Radiation Measurements 32 (2000) 647±652
www.elsevier.com/locate/radmeas
Changes in OSL properties of quartz by preheating: an
interpretation
E. Vartanian*, P. Guibert, C. Roque, F. Bechtel, M. Schvoerer
Centre de Recherche en Physique AppliqueÂe aÁ l'ArcheÂologie (CRPAA), Universite Michel de Montaigne Bordeaux III-CNRS, Maison
de l'archeÂologie, F 33607 Pessac Cedex, France
Received 20 October 1999; received in revised form 2 March 2000; accepted 24 April 2000
Abstract
A study of OSL variation with preheat temperature showed, in a majority of cases, that OSL recorded at room
temperature, increases above 2008C before the normal drainage at higher temperature. To explain this behaviour, an
alternative interpretation to the common `electronic thermal transfer' mechanism is suggested, supported by a study
of hydrothermally grown quartz crystals. This interpretation involves impurities in substitution of Si4+, specially
Al3+, which are associated with species like, in the case of quartz, hydrogen (H+, in fact, OHÿ) and alkali ions
(Li+, Na+, K+). These monovalent ions usually act as charge compensators and are mobile during heating. As a
consequence of the mobility and a possible irreversible exchange between compensators, the number of radiative
recombination centres associated with the OSL trap(s), observable within the detection spectral window used (250±
400 nm), increases during preheating. This phenomenon could lead to a wrong ED determination. 7 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: OSL; Preheating; Quartz; Change compensator; Luminogen centres
1. Introduction
For dating purposes, a preheat is necessary to
remove unstable components of OSL. During preheat,
charges from shallow traps are thermally drained so
that only appropriate long life trapped charges remain.
Diverse preheat conditions, temperature and duration,
have already been adopted: for example, 5 min at
2208C (Rhodes, 1988; Roberts et al., 1994) or 16 h at
1608C (Stokes, 1992, 1996). In practice, adequate pre-
* Corresponding author. Tel.: +33-5-571-24553; fax: +335-571-24550.
E-mail address: [email protected] (E. Vartanian).
heat parameters need to be determined during preliminary studies for any sample to be dated. The
corresponding experiments are carried out with archaeological artefacts which have been anciently heated
(e.g. ceramics or baked earth). Frequently, they exhibit
a signi®cant increase of OSL intensity with preheat
temperature (in the range 200±2508C) with either naturally or naturally plus laboratory irradiated material.
This increase, already observed (Rhodes, 1988), is
sometimes extremely important, up to 400% (Guibert
et al., 2000). Its extent and frequency among the
archaeological heated samples studied led us to search
for a consistent interpretation linked to experimental
facts Ð for example, after a thermal annealing at
5008C and a reirradiation, an increase of OSL intensity
is still observed, but its extent is less important and
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 0 9 - 8
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E. Vartanian et al. / Radiation Measurements 32 (2000) 647±652
Table 1
Sample description, synthesis conditions and measured impurity concentrations in hydrothermally grown quartz. Measurements
were performed by ICP-MS and IR spectrometry for OH. The term a3500 = (IR absorbance at 3500 cmÿ1 ÿ IR absorbance at
3800 cmÿ1)/(sample thickness), is proportional to the OH concentration
Samples
reference
E 35
EG 15
ED 6
ED 7
ED 9
Synthesis conditions
P ˆ 1400 bar
Tcr ˆ 3488C; DT ˆ 258C
NaOH + NaF
P ˆ 1200 bar
Tcr ˆ 4188C; DT ˆ 168C
NaOH
P ˆ 2000 bar
Tcr 13608C; DT ˆ 228C
NaOH
P11500 bar
Tcr 13578C; DT ˆ 258C
NaOH
P11000 bar Tcr 13608C;
DT ˆ 118C NaOH
Dopant
introduction
in % of Si
atoms number
Impurities concentrations (mmol/g)
None
6.91
1.76 0.24
None
4.71
2.65 0.11
[Fe]
[Cu]
a3500
1.97 0.12
0.45
0.05
0.024
3.85 0.07
0.18
0.15
0.001
2% Al
3.62 20.40 0.38 19.58 1.07
0.71
2.02
0.082
2% Al
3.84 14.29 0.30 14.75 0.44
0.30
0.06
0.058
2% Al + 2% Li
0.00
0.24
0.01
0.007
strong predose e€ects are observable (Guibert et al.,
2000).
2. Samples and instrument
Two kinds of samples were studied with OSL and
TL: (i) ®ne grains (3±12 mm) from archaeological ceramics, the major luminescent component of which is
found to be quartz with some amount of feldspars; (ii)
paralellepipedic slides (about 0.5 cm in length and
about 0.5 mm in thickness) cut from hydrothermally
grown synthetic quartz, described in Table 1.
OSL measurements were performed with a variable
wavelength home-made device (Vartanian, 1999; Guibert et al., 2000). The stimulation wavelength was ®xed
at 450210 nm. OSL is detected in the range 250±400
nm (combination of the spectral transmittance of optical ®lters, 2 Schott DUG11, and the spectral eciency
of photomultiplier EMI 9813 QKA).
Impurities in the synthetic quartz crystals have been
determined by ICP-MS (Perkin Elmer ICP/6500 apparatus) after dissolution of crystals in HF 40%, then in
HNO3 (4 N). IR spectrometry measurements (Nicholet
740 spectrometer) have been carried out at room temperature on polished slides.
3. The thermal transfer assumption in question
The `thermal transfer' assumption corresponds to a
[B]
[Al]
[Li] [Na]
1.74 1.63
[K]
Relative intensity
due to OH groups
at 3500 cmÿ1
0.63 0.02
transfer of charge from shallow and non photosensitive
traps to light sensitive thermally stable traps, induced
by preheating (Aitken and Smith, 1988; Rhodes, 1988).
This interpretation implies that regenerating an OSL
signal after optical bleaching followed by preheating is
possible for samples which exhibit an increase of OSL
with preheat temperature. In order to check the existence of a thermal transfer, the following experiment
has been carried out. As an example of many observations in archaeological ceramics or other heated materials, naturally irradiated aliquots of grains from a
Bronze Age ceramic sample (Piano di Sorrento-La Trinita, Italy) were used. A constant b dose (33 Gy) had
Fig. 1. Comparison of TL intensities before and after optical
bleaching on natural plus 33 Gy b irradiated aliquots (ceramic
from the Bronze Age site of Piano di Sorrento-La Trinita).
There is a reduction of TL intensity by a factor of 10 between
150 and 2408C.
E. Vartanian et al. / Radiation Measurements 32 (2000) 647±652
Fig. 2. Correlation between trivalent ions concentration ([Al]
+ [B]) and monovalent ions of alkali type ([Li] + [Na] +
[K]).
649
Fig. 4. Correlation between OSL intensity and the term Y =
[Al][M]/a3500[Fe], with [M] = [Li] + [Na] + [K].
been given to each aliquot: (i) step 1, preheat study
and evaluation of OSL increase, (increase of OSL:
53:724:5 c/s); (ii) step 2, optical bleaching, measurement of OSL …2121 c/s), followed by a preheating at
2508C for 2 min and second measurement of OSL
…2121 c/s): no di€erence is observed which means that
thermal transfer is not detected. However, taking into
account the statistical uncertainties at the 95% probability level, a thermally transferred OSL should be
less than 3 c/s; (iii) step 3, TL measurements on
bleached and unbleached aliquots: in order to predict
the OSL increase assuming that thermal transfer acts
on the unbleachable components, TL intensities in the
low temperature region (less than 2408C) are compared
(Fig. 1). The reduction to a tenth by optical bleaching,
restricts by the same proportion the intensity of a possible thermal transfer (by preheating at 2508C), in comparison with the unbleached material. If thermal
transfer of charges occurred during step 2, we should
have observed an increase of OSL by 520:5 c/s Ð i.e.
15025 c/s (OSL increase, in step 1) 1/10 (bleaching
reduction factor of low temperature trapped charge
population, step 3) Ð which would have been detect-
able. In practice, no variation of the signal was
observed, leading to the conclusion that the solely thermal transfer assumption is unable to explain the
increase of OSL by preheating.
We propose in this paper an alternative interpretation for this phenomenon frequently observed on
quartz. For this purpose, we tried to identify the
defects involved in the OSL sensitivity changes,
specially the radiative recombination centres, exploiting
experimental data obtained from hydrothermally
grown synthetic quartz.
Fig. 3. Correlation between trivalent ions concentration ([Al]
+ [B]) and a3500.
Fig. 5. Correlation between 1108C TL peak intensity and the
term Y = [Al][M]/a3500[Fe], with [M] = [Li] + [Na] + [K].
4. A study of hydrothermally grown synthetic quartz
crystals: correlation between luminescence and impurities
Among the defects in quartz suspected to be active
in electronic processes, those associated to aluminium
impurities seem to be particularly involved in the UV
emission, specially around 380 nm thus in the range
250±400 nm (Stevens Kalce€ and Phillips, 1995). Synthetic quartz crystals have been then studied as physical models, attempting to explain and interpret the
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E. Vartanian et al. / Radiation Measurements 32 (2000) 647±652
OSL behaviour observed on archaeological material.
Description, synthesis conditions, ICP-MS and IR
spectrometry measurements are reported in Table 1.
4.1. Al impurities
Correlations between the molar concentrations of
the main trivalent impurities ([Al] + [B]) and both the
total alkali concentration …‰MŠ ˆ ‰Li Š ‡ ‰NaŠ ‡ ‰KŠ† in
one hand (Fig. 2), and the hydroxyl ion [OHÿ] concentration via the IR term a3500 on the other Ð which is
equal to (IR absorbance at 3500 cmÿ1 ÿ IR absorbance at 3800 cmÿ1)/(sample thickness), and proportional to the OH concentration (Fig. 3) Ð are
observed. These correlations suggest that trivalent
impurities (Al3+ and B3+) in substitution of Si4+, are
linked to monovalent ions (alkali or hydrogen) in
order to compensate for the negative charge in excess.
Note that the hydrogen ion is necessarily linked to an
oxygen ion and it forms a hydroxyl ion. As a consequence, two main kinds of Al defects are considered
according to the nature of the compensator: [Al3+,
H+] and [Al3+, M+] with M+ being an alkali ion.
4.2. Al impurities and luminescence (within the 250±400
nm range)
We have studied the recombination centres involved
in the luminescence processes by OSL and TL with the
1108C peak, assuming that both phenomena are correlated (Stoneham and Stokes, 1991).
Relations between the concentration of impurities
and the luminescence intensities (OSL and TL) have
been drawn up as general tendencies. A relationship
between the OSL or the 1108C TL peak intensity, and
the term Y = [Al][M]/a3500[Fe] ([M] = alkali), can be
observed (Figs. 4 and 5). The term Y [Al][M]/a3500 represents, in a ®rst approach, the concentration of alkali
compensated Al centres, assuming that the charge
compensators, alkali and hydrogen ions, are equally
distributed whatever the nature of the trivalent ion. In
fact, this concentration should be better determined by
the ratio [Al][M]/[M + H], but methods for the direct
measurement of [H] were not available. Furthermore,
we have considered that iron could be a luminescence
killer (Curie, 1960) so we introduced its concentration
into the denominator of Y. Note that the formalism
used in the term Y is the simplest way to correlate
data and seems sucient to give general tendencies.
Then, the trend is that the near-UV luminescence
increases with the Al±M concentration. This relationship means: (i) Al is necessary, and this result agrees
with many previous works (Halliburton et al., 1981;
Alonso et al., 1983; Yang and McKeever, 1990); (ii)
there is a competition between alkali ions and OHÿ.
This could be considered as follows: Al±M (i.e. [Al3+,
M+]) is the precursor of a radiative recombination
centre and AlOH (i.e. [Al3+, H+]) is the precursor of
a non-radiative recombination centre.
5. Interpretation of the OSL increase with preheat
temperature in quartz
We propose that during preheating within a temperature region where sensitisation of OSL occurs,
some of the non-radiative centres irreversibly become
radiative. So, during the optical stimulation that follows preheat, the probability of an electron recombining with a radiative centre is higher and the OSL
intensity is increased. This phenomenon can be
described by the following mechanisms:
Irradiation
Fig. 6. Correlation between the 1108C TL peak sensitisation
(ratio of the 1108C TL peak intensity before and after an irradiation of 135 Gy b followed by an annealing at 5008C (a
run at 48C/s in the TL oven in nitrogen atmosphere) and Y'
= [Al][Li]/a3500 (approximately proportional to the Al±Li
centres content).
Preheat
Al3+ defects can trap a hole (h+), giving
rise to hydrogen or alkali compensated
Al centres: [Al3+, M+] 4 [Al3+, h+] +
M+ and [Al3+, H+] 4 [Al3+, h+] +
H+. However, after a hole has been
trapped, no charge compensation is
needed, so the H+ (i.e. OHÿ) and M+
ions are able to move from their original
position by di€usion. Note that a similar
scenario is possible for other common
trivalent impurities (B3+ and Fe3+) in
substitution of Si4+.
The mobility of the initial charge compensator increases with temperature; an
irreversible replacement of H+ (OHÿ)
coming from Al centres, by alkali ions
coming from other sources (yet unidenti®ed, X3+), may occur: (X3+, M+) +
E. Vartanian et al. / Radiation Measurements 32 (2000) 647±652
Light
stimulation
H+ 4 (X3+, H+) + M+. This process
induces an increase of alkali ions in
comparison with H+ available into the
crystal, so the probability of having a
recombination with an alkali ion is
greater than before the preheating.
When this recombination occurs at an
Al centre, two kinds of Al defects are recreated: those compensated with H+
and those compensated with M+. The
re-creation of [Al3+, M+] defect leads to
the UV luminescence of quartz: …Al3‡ ,
h‡ † ‡ M‡ ‡ e ÿ
ÿ
ÿ4
ÿ
…Al3‡ ,
…from light sensitive traps†
M‡ †‡hn (250±400 nm). On the contrary,
[Al3+, H+] may not produce any luminescence within the spectral range investigated, or no luminescence at all. As the
probability to form [Al3+, M+]
increases after preheating, the OSL
intensity also increases.
This interpretation suggests that monovalent ions
should be mobile through the lattice. It is supported
by previous works and our studies: (i) mobility of the
H+ ion (i.e. OHÿ : H+ jumps from oxygen to oxygen
ions) at room temperature (Malik et al., 1981; Stevens
Kalce€ and Phillips, 1995; Hashimoto et al., 1997),
and consequently at higher temperature (Subramanian
et al., 1984; McKeever et al., 1985); (ii) mobility of
alkali ions specially along the z-axis channels (Markes
and Halliburton, 1979; Edwards and Fowler, 1982;
Jani et al., 1983; McKeever et al., 1985); furthermore,
our experiments on synthetic quartz, indicate that
lithium is the more mobile alkali ion (in accordance
with its dimensions): the correlation between the 1108C
TL peak sensitisation factor (by irradiation and
annealing at 5008C Ð a run at 48C/s in the TL oven
in nitrogen atmosphere) and the term Y ' = [Al][Li]/
a3500 (Fig. 6), shows that the increase in radiative
centre concentration is enhanced by both important
lithium and aluminium contents.
6. Conclusion
The origin of the OSL sensitivity increase in archaeological material by preheating has been investigated.
Experimental evidence leads to the rejection of the
thermal transfer mechanism assumption. Thus, according to studies of synthetic quartz crystals, our interpretation of OSL increase is based on an irreversible
augmentation of the population of radiative recombination centres: (Al-alkali). Following that mechanism,
the sensitisation of OSL can occur at each experimen-
651
tal irradiation/annealing cycle as it is observed from
anciently heated archeomaterials. For dating purposes,
as the phenomenon induced by high preheats (above
2008C during 2 min) is irreversible, care must be taken,
especially when dating procedures are based on the
comparison of natural (or natural + dose) signals and
regenerated signals (as reported in Guibert et al., 2000;
this technique is quite di€erent from SARA (Mejdahl
and Bùtter-Jensen, 1994) or SAR (Murray and
Roberts, 1998) procedures which seem to correct preheat e€ects). In that case, the OSL response to irradiation should behave di€erently between the two
experimental series: the measurement of natural (or
natural + dose) signals requires only one preheating
although the measurement of regenerated OSL generally requires that the crystals have to be preheated
again. Then, high temperature preheat could lead to a
wrong ED determination, and we suggest preheating at
a temperature suciently low to avoid sensitisation.
Acknowledgements
The synthetic quartz crystals were grown up by
P.E.Hickel at the LHP ICMCB-CNRS laboratory, directed by Professor G. Demazeau within the frame of
a common research programme. Special thanks to S.
Augagneur LPPM Bordeaux 1 and B. Desbat LSMC
Bordeaux 1, for ICP-MS and IR spectrometry
measurements, respectively.
References
Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation
after bleaching. Quat. Sci. Rev. 7, 387±393.
Alonso, P.J., Halliburton, L.E., Kohnke, E.E., Bossoli, R.B.,
1983. X-ray and luminescence in crystalline SiO2. J. Appl.
Phys. 54 (9), 5369±5375.
Curie, D., 1960. Champ cristallin et luminescence: application
de la theÂorie des groupes aÁ la luminescence cristalline.
Monographie de Chimie Physique.
Edwards, A.H., Fowler, W.B., 1982. Theory of the peroxyradical defect in a-SiO2. Phys. Rev. B 26, 6649±6660.
Guibert, P., Vartanian, E., Roque C., Bechtel, F., Schvoerer,
M., 2000. Changes in OSL properties of quartz by preheating: an interpretation. Radiat. Meas. 32, 647±652.
Halliburton, L.E., Koumvakallis, M.E., Markes, M.E.,
Martin, J.J., 1981. Radiation e€ects in Crystalline SiO2:
the role of aluminum. J. Appl. Phys. 52, 3565±3574.
Hashimoto, T., Katayama, H., Sakaue, H., Hase, H.,
Arimura, T., Ojima, T., 1997. Dependence of some radiation-induced phenomena from natural quartz on hydroxyl-impurity contents. Radiat. Meas. 27, 243±250.
Jani, M.G., Bossoli, R.B., Halliburton, L.E., 1983. Further
652
E. Vartanian et al. / Radiation Measurements 32 (2000) 647±652
characterization of the E1' center in crystalline SiO2. Phys.
Rev. B 27, 2285±2293.
Malik, D.M., Kohnke, E.E., Sibley, W.A., 1981. Low temperature thermally stimulated luminescence of high quality
quartz. J. Appl. Phys. 52, 3600±3605.
Markes, M.E., Halliburton, L.E., 1979. Defects in synthetic
quartz: radiation-induced mobility of interstitials ions. J.
Appl. Phys. 50, 8172±8180.
McKeever, S.W.S., Chen, C.Y., Halliburton, L.E., 1985.
Point defects and the pre-dose e€ect in natural quartz.
Nucl. Tracks 10, 489±495.
Mejdahl, V., Bùtter-Jensen, L., 1994. Luminescence dating of
archaeological materials using a new technique based on
single aliquot measurements. Quat. Geochron. 13 (5/7),
551±554.
Murray, A.S., Roberts, R.G., 1998. Measurement of the
equivalent dose in quartz using a regenerative-dose single
aliquot protocol. Radiat. Meas. 29, 503±515.
Rhodes, E.J., 1988. Methodological considerations in the
optical dating of quartz. Quat. Sci. Rev. 7 (3/4), 395±400.
Roberts, R.G., Spooner, N.A., Questiaux, D.G., 1994.
Paleodose underestimates caused by extended duration
pre-heats in the optical dating of quartz. Radiat. Meas. 23,
647±653.
Stevens
Kalce€,
M.A.,
Phillips,
M.R.,
1995.
Cathodoluminescence microcharacterization of defect
structure of quartz. Phys. Rev. B 55 (5), 3122±3134.
Stokes, S., 1992. Optical dating of young (modern) sediments
using quartz: results from a selection of depositional environments. Quat. Sci. Rev. 11, 153±159.
Stokes, S., 1996. Further comparisons of quartz OSL additive
dose paleodoses generated using long and short duration
pre-heats. Ancient TL 14 (1), 1±4.
Stoneham, D., Stokes, S., 1991. An investigation of the relationship between the 1108C TL peak and optically stimulated luminescence in sedimentary quartz. Nuclear Tracks
and Radiat. Meas. 18, 119±123.
Subramanian, B., Halliburton, L.E., Martin, J.J., 1984.
Radiation e€ects in crystalline SiO2: infrared absorption
from OH-related defects. J. Phys. Chem. Solids 45, 575±
579.
Vartanian, E., 1999. Ph.D. Thesis, University of Bordeaux 3.
Yang, X.H., McKeever, S.W.S., 1990. The pre-dose e€ect in
crystalline quartz. J. Phys. D: Appl. Phys. 23, 237±244.