Optically stimulated phosphorescence and optically transferred TL as a tool for dating

Quaternary Science Reviews, Vol. 7, pp. 407-410, 1988.

0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc

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OPTICALLY STIMULATED P H O S P H O R E S C E N C E AND OPTICALLY T R A N S F E R R E D TL AS A T O O L F O R DATING

G.C.W.S. Wheeler

Research Laboratory for Archaeology and the History of Art, Oxford University, 6 Keble Rd, Oxford OX1 3Q J, U.K.

Immediately after illumination of quartz or feldspar samples by green light from a laser, there is phosphorescence (optically stimulated phosphorescence, OSP), with a lifetime of a few seconds, the size of the signal being dependent on radiation dose. For both minerals there is also charge transfer into low temperature TL peaks, the amount of transfer being dependent on dose. The feasibility of using these phenomena for dating is studied,

INTRODUCTION Luminescence during bleaching of TL samples has been demonstrated as a dating tool (Huntley et al., 1985). In addition to the prompt luminescence, there is a brief phosphorescence following the cessation of illumination (Smith et al., 1986) and this paper reports on the feasibility of using this phosphorescence as a dating tool. Compared to the use of the prompt luminescence, there is the advantage that the need to filter out the light from the laser scattered by the samples is avoided, allowing a wider choice of wavelengths for observation of the signal, and also that trial of different excitation wavelengths is easier.

EXPERIMENTAL DETAILS The OSP apparatus is mounted on the same cast iron table as that used for the luminescence experiments. Two shutters are used, both of which are iris shutters of 26 mm aperture (Ilex electronic). One is placed early in the beam line, just after the beam spreader, as this has been found to reduce the amount of scattered light seen by the photomultiplier when the shutter is closed. The other shutter is in front of the photomultiplier face, and is used to shield the photomultiplier while the sample is exposed to the laser beam. Both shutters allow a small amount of light to leak through: for this reason apparatus now being constructed will use blade type shutters. Beam power at the sample is 4 mW/cm 2 with the beam-line shutter open. The beam is reflected down on to the sample at an angle of about 40° to the horizontal. The sample, in the form of a monolayer of 100 p~m grains, is held on a rhodium-plated copper disc of 10 mm diameter, and is held in place by the residue of light silicone oil (Rfisch Silkospray). The rhodium surface and the silicone oil have been found to give the minimum interfering signals. The disc is laid horizontally on a standard nichrome heater strip, which has a cooled glycol/water mixture circulated in its support in order to allow

experimentation below room temperature. Above the disc, at a separation of about 12 ram, are two polished quartz (spectrosil) rods of 10 mm diameter, used as light guides. These are coaxial, with the second shutter between them. The gaps between the rods, and between the upper rod and the window of the photomultiplier housing are about 1 mm. The rods are supported by single 'O' rings. The light guides transmit about 80% of the light incident upon them, minimising the loss of photon counts. This is important since the total number of counts from one observation is often less than 50. The shutters are electronically controlled to avoid the possibility of their both being open at the same time. Normally the shutter in the laser path was opened for 1 sec, and the other shutter opened after a delay of 0.1 sec after the other had shut to allow time for complete closure. The photomultiplier used was an EMI 9635QA, having a vapour-blasted front window to improve optical coupling, and no filters. The dark count was 7.5-8.2 Hz. Measurement of the wavelength of emissions was by Corning glass sharp-cut filters successively placed between the top light guide and the photomultiplier. Each of these transmits only beyond a certain wavelength, the 'cut'. The range of filters used had cuts spaced at 10-30 nm intervals in the range 320-600 nm.

OBSERVATIONS The following sequence was normally used to measure OSP. The sample was given a [3 dose when required; electrons in traps not stable over archaeological periods were then removed by holding the sample at 200°C for 2 min. The sample was then placed on the heater strip of the measurement chamber. It was necessary that the sample should be placed at the same azimuthal angle to the beam every time it was placed in the chamber, so that individual crystals were exposed to consistent light intensities: this was accomplished by

407

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G.C.W.S. Wheeler

scratching the sample monolayer and always placing the disc so that the scratch pointed in one particular direction. The sample was then given a 1 sec exposure to the laser, and the phosphorescence was observed until it was not detectable above background (usually about 3 sec). The units of OSP are photon counts.

Because the sharp-cut filters have varying transmissions (85-95%) in their pass-bands, this could be an artifact, the real transmissions being narrower. In addition to the dose-dependent signal, there is a zero-dose signal (i.e. existing after long bleaching) on the red side of the laser wavelength peaking at 540 nm (uncorrected for photomultiplier response) with a small component to the blue side of the laser wavelength. The zero-dose signal was removed with a Corning 7-59 filter, which passes a band centred on 370 nm. This had the added advantage that it removes the small amount of laser leakage through the second shutter. All measurements from here on were made using this filter.

Laser Illumination Time If it is assumed that the number of electrons detrapped by a laser pulse is directly proportional to the length of that pulse (which will be the case providing that the source population is not significantly eroded), and that the decay of the OSP resulting from an instantaneous exposure is approximately exponential in form, it can be seen that part of the OSP from electrons detrapped at the beginning of a non-instantaneous exposure will be produced before the end of the exposure. This OSP will not be observed with the apparatus described because the shutter in front of the photomultiplier is closed during the exposure to prevent damage to the photomultiplier. The proportion of the OSP lost in this way will increase with increasing length of the laser exposure. For observation of OSP in quartz, a pulse length of 1 sec was chosen: it is experimentally estimated that for this pulse duration approximately 50% of the OSP produced is lost because it appears while the photomultiplier shutter is closed.

Growth Curve An additive dose growth curve of 40 points normalised by the natural OSP was produced for the coarse grain fraction (90-125 ixm) of a desert sand from a palaeolithic site (Chaperon Rouge, Morocco). Depletion of the natural OSP due to the normalisation process is about 4%. The growth appeared linear to the maximum added dose of 21 Gy (Fig. 2). Assuming a linear fit, the ED was 12.8 + 1.3 Gy, in satisfactory agreement with the value obtained by Rhodes (1988) using the luminescence. A regeneration growth curve on a further 40 samples, normalised by the natural OSP, then bleached under a solar simulator (Oriel 300W) for 1 ksec before dosing gave an ED of 10.2 _+ 1.3 Gy (Fig. 3). There is more scatter on the points than might be expected from counting statistics (the natural count from a disc was 400-550 counts, with no clear dependence on sample mass). This may be ascribed to loss of grains or a slight change in angle of the sample to the laser beam. Other samples that were tried of quartz, apatite and sanidine feldspar appear too insensitive to be practical for dating by OSP at room temperature even if multiple laser exposures were used to drain more of the trapped electrons. It appears that the quartz used for the growth curve was atypically bright.

Spectrum of Emissions This was measured (Fig. 1) with a bright geological quartz (shallow marine Jurassic sand, annealed at 580°C for 40 min in a nitrogen atmosphere), because the archaeological samples examined so far have been found to be too dim to allow the measurement. The dose-dependent signal peaked at 370 nm after correction for PMT response, with the long-wavelength foot of the transmission peak apparently extending to the laser wavelength at 514 nm, at which point it had an intensity per nm of about 1.5% of the peak value.

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FIG. 1. Block spectra of dose-dependent and zero-dose OSP of quartz compared. The laser wavelength is 514 nm. The zero dose signal can be conveniently eliminated by use of a Corning 7-59 filter. The spectra are corrected for photomultiplier response.

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FIG. 2. Additive dose OSP growth curve for quartz from Chaperon Rouge, normalised by the natural OSP. The natural OSP is in the range 400-550 counts.

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FIG. 3. Regenerated OSP growth curve for the same sample as Fig. 2. The portions were bleached for 1 ksec under an Oriel 300 W solar simulator, then given additive doses, followed by standard preheat. Normalisation was by the natural OSP measured before bleaching.

Optically Transferred TL (OTTL) It is well known that T L minerals have charge transfer into low temperature traps on bleaching. In investigation of this, the procedure used was to expose the sample to the laser for 10 sec, then glow it out to 150°C in situ. The resulting T L peak included a T L peak in quartz only: in the two other samples T L was produced, but the T L peak lay above the maximum temperature reached. The ratio of the amount of T L produced to the amount of optically stimulated luminescence during the laser exposure was 0.1-0.15 for apatite, and 0.6-0.11 for quartz. These values are not much higher than for OSP (0.07-0.08, 0-0.006) but the accuracy of the measurement is improved because a larger number of counts is involved (the laser exposure being longer), measured over a shorter time so that the errors due to counting statistics and subtraction of dark count are reduced. Unlike a single OSP measurement, the procedure significantly erodes the trapped electron population.

An additive dose growth curve was produced for the Chaperon Rouge sample, giving an E D of 12.1 + 1.2 Gy (Fig. 4) for a non-normalised growth curve. After a 1 ksec solar simulator bleach of the same samples, it was found that the signal was reduced to 1.7% of its original value. It appeared that there would be advantages in combining the techniques of OSP and optically transferred T L by exposing the sample to the laser at a constant elevated temperature using thermal phosphorescence to drain the transfer peaks. This is operationally more convenient than separate exposure and glow stages, and it minimises the risk of changes in sensitivity because a lower temperature is reached than in optically transferred TL. Measurement was made on a bleached portion of the Chaperon Rouge quartz, dosed to 15 Gy (Fig. 5). A maximum of OSP was found at 90°C, giving 1750 counts compared to 157 counts at 18°C. R o o m temperature appears to be the worst temperature at which to measure OSP. Interestingly, as the temperature is decreased below 20°C there is a rise in response. It is thought that this corresponds to the transfer T L peak seen at 17°C by Mobbs (1978) for optically transferred T L using exciting wavelengths of 260-360 nm. This rests on the assumption that all OSP is due to thermal detrapping of electrons from shallow traps, into which they have been phototransferred by the laser. If this were the case, one would expect the peak in OSP to lie at a lower temperature than for measurement of TL. Apparatus is being prepared for use of this OSP technique at temperatures down to liquid nitrogen. Preliminary measurements of OSP at 90°C for the Chaperon Rouge sample have shown problems with a residual component of the trapped electron population: that is, a part of the electron population contributing to the OSP is resistant to solar bleaching. Measurement of OSP at elevated temperatures was tried with sanidine also, with less encouraging results.

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FIG. 4. Non-normalised additive dose growth curve for optically transferred TL in quartz.

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Temperature ('C) FIG. 5. Graph of quartz OSP against the temperature at which the sample was exposed to the laser. There is a clear peak at 88°C, probably corresponding to the 110°C TL peak, and an indication of a peak below [/°C, possibly corresponding to a transfer TL peak seen in this region by Mobbs (1978).

A broad maximum was seen in the range 20-110°C, but, this was only about 50% higher than the lowest point of the measurements, which were taken over the range 10-21.5oC.

CONCLUSIONS Although EDs can be produced by OSP at room temperature, this method does not seem promising because of the weakness of the signals produced from most samples. Optically transferred TL seems a more fruitful field: most work by earlier investigators has been done at a shorter exciting wavelength as this has been thought necessary to drain the dating traps, but this does not seem necessary. Another problem these workers had was that the monochromators they used for illumination of samples produced rather weak beams of light: to transfer useful amounts of TL, short wavelength light was necessary. This problem is not so severe with the use of the laser for illumination. Transferred TL does share some problems with TL: the temperatures used may be high enough to cause

changes of sensitivity in some samples, and the trapped electron population is significantly eroded if the sample is exposed to the laser for long enough to produce a reasonably sized TL peak. For these reasons, OSP at constant temperatures just below the peaks of optically transferred TL seems to be advantageous. ACKNOWLEDGEMENTS This work was supported by the Science-based Archaeology Committee of the U.K. Science and Engineering Research Council.

REFERENCES Huntley, D.J., Godfrey-Smith, D.I. and Thewalt, M.L.W. (1985). Optical dating of sediments. Nature, 313, 105-107. Mobbs, S.F. (1978). Low temperature optical re-excitation in thermoluminescence dating. Unpublished M.Sc. thesis, Oxford University. Rhodes, E.J. (1988). Methodological considerations in optical dating of quartz. Quaternary Science Reviews, 7, 395-400. Smith, B.W., Aitken, M.J., Rhodes, E.J.. Robinson, P.D. and Geldard, D.M. (1986). Optical dating: Methodological aspects. Radiation Protection Dosimetry, 17,228-233.