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Dose determination on tooth enamel fragments from two human fossils
Radiation Measurements 32 (2000) 773±779
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Dose determination on tooth enamel fragments from two human fossils Steve Robertson, Rainer GruÈn* Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Received 18 October 1999; received in revised form 3 February 2000; accepted 29 February 2000
Abstract The ESR dose values of two human tooth enamel fragments were measured in 108 steps with an automated goniometer. ESR spectra were deconvoluted using four Gaussian peaks. The central region of the spectra is interfered with by at least one additional line which yields signi®cantly lower dose values than the central, apparently axial region. There is a clear relationship between dose and width of the deconvoluted axial peaks which is attributable to incomplete separation between the axial and interfering peaks. A simple model is presented that coherently explains all observations. We conclude that deconvolution is necessary for the estimation of the most likely dose value of enamel pieces. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction When human remains are to be dated by ESR, it is necessary, for the sake of maximum preservation, to carry out dose determinations of tooth fragments or whole teeth. ESR spectra of tooth fragments are considerably dierent to those of powdered samples because hydroxyapatite is an orthorhombic mineral. Therefore, the components of the COÿ 2 radical, which dominate the ESR spectrum of tooth enamel (Vanhaelewyn et al., 2000), show strong angular dependences in the ESR spectrum. In natural tooth enamel small crystallites may have preferential orientations without approaching anything like a single crystal. This makes it unfeasible to ®t the three components of the COÿ 2 radical to the spectra of the fragments. In this paper we present the results for two enamel
* Corresponding author. Tel.: +61-2-6249-3122; fax: +612-6249-0315. E-mail address: [email protected] (R. GruÈn).
fragments, one from a Neanderthal excavated at Tabun and one from a Homo heidelbergensis at Atapuerca (Arsuaga et al., 1997; Bischo et al., 1997). 2. Experimental An enamel fragment (11 mg) was removed from the lingual surface of the left lower ®rst molar of Tabun C1 at the Natural History Museum London (GruÈn and Stringer, submitted), following existing cracks in the enamel, and was reattached after analysis. Another small enamel fragment (31 mg) was removed from a broken human incisor from the collection in the Museo National de Ciencias Naturales, Madrid. In this study, we used a similar sample holder as that described by GruÈn (1995), except that the holder was mounted in a Bruker ER 218PG1 programmable goniometer, and the fragments were measured at each dose step at 108 angle intervals for 3608. The fragments were mounted such that the enamel surfaces were roughly parallel with the goniometer's axis of rotation,
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 0 7 2 - X
774
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
i.e. perpendicular to the magnetic ®eld. ESR measurements were carried out on a Bruker ECS 106 spectrometer with a 1.5 T magnet and a rectangular 4102 ST cavity and the following measurement conditions: accumulation of 100 (natural sample) and between 10 and 100 scans (higher doses), 0.1015 mTpp modulation amplitude, 10.24 ms conversion factor, 20.48 ms time constant, 2048 bit spectrum resolution, 12 mT (Atapuerca) and 20 mT (Tabun) sweep width, 2 mW microwave power. The Tabun sample was irradiated with cumulative doses of: 0, 13, 26, 52, 104, 208, 416, 624, 832 and 1040 Gy and the Atapuerca sample 0, 26, 52, 104, 208, 416, 832, 1664 and 6656 Gy. Samples
were measured between 3 h and 2 days after irradiation without any preheating. The spectrum deconvolution and dose determination were carried out with the Peak®t Programme (Jandel Scienti®c, 1995) using four Gaussian peaks (Figs. 1 and 3). It was not possible to obtain reproducible convergence when more peaks were postulated. Spectrum preparation and ®tting procedures followed the strategies of GruÈn (1998a). The procedure gives reasonable agreement in powders with dose estimations derived from COÿ 2 ®tting (GruÈn, 2000). The central peak is ®tted with two Gaussian peaks (Ax1 and Ax2) which approximate the COÿ 2 radical, a central Gaussian line
Fig. 1. Natural (A, C) and irradiated (B, D) absorption spectra of Tabun at 608 (A, B) and 3308 (C, D) along with the results of spectrum deconvolution using four Gaussian peaks. Ax1 and Ax2 constitute most of the COÿ 2 radical, G relates most probably to the CO3ÿ 3 radical.
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
(G) and a wider background line (wide); these are closely similar to equivalent lines ®tted to powder spectra (GruÈn, 2000). Dose values and errors (using the ®tting procedures of Brumby, 1992) given henceforth are the average doses of the 36 measurements and the standard deviation of the mean. 3. Results The dose values for various procedures are given in Table 1. 3.1. The Tabun sample The ESR spectrum of the Tabun sample shows a strong angular dependence (see Fig. 1). Initially, doses were derived form the derivative spectrum at positions T1, B1 and B2 (see GruÈn, 2000). The results show a very strong axial dependency for all three positions with respect to both natural intensity and dose [Fig. 2(A)]. Using the integral spectra, the dose values derived from Ax1 and Ax2 show angular dependence with an angular shift of approximately 908; dose values derived from Ax1+Ax2 are considerably less angle dependent [Fig. 2(B)]. The dose peaks of T1, B1 and Ax1 occur at about the same angles as the intensity peaks of T1 and B1; the dose peaks of B2 and Ax2 occur at the intensity peak of B2. The scatter in the dose estimation of Ax1+Ax2 (3.4%) is well within the range expected from the repeated dose measurement of a single sample using spectrum deconvolution, which is in the range of 2±3% for powders (GruÈn, 2000).
775
The dose values derived from the interfering signal, G, are angle dependent and considerably lower than those of Ax1 and Ax2 [Fig. 2(B)]. Ax1 shows little change in ®eld position but an angular dependence in its width. Ax2 shows angular dependence in both ®eld position and width [Fig. 2(C)]. The envelope of the combined peaks is approximately constant. There is a clear relationship between dose values of Ax1 and Ax2 and their width [Fig. 2(D)]. There is an oset of about 10 to 15 Gy between the dose results of the two axial components, the linear regression of the Ax1 data reaching a dose value of about 190 Gy and Ax2 of about 205 Gy for the narrowest peak widths. 3.2. The Atapuerca sample Fig. 3 shows the absorption spectra and deconvolution components of the Atapuerca sample. In contrast to Tabun, the ESR spectra of the Atapuerca sample show signi®cantly less angular dependence. Peak amplitude dose determinations using derivative spectra show a strong and systematic oset between peak positions T1 and B1 compared to B2 [see Fig. 4(A)]. The deconvoluted components show only a small angular dose dependency. The values are similar for Ax1 and Ax2 as well as the combination of both [see Fig. 4(B)]. The interfering signal, G, yields doses around 300 Gy without any apparent angular dependence [Fig. 4(B)]. The dose peaks of T1, B1 and Ax1 occur approximately at the same angles as the intensity peaks of T1 and B1; the dose peaks of B2 and Ax2 occur at the intensity peak of B2. Note that the dierences between Ax1 and Ax2 are considerably smaller than those between T1, B1 and B2. Because the ESR
Table 1 Dose estimations (in Gy; average with s.d. of the 36 measurements (average error in parentheses), Max-6: error weighted pooled mean of the six doses presenting the two dose maxima)a Tabun
T1 B1 B2 Ax1 Ax2 Ax1+Ax2 Ax1-narrow Ax2-narrow
Atapuerca
Average
Max-6
163216 (6) 144241 (6) 180219 (6) 174213 (9) 186216 (8) 17526 (9)
18323 19723 19622 19124 20323 17923 19023 20023
(8) (8) (5) (15) (8) (9) (14) (8)
Average
Max-6
403210 (19) 40828 (19) 33227 (19) 396213 (24) 375213 (26) 381214 (24)
410210 (24) 409211 (28) 33727 (19) 42029 (24) 395210 (28) 404211 (28) 42429 (24) 372210 (26)
a Note added in press: The irradiations of the hominid teeth from Tabun and Atapuerca were carried out with a Cs-137 source which we had to use temporarily whilst our Co-60 source was renovated. For the dose estimations of both samples we have erroneously used the calibration for the rim positions of the rotating sample holder rather than the calibration for the centre. Thus, all dose values are 12% too high. We want to point out that this does not aect any other published age determinations from this laboratory.
776
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
spectra show little change with angle, the width of Ax1 and Ax2 changes little [Fig. 4(C)]. The dose versus width plot shows two distinct clusters where the wider line Ax2 yields smaller average doses than Ax1 (Fig. 4(D)). The line width of Ax1 (between 1.8 and 2.0 10ÿ4 T) corresponds to the narrowest axial peaks of Tabun. 4. Discussion At present, at least three dierent components can
reproducibly be deconvoluted from the ESR spectra of tooth enamel fragments. Ax1 and Ax2 are most probably due to COÿ 2 and this paramagnetic centre is thought to be most suitable for dose estimation. These apparently axial components are interfered with by a wide line and line G, possibly generated to a large extent by the CO3ÿ radical. The wide line is likely to 3 contain predominantly the spectrometer response function (see Peak®t manual by Jandel Scienti®c (1995)). It mostly yields poorly de®ned dose response curves leading to arbitrary dose values, which are usually greater than those from Ax1 and Ax2. Due to the shallow
Fig. 2. Results of peak amplitude dose estimation (T1, B1 and B2 denote positions in the derivative spectra; see GruÈn, 2000) compared to the natural intensity. Note that dose-maxima coincide with maxima of the natural intensity. (B) Dose values of Ax1, Ax2, Ax1 plus Ax2, and G. (C) Field position and width of Ax1, Ax2 and G. (D) Dose versus peak width of Ax1 and Ax2. The relationship between peak width and dose is attributed to insucient separation of the axial components and the interfering line G. The lines show the linear regression of the Ax1 and Ax2 data (width=full width half maximum).
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
slope of the wide line in the axial region, the separation of the wide line from Ax1 and Ax2 should be straightforward. Thus, possible interferences from the wide line are not further discussed. Most other signals observed in ESR spectra of tooth enamel, including G, give either somewhat lower doses than the axial region or are thermally unstable, yielding doses close to zero (see GruÈn, 1998b). For the discussion of the results the following simple model is introduced: the ESR spectrum consists of two centres: Centre A is thermally stable and has an axial symmetry (where Ax1 corresponds to g? and Ax2 to gk ), Centre G is unstable and has an isotropic sym-
Fig. 3. Absorption spectra and deconvolution of the Atapuerca sample: natural (A) and irradiated (B)
777
metry (line G). Any other contributions are ignored. The model can be justi®ed, in principle, by the fact 3ÿ that although both COÿ 2 and CO3 radicals have three principle g-values gx , gy and gz ), the powder ESR 3ÿ spectra of COÿ approximate those of an 2 and CO3 axial and an isotropic centre, respectively (Vanhaelewyn et al., in press). If we were able to extract the pure components of Centre A, all dose estimates should be independent of the rotational angle and give the same value within error. Any dose deviation is caused by Centre G, and has a trend towards smaller dose values. Positions T1 and B1 in the derivative spectrum are related to Ax1; B2 is a measure of Ax2. The Tabun sample shows a very strong angular dependency of the derivative doses. The ®eld orientation of the components of Centre A causes a variable interference from Centre G. This interference is smallest when the natural intensity of the components of Centre A is largest [see Fig. 2(A)]. The dose values closest to the correct value are thus given by the maxima (each component has two maxima and the three dose values around each maximum are taken as a measure for the dose maximum, Max-6 in Table 1). Fig. 2(B) shows that spectrum deconvolution is reasonably ecient in isolating Ax1 and Ax2 from the interference by Centre G. The excursions towards smaller dose values are signi®cantly smaller than in the derivative spectrum, where, of course, no attempt was made to separate the dierent components. However, the occurrence of the angular dose dependency indicates that Ax1 and Ax2 still contain some interference from Centre G. The maxima and minima of the doses from Ax1 and Ax2 occur at about the same angular positions as (T1, B1) and B2, respectively [compare Fig. 2(A) and (B)]. The maxima of Ax1 and Ax2 show a shift in the rotational angle of 908 and the doses derived from the sum (Ax1 and Ax2) are therefore less angle dependent. However, according to our model, the correct dose value results from the maxima of Ax1 and/or Ax2 rather than the sum (Ax1+Ax2), which will contain an average interference component from Centre G. Fig. 2(C) shows that the possible interference of Centre G is larger when the peak widths of Ax1 and Ax2 are wider. Therefore, the narrower peak width should approximate the correct dose values more closely. Interestingly, the dose maxima of (T1, B1) and B2 are virtually the same as those of Ax1 and Ax2. The Atapuerca sample shows little angular dose dependence, but there is a clear oset between (T1, B1) and B2, as well as Ax1 and Ax2. If Ax2 (i.e. gk) is orientated parallel to the rotational axis, or the crystallites of the sample show a random orientation, the Atapuerca sample shows qualitatively the same results as the Tabun sample at about 508 where (T1, B1) and
778
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
Ax1 yield larger dose values than B2 and Ax2. However, either due to a dierent orientation of the sample relative to the external magnetic ®eld or dierent crystallographic properties, the results of Atapuerca show little angle dependency. The peak width of Ax1 is considerably smaller than of Ax2. Following the observations on the Tabun sample, the correct dose value is best approximated by the dose maxima of Ax1. Again, the maxima of T1 and B1 agree well with the deconvoluted results. For dose estimation, deconvolution of the absorption spectra is preferable to the use of the derivative
spectrum because: (1) Ax1 and Ax2 show less angular dependency than T1, B1 and B2 [compare Fig. 2(A) and (B)] and (2) deconvolution is reasonably ecient in separating the COÿ 2 components from interfering signals [compare Fig. 4(A) and (B)]. Based on the arguments above we feel con®dent that the correct dose estimation of the Tabun sample is presented by maxima of Ax1 and Ax2 peaks which also coincide with the narrower peaks, i.e. in the range of 189 and 204 Gy, and the best dose estimation of the Atapuerca sample is given by the Ax1 Max-6 value (42029 Gy).
Fig. 4. (A) Results of peak amplitude dose estimation compared to the natural intensity. (B) Dose values of Ax1, Ax2, Ax1 plus Ax2, and G. (C) Field position and width of Ax1, Ax2 and G. (D) Dose versus peak width of Ax1 and Ax2. The dierences to the Tabun sample (Fig. 2) may be due to a dierent orientation of Ax1 and Ax2 in the external magnetic ®eld.
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
We are aware that more experiments have now to be carried out to validate our conclusions. Unfortunately, these cannot be carried out on fossil human teeth. Acknowledgements We thank Prof. C.B. Stringer, Natural History Museum, London, for providing the Tabun sample, Dr. J.L. Arsuaga, Departamento de Paleontologia, Universidad Compultense de Madrid, Dr. J.M. Bermudez de Castro, Museo Nacional de Ciencias Naturales, Madrid and E. Carbonell, Department de Historia i Geographia, Universitat Rovira i Virgili, Tarragona for providing the Atapuerca sample. Dr. D. Curnoe carried out the ESR measurements on the tooth from Atapuerca. References Arsuaga, J.L., Martinez, I., Garcia, A., Carretero, J.M., Lorenzo, C., Garcia, N., Ortega, A.I., 1997. Sima de los Huesos (sierra de Atapuerca, Spain). The site. Journal of Human Evolution 33, 109±127. Bischo, J.L., Fitzpatrick, J.A., Leon, L., Arsuaga, J.L.,
779
Falguerez, C., Bahain, J.J., Bullen, T., 1997. Geology and preliminary dating of the hominid-bearing sedimentary ®ll of the Sima de los Huesos Chamber, Cueva Mayor of the Sierra de Atapuerca, Burgos, Spain. Journal of Human Evolution 33, 129±154. Brumby, S., 1992. Regression analysis of ESR/TL dose-response data. Nuclear Tracks and Radiation Measurements 20, 595±599. GruÈn, R., 1995. Semi non-destructive, single aliquot ESR dating. Ancient TL 13, 3±7. GruÈn, R., 1998a. Dose determination on fossil tooth enamel using spectrum deconvolution with Gaussian and Lorentzian peak shapes. Ancient TL 16, 51±55. GruÈn, R., 1998b. Reproducibility measurements for ESR signal intensity and dose determination: high precision but doubtful accuracy. Radiation Measurements 29, 177±193. GruÈn, R., 2000. Methods of dose determination using ESR spectra of tooth enamel. Radiation Measurements 32, 767±772. GruÈn, R., Stringer, C.B., submitted. ESR and U-series analyses of dental material from Tabun C1. Journal of Human Evolution submitted. Jandel Scienti®c, 1995. PeakFit2 Peak separation and analysis software. Jandel Scienti®c, San Rafael, CA. Vanhaelewyn, G., Callens, F., GruÈn, R., 2000. EPR spectrum deconvolution and dose assessment of fossil tooth enamel using maximum likelihood common factor analysis. Applied Radiation and Isotopes 52, 1317±1326.
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Dose determination on tooth enamel fragments from two human fossils Steve Robertson, Rainer GruÈn* Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Received 18 October 1999; received in revised form 3 February 2000; accepted 29 February 2000
Abstract The ESR dose values of two human tooth enamel fragments were measured in 108 steps with an automated goniometer. ESR spectra were deconvoluted using four Gaussian peaks. The central region of the spectra is interfered with by at least one additional line which yields signi®cantly lower dose values than the central, apparently axial region. There is a clear relationship between dose and width of the deconvoluted axial peaks which is attributable to incomplete separation between the axial and interfering peaks. A simple model is presented that coherently explains all observations. We conclude that deconvolution is necessary for the estimation of the most likely dose value of enamel pieces. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction When human remains are to be dated by ESR, it is necessary, for the sake of maximum preservation, to carry out dose determinations of tooth fragments or whole teeth. ESR spectra of tooth fragments are considerably dierent to those of powdered samples because hydroxyapatite is an orthorhombic mineral. Therefore, the components of the COÿ 2 radical, which dominate the ESR spectrum of tooth enamel (Vanhaelewyn et al., 2000), show strong angular dependences in the ESR spectrum. In natural tooth enamel small crystallites may have preferential orientations without approaching anything like a single crystal. This makes it unfeasible to ®t the three components of the COÿ 2 radical to the spectra of the fragments. In this paper we present the results for two enamel
* Corresponding author. Tel.: +61-2-6249-3122; fax: +612-6249-0315. E-mail address: [email protected] (R. GruÈn).
fragments, one from a Neanderthal excavated at Tabun and one from a Homo heidelbergensis at Atapuerca (Arsuaga et al., 1997; Bischo et al., 1997). 2. Experimental An enamel fragment (11 mg) was removed from the lingual surface of the left lower ®rst molar of Tabun C1 at the Natural History Museum London (GruÈn and Stringer, submitted), following existing cracks in the enamel, and was reattached after analysis. Another small enamel fragment (31 mg) was removed from a broken human incisor from the collection in the Museo National de Ciencias Naturales, Madrid. In this study, we used a similar sample holder as that described by GruÈn (1995), except that the holder was mounted in a Bruker ER 218PG1 programmable goniometer, and the fragments were measured at each dose step at 108 angle intervals for 3608. The fragments were mounted such that the enamel surfaces were roughly parallel with the goniometer's axis of rotation,
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 0 7 2 - X
774
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
i.e. perpendicular to the magnetic ®eld. ESR measurements were carried out on a Bruker ECS 106 spectrometer with a 1.5 T magnet and a rectangular 4102 ST cavity and the following measurement conditions: accumulation of 100 (natural sample) and between 10 and 100 scans (higher doses), 0.1015 mTpp modulation amplitude, 10.24 ms conversion factor, 20.48 ms time constant, 2048 bit spectrum resolution, 12 mT (Atapuerca) and 20 mT (Tabun) sweep width, 2 mW microwave power. The Tabun sample was irradiated with cumulative doses of: 0, 13, 26, 52, 104, 208, 416, 624, 832 and 1040 Gy and the Atapuerca sample 0, 26, 52, 104, 208, 416, 832, 1664 and 6656 Gy. Samples
were measured between 3 h and 2 days after irradiation without any preheating. The spectrum deconvolution and dose determination were carried out with the Peak®t Programme (Jandel Scienti®c, 1995) using four Gaussian peaks (Figs. 1 and 3). It was not possible to obtain reproducible convergence when more peaks were postulated. Spectrum preparation and ®tting procedures followed the strategies of GruÈn (1998a). The procedure gives reasonable agreement in powders with dose estimations derived from COÿ 2 ®tting (GruÈn, 2000). The central peak is ®tted with two Gaussian peaks (Ax1 and Ax2) which approximate the COÿ 2 radical, a central Gaussian line
Fig. 1. Natural (A, C) and irradiated (B, D) absorption spectra of Tabun at 608 (A, B) and 3308 (C, D) along with the results of spectrum deconvolution using four Gaussian peaks. Ax1 and Ax2 constitute most of the COÿ 2 radical, G relates most probably to the CO3ÿ 3 radical.
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
(G) and a wider background line (wide); these are closely similar to equivalent lines ®tted to powder spectra (GruÈn, 2000). Dose values and errors (using the ®tting procedures of Brumby, 1992) given henceforth are the average doses of the 36 measurements and the standard deviation of the mean. 3. Results The dose values for various procedures are given in Table 1. 3.1. The Tabun sample The ESR spectrum of the Tabun sample shows a strong angular dependence (see Fig. 1). Initially, doses were derived form the derivative spectrum at positions T1, B1 and B2 (see GruÈn, 2000). The results show a very strong axial dependency for all three positions with respect to both natural intensity and dose [Fig. 2(A)]. Using the integral spectra, the dose values derived from Ax1 and Ax2 show angular dependence with an angular shift of approximately 908; dose values derived from Ax1+Ax2 are considerably less angle dependent [Fig. 2(B)]. The dose peaks of T1, B1 and Ax1 occur at about the same angles as the intensity peaks of T1 and B1; the dose peaks of B2 and Ax2 occur at the intensity peak of B2. The scatter in the dose estimation of Ax1+Ax2 (3.4%) is well within the range expected from the repeated dose measurement of a single sample using spectrum deconvolution, which is in the range of 2±3% for powders (GruÈn, 2000).
775
The dose values derived from the interfering signal, G, are angle dependent and considerably lower than those of Ax1 and Ax2 [Fig. 2(B)]. Ax1 shows little change in ®eld position but an angular dependence in its width. Ax2 shows angular dependence in both ®eld position and width [Fig. 2(C)]. The envelope of the combined peaks is approximately constant. There is a clear relationship between dose values of Ax1 and Ax2 and their width [Fig. 2(D)]. There is an oset of about 10 to 15 Gy between the dose results of the two axial components, the linear regression of the Ax1 data reaching a dose value of about 190 Gy and Ax2 of about 205 Gy for the narrowest peak widths. 3.2. The Atapuerca sample Fig. 3 shows the absorption spectra and deconvolution components of the Atapuerca sample. In contrast to Tabun, the ESR spectra of the Atapuerca sample show signi®cantly less angular dependence. Peak amplitude dose determinations using derivative spectra show a strong and systematic oset between peak positions T1 and B1 compared to B2 [see Fig. 4(A)]. The deconvoluted components show only a small angular dose dependency. The values are similar for Ax1 and Ax2 as well as the combination of both [see Fig. 4(B)]. The interfering signal, G, yields doses around 300 Gy without any apparent angular dependence [Fig. 4(B)]. The dose peaks of T1, B1 and Ax1 occur approximately at the same angles as the intensity peaks of T1 and B1; the dose peaks of B2 and Ax2 occur at the intensity peak of B2. Note that the dierences between Ax1 and Ax2 are considerably smaller than those between T1, B1 and B2. Because the ESR
Table 1 Dose estimations (in Gy; average with s.d. of the 36 measurements (average error in parentheses), Max-6: error weighted pooled mean of the six doses presenting the two dose maxima)a Tabun
T1 B1 B2 Ax1 Ax2 Ax1+Ax2 Ax1-narrow Ax2-narrow
Atapuerca
Average
Max-6
163216 (6) 144241 (6) 180219 (6) 174213 (9) 186216 (8) 17526 (9)
18323 19723 19622 19124 20323 17923 19023 20023
(8) (8) (5) (15) (8) (9) (14) (8)
Average
Max-6
403210 (19) 40828 (19) 33227 (19) 396213 (24) 375213 (26) 381214 (24)
410210 (24) 409211 (28) 33727 (19) 42029 (24) 395210 (28) 404211 (28) 42429 (24) 372210 (26)
a Note added in press: The irradiations of the hominid teeth from Tabun and Atapuerca were carried out with a Cs-137 source which we had to use temporarily whilst our Co-60 source was renovated. For the dose estimations of both samples we have erroneously used the calibration for the rim positions of the rotating sample holder rather than the calibration for the centre. Thus, all dose values are 12% too high. We want to point out that this does not aect any other published age determinations from this laboratory.
776
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
spectra show little change with angle, the width of Ax1 and Ax2 changes little [Fig. 4(C)]. The dose versus width plot shows two distinct clusters where the wider line Ax2 yields smaller average doses than Ax1 (Fig. 4(D)). The line width of Ax1 (between 1.8 and 2.0 10ÿ4 T) corresponds to the narrowest axial peaks of Tabun. 4. Discussion At present, at least three dierent components can
reproducibly be deconvoluted from the ESR spectra of tooth enamel fragments. Ax1 and Ax2 are most probably due to COÿ 2 and this paramagnetic centre is thought to be most suitable for dose estimation. These apparently axial components are interfered with by a wide line and line G, possibly generated to a large extent by the CO3ÿ radical. The wide line is likely to 3 contain predominantly the spectrometer response function (see Peak®t manual by Jandel Scienti®c (1995)). It mostly yields poorly de®ned dose response curves leading to arbitrary dose values, which are usually greater than those from Ax1 and Ax2. Due to the shallow
Fig. 2. Results of peak amplitude dose estimation (T1, B1 and B2 denote positions in the derivative spectra; see GruÈn, 2000) compared to the natural intensity. Note that dose-maxima coincide with maxima of the natural intensity. (B) Dose values of Ax1, Ax2, Ax1 plus Ax2, and G. (C) Field position and width of Ax1, Ax2 and G. (D) Dose versus peak width of Ax1 and Ax2. The relationship between peak width and dose is attributed to insucient separation of the axial components and the interfering line G. The lines show the linear regression of the Ax1 and Ax2 data (width=full width half maximum).
S. Robertson, R. GruÈn / Radiation Measurements 32 (2000) 773±779
slope of the wide line in the axial region, the separation of the wide line from Ax1 and Ax2 should be straightforward. Thus, possible interferences from the wide line are not further discussed. Most other signals observed in ESR spectra of tooth enamel, including G, give either somewhat lower doses than the axial region or are thermally unstable, yielding doses close to zero (see GruÈn, 1998b). For the discussion of the results the following simple model is introduced: the ESR spectrum consists of two centres: Centre A is thermally stable and has an axial symmetry (where Ax1 corresponds to g? and Ax2 to gk ), Centre G is unstable and has an isotropic sym-
Fig. 3. Absorption spectra and deconvolution of the Atapuerca sample: natural (A) and irradiated (B)
777
metry (line G). Any other contributions are ignored. The model can be justi®ed, in principle, by the fact 3ÿ that although both COÿ 2 and CO3 radicals have three principle g-values gx , gy and gz ), the powder ESR 3ÿ spectra of COÿ approximate those of an 2 and CO3 axial and an isotropic centre, respectively (Vanhaelewyn et al., in press). If we were able to extract the pure components of Centre A, all dose estimates should be independent of the rotational angle and give the same value within error. Any dose deviation is caused by Centre G, and has a trend towards smaller dose values. Positions T1 and B1 in the derivative spectrum are related to Ax1; B2 is a measure of Ax2. The Tabun sample shows a very strong angular dependency of the derivative doses. The ®eld orientation of the components of Centre A causes a variable interference from Centre G. This interference is smallest when the natural intensity of the components of Centre A is largest [see Fig. 2(A)]. The dose values closest to the correct value are thus given by the maxima (each component has two maxima and the three dose values around each maximum are taken as a measure for the dose maximum, Max-6 in Table 1). Fig. 2(B) shows that spectrum deconvolution is reasonably ecient in isolating Ax1 and Ax2 from the interference by Centre G. The excursions towards smaller dose values are signi®cantly smaller than in the derivative spectrum, where, of course, no attempt was made to separate the dierent components. However, the occurrence of the angular dose dependency indicates that Ax1 and Ax2 still contain some interference from Centre G. The maxima and minima of the doses from Ax1 and Ax2 occur at about the same angular positions as (T1, B1) and B2, respectively [compare Fig. 2(A) and (B)]. The maxima of Ax1 and Ax2 show a shift in the rotational angle of 908 and the doses derived from the sum (Ax1 and Ax2) are therefore less angle dependent. However, according to our model, the correct dose value results from the maxima of Ax1 and/or Ax2 rather than the sum (Ax1+Ax2), which will contain an average interference component from Centre G. Fig. 2(C) shows that the possible interference of Centre G is larger when the peak widths of Ax1 and Ax2 are wider. Therefore, the narrower peak width should approximate the correct dose values more closely. Interestingly, the dose maxima of (T1, B1) and B2 are virtually the same as those of Ax1 and Ax2. The Atapuerca sample shows little angular dose dependence, but there is a clear oset between (T1, B1) and B2, as well as Ax1 and Ax2. If Ax2 (i.e. gk) is orientated parallel to the rotational axis, or the crystallites of the sample show a random orientation, the Atapuerca sample shows qualitatively the same results as the Tabun sample at about 508 where (T1, B1) and
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Ax1 yield larger dose values than B2 and Ax2. However, either due to a dierent orientation of the sample relative to the external magnetic ®eld or dierent crystallographic properties, the results of Atapuerca show little angle dependency. The peak width of Ax1 is considerably smaller than of Ax2. Following the observations on the Tabun sample, the correct dose value is best approximated by the dose maxima of Ax1. Again, the maxima of T1 and B1 agree well with the deconvoluted results. For dose estimation, deconvolution of the absorption spectra is preferable to the use of the derivative
spectrum because: (1) Ax1 and Ax2 show less angular dependency than T1, B1 and B2 [compare Fig. 2(A) and (B)] and (2) deconvolution is reasonably ecient in separating the COÿ 2 components from interfering signals [compare Fig. 4(A) and (B)]. Based on the arguments above we feel con®dent that the correct dose estimation of the Tabun sample is presented by maxima of Ax1 and Ax2 peaks which also coincide with the narrower peaks, i.e. in the range of 189 and 204 Gy, and the best dose estimation of the Atapuerca sample is given by the Ax1 Max-6 value (42029 Gy).
Fig. 4. (A) Results of peak amplitude dose estimation compared to the natural intensity. (B) Dose values of Ax1, Ax2, Ax1 plus Ax2, and G. (C) Field position and width of Ax1, Ax2 and G. (D) Dose versus peak width of Ax1 and Ax2. The dierences to the Tabun sample (Fig. 2) may be due to a dierent orientation of Ax1 and Ax2 in the external magnetic ®eld.
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We are aware that more experiments have now to be carried out to validate our conclusions. Unfortunately, these cannot be carried out on fossil human teeth. Acknowledgements We thank Prof. C.B. Stringer, Natural History Museum, London, for providing the Tabun sample, Dr. J.L. Arsuaga, Departamento de Paleontologia, Universidad Compultense de Madrid, Dr. J.M. Bermudez de Castro, Museo Nacional de Ciencias Naturales, Madrid and E. Carbonell, Department de Historia i Geographia, Universitat Rovira i Virgili, Tarragona for providing the Atapuerca sample. Dr. D. Curnoe carried out the ESR measurements on the tooth from Atapuerca. References Arsuaga, J.L., Martinez, I., Garcia, A., Carretero, J.M., Lorenzo, C., Garcia, N., Ortega, A.I., 1997. Sima de los Huesos (sierra de Atapuerca, Spain). The site. Journal of Human Evolution 33, 109±127. Bischo, J.L., Fitzpatrick, J.A., Leon, L., Arsuaga, J.L.,
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Falguerez, C., Bahain, J.J., Bullen, T., 1997. Geology and preliminary dating of the hominid-bearing sedimentary ®ll of the Sima de los Huesos Chamber, Cueva Mayor of the Sierra de Atapuerca, Burgos, Spain. Journal of Human Evolution 33, 129±154. Brumby, S., 1992. Regression analysis of ESR/TL dose-response data. Nuclear Tracks and Radiation Measurements 20, 595±599. GruÈn, R., 1995. Semi non-destructive, single aliquot ESR dating. Ancient TL 13, 3±7. GruÈn, R., 1998a. Dose determination on fossil tooth enamel using spectrum deconvolution with Gaussian and Lorentzian peak shapes. Ancient TL 16, 51±55. GruÈn, R., 1998b. Reproducibility measurements for ESR signal intensity and dose determination: high precision but doubtful accuracy. Radiation Measurements 29, 177±193. GruÈn, R., 2000. Methods of dose determination using ESR spectra of tooth enamel. Radiation Measurements 32, 767±772. GruÈn, R., Stringer, C.B., submitted. ESR and U-series analyses of dental material from Tabun C1. Journal of Human Evolution submitted. Jandel Scienti®c, 1995. PeakFit2 Peak separation and analysis software. Jandel Scienti®c, San Rafael, CA. Vanhaelewyn, G., Callens, F., GruÈn, R., 2000. EPR spectrum deconvolution and dose assessment of fossil tooth enamel using maximum likelihood common factor analysis. Applied Radiation and Isotopes 52, 1317±1326.