Laboratory fading rates of various luminescence signals from feldspar-rich sediment extracts

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Radiation Measurements 43 (2008) 1474–1486

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Radiation Measurements

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Laboratory fading rates of various luminescence signals from feldspar-richsediment extracts

K.J. Thomsen a,*, A.S. Murray b, M. Jain a, L. Bøtter-Jensen a

a Radiation Research Department, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, P.O. 49, DK-4000 Roskilde, Denmarkb Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Risø DTU, DK-4000 Roskilde, Denmark

a r t i c l e i n f o

Article history:Received 28 April 2008Received in revised form 13 June 2008Accepted 23 June 2008

Keywords:Optically stimulated luminescenceSedimentary feldsparFading rateElevated temperature stimulation

* Corresponding author.E-mail address: [email protected] (K.J. Th

1350-4487/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.radmeas.2008.06.002

a b s t r a c t

Feldspar as a retrospective dosimeter is receiving more and more attention because of its usefulluminescence properties; in particular the dose response curve extends to significantly higher doses thanquartz. However, feldspars have one major disadvantage; both the thermoluminescence (TL) andoptically stimulated luminescence (OSL) signals exhibit anomalous (athermal) fading. Much of the OSLwork carried out on feldspars in recent years has focussed on determining fading rates and correcting forthem. Almost all work has been carried out using IR stimulation at 50 �C detected in the blue region ofthe spectrum. In contrast, we have determined fading rates for various sedimentary feldspar samplesusing different stimulation and detection windows. If the initial part of the OSL signal is used the lowestfading rate is observed with post-IR blue stimulation (UV detection), but if a later part of the signal isused the lowest fading rate is obtained for IR stimulation (blue detection). Daylight bleachingexperiments show that, unlike quartz, the initial and final parts of the feldspar OSL signal bleach atapproximately the same rate. Stimulation at elevated temperatures significantly reduces the apparentfading rate. This signal appears to bleach more rapidly in daylight than the signal observed whenstimulation is at 50 �C (for IR stimulation and blue detection). We conclude that there are OSL signalsfrom sedimentary feldspars which fade at a significantly lower rate than the signal used conventionally,and that further testing of these signals is warranted to see if they are dosimetrically useful.

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1. Introduction

Feldspars and quartz are the most abundant minerals in theregolith, and both can be used as luminescence dosimeters. In thepast decade most retrospective luminescence dosimetry researchhas focused on developing methods based on quartz. Although thishas proved to be a sensitive and reliable dosimeter, it has severalshortcomings. In some places, quartz is nearly absent in the naturalenvironment (e.g. Iceland and Hawaii). In other places quartz ispresent but low sensitivity limits its value as a dosimeter (e.g. muchof Canada). Furthermore, the quartz dose response curve generallysaturates at (or before) 200 Gy, limiting the practical age range toapproximately 100–150 ka. For all these reasons, it is desirable toinvestigate alternative natural luminescence dosimeters. Feldsparshave many advantages compared to quartz; these include: (1)a higher intrinsic luminescence sensitivity implying an improvedminimum detection limit; (2) the luminescence signals from feld-spars have been reported to grow to larger doses than quartz,providing a potential extension of the usable dose range; and (3)

omsen).

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feldspars, unlike quartz, are sensitive to IR stimulation (Hutt et al.,1988). This provides an instrumental way of targeting feldsparalone and allows the detection window to be centred on the mainemission at w400 nm (Huntley et al., 1991). Unfortunately, theluminescence signal has one major disadvantage: the signaldecreases with time faster than expected simply from a consider-ation of trap depth and ambient temperature. This anomalousfading (Wintle, 1973; Spooner, 1994) is only weakly dependent ontemperature. In order to consider the mechanisms behind thisprocess, we first consider the process of charge recombination.

Hutt et al. (1988) suggested that the infrared stimulated lumi-nescence (IRSL) from feldspar arises by excitation of electrons fromjust a single trap type. The IRSL trap in alkali feldspar is believed tolie approximately 2 eV below the conduction band (Poolton et al.,2002b). The IR transition has been reported to occur in the range1.41–1.47 eV (Bailiff and Barnett, 1994; Clark and Sanderson, 1994;Poolton et al., 1995a; Godfrey-Smith and Cada, 1996; Barnett andBailiff, 1997; Trautmann et al., 2000). Hutt et al. (1988) suggestedthat IRSL from feldspars is a thermally assisted process in which IRphotons excite the trapped electrons into an excited state fromwhere the excited electrons are thermally excited into theconduction band. However, the thermal activation energies lie inthe range 0.02–0.15 eV (Rieser et al., 1997; Bailiff and Barnett, 1994)

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Fig. 1. Transmission characteristics of three different filter combinations. Also shown isan emission spectra from a sedimentary K feldspar obtained using IR diode stimulation(redrawn from Huntley et al., 1991).

K.J. Thomsen et al. / Radiation Measurements 43 (2008) 1474–1486 1475

and are thus too small to thermally excite the excited electronsdirectly into the conduction band (this would require approxi-mately 0.5 eV). Poolton et al. (1995b, 2002a,b) suggested that theIRSL from feldspars results from a combination of two processes:(1) tunnelling from the excited state of the IRSL trap to a recombi-nation centre; and (2) transfer of electrons from the excited stateinto low-mobility band-tail states lying beneath the conductionband. The tunnelling component requires no thermal assistancewhereas the non-tunnelling component is thermally assisted.Which of the two components dominates the IRSL productionseems to be sample dependent. When IRSL is produced by directtunnelling the probability of recombination depends on thedistance between the electron donor (the trap) and the acceptor(the recombination centre). The further apart the donor andacceptor are the smaller the probability of recombination. WhenIRSL is produced by excitation into the band-tail states a strongtemperature dependence of the luminescence signal is expected.According to Poolton et al. (2002a) an electron excited into theband-tail states undergoes a random walk through the differentstates. Raising the temperature results in access to more states andthus increases the chance of luminescent recombination.

The combination of tunnelling to charge recombination infeldspars complicates the use of feldspars as dosimeters, since it isbelieved to be responsible for anomalous (athermal) fading (e.g.Wintle, 1973; Spooner, 1994). Anomalous fading is widely acceptedto be caused by direct tunnelling of electrons from the ground stateof the IRSL trap to holes in nearby luminescence centres (e.g.Aitken, 1985; Visocekas, 2002; Poolton et al., 2002a,b). Initial earlywork tried to circumvent anomalous fading by employing variouspreheating procedures (e.g. Clark and Templer, 1988). However,Spooner (1992, 1993, 1994) concluded that the fading process isessentially independent of prior heating. Much of the OSL workcarried out on feldspars in recent years has focussed on deter-mining fading rates and using them to correct apparent feldsparages for anomalous fading (e.g. Huntley and Lamothe, 2001; Buy-laert et al., 2007; Wallinga et al., 2007). Almost all work hasfocussed on using IR stimulation at 50 �C detected in the bluewavelength region. These age corrections are typical 30–40% anddependent on the exact fading model employed, and a satisfactorymodel for correction in the non-linear part of the growth curve hasyet to be presented. Unfortunately this is potentially the mostuseful part of the dose response.

The objective of this work is to determine whether laboratoryfading rates are dependent on experimental conditions, and if so, toidentify the most stable signal(s) from feldspar. The experimentalwork presented here is divided into three main parts. In the firstpart (Section 4) we investigate the effect of using different stimu-lation wavelengths (IR, green and blue) and detection wavelengths(blue and UV) on the laboratory determined fading rate. In thesecond part (Section 5) we investigate the dependence of fadingrate on the location of the signal in the feldspar decay curve, and inthe third part (Section 6) we investigate the fading rate dependenceon the IR stimulation temperature. Finally, we relate our observa-tions to the models for charge recombination discussed above.

2. Experimental details

2.1. Instrumentation

Automated Risø TL/OSL readers, model Risø TL/OSL-DA-15, wereused for the OSL measurements. Several different stimulationsources were employed. (1) Blue light stimulation used a blue(470 nm, FWHM 20 nm) light emitting diode (LED) array delivering50 mW/cm2 at the sample (Bøtter-Jensen et al., 2003). (2) Infraredlight stimulation used an IR (870 nm, FWHM 40 nm) LED arraydelivering approximately 135 mW/cm2 at the sample (Bøtter-

Jensen et al., 2003). (3) Green light stimulation used an array ofgreen LEDs (Luxeon High Power green LEDs) emitting at 530 nm(FWHM ¼ 45 nm). A green long-pass filter (3 mm of GG-495) isfitted in front of each green LED to minimise the short wavelengthtail (Thomsen et al., 2006). A configuration of seven such LEDsprovides a power density of 43 mW cm�2 at the sample.

Various detection windows were used to detect the emittedOSL (see Fig. 1): (1) 7.5 mm of Hoya U-340 filter; (2) 4 mm ofCorning 7–59 in combination with 2 mm Schott BG 39 (the bluefilter pack); (3) the blue filter pack in combination with 6 mmComar 435 IK (25 mm dia., 1 mm thickness) interference filters(this filter combination is used with green stimulation); and (4)6 mm of Chroma D410. Laboratory irradiations were made usingcalibrated 90Sr/90Y beta sources mounted on the readers (quartzdose rates varied between 0.1 Gy/s and 0.3 Gy/s). Samples weremounted in stainless steel cups using silicon oil. (The repro-ducibility of fading measurements improves significantly whenusing cups instead of discs, probably because of improvedthermal reproducibility.)

2.2. Samples

A summary of the samples used in this study is given inTable 1. The sediments were first wet-sieved and treated with HCland H2O2 to remove carbonates and organic material, and thendensity separated using heavy liquid techniques. Three differentdensity fractions were obtained: (i) a quartz-rich fraction(r > 2.62 g/cm�3); (ii) a sodium feldspar-rich fraction (2.62 g/cm�3 > r > 2.58 g/cm�3); and (iii) a potassium feldspar-richfraction (r < 2.58 g/cm�3). Throughout this paper the quartz-richfraction will be called ‘‘Q’’, the sodium feldspar-rich fraction ‘‘NF’’and the potassium feldspar-rich fraction ‘‘KF’’. However, mineralscover a range of densities and heavy liquid separation is not perfect(Godfrey-Smith and Cada, 1996). Quartz fractions were furtherrefined by etching in concentrated HF (40%) for 60 min and theluminescence purity of the quartz samples was monitored by IRstimulation (Smith et al., 1990). The feldspar fractions were etchedfor 40 min in 10% HF to remove outer layers. After etching, anycontaminating fluorides were dissolved using HCl, before finalrinsing and drying.

3. Determination of fading rate

Anomalous fading in feldspars is attributed to loss of electronsfrom thermally stable traps. A commonly accepted explanation for

Table 1Summary of samples used in this study

Sample Fraction Country Grain size (mm) Origin

963806 KF NF Russia 106–212 Glaciofluvial951012 KF NF Greenland 106–212 Glaciofluvial951020 KF NF Q Greenland 212–300 Glaciofluvial970204 KF NF Q Denmark 90–180 Glaciofluvial951019 NF Greenland 106–212 Glaciofluvial972513 NF Russia 180–250 Aeolian cover sand052266 KF Q Portugal 180–250 Aeolian dune sand055642 KF Q Denmark 90–180 Beach sand031734 KF Q India 180–250 Beach sand914807 Q Denmark 180–250 Marine

‘‘KF’’ and ‘‘NF’’ are the potassium and sodium feldspar-rich fractions, respectively.‘‘Q’’ is the quartz-rich fraction.

K.J. Thomsen et al. / Radiation Measurements 43 (2008) 1474–14861476

this phenomenon is quantum mechanical tunnelling of trappedelectrons to nearby acceptors (Spooner, 1994; Aitken, 1998). In thelaboratory, anomalous fading can be detected as a decrease withtime of the IRSL signal measured from artificially irradiatedaliquots. Anomalous fading can be quantified by the g-value, whichcorresponds to the signal loss per decade of logarithmic normalisedstorage time (Aitken, 1985), as defined by Eq. (1)

I ¼ Ic

�1� g

100log10

�ttc

��(1)

where I is the intensity at time t, Ic is the intensity at time tc, g isa constant (termed the g-value) and tc is an arbitrary time (in thiswork tc is 2 days). At least in principle, a knowledge of the g-valueshould allow apparent feldspar ages to be corrected for anomalousfading using various models (e.g. Huntley and Lamothe, 2001).Fig. 2 shows the ratio between the corrected age and the uncor-rected age as a function of g-value for a dose rate of 1 Gy/ka and anapparent age of 100 ka. Also shown (right axis) is the age correctionin percent. A g-value of 3%/decade or 0.5%/decade corresponds toan age correction of w36% or w5%, respectively (based on thecorrection model of Huntley and Lamothe, 2001). Typical g-valuesused for age correction of apparent ages for geological sedimentsare often in the range 3–4%/decade (Buylaert et al., 2008; Martinset al., in press).

In this work, g-values are determined by carrying out a series ofsensitivity corrected measurements (Lx/Tx measurements) usinga modified SAR procedure (Murray and Wintle, 2000; Auclair et al.,2003), where any sensitivity changes are explicitly monitored and

Fig. 2. Ratio between the corrected age and the uncorrected age as a function of g-value(corrections based on Huntley and Lamothe, 2001). Also shown (right axis) is the agecorrection in percent. Inset shows an enlargement of the age correction in percent.A g-value of 3% or 0.5% corresponds to an age correction of w36% or w5%, respectively.

corrected by the insertion of a test dose (Tx). After each delayedmeasurement a prompt measurement was undertaken to ensurethat sensitivity change in the sample was properly corrected for.Optical stimulation was carried out at 50 �C (unless otherwisestated) for 100 s. A preheat of 250 �C for 60 s directly followed eachirradiation (Auclair et al., 2003).

4. Effect of the choice of stimulation and detectionwavelengths on the laboratory fading rate

In this section the fading rate dependence on stimulation anddetection conditions is examined.

4.1. IR stimulation

Most OSL work on feldspars related to dating has been carriedout using IR stimulation (IRSL) and detection in the blue wave-length region (w340–450 nm). Feldspar extracts will usually becontaminated by other minerals, especially quartz, because thedensity separation method is not completely mineral specific.However, quartz does not respond significantly to IR stimulation atroom temperature (e.g. Spooner and Questiaux, 1989; Jain et al.,2005); feldspar does, on the other hand, and so it has alwaysseemed appropriate to use IR stimulation on feldspar samples.Furthermore, stimulating with IR light (compared to blue lightstimulation) allows the detection window to be more closely cen-tred on one of the main emissions at approximately 400 nm (seeFig. 1).

Fig. 3 summarises fading rate data obtained from an aliquot ofsample 951020 KF using IR stimulation at 50 �C detected using theblue filter pack (see Section 2.1). The sum of the initial 1.15 s of IRstimulation was used, less a background based on the sum of thefinal 19.6 s of stimulation (see inset to Fig. 3a); data were collectedfollowing delays of up to 12 h. A g-value of 3.2 � 0.1%/decade wasobtained (see Fig. 3a). Fig. 3b summarises the g-values for six suchaliquots. The average g-value (gAV, the mean of these g-values) is3.3 � 0.1%/decade (n ¼ 6). All values of gAV reported later are basedon the results from six individual aliquots.

A similar average g-value for this sample was then determinedusing various different stimulation sources and detection windows.The results are summarised in Fig. 3c. All stimulations were carriedout at 50 �C with a preheat temperature of 250 �C for 60 s givenimmediately after dosing.

Grun et al. (1989) showed that thermoluminescence (TL) dosesobtained from coarse grain K-feldspars using a blue-transmittingCorning 7–59/HA-3 filter combination (280–475 nm; similar to theblue filter pack used in this study) were significantly higher thanthe TL doses measured using a UV-transmitting UG 11 filter (similarto the U-340 filter employed in this study). A similar observationwas made by Balescu and Lamothe (1992) suggesting that the blueTL emission might be more stable than the UV emission. However,in both these studies fading rates were not explicitly measured andso the difference in dose could be caused by other factors (e.g.differential bleaching). Bøtter-Jensen et al. (1993) compared theanomalous fading (fading factors) of TL measured using Corning 5–60 (peak transmission at 420 nm) and Hoya U-340 detection filtersand reported a reduced fading factor using UV detection. Similarly,IR stimulation detected through the broad band Schott BG 39 filter(330–600 nm) gave a fading factor of 0.87 � 0.02 whereas using theU-340 detection filter gave a fading factor of 0.97 � 0.02, implyingthat for these samples the UV emission was more stable than theemission detected through the broad band a BG 39 detection filter.1

Lang and Wagner (1996) compared fading rates for their

1 A preheat of 260 �C for 40 s was employed before all OSL measurements.

a

b

c

Fig. 3. Fading rate data obtained from sample 951020 KF using IR stimulation at 50 �Cdetected using the blue filter pack. (a) Determination of the fading rate for a singlealiquot by using a series of prompt and delayed sensitivity corrected measurements.A typical decay curve is shown in the inset. (b) The fading rate for six individualaliquots of the same sample; gAV is the average fading rate. (c) Average fading rate asa function of stimulation and detection conditions. The detection filter is given inbrackets. ‘‘pIRB’’ refers to post-IR blue stimulation. ‘‘pIRB BG’’ refers to post-IR bluestimulation using a background lying directly adjacent to the initial part of the decayused as signal (see Section 4.3 for further details).

2 Measurements on a Na feldspar-rich extract and a K feldspar-rich extract haveshown that the intensity of the signal detected using D410 is about 50% of thatdetected using the blue filter combination (data not shown).

3 Exceptions are Blair et al. (2005, 2006) who were working towards developinga single-aliquot, polymineral procedure that works for both quartz and feldspars.However, their work made use of commercially available feldspar specimens.


polymineral fine grain samples using BG 39 and a narrow inter-ference filter centred on 402 nm. These authors could not detectany fading using the 402 nm filter whereas fading was detectedusing the BG 39 filter (5–8% in 4 months). The authors state thatusing a wide detection band (e.g. BG 39) is unfavourable for poly-mineral samples because it probably includes unstable green andyellow emissions. Krause et al. (1997) did not detect anomalousfading in their Antarctic plagioclase feldspars for any of the fouremissions investigated (280 nm, 330 nm, 410 nm and 560 nm).

We have measured the fading rates in sample 951020 KF usingthe U-340 detection filter. The average fading rate is 4.7 � 0.2%/decade (n ¼ 6) for UV detection, which can be compared with the3.3 � 0.1%/decade (n ¼ 6) for detection using the blue filter

combination reported above. This suggests that for this sample thesignal emitted in UV is less stable than that in the blue.

The blue filter combination transmits part of the UV emission, aswell as the blue emission. Thus, by choosing a more restrictive filtercombination, that limits detection to the blue emission it may bepossible to reduce the influence of any UV component transmittedby the blue filter combination. We have used a D410 filter (sug-gested by M.R. Krbetschek; this only transmits the blue emission –see Fig. 1) to determine whether using a narrow blue detectionwindow further reduces the fading rate for our samples. Using6 mm of the D410 detection filter gives an average g-value of3.11 � 0.04%/decade (n ¼ 6), indistinguishable from the g-valueobtained using the standard blue filter combination.2 The exactemission spectra from these samples are unknown, but the resultsseem to imply that there is no significant UV contribution whendetection is through the blue filter combination. Thus, there seemsto be no significant advantage with respect to fading in using theD410 filter combination with this sample (or any of the othersamples investigated in this study, see Table 2). In the followingsections we assume that the blue filter combination is dominantlytransmitting the blue luminescence emission.

4.2. Green and blue stimulation

Given that the UV emission fades more than the blue emissionusing IR stimulation it is interesting to investigate whether thefading rate also depends on stimulation wavelength. Stimulationwith green light and detection in the UV gives an average fadingrate of 3.3 � 0.2%/decade, about 30% lower than the gAV determinedfor IR stimulation detected in the UV (4.7 � 0.2%/decade).

It is not possible to use the standard blue filter combination withthe green light stimulation employed here because of the closeproximity of the stimulation and detection windows, so the bluefilter combination was modified by introducing an additional 6 mmof Comar IK 435 (see Fig. 1). The average g-value measured usingthis filter combination is 2.8 � 0.1%/decade (n ¼ 6), about 15% lowerthan the gAV determined for IR stimulation detected in the blue(3.3 � 0.2%/decade).

We conclude that there does seem to be a reduction in fadingrate when green light stimulation is used instead of IR stimulation.

Even though detection of the blue emission when stimulatingwith blue light is not possible using continuous wave stimulation, itis still useful to determine the signal stability of the blue stimulatedOSL detected in the UV. This, yields an average fading rate of3.2 � 0.2%/decade (n ¼ 6) indistinguishable from that measuredusing green light stimulation. Duller and Bøtter-Jensen (1993);Ditlefsen and Huntley (1994); Jain and Singhvi (2001) have sug-gested that the feldspar signal originates from a least two differenttrap types; one stimulatable by blue/green light only and anotherstimulatable by both blue/green and IR light. Thus, it is possible thatthe IR sensitive signal is more unstable than the non-IR sensitivesignal. This hypothesis is examined in the following section.

4.3. Post-IR blue stimulation

Although several papers in the literature (e.g. Jain and Singhvi,2001; Banerjee et al., 2001) recommend using post-IR blue stim-ulation, the intention was almost always to obtain a quartz-richsignal from a mixed sample.3 The objective of using post-IR blue

Fig. 4. Normalised OSL decay curves for the quartz and feldspar fractions for sample951020 detected using the U-340 filter combination. (1) Quartz post-IR blue decay for951020 Q. (2) Feldspar blue decay for 951020 KF. (3) Feldspar post-IR blue decay of951020 KF. The inset displays the same data using a logarithmic abscissa. Also shownare the standard summation intervals used for signal and background calculation.

Table 2Average fading rates for various stimulation and detection modes

Stimulation Detection KF initial KF final NF initial NF final

pIRB BG UV 0.9 � 0.8 – 2.4 � 0.5 –pIRB UV 2.1 � 0.2 2.2 � 0.2 2.9 � 0.5 2.5 � 0.3Blue UV 3.5 � 0.3 2.0 � 0.2 4.7 � 0.3 2.3 � 0.3Green UV 3.7 � 0.4 2.2 � 0.2 4.7 � 0.3 2.6 � 0.3IR UV 4.7 � 0.3 2.6 � 0.1 6.2 � 0.4 3.2 � 0.5Green Blue 2.9 � 0.4 1.8 � 0.3 3.1 � 0.1 1.9 � 0.1IR Blue 3.0 � 0.1 1.4 � 0.1 3.1 � 0.2 1.4 � 0.1IR D410 3.0 � 0.3 1.2 � 0.1 3.3 � 0.1 1.5 � 0.1

The numbers given are average gAV-values determined for four potassium-richsamples (963806 KF, 951012 KF, 951020 KF and 970204 KF) and six sodium-richsamples (963806 NF, 951012 NF, 951020 NF, 970204 NF, 951019 NF and 972513 NF).All stimulations were for 100 s and carried out at 50 �C. ‘‘Initial’’ refers to the fadingrate obtained using the initial 1.15 s of stimulation. ‘‘Final’’ refers to the fading rateobtained using the signal summed from 28.8 s to 72.3 s of stimulation (see Section 5for further details).


stimulation in this work is to investigate if the IR sensitive signal isless stable than the non-IR sensitive signal. The average fading rateof post-IR blue stimulation (IRSL for 100 s at 50 �C followed by 100 sblue stimulation at 50 �C) detected in the UV window is 1.6 � 0.3%/decade for sample 951020 KF. However, it must be acknowledgedthat this measurement may include a quartz contribution andquartz does not exhibit anomalous fading (Murray and Wintle,1999). This possibility is considered in Section 4.4.

In Fig. 4, typical normalised post-IR blue and blue OSL signalsfrom sample 951020 KF are shown. The post-IR blue OSL signalshows a weak initial rapid decay followed by a slower decay. In allmeasurements described so far the final 19.6 s of the OSL signal hasbeen summed and subtracted from the initial 1.15 s of the OSLsignal (after normalisation to the same collection time interval). Ifthe background summation is moved to lie adjacent to the initialsignal (i.e. from 1.15 s to 3.8 s) in an attempt to isolate the initialcomponent the average fading rate reduces to 0.0 � 1.0% (n ¼ 6).

Fig. 3c summarises the average g-values for all the signalsinvestigated from sample 951020 KF. Table 2 shows the averagefading rates for four potassium feldspar-rich samples and sixsodium feldspar-rich samples. It is interesting to note that thefading rate in sodium-rich samples appear to be higher than in thepotassium-rich. It is also interesting to note that the fading ratesfound in the different samples are remarkably similar (as indicatedby the uncertainties on the individual data points).

4.4. Quartz contamination?

Using blue/green light stimulation on feldspar samples mightgive rise to mixed signals because it is very difficult to obtain a cleanfeldspar extract from a sedimentary sample. It is very likely that anyfeldspar-rich extract will contain at least some quartz. Thus, thefading rate decrease observed when using blue/green stimulationcompared to IR stimulation may arise from a larger contribution tothe total signal from a stable quartz signal. This possibility isexamined below.

4.4.1. Initial decay rateIn Fig. 4 typical normalised post-IR blue and blue OSL signals from

sample 951020 KF are shown together with the post-IR blue OSLdecay curve from the quartz-rich extract from sample 951020. It isapparent that the quartz signal decays significantly faster than boththe feldspar signals. If a signal is contaminated with that from quartzthen one would expect the initial signal to be the most affected partof the decay curve. However, the initial decay rate of the post-IR bluesignal from 951920 KF is slower than that from both the quartz andblue stimulated feldspar, suggesting that the post-IR blue signalfrom the feldspar extract is not dominated by quartz.

4.4.2. Time resolved OSL measurementsTime resolved OSL (TR-OSL) measurements were undertaken

using the photon timer described by Denby et al. (2006) to furtherexamine if quartz contamination could be the cause of the observeddecrease in fading rate using post-IR blue stimulation. It is well-established (e.g. Sanderson and Clark, 1994; Clark et al., 1997;Bailiff, 2000; Denby et al., 2006) that quartz and feldspar havesignificantly different luminescence lifetimes. Thus, pulsing thestimulation light can be used as a method of discriminatingbetween quartz and feldspar OSL signals. Thomsen et al. (2008)successfully applied the pulsing technique to separate feldspar andquartz OSL signals in mixed sediment samples. Thus, by under-taking TR-OSL measurements it should be possible to determine ifthere is a significant quartz contamination in the post-IR blue signalfrom the feldspar extract.

4.4.2.1. Can TR-OSL be used to determine quartz contamination? Todemonstrate the potential of using TR-OSL decay shapes todetermine quartz contamination in feldspar samples TR-OSLmeasurements using pulsed stimulation with on- and off-times of50 ms were carried out using two portions of feldspar sample951020 KF. One portion was bleached and dosed in the reader(unheated feldspar sample) and the other portion was heated to500 �C for 1 h before dosing in the reader (heated feldspar sample).In the process of heating, the quartz contaminating the feldsparsample should be sensitised and the feldspar itself desensitised. InFig. 5b the normalised IR signal from the unheated feldspar sampleis compared to that of the heated feldspar sample. These data wereobtained by summing all counts from 100 s of stimulation (i.e. thedata from both the on- and off-times were included). As expected,heating the feldspar sample does not change the decay shape of theIRSL signal. In Fig. 5c the decay shapes for the post-IR blue signal forthe unheated and heated feldspar sample are compared. The initialdecay of the heated sample is more quartz like (i.e. it decayssignificantly faster) than that of the unheated sample. The heatedsample has decayed to half of its initial value in about 1.5 s whereasthe same value is reached in 45 s for the natural sample. Thus, byheating the sample we have obtained a signal which initially isdominated by quartz.

In Fig. 5a normalised TR-OSL curves for quartz (914807 Q) andnatural (unheated) feldspar (951920 KF) obtained using pulsed bluestimulation are compared. The on- and off-times were both 50 ms

a

b c e

d

Fig. 5. TR-OSL measurements from quartz (914807 Q) and feldspar (951020 KF) obtained using pulsed stimulation with on- and off-times of 50 ms. Detection is through 7.5 mm ofU-340. (a) 1. post-IR blue TR-OSL data from a heated quartz sample (914807 Q). Data is obtained from the first 0.4 s of stimulation (40,000 pulses). 2. Post-IR blue TR-OSL data froma natural (unheated) feldspar sample (951020 KF). Data is obtained from the first 0.4 s of stimulation (40,000 pulses). Also shown are TR-OSL data from a heated portion of thefeldspar sample. These data have been obtained using 1 s of stimulation. 3. First 1 s of stimulation. 4. Data from 3 s to 4 s of stimulation. 5. Data from 7 s to 8 s of stimulation. 6. Datafrom 31 s to 32 s of stimulation. (b) Normalised IRSL decay curves derived from the post-IR blue TR-OSL data for the natural and heated portions of 951020 KF. The data were derivedby summation of the entire TR-IRSL signal (both on- and off-time). 7. Natural. 8. Heated. (c) Normalised post-IR blue decay curves for the natural (9) and the heated (10) portions.Inset shows the blue decay curves (11 and 12, respectively). The data were derived by summation of the entire TR-OSL signal (both on- and off-time). (d) The first 0.4 s ofstimulation. 1. Post-IR blue stimulation of quartz (diamonds). 2. Post-IR blue stimulation of feldspar (closed circles). 13. Blue stimulation of feldspar (squares). (e) Comparison ofon- and off-time signals for post-IR blue stimulation. The curves are derived by summing over the entire 100 s of stimulation. 14. On-time signal from quartz. 15. Off-time signal fromquartz. 16. On-time signal from feldspar. 17. Off-time signal from feldspar.


and the data were recorded for 100 s (corresponding to approxi-mately 50 s of continuous wave (CW) stimulation). A total of 106

stimulation pulses arrived at each sample. The data are integratedover the first 4000 pulses (0.4 s) corresponding to approximately thefirst 0.2 s of an equivalent CW stimulation. As expected, the responseof the two phosphors to blue stimulation is markedly different. Alsoshown in Fig. 5a are the TR-OSL data for post-IR blue stimulation ofthe heated feldspar sample. Data are displayed for various timesdown the decay curve. Summing the first 1 s of data (correspondingto the first 10,000 pulses) gives a quartz like TR-OSL decay shape. Inthe subsequent summations the TR-OSL decay shapes become moreand more feldspar like. After about 4 s of stimulation (correspondingto 40,000 pulses) the TR-OSL signal is completely dominated byfeldspar, because the signal from quartz has decayed to a negligiblelevel. Thus this experiment gives us confidence that TR-OSLmeasurements can be used to determine whether the natural post-IR blue signal from feldspar is contaminated by quartz.

4.4.2.2. TR-OSL of a natural feldspar sample. We are now in a posi-tion to evaluate whether the post-IR blue signal from our natural(unheated) feldspar sample has a significant contribution fromquartz. In Fig. 5d normalised TR-OSL curves for the quartz sampleand the natural (unheated) feldspar sample obtained using pulsedblue stimulation (data taken from Fig. 5a) are compared with thepost-IR blue response from the feldspar sample. If the post-IR bluesignal from the feldspar sample included a significant quartzcontamination, we would expect that the shape of both the on-time(first 50 ms) and the off-time (last 50 ms) TR-OSL signals to bedifferent for the blue and post-IR blue feldspar signals. However,the post-IR blue response is indistinguishable from the response toblue light stimulation implying that any quartz contamination ofthe feldspar signal is undetectable. It could still be argued that anyquartz contamination would only be detectable in the off-time andthat both the signals observed in this period are completely

dominated by quartz signals. If this was the case, we would notexpect to see a significant difference between the blue and thepost-IR blue signal. To place an upper limit on the amount of quartzcontamination in the post-IR blue signal we assume the limitingpossibility that the off-time signal from feldspar is zero. Thus, weassume that all recombination processes in feldspar take place ona time scale � 1 ms (the resolution of the photon timer). Then themaximum quartz contribution can be calculated by taking the ratiobetween the observed post-IR blue signal from feldspar to that ofquartz. The average maximum possible contribution to the total off-time signal from quartz, based on this ratio, is 5.5 � 0.4%.

However, if the off-time signal was completely dominated byquartz then we would expect that the off-time signal would decayin a manner similar to the off-time decay of a pure quartz sample(Fig. 5a,d). In Fig. 5e the integrated on- and off-time TR-OSL signalsare shown as a function of stimulation time for the post-IR bluesignal from quartz and feldspar. For both samples the rate of decayis the same for both the on- and off-time signals, and the decayrates observed from the feldspar sample are very different fromthose observed from quartz. This indicates that both the on- andoff-time post-IR blue signals from the feldspar-rich extracts arecompletely dominated by feldspar. Thus, we can safely deduce thatfor the samples investigated here the off-time feldspar signalsoriginate with feldspar and not quartz.

From these observations, we are confident that we can concludethat the observed reduction in fading rate for the CW post-IR bluesignal (Section 4.3) is not caused by quartz contamination.

5. Summation intervals

In all the work presented so far the initial part of the decay curveis used to derive the signal of interest. In the following sections weinvestigate whether the signals derived from later parts of the


decay curve fades at the same rate as the initial signal, and whetherthe later parts of the decay curve are likely to be bleached in nature.

5.1. Fading rate as a function of stimulation time

Fig. 6a shows a typical IRSL decay curve from sample 951020 KFdetected through the blue filter pack. In Fig. 6b the average fadingrate for this sample is shown as a function of stimulation time(closed circles). Each summation interval is 1.15 s long. As thestimulation time increases the fading rate appears to decrease,indicating that the later part of the decay curve (after backgroundsubtraction; see caption to Fig. 6) is more stable than the initialpart. The uncertainties on the fading rates become large as a func-tion of stimulation time because the uncertainties on the individualLx/Tx measurements become larger as a result of signal depletion.Initially, the signal stability increases rapidly but becomesapproximately constant after about 20 s of stimulation. In whatfollows, the initial fading rate is defined as the fading rate calcu-lated using the first 1.15 s of stimulation and the final fading rate isdefined as the fading rate calculated using the signal summed from28.8 s to 72.3 s of stimulation (between the dashed lines in Fig. 6b).The average fading rate obtained using this integration interval isindicated by the closed diamond symbol. In both cases the last 19 sof stimulation is used for background subtraction. The initial andfinal fading rates for this sample are 3.04 � 0.16%/decade and1.18 � 0.09%/decade, respectively; the fading rate has decreased byabout 60% by using the later part of the signal.

Fig. 7a shows a summary of the average initial and final fadingrates for all sodium and potassium feldspar samples using variouscombinations of stimulation and detection windows. Fig. 7b

a

b

Fig. 6. Data from sample 951020 KF. (a) Typical IRSL decay curve (at 50 �C) detectedusing the blue filter pack. (b) Average fading rate as a function of stimulation time.Each stimulation interval is 1.15 s in duration (closed circles). Also shown is the averagefading rate obtained using an integration interval of 43.5 s duration (indicated by thedashed lines) centred on 50.6 s (diamond symbol). This fading rate is referred to as thefinal fading rate. The initial fading rate is the fading rate calculated using the initial1.15 s of stimulation (indicated by the solid lines). Background signal is based on thelast 19 s of stimulation.

shows the ratio of the final fading rate to the initial fading ratefor each signal. For all combinations (with the exception of post-IR blue stimulation for which the fading rate is approximatelyconstant as a function of stimulation time) the final fading rate issignificantly lower than the initial fading rate. The lowest fadingrates are observed for IR stimulation detected in the blue usingeither the blue filter pack or the D410 filter combination. Thus,from a signal stability point of view it would seem to be better touse the final signal rather than the initial signal. However, byanalogy with the behaviour of quartz, it would seem reasonableto expect that the later parts of the feldspar stimulation curve aremore difficult to bleach in nature, and so the reduced fading ratefor larger stimulation times may not be a useful observation inpractice. This assumption of lower bleaching rates is tested in thenext section.

5.2. Bleaching experiments

Previously published bleaching experiments (e.g. Godfrey-Smith et al., 1988) have shown that the initial quartz OSL signalbleaches significantly faster than the initial feldspar signal. Onemight thus expect that the problem of incomplete bleaching is, ingeneral, worse for feldspar than for quartz, and that this problembecomes even more severe further down the feldspar decay curve.In this section, we test this expectation of the bleaching behaviourof feldspar when exposed to simulated sunlight. As a reference, wefirst present the behaviour of quartz (Fig. 8a, diamonds).

Twenty four natural quartz aliquots (lab code 970204 Q) wereprepared and exposed to a daylight simulator (Honle Sol2) forvarying durations of time in batches of three aliquots. The residualdoses were subsequently determined using a standard double SARprocedure (preheat of 260 �C for 10 s and cutheat of 220 �C)employing post-IR blue light stimulation. Similar experiments(preheat equal to cutheat of 250 �C for 60 s) were carried out usingthe natural feldspar sample (970204 NF) using post-IR blue (circlesymbols), blue (downward pointing triangles), and IR stimulation(leftward pointing triangles and hexagonal symbols) and the

a

b

Fig. 7. (a) Summary of initial and final fading rates for all 10 samples for all combi-nations of stimulation and detection windows. Uncertainties indicated are standarderrors on the mean. (b) Ratio of final fading rate and initial fading rate.

a

b

c

Fig. 8. Bleaching curves for a quartz extract (970204 Q) and a sodium-rich feldsparextract (970204 NF). (a) Normalised natural doses calculated using the initial (first 0.8 sof stimulation) OSL for post-IR blue stimulation of quartz and various stimulation anddetection windows for feldspar (all measured at 50 �C). (b) Normalised natural quartzdoses using post-IR blue stimulation for three different stimulation periods. (c) Nor-malised natural feldspar doses using IR stimulation at 50 �C and detected using theblue filter pack for three different stimulation periods. Inset shows the measureddoses.

4 It should be noted here that recycling ratios are constant as a function ofstimulation time. This is demonstrated in Figs. 10, 12.


results are also shown in Fig. 8a. These data indicate that, asexpected, the initial feldspar signal bleaches significantly moreslowly than that from quartz for all combinations of stimulationand detection windows investigated here. It is also clear that all thefeldspar signals investigated here bleach at approximately the samerate.

Usually the initial part of the OSL signal is used for dosedetermination, because this is the most sensitive to laboratorystimulation, and so it is assumed that this part of the signal is alsobest bleached in nature. This is known to be true for quartz (e.g.Aitken, 1998; Singarayer et al., 2005), but has not been explicitlydemonstrated for feldspar. To illustrate the known quartz behav-iour, Fig. 8b shows the normalised residual doses for a naturalquartz sample for three different integration intervals as a functionof daylight simulator exposure time. Each integration interval is0.8 s long and begins: (i) at the on-set of stimulation (diamondsymbols); (ii) after 6.4 s of stimulation (triangles); and (iii) after32 s of stimulation (squares). As expected, the initial signal bleachessignificantly faster than the later part of the decay curves. Fig. 8cshows the corresponding residual doses measured using IR stim-ulation (detection in the blue) from a natural feldspar sample

(970204 NF).4 Surprisingly, there seems to be no significantdifference in the bleaching rate using later parts of the feldsparsignal; there is no suggestion in the data that the initial part of thefeldspar signal is better bleached than later parts. Despite this, theinset to Fig. 8c shows that the absolute doses increase when laterparts of the signal are used for dose determination. If this increase isnot caused by incomplete bleaching then it may arise from thelower fading rates identified in the previous section.

From first principles, it is expected that optical emptying ofa single trap (with no significant retrapping) should produce anexponential reduction in trapped charge (and thus luminescencesignal) with stimulation time. However, this has never beenobserved using feldspars. Bailiff and Poolton (1991) and Bailiff andBarnett (1994) showed that the IRSL decay curve follow a powerlaw decay of the form A(1 þ Bt)�P where A and B are constants, t isstimulation time and P is a constant between 1 and 2. However, ourdose reduction with stimulation time (Fig. 8c) can be representedwell by a single exponential decay. The differences in these curvesfor the three stimulation periods are minor. Thus, we are led toconclude that a single trap is being emptied, even though theluminescence decay curves are non-exponential. This suggests thatthe decay curve shape is influenced by the recombination proba-bility as well as detrapping probability, and that the same trap isbeing emptied throughout the stimulation period.

6. Effect of stimulation temperature

In all the work discussed so far optical stimulation was carriedout at 50 �C. In this section we discuss the effect of increasing thestimulation temperature on the fading rate. We also examine theeffect of stimulation temperature on daylight bleaching.

6.1. Fading rate as a function of stimulation temperature

In feldspars the intensity of the total integrated IRSL signal isobserved to increase with increasing stimulation temperature (e.g.Bailiff and Poolton, 1989; Duller and Wintle, 1991; Wiggenhorn,1994; McKeever et al., 1997). Duller and Wintle (1991) found thatthe total light sum increased by a factor to 2 over the temperaturerange 25–200 �C. In our samples both the initial as well as the totalIRSL signal increase similarly as a function of stimulation temper-ature (data not shown). The IRSL intensity increases approximatelyfive times over the temperature range 50–225 �C for the KF samplesand approximately three times for the NF samples.

Duller and Wintle (1991) and McKeever et al. (1997) alsoreported that the decay rate of the IRSL signal increased withstimulation temperature. McKeever et al. (1997) argued that thisincrease is consistent with the expected dependence for a ther-mally assisted process, i.e. the same traps were being accessed butthe elevated temperature increased the optical cross-section.However, Poolton et al. (2002a) found that the form of the decay islargely unaffected by stimulation temperature. We have alsoexamined the dependency on stimulation temperature for our Na-and K-rich extracts.

This is illustrated in Fig. 9a for sample 970204 KF. The samealiquot was dosed, preheated (250 �C, 60 s) and stimulated with IRlight at different temperatures (50 �C, 80 �C, 110 �C, 140 �C, 170 �C,200 �C and 225 �C). A test dose cycle using IR stimulation at 50 �Cwas inserted in between the regeneration cycles, and the emittedluminescence was detected using a Hoya U-340 filter. The nor-malised decay curves for four different stimulation temperaturesare shown in Fig. 9a. In contrast to the results of Duller and Wintle

a b

c d

Fig. 9. Effect of stimulation temperature on IRSL decay shape and signal intensity for sample 970204 KF. The data displayed are for IR stimulation detected through the Hoya U-340filter. (a) Normalised IRSL decay curves stimulated at 50 �C, 110 �C, 170 �C and 225 �C. (b) Sensitivity corrected IRSL as a function of stimulation temperature for two differentsummation intervals each of 1.44 s and beginning at (i) the on-set of stimulation (0 s; squares), and (ii) after 32.16 s of stimulation (32.16 s; circles). Also shown are recycling pointscarried out at 50 �C after stimulation at 225 �C (open symbols). (c) Same as a) but for post-IR IR stimulation. Also shown is an IRSL decay curve stimulated at 50 �C without any priorstimulation ([50]*). (d) Same as (b) but for post-IR IR stimulation.


(1991) and McKeever et al. (1997), but supporting those of Pooltonet al. (2002a), we do not see any significant change in decay ratewith stimulation temperature. The sensitivity corrected IRSLintensity is shown as a function of stimulation temperature for twodifferent summation intervals in Fig. 9b. In this sample the initialsignal increases approximately by a factor of 4 and the later signalby a factor of 3 between 50 and 225 �C.

Fig. 10 shows the average initial and final fading rates asa function of stimulation temperature for both the sodium andpotassium fractions of sample 963806, using IR stimulation andthe blue filter pack. The fading rate decreases monotonically asa function of increasing stimulation temperature for all data sets.The lowest fading rate (0.72 � 0.06%/decade) is observed fromthe potassium fraction using the final signal stimulated at 225 �C.The preheat employed in all these experiments was 250 �C for60 s. To ensure that the decrease in fading rate is not caused bythermal erosion of unstable traps the preheat was changed to

Fig. 10. Effect of stimulation temperature on average fading rate for both the sodiumand potassium fractions of feldspar sample 953806. The data displayed are for IRstimulation detected through the blue filter pack.

310 �C for 10 s in one experiment. The fading rates observed wereindistinguishable from those displayed in Fig. 10 (data notshown).

One explanation for the observed decrease in fading rate asa function of stimulation time and temperature is that a differentsuite of traps is being accessed and that these more difficult tostimulate traps are more stable than the traps accessed by stimu-lation at 50 �C. However, if a different suite of traps were beingaccessed at elevated temperatures one might expect that the decayrate would change as a function of stimulation temperature. This isnot observed in these samples. Thus, it seems more likely that thetrapped electron population being accessed is the same (i.e. a singletrap) but that either the luminescence efficiency or the number ofelectrons available for decay increases as a function of temperature.

6.2. Post-IR IR stimulation

As mentioned above, the IRSL signal increases as a function ofstimulation temperature. This opens up the possibility ofmeasuring a post-IR IR signal (where the second IR stimulation is atsome temperature above the first) instead of a post-IR blue (orpost-IR green) signal. Jain and Singhvi (2001) showed that it ispossible to access some residual trap population after near roomtemperature IR bleaching by using high temperature (w220 �C) IRstimulation. This is illustrated in Fig. 9c where normalised decaycurves for post-IR IR stimulation detected in the UV are shown. Thefirst IR bleach is carried out at 50 �C for 100 s and the subsequent IRmeasurement is carried out at four different temperatures (50 �C,110 �C, 170 �C and 225 �C) for 100 s. It is interesting to note thatnow a marked increase in decay rate is observed as a function ofstimulation temperature (in contrast to Fig. 9a, where there was noprior IR stimulation). The increase in light intensity of the initialpost-IR IR signal at 225 �C compared to that for IR stimulation at50 �C is 1.2, whereas the later part of the post-IR IR signal at 225 �Cincrease by at factor of three compared to the IR intensity at 50 �C.

Jain and Singhvi (2001) speculated that the residual populationmeasured in a post-IR IR stimulation at elevated temperature mayshow less fading since the residual charge after IR bleaching islikely to have a greater trap depth (their Type B traps). We now

a

b

Fig. 11. Normalised dose from bleaching experiments carried out at 225 �C for 970204NF and KF. (a) Initial signal (first 1.15 s of stimulation). (b) Final signal (signal integratedfrom 28.8 s to 72.3 s). Both signals have had a background based on the last 19 s ofstimulation subtracted.

a


investigate if such a post-IR IR signal has a lower fading rate. In ourpost-IR IR stimulation the first IR measurement is carried out at50 �C for 100 s and the subsequent IR measurement is carried out at225 �C for 100 s. A summary of the average fading rates obtained inthe elevated temperature experiments is shown in Table 3. Theintensity of the post-IR IR light emitted at 225 �C is approximately 3times the light emitted at 50 �C (without prior IR bleaching) whendetected through the blue filter combination (initial signal). Theaverage initial fading rates for post-IR IR stimulation for three KFsamples (962806, 951012 and 951020) is 1.28 � 0.07%/decade,which is about 60% lower than the initial average fading ratedetermined employing IR stimulation at 50 �C (3.0 � 0.1%/decade).The final average fading rates for post-IR IR stimulation are0.66 � 0.02%/decade, which is about 80% lower than the initialaverage fading rate determined employing IR stimulation at 50 �C.The final average fading rates for both IR and post-IR IR stimulationat 225 �C are indistinguishable from each other. For comparison,the final average fading rate for post-IR blue stimulation (bothstimulations at 50 �C) detected in the UV is 2.2 � 0.2%/decade.

6.3. Bleaching at room temperature and stimulation atelevated temperature

Stimulation at elevated temperature is generally avoided in OSLdose determinations because it has been shown that raising thestimulation temperature may access traps which are difficult tobleach in nature (Poolton et al., 2002a). In Fig. 11 the results frombleaching experiments carried out using natural aliquots areshown. The graph displays the normalised dose measured usinga SAR protocol as a function of daylight simulator exposure time.Each point is an average of three aliquots. Data are presented forsample 970204 NF stimulated at 50 �C (diamonds) and 225 �C(squares). Also shown are data for sample 970204 KF stimulated at225 �C (circles). In Fig. 11a the initial signal has been used for dosecalculation whereas the final signal has been used in Fig. 11b. Anydifferences in bleaching rate are subtle, suggesting that stimulationat elevated temperature does not access traps significantly morelikely to be incompletely bleached in nature.

7. Modern samples

It has been shown that feldspar bleaches much more slowlythan quartz, which probably makes the problem of incompletebleaching more important when feldspar is used as a dosimeter. Wehave measured the residual doses in both quartz- and feldspar-extracts from three modern beach sands (052266, 055642 and031734). The doses measured in the quartz fractions were10 � 1 mGy, �1 � 3 mGy and 39 � 5 mGy, respectively. In Fig. 12athe doses measured for the feldspar fractions (using IR stimulationat either 50 �C or 225 �C and the blue filter pack) are shown asa function of stimulation time. The relevant recycling ratios areshown in Fig. 12b. They all lie within �5% of unity, supporting theuse of our SAR protocol at both stimulation temperatures and overthe range of stimulation times examined here. The doses displayed

Table 3Average fading rates (%/decade) for IR stimulation detected using the blue filter pack

Stimulation �C KF initial KF final NF initial NF final

IR 50 3.0 � 0.1 1.4 � 0.1 3.1 � 0.2 1.4 � 0.1IR 225 1.95 � 0.13 0.77 � 0.10 2.35� 0.10 0.73 � 0.04Post-IR IR 225 1.28 � 0.07 0.66 � 0.02 1.40 � 0.20 1.00 � 0.60

For IR stimulation at 50 �C these data are the same as in Table 2. For IR stimulation at225 �C the values are averages of the results obtained from samples 963806 KF andNF, 951012 KF and NF, 951020 KF and NF, 970204 KF and NF, 972502 NF and 972516KF. For post-IR IR stimulation the values are averages of the results obtained fromsamples 963806 KF and NF, 951012 KF and 951020 KF.

are each the average from three aliquots. The initial dose in Fig. 12afor a stimulation temperature of 50 �C is about 0.5 Gy for allsamples and the final dose is about 0.7 Gy confirming that in thesewater-lain samples the feldspar signal is less well bleached than thequartz signal. Employing a stimulation temperature of 225 �C givesan initial dose less than 2 Gy and a final dose of less than 4 Gy. Thus,we do observe an increase in dose as a function of both stimulationtime and stimulation temperature implying that in nature the finalsignal accessed using higher stimulation temperature is slightly

b

Fig. 12. (a) Doses measured in the K-rich extract from three modern beach sands asa function of stimulation time. Each sample has been measured using IR stimulation atboth 50 �C and 225 �C, with the blue filter pack. (b) Recycling ratios from the dosemeasurements as a function of stimulation time.


more difficult to bleach than the initial signal. This suggests thatfeldspar is probably most useful as a natural dosimeter in thehigher dose range where a 4 Gy off-set is insignificant.

Fig. 13. Average fading rate as a function of uncorrected age for sample 951020 KF. Allstimulations employed IR light and detection was through the blue filter pack. The ageof the sample predicted from quartz is shown as a shaded band. Also shown are thepredicted relationship between fading rate and uncorrected age calculated using thedose rate determined for this sample.

8. Fading rate as a function of uncorrected age

In Fig. 13 the average fading rate is shown as a function ofuncorrected age for sample 951020 KF.5 The data were obtainedusing IR stimulation and detection was through the blue filter pack.The square symbols show data from stimulation at 50 �C obtainedat various stimulation times; the point plotting at the highestfading rate is calculated using the initial signal, whereas the pointplotting at the lowest fading rate is calculated using the final signal.The grey shaded area is centred on the age determined using thequartz fraction of sample 951020.6 The solid line shows the pre-dicted correlation between fading rate and uncorrected age, withthe latter calculated using the dose rate determined for this feld-spar sample, the quartz age and the Huntley and Lamothe (2001)fading correction model. The two dotted lines represent the valuesthat would give � one standard error on the known (quartz) age.Thus, if the age calculated using the initial signal (top square) wascorrected for fading (i.e. following the lower dashed line) an agejust below that predicted from quartz would be derived. However,the final signal (lowest fading rate) would result in an age tooyoung compared to that predicted by quartz, i.e. the horizontaldistance between the final point and the predicted correlationbetween fading rate and uncorrected age is larger than that for theinitial point. The slope of the data points is larger than that pre-dicted from model calculations. The closed circles represent thedata obtained from stimulation at 225 �C. Fading correction of alldata points would result in an age consistent with the quartz age(corrected ages would lie between the quartz mean age and themean � 1 standard error), but the slope of the data points is toosmall compared to that predicted by the fading model. The trianglesrepresent the data obtained from a post-IR IR stimulation, wherethe first IR stimulation is carried out at 50 �C and the second IRstimulation (from which the data are obtained) is carried out at225 �C. Fading correcting this data set would result in an over-estimation of the quartz age by about 20%, but interestingly theslope of these data points closely match that predicted from modelcalculations (dashed line).

Unfortunately, we have no independent age control for thissample (or any of the other samples investigated in this study), so itis not possible to determine which stimulation mode (if any) givesthe correct answer. The measured quartz dose is w200 Gy makingit difficult to be sure that we know the age of this sample. Futurework will focus on using known-age samples. The importantconclusion here is simply that we do see a correlation betweenfading rate and age.

9. Summary and discussion

Anomalous fading has for many years been the greatest obstacleto the use of natural feldspar as a reliable dosimeter. The workdescribed in this paper shows how different experimentalconditions can affect the anomalous fading rate observed in thelaboratory for a range of sedimentary sodium and potassium feld-spar-rich samples. The fading rate seems to be dependent on everyexperimental parameter investigated. In this section we summarise

5 The dose rate used is 3.29 � 0.13 Gy/ka. The contribution from internal betaactivity from 40K is based on an effective potassium content of 12.5 � 0.5% Huntleyand Baril (1997) and the beta dose rate contribution from 87Rb is calculatedassuming a Rb content of 400 � 100 ppm Huntley and Hancock (2001).

6 The quartz dose rate used is 2.27 � 0.11 Gy/s.

the most important results and discuss them in relation to thepublished models for luminescence production in feldspars. Themain results are:

(1) In the UV emission (detected through Hoya U-340) the fadingrate is highest for IR stimulation followed by green and bluelight stimulation. The difference between green and blue lightstimulation is small. If IR stimulation occurs before blue stim-ulation (e.g. post-IR blue stimulation) a lower fading rate isobserved; i.e. gIR > gGreen > gBlue > gpost-IRBlue.

(2) The fading rate decreases as a function of stimulation time forall stimulation and detection conditions (with the exception ofpost-IR blue stimulation where the fading rate is constant asa function of stimulation time). Thus, in general, the signal fromthe later part of the decay curve is more stable than that fromthe initial part.

(3) The fading rate decreases as a function of increasing stimula-tion temperature. The lowest fading rate in this study isobserved for post-IR IR stimulation at 225 �C using the laterpart of the signal.

(4) The UV emission seems to fade somewhat more than the blueemission.

(5) The IRSL signal from feldspar decays non-exponentially but UVbleaching (using a daylight simulator) produces a singleexponential reduction in residual dose, implying that a singletrap is being emptied.

In the simplest luminescence model (a system comprisinga single trap and a single recombination centre) the time betweenabsorption of a stimulating photon and the emission of a lumines-cence photon can be subdivided into: (1) the time required to evictthe electron; (2) the time required for lattice migration; and (3) therelaxation life time of the recombination centre. In the CW-OSL fromquartz the lattice migration time and the relaxation life time aresufficiently short that the observed CW decay is dominated by theeviction time (proportional to the photoionisation cross-section).Therefore, if retrapping is neglected a single exponential decay isexpected. Murray and Wintle (1999) showed that in their quartzsample a single trap explained w99% of the natural OSL. Other quartzsamples show non-exponential OSL decay, but this has beenexplained in terms of the linear superposition of several first orderOSL components derived from traps with different characteristics,particularly the optical cross-section (e.g. Jain et al., 2003).

The IRSL signal from feldspar also decays in a non-exponentialmanner and a possible explanation is that the signal arises from


a distribution of traps with different optical cross-sections. Bailiffand Poolton (1991), however, showed that IRSL decay followsa power law. Subsequently, the production of IRSL in feldspar wasexplained using a donor-acceptor model (Poolton et al.,1994), whereelectron tunnelling occurs from the excited state of the IRSL trap atabout 1.4 eV. This implies that the IRSL decay curve is a function oftunnelling probability, which is exponentially related to the distancebetween the donor and the acceptor recombination pair. In thismodel, the beginning of the IRSL decay curve originate with theluminescence emitted from close donor-acceptor pairs and the endof the IRSL curve represent the tunnelling of distant pairs.

Poolton et al. (1995b, 2002a,b) refined this model by includingthe possibility of transfer from the excited state of the IRSL trap intothe band-tail states. The more energetic an electron in the band-tailstates, the greater the probability of gaining access to distantrecombination centres.

Based on this view one would expect the fading rate of the initialpart of the signal to be greater than that of the later part of thesignal. This is what we have observed; the fading rate for IR stim-ulation detected both in blue and UV is about 50% lower using thefinal signal than using the initial part (see Fig. 7).

As the stimulation energy is increased (IR / green / blue)both the initial and final fading rates decrease. This is probablybecause at higher stimulation energies the probability of trans-ferring an electron from the ground state or excited state in a trapdirectly into the conduction band (or band-tail states) is increasedcompared to that for IR stimulation at the same temperature. Thusgreen/blue light stimulation will excite some trapped electronwhich otherwise have a low probability of giving luminescence (bytunnelling recombination) during IR stimulation, because the trap-recombination centre distance is too great. Excitation into theconduction band allows these electrons to access distant recom-bination centres on a much faster time scale. Nevertheless, becauseof slow hopping in the band-tails (and because of some contribu-tion from tunnelling recombination), the initial signal will againarise from more local recombination compared to the later signal.Thus, the initial blue or green stimulated signal should fade fasterthan the later signal, as observed here.

Similarly, a more stable signal will be obtained if one gives an IRwash to recombine all the closely spaced electron/hole pairs andthen uses blue light to access the distant centres via the conductionband (or band-tail states). This is observed in our post-IR blue OSLdata. In addition, the fading rate is apparently constant as a functionof post-IR blue stimulation time, suggesting that the post-IR bluesignal is almost entirely produced from charge transport via theconduction band. This model is further supported by the observa-tion that the fading rate for blue stimulation (without prior IRstimulation) using the final signal, is indistinguishable from thefading rate observed for post-IR blue stimulation (see Table 2).

Based on this model it is to be expected that IR stimulation atelevated temperature would produce a similar effect to blue lightstimulation. By increasing the IR stimulation temperature theprobability of promotion to the band-tail states (or to higher energyexcited states within the trap) increases and thus the electronswithout nearby centres have a greater probability of recombining atdistant centres either by conduction band transport or increasedprobability of tunnelling. However, the decay rate of IR stimulationas a function of stimulation temperature is constant. This suggeststhat in fact tunnelling recombination still dominates the lumines-cence production (because tunnelling is not thermally assisted).

By employing an IR wash (to recombine close electron/hole pairs)prior to IR stimulation at elevated temperature one might anticipatethat recombination after passage through the band-tail states woulddominate the luminescence production. This is supported by theobserved change in decay rate for post-IR IR stimulation as a functionof stimulation temperature (tunnelling recombination should not be

temperature sensitive). Thus, using post-IR IR stimulation can beexpected to produce a smaller fading rate than either IR at roomtemperature or IR at elevated temperature. This is what is experi-mentally observed. Interestingly, however, the post-IR IR fading ratealso decreases with stimulation time; unlike post-IR blue stimula-tion. This may simply be a reflection of the difference betweentransport via the band-tail states (post-IR IR) and transport throughthe conduction band (post-IR blue).

Blue emission seems to fade somewhat less than UV emissionand, using the initial signal, the sodium-rich samples appear to fademore than the potassium-rich samples. Generally (although notinvariably) sodium-rich feldspars tend to emit more strongly in theUV than potassium-rich feldspars. One can reasonably concludethat electron/hole distances are usually shorter if the hole is trap-ped in a UV centre, than if it is trapped in a blue centre. It may bethat UV centres are located predominately in regions of the crystalwith a high trap/recombination centre density and that the bluecentres predominately appear in areas which cannot build up sucha high donor-acceptor population. This speculation is supported bythe fact that the final fading rates are indistinguishable for sodium-rich and potassium-rich feldspars.

The reduction in residual signal by bleaching (Fig. 8) can berepresented well by a single exponential, implying that emptying ofa single trap dominates the decrease in luminescence. The singletrap hypothesis is further supported by flat preheat plateaus above200 �C determined for these samples (data not shown) and alsoreported in the literature for other samples (e.g. Wallinga et al.,2000; Preusser, 2003). If we are indeed looking at a single trap thenthe non-exponential luminescence decay curves universallyobserved from feldspar must be caused by variation in lumines-cence efficiency, perhaps by varying competition betweenpopulated luminescence and non-luminescence centres. It hasbeen reported that exposure to IR light decreases the entire TLcurve (Duller and Bøtter-Jensen, 1993) and this was interpreted asimplying that the IRSL signal arises from a continuum of traps. If weassume instead that a single trap decays exponentially duringoptical stimulation, then the decrease in the TL signal would haveto result from a change in recombination probability as a conse-quence of IR stimulation.

10. Implications for dosimetry

The observation that the later part of the feldspar luminescencecurve is as readily bleached by simulated daylight as the initial parthas important implications for the use of feldspars as dosimeters. Itappears that there is no ‘‘a priori’’ reason to select one part of theluminescence curve in preference to another. This is in markedcontrast to quartz OSL, where it is clear that the initial part of theOSL curve is most readily bleached in nature. Thus one mayconsider identifying the most useful feldspar signal on the basis onfading rate: the smaller the fading rate the smaller the correction tothe observed age, and so the smaller the dependence on the modelused to correct the age. On the basis of the work presented here, thelater part of the post-IR blue, or the post-IR (elevated temperature)IR luminescence curves are apparently the most stable, and thuslikely to be the most accurate dosimeters (at least from the point ofview of anomalous fading). Further optimisation of measurementparameters may well lead to even lower fading rates, but it remainsto be seen if these much reduced laboratory fading rates arerelevant to geological timescales.

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Laboratory fading rates of various luminescence signals from feldspar-rich sediment extracts

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