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The use of electron spin resonance (ESR) for the determination of prehistoric lithic heat treatment
The Use of Electron Spin Resonance (ESR) for the Determination of Prehistoric Lithic Heat Treatment S. TOYODA,
M. IKEYA,
Department of Earth and Space Science, Faculty of Science, Osaka University, Toyonaka, Osaka, 560, JAPAN R.C. DUNNELL, Department
and P.T. MCCUTCHEON
of Anthropology, University of Washington, Seattle, WA 98195, U.S.A.
The ratio between the concentration of E’ centers and of oxygen vacancies in quartz was used to estimate the heating temperature of ancient lithic tool fragments made of cherts; the latter concentration was estimated from the signal intensity of E’ centers after converting diamagnetic oxygen vacancies with two electrons into paramagnetic E’ centers. Control experiments of temperature estimation was made by granitic quartz grains heated in a laboratory. The results for some lithic tool fragments were consistent with the one by conventional qualitative assessment using the color changes of the samples. KEWORDS:
ESR dating; heat treatment;
lithic tool; E’, center; quartz; chert.
INTRODUCTION The lithic tool fragments made of heated chert have been used to investigate the firing technology for studying the history of ancient human beings. Although heat treatment as a frequent component in lithic technology has been well established for two decades, there are major problems in the identification of heat treatment in archaeological assemblages using macroscopic attributes such as color and luster (Collins and Fenwick, 1974). Even when raw material from the geologic source is available for comparison, the range of these attributes may overlap between heated and unheated materials. Thermoluminescence analysis is an established method for the secure identification of heat treatment (Melcher and Zimmerman, 1977; Pawlish and Sheppard, 1983). The method simply contrasts the relatively modern thermoluminescence of artificial heating with the ancient luminescence centers induced by radiation in the chert. Electron spin resonance (ESR) should offer an equally secure method by observing trapped electrons (or holes). However, ESR signals other than from carbon radicals (Robins ef al., 1978; Griffiths er al., 1982) have not been utilized for identifying heat treatment. A new method to assess the ancient heat treatment using E’ centers is proposed in this paper. It is based on the thermal property of E’ centers in quartz contained in chert. E’ centers are paramagnetic defects in quartz, where an electron is at an oxygen vacancy. The change of signal intensity of E’ centers for heat treatment and for gamma ray irradiation is governed by thermal stability of the E’ centers itself as well as by that of oxygen vacancies and by a charge transfer process. The change of E’ centers on heating was recently discussed quantitatively by Toyoda and Ikeya (1991a). It was found that the E’ intensity increases on heating below 3OO”C, while it decreases above 300°C. This behavior is in contrast to impurity centers, whose intensities only decrease on heating. A charge transfer process has been proposed for the increase, namely that an oxygen vacancy with two electrons traps a hole released from a hole center. The increment of E’ centers has been shown to be correlated with the decrement of Al center (Jani et al., 1983; Toyoda and Ikeya, 1991b). A method to 227
228
ESR dosimetry and applications
obtain the number of oxygen vacancies as the intensity of E’ center was proposed (Toyoda and Ikeya, 1991a). The procedure used is the irradiation of the quartz sample by gamma rays at a high dose (e.g., 3 kGy) following heating at 300°C for 15 minutes to convert oxygen vacancies with two electrons into E’ centers. The E’ intensity is proportional to the amount of oxygen vacancies. Toyoda and Ikeya (1991a) showed that oxygen vacancies decrease in number when the sample is heated above 400°C. The behavior of the E’ center on heating is dependent on the structure of the quartz. The lineshape of the powder spectrum of E’ centers in crystalline quartz (from granite) is different from the one observed in amorphous silica, shown by comparing the spectrum shown in Toyoda and Ikeya (1991a) with the one in Griscom (1979). The signal observed in the chert samples presented in this work has the same lineshape as the one in crystalline quartz from granite. Therefore, we interpret the signal observed as being from crystalline quartz, with a negligible contribution from amorphous silica. EXPERIMENTAL Quartz grains were extracted from Takefushi granite at Okazaki city, Aichi prefecture, Japan for an experiment to confirm the theoretical basis outlined above. ESR measurements were made with commercial ESR spectrometer (JEOL RE-1X). The E’ signal was observed with microwave power of lO%nW at room temperature with a field modulation of 0.1 mT and 100 kHz. The peak-to-peak height at g=2.001 was taken as the intensity. Heating experiments confirmed that the effect on E’ intensity of heating a sample stepwise through the range of interest is the same as heating aliquots of the sample to various temperatures within this range. Therefore in the initial control experiments, sample aliquots were heated to a given temperature and their E’ intensity measured (I,). The samples were then gamma-irradiated to 3 kCy and annealed at 300°C for 15 minutes. The E’ intensity after this procedure is I,. The 1,/I, ratio is plotted as a function of temperature (Fig. 1). The change in the ratio shows a monotonic increase below 3OO”C, and a monotonic decrease above 300°C.
300 400 500 “C‘ Temperature Fig. 1. E’ to oxygen vacancy ratio plotted as a function of heating temperature for granitic 100
200
can be estimated from E’ center to oxygen vacancy ratio if one knows the temperature range. A control experiment (TST-I, 2, 3) shows that the estimated temperatures agree with the actual ones (see Table 1 for summary). quartz. The heating temperature
Therefore, the temperature at which a sample has been heated in the past can be obtained using the curve shown in Fig. 1. The E’ intensity before any laboratory treatment is I,. The range, above or below 3OO”C, can be determined by comparing the E’ intensities before and after annealing at 300°C. If the sample had been heated below 3OO”C, the E’ intensity would increase by annealing because remaining holes would be transferred to oxygen vacancies forming E’ centers. If the sample had been heated above 3OO”C, the E’ intensity would be the same or decrease slightly because there are no remaining holes to be transferred. To determine the heating temperature more precisely, the aliquot should be irradiated and annealed (as described above), and the ratio plotted in Fig. 1. Toyoda and Ikeya (1991a) interpreted the experimental results obtained by Wieser and Regulla (1989) as showing that gamma-ray irradiation to doses in the “giga-rad” (10 MGy) range created oxygen vacancies.
ESR dosimetry and applications
229
However, the number of oxygen vacancies created by gamma-ray irradiation (3 kGy) is much less than the inherent one presumably created by cr-recoils in quartz (Rink and Cklom, 1991). If E’ centers are not observed in the initial sample aliquot, only an approximate temperature range can be determined, as follows. If an E’ center is observed after heating irradiation/annealing, it indicates that not all the oxygen vacancies had been annealed by previous heating. Since heating to 600°C anneals oxygen vacancies, a sample of this sort has been heated in the past to between 450°C and 600°C. Alternatively, if no E’ signal is observed after irradiation/annealing, the past heating has been in the range above 600°C. The algorithm to estimate the heating temperature is summarized in Fig. 2. RESULTS AND DISCUSSION A Control ExDeriment. Several aliquots of quartz grains from a granite were heated at temperatures shown in Table 1 for 30 minutes. The heating and irradiation procedures followed the algorithm shown in Fig. 2. The procedure of temperature estimation using obtained 1,/12ratio is shown in Fig. 1. The results are summarized in Table 1. Actual heating temperatures and those estimated by the procedure agree very well. SAMPLES I
7
ESR Measurements. I1 = Intensity of E' centers 11'0 Il>O Degree0 : unheatedor heared Heating
at
below 300°C if it has been heated
300°C
I
.
ESR Measurements. Does E' intensity increase? No Degree 1
Yes Degree 0
Degree 1 Degree2-1
: heated behveen 300 and 450°C
:heatedhehveen450and600°C : heated above 600°C Degree2-2
Degree 2
y ray Irradiation (3 kGy) Heating at 300'C for 15 minutes I
I
I
ESR Measurements. I2 = Intensity of E' centers
ESR Measurements Are E' centers Observed?
I
I
Yes
No
value of 11/12 is obtained Degree 2-1
Degree 2-2
Heating temperature is estimated using the curve between 11/12 and beating temperature
,
Fig. 2. The
algorithm proposedto estimatethetemperature ofancient heat treatment.
1
I, ‘?;
0.5 j e!
?7? I
100
\
,
To
. . .._ ;,ho_
I
200
I
300 Temperature
400
500
“C
, 100
200
I 300
‘0 ,‘o400
/ 500
Temperature
Fig. 3. E’ to oxygen vacancy ratio plotted as a function of heating temperature for (a) Mill creek and for (b) Dover cherts. The heating temperatures were estimated for CL253 and LAN279.
‘C
ESR
230
dosimetry and applications
Table 1. The results of a control experiment. temperatures.
A e.
Estimated temperatures
Heating Temperature 315
agreed with heating
Estimated Temperature 310
Sample
WI2
TST-1 TST-2 TST-3
0.60
373
370
0.04
445
440
0.95
( OC )
( OC)
lication
Our initial application involved the examination of ten specimens from the central Mississippi valley
representing two distinctive and well known cherts, the Mill Creek chert from southern Illinois and the Dover chert from western Tennessee. Both cherts are tough materials and were widely traded in the form of hoes in the Late Prehistoric period (Winters 1981). Two of our samples represent the unheated raw materials. The Mill Creek chert, typically a light gray (lOYR7/1), does change color quite markedly upon heating to a light red (7.5YR6/6) (Dunnell ef al., submitted). The Dover chert, on the other hand, is a chert that changes little upon heat treatment, either in color or luster (Dunnell et al., submitted). The remaining eight specimens are archaeological and originate from two late Mississippian sites, County Line and Langdon, on the Malden Plain in the Eastern Lowlands in the Mississippi valley of Southeast Missouri. About 200 mg of raw cherts were heated with a stepwise temperature elevation. The heating method did not affect the final E’ intensities. The curves relating heating temperature to 1,/I, were obtained as shown in Figs. 3a, b. The temperature estimations were made using these curves and the algorithm shown in Fig. 2. The heating temperatures of CL253 and LAN279 were estimated after determining the heating degrees to be one. E’ centers were not observed in LAN10351 and in LAN88, but oxygen vacancies were not annealed. The range of heating temperature was judged to be 450-600°C. The increments of E’ intensities by heating up to 300°C were rather small for these source cherts. Therefore, the heating temperature for the samples classed as degree could not be determined precisely. Al centers in these samples gave complementary information for temperature estimations of these samples. Al centers were observed in LAN946, CL176, and LAN1038, and in both cherts, but not in other samples. After gamma-ray irradiation, they were observed in all samples, and annealed after subsequent heating at 300°C. These observations imply that Al centers were observed in former samples because of no heat treatment. However, it is because of moderate heating below 300°C for LAN1052 that Al centers were not observed although its heating degree was judged to be zero. The results summarized in Table 2 are consistent with qualitative estimation using color changes (Dunnell et al., submitted). Table 2. The results of an application to lithic tool fragments. The observation each step of the procedures, that of Al centers, and the estimated temperatures Sample
Source
Change
Chert
after heating
‘PI
Al center
LAN946
M
i
0.71
CL176
M
i
0.78
+ +
LAN1035-1
M M
d
0 0.85
-
d i
0.17 0.86
+
i
0 0.97
-
CL253 LAN279 LAN1038 LAN88 LAN1052
M D D D
D: M:
Dover Chert Mill Creek Chert
i: d:
The E’ intensity increased
by heating at 300°C.
of E’ centers at are shown.
Estimated Temperature (“C ) not not
heated heated
450-600 340 400 not heated 450-600 t300
t:
observed
-:
not observed
1,/I,: E’ to oxygen vacancy ratio The E’ intensity slightly decreased or did not change by heating at 300°C.
ESR dosimetry and applications
231
Some Problems with the Method. The reduction of E’ intensity by gamma-ray irradiation for heated quartz was reported by Sato ef. al. (1985). Their sample had been irradiated > 100 Gy after heating, in which case the E’ intensity would have been affected. For the lithic samples in this study, the change in intensity by natural radiation can be neglected because the accumulated dose would be within 5 Gy. This is derived from an annual dose rate of 5 mGy/a, based on the hoe fragments being from the 10th century (Dunnell et al., submitted). The duration of heating is assumed to be 30 minutes. However, calculations suggest that the results are not very sensitive to heating time. A temperature range from 380°C to 355°C is obtained when the duration ranges from 20 to 60 minutes for 1,/1,=0.5 using the parameters of thermally stable E’ centers (Toyoda and Ikeya, 1991a).
ACKNOWLEDGEMENTS We are indebted to Ms. S. Geneise of Knoxville, TN for samples of the Dover quarry material, Dr. P. A. Teuser of the Department of Anthropology, Southern Illinois University, for the County Line Specimens, and Mr. and Mrs. W. L. Pavidson of Kinnet Montana, for the Langdon samples. We also thank to Dr. N. Kimura at Institute of Industrial Science, Osaka University for gamma-ray irradiations. This work was supported in part by Grant-in-Aid for Encouragement of Young Scientists (No. 03854034) from the Ministry of Education, Science and Culture and in part by Inoue Foundation for Science. REFERENCES Collins M.B. and Fenwick J.M. (1974) Heat treatment of chert: Methods of interpretation and their application. Plains Anthropologist, 19, 134-145. Dunnell R.C., Ikeya M., McCutcheon P.T. and Toyoda S. (submitted, J. Arch. Sci.). Heat treatment of Mill creek and Dover cherts on the Malden Plain, Southeast Missouri. Griffitbs D.R., Robins G.U., Seeley N.J., Chandra D.A.C. and Symons M.C.R. (1982) Trapped methyl radicals in chert. Nature, 300, 435-436. Griscom D.L. (1979) E’ center in glassy SiO,: Microwave saturation properties and confirmation of the primary =Si hyperfine structure. Phys. Rev. B, 20, 1823-1834. Jani M.G., Bossoli R.B. and Halliburton L.E. (1983) Further characterization of the E’, center in crystalline SiO,. Phys. Rev. B, 27, 2285-2293. Melcher C.L. and Zimmerman D.W. (1977) Thermoluminescence Determination of prehistoric heat treatment chert artifacts. Science, 197, 1359-1362. Pawlish L.A. and P.J. Sheppard (1983) Thermoluminescent determination of paleoindian heat treatment in Ontario, Canada. American Antiquiry, 48, 793-799. Rink W.J. and Odom A.L. (1991) Natural cr-recoil particle radiation and ionizing radiation sensitivities in quartz detected with EPR: implications for geochronometry. Nucl. Trucks R&at. Meus. 18, 163-173.
Robins G.V., Seeley N.J., McNeil D.A.C. and Symons M.R.C. (1978) Identification of ancient heat treatment in flint artifacts by ESR spectroscopy. Nature, 276, 703-704. Sato T., Suit0 K. and Ichikawa Y. (1985) Characteristics of ESR and TL signals on quartz from fault regions. ESR Dating and Dosimetry, 267-273. Toyoda S. and Ikeya M. (1991a) Thermal stabilities of paramagnetic defect and impurity centers in quartz: Basis of ESR dating of thermal history. Geochem. J., 25, 427-435. Toyoda S. and Ikeya M. (1991b) ESR dating of quartz and plagioclase from volcanic ashes using E’, Al, and Ti centers. Nucl. Tracks Radiat. Meas., 18, 179-184. Wieser A. and Regulla D.F. (1989) ESR dosimetry in the “Gigarad” range. Appl. Radiat. Isot., 40, 91 l- 913. Winters, H. D. (1981). Excavating in Museums: Notes on Mississippian hoes and Middle Woodland copper Gouges and cherts. In: Research Potential of Anthropological Museum Collections (A. E. Cantwell, J. B. Griffin and N.A. Rothschild, ed.), pp. 17-34. Annals of the New York Academy of Science, No.376. New York.
M. IKEYA,
Department of Earth and Space Science, Faculty of Science, Osaka University, Toyonaka, Osaka, 560, JAPAN R.C. DUNNELL, Department
and P.T. MCCUTCHEON
of Anthropology, University of Washington, Seattle, WA 98195, U.S.A.
The ratio between the concentration of E’ centers and of oxygen vacancies in quartz was used to estimate the heating temperature of ancient lithic tool fragments made of cherts; the latter concentration was estimated from the signal intensity of E’ centers after converting diamagnetic oxygen vacancies with two electrons into paramagnetic E’ centers. Control experiments of temperature estimation was made by granitic quartz grains heated in a laboratory. The results for some lithic tool fragments were consistent with the one by conventional qualitative assessment using the color changes of the samples. KEWORDS:
ESR dating; heat treatment;
lithic tool; E’, center; quartz; chert.
INTRODUCTION The lithic tool fragments made of heated chert have been used to investigate the firing technology for studying the history of ancient human beings. Although heat treatment as a frequent component in lithic technology has been well established for two decades, there are major problems in the identification of heat treatment in archaeological assemblages using macroscopic attributes such as color and luster (Collins and Fenwick, 1974). Even when raw material from the geologic source is available for comparison, the range of these attributes may overlap between heated and unheated materials. Thermoluminescence analysis is an established method for the secure identification of heat treatment (Melcher and Zimmerman, 1977; Pawlish and Sheppard, 1983). The method simply contrasts the relatively modern thermoluminescence of artificial heating with the ancient luminescence centers induced by radiation in the chert. Electron spin resonance (ESR) should offer an equally secure method by observing trapped electrons (or holes). However, ESR signals other than from carbon radicals (Robins ef al., 1978; Griffiths er al., 1982) have not been utilized for identifying heat treatment. A new method to assess the ancient heat treatment using E’ centers is proposed in this paper. It is based on the thermal property of E’ centers in quartz contained in chert. E’ centers are paramagnetic defects in quartz, where an electron is at an oxygen vacancy. The change of signal intensity of E’ centers for heat treatment and for gamma ray irradiation is governed by thermal stability of the E’ centers itself as well as by that of oxygen vacancies and by a charge transfer process. The change of E’ centers on heating was recently discussed quantitatively by Toyoda and Ikeya (1991a). It was found that the E’ intensity increases on heating below 3OO”C, while it decreases above 300°C. This behavior is in contrast to impurity centers, whose intensities only decrease on heating. A charge transfer process has been proposed for the increase, namely that an oxygen vacancy with two electrons traps a hole released from a hole center. The increment of E’ centers has been shown to be correlated with the decrement of Al center (Jani et al., 1983; Toyoda and Ikeya, 1991b). A method to 227
228
ESR dosimetry and applications
obtain the number of oxygen vacancies as the intensity of E’ center was proposed (Toyoda and Ikeya, 1991a). The procedure used is the irradiation of the quartz sample by gamma rays at a high dose (e.g., 3 kGy) following heating at 300°C for 15 minutes to convert oxygen vacancies with two electrons into E’ centers. The E’ intensity is proportional to the amount of oxygen vacancies. Toyoda and Ikeya (1991a) showed that oxygen vacancies decrease in number when the sample is heated above 400°C. The behavior of the E’ center on heating is dependent on the structure of the quartz. The lineshape of the powder spectrum of E’ centers in crystalline quartz (from granite) is different from the one observed in amorphous silica, shown by comparing the spectrum shown in Toyoda and Ikeya (1991a) with the one in Griscom (1979). The signal observed in the chert samples presented in this work has the same lineshape as the one in crystalline quartz from granite. Therefore, we interpret the signal observed as being from crystalline quartz, with a negligible contribution from amorphous silica. EXPERIMENTAL Quartz grains were extracted from Takefushi granite at Okazaki city, Aichi prefecture, Japan for an experiment to confirm the theoretical basis outlined above. ESR measurements were made with commercial ESR spectrometer (JEOL RE-1X). The E’ signal was observed with microwave power of lO%nW at room temperature with a field modulation of 0.1 mT and 100 kHz. The peak-to-peak height at g=2.001 was taken as the intensity. Heating experiments confirmed that the effect on E’ intensity of heating a sample stepwise through the range of interest is the same as heating aliquots of the sample to various temperatures within this range. Therefore in the initial control experiments, sample aliquots were heated to a given temperature and their E’ intensity measured (I,). The samples were then gamma-irradiated to 3 kCy and annealed at 300°C for 15 minutes. The E’ intensity after this procedure is I,. The 1,/I, ratio is plotted as a function of temperature (Fig. 1). The change in the ratio shows a monotonic increase below 3OO”C, and a monotonic decrease above 300°C.
300 400 500 “C‘ Temperature Fig. 1. E’ to oxygen vacancy ratio plotted as a function of heating temperature for granitic 100
200
can be estimated from E’ center to oxygen vacancy ratio if one knows the temperature range. A control experiment (TST-I, 2, 3) shows that the estimated temperatures agree with the actual ones (see Table 1 for summary). quartz. The heating temperature
Therefore, the temperature at which a sample has been heated in the past can be obtained using the curve shown in Fig. 1. The E’ intensity before any laboratory treatment is I,. The range, above or below 3OO”C, can be determined by comparing the E’ intensities before and after annealing at 300°C. If the sample had been heated below 3OO”C, the E’ intensity would increase by annealing because remaining holes would be transferred to oxygen vacancies forming E’ centers. If the sample had been heated above 3OO”C, the E’ intensity would be the same or decrease slightly because there are no remaining holes to be transferred. To determine the heating temperature more precisely, the aliquot should be irradiated and annealed (as described above), and the ratio plotted in Fig. 1. Toyoda and Ikeya (1991a) interpreted the experimental results obtained by Wieser and Regulla (1989) as showing that gamma-ray irradiation to doses in the “giga-rad” (10 MGy) range created oxygen vacancies.
ESR dosimetry and applications
229
However, the number of oxygen vacancies created by gamma-ray irradiation (3 kGy) is much less than the inherent one presumably created by cr-recoils in quartz (Rink and Cklom, 1991). If E’ centers are not observed in the initial sample aliquot, only an approximate temperature range can be determined, as follows. If an E’ center is observed after heating irradiation/annealing, it indicates that not all the oxygen vacancies had been annealed by previous heating. Since heating to 600°C anneals oxygen vacancies, a sample of this sort has been heated in the past to between 450°C and 600°C. Alternatively, if no E’ signal is observed after irradiation/annealing, the past heating has been in the range above 600°C. The algorithm to estimate the heating temperature is summarized in Fig. 2. RESULTS AND DISCUSSION A Control ExDeriment. Several aliquots of quartz grains from a granite were heated at temperatures shown in Table 1 for 30 minutes. The heating and irradiation procedures followed the algorithm shown in Fig. 2. The procedure of temperature estimation using obtained 1,/12ratio is shown in Fig. 1. The results are summarized in Table 1. Actual heating temperatures and those estimated by the procedure agree very well. SAMPLES I
7
ESR Measurements. I1 = Intensity of E' centers 11'0 Il>O Degree0 : unheatedor heared Heating
at
below 300°C if it has been heated
300°C
I
.
ESR Measurements. Does E' intensity increase? No Degree 1
Yes Degree 0
Degree 1 Degree2-1
: heated behveen 300 and 450°C
:heatedhehveen450and600°C : heated above 600°C Degree2-2
Degree 2
y ray Irradiation (3 kGy) Heating at 300'C for 15 minutes I
I
I
ESR Measurements. I2 = Intensity of E' centers
ESR Measurements Are E' centers Observed?
I
I
Yes
No
value of 11/12 is obtained Degree 2-1
Degree 2-2
Heating temperature is estimated using the curve between 11/12 and beating temperature
,
Fig. 2. The
algorithm proposedto estimatethetemperature ofancient heat treatment.
1
I, ‘?;
0.5 j e!
?7? I
100
\
,
To
. . .._ ;,ho_
I
200
I
300 Temperature
400
500
“C
, 100
200
I 300
‘0 ,‘o400
/ 500
Temperature
Fig. 3. E’ to oxygen vacancy ratio plotted as a function of heating temperature for (a) Mill creek and for (b) Dover cherts. The heating temperatures were estimated for CL253 and LAN279.
‘C
ESR
230
dosimetry and applications
Table 1. The results of a control experiment. temperatures.
A e.
Estimated temperatures
Heating Temperature 315
agreed with heating
Estimated Temperature 310
Sample
WI2
TST-1 TST-2 TST-3
0.60
373
370
0.04
445
440
0.95
( OC )
( OC)
lication
Our initial application involved the examination of ten specimens from the central Mississippi valley
representing two distinctive and well known cherts, the Mill Creek chert from southern Illinois and the Dover chert from western Tennessee. Both cherts are tough materials and were widely traded in the form of hoes in the Late Prehistoric period (Winters 1981). Two of our samples represent the unheated raw materials. The Mill Creek chert, typically a light gray (lOYR7/1), does change color quite markedly upon heating to a light red (7.5YR6/6) (Dunnell ef al., submitted). The Dover chert, on the other hand, is a chert that changes little upon heat treatment, either in color or luster (Dunnell et al., submitted). The remaining eight specimens are archaeological and originate from two late Mississippian sites, County Line and Langdon, on the Malden Plain in the Eastern Lowlands in the Mississippi valley of Southeast Missouri. About 200 mg of raw cherts were heated with a stepwise temperature elevation. The heating method did not affect the final E’ intensities. The curves relating heating temperature to 1,/I, were obtained as shown in Figs. 3a, b. The temperature estimations were made using these curves and the algorithm shown in Fig. 2. The heating temperatures of CL253 and LAN279 were estimated after determining the heating degrees to be one. E’ centers were not observed in LAN10351 and in LAN88, but oxygen vacancies were not annealed. The range of heating temperature was judged to be 450-600°C. The increments of E’ intensities by heating up to 300°C were rather small for these source cherts. Therefore, the heating temperature for the samples classed as degree could not be determined precisely. Al centers in these samples gave complementary information for temperature estimations of these samples. Al centers were observed in LAN946, CL176, and LAN1038, and in both cherts, but not in other samples. After gamma-ray irradiation, they were observed in all samples, and annealed after subsequent heating at 300°C. These observations imply that Al centers were observed in former samples because of no heat treatment. However, it is because of moderate heating below 300°C for LAN1052 that Al centers were not observed although its heating degree was judged to be zero. The results summarized in Table 2 are consistent with qualitative estimation using color changes (Dunnell et al., submitted). Table 2. The results of an application to lithic tool fragments. The observation each step of the procedures, that of Al centers, and the estimated temperatures Sample
Source
Change
Chert
after heating
‘PI
Al center
LAN946
M
i
0.71
CL176
M
i
0.78
+ +
LAN1035-1
M M
d
0 0.85
-
d i
0.17 0.86
+
i
0 0.97
-
CL253 LAN279 LAN1038 LAN88 LAN1052
M D D D
D: M:
Dover Chert Mill Creek Chert
i: d:
The E’ intensity increased
by heating at 300°C.
of E’ centers at are shown.
Estimated Temperature (“C ) not not
heated heated
450-600 340 400 not heated 450-600 t300
t:
observed
-:
not observed
1,/I,: E’ to oxygen vacancy ratio The E’ intensity slightly decreased or did not change by heating at 300°C.
ESR dosimetry and applications
231
Some Problems with the Method. The reduction of E’ intensity by gamma-ray irradiation for heated quartz was reported by Sato ef. al. (1985). Their sample had been irradiated > 100 Gy after heating, in which case the E’ intensity would have been affected. For the lithic samples in this study, the change in intensity by natural radiation can be neglected because the accumulated dose would be within 5 Gy. This is derived from an annual dose rate of 5 mGy/a, based on the hoe fragments being from the 10th century (Dunnell et al., submitted). The duration of heating is assumed to be 30 minutes. However, calculations suggest that the results are not very sensitive to heating time. A temperature range from 380°C to 355°C is obtained when the duration ranges from 20 to 60 minutes for 1,/1,=0.5 using the parameters of thermally stable E’ centers (Toyoda and Ikeya, 1991a).
ACKNOWLEDGEMENTS We are indebted to Ms. S. Geneise of Knoxville, TN for samples of the Dover quarry material, Dr. P. A. Teuser of the Department of Anthropology, Southern Illinois University, for the County Line Specimens, and Mr. and Mrs. W. L. Pavidson of Kinnet Montana, for the Langdon samples. We also thank to Dr. N. Kimura at Institute of Industrial Science, Osaka University for gamma-ray irradiations. This work was supported in part by Grant-in-Aid for Encouragement of Young Scientists (No. 03854034) from the Ministry of Education, Science and Culture and in part by Inoue Foundation for Science. REFERENCES Collins M.B. and Fenwick J.M. (1974) Heat treatment of chert: Methods of interpretation and their application. Plains Anthropologist, 19, 134-145. Dunnell R.C., Ikeya M., McCutcheon P.T. and Toyoda S. (submitted, J. Arch. Sci.). Heat treatment of Mill creek and Dover cherts on the Malden Plain, Southeast Missouri. Griffitbs D.R., Robins G.U., Seeley N.J., Chandra D.A.C. and Symons M.C.R. (1982) Trapped methyl radicals in chert. Nature, 300, 435-436. Griscom D.L. (1979) E’ center in glassy SiO,: Microwave saturation properties and confirmation of the primary =Si hyperfine structure. Phys. Rev. B, 20, 1823-1834. Jani M.G., Bossoli R.B. and Halliburton L.E. (1983) Further characterization of the E’, center in crystalline SiO,. Phys. Rev. B, 27, 2285-2293. Melcher C.L. and Zimmerman D.W. (1977) Thermoluminescence Determination of prehistoric heat treatment chert artifacts. Science, 197, 1359-1362. Pawlish L.A. and P.J. Sheppard (1983) Thermoluminescent determination of paleoindian heat treatment in Ontario, Canada. American Antiquiry, 48, 793-799. Rink W.J. and Odom A.L. (1991) Natural cr-recoil particle radiation and ionizing radiation sensitivities in quartz detected with EPR: implications for geochronometry. Nucl. Trucks R&at. Meus. 18, 163-173.
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