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Formation and decay of the E1′ center and of its precursor
Applied Radiation and Isotopes 52 (2000) 1351±1356
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Formation and decay of the E1 ' center and of its precursor Shin Toyoda a,*, Wataru Hattori b a
Department of Applied Physics, Faculty of Science, Okayama University of Science, 1-1 Ridai, Okayama, 700-0005, Japan Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikeneyama, Toyonaka, Osaka, 560-0043, Japan
b
Abstract The E1 ' center has been used for ESR dating of quartz with assuming that the signal intensity increases with natural radiation dose as those of other ESR signals do. However, this simple assumption is not necessarily correct. Formation and decay of the E1 ' center are closely related with its precursor, diamagnetic oxygen vacancies. Gamma ray of large dose (>100 kGy) creates oxygen vacancies giving little dose rate eect, which, therefore, might be useful for dating of granites and high dose dosimetry. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The E1 ' center, an unpaired electron in a single silicon sp3 orbital oriented along a bond direction into an oxygen vacancy (Feigl et al., 1974), has been believed to be useful for ESR dating and dosimetry. It has been assumed that the E1 ' center is enhanced by gamma ray. Ikeya et al. (1982) ®rst used this signal for ESR dating together with gamma ray irradiation to obtain the amount of accumulated natural doses, while Garrison et al. (1981) tried to date ¯int using fast neutron irradiation. Ikeya et al. (1982) believed that the fault movement zeroes the signal of the E1 ' center and that the signal is enhanced by natural radiation since the fault movement. The following studies aimed to establish the method which con®rms the complete zeroing at the time of fault movements (e.g., Ariyama, 1985;
* Corresponding author. Tel.: +81-86-256-9608; fax: +8186-255-7700. E-mail address: [email protected] (S. Toyoda).
Tanaka, 1989; Fukuchi et al., 1986) as well as recent study by Lee and Schwarcz (1994) where the ages of faults in California were successfully obtained based on the grain size plateau criterion. These studies also simply assumed that the E1 ' center was enhanced by gamma ray irradiation but did not investigate the characteristics of formation of the E1 ' center precisely. Dating of burnt ¯ints also employed the E1 ' center with the same assumption (Porat and Schwarcz, 1991; Porat et al., 1994) as well as dating of tephra (Toyoda and Ikeya, 1991a) and investigation of the thermal eect given by intrusion of a dike (Ikeya and Toyoda, 1991). However, it was found that the formation of the E1 ' center is not so simple as has been assumed (Jani et al., 1983; Toyoda and Ikeya, 1991b). Its formation and decay are closely related with its precursor, an oxygen vacancy, as being discussed in the present paper. The intensity of the E1 ' center looked as if it was enhanced by gamma ray irradiation like ordinary ESR signals presumably due to formation of the counterfeit E1 ' signal (Toyoda and Schwarcz, 1997a). In addition, the
0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 0 9 4 - 4
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characteristics of formation of the precursors (oxygen vacancies) are summarized in the present paper together with the presentation of new data on the dose rate dependence of the oxygen vacancies formation.
2. The E1 ' center and its precursor 2.1. Increase of the E1 ' center intensity on heating An ordinary ESR dating signal is enhanced by gamma ray irradiation and decreases with heating the sample (e.g., impurity centers in quartz (Toyoda and Ikeya, 1994)). However, the intensity of the E1 ' center increases on heating as was already found in 1960 by Weeks and Nelson (1960). The electronic process to cause this feature was proposed by Jani et al. (1983) that thermally activated electronic holes are released from the Al centers and are supplied to diamagnetic oxygen vacancies with two electrons so that a hole can recombine with one of the electrons to leave an unpaired electron in the oxygen vacancies (forming the E1 ' center). Typical change on heating of the signal intensity in granitic quartz is shown in Fig. 1 (Toyoda and Schwarcz, 1997a) where the E1 ' center signal increased up to 3008C and then decease until the signal disappears at 5008C.
Fig. 1. The change of the signal intensities observed at the position of the E10 center (after Toyoda and Schwarcz, 1997a). The natural granitic quartz sample showed the increase in the intensity up to 3008C then decrease above the temperature. On the other hand, the irradiated quartz sample showed decrease up to 1708C before increase because the thermally unstable counterfeit E10 signal is generated by the gamma ray irradiation and overlapped the ``real '' E10 , making it to look as if the signal increases on irradiation. That counterfeit signal decays up to 1708C.
2.2. Factors that limit the intensity of the E1 ' center: Experiment 1 If the process proposed by Jani et al. (1983) is correct, the intensity of the E1 ' center should be a function of both the amount of hole centers (to transform the diamagnetic oxygen vacancies to the E1 ' center) and the amount of its precursor, oxygen vacancies (that limits the amount of the E1 ' center). The following experiment was performed in order to con®rm the idea. Quartz grains were extracted from Mannari granite, Okayama, Japan. After crushing a block gently, the sample was soaked in 6 N HCl for one night, then, magnetic minerals were removed by a portable magnetic separator. The grains were then soaked in sodium polytungstate in order to remove minerals lighter than quartz. Finally, the grains were treated with 20% HF for two hours to dissolve contaminant feldspar. The extracted quartz grains were then gently crushed into 75 to 250 mm. Nine aliquots of 100 mg were prepared after heating the sample at 4308C for 30 min to anneal the E1 ' center signal. The oxygen vacancies are not annealed at this temperature because the signal of the E1 ' center is regenerated after the following experimental procedure. The sample aliquots were irradiated by 60 Co gamma ray to the doses ranging from 0 to 880 Gy with a dose rate of 44 Gy/h. After leaving for three days, ESR signal intensity of the Al center was measured for each aliquot. The intensities of the regenerated E1 ' center were measured after the samples were subsequently heated at 3008C for 15 min. The ESR intensity of the Al center was measured at
Fig. 2. The dose dependence of the regenerated signal intensity of the E10 center (the heat treated E10 intensity, see text for the experimental procedure). The E10 center intensity saturates above 200 Gy while no saturation occur for the Al center, indicating that the E10 center intensity saturates due to the limit of the amount of the precursors (oxygen vacancies). This level denotes the amount oxygen vacnacies in quartz relatively.
S. Toyoda, W. Hattori / Applied Radiation and Isotopes 52 (2000) 1351±1356
liquid nitrogen temperature with an ESR spectrometer, JEOL RE1-X. The sample in a quartz tube was dipped in a ®nger dewar inserted into the microwave cavity. The microwave power was 5 mW with a ®eld modulation amplitude of 0.1 mT. The width of the magnetic ®eld was 10 mT taking 8 min to scan with a time constant of 0.3 s. The signal of the E1 ' center was observed with a microwave power of 0.01 mW at room temperature. The other conditions were the same as above except for the scan range of 5 mT. The intensity of Al center was taken as the peak height from the top of the ®rst peak to the bottom of the 16th peak of the main hyper®ne structure (Yokoyama et al., 1985). The results are shown in Fig. 2 as a function of gamma ray dose. The intensity of the Al center (before heating) shows monotonic increase at least up to 1 kGy. On the other hand, the intensity of the regenerated E1 ' center increases up to 200 Gy and shows a ¯at response, being consistent with the previous results obtained by Toyoda and Ikeya (1991b) where they neither measured Al center nor discussed the reason well enough for that response. The results are interpreted as below following the idea proposed by Jani et al. (1983). When the gamma ray dose is smaller than 200 Gy, the amount of the hole centers such as Al center which can release holes on heating would be small. Therefore, the amount of the regenerated E1 ' center is smaller, depending on the dose, although there would still be diamagnetic oxygen vacancies that could become the E1 ' center. On the other hand, when the dose is larger than 200 Gy, the intensity of the E1 ' center shows saturation because of the limitation of the amount of diamagnetic oxygen vacancies. The intensity of Al center before heating does not saturate but monotonically increases, therefore, the amount of holes released get larger with dose, but the E1 ' center intensity saturates because all diamagnetic oxygen vacancies would be converted to E1 ' center in this dose range.
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radiation or when it is a natural sample. It might be because the electrons of electron-hole pairs created at the time of irradiation were trapped by the E1 ' centers to become diamagnetic oxygen vacancies where there would have been much more paramagnetic oxygen vacancies (the E1 ' center) than diamagnetic oxygen vacancies in those heated or natural samples. If this discussion is correct, the intensity of the E1 ' center in natural quartz would strongly depend on the environment, temperature and dose rate. It might reach a steady state with two eects, heat (temperature) which enhances the intensity and radiation which would rather decrease the intensity. The characteristics of formation of the E1 ' center revealed here is completely dierent from other ESR signals which have been used for dating. Therefore, the authors do not recommend using the E1 ' center signal for dating as long as the conventional dating proceedure is applied. The formation and decay characteristics of the E1 ' center should be precisely investigated at least for the speci®c sample to be dated before using this signal for dating.
2.3. Processes that aect the intensity of the E1 ' center The above experimental results indicate that formation of the E1 ' center should involve at least two processes, formation of the precursors (oxygen vacancies) and the electronic process that makes the precursors paramagnetic. The former process in nature will be discussed later in this paper. The latter electronic process can also go on in nature, which might enhance the intensity and enable us to observe the E1 ' center in natural quartz. As shown by Sato et al. (1985); Ozeki (1990), and Fig. 2 of Toyoda and Schwarcz (1997a), the intensity of the E1 ' center decreases with gamma ray irradiation when the intensity has been enhanced by heating before the ir-
Fig. 3. The change with dose of signal shapes observed at the position of the E10 center in granitic quartz. (a) no irradiation, (b) 3 kGy, (c) 6 kGy, (d) 15 kGy. The two peaks signal of the E10 center turns into one peak signal with dose, indicating that another signal (the counterfeit E10 signal.
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3. The counterfeit E1 ' signal The papers on ESR dating of quartz using the E1 ' center reported that the signal was enhanced by gamma ray and that the equivalent doses were obtained by extrapolating the increase with dose. However, Toyoda and Schwarcz (1997a) found that another signal is created by irradiation and overlaps the real E1 ' center. The signal is characterized by the single peak, microwave power dependence similar to the real E1 ' center, and thermal stability; decay below 1708C. When irradiated by gamma rays, the signal shape of the E1 ' center changes as shown in Fig. 3 (Toyoda and Schwarcz, 1997a), gradually appearing a single peak of the counterfeit E1 ' signal as the dose gets larger. On heating the irradiated sample, the signal intensity once decreases up to 1708C, then increase up to 3008C, and ®nally decrease again to the temperature as shown in Fig. 1. The increase of the E1 ' center by gamma ray irradiation in the previous dating works would possibly be due to this counterfeit signal. Porat et al. (1994) pointed out that the ages of ¯ints obtained for the E1 ' center are systematically younger than those for Al center. Their following work (Porat and Schwarcz, 1995) also found that the lifetime of the E1 ' center in ¯ints after gamma ray irradiation is much smaller than that of natural signal. These results on ¯ints might be consistent with the idea that the generation of the counterfeit E1 ' signal made the E1 ' center appear to increase with gamma ray irradiation. Toyoda and Schwarcz (1997b) also found the generation of this counterfeit E1 ' signal in fault gouge from California. Therefore, this is another reason why the conventional dating method should not be applied to the E1 ' center.
method would evaluate the amount vacancies in quartz at least relatively.
of oxygen
4.2. The processes that create the oxygen vacancies Correlation was found between the intensities of the E1 ' center in granites and their ages by Odom and Rink (1989) in the range of 10±1000 Ma. They attributed the correlation to the formation of the oxygen vacancies due to alpha recoil nuclei contained in the quartz matrix in ppb order and pointed out the possibility to date granites with the E1 ' center. Their theoretical estimate of the amount created by this process was not successful (Rink and Odom, 1991) possibly because they did not consider the amount of diamagnetic oxygen vacancies. Being considered the existence of the diamagnetic oxygen vacancies, the correlation they observed should have two interpretations: 1. The amount of oxygen vacancies increases with age as Odom and Rink (1989) suggested. 2. The paramagnetic fraction (the E1 ' center) of oxygen vacancies increases with age. Toyoda (1992) examined the correlation between the ages of granites and the intensities of the heat treated E1 ' center (after irradiation and subsequent heating, relative amount of oxygen vacancies) and obtained a result shown in Fig. 4. This correlation clearly indicates that (1) is the case. In the following work, Toyoda et al. (1996) showed that the intensities are consistent with the heat treated E1 ' intensity created
4. Formation of the oxygen vacancies 4.1. The heat treated E1 ' center as an indicator of relative amount of oxygen vacancies Toyoda and Ikeya (1991b) proposed a method to evaluate the amount of oxygen vacancies in quartz, which is the intensity of the E1 ' center after heating at 3008C for 15 min following gamma ray irradiation to above 200 Gy (the heat treated E1 ' center intensity). As discussed in the above section, the intensity of the heat treated E1 ' center saturates above 200 Gy as results of the Experiment 1 because of the limitation of the amount of precursors, oxygen vacancies. Therefore, this saturation level would denote the amount of oxygen vacancies in quartz, supporting the proposal made by Toyoda and Ikeya (1991b). As long as the same procedure is applied, for a series of samples, this
Fig. 4. The correlation between the ages of granites and the heat treated E10 center intensities (after Toyoda, 1992). It indicates that the amount of oxygen vacancies in quartz increases with age.
S. Toyoda, W. Hattori / Applied Radiation and Isotopes 52 (2000) 1351±1356
by gamma ray irradiation. They pointed out that the oxygen vacancies in natural quartz might have been created by external (outside of quartz grains) beta and gamma rays. On the other hand, the amount of the oxygen vacancies created by alpha recoil nuclei was merely theoretically estimated, which did not coincide with those in natural quartz (Rink and Odom, 1991). For the purpose of dosimetry, Wieser and Regulla (1989) already observed the response of the heat treated E1 ' center intensity to dose in the range of several MGy without considering the relation between the E1 ' center and oxygen vacancies. 4.3. Factors that control the formation of the oxygen vacancies Temperature during irradiation and dose rate are the factors which could aect the formation of lattice defects and should be examined before applying the signal for practical dating and dosimetry. Wieser and Regulla (1989) ®rst pointed out the temperature dependence with showing the preliminary results. Toyoda et al. (1996) made precise experiments to show the strong temperature dependence for the formation of the oxygen vacancies. The oxygen vacancy formation is ten times larger at 2508C than that at room temperature. They also obtained an activation energy for the formation process. On the other hand, dose rate dependence has not yet been examined.
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4.4. Dose rate dependence of the formation of the oxygen vacancies in quartz: Experiment 2 The dose rate dependence is an important factor for dosimetry as well as for dating because the one used for gamma ray irradiation for calibration is much larger than the actual or natural dose rate. Quartz grains were extracted by the same method as described in the section above from Mannari granite and from a Quaternary volcanic tephra collected in New Mexico, USA Six aliquots were prepared for each sample after heating at 6008C for 1 h in order to erase all oxygen vacancies (Toyoda et al., 1992). Samples were, then, irradiated by gamma rays of 60 Co source to the total irradiation dose, 2.4 106 Gy, with varying dose rates ranging from 3.5 to 20 kGy/h. The ESR intensities of the heat treated E1 ' center were measured with the same conditions as Experiment 1. There is slight dose rate dependence observed as shown in Fig. 5. The dierence in the signal intensity between the largest and smallest exposure rates in the present experiment was 16% for the granitic quartz and 6% for the volcanic quartz. Such a rather small dose rate dependence would be less than the accuracy with which we are in discussion (Fig. 4). The temperature dependence would cause larger ambiguity in obtaining the age from the intensity of the heat treated E1 ' center while rock bodies decrease its temperature. However, that dependence might need to be investigated more precisely for the purpose of accurate dosimetry where the temperature of the dosimeter is well controlled.
5. Conclusion
Fig. 5. The dose rate dependence of the formation of oxygen vacancies in quartz. The total dose of 2.4 106 Gy was given to all samples with varying the dose rate. There are slight dose rate dependence which would give much smaller eect on the formation that the temperature dependence. However, it might be needed to consider for the precise dosimetry where the temperature of the dosimeter is well controlled.
The E1 ' center intensity is closely related with the amount of its precursors, oxygen vacancies as well as the amount of electronic holes to be transferred, as shown by the Experiment 1. The heat treated E1 ' center intensity corresponds to the amount of the oxygen vacancies. The counterfeit E1 ' signal may make the E1 ' center appear to increase on irradiation. Therefore, the conventional dating procedure would possibly mislead wrong ESR ages when applied to the E1 ' center signal. The heat treated E1 ' center would be a new indicator which might be useful for new applications of dating and dosimetry. Dating of granite in the age range of 10±1000 Ma as well as dosimetry of extremely high dose have been proposed. Experiment 2 revealed that the dose rate dependence of the formation of the oxygen vacancies is rather small and would give much smaller ambiguity than temperature dependence in the practical application but might be a factor to be con-
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sidered as a possible source of error in precise dosimetry.
Acknowledgements The present study was supported in part by InterUniversity Joint Research program using the JAERI facilities, and by Grants-in-AID for Encouragement of Young Scientists (No. 06854026).
References Ariyama, T. 1985. Conditions of resetting the ESR clock during faulting. In: Ikeya, M., Miki, T. (Eds.), ESR Dating and Dosimetry. IONICS, Tokyo, pp. 251±258. Feigl, F.J., Fowler, W.B., Yip, K.L., 1974. Oxygen vacancy model for the E1 ' centre in SiO2. Solid State Commun. 14, 225. Fukuchi, T., Imai, N., Shimokawa, K., 1986. ESR dating of fault movement using various defect centers in quartz; the case in the western south Fossa Magna. Japan. Earth Planet. Sci. 78, 121. Garrison, E.G., Rowlett, R.M., Cowan, D.L., Holroyd, L.V., 1981. ESR dating of ancient ¯ints. Nature 290, 44. Ikeya, M., Toyoda, S., 1991. Thermal eects in metamorphic rocks around an intrusion. Appl. Magn. Reson. 2, 69. Ikeya, M., Miki, T., Tanka, K., 1982. Dating of a fault by electron spin reonance on intrafault materials. Science 215, 1392. Jani, M.G., Bossoli, R.B., Halliburton, L.E., 1983. Further characterization of the E1 ' centre in crystalline SiO2. Phys. Rev. B 27, 2285. Lee, H.K., Schwarcz, H.P., 1994. Criteria for complete zeroing of ESR signals during faulting of the San Gabriel fault zone, southern California. Tectonophys. 235, 317. Ozeki, T., 1990. The change on g-ray irradiation of the intensity of the E1 ' center in quartz (in Japanese). Unpublished graduate thesis, Osaka University, Japan. Odom, A.L., Rink, J.W., 1989. Natural accumulation of Shottky-Frenkel defects: implications for a quartz geochronometer. Geology 17, 55. Porat, N., Schwarcz, H.P., 1991. Use of signal subtraction methods in ESR dating of burned ¯int. Nucl. Tracks Radiat. Meas. 18, 203. Porat, N., Schwarcz, H.P., 1995. Problems in determining life-
times of ESR signals in natural and burned ¯int by isothermal annealing. Radiat. Meas. 24, 161. Porat, N., Schwarcz, H.P., Valladas, H., Bar-Yosef, O., Vandermeersch, B., 1994. Electron spin resonance dating of burned ¯int from Kebara cave Israel. Geoarchaeology 9, 393. Rink, J.W., Odom, A.L., 1991. Natiural alpha recoil particle radiation and ionizing radiation sensitivities in quartz detected with EPR: implications for geochronometry. Nucl. Tracks Radiat. Meas. 18, 163. Sato, T., Suito, K., Ichikawa, Y. 1985. Characteristics of ESR and TL signals on quartz from fault regions. In: Ikeya, M., Miki, T. (Eds.), ESR Dating and Dosimetry. IONICS, Tokyo, pp. 267±273. Tanaka, K., 1989. Dating of fault movement by ESR method Ð basis and application. Ph.D. dissertation, University of Kyushu, Japan. Toyoda, S., Ikeya, M., 1991a. ESR dating of quartz and plagioclase from volcanic ashes using E1 ', Al and Ti centers. Nucl. Tracks. Radiat. Meas. 18, 179. Toyoda, S., Ikeya, M., 1991b. Thermal stabilities of paramagnetic defect and impurity centers in quartz: basis for ESR dating of thermal history. Geochem. J. 25, 437. Toyoda, S., 1992. Production and decay characteristics of paramagnetic defects in quartz: applications to ESR dating. Ph.D. dissertation, Osaka University, Toyonaka, Osaka, Japan. Toyoda, S., Ikeya, M., 1994. ESR dating of quartz with stable component of impurity centers. Quatern. Geochronol. 13, 625. Toyoda, S., Rink, W.J., Schwarcz, H.P., Ikeya, M., 1996. Formation of E1 ' precursors in quartz: applications to dating and dosimetry. Appl. Radiat. Isot. 47, 1393. Toyoda, S., Schwarcz, H.P., 1997a. Counterfeit E1 ' signal in quartz. Radiat. Meas. 27, 59. Toyoda, S., Schwarcz, H.P., 1997b. The hazard of the Counterfeit E1 ' signal in quartz to ESR dating of fault movements. Quartern. Geochronol. 16, 483. Toyoda, S., Ikeya, M., Morikawa, J., Nagatomo, T., 1992. Enhancement of oxygen vacancies in quartz by natural external and ray dose: a possible ESR geochronometer of Ma±Ga range. Geochem. J. 26, 111. Weeks, R.A., Nelson, C.M., 1960. Trapped electrons in irradiated quartz and silica: II, electron spin resonance. J. Am. Ceram. Soc. 43, 399. Wieser, A., Regulla, D.F., 1989. ESR dosimetry in the `Gigarad' range. Appl. Radiat. Isot. 40, 911. Yokoyama, Y., FalgueÁres, C., Quaegebeur, J.P., 1985. ESR dating of quartz from Quaternary sediments: ®rst attempt. Nucl. Tracks Radiat. Meas. 10, 921.
www.elsevier.com/locate/apradiso
Formation and decay of the E1 ' center and of its precursor Shin Toyoda a,*, Wataru Hattori b a
Department of Applied Physics, Faculty of Science, Okayama University of Science, 1-1 Ridai, Okayama, 700-0005, Japan Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikeneyama, Toyonaka, Osaka, 560-0043, Japan
b
Abstract The E1 ' center has been used for ESR dating of quartz with assuming that the signal intensity increases with natural radiation dose as those of other ESR signals do. However, this simple assumption is not necessarily correct. Formation and decay of the E1 ' center are closely related with its precursor, diamagnetic oxygen vacancies. Gamma ray of large dose (>100 kGy) creates oxygen vacancies giving little dose rate eect, which, therefore, might be useful for dating of granites and high dose dosimetry. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The E1 ' center, an unpaired electron in a single silicon sp3 orbital oriented along a bond direction into an oxygen vacancy (Feigl et al., 1974), has been believed to be useful for ESR dating and dosimetry. It has been assumed that the E1 ' center is enhanced by gamma ray. Ikeya et al. (1982) ®rst used this signal for ESR dating together with gamma ray irradiation to obtain the amount of accumulated natural doses, while Garrison et al. (1981) tried to date ¯int using fast neutron irradiation. Ikeya et al. (1982) believed that the fault movement zeroes the signal of the E1 ' center and that the signal is enhanced by natural radiation since the fault movement. The following studies aimed to establish the method which con®rms the complete zeroing at the time of fault movements (e.g., Ariyama, 1985;
* Corresponding author. Tel.: +81-86-256-9608; fax: +8186-255-7700. E-mail address: [email protected] (S. Toyoda).
Tanaka, 1989; Fukuchi et al., 1986) as well as recent study by Lee and Schwarcz (1994) where the ages of faults in California were successfully obtained based on the grain size plateau criterion. These studies also simply assumed that the E1 ' center was enhanced by gamma ray irradiation but did not investigate the characteristics of formation of the E1 ' center precisely. Dating of burnt ¯ints also employed the E1 ' center with the same assumption (Porat and Schwarcz, 1991; Porat et al., 1994) as well as dating of tephra (Toyoda and Ikeya, 1991a) and investigation of the thermal eect given by intrusion of a dike (Ikeya and Toyoda, 1991). However, it was found that the formation of the E1 ' center is not so simple as has been assumed (Jani et al., 1983; Toyoda and Ikeya, 1991b). Its formation and decay are closely related with its precursor, an oxygen vacancy, as being discussed in the present paper. The intensity of the E1 ' center looked as if it was enhanced by gamma ray irradiation like ordinary ESR signals presumably due to formation of the counterfeit E1 ' signal (Toyoda and Schwarcz, 1997a). In addition, the
0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 0 9 4 - 4
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S. Toyoda, W. Hattori / Applied Radiation and Isotopes 52 (2000) 1351±1356
characteristics of formation of the precursors (oxygen vacancies) are summarized in the present paper together with the presentation of new data on the dose rate dependence of the oxygen vacancies formation.
2. The E1 ' center and its precursor 2.1. Increase of the E1 ' center intensity on heating An ordinary ESR dating signal is enhanced by gamma ray irradiation and decreases with heating the sample (e.g., impurity centers in quartz (Toyoda and Ikeya, 1994)). However, the intensity of the E1 ' center increases on heating as was already found in 1960 by Weeks and Nelson (1960). The electronic process to cause this feature was proposed by Jani et al. (1983) that thermally activated electronic holes are released from the Al centers and are supplied to diamagnetic oxygen vacancies with two electrons so that a hole can recombine with one of the electrons to leave an unpaired electron in the oxygen vacancies (forming the E1 ' center). Typical change on heating of the signal intensity in granitic quartz is shown in Fig. 1 (Toyoda and Schwarcz, 1997a) where the E1 ' center signal increased up to 3008C and then decease until the signal disappears at 5008C.
Fig. 1. The change of the signal intensities observed at the position of the E10 center (after Toyoda and Schwarcz, 1997a). The natural granitic quartz sample showed the increase in the intensity up to 3008C then decrease above the temperature. On the other hand, the irradiated quartz sample showed decrease up to 1708C before increase because the thermally unstable counterfeit E10 signal is generated by the gamma ray irradiation and overlapped the ``real '' E10 , making it to look as if the signal increases on irradiation. That counterfeit signal decays up to 1708C.
2.2. Factors that limit the intensity of the E1 ' center: Experiment 1 If the process proposed by Jani et al. (1983) is correct, the intensity of the E1 ' center should be a function of both the amount of hole centers (to transform the diamagnetic oxygen vacancies to the E1 ' center) and the amount of its precursor, oxygen vacancies (that limits the amount of the E1 ' center). The following experiment was performed in order to con®rm the idea. Quartz grains were extracted from Mannari granite, Okayama, Japan. After crushing a block gently, the sample was soaked in 6 N HCl for one night, then, magnetic minerals were removed by a portable magnetic separator. The grains were then soaked in sodium polytungstate in order to remove minerals lighter than quartz. Finally, the grains were treated with 20% HF for two hours to dissolve contaminant feldspar. The extracted quartz grains were then gently crushed into 75 to 250 mm. Nine aliquots of 100 mg were prepared after heating the sample at 4308C for 30 min to anneal the E1 ' center signal. The oxygen vacancies are not annealed at this temperature because the signal of the E1 ' center is regenerated after the following experimental procedure. The sample aliquots were irradiated by 60 Co gamma ray to the doses ranging from 0 to 880 Gy with a dose rate of 44 Gy/h. After leaving for three days, ESR signal intensity of the Al center was measured for each aliquot. The intensities of the regenerated E1 ' center were measured after the samples were subsequently heated at 3008C for 15 min. The ESR intensity of the Al center was measured at
Fig. 2. The dose dependence of the regenerated signal intensity of the E10 center (the heat treated E10 intensity, see text for the experimental procedure). The E10 center intensity saturates above 200 Gy while no saturation occur for the Al center, indicating that the E10 center intensity saturates due to the limit of the amount of the precursors (oxygen vacancies). This level denotes the amount oxygen vacnacies in quartz relatively.
S. Toyoda, W. Hattori / Applied Radiation and Isotopes 52 (2000) 1351±1356
liquid nitrogen temperature with an ESR spectrometer, JEOL RE1-X. The sample in a quartz tube was dipped in a ®nger dewar inserted into the microwave cavity. The microwave power was 5 mW with a ®eld modulation amplitude of 0.1 mT. The width of the magnetic ®eld was 10 mT taking 8 min to scan with a time constant of 0.3 s. The signal of the E1 ' center was observed with a microwave power of 0.01 mW at room temperature. The other conditions were the same as above except for the scan range of 5 mT. The intensity of Al center was taken as the peak height from the top of the ®rst peak to the bottom of the 16th peak of the main hyper®ne structure (Yokoyama et al., 1985). The results are shown in Fig. 2 as a function of gamma ray dose. The intensity of the Al center (before heating) shows monotonic increase at least up to 1 kGy. On the other hand, the intensity of the regenerated E1 ' center increases up to 200 Gy and shows a ¯at response, being consistent with the previous results obtained by Toyoda and Ikeya (1991b) where they neither measured Al center nor discussed the reason well enough for that response. The results are interpreted as below following the idea proposed by Jani et al. (1983). When the gamma ray dose is smaller than 200 Gy, the amount of the hole centers such as Al center which can release holes on heating would be small. Therefore, the amount of the regenerated E1 ' center is smaller, depending on the dose, although there would still be diamagnetic oxygen vacancies that could become the E1 ' center. On the other hand, when the dose is larger than 200 Gy, the intensity of the E1 ' center shows saturation because of the limitation of the amount of diamagnetic oxygen vacancies. The intensity of Al center before heating does not saturate but monotonically increases, therefore, the amount of holes released get larger with dose, but the E1 ' center intensity saturates because all diamagnetic oxygen vacancies would be converted to E1 ' center in this dose range.
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radiation or when it is a natural sample. It might be because the electrons of electron-hole pairs created at the time of irradiation were trapped by the E1 ' centers to become diamagnetic oxygen vacancies where there would have been much more paramagnetic oxygen vacancies (the E1 ' center) than diamagnetic oxygen vacancies in those heated or natural samples. If this discussion is correct, the intensity of the E1 ' center in natural quartz would strongly depend on the environment, temperature and dose rate. It might reach a steady state with two eects, heat (temperature) which enhances the intensity and radiation which would rather decrease the intensity. The characteristics of formation of the E1 ' center revealed here is completely dierent from other ESR signals which have been used for dating. Therefore, the authors do not recommend using the E1 ' center signal for dating as long as the conventional dating proceedure is applied. The formation and decay characteristics of the E1 ' center should be precisely investigated at least for the speci®c sample to be dated before using this signal for dating.
2.3. Processes that aect the intensity of the E1 ' center The above experimental results indicate that formation of the E1 ' center should involve at least two processes, formation of the precursors (oxygen vacancies) and the electronic process that makes the precursors paramagnetic. The former process in nature will be discussed later in this paper. The latter electronic process can also go on in nature, which might enhance the intensity and enable us to observe the E1 ' center in natural quartz. As shown by Sato et al. (1985); Ozeki (1990), and Fig. 2 of Toyoda and Schwarcz (1997a), the intensity of the E1 ' center decreases with gamma ray irradiation when the intensity has been enhanced by heating before the ir-
Fig. 3. The change with dose of signal shapes observed at the position of the E10 center in granitic quartz. (a) no irradiation, (b) 3 kGy, (c) 6 kGy, (d) 15 kGy. The two peaks signal of the E10 center turns into one peak signal with dose, indicating that another signal (the counterfeit E10 signal.
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3. The counterfeit E1 ' signal The papers on ESR dating of quartz using the E1 ' center reported that the signal was enhanced by gamma ray and that the equivalent doses were obtained by extrapolating the increase with dose. However, Toyoda and Schwarcz (1997a) found that another signal is created by irradiation and overlaps the real E1 ' center. The signal is characterized by the single peak, microwave power dependence similar to the real E1 ' center, and thermal stability; decay below 1708C. When irradiated by gamma rays, the signal shape of the E1 ' center changes as shown in Fig. 3 (Toyoda and Schwarcz, 1997a), gradually appearing a single peak of the counterfeit E1 ' signal as the dose gets larger. On heating the irradiated sample, the signal intensity once decreases up to 1708C, then increase up to 3008C, and ®nally decrease again to the temperature as shown in Fig. 1. The increase of the E1 ' center by gamma ray irradiation in the previous dating works would possibly be due to this counterfeit signal. Porat et al. (1994) pointed out that the ages of ¯ints obtained for the E1 ' center are systematically younger than those for Al center. Their following work (Porat and Schwarcz, 1995) also found that the lifetime of the E1 ' center in ¯ints after gamma ray irradiation is much smaller than that of natural signal. These results on ¯ints might be consistent with the idea that the generation of the counterfeit E1 ' signal made the E1 ' center appear to increase with gamma ray irradiation. Toyoda and Schwarcz (1997b) also found the generation of this counterfeit E1 ' signal in fault gouge from California. Therefore, this is another reason why the conventional dating method should not be applied to the E1 ' center.
method would evaluate the amount vacancies in quartz at least relatively.
of oxygen
4.2. The processes that create the oxygen vacancies Correlation was found between the intensities of the E1 ' center in granites and their ages by Odom and Rink (1989) in the range of 10±1000 Ma. They attributed the correlation to the formation of the oxygen vacancies due to alpha recoil nuclei contained in the quartz matrix in ppb order and pointed out the possibility to date granites with the E1 ' center. Their theoretical estimate of the amount created by this process was not successful (Rink and Odom, 1991) possibly because they did not consider the amount of diamagnetic oxygen vacancies. Being considered the existence of the diamagnetic oxygen vacancies, the correlation they observed should have two interpretations: 1. The amount of oxygen vacancies increases with age as Odom and Rink (1989) suggested. 2. The paramagnetic fraction (the E1 ' center) of oxygen vacancies increases with age. Toyoda (1992) examined the correlation between the ages of granites and the intensities of the heat treated E1 ' center (after irradiation and subsequent heating, relative amount of oxygen vacancies) and obtained a result shown in Fig. 4. This correlation clearly indicates that (1) is the case. In the following work, Toyoda et al. (1996) showed that the intensities are consistent with the heat treated E1 ' intensity created
4. Formation of the oxygen vacancies 4.1. The heat treated E1 ' center as an indicator of relative amount of oxygen vacancies Toyoda and Ikeya (1991b) proposed a method to evaluate the amount of oxygen vacancies in quartz, which is the intensity of the E1 ' center after heating at 3008C for 15 min following gamma ray irradiation to above 200 Gy (the heat treated E1 ' center intensity). As discussed in the above section, the intensity of the heat treated E1 ' center saturates above 200 Gy as results of the Experiment 1 because of the limitation of the amount of precursors, oxygen vacancies. Therefore, this saturation level would denote the amount of oxygen vacancies in quartz, supporting the proposal made by Toyoda and Ikeya (1991b). As long as the same procedure is applied, for a series of samples, this
Fig. 4. The correlation between the ages of granites and the heat treated E10 center intensities (after Toyoda, 1992). It indicates that the amount of oxygen vacancies in quartz increases with age.
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by gamma ray irradiation. They pointed out that the oxygen vacancies in natural quartz might have been created by external (outside of quartz grains) beta and gamma rays. On the other hand, the amount of the oxygen vacancies created by alpha recoil nuclei was merely theoretically estimated, which did not coincide with those in natural quartz (Rink and Odom, 1991). For the purpose of dosimetry, Wieser and Regulla (1989) already observed the response of the heat treated E1 ' center intensity to dose in the range of several MGy without considering the relation between the E1 ' center and oxygen vacancies. 4.3. Factors that control the formation of the oxygen vacancies Temperature during irradiation and dose rate are the factors which could aect the formation of lattice defects and should be examined before applying the signal for practical dating and dosimetry. Wieser and Regulla (1989) ®rst pointed out the temperature dependence with showing the preliminary results. Toyoda et al. (1996) made precise experiments to show the strong temperature dependence for the formation of the oxygen vacancies. The oxygen vacancy formation is ten times larger at 2508C than that at room temperature. They also obtained an activation energy for the formation process. On the other hand, dose rate dependence has not yet been examined.
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4.4. Dose rate dependence of the formation of the oxygen vacancies in quartz: Experiment 2 The dose rate dependence is an important factor for dosimetry as well as for dating because the one used for gamma ray irradiation for calibration is much larger than the actual or natural dose rate. Quartz grains were extracted by the same method as described in the section above from Mannari granite and from a Quaternary volcanic tephra collected in New Mexico, USA Six aliquots were prepared for each sample after heating at 6008C for 1 h in order to erase all oxygen vacancies (Toyoda et al., 1992). Samples were, then, irradiated by gamma rays of 60 Co source to the total irradiation dose, 2.4 106 Gy, with varying dose rates ranging from 3.5 to 20 kGy/h. The ESR intensities of the heat treated E1 ' center were measured with the same conditions as Experiment 1. There is slight dose rate dependence observed as shown in Fig. 5. The dierence in the signal intensity between the largest and smallest exposure rates in the present experiment was 16% for the granitic quartz and 6% for the volcanic quartz. Such a rather small dose rate dependence would be less than the accuracy with which we are in discussion (Fig. 4). The temperature dependence would cause larger ambiguity in obtaining the age from the intensity of the heat treated E1 ' center while rock bodies decrease its temperature. However, that dependence might need to be investigated more precisely for the purpose of accurate dosimetry where the temperature of the dosimeter is well controlled.
5. Conclusion
Fig. 5. The dose rate dependence of the formation of oxygen vacancies in quartz. The total dose of 2.4 106 Gy was given to all samples with varying the dose rate. There are slight dose rate dependence which would give much smaller eect on the formation that the temperature dependence. However, it might be needed to consider for the precise dosimetry where the temperature of the dosimeter is well controlled.
The E1 ' center intensity is closely related with the amount of its precursors, oxygen vacancies as well as the amount of electronic holes to be transferred, as shown by the Experiment 1. The heat treated E1 ' center intensity corresponds to the amount of the oxygen vacancies. The counterfeit E1 ' signal may make the E1 ' center appear to increase on irradiation. Therefore, the conventional dating procedure would possibly mislead wrong ESR ages when applied to the E1 ' center signal. The heat treated E1 ' center would be a new indicator which might be useful for new applications of dating and dosimetry. Dating of granite in the age range of 10±1000 Ma as well as dosimetry of extremely high dose have been proposed. Experiment 2 revealed that the dose rate dependence of the formation of the oxygen vacancies is rather small and would give much smaller ambiguity than temperature dependence in the practical application but might be a factor to be con-
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sidered as a possible source of error in precise dosimetry.
Acknowledgements The present study was supported in part by InterUniversity Joint Research program using the JAERI facilities, and by Grants-in-AID for Encouragement of Young Scientists (No. 06854026).
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