The E1’ centre, an unpaired electron at an oxygen vacancy (Feigl
It was already known that the intensity of the E1’ centre increased on heating when the signal was first found (Weeks and Nelson, 1960). In one of the later works, Jani
These previous studies focusing on the formation of the oxygen vacancies and on the transferring electronic process, however, did not really answer the question of how a natural quartz sample has a specific signal intensity of the E1’ centre. In the natural environment, the sample receives the natural radiation, but at the same time, the trapped electron and holes created by the natural radiation would be transferred, in the geological time scale, by thermal activation processes. The previous works have investigated the latter thermal activation process with heating experiments in the laboratory time scale; however, the effect of radiation on the E1’ centre was not systematically investigated.
One caution is needed on the “counterfeit” E1’ centre formed by gamma ray irradiation. Toyoda and Schwarcz (1997a) found that a signal very similar to the E1’ centre is formed by gamma ray irradiation. The signal has the g factor (g=2.001) same as the E1’ centre and similar microwave power dependence; therefore is easily misidentified as the E1’ centre. The signal can be distinguished by its isotropic signal shape (one peak signal) (the “real” E1’ centre signal has two peaks due to almost axial symmetry) and lower thermal stability (erased by heating at 170°C for 15 minutes). As this counterfeit signal is formed by gamma ray irradiation, one can observe the increase of the “E1’ centre” by gamma ray irradiation to be erroneously used for ESR dating. In the present paper, the dose response of the E1’ centre is examined after removing the contribution of the counterfeit signal.
Quartz grains of 0.25 to 1 mm were extracted from Mannari Granite, Okayama, Japan. After crushing the grains to 0.25 to 0.5 mm, they were separated into six portions. Five of them were heated at 250, 300, 400, 520, and 600°C (preheating), leaving one without heating. Aliquots of about 100 mg each were prepared of each portion to be irradiated by the gamma rays up to 9.5 kGy with a dose rate of 100 Gy/h by a 60Co source at Takasaki Advanced Radiation Research Institute of National Institutes for Quantum and Radiological Science and Technology. Stepwise heating experiments were done with the four aliquots (natural, preheating at 400°C without irradiation, preheating at 400°C and subsequent irradiation to 430 Gy, and preheating at 520°C and 430 Gy) up to 480°C with a heating duration of 15 minutes using a muffle furnace. The dose responses of the E1’ centre were examined with the aliquots without heating and subsequent post-heating at 170°C and at 300°C for 15 minutes. Without post-heating, the signal intensity includes the contribution from the counterfeit E1’ centre signal, while heating at 170°C removes that contribution and heating at 300°C makes the intensity maximized due to the transfer of the holes.
ESR measurements were made at room temperature with an ESR spectrometer, PX-2300, JEOL, at the Okayama University of Science with the following conditions, a microwave power of 0.01 mW (Toyoda
The sensitivity variation of the spectrometer was checked with MgO:Mn2+ marker to find the day to day variation was typically within 1.7% in a week. It was assumed that the signal intensity of the E1’ centre has similar variability as aliquots of the almost same amount (about 100 mg) of quartz samples extracted from a granite were measured in the present study.
The spectrum of the E1’ centre changed with irradiation, as shown in
The results of stepwise heating experiments are shown in
It would be noted in
The dose response of the E1’ centre signal intensity of aliquots without preheating is shown in
The dose responses of the E1’ centre after post-heating at 170°C are shown in
The counterfeit signal was not formed in natural aliquots as shown in
Toyoda and Schwarcz (1997a) showed that the counterfeit E1’ centre signal was created by gamma ray irradiation in granitic quartz and their subsequent paper (Toyoda and Schwarcz, 1997b) indicated that the counterfeit E1’ centre signal was observed in quartz extracted from a fault gouge after gamma ray irradiation. The present result would indicate that this counterfeit E1’ centre signal is not always created by gamma ray irradiation in quartz but depends on the pre-treatment of the samples. In
Both, irradiation with ionizing radiation and heating, induce electronic processes in the minerals. Ionizing radiation creates pairs of electrons and holes, a part of which are trapped by lattice defects or by impurities. Those trapped electrons and holes are measured by ESR or by luminescence to deduce the accumulated radiation doses and hence the ages. Heating induces the activation of those trapped electrons and holes to recombine through thermal activation processes. In the geological time scale, they limit the dating range while they are experimentally used to erase the unstable electrons and holes. As for the formation of the E1’ centre in quartz, Jani
On heating at 300°C or at a lower temperature, holes are supplied to diamagnetic oxygen vacancies with two electrons to recombine one of the electrons to form the E1’ centre. If holes are still supplied to the E1’ centre at a higher temperature, they will possibly also recombine with the electron at the E1’ centre to form oxygen vacancies without electrons, which are ESR insensitive. This process would explain the decrease in the E1’ centre on heating at a temperature higher than 300°C in
By irradiation, electrons are supplied to the E1’ centre in the sample preheated at 300°C or at a lower temperature to make the E1’ centre the oxygen vacancy with two electrons, which are ESR insensitive, explaining the decrease in the dose responses in
Quartz is one of the most abundant minerals on the surface of the Earth. The grains stay in the stratigraphic layers or in the rock for a geological time scale. During the time, quartz grains receive natural radiation and at the same time, thermal activation processes occur in the minerals even at the environmental temperature. The effect of the thermal activation processes in the geological time scale would be equivalent to that in the laboratory time scale occurring at a higher temperature. As these two processes, natural radiation and thermal activation process, may induce the reactions to the opposite directions as shown in the present study. The natural signal intensity of the E1’ centre in quartz would be the result of the balance of these processes. It would imply that the natural environment might be possibly estimated by the natural signal intensity of the E1’ centre. Further studies would be necessary to examine quartz being at various temperatures in the natural environment, such as in drilled cores and that with various natural dose rates.
The dose response to gamma ray dose of the E1’ centre in quartz of granite was examined after removing the possible interference of the counterfeit E1’ centre signal by heating at 170°C for 15 minutes. It was found that the dose response depends on the pre-treatment of the sample. When the sample has been preheated at 300°C or at a lower temperature, the signal intensity decreases with dose while it once increases with dose up to 50 or 100 Gy then decrease above the dose when the sample was pre-heated at 400°C. These phenomena can be explained by the electronic processes that heating supplies electronic holes to the oxygen vacancies and that gamma ray irradiation supplies electrons. The natural intensity of the E1’ centre in quartz would probably be in the balance of these two processes.
The counterfeit E1’ centre signal was not created in the present particular sample without preheating but the one preheated. The condition for the formation of the counterfeit E1’ centre signal is an issue further to be investigated.