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Geophys. J. R . ash. Soc. (1975) 41, 107-113. Thermal Quenching of Thermoluminescence in Quartz A. G . Wintle (Received 1974 November 12)* Summary Studies of the stability and kinetics of the 325 "C thermoluminescence peak in quartz are described which show the occurrence of thermal quenching, the decrease in luminescence efficiency with rise in temperature. This phenomenon causes the initial rise ' method of trap depth determination to give spuriously low results which erroneously implies thermal instability of a peak which is suitable for dating. Its presence is confirmed by radioluminescence studies and is shown not to preclude the use of other methods of trap depth determination. 1. Introduction Quartz is one of the most widely studied naturally occurring thermoluminescent (TL) minerals. Quartz grains are present in nearly all types of pottery and a TLdating method has been developed using them. One of the basic assumptions in TL dating is that there are traps in the material which are capable of retaining electrons over archaeological time. Quartz has two traps which give rise to TL peaks between 300 and 400 "C, the region of the glow curve usually used for archaeological dating. In this paper I shall discuss the properties of the lower peak which occurs at 325 "C when a heating rate of 20"C/second is used, Fig. 1. The sample studied is from a fragment of Romano-British pottery which was included in an earlier quartz inclusion dating test programme (Fleming 1970). Fleming showed that the naturally acquired TL dose deduced for the 325 "C peak was within 23 per cent of the values obtained at higher glow curve temperatures, implying that negligible emptying of the traps giving rise to the 325°C peak has taken place during archaeological time, 1900 years. On the other hand a determination by the initial rise method of the trap depth and associated frequency factor (Aitken & Fleming 1972) yielded a mean life for electrons in these traps of only 3000 years. In this paper I describe my investigations of the stability and related kinetics of the 325 "C peak in quartz and show that the above discrepancy can be attributed to the occurrence of thermal quenching which causes trap depth determinations by the initial rise method to be erroneous. I have commented on similar discrepancies in connection with TL phosphors (Wintle 1974). 2. Kinetic discussion The thermoluminescence characteristics of geological materials are extremely complex since they are affected by all the impurities and other lattice defects. However, if the glow curve contains a well-defined peak, it is often possible to apply simple * Received in original form 1974 August 12. 107 108 A. G. Wintle c Temperature /"C FIG.1. Quartz high temperature glow curve. kinetic theory and obtain useful results (Braunlich 1968). The method most widely used in the determination of the trap depth, E, is the initial rise method (Garlick & Gibson 1948); it is independent of the kinetics of the untrapping process and can be used where peaks are overlapping. The log of the light intensity, I , is plotted against T-' for the initial portion of the rising part of the peak for which I ( t ) = Cf(n) exp ( - E / k T ) (1) wheref(n) represents the type of kinetics governing the remainder of the glow curve and C is a constant related to the luminescence efficiency. For first order kinetics the above equation can be rewritten I = -C- dn dt = Cns exp [ - E / k T ] where s is a frequency factor. Hence the number of trapped electrons decays exponentially, their mean life being 1 z =S exp [E/kT] (3) and hence E can be found from isothermal decay measurements. Differentiation of equation (2) with respect to T leads to the following relation between peak temperature T, and heating rate /l E S E (4) which is the basis of another method of trap depth determination (Hoogenstraaten 1958). 3. Kinetic studies The TL measurements were made using the equipment described by Aitken & Fleming. Except where otherwise stated an EM1 9635 photomultiplier tube was used Thermal quenching of thermoluminescencein quartz 109 in conjunction with a Corning (7-51) ultraviolet filter and a Chance-Pilkington heat reflecting (HA-3) filter. Artificial irradiations were given using a 40mCi 9oSr/90Y beta source. That the 325 "C peak obeyed first order kinetics was shown by the linearity of a plot, which extended beyond the linear initial rise region right up logZ/n versus T to the peak. Subsequently the three methods of trap depth determination described above were applied giving the following results: -' (a) isothermal decay method--E = 1.7 & 0.1 eV, (b) Hoogenstraaten's method-E = 1.69+ 0.02 eV, (c) initial rise method-E = 1-05k0.03 eV. These values of E predict a mean life at 20 "C of - 3 x lo7 yr for (a) and (b) but only 200 years for (c); this low value is in disagreement with the known stability of the peak over archaeological time. The plot of logI/n versus 3"-' had the same slope as the initial rise plot of log I versus T-'. This suggests that the kinetics are first order but the intensity observed is not solely dependent on them. This is to be expected as TL is due to two processes, trap emptying and subsequent recombination at luminescence centres, suggesting that the efficiency of radiative recombination is not the simple constant C as assumed in equation (I). 3( 2c 1, 10 0 FIG.2. (a) , Z versus f3/Tm2,n constant; (b) I, exp f-,9/kTm)versus j3/Tmz. 110 A. C . Wintle A plot of log1 versus (kT)-' having a slope of 1.05eV for a trap depth E of 1.69 eV can be explained by assuming a luminescent efficiency of the form q = K 1exp ( W / k T ) (5) with W = 1.69-1.05 = 0.64eV. This would not affect the isothermal decay measurements, and it can be shown theoretically that the peak shift method would still give a trap depth of E = 1.69 eV for the range of heating rates used. Such a form for the temperature dependence of the luminescence efficiency can be checked by observing the variation of I , (the intensity at the peak), with T,. For a constant Fig. 2(a), should be linear; for an efficiency obeying efficiencya plot of I,,, versus /?/Tm2, equation (5) a plot of I , exp (- W / k T , )versus /I/Tm2should be. It will be seen that the former plot is far from linear, whereas the latter is a straight line passing through the origin, supporting equation (5). 4. Thermal quenching The decrease of luminescence efficiency with temperature increase due to the increased probability of non-radiative transitions is known as thermal quenching (Curie 1963). If there is only one type of luminescence centre then the luminescence efficiency may be written 1 (6) q ( T ) = l+Kexp(-W/kT) where W is an energy depth characterising the non-radiative process and K is a dimensionless constant. Studies of thermal quenching in six TL dosimetry phosphors (Gorbics, Nash & Attix 1963) aimed at measuring the temperature dependence of their efficiency by observing X-ray excited radioluminescence (RL) failed because of discontinuities at the TL peak temperatures. These workers did however observe variation of glow peak height with heating rate in some phosphors, e.g. CaF,:Mn, that could be explained by thermal quenching. If we compare equation (5) which is arrived at experimentally, with equation (6) which is derived theoretically, we see that when K exp (- W / k T ) is large, q ( T ) converges to K-' exp ( W / k T ) . This suggests that one ought, by making measurements at low enough temperatures, to be able to see a breakdown in the simple exponential approach of equation (5). This could be observed in two ways; firstly by studying the radioluminescence, provided one is observing only the centres that are being thermally quenched, and secondly by considering determinations of E for lower temperature glow peaks by the methods already used for the 325°C peak provided that they also obey first order kinetics and that the same luminescence centres are being used. 5. Radioluminescence The prompt luminescence (RL) observed during irradiation with a 40 millicurie 90Sr/90Ybeta source (dose rate = 10 rad/minute) was observed using an EM1 6256 photomultiplier in conjunction with several different broad band interference filters in a modified filter spectrometer (Fleming 1968). A typical RL curve is shown in Fig. 3. (a) is the curve as observed using a filter centred on 465nm; above 275°C the RL intensity is constant; (b) is obtained by subtracting this constant value which is presumed to be due to different luminescent centres which are not affected by thermal quenching. Fig. 4 shows a theoretical curve for ' 1 = 1 K exp (- WVjkT) + Thermal quenching of thermolum'nescence in quartz T/ "C Fro. 3. Radioluminescence (a) as observed; (b) after subtraction of steady emission. T-' x IOVK-I Fro. 4. Theoretical plot of In 1) versus T- for K = 2 . 8 x lo7 and W = 0.64 eV; the crosses are experimental points for RL (456 nm). 111 112 A. G. Wintle where W = 0.64 eV, obtained from the difference between the initial rise determination of E and that by the other methods, and K is chosen such that K exp (- W / k T ) = 1 where T is the temperature at which the RL curve reaches half its initial intensity and hence K = 2.8 x 10'. The experimental points are those obtained by replotting curve (b) from Fig. 3 after normalizing with respect to the initial intensity. Similar thermal quenching was seen using filters centred at 410 nm, 370 nm and 310 nm but only very slight quenching was observed for TL at 495 nm. This suggests that certain luminescence centres are free of thermal quenching and that it might be possible to use these for the initial rise determinations by using more selective optical filters. Initial rise measurements were subsequently carried out using an EM1 9558 photomultiplier in conjunction with a Corning (4-96) blue-green filter and a Corning (3-70) sharp cut filter. A value of E = 1.25 eV was measured for the 325 "C peak; this is greater than the 1.05 eV initially obtained but is still well below 1.69 eV which was obtained by the methods based on first order kinetics. 6. Kinetic studies of 110 "C and 230 "C TL Similar experiments were performed on the 110 "C and 230 "C peak. The trap depth of the 230°C peak as obtained from the slope of log Tm2/Pversus T,-' is E = 1.79+0*03eV, but the initial rise method gave E = 1.10k0.05 eV. A plot of I , versus P/Tm2was found to be non-linear (as in Fig. 2(a)) for the 325 "C peak) but linear when I , exp (- W / k T )was plotted versus fi/Tm2(as in Fig. 2(b)). In the case of the 110 "C peak, E = 0.99 &Om02eV was obtained for initial rise, for isothermal decay and for variations of log Tm2/Pwith T,-' and furthermore I , plotted against P/Tm2was linear. These observations point to this peak being somewhat below the onset of thermal quenching. 7. Conclusion From the study carried out, it can be seen that the initial rise method of trap determination cannot be applied without considering whether thermal quenching could occur. In the preliminary study of a mineral to be used for geological dating by thermoluminescence the presence of thermal quenching would give too low a value for the trap depths as measured by the initial rise method and hence it could be thought unsuitable for dating because of its apparent thermal instability. Thermal quenching can be detected by RL studies and may also be overcome by other methods of trap depth determination if the peak obeys first order kinetics. Research Laboratory for Archaeology, 6 Keble Road, Oxford OX1 3QJ. References Aitken, M. J. & Fleming, S. J., 1972. Thermoluminescence dosimetry in archaeological dating, Topics in radiation dosimetry Supplement 1, 1, ed. F. H. Attix. Academic Press, London. Braunlich, P., 1968. Thermoluminescence and thermally stimulated current-tools for the determination of trapping parameters, Thermoluminescence of geological materials, 241, ed. D. J. McDougall, Academic Press, London. Curie, D., 1963. Luminescence in crystals, Wiley, New York. Thermal quenching of thermoluminescencein quartz 113 Fleming, S . J., 1968. The colour of spurious thermoluminescence in dosimetry phosphors, Proc. Int. Con$ Lumin. Dosim., 2nd, 266, Gatlinburg, Tennessee. Fleming, S . J., 1970. Thermoluminescent dating; refining of the quartz inclusion method, Archaeometry, 12, 133. Garlick, G. F. J. & Gibson, A. F., 1948. The electron trap mechanism of luminescence in sulphide and silicate phosphors, Proc. Phys. SOC.,60, 514. Gorbics, S . G., Nash, A. E. & Attix, F. H., 1969. Thermal quenching of luminescence in six thermoluminescent dosimetry phosphors, I, Znt. J. appl. Radiat. Isotopes, 20, 829. Hoogenstraaten, W., 1958. Electron traps in zinc sulphide phosphors, Philips Res. Rep., 13, 515. Wintle, A. G., 1974. Comment on a letter by Ziniker, Rusin and Stoebe, J. mater. Sci., in press. H

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