The stimulated luminescence properties of natural quartz, the most reliable natural phosphor, have been widely used in variety of applications in general and retrospective dosimetry and dating of sediments in particular. Quartz grains, once irradiated, naturally or artificially, release luminescence by stimulation with heat or light in the visible range. The emissions due to thermal and optical stimulations result in thermoluminescence (TL) and optically stimulated luminescence (OSL) respectively. These emissions occur due to recombination of charge released from the traps due to thermal or optical stimulation with the luminescence centre. Quartz has been observed to have a variety of traps, ranging from shallow to deep, which participate in stimulated luminescence process in some way or other (Sankaran
The OSL emission of quartz has been found to possess in general three components (fast, medium and slow), though more than three components have also been reported in literature (Jain
Studies carried out to see the thermal stability of different OSL components of quartz showed the removal of both, the fast and medium component with preheating at higher temperatures, around 400°C (Jain
Thermal treatment, firing, has been reported to have a profound influence on the luminescence properties of quartz (Schilles
The survival of the OSL decaying faster than the slow component, with pre-heating at such high temperatures (> 400°C) was reported recently by Koul
The work reported in this paper tried to investigate the post 500°C blue stimulated OSL emission of fired quartz specimen. This signal was seen to exist in all the samples studied in this work, a local brick sample, Indian dessert sample (Thar), geological Greek sample (Koupa) and Polish sedimentary sample. Subsequently, one of these samples, namely the Greek sample, studied in detail in this work, acted as a representative sample of all the samples which yielded this signal. The deep trap responsible for the post-500°C post-blue OSL has been identified and characterized. The observed signal was found to be dominated by initial medium component. The trap was found to be bleachable, the signal getting bleaching out completely with a bleaching of 100 s with blue light. Pulse annealing and impact of bleaching on glow curve suggested the 510°C TL peak (heating rate 2°C/s) to be responsible for the post-500°C pre-heat OSL. This signal would be referred to as deep trap OSL (DT-OSL), emission arising from a trap deeper than 325°C TL trap, in this paper. DT-OSL simply implies that the charge responsible for emission originates from the deep trap and not the mechanism involved in OSL process. This is necessary to differentiate from the conventional OSL (325°C TL Peak). The dosimetric properties of DT-OSL were found to be good in terms of linearity and upper limit of dose estimation. The signal also satisfied various tests needed for reliable use dose estimation.
The luminescence measurements were carried out on automatic Risø TL/OSL, TL-DA-15 system having a blue light-emitting diodes (λ=470 ± 30nm ) stimulation source with power level set at the 90% of the maximum stimulation intensity for the blue LEDs (40 mW/cm2) (Bøtter-Jensen
The DT-OSL was measured in case of a Greek geological sample, local ceramic sample and Polish sedimentary sample. Thereafter, the detailed observations were carried out on fired geological Greek sample, Koupa, which acted as a representative sample. It is a milky sedimentary quartz sample collected from the Northern Greece. Apart from utilizing this sample in our recent work (Koul
The samples were fired at 800°C in air in a resistance furnace for one hour and then allowed to cool down in the furnace. Thereafter, all the thermal treatments reported in this work were carried out on the heater of the Risø system. Discs were loaded on alternate slots to reduce the inter-disc interference (Bray
To observe the nature of post 500°C OSL or deep trap OSL (DT-OSL), three protocols were devised (i) recording OSL after 500°C pre-heat, (ii) bleaching with blue light of Risø system for 100 s and recording OSL measurement and (iii) recording OSL after bleaching and 500°C pre-heating. The first protocol employed was similar to the one reported in literature (Polymeris
All the four samples studied here behaved in a similar way with the above-mentioned protocols as shown in
OSL decay curves of fired (a) Greek sample, (b) local brick sample (c) Polish sample and (d) Thar dessert samples after 500°C pre-heating (curve ‘A’), bleaching with blue light of RISO system for 100 s (curve ‘B’) and bleaching and 500°C pre-heating (curve ‘C’). The OSL was recorded at a stimulation temperature of 125°C after administering a dose of 200 Gy to the sample. Fresh disc was used for each protocol.Fig. 1
To compare various features of the 510°C TL trap OSL (DT-OSL) with the 325°C TL trap OSL (fast OSL) a protocol was devised which allowed observation of the signals with pre-heating at, both, 260 and 500°C. The Greek sample, Koupa, was irradiated with a dose of 200 Gy, pre-heated at 260°C for 10 s and then stimulated for 200 s at 125°C to record the fast OSL. Thereafter, in order to record the DT-OSL the aliquot was subjected to the same treatment as mentioned above except for the pre-heat, a pre-heat at 500°C instead of 260°C was incorporated. A test dose OSL for sensitivity correction of the two emissions was also recorded using a test dose of 1.5 Gy and cut heat of 160°C subsequent to the OSL measurements.
The OSL decay curves recorded with pre-heats at 260°C and 500°C are shown in
OSL shine down curves recorded with pre-heating temperatures of 260°C and 500°C (a) sensitization corrected and (b) the corrected curve normalized with the respective initial data point. The OSL in these plots represent fast OSL (OSL measured with pre-heat at 260°C) and DT-OSL (OSL measured with pre-heat at 500°C). In order to understand the decaying nature of DT-OSL, its initial 10 data points are compared with that of fast OSL in the inset of (b). The shine down curves were corrected for sensitization with respective test dose OSL. These curves were recorded after administering a dose of 200 and test dose of 1.5 Gy. All OSL measurements were done at a stimulation temperature of 125°C.Fig. 2
To know more about the nature of the DT-OSL a component analysis of the two signals was done using computerized curve deconvolution analysis (CCDA) (Afouxenidis
Component analysis of CW-OSL curves recorded with preheats (a) 260°C and (b) 500°C. The samples were irradiated with the dose oi 200 Gy and the OSL was recorded at a stimulation temperature oi 125°C.Fig. 3
In order to identify the trap responsible for DT-OSL phenomenon it was found necessary to observe the (i) nature of the TL glow curve, (ii) impact of bleaching on the TL glow curve and (iii) the pulse annealing behaviour of the OSL signal. The details of these measurements are described as under.
Glow curve gives information about the traps in terms of their number and trap depth. OSL emission has to originate from one or more than one traps corresponding to the glow peaks of the glow curve. TL glow curve of the Koupa sample, as shown in
(a) TL glow curve of the Koupa, Greek, sample recorded after administration of a dose of 200 Gy and heating up to a temperature of 625°C with a heating rates 2°C/s. The inset in the Fig. 2a shows the high temperatures peaks in a better way by plotting the data in linearlog scale. (b) to have a better feel of the 5l0°C TL glow peak, it was recorded after thermal cleaning of the glow curve by heating the sample up to 475°C.Fig. 4
In order to understand the nature of trap responsible for DT-OSL the impact of bleaching on TL glow curve was undertaken. The sample was irradiated with a dose of 200 Gy and then heated to a temperature of 475°C to remove charge from all traps except for the deeper ones, the traps of interest here. Thereafter, the glow curve was recorded up to 575°C at a heating rate of 2°C/s. This curve would be referred to as TL glow curve without bleaching. A test dose 110°C TL was also recorded with a dose of 1.5 Gy for sensitization correction of the glow curve. Since the sample is pre-heated to high temperature of 475°C, before recording of the glow curve, it is expected to get sensitized. Another TL glow curve was measured in an identical way as mentioned above but with a bleaching step of 100 s with blue light of Risø system, at a stimulation temperature of 125°C, before recording the glow curve. This curve would be referred to as glow curve with bleaching and it was, again, corrected for the sensitization the same way as mentioned above in case of TL glow curve measured without bleaching.
(a) Sensitization corrected 510°C TL glow peak of Greek sample with and without bleaching treatment. Prior to the TL measurement the sample was heated up to 475°C to remove the traps lower than this glow peak. Heating in both cases, TL measurement and preheating, was done with heating rate of 2°C/s. The bleaching was done for 100 s using the blue light of the Riso system at a stimulation temperature of 125°C. The glow curves were measured with a radiation dose of200 Gy and sensitization corrected with a test dose OSL of 1.5 Gy.Fig. 5
This observation involves observing the behaviour of OSL decay curves with different increasing pre-heating temperatures and, thereby, determines the thermal stability of the trap responsible for OSL emission. The observations reported above, suggested that the 510°C TL glow peak could be responsible for DT-OSL. So, accordingly the pre-heating temperature window for the OSL measurements, DT-OSL emissions, was set around this temperature, 450 to 625°C, in steps of 25°C. The protocol devised for this measurement is listed in
Protocol to record the thermal pulse annealing of deep trap.Step Treatment 1 Administer a radiation dose of 100 Gy 2 Heat at different temperatures starting with 450°C (Pre-heat) 3 Stimulate for 100 s at 125°C to record OSL, Lx 4 Administer a test dose of 1.5 Gy 5 Heat at 160°C for 10 s (Cut- heat) 6 Stimulate for 100 s at 125°C to record OSL, Tx 7 Repeat the steps from 1 to 6 using various pre-heating temperature up to 625°C in steps of 25°C
The sensitization corrected OSL decay curves recorded with various pre-heating temperatures are depicted in
(a) Plot of sensitization corrected OSL shine down curves as a function of pre-heating temperatures (the inset depicting the uncorrected curves), (b) TL glow curves recorded during pre-heating of 475, 525, 575 and 625°C, applied prior to OSL measurement and (c) pulse annealing curves; plots of sensitization corrected fast OSL signal as a function of pre-heating temperatures, the inset shows a comparison of the sensitization corrected and un-corrected pulse annealing curves. The signal was represented by the sum of counts in the initial 5 channels (2.5 s) minus the sum of counts in the final 5 s of the shine down curve. The sensitization correction was done with a test dose OSL signal. A radiation dose of 100 Gy and a test dose of 1.5 Gy were employed in this measurement. The TL glow curves were recorded with a heating rate of 2°C/s and OSL was recorded at a stimulation temperature of 125°C.Fig. 6
The depletion in the sensitization corrected OSL signal as a function of pre-heating temperature,
The nature of bleaching of the trap responsible for the DT-OSL was observed by measuring the impact of different optical stimulation time periods on the signal. The sample was subjected to a radiation dose of 150 Gy and heated at 500°C, to ensure the removal of charges from traps except for the deep traps. It was then bleached for various time periods starting with 0 s (no bleaching) using the blue light of the Risø system at 125°C. Subsequent to the bleaching, the specimen was stimulated for 200s, again with blue light, at 125°C to record the DT-OSL emission. Finally, the test dose OSL was measured after the administration of a test dose of 5 Gy and cut heat of 160°C. This signal was used to undertake the sensitization correction of the DT-OSL emission. This sequence of measurement was recorded with other bleaching time periods of 0.5, 2, 5, 20, 50 and 100 s.
The sensitization corrected DT-OSL signals observed after various bleaching time durations are depicted in Fig. 7. The correction, as stated above, has been done with the test dose OSL emission. The data in the figure has been divided by the first data point in order to have better feel of the curve. The magnitude of DT-OSL can be seen decaying sharply within first few seconds and reaching almost negligible value after a bleaching of 100 s. This measurement corroborates with the earlier finding, described in Section 3, that the trap responsible for DT-OSL trap is an easily bleachable trap.
Impact of bleaching on DT-OSL signal with bleaching time periods of 0, 2, 10, 20, 50 and 100 s using blue light of Riso system. The sample was irradiated with a dose of 150 Gy and the bleached at a stimulation temperature of 125°C. The signal was represented by the sum of counts in the initial 5 channels (2.5 s) minus the sum of counts in the final 5 s of the shine down curve. The signal was corrected with a test dose OSL. A dose of 150 Gy was administered to the sample prior to each optical stimulation and a test dose of 5 Gy was used for sensitization correction.Fig. 7
The feasibility of any luminescence for dose estimation using depends on its (i) nature of growth with radiation dose,
The protocol employed for recording the growth curve of DT-OSL involved (i) administration of dose, (ii) heating at 500°C, (iii) recording of OSL at a stimulation temperature of 125°C (Lx), (iv) administrating a test dose of 1.5 Gy and (v) recording of OSL at a stimulation temperature of 125°C and a cut heat of 160°C (Tx). The dose values of 0.006, 0.05, 0.3, 0.6, 1, 1.5, 2.5 and 10 kGy and a test dose of 1.5 Gy were employed in this study. To compare the growth curve of this emission from deep trap, DT-OSL, with the 325°C TL trap OSL, fast OSL, the growth curve of the latter was also measured in a similar way as described above but with dose values of 6, 30, 60, 120, 300 and 600 Gy and pre-heat at 260°C. Since the fast OSL typically saturates at around 100 Gy the growth curve was measured up to 600 Gy, only (Aitken, 1998).
The sensitization corrected growth curves (Lx/Tx versus dose) corresponding to DT-OSL and fast OSL emissions with administered dose values up to 1000 and 600 Gy, respectively, are depicted in
The plot of sensitization corrected signals corresponding to (a) deep trap OSL (DT-OSL) and fast OSL with dose administered up to a dose of 1000 and 600 Gy respectively and (b) only DT-OSL up to highest administered dose value of 10 kGy, the inset in this figure shows plot of the data till a dose of 1 kGy. Since the fast OSL saturated much early than the DT-OSL, so, its growth curve was measured up to 600 Gy only. The data of the two curves in (a) was normalized in such a way that the data points of the two curves corresponding to 600 Gy matched with each other. The protocol involved pre-heats of 260 and 500°C for measurements of fast OSL and DT-OSL respectively and a cut heat of 160°C. The data in DT-OSL could be fitted with a linear fit till a dose of 1 kGy. and latter beyond it till the last administered dose of 2.5 kGy, as shown in (b). The signal was represented by the sum of counts in the initial 5 channels (2.5 s) minus the sum of counts in the final 5 s of the shine down curve. These curves were measured at a stimulation temperature of 125°C.Fig. 8
Single aliquot regenerative (SAR) protocol has been found to be reliable and efficient in dose estimation using OSL signal (Wintle and Murray, 2006). Good reproducibility of the signal and absence of the thermally transferred charge (termed as recuperation as per the convention of the protocol) determine the reliability of SAR technique. The reproducibility of DT-OSL was undertaken by observing sensitization corrected DT-OSL signal with (i) four identical dose values, (ii) no dose or zero dose and (iii) same dose as administered above once more. This procedure used here is identical to the one employed in SAR, but instead of different regenerative doses an identical dose was employed here. A radiation dose of 100 Gy and test dose of 3 Gy were used in this observation. The measurements were carried out done with a pre-heat and cut-heat of 500 and 160°C respectively. As can be seen from the
The reproducibility of DT-OSL was undertaken by observing its four repeated sensitization corrected measurements with the same dose value of 100 Gy and then repeating it once more with the same dose, but, after a measurement with administration of a zero dose, i.e. no dose, (run number 5). A test dose of 3 Gy was used in this observation. The measurement was done with a pre-heat and cut-heat of 500 and 160°C respectively. The signal was represented by the sum of counts in the initial 5 channels (2.5 s) minus the sum of counts in the final 5 s of the shine down curve. These curves were measured at a stimulation temperature of 125°C.Fig. 9
The recuperation term used in SAR protocol, essentially consists of the thermal transfer which might happen during different heating steps of the protocol. The magnitude of this signal is evaluated by measuring the OSL signal with no dose,
Though the DT-OSL has been found above to qualify for dose estimation using SAR, it is the dose recovery test which guarantees the applicability of this protocol for dose estimation. Accordingly, the dose recovery of a fired geological sample utilizing the DT-OSL emission was undertaken. An identical dose of 200 Gy, the dose to be recovered, was administered to three aliquots. The regenerative doses used in the protocol were 100, 300 and 400Gy. A test dose of 1.5 Gy, pre-heat of 500°C (without holding it) and cut heat of 160°C was incorporated in the measurements. The test use signal is used for sensitization correction of the signals.
The equivalent dose estimation using SAR was seen to yield similar results in case of all the three discs employed here. These recovered dose values were 208.06, 204.06 and 206.06 Gy. The average of these numbers is 206.1 ± 1.2 Gy (n=3), which is very close to the administered dose of 200 Gy, giving a dose recovery ratio of 1.030 ± 0.006. This measurement, again, was seen to reproduce the above-mentioned results,
The variation in luminescence sensitization during different runs of the SAR. The data represents the average values of the three discs which were administered an identical dose of 200 Gy, i.e. the dose to be recovered. The regenerative doses used in the protocol were 100, 300 and 400 Gy. A test dose of 1.5 Gy and cut heat of 160°C was incorporated in the measurement. The protocol was modified in terms of pre-heating temperature, a heating at 500°C, without holding it for anytime.Fig. 10
Based on the nature of the DT-OSL (i) bleachability, (ii) large dynamic range of the growth curve and (iii) reliability for dose estimation, it will be useful for dating of fired quartz samples. Since the pre-heat of 500°C is incorporated in the measurement of the signal the sensitization during the regenerative protocol would not be appreciable as is the case with conventional OSL. Also based on the dose response curve, as shown in
Pre-heat the sample with natural dose at 500°C for 10 s Stimulate for 100 s at 125°C to record the OSL signal (DT-OSLN) Administer a test dose Heat at 160°C for 10 s Stimulate for 100 s at 125°C to record the test dose OSL (OSLTD) Stimulate at 125°C for 200 s to remove any residual signal Repeat steps 1) to 7) with different three artificially administered dose values at step 1)
The nature of optically stimulated luminescence signal of quartz recorded with high pre-heating temperatures (> 400°C) has been studied in detail by many scientific workers (Jain
The thermal pulse annealing characteristics of the luminescence with pre-heat showed an appreciable decrease in the DT-OSL between heating temperatures of 450 and 550°C (
The nature of growth curve of DT-OSL, undertaken to look into its feasibility for equivalent dose estimation, was seen to increase up to the largest dose applied in this study, 10 kGy as shown in
The DT-OSL signal was observed to fulfill the necessary conditions required for the reliable dose estimation. Both, reproducibly and dose recovery was observed to be quite good. There was almost negligible recuperation,
Thermal treatment, firing, has been observed to affect luminescence properties of quartz (Sankaran
In a recent study involving the impact of firing on the nature of CW-OSL of quartz it was observed that the composition of the OSL signal changed significantly (Koul