- Email: [email protected]
Charge movement in grains of quartz studied using exo-electron emission
Radiation Measurements 43 (2008) 273 – 277 www.elsevier.com/locate/radmeas
Charge movement in grains of quartz studied using exo-electron emission C. Ankjærgaard a,∗ , P.M. Denby a , A.S. Murray b , M. Jain a a Radiation Research Department, Ris National Laboratory, Technical University of Denmark, DK-4000 Roskilde, Denmark b Nordic Laboratory for Luminescence Dating, Department of Earth Science, Aarhus University, Ris National Laboratory, DK-4000 Roskilde, Denmark
Abstract A flow-through Geiger-Müller pancake electron detector attachment has been fitted to the RisZ TL/OSL reader, enabling optically stimulated electrons (OSE) to be measured simultaneously with optically stimulated luminescence (OSL). This detector has been used to provide new insights into charge movement in luminescence phosphors. Here we show that OSE from natural quartz grains gives an easily detectable, reproducible and light sensitive signal, although it is not as intense as OSL. A single sample of natural quartz grains extracted from a sediment is used to investigate the thermal stability and dependence on stimulation temperature of the OSE and OSL signals. The response to dose of these signals is also investigated, and conclusions drawn on the movement of electrons in this natural phosphor. © 2007 Elsevier Ltd. All rights reserved. Keywords: Optically stimulated exo-electron emission; OSE; Optically stimulated luminescence; OSL; Quartz; Electron detector; Charge movement
1. Introduction Luminescence is widely used to investigate charge movement and determine absorbed doses in retrospective dosimetry. However, the observed signal is a product of charge stimulation and recombination; only some fraction of the stimulated electrons will recombine in the measured waveband interval. As a result, there is not always a simple relationship between the amount of trapped charge and the observed luminescence. An alternative approach to examining trapped charge populations in natural insulators such as quartz is by the measurement of exo-electron emission (Fig. 1), in which electrons either (a) overcome the work function, , of the dosimeter surface and escape from a trap without residing in the conduction band or (b) are derived from evicted electrons already in the conduction band; these then acquire sufficient energy to overcome from ambient temperature (Bohun, 1970). These processes happen simultaneously with the emission of luminescence. The principle advantage of measurement of exo-electrons is that the electron signal comes directly from trapped charge, and so the relationship to the trapped population is less complex than in the case of luminescence. However, it should be noted that OSE ∗ Corresponding author. Tel.: +45 4677 4920; fax: +45 4677 4959.
E-mail address: [email protected] (C. Ankjærgaard). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.044
and TSE originate from a surface layer of ∼ 1 m deep (Becker, 1973), whereas OSL and TL come from throughout the body of the crystal. In Ankjærgaard et al. (2006) measurements of dose made both before and after etching of the surface of different materials showed that surface etching leaves the population density of trapped charge and the trap sensitivity unchanged. It was concluded there that the dosimetric properties of bulk and surface traps are identical, enabling electron and luminescence signals to be compared directly. In this paper, we will first consider the comparison of OSE and OSL signals from 12 different sedimentary quartz samples. Then a single sample of quartz extracted from a sandstonederived sediment (WIDG8) is used to investigate the dependence of OSE and OSL signals on preheat temperature and stimulation temperature as well as the response of these signals to dose. Conclusions are then drawn on the movement of electrons in this natural phosphor. 2. Instrumentation, samples and signals To allow measurements on sand-sized grains of natural phosphors of interest in retrospective dosimetry, a windowless flowthrough Geiger-Müller pancake detector has been added to the stimulation head of a RisZ TL/OSL reader beneath the PM-tube and filter basket (Fig. 2a). Stimulation light from the
274
C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
Energy
OSE,TSE (b)
E0
φ
(a)
CONDUCTION BAND OSL,TL
T
Heat, light
L
Non-L
ET
VALENCE BAND Fig. 1. Band gap diagram showing the OSE/TSE and the OSL/TL processes. represents the work-function, T is a trap with energy ET , and L and Non-L are luminescent and non-luminescent centres respectively. (E0 − ET ) is the minimum energy required to produce a free electron.
surrounding blue (50 mW cm−2 ) and infrared (∼100 mW cm−2 ) LED arrays penetrate the transparent plastic walls of the counter to reach the sample. Electrons are emitted into the counting gas (99% argon, 1% isobutane), and photons reach the detection filters through the gridded cathode at the back of the electron-sensitive counting volume. This makes it possible to measure OSE simultaneously with OSL and TSE with TL. A close-up diagram of the detector alone is shown in Fig. 2b. The detector volume is sealed at the top against the detection filter face so the counting volume (and the internal volume of the TL/OSL reader) can be kept at a constant overpressure using the counting gas. A more detailed description of the detector is given in Ankjærgaard et al. (2006). All work reported here was undertaken using 180.250 m sedimentary quartz grains (90–125 m for WIDG9), cleaned using standard laboratory procedures (Aitken, 1985), and checked for the absence of feldspar contamination using infrared stimulation. Fig. 3 shows OSE and OSL decay curves from a sedimentary quartz extract from Australia (WIDG8) stimulated using blue light; OSE signals are typically much less intense than OSL, but the characteristic decay times of the initial signal are similar. 3. Comparison of signals from various samples
anode
gridded cathode
Fig. 2. (a) Cross-section showing detector inserted into a standard OSL/TL RisZ reader head. (b) Enlarged view of electron detector.
To provide insight into the reasons for the considerable variation in the absolute luminescence sensitivity of quartz, the ratio of OSL to OSE signal for 12 quartz samples extracted from different natural sediments were examined. All samples were given a dose of 220 Gy, preheated to 260 ◦ C and blue light stimulated at 200 ◦ C; the initial intensity of OSE against OSL is shown in Fig. 4. The solid straight line is a best fit through the origin (note that both axes are on logarithmic scale); the correlation between the OSE and OSL signals is weak, perhaps because the surface area to volume ratios vary from sample to sample. Typically OSL is about 103 stronger than OSE for our geometry. 107
400
2x10
6
1x106
106 OSL
OSL, s-1
OSL, (0.5 s)-1
OSE, (0.5 s)-1
600
0 200
0
10
20
30
40
105
Stimulation time, s OSE 0 0
10
20
30
40
50
Stimulation time, s Fig. 3. OSE and OSL decay curves from loose 90.125 m quartz grains (sample WIDG8). The OSE and OSL decay curves were measured simultaneously during blue light stimulation at 125 ◦ C, following a 440 Gy dose and preheat to 250 ◦ C. Background count rate is ∼ 1 s−1 for OSE and ∼ 100 s−1 for OSL.
104 102
103 OSE,
104
s-1
Fig. 4. Initial intensity (0–1 s) of OSE plotted against OSL for 12 different quartz samples (180.250 m grains); all were given a dose of 220 Gy, preheated to 260 ◦ C and stimulated at 200 ◦ C. The straight line represents a best fit through the origin. Note that both axes are logarithmic.
C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
OSE
750
OSL
2
500 Total OSE/5,
250
1
(100 s)-1
0 0
100
200
300
0 400
Stimulation temperature, °C Fig. 5. OSE and OSL initial 0.5 s signals from an aliquot of quartz (WIDG8) together with the total integrated OSE signal as a function of stimulation temperature. The aliquot was given a dose of 44 Gy and preheated to 260 ◦ C before each measurement; corresponding test doses of 18 Gy were preheated to 260 ◦ C before measurement at 125 ◦ C. Repeat points are shown as open symbols. Note that the total OSE curve has been divided by a factor 5 for display; the OSL data have been corrected for sensitivity change by dividing by the response to a test dose.
4. Signal dependence on stimulation temperature In the remainder of this paper, only the quartz sample WIDG8 will be used; we first consider the dependence of the OSE and OSL signals on stimulation temperature. A single aliquot of WIDG8 was given a dose of 44 Gy and preheated to 260 ◦ C before blue light stimulation at some temperature. This was followed by a test dose measurement where the same aliquot was given a dose of 18 Gy, preheated to 260 ◦ C and stimulated at 125 ◦ C. At the end of the cycle, the aliquot was stimulated at 280 ◦ C with blue light for 100 s. This cycle, including the 44 Gy dose, was repeated for different stimulation temperatures. In Fig. 5, the initial (0–0.5 s) sensitivity corrected OSL signal together with the initial (0–0.5 s) OSE and integrated (0–100 s) OSE signals are shown as a function of stimulation temperature. Note that the OSE signals are not corrected for sensitivity changes; these are negligible, as can be seen from the agreement between the repeated points on the curve. The decrease in OSL as a function of stimulation temperature is usually attributed to thermal quenching (e.g. Murray and Wintle, 1998). The increase in OSE is not due to thermal assistance because the total electron sum (integrated over the total decay curve) follows the trend shown for the initial signal (thermal assistance should change the rate of stimulation, but not the total number of stimulated electrons). Presumably the increase arises simply from the increased mean energy of electrons in the conduction band at higher temperatures. Above ∼ 280 ◦ C, thermal erosion of the light sensitive trap begins to dominate. 5. Thermal stability It has previously been shown (Ankjærgaard et al., 2006, using a different sample) that the thermal stability of the OSE signal differs from the thermal stability of the OSL signal; this
OSE, (2.5 s)-1
(0.5 s)-1
Sensitivity corrected OSL
OSL
1200
1.0
3x107
2x107 200
300
Preheat temp., °C
Murray, 1998
600
OSL/ OSE
0.5
0.0 100
Wintle &
OSE
1x107
OSL, (2.5 s)-1
1800
3 Initial OSE,
Fractional change
1000
275
OSE
0 100
200
300
400
0 500
Preheat temperature, °C
Fig. 6. Thermal stability of the OSE and OSL signal from an aliquot of quartz (WIDG8) which was given a dose of 330 Gy, preheated to a given temperature for 10 s and stimulated at 125 ◦ C. Repeat points are shown as open symbols. Published WIDG8 data from Wintle and Murray (1998) (where brief stimulations of 0.1 s blue/green light at 125 ◦ C following a dose of 56 Gy are used to monitor the remaining trap population) are also shown (square data points). Note that neither of the OSL data sets have been corrected for sensitivity changes. Inset: OSE (electron detrapping) and OSL/OSE (luminescence sensitivity) as a function of preheat temperature.
observation suggested that the rapid depletion of the OSL signal around 300 ◦ C may not arise only because of the depletion of the OSL 325 ◦ C trap, as discussed in Wintle and Murray (1998) using WIDG8. To test whether this observation was sample specific, the experiment is repeated here using WIDG8. An aliquot was dosed with 330 Gy, preheated to a given temperature for 10 s and stimulated at 125 ◦ C. This cycle was repeated for different preheat temperatures using the same aliquot. The resulting data are shown in Fig. 6, together with the original data from Wintle and Murray (1998); both curves are normalized at the 250 ◦ C point on the OSL curve. Although the experimental details were different from those of Wintle and Murray (1998), both experiments were intended to measure the OSL remaining after a given preheat. The shape of the two OSL curves is very similar, especially the rapid decrease from about 300 ◦ C (incidentally confirming that the differences in experimental approach are not important). It is therefore appropriate to compare this measured OSE and OSL to other published data using WIDG8. The OSE signal above 130 ◦ C decreases steadily, presumably due to trap emptying (assuming the OSE curve reflects electron detrapping behaviour), while the OSL curve first increases in sensitivity (not because of thermal transfer, since the OSE signal does not increase at the same time) after which trap emptying dominates and the OSL curve decreases rapidly. The decrease in OSL > 260 ◦ C is usually attributed to thermal depletion of the 325 ◦ C trap as described above, but as is seen from the OSE curve, electron depletion is apparently much slower and happens over a much wider temperature range. The inset shows electron detrapping (OSE) and luminescence sensitivity change (calculated by dividing the OSL by the OSE data). The luminescence sensitivity sensitizes by almost a factor of 20 before decreasing rapidly at about 300 ◦ C, above which the chargetrap is presumably completely empty. It may be that when the trap is empty, the relevant
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C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
4 OSL
10
2
1
100
1000
4
OSE Counts, (2.5 s)-1
Counts, (2.5 s)-1
3
Dose, Gy OSL 2
OSE
0 0
1000
2000
3000
4000
Dose, Gy Fig. 7. Simultaneously measured OSE and OSL growth curves from WIDG8 using a preheat of 260 ◦ C and stimulation at 200 ◦ C. Repeat points are shown as open symbols. Note that both OSE and OSL are corrected for sensitivity change. Inset: Growth curves shown on a logarithmic scale.
luminescence centres have all recombined, leaving effectively no luminescence sensitivity. The initial rise in the OSL curve can therefore be explained as a combination of trap emptying and sensitization and the decrease as a collapse in luminescence sensitivity. 6. Growth curves Fig. 7 presents OSE and OSL growth curves measured simultaneously from a single aliquot of quartz (WIDG8). The curves show similar behaviour up to about a few hundred Gy above which the OSL curve decreases slightly whereas the OSE curve becomes independent of dose. Since the latter is presumed to reflect the behaviour of the trapped electron population, the decrease observed in the OSL curve must result from a dose-dependent change in the luminescence recombination efficiency. 7. Discussion and conclusions The electron detector attachment allows OSE and OSL to be measured simultaneously, and sample treatment (i.e. irradiation, heating, light stimulation) can be performed without removing the sample from the counting gas; this configuration appears to result in excellent measurement reproducibility. Comparing the initial intensity (0–1 s) of the decay curves from 12 different natural granular quartz samples, the OSE sensitivity is in general ∼ 103 times weaker than that of the OSL, but because the background noise level is ∼ 1 s−1 for OSE and ∼ 100 s−1 for OSL, the overall signal to noise ratio is only a factor of 10 different between OSE and OSL signals. Fig. 4 also shows that there exists only a weak correlation between the two signals, probably because the surface areas to volume ratio vary from sample to sample. Nevertheless, OSE signals are easy to
detect and permit studies of electron movement directly instead through the proxy of luminescence. The shape of the OSL curve as a function of stimulation temperature (Fig. 5) has been attributed by Murray and Wintle (1998) to a combination of two independent processes: thermal assistance (which is seen as a slight initial rise in curve) and thermal quenching. As the temperature is increased, thermal quenching of the luminescence recombination becomes the dominant factor, and the curve decreases steadily. However, the increase in OSE with temperature is not consistent with thermal assistance. Thermal assistance increases the rate of optical ejection of charge (and thus should affect the initial OSE) but it should not affect the total number of electrons available for eviction, and so the integrated signal should be insensitive to changes in eviction rate. In our data set, we do, however, see an increase in the integrated signal; in fact, the total electron sum (integrated over the entire decay curve) closely matches the shape of the change in the initial (0–0.5 s) part of the signal. This increase in OSE signal may arise because a larger fraction of the evicted electrons is able to overcome the workfunction at higher sample temperatures. This suggests that at least some of the initial rise in the OSL curve might be caused by processes other than thermal assistance. The thermal depletion of the 325 ◦ C trap is usually thought to account for the rapid decrease in the OSL curve > 260 ◦ C (Wintle and Murray, 1998). This model seems to be inconsistent with the behaviour of the OSE signal, which decreases over a much broader temperature range than the OSL (Fig. 6). Based on these data, the inset to Fig. 6 shows electron detrapping and luminescence sensitivity change; the latter increases by almost a factor of 20 before decreasing rapidly around 300 ◦ C. Note that these data are broadly similar to those shown by Murray and Wintle (1998); (Fig. 7b, solid circles), except that their sensitivity does not decrease so rapidly above 300 ◦ C. However, their sensitivity changes were monitored using the response of the 110 ◦ C. TL peak as a surrogate, whereas our data are directly based on the OSL behaviour itself. From our data, at ∼ 300 ◦ C the charge trap is presumably completely empty and the relevant luminescence centres have all recombined, resulting in a very rapid decrease in sensitivity. This offers a possible development in the explanation for the rapid decrease in the OSL curve at > 260 ◦ C; the decrease may be a combination of trap depletion and rapid luminescence de-sensitization. Finally, the OSE and OSL dose response curves (Fig. 7) behave very similarly at low doses, but at high doses, the OSE signal saturates while the OSL signal decreases due to changes in the probability of luminescent recombination. Thus the OSL data appear to accurately track the growth of the trapped electron population at low does, but at higher doses the influence of luminescence centres becomes important. We conclude that the measurement of exo-electron signals has considerable potential to contribute to our understanding of charge movement in natural phosphors and has shown that several processes thought to be caused by trap behaviour may in fact be attributed, at least in part, to changes in luminescence centre recombination probability.
C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
Acknowledgements This work was supported by the Joint Committee of the Nordic Natural Science Research Councils through the Nordic Centre of Excellence Programme. References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London.
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Ankjærgaard, C., Murray, A.S., Denby, P.M., BZtter-Jensen, L., 2006. Measurement of optically and thermally stimulated electron emission from natural minerals. Radiat. Meas. 41, 780–786. Becker, K., 1973. Solid State Dosimetry. CRC-Press, Ohio, 334pp. Bohun, A., 1970. The physics of exoelectron emission on ionic crystals. PTB-Mitt. 80, 329–333. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in quartz. Radiat. Meas. 29, 65–79. Wintle, A.G., Murray, A.S., 1998. Towards the development of a preheat procedure for OSL dating of quartz. Radiat. Meas. 29, 81–94.
Charge movement in grains of quartz studied using exo-electron emission C. Ankjærgaard a,∗ , P.M. Denby a , A.S. Murray b , M. Jain a a Radiation Research Department, Ris National Laboratory, Technical University of Denmark, DK-4000 Roskilde, Denmark b Nordic Laboratory for Luminescence Dating, Department of Earth Science, Aarhus University, Ris National Laboratory, DK-4000 Roskilde, Denmark
Abstract A flow-through Geiger-Müller pancake electron detector attachment has been fitted to the RisZ TL/OSL reader, enabling optically stimulated electrons (OSE) to be measured simultaneously with optically stimulated luminescence (OSL). This detector has been used to provide new insights into charge movement in luminescence phosphors. Here we show that OSE from natural quartz grains gives an easily detectable, reproducible and light sensitive signal, although it is not as intense as OSL. A single sample of natural quartz grains extracted from a sediment is used to investigate the thermal stability and dependence on stimulation temperature of the OSE and OSL signals. The response to dose of these signals is also investigated, and conclusions drawn on the movement of electrons in this natural phosphor. © 2007 Elsevier Ltd. All rights reserved. Keywords: Optically stimulated exo-electron emission; OSE; Optically stimulated luminescence; OSL; Quartz; Electron detector; Charge movement
1. Introduction Luminescence is widely used to investigate charge movement and determine absorbed doses in retrospective dosimetry. However, the observed signal is a product of charge stimulation and recombination; only some fraction of the stimulated electrons will recombine in the measured waveband interval. As a result, there is not always a simple relationship between the amount of trapped charge and the observed luminescence. An alternative approach to examining trapped charge populations in natural insulators such as quartz is by the measurement of exo-electron emission (Fig. 1), in which electrons either (a) overcome the work function, , of the dosimeter surface and escape from a trap without residing in the conduction band or (b) are derived from evicted electrons already in the conduction band; these then acquire sufficient energy to overcome from ambient temperature (Bohun, 1970). These processes happen simultaneously with the emission of luminescence. The principle advantage of measurement of exo-electrons is that the electron signal comes directly from trapped charge, and so the relationship to the trapped population is less complex than in the case of luminescence. However, it should be noted that OSE ∗ Corresponding author. Tel.: +45 4677 4920; fax: +45 4677 4959.
E-mail address: [email protected] (C. Ankjærgaard). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.044
and TSE originate from a surface layer of ∼ 1 m deep (Becker, 1973), whereas OSL and TL come from throughout the body of the crystal. In Ankjærgaard et al. (2006) measurements of dose made both before and after etching of the surface of different materials showed that surface etching leaves the population density of trapped charge and the trap sensitivity unchanged. It was concluded there that the dosimetric properties of bulk and surface traps are identical, enabling electron and luminescence signals to be compared directly. In this paper, we will first consider the comparison of OSE and OSL signals from 12 different sedimentary quartz samples. Then a single sample of quartz extracted from a sandstonederived sediment (WIDG8) is used to investigate the dependence of OSE and OSL signals on preheat temperature and stimulation temperature as well as the response of these signals to dose. Conclusions are then drawn on the movement of electrons in this natural phosphor. 2. Instrumentation, samples and signals To allow measurements on sand-sized grains of natural phosphors of interest in retrospective dosimetry, a windowless flowthrough Geiger-Müller pancake detector has been added to the stimulation head of a RisZ TL/OSL reader beneath the PM-tube and filter basket (Fig. 2a). Stimulation light from the
274
C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
Energy
OSE,TSE (b)
E0
φ
(a)
CONDUCTION BAND OSL,TL
T
Heat, light
L
Non-L
ET
VALENCE BAND Fig. 1. Band gap diagram showing the OSE/TSE and the OSL/TL processes. represents the work-function, T is a trap with energy ET , and L and Non-L are luminescent and non-luminescent centres respectively. (E0 − ET ) is the minimum energy required to produce a free electron.
surrounding blue (50 mW cm−2 ) and infrared (∼100 mW cm−2 ) LED arrays penetrate the transparent plastic walls of the counter to reach the sample. Electrons are emitted into the counting gas (99% argon, 1% isobutane), and photons reach the detection filters through the gridded cathode at the back of the electron-sensitive counting volume. This makes it possible to measure OSE simultaneously with OSL and TSE with TL. A close-up diagram of the detector alone is shown in Fig. 2b. The detector volume is sealed at the top against the detection filter face so the counting volume (and the internal volume of the TL/OSL reader) can be kept at a constant overpressure using the counting gas. A more detailed description of the detector is given in Ankjærgaard et al. (2006). All work reported here was undertaken using 180.250 m sedimentary quartz grains (90–125 m for WIDG9), cleaned using standard laboratory procedures (Aitken, 1985), and checked for the absence of feldspar contamination using infrared stimulation. Fig. 3 shows OSE and OSL decay curves from a sedimentary quartz extract from Australia (WIDG8) stimulated using blue light; OSE signals are typically much less intense than OSL, but the characteristic decay times of the initial signal are similar. 3. Comparison of signals from various samples
anode
gridded cathode
Fig. 2. (a) Cross-section showing detector inserted into a standard OSL/TL RisZ reader head. (b) Enlarged view of electron detector.
To provide insight into the reasons for the considerable variation in the absolute luminescence sensitivity of quartz, the ratio of OSL to OSE signal for 12 quartz samples extracted from different natural sediments were examined. All samples were given a dose of 220 Gy, preheated to 260 ◦ C and blue light stimulated at 200 ◦ C; the initial intensity of OSE against OSL is shown in Fig. 4. The solid straight line is a best fit through the origin (note that both axes are on logarithmic scale); the correlation between the OSE and OSL signals is weak, perhaps because the surface area to volume ratios vary from sample to sample. Typically OSL is about 103 stronger than OSE for our geometry. 107
400
2x10
6
1x106
106 OSL
OSL, s-1
OSL, (0.5 s)-1
OSE, (0.5 s)-1
600
0 200
0
10
20
30
40
105
Stimulation time, s OSE 0 0
10
20
30
40
50
Stimulation time, s Fig. 3. OSE and OSL decay curves from loose 90.125 m quartz grains (sample WIDG8). The OSE and OSL decay curves were measured simultaneously during blue light stimulation at 125 ◦ C, following a 440 Gy dose and preheat to 250 ◦ C. Background count rate is ∼ 1 s−1 for OSE and ∼ 100 s−1 for OSL.
104 102
103 OSE,
104
s-1
Fig. 4. Initial intensity (0–1 s) of OSE plotted against OSL for 12 different quartz samples (180.250 m grains); all were given a dose of 220 Gy, preheated to 260 ◦ C and stimulated at 200 ◦ C. The straight line represents a best fit through the origin. Note that both axes are logarithmic.
C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
OSE
750
OSL
2
500 Total OSE/5,
250
1
(100 s)-1
0 0
100
200
300
0 400
Stimulation temperature, °C Fig. 5. OSE and OSL initial 0.5 s signals from an aliquot of quartz (WIDG8) together with the total integrated OSE signal as a function of stimulation temperature. The aliquot was given a dose of 44 Gy and preheated to 260 ◦ C before each measurement; corresponding test doses of 18 Gy were preheated to 260 ◦ C before measurement at 125 ◦ C. Repeat points are shown as open symbols. Note that the total OSE curve has been divided by a factor 5 for display; the OSL data have been corrected for sensitivity change by dividing by the response to a test dose.
4. Signal dependence on stimulation temperature In the remainder of this paper, only the quartz sample WIDG8 will be used; we first consider the dependence of the OSE and OSL signals on stimulation temperature. A single aliquot of WIDG8 was given a dose of 44 Gy and preheated to 260 ◦ C before blue light stimulation at some temperature. This was followed by a test dose measurement where the same aliquot was given a dose of 18 Gy, preheated to 260 ◦ C and stimulated at 125 ◦ C. At the end of the cycle, the aliquot was stimulated at 280 ◦ C with blue light for 100 s. This cycle, including the 44 Gy dose, was repeated for different stimulation temperatures. In Fig. 5, the initial (0–0.5 s) sensitivity corrected OSL signal together with the initial (0–0.5 s) OSE and integrated (0–100 s) OSE signals are shown as a function of stimulation temperature. Note that the OSE signals are not corrected for sensitivity changes; these are negligible, as can be seen from the agreement between the repeated points on the curve. The decrease in OSL as a function of stimulation temperature is usually attributed to thermal quenching (e.g. Murray and Wintle, 1998). The increase in OSE is not due to thermal assistance because the total electron sum (integrated over the total decay curve) follows the trend shown for the initial signal (thermal assistance should change the rate of stimulation, but not the total number of stimulated electrons). Presumably the increase arises simply from the increased mean energy of electrons in the conduction band at higher temperatures. Above ∼ 280 ◦ C, thermal erosion of the light sensitive trap begins to dominate. 5. Thermal stability It has previously been shown (Ankjærgaard et al., 2006, using a different sample) that the thermal stability of the OSE signal differs from the thermal stability of the OSL signal; this
OSE, (2.5 s)-1
(0.5 s)-1
Sensitivity corrected OSL
OSL
1200
1.0
3x107
2x107 200
300
Preheat temp., °C
Murray, 1998
600
OSL/ OSE
0.5
0.0 100
Wintle &
OSE
1x107
OSL, (2.5 s)-1
1800
3 Initial OSE,
Fractional change
1000
275
OSE
0 100
200
300
400
0 500
Preheat temperature, °C
Fig. 6. Thermal stability of the OSE and OSL signal from an aliquot of quartz (WIDG8) which was given a dose of 330 Gy, preheated to a given temperature for 10 s and stimulated at 125 ◦ C. Repeat points are shown as open symbols. Published WIDG8 data from Wintle and Murray (1998) (where brief stimulations of 0.1 s blue/green light at 125 ◦ C following a dose of 56 Gy are used to monitor the remaining trap population) are also shown (square data points). Note that neither of the OSL data sets have been corrected for sensitivity changes. Inset: OSE (electron detrapping) and OSL/OSE (luminescence sensitivity) as a function of preheat temperature.
observation suggested that the rapid depletion of the OSL signal around 300 ◦ C may not arise only because of the depletion of the OSL 325 ◦ C trap, as discussed in Wintle and Murray (1998) using WIDG8. To test whether this observation was sample specific, the experiment is repeated here using WIDG8. An aliquot was dosed with 330 Gy, preheated to a given temperature for 10 s and stimulated at 125 ◦ C. This cycle was repeated for different preheat temperatures using the same aliquot. The resulting data are shown in Fig. 6, together with the original data from Wintle and Murray (1998); both curves are normalized at the 250 ◦ C point on the OSL curve. Although the experimental details were different from those of Wintle and Murray (1998), both experiments were intended to measure the OSL remaining after a given preheat. The shape of the two OSL curves is very similar, especially the rapid decrease from about 300 ◦ C (incidentally confirming that the differences in experimental approach are not important). It is therefore appropriate to compare this measured OSE and OSL to other published data using WIDG8. The OSE signal above 130 ◦ C decreases steadily, presumably due to trap emptying (assuming the OSE curve reflects electron detrapping behaviour), while the OSL curve first increases in sensitivity (not because of thermal transfer, since the OSE signal does not increase at the same time) after which trap emptying dominates and the OSL curve decreases rapidly. The decrease in OSL > 260 ◦ C is usually attributed to thermal depletion of the 325 ◦ C trap as described above, but as is seen from the OSE curve, electron depletion is apparently much slower and happens over a much wider temperature range. The inset shows electron detrapping (OSE) and luminescence sensitivity change (calculated by dividing the OSL by the OSE data). The luminescence sensitivity sensitizes by almost a factor of 20 before decreasing rapidly at about 300 ◦ C, above which the chargetrap is presumably completely empty. It may be that when the trap is empty, the relevant
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C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
4 OSL
10
2
1
100
1000
4
OSE Counts, (2.5 s)-1
Counts, (2.5 s)-1
3
Dose, Gy OSL 2
OSE
0 0
1000
2000
3000
4000
Dose, Gy Fig. 7. Simultaneously measured OSE and OSL growth curves from WIDG8 using a preheat of 260 ◦ C and stimulation at 200 ◦ C. Repeat points are shown as open symbols. Note that both OSE and OSL are corrected for sensitivity change. Inset: Growth curves shown on a logarithmic scale.
luminescence centres have all recombined, leaving effectively no luminescence sensitivity. The initial rise in the OSL curve can therefore be explained as a combination of trap emptying and sensitization and the decrease as a collapse in luminescence sensitivity. 6. Growth curves Fig. 7 presents OSE and OSL growth curves measured simultaneously from a single aliquot of quartz (WIDG8). The curves show similar behaviour up to about a few hundred Gy above which the OSL curve decreases slightly whereas the OSE curve becomes independent of dose. Since the latter is presumed to reflect the behaviour of the trapped electron population, the decrease observed in the OSL curve must result from a dose-dependent change in the luminescence recombination efficiency. 7. Discussion and conclusions The electron detector attachment allows OSE and OSL to be measured simultaneously, and sample treatment (i.e. irradiation, heating, light stimulation) can be performed without removing the sample from the counting gas; this configuration appears to result in excellent measurement reproducibility. Comparing the initial intensity (0–1 s) of the decay curves from 12 different natural granular quartz samples, the OSE sensitivity is in general ∼ 103 times weaker than that of the OSL, but because the background noise level is ∼ 1 s−1 for OSE and ∼ 100 s−1 for OSL, the overall signal to noise ratio is only a factor of 10 different between OSE and OSL signals. Fig. 4 also shows that there exists only a weak correlation between the two signals, probably because the surface areas to volume ratio vary from sample to sample. Nevertheless, OSE signals are easy to
detect and permit studies of electron movement directly instead through the proxy of luminescence. The shape of the OSL curve as a function of stimulation temperature (Fig. 5) has been attributed by Murray and Wintle (1998) to a combination of two independent processes: thermal assistance (which is seen as a slight initial rise in curve) and thermal quenching. As the temperature is increased, thermal quenching of the luminescence recombination becomes the dominant factor, and the curve decreases steadily. However, the increase in OSE with temperature is not consistent with thermal assistance. Thermal assistance increases the rate of optical ejection of charge (and thus should affect the initial OSE) but it should not affect the total number of electrons available for eviction, and so the integrated signal should be insensitive to changes in eviction rate. In our data set, we do, however, see an increase in the integrated signal; in fact, the total electron sum (integrated over the entire decay curve) closely matches the shape of the change in the initial (0–0.5 s) part of the signal. This increase in OSE signal may arise because a larger fraction of the evicted electrons is able to overcome the workfunction at higher sample temperatures. This suggests that at least some of the initial rise in the OSL curve might be caused by processes other than thermal assistance. The thermal depletion of the 325 ◦ C trap is usually thought to account for the rapid decrease in the OSL curve > 260 ◦ C (Wintle and Murray, 1998). This model seems to be inconsistent with the behaviour of the OSE signal, which decreases over a much broader temperature range than the OSL (Fig. 6). Based on these data, the inset to Fig. 6 shows electron detrapping and luminescence sensitivity change; the latter increases by almost a factor of 20 before decreasing rapidly around 300 ◦ C. Note that these data are broadly similar to those shown by Murray and Wintle (1998); (Fig. 7b, solid circles), except that their sensitivity does not decrease so rapidly above 300 ◦ C. However, their sensitivity changes were monitored using the response of the 110 ◦ C. TL peak as a surrogate, whereas our data are directly based on the OSL behaviour itself. From our data, at ∼ 300 ◦ C the charge trap is presumably completely empty and the relevant luminescence centres have all recombined, resulting in a very rapid decrease in sensitivity. This offers a possible development in the explanation for the rapid decrease in the OSL curve at > 260 ◦ C; the decrease may be a combination of trap depletion and rapid luminescence de-sensitization. Finally, the OSE and OSL dose response curves (Fig. 7) behave very similarly at low doses, but at high doses, the OSE signal saturates while the OSL signal decreases due to changes in the probability of luminescent recombination. Thus the OSL data appear to accurately track the growth of the trapped electron population at low does, but at higher doses the influence of luminescence centres becomes important. We conclude that the measurement of exo-electron signals has considerable potential to contribute to our understanding of charge movement in natural phosphors and has shown that several processes thought to be caused by trap behaviour may in fact be attributed, at least in part, to changes in luminescence centre recombination probability.
C. Ankjærgaard et al. / Radiation Measurements 43 (2008) 273 – 277
Acknowledgements This work was supported by the Joint Committee of the Nordic Natural Science Research Councils through the Nordic Centre of Excellence Programme. References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London.
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