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Spectral measurements of loess TL
Nucl. Tracks Rodiat. Mea$., Vol. 14, Nos I/2, pp. 63-72, 1988 Int. J. Radiat, Appl. Instrum., Part D
0191-278X/88 $3.00 + 0.00 Pergamon Press pk
Printed in Great Britain
SPECTRAL MEASUREMENTS OF LOESS TL H. M. RENDELL,S. J. M^NN Geography Laboratory, University of Sussex, Falmer, Brighton, BNI 9QH and P. D. TOWNSEND School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton, BN1 9QH (Received 23 November 1987)
Abstract--Variations in TL glow curves are reported for two loess samples when examined with broad band filters in the range 275-650 nm. Samples show striking differences in bleaching behaviour, when their TL emissions are observed in the u.v., blue, green and yellow spectral regions. The age estimates, given by the equivalent dose (ED) values, differ by up to a factor of two for analyses using the green and u.v. TL signals. These ED values also vary with prolonged room temperature storage between the bleaching and irradiation steps. The anomalies in the bleaching behaviour are interpreted in terms of changes in TL efficiency. The results have major implications for the regeneration method of TL dating for these fine-grained sediments and suggest that reliable dates obtained by it may be fortuitous.
1. I N T R O D U C T I O N TL MEASURE~NTS of sediments traditionally favour emissions in the blue to ultraviolet spectral region. The choice of this spectral region has undoubtedly been influenced by the fact that quartz emits strongly in the blue region (440-480 nm) and, rather more pragmatically, by the need to reject black body radiation, and by the availability of low-noise bluesensitive photomultiplier tubes operating at room temperature. The relatively low intensity of TL emissions from fine-grained sediments, compared with emissions from single crystals of feldspar or zircon, militates against the use of most 3-D spectral systems currently available. The system at Sussex, for instance, employs a scanning monochromator and f/4.5 collection optics, so by comparison with a conventional broad band filter system and f/l.0 optics, the spectral system is up to 1500 times less sensitive. To retain the f/l.0 light gathering and accept the low spectral resolution of broad band filters, the present study separately monitored TL emissions in the u.v., blue, green and yellow regions using a range of optical filters. The main aim of the study was to assess the regeneration method of TL dating, and bleaching behaviour, across a wider spectral range than in more conventional studies. Also, given that emissions from feldspars tend to dominate TL signals from fine-grain polymineral mixtures from loess, and that feldspars may also emit strongly at longer wavelengths, the choice of unconventional spectral regions might prove illuminating. Note however that this present approach with low spectral resolution has still not been extended into the red end of the spectrum even though all feldpars seem to exhibit strong emissions in the red region (Dalai et al., 1988). This study provides an interesting corollary to existing 63
studies on the bleaching effects of different wavelengths of light on TL as monitored by emissions in one spectral region (normally the blue or u.v.).
2. M E T H O D S The two loess samples chosen for analysis were from the Indian subcontinent; KB2 is from the Khanchikol I (Karapur) section in Kashmir, and RMG7 is from the Riwat area of the Potwar Plateau in northern Pakistan. The sampling sites are 150 km apart, separated by the Pir Panjal Range of the Himalayas. On the basis of their stratigraphic positions both samples are relatively old. Fine-grain, 2-10 micron, polymineral fraction of the samples were prepared for TL measurements. A suspension of the fine-grain fraction in acetone was pipetted on to clean I cm diameter aluminium discs and allowed to evaporate to dryness at room temperature. Sample discs were glowed out in an argon atmosphere at a heating rate of 150°C rain -I. No initial pre-heating was used. TL emissions were measured using an EMI 9635Q photo-multiplier tube and the following range of filters: u. v. emissions Schott U G I 1 + Chance Pilkington HA3 Blue emissions Corning 5 - 58 + Chance Pilkington HA3 Green emissions Chance Pilkington O G R I + HA3 Yellow emissions Chance Piikington OY2 + HA3. The u.v., blue and green filters have their peak transmission at 325nm, 405 nm and 540nm, respectively. The yellow filter effectively excludes all
64
H . M . R E N D E L L et al.
emissions below 520 nm and reaches a peak by 600 nm. The HA3, heat-absorbing filter, has 90% transmission through the region 370-640 nm with cut-offs at 275 nm and 750 nm. The characteristics of the filter combinations used in the study are given in Fig. 1. The response of the 9635Q photomultiplier tube peaks at about 400 nm and responses declined steadily to zero beyond 600 nm. Two sets of measurements were made. First, the regeneration method was used to obtain estimates of equivalent dose (ED) for both samples. After measuring the natural TL signals (NTL) and the response to additive laboratory radiation dose (N + beta) a number of sample discs were bleached under a 300 W sunlamp for 450 min before being given additive radiation doses to build the TL signal up to a level just beyond that of the NTL. A second set of measurements was made of TL remaining after different bleaching times in order to construct bleaching curves for both samples. 3. RESULTS The resultant glow curves show pronounced differences between the samples (Figs 2a, b and c). The NTL peak for the Kashmir loess sample (K2B) occurs at about 250°C in the blue and u.v. regions and at a slightly lower temperature in the green
(242°C) whereas the NTL peak for RMG7 is at 280°C for all wavelengths measured. The Pakistan loess sample also shows a second peak, visible as a higher temperature "shoulder" on the NTL glow curve at about 375°C in the blue and u.v. regions. A sample of RMG7 was treated with hexafluorosilicic acid, to remove all but the quartz component. The resultant material had an NTL peak (u.v. emissions) at about 310°C with some indication of a peak at a higher temperature. Glow curves for bleached samples (Fig. 2b) again show major differences between the samples, with K2B showing a broad peak at 325°C in the blue and u.v. and a sharp peak at 250°C in the green, The other sample, RMG7, exhibits broad peaks at about 310°C in all three spectral regions. The shape and relative size of the residual signal for R M G 7 exerts a strong influence on the glow curve shape for the regenerated TL (Fig. 2c). The bleaching also populates shallow traps to give a peak near 150°C in both samples. The results of regeneration measurements show major differences between samples. For K2B a similar range of values was obtained for both u.v. and green emissions (Figs 3a, b). EDs for glow curve temperatures 290°C, 310°C and 330°C were 440 Gy, 465 Gy and 450 Gy in the u.v. and 380 Gy, 440 Gy and 420 Gy in the green region, respectively. Data for RMG7 given in Figs 4a and 4b, indicate different TL
OY2 +
HA3
I
o o. O
2. 200
300
400
500
600
700
Wavelength (nm)
FIG. I. Optical densities of filter combinations used in the study.
800
SPECTRAL MEASUREMENTS
O F LOESS TL
K2B N A T U R A L TL
65
R M G 7 N A T U R A L TL
20
30
16,
UV
UV
¢1
m c o
g 20
12
dBLUE YELLOW 10
BLUE
J '-
0
16o
o
260
0
360
45o Temperature (e C)
o
160
260
~
GREEN
360
460
Temperature (0 C )
(a)
RMG7 B L E A C H E D NTL
K2B B L E A C H E D NTL 16
4 5 0 rain bleach
30.
UV
UV 12
BLUE
20 .
BLUE
m ¢=
GREEN
,.J
#~'F..EN
16o
260
abe
10
0
460
160
0
260
Temperature (" C)
360 4()0 Temperature (= C)
(b)
K2B R E G E N
RMG7 REGEN
NTL + 7 . 5 h r b l e a c h + 1 8 5 G y beta
NTL + 7 . 5 h r b l e a c h + 1 8 5 G y beta
24" 12
20:
10
16 .i
m
-L2:
~6 o
8:
4.
2.
0
0
16o
260
16o
360 460 Temperature (" C)
260
3be
460
Temperature (0 C)
(c) FIG. 2. (a) Natural TL for K2B and RMG7; Co) TL emissions after optical bleaching of K2B and RMG7; and (c) rcgencratcxi TL (N + sunlamp + beta) for K2B and RMG7.
66
H.M.
RENDELL
et aL
K2B REGEN UV EMISSIONS
,~ 2 9 0 ° C
50
o 310 ° C
40
3O _J I--
20
10
100
200
300
400
I 500
i 600
Laboratory beta dose (Gy) (a)
K2B REGEN GREEN EMISSIONS
290 ° C
[] 3 1 0 o C
_J F--
i
i
i
100
200
300
"
i
~
T
400
5 0
600
L a b o r a t o r y b e t a d o s e (Gy)
(b) FIG. 3. (a) Regenerated TL, u.v. emissions K2B; and (b) regenerated TL, green emissions K2B
SPECTRAL MEASUREMENTS OF LOESS TL
67
RMG7 REGEN UV EMISSIONS
16-
C 14
10 ¢0
F=
"~ 8 --I
I-
6
4
0
0
100
200
300
400
500
600
700
800
900
10'00
Laboratory beta dose (Gy)
Ca) RMG7 REGEN GREEN EMISSIONS
30'
C
300" C
Y
20
.J
10.
o
o
1~o
200
300
400
500
600
~0o
8~o
Laboratory beta dose (Gy)
(b) FIo. 4. (a) Regenerated TL, u.v. emissions RMGT; and (b) regenerated TL, green emissions RMGT.
68
H . M . R E N D E L L et al.
responses, and different EDs for the spectral regions examined, with EDs in the green region of the order of 50% of those in the u.v. region. A further set of measurements~ with a 5 week delay introduced between bleaching and subsequent irradiation, gave a measurable increase in ED in the green, but no change, within the errors, for the u.v. emissions. The data are summarised in Table 1. The bleaching behaviour of the two samples differs in a radical manner. Whereas the amount of TL remaining after a given bleaching time (expressed as a percentage of the NTL) shows a rapid initial decline and then a more gradual change for u.v. and green emissions for K2B (Figs 5a, b), the initial change for green and yellow emissions for RMG7 is slow and is then followed by a more rapid decline (Figs 6a, b). The reverse is true for the blue and u.v. emissions from the same sample. The data suggest that as many as three distinct decay stages may occur and lines are sketched to emphasise this. Data for signal levels in the different spectral regions after a 450 min bleach are summarized in Table 2.
value, but had no apparent bleaching effect on the green emission. One might argue that regenerations will underestimate the ED value if bleaching is not carried to completion, and because the green and u.v. signals have reached different degrees of bleaching, the less bleached signal will give a greater underestimate of the ED. For sample K2B, this argument is tempting as the green signals bleach more slowly and give ED values ca 10% less than those measured with the u.v. filter. For RMG7 the ED values obtained for green emissions at 270°C and 310°C are only 57% and 63% of the ED values obtaining using u.v. emissions. Although the greater percent bleaching was noted for the 270°C green signal one cannot readily use these values to predict a correction factor since the form of the bleaching curves differs at the two temperatures. Note also that, as discussed later, the ED value is a function of length of storage time, at room temperature, between bleaching and subsequent laboratory irradiation, and this further complicates the analysis. The use of the regeneration method for samples with such bleaching characteristics is questionable. The resolution of the problems presented by the bleaching and regeneration behaviour of the two fine-grain polymineral mixtures considered in this study is fraught with difficulties. Even if the samples were simply composed of grains of orthoclase feldspar there is no reason to expect that the emission spectra of the individual grains would be identical (see Dalai et al., 1988). Nevertheless, faced with the data available, however imperfect, we may attempt to explain the observed behaviour in terms of a simple model based on the assumption that all TL emission features are common to a dominant TL emitter. With these caveats, one possible explanation of the bleaching behaviour of RMG7 is now discussed with reference to Fig. 7 in which a simplified set of electron traps and recombination centres is considered. The normal situation after natural or
4. DISCUSSION The two loess samples exhibit distinct differences in position and number of peaks in their natural and bleached TL signals, and whereas EDs obtained for K2B are in tolerable agreement (ie. _ 12%) for the u.v. and green spectral regions, those for RMG7 are not. In addition RMG7 exhibits radically different bleaching behaviour in the different spectral regions monitored. It is clear from the bleaching curves that the natural TL does not fall to zero or to a constant value with the bleaching times used here. These data exemplify the problem of defining the "completeness" of bleaching. Such complex decay of signals has analytical implications for all TL dating strategies. For example, with RMG7 a short bleach of 5 min reduced the u.v. signal at 270°C to 20% of the natural
Table 1. Summary of ED values (Gy) obtained by regeneration K2B
RMG7
Temperature (°C)
Green
290 310 330
380 440 420
u.v.
Temperature (°C)
Green
Green (delay)
u.v.
440 465 450
270 310 330
466 555 462
740 925 930
805 905 905
Errors are: __+10% for K2B (green, u.v.) and RMG7 (u.v.); +20% for RMG7 (green). Table 2. Summary of level of residual signal, after 450 min bleach, as a percentage of Natural TL K2B
RMG7
Temperature (°C)
u.v.
Green
u.v.
Blue
Green
Yellow
270 300
5.3 9.8
13.2 22.2
5.2 7.5
19.0 24.3
27.2 55.0
37.9 59.0
SPECTRAL
MEASUREMENTS
OF LOESS
TL
69
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SPECTRAL M E A S U R E M E N T S O F LOESS TL
71
Conduction Band
Tt
Y
T
Y B
T E
~
B
E
F2
F1
Valence Band
FIG. 7. Simple model of changes in recombination routes induced by optical bleaching.
laboratory irradiation is that the electron traps capture charge at level T (a set of hole traps will also be filled. At this stage the traps T are full and are above the normal quasi-Fermi level EFt; and both the recombination centres Y and B are empty. On heating the trap T relaxes and the configuration energy diagram shifts the ground state to T* so that it thermally empties. Electrons wander through the conduction band and fall into recombination centres Y or B giving yellow or blue emission. Emission may occur as a radiative transition from the conduction band or as a secondary step within the recombination centres. Hence the energy spacing of the B a n d Y bands is not known. When the sample is optically bleached there is no possibility of ionisation as the photon energy is much less than the band gap, however, both electrons and holes can be redistributed among the various traps and if the light favours the release of only one charge carrier then this may result in a charge capture at B and an upward shift in the Fermi level to E~2. On the next TL run T moves to T* and releases an electron, but the path to B is not favourable so only the Y route remains (excluding the non-radiative routes and the release of holes). The net effect is that the "apparent" TL signal monitored by blue light is reduced. The Y signal is only slightly reduced as the Fermi level is still beneath it. However, on further bleaching the Fermi level can rise up into the Y region and then the rate of yellow TL will decay more rapidly with further bleaching. Overall, this model gives a blue decay curve with a fast and a slow stage in contrast with the yellow one which surprisingly has a slow first stage and a fast second stage. This model is simplified so as to ignore the wide range of other possible traps or stimulated migration of defects. It is worth noting
that Fermi-Dirac statistics do not give such a sharp cut off between empty or full traps with shifts in the Fermi levels as described above, and this distribution function explains the presence of reduced blue and yellow signals in different proportions. The model described here emphasises that measurement of the TL intensity does not immediately give the number of trapped charges which are released from shallow traps. Rather, the signal is a product of the number of liberated charges and an efficiency term which measures light production at a particular wavelength. The efficiency may change if the ratio of radiative to non-radiative decay paths is altered. In the present case, the blue emission is quenched faster than the green signal. Changes in TL efficiency may also occur if non-radiative recombination centres are converted to radiative ones, as discussed by Rendell and Townsend (1988) as a possible explanation of the failure of the regeneration method for older loess samples from Pakistan. The major problem with TL analyses is that one is concerned with a complex range of defect sites and the bleaching not only empties traps but also fills them, as shown by Fig. 2b. Further, these new charge distributions may not be stable. In the present case with sample R M G 7 the regeneration curve of the green signal underwent a noticeable change with room temperature storage between the bleaching and irradiation steps. Whilst immediate irradiation and TL measurement after bleaching gave an ED value of 462 Gy at 330°C, a 5 week storage increased the ED to 930Gy. This trend has been noted for other samples. Once again one must question these ED values as whilst the blue or u.v. signals corresponded to a well bleached (i.e. 90%) state, the yellow/green signals did not (only 50% N T L bleached). Yet both
72
H . M . R E N D E L L et al.
wavelength regions gave similar values of ED for stored samples. This problem may be resolved if one appreciates that the TL signal for the u.v./blue has decreased in a different manner from the green signal because of a change in TL efficiency at these wavelengths. In fact the population of the original traps which produce the 300°C TL signal is not directly monitored at any stage and the optical bleaching of these traps is merely an assumption, albeit a sensible one. Indeed, the only conclusion to be drawn from the bleaching data is that the efficiency of light production is reduced. This could be solely due to changes in the types of recombination centre, without any change in the populations of the traps which are emptied during TL. The major factor to note is that ED values obtained by the regeneration method for the fine grain polymineral mixtures considered in this study are strongly dependent on the experimental technique
employed. The ED values may vary by a factor of 2 for the same sample, as is the case for sample RMG7. One must assume that in those cases where the regeneration method gives consistent results, with dates confirmed by other methods, that there is a fortuitous combination of choice of experimental conditions and nature of defect states in the material dated. Acknowledgements--The financial support of SERC Grants
GR/C/56399 and GR/D/31805 is gratefully acknowledged. REFERENCES Dalai M., Kirsh Y., Rendell H. M. and Townsend P. D. (1988) TL emission spectra of natural feldspar. Nucl. Tracks 14, 57-62. Rendell H. M. and Townsend P. D. (1988) Thermoluminescence dating of a 10m loess section in Pakistan. Quatern. Sci. Rev. 7.
0191-278X/88 $3.00 + 0.00 Pergamon Press pk
Printed in Great Britain
SPECTRAL MEASUREMENTS OF LOESS TL H. M. RENDELL,S. J. M^NN Geography Laboratory, University of Sussex, Falmer, Brighton, BNI 9QH and P. D. TOWNSEND School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton, BN1 9QH (Received 23 November 1987)
Abstract--Variations in TL glow curves are reported for two loess samples when examined with broad band filters in the range 275-650 nm. Samples show striking differences in bleaching behaviour, when their TL emissions are observed in the u.v., blue, green and yellow spectral regions. The age estimates, given by the equivalent dose (ED) values, differ by up to a factor of two for analyses using the green and u.v. TL signals. These ED values also vary with prolonged room temperature storage between the bleaching and irradiation steps. The anomalies in the bleaching behaviour are interpreted in terms of changes in TL efficiency. The results have major implications for the regeneration method of TL dating for these fine-grained sediments and suggest that reliable dates obtained by it may be fortuitous.
1. I N T R O D U C T I O N TL MEASURE~NTS of sediments traditionally favour emissions in the blue to ultraviolet spectral region. The choice of this spectral region has undoubtedly been influenced by the fact that quartz emits strongly in the blue region (440-480 nm) and, rather more pragmatically, by the need to reject black body radiation, and by the availability of low-noise bluesensitive photomultiplier tubes operating at room temperature. The relatively low intensity of TL emissions from fine-grained sediments, compared with emissions from single crystals of feldspar or zircon, militates against the use of most 3-D spectral systems currently available. The system at Sussex, for instance, employs a scanning monochromator and f/4.5 collection optics, so by comparison with a conventional broad band filter system and f/l.0 optics, the spectral system is up to 1500 times less sensitive. To retain the f/l.0 light gathering and accept the low spectral resolution of broad band filters, the present study separately monitored TL emissions in the u.v., blue, green and yellow regions using a range of optical filters. The main aim of the study was to assess the regeneration method of TL dating, and bleaching behaviour, across a wider spectral range than in more conventional studies. Also, given that emissions from feldspars tend to dominate TL signals from fine-grain polymineral mixtures from loess, and that feldspars may also emit strongly at longer wavelengths, the choice of unconventional spectral regions might prove illuminating. Note however that this present approach with low spectral resolution has still not been extended into the red end of the spectrum even though all feldpars seem to exhibit strong emissions in the red region (Dalai et al., 1988). This study provides an interesting corollary to existing 63
studies on the bleaching effects of different wavelengths of light on TL as monitored by emissions in one spectral region (normally the blue or u.v.).
2. M E T H O D S The two loess samples chosen for analysis were from the Indian subcontinent; KB2 is from the Khanchikol I (Karapur) section in Kashmir, and RMG7 is from the Riwat area of the Potwar Plateau in northern Pakistan. The sampling sites are 150 km apart, separated by the Pir Panjal Range of the Himalayas. On the basis of their stratigraphic positions both samples are relatively old. Fine-grain, 2-10 micron, polymineral fraction of the samples were prepared for TL measurements. A suspension of the fine-grain fraction in acetone was pipetted on to clean I cm diameter aluminium discs and allowed to evaporate to dryness at room temperature. Sample discs were glowed out in an argon atmosphere at a heating rate of 150°C rain -I. No initial pre-heating was used. TL emissions were measured using an EMI 9635Q photo-multiplier tube and the following range of filters: u. v. emissions Schott U G I 1 + Chance Pilkington HA3 Blue emissions Corning 5 - 58 + Chance Pilkington HA3 Green emissions Chance Pilkington O G R I + HA3 Yellow emissions Chance Piikington OY2 + HA3. The u.v., blue and green filters have their peak transmission at 325nm, 405 nm and 540nm, respectively. The yellow filter effectively excludes all
64
H . M . R E N D E L L et al.
emissions below 520 nm and reaches a peak by 600 nm. The HA3, heat-absorbing filter, has 90% transmission through the region 370-640 nm with cut-offs at 275 nm and 750 nm. The characteristics of the filter combinations used in the study are given in Fig. 1. The response of the 9635Q photomultiplier tube peaks at about 400 nm and responses declined steadily to zero beyond 600 nm. Two sets of measurements were made. First, the regeneration method was used to obtain estimates of equivalent dose (ED) for both samples. After measuring the natural TL signals (NTL) and the response to additive laboratory radiation dose (N + beta) a number of sample discs were bleached under a 300 W sunlamp for 450 min before being given additive radiation doses to build the TL signal up to a level just beyond that of the NTL. A second set of measurements was made of TL remaining after different bleaching times in order to construct bleaching curves for both samples. 3. RESULTS The resultant glow curves show pronounced differences between the samples (Figs 2a, b and c). The NTL peak for the Kashmir loess sample (K2B) occurs at about 250°C in the blue and u.v. regions and at a slightly lower temperature in the green
(242°C) whereas the NTL peak for RMG7 is at 280°C for all wavelengths measured. The Pakistan loess sample also shows a second peak, visible as a higher temperature "shoulder" on the NTL glow curve at about 375°C in the blue and u.v. regions. A sample of RMG7 was treated with hexafluorosilicic acid, to remove all but the quartz component. The resultant material had an NTL peak (u.v. emissions) at about 310°C with some indication of a peak at a higher temperature. Glow curves for bleached samples (Fig. 2b) again show major differences between the samples, with K2B showing a broad peak at 325°C in the blue and u.v. and a sharp peak at 250°C in the green, The other sample, RMG7, exhibits broad peaks at about 310°C in all three spectral regions. The shape and relative size of the residual signal for R M G 7 exerts a strong influence on the glow curve shape for the regenerated TL (Fig. 2c). The bleaching also populates shallow traps to give a peak near 150°C in both samples. The results of regeneration measurements show major differences between samples. For K2B a similar range of values was obtained for both u.v. and green emissions (Figs 3a, b). EDs for glow curve temperatures 290°C, 310°C and 330°C were 440 Gy, 465 Gy and 450 Gy in the u.v. and 380 Gy, 440 Gy and 420 Gy in the green region, respectively. Data for RMG7 given in Figs 4a and 4b, indicate different TL
OY2 +
HA3
I
o o. O
2. 200
300
400
500
600
700
Wavelength (nm)
FIG. I. Optical densities of filter combinations used in the study.
800
SPECTRAL MEASUREMENTS
O F LOESS TL
K2B N A T U R A L TL
65
R M G 7 N A T U R A L TL
20
30
16,
UV
UV
¢1
m c o
g 20
12
dBLUE YELLOW 10
BLUE
J '-
0
16o
o
260
0
360
45o Temperature (e C)
o
160
260
~
GREEN
360
460
Temperature (0 C )
(a)
RMG7 B L E A C H E D NTL
K2B B L E A C H E D NTL 16
4 5 0 rain bleach
30.
UV
UV 12
BLUE
20 .
BLUE
m ¢=
GREEN
,.J
#~'F..EN
16o
260
abe
10
0
460
160
0
260
Temperature (" C)
360 4()0 Temperature (= C)
(b)
K2B R E G E N
RMG7 REGEN
NTL + 7 . 5 h r b l e a c h + 1 8 5 G y beta
NTL + 7 . 5 h r b l e a c h + 1 8 5 G y beta
24" 12
20:
10
16 .i
m
-L2:
~6 o
8:
4.
2.
0
0
16o
260
16o
360 460 Temperature (" C)
260
3be
460
Temperature (0 C)
(c) FIG. 2. (a) Natural TL for K2B and RMG7; Co) TL emissions after optical bleaching of K2B and RMG7; and (c) rcgencratcxi TL (N + sunlamp + beta) for K2B and RMG7.
66
H.M.
RENDELL
et aL
K2B REGEN UV EMISSIONS
,~ 2 9 0 ° C
50
o 310 ° C
40
3O _J I--
20
10
100
200
300
400
I 500
i 600
Laboratory beta dose (Gy) (a)
K2B REGEN GREEN EMISSIONS
290 ° C
[] 3 1 0 o C
_J F--
i
i
i
100
200
300
"
i
~
T
400
5 0
600
L a b o r a t o r y b e t a d o s e (Gy)
(b) FIG. 3. (a) Regenerated TL, u.v. emissions K2B; and (b) regenerated TL, green emissions K2B
SPECTRAL MEASUREMENTS OF LOESS TL
67
RMG7 REGEN UV EMISSIONS
16-
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68
H . M . R E N D E L L et al.
responses, and different EDs for the spectral regions examined, with EDs in the green region of the order of 50% of those in the u.v. region. A further set of measurements~ with a 5 week delay introduced between bleaching and subsequent irradiation, gave a measurable increase in ED in the green, but no change, within the errors, for the u.v. emissions. The data are summarised in Table 1. The bleaching behaviour of the two samples differs in a radical manner. Whereas the amount of TL remaining after a given bleaching time (expressed as a percentage of the NTL) shows a rapid initial decline and then a more gradual change for u.v. and green emissions for K2B (Figs 5a, b), the initial change for green and yellow emissions for RMG7 is slow and is then followed by a more rapid decline (Figs 6a, b). The reverse is true for the blue and u.v. emissions from the same sample. The data suggest that as many as three distinct decay stages may occur and lines are sketched to emphasise this. Data for signal levels in the different spectral regions after a 450 min bleach are summarized in Table 2.
value, but had no apparent bleaching effect on the green emission. One might argue that regenerations will underestimate the ED value if bleaching is not carried to completion, and because the green and u.v. signals have reached different degrees of bleaching, the less bleached signal will give a greater underestimate of the ED. For sample K2B, this argument is tempting as the green signals bleach more slowly and give ED values ca 10% less than those measured with the u.v. filter. For RMG7 the ED values obtained for green emissions at 270°C and 310°C are only 57% and 63% of the ED values obtaining using u.v. emissions. Although the greater percent bleaching was noted for the 270°C green signal one cannot readily use these values to predict a correction factor since the form of the bleaching curves differs at the two temperatures. Note also that, as discussed later, the ED value is a function of length of storage time, at room temperature, between bleaching and subsequent laboratory irradiation, and this further complicates the analysis. The use of the regeneration method for samples with such bleaching characteristics is questionable. The resolution of the problems presented by the bleaching and regeneration behaviour of the two fine-grain polymineral mixtures considered in this study is fraught with difficulties. Even if the samples were simply composed of grains of orthoclase feldspar there is no reason to expect that the emission spectra of the individual grains would be identical (see Dalai et al., 1988). Nevertheless, faced with the data available, however imperfect, we may attempt to explain the observed behaviour in terms of a simple model based on the assumption that all TL emission features are common to a dominant TL emitter. With these caveats, one possible explanation of the bleaching behaviour of RMG7 is now discussed with reference to Fig. 7 in which a simplified set of electron traps and recombination centres is considered. The normal situation after natural or
4. DISCUSSION The two loess samples exhibit distinct differences in position and number of peaks in their natural and bleached TL signals, and whereas EDs obtained for K2B are in tolerable agreement (ie. _ 12%) for the u.v. and green spectral regions, those for RMG7 are not. In addition RMG7 exhibits radically different bleaching behaviour in the different spectral regions monitored. It is clear from the bleaching curves that the natural TL does not fall to zero or to a constant value with the bleaching times used here. These data exemplify the problem of defining the "completeness" of bleaching. Such complex decay of signals has analytical implications for all TL dating strategies. For example, with RMG7 a short bleach of 5 min reduced the u.v. signal at 270°C to 20% of the natural
Table 1. Summary of ED values (Gy) obtained by regeneration K2B
RMG7
Temperature (°C)
Green
290 310 330
380 440 420
u.v.
Temperature (°C)
Green
Green (delay)
u.v.
440 465 450
270 310 330
466 555 462
740 925 930
805 905 905
Errors are: __+10% for K2B (green, u.v.) and RMG7 (u.v.); +20% for RMG7 (green). Table 2. Summary of level of residual signal, after 450 min bleach, as a percentage of Natural TL K2B
RMG7
Temperature (°C)
u.v.
Green
u.v.
Blue
Green
Yellow
270 300
5.3 9.8
13.2 22.2
5.2 7.5
19.0 24.3
27.2 55.0
37.9 59.0
SPECTRAL
MEASUREMENTS
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FIG. 7. Simple model of changes in recombination routes induced by optical bleaching.
laboratory irradiation is that the electron traps capture charge at level T (a set of hole traps will also be filled. At this stage the traps T are full and are above the normal quasi-Fermi level EFt; and both the recombination centres Y and B are empty. On heating the trap T relaxes and the configuration energy diagram shifts the ground state to T* so that it thermally empties. Electrons wander through the conduction band and fall into recombination centres Y or B giving yellow or blue emission. Emission may occur as a radiative transition from the conduction band or as a secondary step within the recombination centres. Hence the energy spacing of the B a n d Y bands is not known. When the sample is optically bleached there is no possibility of ionisation as the photon energy is much less than the band gap, however, both electrons and holes can be redistributed among the various traps and if the light favours the release of only one charge carrier then this may result in a charge capture at B and an upward shift in the Fermi level to E~2. On the next TL run T moves to T* and releases an electron, but the path to B is not favourable so only the Y route remains (excluding the non-radiative routes and the release of holes). The net effect is that the "apparent" TL signal monitored by blue light is reduced. The Y signal is only slightly reduced as the Fermi level is still beneath it. However, on further bleaching the Fermi level can rise up into the Y region and then the rate of yellow TL will decay more rapidly with further bleaching. Overall, this model gives a blue decay curve with a fast and a slow stage in contrast with the yellow one which surprisingly has a slow first stage and a fast second stage. This model is simplified so as to ignore the wide range of other possible traps or stimulated migration of defects. It is worth noting
that Fermi-Dirac statistics do not give such a sharp cut off between empty or full traps with shifts in the Fermi levels as described above, and this distribution function explains the presence of reduced blue and yellow signals in different proportions. The model described here emphasises that measurement of the TL intensity does not immediately give the number of trapped charges which are released from shallow traps. Rather, the signal is a product of the number of liberated charges and an efficiency term which measures light production at a particular wavelength. The efficiency may change if the ratio of radiative to non-radiative decay paths is altered. In the present case, the blue emission is quenched faster than the green signal. Changes in TL efficiency may also occur if non-radiative recombination centres are converted to radiative ones, as discussed by Rendell and Townsend (1988) as a possible explanation of the failure of the regeneration method for older loess samples from Pakistan. The major problem with TL analyses is that one is concerned with a complex range of defect sites and the bleaching not only empties traps but also fills them, as shown by Fig. 2b. Further, these new charge distributions may not be stable. In the present case with sample R M G 7 the regeneration curve of the green signal underwent a noticeable change with room temperature storage between the bleaching and irradiation steps. Whilst immediate irradiation and TL measurement after bleaching gave an ED value of 462 Gy at 330°C, a 5 week storage increased the ED to 930Gy. This trend has been noted for other samples. Once again one must question these ED values as whilst the blue or u.v. signals corresponded to a well bleached (i.e. 90%) state, the yellow/green signals did not (only 50% N T L bleached). Yet both
72
H . M . R E N D E L L et al.
wavelength regions gave similar values of ED for stored samples. This problem may be resolved if one appreciates that the TL signal for the u.v./blue has decreased in a different manner from the green signal because of a change in TL efficiency at these wavelengths. In fact the population of the original traps which produce the 300°C TL signal is not directly monitored at any stage and the optical bleaching of these traps is merely an assumption, albeit a sensible one. Indeed, the only conclusion to be drawn from the bleaching data is that the efficiency of light production is reduced. This could be solely due to changes in the types of recombination centre, without any change in the populations of the traps which are emptied during TL. The major factor to note is that ED values obtained by the regeneration method for the fine grain polymineral mixtures considered in this study are strongly dependent on the experimental technique
employed. The ED values may vary by a factor of 2 for the same sample, as is the case for sample RMG7. One must assume that in those cases where the regeneration method gives consistent results, with dates confirmed by other methods, that there is a fortuitous combination of choice of experimental conditions and nature of defect states in the material dated. Acknowledgements--The financial support of SERC Grants
GR/C/56399 and GR/D/31805 is gratefully acknowledged. REFERENCES Dalai M., Kirsh Y., Rendell H. M. and Townsend P. D. (1988) TL emission spectra of natural feldspar. Nucl. Tracks 14, 57-62. Rendell H. M. and Townsend P. D. (1988) Thermoluminescence dating of a 10m loess section in Pakistan. Quatern. Sci. Rev. 7.