Second interlaboratory-comparison project on ESR dating

Second Interlaboratory-Comparison Project on ESR Dating

M. BARABAS, R. WALTHER Heidelberger Akademie der Wissenschaften

c/o Institut fur Umweltphysik, INF 366, D-6900 Heidelberg, Germany

A. WIESER GSF, Ingolstadter Landstrasse 1, D-8042 Neuherberg, Germany

U. RADTRE Geographisches Institut UniversitZt Diisseldorf, D-4000 Dusseldorf 1, Germany

R. GRUN Research Laboratory for Radiocarbon Dating, ANV,

P.O. Box 4, Canberra A.C.T. 2601, Australia

During the TL- and ESR-Conference in Clermont-Ferrand in 1990 we initiated a second “Intercomparison project on ESR dating” in which 14 ESR groups participated. Each group was provided with two samples of a coral powder, one of which (Sample A) was a fossil coral with a mass spectrometric determined U/Th age. Sample B was a recent coral, irradiated with a definite y-dose (checked by alanine dosimetry). In both cases the AD had to be determined (for sample A the U-content and age as well). Additionally, the chance of a calibration of y-sources by alanine dosimeters was offered (GSF). The results show that (i) the y-source calibration is better than f5 R, (ii) the mean value of the AD from sample A seems to agree with the expected AD but the mean AD value from sample B is overestimated, (iii) systematic errors occur due to the fitting procedure: the AD estimate depends on the maximum y-dose used for the irradiation curve, (iv) the AD determination including the smallest systematic error gives correct values for sample B but too low values for sample A which may be caused by fading of the signal g = 2.0006.

KEYWORDS: Intercomparison; ESR-dating; y-source; calibration; ADdetermination; fading; corals

FIRST ICP - A REVIEW

After ESR spectroscopy was first applied as a dating technique in 1975, it promised to be a relatively simple technique and hence has been applied to a wide range of materials. In many cases, however, very basic principles were not thoroughly investigated and the dating procedure was based on oversimplified models albeit “good results” could be obtained. In 1982 a first interlaboratory- comparison project (ICP) was initialized (see Hennig et al., 1983, 1985). In a first phase of this project, the laboratories engaged in ESR dating were evaluated and their respective research fields and further aims were summarized and published (ibid). Thereafter, four homogenized fossil samples were distributed and 16 of these ESR-laboratories submitted their results. The task was to determine the accumulated dose, AD, of each sample. However, as the true AD value was not exactly known, only the statistical scatter and the range of the AD value could be evaluated.

The results of this first project were rather discouraging (Hennig et al., 1985), because of the large scatter of the calculated AD’s (more than 25% up to 160%), although the general range of the AD estimates agreed roughly with the (geological) expectations. In the meantime some reasons for this

I19

120 ESR dosimetry and applications

bad result have been identified. Systematic deviations occurred because of the use of improper ESR signals (g = 2.0001/g = 2.0023) or incorrect evaluation of ESR spectra. Further systematic errors have been assumed. Nevertheless, some progress resulted from this first ICP:

l Stable signals in carbonates have been identified at g = 2.0006 (and in some cases at g = 2.0036) (see Barabas ef al., 1988a, 1988b);

l Unsuitable ESR signals were found (at 2.0001,2.0023) and methods for their elimination have been reported (Grtin and DeCanniere 1984, Smith ef al., 1985);

l Basic conditions for reliable AD-determination (number of aliquots, contamination, normalization) have been quoted (i.e., Grtln 1989, Grtln and Rhodes 1991).

Although this ICP was not a success from the view of reliable ESR dating, it gave impulses for further research and could have been the basis of continuing the efforts to improve ESR results in a cooperative process. This chance, however, was partly missed.

SECOND ICP

Background

Although significant progress has been achieved since then, the confidence of many laboratories in ESR dating is still rather low.

Specific problems in AD determination remain:

l Definition of suitable ESR signals (e.g., in aragonitic shells; Radtke et al., 1985; Molodkov 1988; Barabas er al., 1988a; Katzenberger et al., 1989;)

l Interferences of signals with different radiation response and thermal stability (Katzenberger & Willems 1988; Lyons et al., 1988);

l The choice of the mathematical procedure to fitting the data points (Apers et al., 1981; Barabas et al., 1988b; Grtin and Macdonald 1989);

l Definition of conditions for artificial irradiation and calibration of radioactive sources.

In order to overcome at least some of these problems, we initiated a (second) laboratory comparison project. The main goals were:

l Source calibration and definition of uniform irradiation conditions; l Assessment of systematic and statistical errors in determining the accumulated dose (AD); l Comparison between U/Th and ESR ages.

In order to keep the procedure simple we distributed just two samples (about 4 g each; size fraction 100-200 pm) of aragonitic coral. This material has a large and radiation sensitive signal at g = 2.0006 which is - from the present knowledge - the most reliable signal in ESRdating of carbonates and in case of corals almost free of interference from any other signal. Furthermore U-series dating is a reliable method to date corals which allows one to check the ESR-results.

Participants

The two coral samples were distributed to the interested research groups during the TL- and ESR- Seminar in Clermont-Ferrand in July 1990. The following 12 groups have participated in the ICP and submitted their results (including data points, sample treatment etc.) through the end of April 1991:

0 B. Blackwell, McMaster University, Hamilton, Canada l S. Brumby, Research School of Chemistry, Canberra, Australia l F. Dejehet, Universite Catholique de Louvain, Louvain-la-Neuve, Belgique l C. Falgueres, Institut de Paleontologie Humaine, Paris, France l R. Grtin, Subdepartment of Quaternary Research, Cambridge, UK

ESR dosimetry and applications 121

l G. Hiitt, Institute of Geology, Tallinn, Estonia l A. Molodkov, Institute of Geology, Tallinn, Estonia l A. M. Gzer, Middle East Technical University, Ankara, Turkey l U. Radtke, Geographisches Institut, Dusseldorf, FRG l A. Skinner, Williams College, Williamstown, USA l R. Walther, Heidelberg Academy of Sciences, Heidelberg, FRG l A. Wieser, GSF, Miinchen, FRG

Three groups submitted two or three different measurement sets of the samples. We therefore obtained 17 (sample A) and 15 (sample B) AD-evaluations, respectively. The authors wish to express their gratitude to all colleagues that have been involved in the ICP. Their thorough investigation enabled us to evaluate the following results.

Samples/Procedure

Sample A: A fossil coral (< 150 ka; except grinding no further pretreatment) for which the precise Th/U age was determined (mass spectroscopy). The main goal was to determine the AD (and its uncertainty) by ESR using the signal at g = 2.0006. The U-content (initial 2%/238U = 1.14) of the fossil sample (A) had to be determined (if possible; otherwise it was sufficient to quote the AD). Using the U-content, a cosmic dose rate of P = 200 PGyla and the AD, the age of the sample had to be evaluated.

Sample B: This sample was a recent coral that was irradiated with a precisely determined y-dose of 37.6f 1 Gy at GSF (this dose was unknown to the participants). The task was to determine the accumulated dose with ESR from the signal at g = 2.0006.

Participants were offered the opportunity to calibrate their y-sources with alanine dosimeters which were provided and evaluated by A. Wieser (GSF, Miinchen).

RESULTS

y-Source calibration

Seven groups have taken the opportunity to calibrate their y-sources by alanine dosimetry. Deviations from the assumed dose rate have been found to be smaller than f 5%, which indicates that this is not a main source of uncertainty. However, it is reasonable to assume that the error introduced from the source calibration contributes 3 -5% to the error in AD determination.

During the ICP another possible source of error has been identified: the correction factor for calcite that has to be applied if the y-source is calibrated in terms of water equivalent dose. The dose rate absorbed in calcite is 10% smaller as compared to water (for calculation of conversion factors, see Hubbell, 1982; Krieger and Petzold, 1989; or communicate with one of the present authors).

Two laboratories did not state whether they used calcite- or water-equivalent dose rates (marked with a ? in Table 1). In these cases we assumed a water equivalent dose rate, which may have introduced an additional uncertainty. All doses given in this report have been calculated as calcite-equivalent doses.

Sample pretreatment, measurement parameters, dose ranges

All laboratories used the dating signal at g = 2.0006 for the determination of the AD and irradiated the aliquots with y-sources. The number of aliquots (weight between 200 and 300 mg) ranged between 7 and 20, although most of the laboratories used about 12- 15 irradiation steps. The maximum irradiation dose varied between 140 Gy and about 2000 Gy.


After irradiation samples were stored for a few weeks at room temperature or for several hours at 80- 100 “C. Most of the laboratories did not apply any further pretreatment. Details are given in Table 1 and 2. No influence of the pretreatment procedure could be derived from the measurements.

Tab.1: Data for samde A

Lab Data max.irr. MW Power I-max saturat. scatter No points dose [Gy] [mW] [a.u.] dose [Gy] PI

1 12 720 7

2 23 383.1

3, 30 1683.5

3b 20 244.8

3C 20 244 .a

4 16 405

5 13 160.7

6 9 1000

7 14 468 ?

a 20 450

9a 13 2112

9b 13 2112

10 19 733.1

11 14 715.5

12a 12 320.6

12b 12 320.6

12c 12 320.6

Tab.2: Data for samde B

10

15

20

20

20

10

10

15

15

50

5

20

100

200

6417.8 2259.7 135.3 f 16.1 3.97

11 479.3 145.7 t 46.2 7.57

22.7 906.8 139.7 * to.8 1.92

11.95 419.6 92.3 t 14.6 3.26

11.8 466.2 106.2 t 21.2 2.aa

325 1084 137.5 t 20.3 3.22

25680 2221 132.4 t 28.9 2.28

122.2 702.2 118.7 i 20.7 (0.8) 20.7 844.6 116.6 f 22.a 5.76

6.33 641.7 113.3 t 6.6 2.03

10.45 1609 181.4 t 19.5 (0.8) 6.95 1214 157.6 f 13 (1.79)

42.6 509 101.8 t 4.2 2.23

71.9 734.3 128.7 t 12.5 2 25

4.89 349.8 77.7 Lt 6 l.B6

5.58 475.5 97.1 t 10.9 1.34

4.97 403.6 102.8 t 20.3 4.48

Lab Data max.irr. MW Power I-max saturat. scatter No points dose [Gy] [mW] [a.u.] dose [Gy] WI

1 11 540

2 20 330.9

3 20 336.7

4 16 405

5 13 160.7

6 7 500

7 13 ia0

a 20 450

9a 13 2112

9b 13 2112

10 9 733.1

11 14 715.5

12a 12 320.6

12b 12 320.6

12c 12 320.6

? 10

15

20

10

10

? 15

15

50

5

20

100

200

53.6 473.9 53.9 f 25.1 a.12

12.5 227.4 39.5 i 12.4 7.99

14 400.5 46.03 f 2.7 1.91

177.6 362 41.9 t a.05 2.18

6048 263 40.36 i 1.95 1.25

112.1 402.3 35.2 i 2.3 (2.27)

7.1 141 33.5 5 7.32 6.2

5.45 412.7 45.99 * 2.5 2.47

21.2 148.4 95.7 f 9.1 (0.6)

18.6 1332 95.22 t 10.3 (2.14)

85.2 408.8 48.86 t 2.9 1.94

148.6 644.9 64.5 * 5.6 1.19

7.07 258.4 40.8 * 3.4 1.18

7.15 271.5 41.86 t 4.1 1.34

7.54 328.8 50.32 f 4.5 4.48

The deviation of the single ESR measurement from the fit was calculated for all irradiation curves up

to 300 Gy (for some data sets this may be a bad estimation because of too few points (< 6) below 300 Gy, these values are given in brackets). The scatter is the percentage standard deviation of the measured ESR intensities around the best fit for maximum irradiation dose 5300 Gy.

AD determination


To standardize the evaluation of the signal growth curves we applied a fitting function based on the single exponential saturation function given by Apets et al. (1981) (see also: Barabas et al. 1988b): S = S_ (1 -exp [ - @ + AD)/D J), where S is the ESR signal amplitude, S, is the maximum value, D is the (additional) dose, and D, is the so-called saturation dose, describing the curvature of the signal growth curve (Fig. 1).

1E

isaturation dose Do

-100 100 300 500 700 9 Dose [Gy]

IO

Fig. 1. Signal growth curve with prhciple parame ters and AD-extrapolation.

The AD results (including standard errors) have been determined using software for scientific analysis (“Asystant” by Asyst Software Technologies) and by a program based on an algorithm of Garnier and Garnier-Monjoe (1981). Both programs gave identical results for the AD compared to the algorithm published by Griin and MacDonald (1989). The errors calculated by the different programs were comparable, whereas the jackknifing gives slightly larger errors than the “Asystant”.

There was no significant difference between AD values determined by our programs and AD values evaluated by the authors. We decided to present the results of our re-evaluation of the original data for the sake of comparison.

Fig. 2a and 2b show the results of the AD determination of both samples with their respective standard errors. For the AD determination all available data points (i.e., the whole dose range) were used.

The following conclusions can be drawn from these results:

The scattering of the AD data (derived from the variance of the data) is around 20% for both samples (> 30% if the outliers are included). However, the AD determinations exhibited standard errors between 5 % and up to 30%. At least two samples (#9a and #9b) deviate distinctly from the mean value of all measurements (A: 116.4k4.7 Gy; B: 44.8& 1.9 Gy). The mean value of sample A agrees very well with the expected dose of 116 Gy as derived from the U-series age, the U-content of 3.12 ppm and the cosmic dose rate. The mean value of the AD of sample B is about 20% larger than the expected dose of 37.6 Gy, which the sample has received.

Statistical scattering

Parameters that influence the standard error of the AD have been analyzed. First, the statistical scattering of a single ESR measurement has been derived from the mean deviation between fitted curves and data points. We estimated a l-u-error for the signal amplitude between 1 and 5% (mean: 2%) for a single measurement (Fig. 3). However, the data points of some laboratories showed much larger statistical uncertainties (up to 8%).


A-whole range 4.

mean AD (without No Sa, 9b): 116.4 +- 4.7 Gy I

0

Data Set Fig. 2a. Results of sample A, the arrows indicate the expected AD of 116 Gy.

1 B-whole range 80, * -

mean AD (without No 9a. 9b): 41.8 +- 1.9 Gy

Data Set

Fig. 2b. Results of sample B, the arrows indicate the expected AD of 37.6 Gy.

..-.a sample .4 *--** sample B

different labs

Fig. 3. Estimation of the standard error of a single ESR measurement as derived from the mean deviation of the measured amplitude from the fitted curve.

This error of a single ESR-measurement is correlated to the accuracy of the AD-value, as expected (? = 0.65). Even if the statistical l-u-error of the single ESR-measurement is only about 2%) the AD precision will be in the range of 10% under reasonable experimental efforts. The number of irradiation steps is of less influence on the AD precision (? = 0.002), however, a minimum of lo- 12 data points is essential.


Possible reasons for a larger scattering may be due to:

- use of different glass tubes for the different aliquots, - drift of the spectrometer parameters (standard!), - electrostatically charged samples which may influence the density of the sample, - and different irradiation positions.

For a reliable AD determination the reproducibility of a single measurement should be better than 2%.

Systematic influences on the AD

Two possible origins for systematic errors of the AD were checked:

a) The microwave power: The microwave power used for the ESR measurements does not show a significant correlation with the AD derived (in order to eliminate other systematic influences, we omitted data points which were irradiated higher than 300 Gy).

b) The maximum irradiation dose: For both samDIes there is a deuendencv of the evaluated AD on the maximum dose of the irradiation curve (Fig i).

,““““‘1”“““11”“‘,,‘,1”_ 500 1000 1500 2000 2

maximum irradiation dose

20;

OO

B ,~,~~~~~~~I’~~,~~‘,‘,/~~~,‘~~1

500 1000 1500 2000 El maximum irradiation dose

00

10

Fig. 4. The dependency of the AD from the maximum dose used for irradiation.

The AD derived from the data increases with increasing maximum y-dose applied in the dose response curve. The reason for this observation is most probably the deviation of the signal growth curve from a single exponential saturation function that has already been reported e.g., Griin, 1990; Barabas ef al., 1991; Walther et al., 1991).


We re-evaluated the AD values, omitting all data points above 300 Gy and, secondly, above 150 Gy. In the last case for some data sets less than 5 points were available and these results were omitted. The resulting mean AD values were found to decrease significantly as the dose range became smaller (Fig. 5). Also, for these smaller dose ranges the scattering of the AD values ranges between 10% (sample B) and 15% (sample A), which is slightly smaller than for the whole dose range results. The difference can be explained by the systematic deviations introduced by the different dose ranges.

COMPARISON WITH KNOWN AIYAGEVALVES

Sample A: There was a trend found for the AD values of the sample A (Fig. 6a): the smaller the maximum dose for the irradiation curve was the smaller was the derived AD. Best agreement with the expected AD was obtained by using the full dose range.

200 I loo7 /

B - ( 150 Gy /

mean AD: 03.3 +- 3.6 Gy rnc~l,, AU: 39.1 f- 2.L CJ

mean AD: 105.9 +- 6.3 Gy mean AD: 43.7 +- 1.4 Gy 0 0

Fig. 5. The AD results for samples A and B calculated by fitting up to (A) 300 Gy and (B) 150 Gy of the irradiation curves.

Sample B: As this sample had been irradiated with a predose of 37.6f 1 Gy controlled by alanine dosimetry (GSF), we are able to check the systematic deviations from this true value by comparing the mean AD’s of all measurements with this value (Fig. 5b). Obviously, the smaller the dose range used for fitting of the signal growth curve, the smaller is the deviation from the true value (see Fig. 6b). Taking into account a “zero” signal of the recent sample corresponding to ~3 Gy, the result derived with a fitting range I150 Gy equals the absorbed dose within the limits of uncertainty. However, this result has the rather uncomfortable implication that the correct dose can only be estimated by using a relative small additional y-dose (< 150 Gy). When this procedure is applied to sample A, it causes a severe underestimation of the dose, and hence the age of 28% (see Fig. 7).

150- 60 _

0- 1 20 _1

0 0

Fig. 6. a) The AD’s for sample A compared with the expected AD corresponding to the U/l% age (dashed line). b) The mean AD results of all measurements of sample B for the different dose ranges compared with the pre-dose of 37.6 Gy (dashed line).


6. ,..;::::;::..,’ ; 5 :- : ,:; ,_.,,_.

._.~_‘,_.’ :ul \g ;2

__..... :- in is _.‘.

: v : ” ; F 60,, ,..,,,.I, ~~‘~.-...‘..:.,~.:......I~~~~~~~~n

50 70 90 110 130 150

AD [CY]

Fig. 7. The ESR ages derived from the mean AD’s of sample A compared Th/U age for different cosmic dose rates.

the

CONCLUSIONS

y-source calibration

The deviation from the expected dose rate was less than 5% for all laboratories (7) according to the evaluation of the alanine dosimeters. This range of uncertainty can hardly be improved under usual “dating” conditions and is therefore a rather satisfying result. However, this general error has to be included into the statistical uncertainty of any AD determination and therefore the calibration of the y-source (including the exact positioning of the sample) is of importance for reliable dating results.

Statistical precision of AD determination

The uncertainty of the AD determination is mainly depending on the precision and reproducibility of a single ESR measurement. The number of irradiation steps is of less importance, although a minimum of 10 - 12 aliquots is be essential.

The mean standard deviation of single ESR measurements was in the range of 2-3% (except for a very few laboratories). This is close to the reproducibility that can be achieved with ESR measurements at best. The resulting error in AD estimation was in the range of lo- 12%.

Very similar results have been reported by Grtin and Rhodes 1991, who estimated the influence of the maximum irradiation dose, precision of ESR measurement and number of data points in computer simulations of dose response curves.

We therefore conclude that to ensure that the AD determination results in a maximum precision, it is most important to keep the reproducibility of the single ESR measurement at a level close to 2% or less, which means: exact aliquots and identical glass tubes as well as a standard and constant spectrometer performance. Even under optimum conditions the statistical uncertainty of the AD will be about 10%.

Systematic deviations

a) Dose range dependency. The absolute values of the AD determination showed a strong dependency on the maximum dose used for the creation of the dose response curves. Only when applying the fitting procedure to a dose range not exceeding about 150 Gy was the mean experimental AD close to the correct value. This finding supports earlier observations that the dose response curve


can only be approximated by a single exponential saturation function. In order to minimize a systematic overestimation of the AD, the maximum irradiation dose should not exceed 100-200 Gy. This will, of course, limit the determinable AD range to about 200 Gy as for larger AD values, the statistical error increases dramatically (see also Grtin and Rhodes 1991b).

The application of more sophisticated fitting functions with more variables was tested on our data sets. However, this procedure did not deliver better results because of large statistical errors. An exponential saturation function added by a linear function (Grim, 1990; Walther et al., 1991), for example, gives the AD values 104.9f 11.0 for sample A and 42.5k5.4 for sample B (calculated for the whole dose ranges).

A systematic influence of microwave power was not supported by the data sets.

b) Fading. The AD of the fossil sample turned out to be about 28% too small when excluding other sources of systematic errors. Assuming that this difference is due to fading, the lifetime of the signal at g = 2.0006 in corals can be estimated to be in the range of 400 ka, which would limit the reliable dating range to less than 100 ka. Similar observations have been made during systematic studies of the signal at g = 2.0006 of foraminifera (Siegele et al., 1985; Mudelsee et al., 199 1). This result, however, has to be carefully checked by further investigation.

ACKNOWLEDGEMENT

The coral sample A with the mass spectrometric U-series result was kindly provided by J.H. Chen, Caltech, USA.

REFERENCES

Apers, D., R. Debuyst, P. DeCanniere, F. Dejehet and E. Lombard (1981). A criticism of the dating by ESR of the stalagmitic floors of the Caune de 1’Arago at Tautavel. In: DeLumly H., Labeyrie J. (eds.) Absolute dating and isotope analysis in prehistory - methods and limits. Preprint, 533.

Barabas, M.,A. Bach, A. Mangini (1988a). An analytical model for ESR-signals in calcite. Nuclear Tracks, 14, 231-235.

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Griin, R. (1989). Die ESR Altersbestimmungsmethode. Springer Verlag Heidelberg, 132 p. Griin, R. (1990). Dose response of the paramagnetic centre at g = 2.0007 in corals. Ancient 72, 8,

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Katzenberger, 0. and N. Willems (1988). Interferences encountered in the determination of AD of mollusc samples. Qmernary Science Review, 7, 485 -489.

Katzenberger, O., R. Debuyst, P. De&m&e, F. Dejehet D. Apers and M. Barabas (1989). Temperature experiments on powdered and non-powdered mollusc samples. An approach to signal identification. Applied Radiation and Isotopes, 40, 1113 - 1118.

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Molodkov, A. (1988). ESR dating of quatemary shells: recent advances. Quaternary Science Review, 7, 477.

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Second interlaboratory-comparison project on ESR dating