Dose Response of the E₁’ Centre in Quartz

The spectrum of the E₁’ centre changed with irradiation, as shown in Fig. 1a. The anisotropic principal g factors were reported as 2.00179, 2.00053, and 2.00030 (Jani et al., 1983). As the latter two g factors are so close, the spectrum shows that of the axial symmetry when the modulation amplitude is 0.1 mT, having two peaks as shown at the top in Fig. 1a before irradiation. The peak at the lower magnetic field became ambiguous with irradiation, so that it almost vanished with 9.5 kGy in Fig. 1a. The change of the shape of the spectra is consistent with previous work by Toyoda and Schwarcz (1997a); however, the overall signal intensity decreased with irradiation, which was different. After post heating at 170°C, all peaks at the lower magnetic field are clearer in Fig. 1b, indicating that the contribution from the counterfeit signal was removed, but the contribution in the present particular sample would probably be much smaller than the previous work (Toyoda and Schwarcz, 1997a).

Fig. 1

The results of stepwise heating experiments are shown in Fig. 2. The natural aliquot shows the increase of the signal intensity of the E₁’ centre on the heating before decreasing above 300°C as already reported (e.g. Toyoda and Ikeya, 1991). The E₁’ centre signal intensity of the aliquot preheated at 400°C without irradiation did not change up to 380°C. However, with irradiation, the intensity increased on heating and at 300°C to the same level as natural aliquot. This is because the electronic holes created by radiation are transferred to oxygen vacancies again to create the E₁’ centre where the number of the precursor oxygen vacancies is the same as natural aliquot. When preheating temperature was 520°C with irradiation, the shape of the increase and decrease on stepwise heating is the same as the one with preheating at 400°C, but the signal intensity was smaller. This can be explained by the decay of the oxygen vacancies on preheating. The number of precursors decreased on preheating at 520°C as the decay was previously reported by Toyoda and Ikeya (1991); therefore, the maximum intensity around 300°C on stepwise heating is smaller.

Fig. 2

It would be noted in Fig. 2 that irradiated aliquots show a small dip of the signal intensity at 170°C. This is due to the decay of the counterfeit E₁’ centre, where the signal shape of these aliquots indicates that contribution, but above the temperature, the signal shape was the usual E₁’ centre with two peaks.

The dose response of the E₁’ centre signal intensity of aliquots without preheating is shown in Fig. 3. The signal intensities of the E₁’ centre decreased with the dose. The intensities did not change on post-heating at 170°C, indicating no contribution from the counterfeit signal in this dose range. After heating at 300°C, the E₁’ centre signal intensity slightly increased with dose and stayed at the same level at a higher dose range. This would be due to a slight increase in the number of transferring holes in the lower dose range and to the limitation of the number of oxygen vacancies, which does not change with irradiation in this dose range.

Fig. 3

The dose responses of the E₁’ centre after post-heating at 170°C are shown in Fig. 4, where the temperatures indicated in the figure are those of preheating. As heating at 170°C removes the contribution of the counterfeit signal, these are the dose responses of the “real” E₁’ centre. It is interesting that the signal intensities in aliquots without irradiation and with preheating at 300°C decrease monotonically with dose while those heated at 400°C once increase up to 100 Gy and then decrease. The one heated at 520°C shows also increase, then saturated, but the level is much lower than those post-heated at 300°C (Fig. 2). The dose response of the signal in the aliquots preheated at 250°C was between natural (without heating) and 300°C.

Fig. 4

The counterfeit signal was not formed in natural aliquots as shown in Fig. 2, however, the contribution was clearly seen in the aliquots preheated at 400°C as shown in Fig. 5. The signal intensity systematically decreased by post-heating at 170°C due to the decay of the counterfeit signal. The gamma ray irradiation did not create the counterfeit signal in the natural aliquots of the present particular sample but did in preheated aliquots.

Fig. 5

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 Fig. 2, a dip at 170°C in the stepwise heating experiment corresponds to the decay of the counterfeit E₁’ centre signal for the sample preheated at 400°C. A similar dip was also observed for that preheated at 520°C, with a smaller amount. As already described in Toyoda and Schwarcz (1997a), the counterfeit E₁’ centre signal may be related to the number of the oxygen vacancies, which would support the suggestion by Mashkovtsev and Pan (2018) that this signal corresponds to the newly found E₉’ or E₁₃’ centre in quartz. The condition for the formation of the counterfeit E₁’ centre signal is an issue further to be investigated.

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 et al. (1983) proposed that holes trapped at the Al hole centre are transferred on heating to diamagnetic neutral oxygen vacancies with two electrons (Si-Si) bond to recombine one of the electrons to form the centre. In the present study, it was found that irradiation reduces the signal intensity of the E₁’ centre when the sample has been preheated at 300°C or at a lower temperature, while the intensity once increased and then decrease when the pre-heating has been 300°C or higher. These phenomena could be explained if irradiation supplies electrons while heating supplies electronic holes to oxygen vacancies, where the latter was already proposed by Jani et al. (1983).

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 Fig. 2.

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 Fig. 4. When the sample has been preheated at 400°C or at a higher temperature, the samples would have only oxygen vacancies without any electrons according to the above discussion, and by supplying electrons by irradiation of smaller dose, the oxygen vacancies will trap one electron to become the E₁’ centre, which explains the increases of the intensity up to 50 or 100 Gy in Fig. 4. With higher doses, electrons are further supplied so that the E₁’ centre will trap another electron to become a neutral oxygen vacancy, ESR insensitive, which cause the decrease observed at the higher dose range.

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.