are summarized in the following: (1) All 14C ages obtained for charred bone remains contained in urns were consistent with archeological estimation based on typological analysis of respective urns. (2) Some 14C ages for wood–charcoal remains contained in urns, which had been produced from cremation pyres or a wooden coffin, were also consistent with archeological estimation of respective urns. However, some 14C ages were older or younger to some extent than archeological estimation (Table 1), suggesting that the charcoal remains are not

The authors would like to thank the staff of the Technical Center of Nagoya University, for their kind support in maintaining a HVE Tandetron 14C-AMS system at Nagoya University. This work was supported partly by the ‘Grant-In Aid for Scientific Research’ of the Japan Society of the Promotion of Science (JSPS) (subject #: 19300300, 16320108 and 15068206). References [1] J.N. Lanting, A.L. Brindley, J. Irish Archaeol. 9 (1998) 1. [2] J.N. Lanting, A.T. Berts-Bijima, J. van der Plicht, Radiocarbon 43 (2001) 249. [3] P. Naysmith, E.M. Scott, G.T. Cook, J. Heinemeier, J. van der Plicht, M.V. Strydonck, C.B. Ramsey, P.M. Grootes, S.P.H.T. Freeman, Radiocarbon 49 (2007) 403. [4] T. Nakamura, Y. Taniguchi, S. Tsuji, H. Oda, Radiocarbon 43 (2001) 1129. [5] Ichinoniya City Board of Education, Excavation Report of Medieval Cemetery at Hoenji Temple at Ichinomiya-city in Aichi Prefecture, 1995, p. 43 (in Japanese). [6] T. Nakamura, E. Niu, H. Oda, A. Ikeda, M. Minami, H. Takahashi, M. Adachi, L. Pals, A. Gottdang, N. Suya, Nucl. Instr. and Meth. B172 (2000) 52. [7] T. Nakamura, E. Niu, H. Oda, A. Ikeda, M. Minami, T. Ohta, T. Oda, Nucl. Instr. and Meth. B223–224 (2004) 124. [8] H. Kitagawa, T. Masuzawa, T. Nakamura, E. Matsumoto, Radiocarbon 35 (2) (1993) 295. [9] W.G. Mook, J. van der Plicht, Radiocarbon 41 (3) (1999) 227. [10] M. Stuiver, P.J. Reimer, Radiocarbon 35 (1) (1993) 215. [11] P.J. Reimer, M.G.L. Baillie, E. Bard, A. Bayliss, J.W. Beck, C. Bertrand, P.G. Blackwell, C.E. Buck, G. Burr, K.B. Cutler, P.E. Damon, R.L. Edwards, R.G. Fairbanks, M. Friedrich, T.P. Guilderson, K.A. Hughen, B. Kromer, F.G. McCormac, S. Manning, C. Bronk Ramsey, R.W. Reimer, S. Remmele, J.R. Southon, M. Stuiver, S. Talamo, F.W. Taylor, J. van der Plicht, C.E. Weyhenmeyer, Radiocarbon 46 (3) (2004) 1029. [12] M. Minami, H. Muto, T. Nakamura, Nucl. Instr. and Meth. B223–224 (2004) 302. [13] P.E. Hare, D. von Endt, Annual Report of Director of the Geophysical Laboratory 1989–1990, Carnegie Institute, Washington, 1990, p. 115. [14] S. Mihara, K. Miyamoto, T. Nakamura, H. Koike, Nucl. Instr. and Meth. B223– 224 (2004) 700.