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Characteristics of time-resolved luminescence in quartz
Radiation Measurements 32 (2000) 401±405
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Characteristics of time-resolved luminescence in quartz I.K. Baili* Luminescence Dating Laboratory, Department of Archaeology, Environmental Research Centre, University of Durham, Woodside Building, South Road, Durham DH1 3LE, UK Received 30 October 1999; received in revised form 6 March 2000; accepted 18 May 2000
Abstract Time-resolved luminescence measurements were performed with samples of synthetic quartz (Sawyer Premium Q) and granular quartz extracted from ceramics and sediment samples under pulsed (05 ns) laser stimulation (OPO). The luminescence was detected in the UV region using colour glass ®lters (FWHM 280±380 nm). The time-resolved spectrum is dominated by a single exponential decay that remains substantially unaltered when the stimulation wavelength is changed from 600 to 450 nm indicating that the same recombination process is being observed. The lifetime measured at room temperature was 4020:6 ms for the synthetic quartz; at elevated temperatures the measured lifetime is reduced in a manner that is consistent with a competitive non-radiative recombination process (thermal quenching). An average lifetime of 3320:3 ms was obtained for seven samples of granular quartz, indicating a common recombination process in these natural samples that diers from the value for synthetic quartz. 7 2000 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Previous measurements of transient time-resolved luminescence (TRL) with various feldspar samples (Sanderson and Clark, 1994; Clark et al., 1997; Clark and Baili, 1998) have shown that the observed recombination luminescence (UV-red) is characterised by very fast processes, being generally less than several hundred nanoseconds. In combination with wavelength-resolved measurements to identify spectral emission bands, TRL spectra have the potential to provide information concerning the dynamics of radiative recombination processes associated with speci®c bands and ultimately the nature of the defect where recombination occurs. In this paper, we discuss the measurement of TRL in samples of a synthetic quartz, and
* Fax: +44-191-374-3741. E-mail address: ian.baili@durham.ac.uk (I.K. Baili).
granular quartz samples of extracted from dating samples.
sedimentary
origin
2. Samples The samples used in this study comprised a synthetic single crystal quartz (Sawyer Premium Q) and granular quartz extracted from a selection of ceramics and sedimentary deposits. The samples of synthetic quartz were polished z-cut plates of 0.5 mm thickness and of areas ranging from 25 to 100 mm2 (for further details see Petrov and Baili, 1997). The plates were annealed in air at 12008C for 24 h before use. The granular samples were extracted from 10 ceramic and sediment samples extracted from: (i) a water-lain sand deposit from a Mesolithic site, (ii) pottery from a settlement site of the ®rst millennium BC, (iii) a Roman brick, (iv) bricks from a medieval building, and (v) a modern brick. In each case, the samples were taken from the
1350-4487/00/$ - see front matter 7 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 2 6 - 8
402
I.K. Baili / Radiation Measurements 32 (2000) 401±405
material prepared for dose evaluation measurements, where standard preparation procedures for the quartz inclusion technique had been applied. Unless stated otherwise they comprised 90±150 mm sieved material that had been etched in 40% HF for 45 min, followed by immersion in HCl to remove precipitates. Samples were also density separated using a solution of sodium polytungstate if measurable IRSL was detected in sub samples of the prepared material. 3. Measurements Measurements of time-resolved luminescence were performed with a pulsed and tunable laser stimulation source (Quanta Ray MOPO type 710) that has been described previously (Clark et al., 1997). The laser produces pulses of 5 ns width, of energy ranging from 10 to 50 mJ and at a repetition rate of 10 Hz. The transient TRL was stimulated using radiation of wavelengths ranging from 450 nm (2.75 eV) to 600 nm (2.07 eV). The intensity of the laser beam was reduced by at least two orders of magnitude when observing transient TRL by placing in the beam path colour glass and neutral density ®lters to reduce the beam intensity and to remove residual pump beam radiation. After passing through a circular aperture of 04 mm diameter, the beam was directed, via two quartz prisms, onto the sample which was held at an angle of 458, rotated about the vertical axis such that the scattered radiation was directed away from the ®rst aperture of the detection system. Typically the incident energy on the sample was 20 mJ per pulse or less. The luminescence was detected by a fast linear focused photomultiplier (PMT; EMI 9831QB) con®gured for single proton counting. The optical ®lters placed in the detection system comprised two Russian type YFC2 UV-transmitting glass ®lters (80% peak transmission, FWHM 0280±380 nm; 400±650 nm suppression > 1010; 1% peak transmission; FWHM 680± 780 nm). The pulse output of the PMT was fed, via a fast charge-coupled ampli®er to a multi-channel scalar (MCS) with a bandwidth of 200 MHz (FAST ComTec 7885) operated under PC control. MCS dwell times used in these experiments were typically 20 ns, the length used depending on the characteristics of TRL; each sweep of the MCS is started by an advanced Qswitch signal from the laser. The power of the incident laser beam was adjusted such that the count-rate in the peak channel was R1 cps. With this experimental con®guration, the measured TRL represents what is essentially a cumulative recombination spectrum in the time domain for the duration of the acquisition. Absorbed doses of ionising radiation were administered to samples with a 90Sr/90Y beta source calibrated for aliquots of grains. The doses to the plates, ranging
between 5 and 50 Gy depending on the sensitivity of the sample, have an estimated uncertainty of 215%. When samples were beta-irradiated in the laboratory, thermal annealing was performed before measurement of TRL by preheating the sample at 2008C for 10 s unless speci®ed otherwise. Curve ®tting to the TRL data was performed using non-linear regression analysis (employing a Marquardt±Levenberg algorithm); the best ®ts to the data were obtained with a combination of exponentials given by an equation of the general form: I
X ai
exp ÿ t=ti k
1
i
where, I is the intensity, ai is the pre-exponential scaling factor, ti is the lifetime of the ith exponential component and k is a constant. The errors associated with the lifetime evaluations only re¯ect the uncertainty in curve ®tting. 4. Results and discussion 4.1. Background spectrum A background spectrum not related to trapped charge population in the sample with emission was detected. Further investigation indicated that the emission is stimulated by scattered laser light within the detection system colour glass ®lters; and thus, is partly sample dependent. Although the speci®c cause could
Fig. 1. Time-resolved background spectrum recorded with a stainless steel disc. Stimulated at 470 nm, incident energy 020 mJ/pulse; spectrum accumulated for 100 s.
I.K. Baili / Radiation Measurements 32 (2000) 401±405
not be identi®ed and eliminated, the spectra (see Fig. 1) were similar in form with a dominant component (98%) of lifetime 01 ms and a minor component (02%) of lifetime 5±10 ms. The TRL spectra that are discussed below for dosed quartz were analysed without subtraction of the background spectrum since the multiple exponential curve ®tting procedure yielded both `background' and `signal' lifetime components. Although the background is more pronounced in samples where no additional laboratory dose was administered (as for sample 232-1 discussed below where the palaeodose was only 5 Gy), the luminescence associated with an 1-ms lifetime component falls to less than 1% within 5 ms. 4.2. Synthetic quartz The TRL spectrum obtained at ambient temperatures with the Sawyer quartz is shown in Fig. 2. The spectrum, measured with 470 nm stimulation, is dominated by a single decay of lifetime 4020:6 ms (s.d. 8) at room temperature. For the data shown in Fig. 2, measurements using a shorter dwell time indicated that any faster decay components, if present, contribute only a minor part of the emission detected. The possibility that weak luminescence from quartz with lifetime components similar in value to those associated with the background emission cannot be ruled out but it has been assumed that in these experiments it makes only a minor contribution to the measured signal. Comparison of TRL spectra, measured initially and
Fig. 2. Time-resolved spectrum recorded with an aliquot of Sawyer synthetic quartz. Stimulated at 470 nm, incident energy 010 mJ/pulse; spectrum accumulated for 250 s.
403
following 090% reduction of the integrated luminescence, indicated no signi®cant change in the measured lifetime. This is surprising given much of the general discussion of the charge transfer mechanisms associated with OSL (under CW stimulation) implies a considerable amount of charge movement with intermediate trapping in metastable states. The form of the spectra also remained substantially unchanged following dierent preheating treatments (ranging from 1808C for 10 s to 2808C for 10 s), apart from dierences in the luminescence yield and consequently the contribution of the background signal. The TRL spectra measured with the same sample using stimulation wavelengths that were increased at 50 nm intervals in the range of 450±650 nm were similar within experimental error. This indicates that over a wide range of stimulation energies (2.75±1.90 eV) a similar recombination process is being observed at sites where the emission is within the spectral range of the detection window (presumed to be the 360 nm emission band observed in CW stimulation measurements; Huntley et al., 1991). The TRL spectra were also recorded at dierent sample temperatures to examine the eect of thermal quenching. The latter has a marked eect on the radioluminescence (RL) from quartz at temperatures greater than 1008C and this has been measured with
Fig. 3. Evaluated lifetime (ms) for Sawyer synthetic quartz measured using 470 nm stimulation (open circles) and 600 nm (open diamonds) for dierent sample temperatures. The dotted line represents the variation of RL (360 nm) measured as a function of sample temperature for the same type of quartz.
404
I.K. Baili / Radiation Measurements 32 (2000) 401±405
quartz cut from the same bar (Petrov and Baili, 1997). The TRL spectra also show a reduction in integrated intensity with increasing temperature above 1008C which is accompanied by a reduction in the measured lifetime, t, as shown in Fig. 3, with the RL data shown for comparison. Such behaviour is consistent with expectation for competing radiative and nonradiative transitions within a given recombination centre since it can be shown that t Ztrad , where trad is the radiative (recombination) lifetime and Z is the quantum eciency. In the case of quartz, Z is a function of temperature (due to thermal quenching; see Wintle (1975), Wintle and Murray (1999) and also Petrov and Baili (1996)) for the 0360 nm emission band. Consequently, the measured lifetime is expected to reduce with increasing sample temperature in a manner that is similar to that observed with 360 nm RL. 4.3. Granular quartz It was possible to measure TRL spectra without additional laboratory dose with eight of the samples tested. Under 470 nm stimulation, they all showed a dominant singular decay component as measured with the Sawyer synthetic quartz; as shown in Fig. 4, the evaluated mean lifetime is 28 ms. The average of the mean lifetimes measured at RT for eight samples is 2721:5 ms (s.d. 8). The TRL spectra measured following the administration of a beta dose (the equivalent of
Fig. 4. Time-resolved spectrum recorded with an aliquot of quartz extracted from Roman brick. Stimulated at 470 nm, incident energy 020 mJ/pulse; spectrum accumulated for 800 s.
a regenerative measurement) showed little qualitative change in terms of shape; the mean value of the recombination lifetime is 3320:3 ms (s.d. 7). Further investigation of changes in the evaluated lifetime with dose suggests that the lower values of lifetime are probably an artefact of the curve ®tting procedure when the photon statistics are poor, rather than being due to real dierences in the recombination process. Assuming that the dierences in the physical form of the samples (i.e. plates versus grains) will have a negligible eect of lifetime evaluation on a microsecond timescale, the results suggest that there may be a signi®cant dierence between the details of recombination dynamics with synthetic and natural granular quartz. Since they represent extremes of high and low purity quartz this is initially not surprising. However, it points to subtle dierences in the localised environment of the defect where recombination takes place and merits further investigation. In spectral terms the emission band is generally deemed to be singular and common to both synthetic and natural quartz, as discussed in Krbetschek et al. (1997). 5. Summary The measurements performed with a selection of granular natural quartz samples and a sample of high purity synthetic quartz indicate that the transient TRL is dominated by a singular recombination process when detected in the UV for a wide range of stimulation energies (2.75±1.90 eV). The radiative transitions observed have lifetimes at room temperature of 030± 40 ms depending on the type of quartz. Lifetimes of this magnitude are characteristic of a partially forbidden transition between triplet and singlet states, and such information is relevant to distinguishing between the various models proposed for the recombination site associated with 360±380 nm luminescence in quartz. At elevated temperatures the eects of competition from non-radiative recombination were evident and a shortening of the measured lifetime occurs, consistent with expectation. These results are also relevant to practical application in luminescence dating and dosimetry whereas, the recombination lifetimes timescale measured so far with feldspars are on a nanosecond timescale the values for quartz, being about three orders of magnitude slower, allow greater scope for the exploitation of pulse stimulation combined with single photon counting detection systems. Acknowledgements The work described in this paper was ®nancially supported by the University of Durham and the
I.K. Baili / Radiation Measurements 32 (2000) 401±405
NERC (grant GST/02/0756) by provision of the laser system within the LOEPS core programme and as part of the Land±Ocean Interaction Study (LOIS). This paper is LOIS publication 719. The author is grateful to Prof. A.M. Stoneham for advice given and to Dr. S.A. Petrov for preparation of the synthetic quartz samples.
References Clark, R.J., Baili, I.K., 1998. Fast time-resolved luminescence emission spectroscopy in some feldspars. Radiation Measurements 29, 553±560. Clark, R.J., Baili, I.K., Tooley, M.J., 1997. A preliminary study of time-resolved luminescence in some feldspars. Radiation Measurements 27, 211±220. Huntley, D.J., Godfrey-Smith, D.I., Haskell, E.H., 1991.
405
Light induced emission spectra from some quartz and feldspars. Nucl. Tracks Radiat. Meas. 18, 127±131. Krbetschek, M.R., GoÈtze, J., Dietrich, A., Trautman, T., 1997. Spectral information from minerals relevant for luminescence dating. Radiation Measurements 27, 695± 748. Petrov, S.A., Baili, I.K., 1996. Thermal quenching and the Initial Rise technique of trap depth evaluation. J. Luminescence 65, 289±291. Petrov, S.A., Baili, I.K., 1997. Determination of trap depths associated with TL peaks in synthetic quartz (350±550 K). Radiation Measurements 27, 185±191. Sanderson, D.C.W., Clark, R.J., 1994. Pulsed photostimulated luminescence in alkali feldspars. Radiation Measurements 23, 633±639. Wintle, A.G., 1975. Thermal quenching of thermoluminescence in quartz. Geophys. Journal of the Royal Astronomical Society 41, 107±113. Wintle, A.G., Murray, A.S., 1999. Isothermal decay of optically stimulated luminescence. Radiation Measurements 30, 119±125.
www.elsevier.com/locate/radmeas
Characteristics of time-resolved luminescence in quartz I.K. Baili* Luminescence Dating Laboratory, Department of Archaeology, Environmental Research Centre, University of Durham, Woodside Building, South Road, Durham DH1 3LE, UK Received 30 October 1999; received in revised form 6 March 2000; accepted 18 May 2000
Abstract Time-resolved luminescence measurements were performed with samples of synthetic quartz (Sawyer Premium Q) and granular quartz extracted from ceramics and sediment samples under pulsed (05 ns) laser stimulation (OPO). The luminescence was detected in the UV region using colour glass ®lters (FWHM 280±380 nm). The time-resolved spectrum is dominated by a single exponential decay that remains substantially unaltered when the stimulation wavelength is changed from 600 to 450 nm indicating that the same recombination process is being observed. The lifetime measured at room temperature was 4020:6 ms for the synthetic quartz; at elevated temperatures the measured lifetime is reduced in a manner that is consistent with a competitive non-radiative recombination process (thermal quenching). An average lifetime of 3320:3 ms was obtained for seven samples of granular quartz, indicating a common recombination process in these natural samples that diers from the value for synthetic quartz. 7 2000 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Previous measurements of transient time-resolved luminescence (TRL) with various feldspar samples (Sanderson and Clark, 1994; Clark et al., 1997; Clark and Baili, 1998) have shown that the observed recombination luminescence (UV-red) is characterised by very fast processes, being generally less than several hundred nanoseconds. In combination with wavelength-resolved measurements to identify spectral emission bands, TRL spectra have the potential to provide information concerning the dynamics of radiative recombination processes associated with speci®c bands and ultimately the nature of the defect where recombination occurs. In this paper, we discuss the measurement of TRL in samples of a synthetic quartz, and
* Fax: +44-191-374-3741. E-mail address: ian.baili@durham.ac.uk (I.K. Baili).
granular quartz samples of extracted from dating samples.
sedimentary
origin
2. Samples The samples used in this study comprised a synthetic single crystal quartz (Sawyer Premium Q) and granular quartz extracted from a selection of ceramics and sedimentary deposits. The samples of synthetic quartz were polished z-cut plates of 0.5 mm thickness and of areas ranging from 25 to 100 mm2 (for further details see Petrov and Baili, 1997). The plates were annealed in air at 12008C for 24 h before use. The granular samples were extracted from 10 ceramic and sediment samples extracted from: (i) a water-lain sand deposit from a Mesolithic site, (ii) pottery from a settlement site of the ®rst millennium BC, (iii) a Roman brick, (iv) bricks from a medieval building, and (v) a modern brick. In each case, the samples were taken from the
1350-4487/00/$ - see front matter 7 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 2 6 - 8
402
I.K. Baili / Radiation Measurements 32 (2000) 401±405
material prepared for dose evaluation measurements, where standard preparation procedures for the quartz inclusion technique had been applied. Unless stated otherwise they comprised 90±150 mm sieved material that had been etched in 40% HF for 45 min, followed by immersion in HCl to remove precipitates. Samples were also density separated using a solution of sodium polytungstate if measurable IRSL was detected in sub samples of the prepared material. 3. Measurements Measurements of time-resolved luminescence were performed with a pulsed and tunable laser stimulation source (Quanta Ray MOPO type 710) that has been described previously (Clark et al., 1997). The laser produces pulses of 5 ns width, of energy ranging from 10 to 50 mJ and at a repetition rate of 10 Hz. The transient TRL was stimulated using radiation of wavelengths ranging from 450 nm (2.75 eV) to 600 nm (2.07 eV). The intensity of the laser beam was reduced by at least two orders of magnitude when observing transient TRL by placing in the beam path colour glass and neutral density ®lters to reduce the beam intensity and to remove residual pump beam radiation. After passing through a circular aperture of 04 mm diameter, the beam was directed, via two quartz prisms, onto the sample which was held at an angle of 458, rotated about the vertical axis such that the scattered radiation was directed away from the ®rst aperture of the detection system. Typically the incident energy on the sample was 20 mJ per pulse or less. The luminescence was detected by a fast linear focused photomultiplier (PMT; EMI 9831QB) con®gured for single proton counting. The optical ®lters placed in the detection system comprised two Russian type YFC2 UV-transmitting glass ®lters (80% peak transmission, FWHM 0280±380 nm; 400±650 nm suppression > 1010; 1% peak transmission; FWHM 680± 780 nm). The pulse output of the PMT was fed, via a fast charge-coupled ampli®er to a multi-channel scalar (MCS) with a bandwidth of 200 MHz (FAST ComTec 7885) operated under PC control. MCS dwell times used in these experiments were typically 20 ns, the length used depending on the characteristics of TRL; each sweep of the MCS is started by an advanced Qswitch signal from the laser. The power of the incident laser beam was adjusted such that the count-rate in the peak channel was R1 cps. With this experimental con®guration, the measured TRL represents what is essentially a cumulative recombination spectrum in the time domain for the duration of the acquisition. Absorbed doses of ionising radiation were administered to samples with a 90Sr/90Y beta source calibrated for aliquots of grains. The doses to the plates, ranging
between 5 and 50 Gy depending on the sensitivity of the sample, have an estimated uncertainty of 215%. When samples were beta-irradiated in the laboratory, thermal annealing was performed before measurement of TRL by preheating the sample at 2008C for 10 s unless speci®ed otherwise. Curve ®tting to the TRL data was performed using non-linear regression analysis (employing a Marquardt±Levenberg algorithm); the best ®ts to the data were obtained with a combination of exponentials given by an equation of the general form: I
X ai
exp ÿ t=ti k
1
i
where, I is the intensity, ai is the pre-exponential scaling factor, ti is the lifetime of the ith exponential component and k is a constant. The errors associated with the lifetime evaluations only re¯ect the uncertainty in curve ®tting. 4. Results and discussion 4.1. Background spectrum A background spectrum not related to trapped charge population in the sample with emission was detected. Further investigation indicated that the emission is stimulated by scattered laser light within the detection system colour glass ®lters; and thus, is partly sample dependent. Although the speci®c cause could
Fig. 1. Time-resolved background spectrum recorded with a stainless steel disc. Stimulated at 470 nm, incident energy 020 mJ/pulse; spectrum accumulated for 100 s.
I.K. Baili / Radiation Measurements 32 (2000) 401±405
not be identi®ed and eliminated, the spectra (see Fig. 1) were similar in form with a dominant component (98%) of lifetime 01 ms and a minor component (02%) of lifetime 5±10 ms. The TRL spectra that are discussed below for dosed quartz were analysed without subtraction of the background spectrum since the multiple exponential curve ®tting procedure yielded both `background' and `signal' lifetime components. Although the background is more pronounced in samples where no additional laboratory dose was administered (as for sample 232-1 discussed below where the palaeodose was only 5 Gy), the luminescence associated with an 1-ms lifetime component falls to less than 1% within 5 ms. 4.2. Synthetic quartz The TRL spectrum obtained at ambient temperatures with the Sawyer quartz is shown in Fig. 2. The spectrum, measured with 470 nm stimulation, is dominated by a single decay of lifetime 4020:6 ms (s.d. 8) at room temperature. For the data shown in Fig. 2, measurements using a shorter dwell time indicated that any faster decay components, if present, contribute only a minor part of the emission detected. The possibility that weak luminescence from quartz with lifetime components similar in value to those associated with the background emission cannot be ruled out but it has been assumed that in these experiments it makes only a minor contribution to the measured signal. Comparison of TRL spectra, measured initially and
Fig. 2. Time-resolved spectrum recorded with an aliquot of Sawyer synthetic quartz. Stimulated at 470 nm, incident energy 010 mJ/pulse; spectrum accumulated for 250 s.
403
following 090% reduction of the integrated luminescence, indicated no signi®cant change in the measured lifetime. This is surprising given much of the general discussion of the charge transfer mechanisms associated with OSL (under CW stimulation) implies a considerable amount of charge movement with intermediate trapping in metastable states. The form of the spectra also remained substantially unchanged following dierent preheating treatments (ranging from 1808C for 10 s to 2808C for 10 s), apart from dierences in the luminescence yield and consequently the contribution of the background signal. The TRL spectra measured with the same sample using stimulation wavelengths that were increased at 50 nm intervals in the range of 450±650 nm were similar within experimental error. This indicates that over a wide range of stimulation energies (2.75±1.90 eV) a similar recombination process is being observed at sites where the emission is within the spectral range of the detection window (presumed to be the 360 nm emission band observed in CW stimulation measurements; Huntley et al., 1991). The TRL spectra were also recorded at dierent sample temperatures to examine the eect of thermal quenching. The latter has a marked eect on the radioluminescence (RL) from quartz at temperatures greater than 1008C and this has been measured with
Fig. 3. Evaluated lifetime (ms) for Sawyer synthetic quartz measured using 470 nm stimulation (open circles) and 600 nm (open diamonds) for dierent sample temperatures. The dotted line represents the variation of RL (360 nm) measured as a function of sample temperature for the same type of quartz.
404
I.K. Baili / Radiation Measurements 32 (2000) 401±405
quartz cut from the same bar (Petrov and Baili, 1997). The TRL spectra also show a reduction in integrated intensity with increasing temperature above 1008C which is accompanied by a reduction in the measured lifetime, t, as shown in Fig. 3, with the RL data shown for comparison. Such behaviour is consistent with expectation for competing radiative and nonradiative transitions within a given recombination centre since it can be shown that t Ztrad , where trad is the radiative (recombination) lifetime and Z is the quantum eciency. In the case of quartz, Z is a function of temperature (due to thermal quenching; see Wintle (1975), Wintle and Murray (1999) and also Petrov and Baili (1996)) for the 0360 nm emission band. Consequently, the measured lifetime is expected to reduce with increasing sample temperature in a manner that is similar to that observed with 360 nm RL. 4.3. Granular quartz It was possible to measure TRL spectra without additional laboratory dose with eight of the samples tested. Under 470 nm stimulation, they all showed a dominant singular decay component as measured with the Sawyer synthetic quartz; as shown in Fig. 4, the evaluated mean lifetime is 28 ms. The average of the mean lifetimes measured at RT for eight samples is 2721:5 ms (s.d. 8). The TRL spectra measured following the administration of a beta dose (the equivalent of
Fig. 4. Time-resolved spectrum recorded with an aliquot of quartz extracted from Roman brick. Stimulated at 470 nm, incident energy 020 mJ/pulse; spectrum accumulated for 800 s.
a regenerative measurement) showed little qualitative change in terms of shape; the mean value of the recombination lifetime is 3320:3 ms (s.d. 7). Further investigation of changes in the evaluated lifetime with dose suggests that the lower values of lifetime are probably an artefact of the curve ®tting procedure when the photon statistics are poor, rather than being due to real dierences in the recombination process. Assuming that the dierences in the physical form of the samples (i.e. plates versus grains) will have a negligible eect of lifetime evaluation on a microsecond timescale, the results suggest that there may be a signi®cant dierence between the details of recombination dynamics with synthetic and natural granular quartz. Since they represent extremes of high and low purity quartz this is initially not surprising. However, it points to subtle dierences in the localised environment of the defect where recombination takes place and merits further investigation. In spectral terms the emission band is generally deemed to be singular and common to both synthetic and natural quartz, as discussed in Krbetschek et al. (1997). 5. Summary The measurements performed with a selection of granular natural quartz samples and a sample of high purity synthetic quartz indicate that the transient TRL is dominated by a singular recombination process when detected in the UV for a wide range of stimulation energies (2.75±1.90 eV). The radiative transitions observed have lifetimes at room temperature of 030± 40 ms depending on the type of quartz. Lifetimes of this magnitude are characteristic of a partially forbidden transition between triplet and singlet states, and such information is relevant to distinguishing between the various models proposed for the recombination site associated with 360±380 nm luminescence in quartz. At elevated temperatures the eects of competition from non-radiative recombination were evident and a shortening of the measured lifetime occurs, consistent with expectation. These results are also relevant to practical application in luminescence dating and dosimetry whereas, the recombination lifetimes timescale measured so far with feldspars are on a nanosecond timescale the values for quartz, being about three orders of magnitude slower, allow greater scope for the exploitation of pulse stimulation combined with single photon counting detection systems. Acknowledgements The work described in this paper was ®nancially supported by the University of Durham and the
I.K. Baili / Radiation Measurements 32 (2000) 401±405
NERC (grant GST/02/0756) by provision of the laser system within the LOEPS core programme and as part of the Land±Ocean Interaction Study (LOIS). This paper is LOIS publication 719. The author is grateful to Prof. A.M. Stoneham for advice given and to Dr. S.A. Petrov for preparation of the synthetic quartz samples.
References Clark, R.J., Baili, I.K., 1998. Fast time-resolved luminescence emission spectroscopy in some feldspars. Radiation Measurements 29, 553±560. Clark, R.J., Baili, I.K., Tooley, M.J., 1997. A preliminary study of time-resolved luminescence in some feldspars. Radiation Measurements 27, 211±220. Huntley, D.J., Godfrey-Smith, D.I., Haskell, E.H., 1991.
405
Light induced emission spectra from some quartz and feldspars. Nucl. Tracks Radiat. Meas. 18, 127±131. Krbetschek, M.R., GoÈtze, J., Dietrich, A., Trautman, T., 1997. Spectral information from minerals relevant for luminescence dating. Radiation Measurements 27, 695± 748. Petrov, S.A., Baili, I.K., 1996. Thermal quenching and the Initial Rise technique of trap depth evaluation. J. Luminescence 65, 289±291. Petrov, S.A., Baili, I.K., 1997. Determination of trap depths associated with TL peaks in synthetic quartz (350±550 K). Radiation Measurements 27, 185±191. Sanderson, D.C.W., Clark, R.J., 1994. Pulsed photostimulated luminescence in alkali feldspars. Radiation Measurements 23, 633±639. Wintle, A.G., 1975. Thermal quenching of thermoluminescence in quartz. Geophys. Journal of the Royal Astronomical Society 41, 107±113. Wintle, A.G., Murray, A.S., 1999. Isothermal decay of optically stimulated luminescence. Radiation Measurements 30, 119±125.