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Radiation Measurements 47 (2012) 210e218

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Radiation Measurements

journal homepage: www.elsevier .com/locate/radmeas

Comparison of the properties of various optically stimulated luminescence signalsfrom potassium feldspar

Xiao Fu, Jia-Fu Zhang*, Li-Ping ZhouMOE Laboratory for Earth Surface Processes, Department of Geography, College of Urban and Environmental Sciences, Peking University, Haidian District, Beijing 100871, China

a r t i c l e i n f o

Article history:Received 6 December 2010Received in revised form30 June 2011Accepted 23 December 2011

Keywords:K-feldspar specimenVarious optically stimulated luminescencesignalsPost-IR IRSLThermal stabilityBleaching

* Corresponding author. Tel./fax: þ86 10 82754411.E-mail addresses: jfzhang@pku.edu.cn, jfzhang@gr

1350-4487/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.radmeas.2011.12.007

a b s t r a c t

Various optically stimulated luminescence signals from K-feldspar have been used to determine theequivalent doses of sediment samples. Understanding the properties of these optical signals is critical toevaluate their applicability and limitations to optical dating. In this paper, some properties of IRSL, post-IR OSL and post-IR IRSL signals (detected in the UV region using U-340 filters) from a museum sample ofK-feldspar were investigated by analyzing the relationships between optical and TL signals, and the effectof optical bleaching and heating on optical signals. The trap parameters of the different optical signalswere calculated using the pulse annealing method. The results show that this sample exhibits tworegenerated TL peaks at w140 and w330 �C. Corresponding to the low temperature TL peak, the OSL andpost-IR OSL signals appear to be more associated with lower temperature TL than the IRSL signalmeasured at 50 �C. Corresponding to the high temperature TL peak, the post-IR IRSL signals mainlyoriginate from the more thermally stable traps associated with the high temperature TL, compared withthe IRSL and post-IR OSL signals. However, the post-IR IRSL225 �C signal is shown to be hard to bebleached by blue light and simulated sunlight, compared with the IRSL50 �C and low temperature post-IRIRSL signals. The implication for optical dating is that the elevated temperature post-IR IRSL signals canbe preferentially applied over other signals from K-feldspar, but it is desirable that the effectiveness ofthe pre-depositional zeroing of these signals is assessed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In luminescence dating (thermally stimulated luminescence(TL) dating, optically stimulated luminescence (OSL) dating) tech-niques, which directly date the deposition of sediments, detritalquartz or feldspar grains extracted from sediments are commonlyused as palaeodosemeters (Aitken, 1985, 1998). Since the single-aliquot regeneration-dose (SAR) method for quartz was proposed(Murray and Wintle, 2000; Wintle and Murray, 2006), quartz hasbeen widely used in the OSL dating of sediments. On the otherhand, K-feldspar has some obvious advantages in OSL dating overquartz. K-feldspar is brighter, and appears to saturate at a higherdose. It can thus be used for extending the age range of OSL dating(Aitken, 1998). Unfortunately, anomalous fading of luminescencefrom feldspar has hindered its extensive use, although some fadingcorrections have been employed (e.g. Huntley and Lamothe, 2001;Lamothe et al., 2003; Kars et al., 2008). Recently, it is reported thatthe effect of anomalous fading on dating can be significantly

aduate.hku.hk (J.-F. Zhang).

All rights reserved.

reduced if an isochron dating approach is applied (Li et al., 2008), orthe luminescence components with lower fading rates are used(Tsukamoto et al., 2006; Thomsen et al., 2008; Buylaert et al., 2009;Thiel et al., 2011; Jain and Ankjærgaard, 2011).

Compared to quartz, K-feldspar can emit different opticallystimulated luminescence signals (optical signals) by using differentstimulation modes. These signals include IRSL determined usinginfrared (IR) light stimulation (Hütt et al., 1988), OSL using blue/green-light stimulation (Huntley et al., 1985), post-IR OSL obtainedby a blue-light stimulation following an IR stimulation (Roberts andWintle, 2001; Banerjee et al., 2001; Zhang and Zhou, 2007; Zhanget al., 2007), elevated temperature IRSL (Poolton et al., 2002) andpost-IR IRSL measured using an elevated temperature IR stimula-tion following a low temperature IR stimulation (Thomsen et al.,2008; Buylaert et al., 2009). These signals have different proper-ties and can be employed to determine the equivalent doses (De) ofK-feldspar samples. In order to decide which signal can providebest De estimates, their properties such as thermal stabilities andbleachability should be better understood. Previously, such inves-tigations were mainly performed on sedimentary feldspar (Dullerand Bøtter-Jensen, 1993; Duller, 1994, 1995; Li and Tso, 1997;Murray et al., 2009; Li and Li, 2011; Thomsen et al., 2011), and

X. Fu et al. / Radiation Measurements 47 (2012) 210e218 211

the behaviors of these signals have been interpreted using differentphysical models (e.g. Li and Li, 2011; Jain and Ankjærgaard, 2011;Thomsen et al., 2011). In this paper, a museum specimen of K-feldspar was investigated to determine the thermal stabilities andrelationships of the optical signals including IRSL, post-IR OSL andpost-IR IRSL signals, and the trap parameters associated with theseoptical signals were determined using the pulse annealing method(Li et al., 1997; Wintle and Murray, 1998).

2. Sample description and experimental details

The studied sample is one part of a specimen (Fig. S1) obtainedfrom museum collections. X-ray powder diffraction analysis indi-cated that the specimen is mainly composed of microcline, witha very small amount of contaminant albite (Fig. 1). No plagioclasesin a thin section were observed under a petrologic microscope(with transmitted polarized light). In the laboratory, the samplewas carefully crushed by hand using an agate mortar and pestle.Coarse grains (90e125 mm) were then obtained by wet sieving, andtreated with 10% HCl for 30 min and washed with deionized water.All the grains obtained were heated to 500 �C in a muffle furnaceand held at that temperature for 1 h before cooling to roomtemperature, in order to remove natural luminescence signals andsensitize the K-feldspar grains. After this thermal treatment, bothOSL and TL sensitivities remain constant, indicated by repeatedlyirradiating, preheating and measuring OSL or TL signals on analiquot. Therefore, the correction for sensitivity changes does notneed to be taken into account for the following experiments.

All luminescence measurements, beta irradiation, preheat andannealing treatments were performed in an automated Risø TL/OSL-DA-15 reader equipped with a 90Sr/90Y beta source (Bøtter-Jensen et al., 2000). Blue LED (470 � 30 nm, 40 mW/cm2 in fullpower) and IR laser diode (830 � 10 nm, 250 mW/cm2 in fullpower) were used for blue-light and IR stimulation, respectively.Luminescence was detected by an EMI 9235QA photomultipliertube with three 2.5 mm Hoya U-340 filters in front of it. In order tocompare different optical signals, emissions were detected in theUV region using U-340 filters (detection window 290e370 nm) forboth IR and blue-light stimulations, although the IRSL signals fromK-feldspar are usually detected in the VIS region with BG-39 filters(330e620 nm).

All TL signals weremeasured to 500 �C at a heating rate of 5 �C/s,followed by a second measurement to determine the background.All IRSL and OSL measurements were made at a sample tempera-ture of 50 �C except for post-IR IRSL measurements which wascarried out at different elevated temperatures. IR-stimulation timefor IRSL and post-IR IRSL measurements were set to 100 and 200 s,

10 20 30 40 50 600

10

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40

AM

A

M MMMM

Inte

nsity

, CPS

, x10

3

2 Theta

M M: MicroclineA: Albite

Fig. 1. XRD spectra of the studied sample. The measurement was performed usinga Dmax 12 kw X-ray powder diffractometer.

respectively, and blue-light stimulation time for post-IR OSLmeasurements to 200 s. These stimulation times were chosen asthey can deplete the signals to a negligible level. The signal inten-sities were taken the first 2.08 s integral of the initial OSL signal ofthe decay curves minus a background estimated from the last 3.2 sintegral. Unless otherwise specified, all heating rates in this studywere 5 �C/s.

3. Results and discussion

3.1. Relationships between TL and optical signals

3.1.1. Effect of optical bleaching on TLThe relationships between OSL and TL signals are commonly

investigated by observing the effect of optical bleaching on TL glowcurves (e.g. Duller and Wintle, 1991; Duller, 1995, 1997; Murrayet al., 2009). Here similar experiments were carried out on themuseum K-feldspar sample. The effects of IR50 �C (the subscriptdenotes the IR-stimulation temperature) and post-IR blue-lightstimulation (IR þ blue-light stimulation: blue-light stimulation at50 �C following a 100 s IR50 �C stimulation) on TL were firstlyinvestigated, the results are shown in Fig. 2a and c. The lost TLsignal due to optical bleaching was also calculated and shown inFig. 2b and d. It can be seen that the regenerated TL curves for thissample have two distinct TL peaks atw140 andw330 �C. Based onthe TL peaks, the TL glow curve can be divided into two parts: thelow temperature TL corresponding to the w140 �C TL peak and thehigh temperature TL corresponding to the w330 �C TL peak. Thefigures show that all these optical stimulations reduced the TLsignal over the whole temperature range (50e500 �C), mainly fromthe two main peaks, as reported in previous studies (e.g. Duller,1995, 1997; Murray et al., 2009). The comparison of Fig. 2a and cindicates that the post-IR blue-light bleaching is much more effi-cient in reducing TL from the two TL peaks than the IR bleaching.The TL reduction in the whole temperature range due to opticalbleaching might be caused by either that the traps associated withthe TL peaks give rise to IRSL/post-IR OSL signals, or that the TLrecombination probability is reduced by prior optical stimulation(Murray et al., 2009; Jain and Ankjærgaard, 2011).

The effects of IR50 �C, post-IR IRT �C (IR50 �C þ IRT �C stimulation:200 s IR stimulations at elevated temperatures (T �C) after a 100 sIR50 �C stimulation) and post-IR blue-light stimulation on the hightemperature TL were investigated separately. The sample waspreheated at 300 �C for 10 s after irradiation and before opticalstimulation, the remaining TL after various optical bleaches and thecorresponding lost TL are shown in Fig. 2e and f, respectively. Thefigures show that the maximum reductions in TL due to the IR50 �Cand post-IR blue-light bleaches are centered atw315 andw330 �C,respectively, and those due to the post-IR IR100 �C, post-IR IR150 �Cand post-IR IR225 �C bleaches occur at w360 �C. The thermaldepletion of the TL signals during the 200 s post-IR IR stimulation athigh temperatures was examined by holding the sample at hightemperatures for 200 s without IR stimulation, and the thermalerosion was found to be negligible (insets to Fig. 2e and f). This canbe explained by the fact that the IR stimulation temperatures (nohigher than 225 �C) is much lower than the preheat temperature of300 �C. The reduction peak shifts in Fig. 2f are thus considered notto be caused by thermal erosion. It should also be noted that noobvious phototransfer was observed in this study.

The difference between the peak positions in Fig. 2f indicatesthat the post-IR IR signals are mainly associated with the trapsresponsible for the higher temperature TL compared with the IRSLand post-IR OSL signals. Based on a multiple-trap model, in whichfeldspar TL peaks are believed to represent various traps withdifferent depths, Fig. 2f may indicate that the traps sensitive to the

Fig. 2. Remaining TL after optical bleaching. In (a) an aliquot was given a laboratory beta dose of 55 Gy, the regenerated TL signal was then measured following an IR stimulation at50 �C for various time periods. In (c) an aliquot was given a dose of 55 Gy followed by a 100 s IR stimulation at 50 �C, a blue-light (post-IR blue light) stimulation at 50 �C for varioustime periods was then performed and the TL was finally measured. In (e) an aliquot was given a dose of 55 Gy followed by a preheat of 300 �C for 10 s, and then bleached at 50 �Cwith IR for 100 s followed by another 200 s IR (post-IR IR) bleach at an elevated temperature (100, 150 and 225 �C), the remaining TL was finally measured; for the post-IR blue-lightbleaching, the post-IR IR bleach in the above procedure was replaced by a 200 s blue-light (post-IR blue light) stimulation at 50 �C. (b), (d) and (f) are the lost TL (DTL) due to opticalbleaching, calculated from the data in (a), (c) and (e): (b) DTLIR ¼ TL0-IR�TLn-IR (the subscripts n and the number denote the bleaching time in seconds); (d) DTLpost-IR_blue ¼ TL100-IRþ0-

post-IR_blue�TL100-IRþn-post-IR_blue; (f) For the IR50 �C bleaching, DTLIR ¼ TL0-IR�TL100-IR; for the post-IR IR bleaching, DTLpost-IR_IR ¼ TL100-IR�TL100-IRþ200-post-IR_IR; for the post-IR blue-light bleaching, DTLpost-IR_blue ¼ TL100-IR�TL100-IRþ200-post-IR_blue. In the inset to (e), an aliquot was given a dose of 55 Gy followed by a preheat of 300 �C for 10 s, and then bleached at50 �C with IR for 100 s followed by holding the aliquot at an elevated temperature (100, 150 and 225 �C) for 200 s without light stimulation, the remaining TL was finally measured.The lost TL due to the thermal depletion is shown in the inset to (f).

Table 1A procedure used for testing the effect of preheating on various optical signals.a

Step Treatment Observed

1 Give a beta dose of 55 Gy2 Preheat for 10 s at Ti (Ti ¼ 100e500 �C)3 Stimulate with IR light at 50 �C for 100 s IRSL50 �C

4 Stimulate with blue light at 50 �C for 200 s Post-IR OSL8 Heat to 500 �C Remove all

residual signals9 Return to 1

a For only OSL signal, step 3 was omitted. For post-IR IRSL100 �C, post-IR IRSL150 �C

and post-IR IRSL225 �C signals, Ti ¼ 300e500 �C at step 2, blue-light stimulation atstep 4 was replaced by successive 200 s IR stimulations at 100, 150 and 225 �C.

X. Fu et al. / Radiation Measurements 47 (2012) 210e218212

IR50 �C, post-IR blue-light and higher temperature post-IR IR stim-ulations are slightly different, and the post-IR IR stimulationdepletes the deeper traps with higher thermal stability. Alterna-tively, the reduction peaks in Fig. 2f can also be explained witha single-trap model, in which different TL peaks are considered tooriginate from a single trap, but reflect different donor-acceptordistances (Jain and Ankjærgaard, 2011). The TL reduction peakshifts to higher temperatures for the post-IR IRSL signals can thusbe interpreted as that the prior IR stimulation has used up most ofthe nearby trap-hole pairs, then the post-IR IR stimulation mainlydeplete those distant trap-hole pairs.

3.1.2. Effect of preheating on optical signalsThe influence of annealing or preheating on optical signals has

beenwidely investigated (e.g. Duller, 1994, 1995, 1997; Zhang et al.,2005; Murray et al., 2009; Thomsen et al., 2011). Here a similarprocedure (Table 1) was also used for this heat-treated sample. The

results for the remaining IRSL50 �C, post-IR OSL and OSL (onlystimulated with blue light) signals after preheats of 100e500 �C for10 s are shown in Fig. 3a. It shows that the IRSL50 �C signal remainsroughly constant in the preheat temperature range of 100e160 �C,

100 200 300 400 5000.0

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Post-IR IRSL150°C

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a

b

Fig. 3. Effect of preheating on (a) the IRSL, post-IR OSL and OSL signals in the preheattemperature range of 100e500 �C, and (b) the IRSL, post-IR OSL and post-IR IRSLsignals in the range of 300e500 �C. See the text for details. All the data werenormalized to unity at the first point of each curve.

X. Fu et al. / Radiation Measurements 47 (2012) 210e218 213

and begin at 160e180 �C to decrease. The plateau between 100 and160 �C indicates that the preheating in this temperature range doesnot significantly affect the hole concentration, thus there is negli-gible sensitivity reduction for the IRSL signals. This also suggeststhat the IRSL50 �C signal cannot come from the traps associated withthe w140 �C TL peak, and are most likely to come from the trapsassociated with the higher temperature TL, as reported previously(e.g. Duller, 1995; Murray et al., 2009). But Fig. 2a shows that IR50 �Cbleaching does remove TL from the low temperature TL around140 �C. The comparison of Figs. 2a and 3a indicates that the loss ofTL from thew140 �C TL peak due to IR bleaching may be caused bydecrease of recombination probability (Murray et al., 2009) orrecombination with non-radiative centers (Duller, 1995).

Fig. 3a shows that the post-IR OSL signal starts to decrease atpreheat temperature of 100e120 �C. Considering the facts that (1)previous IR stimulation has used up most of the proximal holes,and (2) the post-IR OSL signal mainly originates from the recom-bination via the conduction band, we argue that the luminescencerecombination efficiency are not significantly influenced by pre-heating. Thus, the continuous decrease with preheat temperaturesuggests that the post-IR OSL signal originates from the trapsassociated with both of the low and high temperature TL, asdemonstrated by Fig. 2c and d. Fig. 3a also shows that the OSLdepletion rate due to preheating is between those of the IRSL50 �Cand post-IR OSL signals. The most likely explanation for this is thatthe OSL signal originates from the traps associated with both theIRSL50 �C and post-IR OSL signals. This is consistent with previouswork by Duller and Bøtter-Jensen (1993) who showed that part ofOSL signals come from the traps associated with low temperatureTL peaks.

The effect of preheating on the optical signals from the trapsassociated with the high temperature TL was also tested using theprocedure in Table 1. The remaining IRSL50 �C, post-IR OSL and post-IR IRSL signals obtained are shown in Fig. 3b. The figure indicatesthat the thermal depletion of these optical signals are changed withincreasing preheat temperature and the change for each signal isdifferent. For example, the IRSL50 �C, post-IR OSL, post-IR IRSL100 �C,post-IR IRSL150 �C and post-IR IRSL225 �C signals measured after thepreheat temperature of 400 �C are 1%, 8%, 15%, 33% and 45% of thefirst point (300 �C) of each curve, respectively. This implies that thepost-IR IRSL signals aremore thermally stable than the IRSL50 �C andpost-IR OSL signals.

Considering that the blue OSL signal is a direct ionizationprocess (Jain and Ankjærgaard, 2011) and therefore may sampletraps of different stabilities, the pulse annealing curve for the post-IR OSL signal (Fig. 3b) may reflect the thermal stability of all bluelight sensitive traps. The difference in thermal stability between theIRSL50 �C and post-IR IRSL signals can be explained with either themultiple-trap or the single trap model. Based on the multiple-trapmodel, the IRSL50 �C and post-IR IRSL signals are associated withdifferent traps. In the temperature range of 300e500 �C, the highertemperature post-IR IRSL signals such as post-IR IRSL225 �C originatefrom the deeper traps associated with the higher temperature TL,while the IRSL50 �C signal is mainly associated with relativelyshallower traps responsible for the lower temperature TL. Based onthe single-trap model (Jain and Ankjærgaard, 2011; Thomsen et al.,2011), the influence of preheat on optical signals can be interpretedas a reflection of recombination efficiency changes. In this model,all IR-sensitive signals are considered to originate from an identicaltrap, but represent different recombination routes. The IRSL50 �Csignal represents the recombination with proximal holes viaexcited state, and the post-IR IRSL signals represent the recombi-nation with more distant holes via band tail state (the highertemperature post-IR IRSL signals represent recombination withmore distant trap-hole pairs), i.e. the accessible crystal volume r3

(r is the radius vector) for IRSL50 �C and post-IR IRSL signals is in theorder r3IRSL < r3low temperature post-IR IRSL < r3high temperature post-IR IRSL.The hole population is proportional to the accessible crystalvolume. During preheating, the near-by recombination sites arepreferentially used up. The IRSL50 �C signal, in the pulse annealingexperiments is significantly reduced with increasing preheattemperature due to reduction in available recombination sites. Thiscan be explained by (1) the related near-by holes are preferentiallyfilled and (2) the hole population is limited. For the post-IR IRSLsignals, more distant holes can be access with increasing stimula-tion temperature, thus such efficiency reduction caused by pre-heating becomes less severe. As a result, the post-IR IRSL signalobtained at higher stimulation temperature shows a higherthermal stability (Fig. 3b).

3.2. Bleaching of post-IR IRSL signals

In order to understand the bleaching properties of the post-IRIRSL signals, bleaching experiments were carried out using theprocedure in Table 2, with the IR and blue lights in the reader andsunlight from an ORIEL sunlight simulator. Additionally, thethermal transfer due to the 300 �C preheat in step 2 was alsoevaluated by comparing the optical signals obtained using thefollowing procedures. The IRSL50 �C, post-IR IRSL100 �C, post-IRIRSL150 �C and post-IR IRSL225 �C signals from an aliquot weremeasured using the procedure in Table 2, in which a 1000 s IRbleach at 225 �C is inserted between steps 1 and 2, and step 3 isomitted. These signals (S1) obtained are considered to be thosecaused by thermal transfer, and compared to the signals (S2) ob-tained using the procedure in Table 2 without step 3. The ratios of

Table 3The remaining optical signals after 5000 s bleach using various light sources.

Bleaching light Remaining optical signals, %

IRSL50 �C Post-IRIRSL100 �C

Post-IRIRSL150 �C

Post-IRIRSL225 �C

IR light 0 4.1 54.3 83.1Blue light 0 1.7 8.3 21.4Simulator sunlight 0.4 0.5 0.8 4.3

Table 2A procedure employed to test the bleaching of the post-IR IRSL signals.

Step Treatment Observed

1 Give a beta dose of 138 Gy2 Preheat at 300 �C for 10 s3 Stimulate with IR light at 50 �C for ti

(ti ¼ 0e5000 s)IR bleachinga

4 Stimulate with IR light at 50 �C for 100 s Remaining IRSL50 �C

5 Stimulate with IR light at 100 �C for 200 s Post-IR IRSL100 �C

6 Stimulate with IR light at 150 �C for 200 s Post-IR IRSL150 �C

7 Stimulate with IR light at 225 �C for 200 s Post-IR IRSL225 �C

8 Heat to 500 �C Remove all residualsignals

9 Return to 1

a For blue-light and sunlight bleaching, the IR light used at step 3 was replaced byblue light and simulator sunlight, respectively.

X. Fu et al. / Radiation Measurements 47 (2012) 210e218214

S1 (thermal transfer signals) to S2 are 0.2%, 0.4%, 0.7% and 0.8% forthe IRSL50 �C, post-IR IRSL100 �C, post-IR IRSL150 �C and post-IRIRSL225 �C signals, respectively, suggesting that the thermal trans-fer effects in this bleaching experiment are negligible.

Fig. 4 shows the remaining post-IR IRSL signals after bleaches ofIR, blue-light and simulated sunlight, the remaining IRSL50 �C signalobtained at step 4 is also given for comparison. In addition, theremaining post-IR IRSL signals after 5000 s bleaching are alsosummarized in Table 3. It can be seen that the higher temperaturepost-IR IRSL signals are harder to bleach than the lower tempera-ture signals. Fig. 4a and Table 3 also indicate that the intensity of thepost-IR IRSL signals is dependent on prior IR50 �C stimulation time,

0.01

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Fig. 4. Effect of IR (a), blue-light (b) and simulated sunlight (c) bleaching on the post-IRIRSL signals. The IRSL50 �C signals (see text) are given for comparison. All data arepresented as the percentage of the first point of each curve.

the post-IR IRSL150 �C and post-IR IRSL225 �C signals are difficult tobleach using IR50 �C stimulation.

Fig. 4b and Table 3 demonstrate that the blue-light bleaching at50 �C can easily zero the IRSL50 �C and post-IR IRSL100 �C signals, butis less effective in removing the post-IR IRSL150 �C and post-IRIRSL225 �C signals. This suggests that some traps responsible forpost-IR IRSL signals in K-feldspar may not be sensitive to blue light.This thus probably indicate that at least a part of the post-IR IRSLsignals is derived from the traps different from those for the post-IROSL or OSL signals. On the other hand, elevated temperature IRstimulations do not completely remove the post-IR OSL signal evenfor long IR-stimulation time (Jain and Singhvi, 2001), suggestingthat either some traps in K-feldspar are only sensitive to blue light(Jain and Singhvi, 2001; Wallinga et al., 2002), or a highertemperature IR simulation is needed to release those traps.Comparatively, the sunlight bleaching is more effective in removingthe post-IR IRSL signals, even for the post-IR225 �C signal (Fig. 4c andTable 3). However, the small remaining post-IR IRSL225 �C signal(4.3%) after 5000 s simulated sunlight bleaching still suggests thatthe degree of bleaching of samples should be taken into consider-ation when higher temperature post-IR IRSL signals are used fordating.

3.3. Pulse annealing curves

A high temperature preheat (>250 �C) is now widely used toisolate more stable optical signals in optical dating of K-feldspar(e.g. Aulair et al., 2003; Buylaert et al., 2009), although the intensityof IRSL signals is dramatically reduced due to such a high temper-ature preheat, as indicated by Fig. 3a.To better understand thethermal stability of these optical signals used for dating from thetraps associated with the high temperature TL, pulse annealingexperiments with various heating rates (Duller, 1994; Li et al., 1997;Li and Tso, 1997; Li and Chen, 2001; Fan et al., 2009) were carriedout on this museum sample. The procedure in Table 4 was used forconstructing pulse annealing curves with various heating rates forthe IRSL50 �C, post-IR OSL and post-IR IRSL225 �C signals. It is notedthat the sample had been preheated at 250 �C for 60 s prior to pulseannealing measurements. The results are shown in Fig. 5a, c and e,and the reduced optical signals due to increasing annealingtemperature are shown as reduction curves in Fig. 5b, d and f.

3.3.1. Interpretation using multiple-trap modelThe shapes of the reduction curves in Fig. 5b, d and f appear to

be composed of at least two peaks. In order to distinguish thesesignals and analyze their components, here we assume that thereduction curves are composed of two peaks, which follow first-order kinetics. It should be noted that although the presence ofthe excited state and band-tail state (Poolton et al., 2009) indicatesthat the kinetic order of feldspar is more complex, the first orderassumption is commonly used in previous studies (e.g. Li et al.,1997; Murray et al., 2009; Li and Li, 2011), because the analysis,including the multiple energy levels in feldspar, is very compli-cated. A TL glow-curve deconvolution software GlowFit (Puchalska

Table 4A procedure used for pulse annealing experiments.

Step Treatment Observed

1 Give a beta dose of 173 Gy2 Preheat at 250 �C for 60 s3 Stimulate with IR light at

50 �C for 100 s4 Heat (annealing) to Ti

(Ti ¼ 200e500 �C) at aheating rate

5 Stimulate with IR light at225 �C for 0.1 s (50% of thelaser diode power)

Post-IR IRSL225 �Ca

6 Repeat steps 4 and 5 usingvarious annealing temperaturesfrom 200 to 500 �C with anincrement of 10 �C.

a For post-IR OSL signal, step 5 (0.1 s IR stimulation at 225 �C) was replaced by0.1 s blue-light stimulation at 50 �C. For IRSL50 �C signal, the 100 s IR stimulation at50 �C at step 3 was omitted, and the 0.1 s IR stimulation at step 5 was performed at50 �C.

X. Fu et al. / Radiation Measurements 47 (2012) 210e218 215

and Bilski, 2006) was employed to fit the reduction curves. Exam-ples of the fits to the three signals obtained using the heating rate of3 �C/s are shown in Fig. 6. It can be seen that the data in thetemperature range of >280 �C are fitted very well with two peaks,

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Temperature, °C

a b

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fe

Fig. 5. Pulse annealing curves obtained using different linear heating rates for the IRSL50normalized to the first point of each curve. The signal reduction (DI ¼ IT�ITþ10 �C) duetemperature for IRSL50 �C (b), post-IR OSL (d) and post-IR IRSL225 �C (f) signals, respectively. Tobtained from an aliquot on which successive 0.1 s pulse stimulation at 50 �C was carried

however, this is not the case for the data in the lower temperaturerange of<280 �C. This is probably because the preheat of 250 �C for60 s before pulse annealing had erased some relatively lowertemperature (<280 �C) TL signals, or the first order assumption isflawed.

Based on the fitted peaks in Fig. 6, the traps associated with thehigh temperature TL can be divided into two groups: the traps (TrapA) associated with relatively low temperature TL and those (Trap B)with relatively high temperature TL. The division of trap A and trapB is just based on the difference in the thermal stability, so theorigins of traps A and B associated with different signals are notnecessarily identical. Based on the integral of the correspondingpeaks in Fig. 6, the ratios of Traps A to Bwere calculated to be 3.7,1.3and 0.4 for the IRSL50 �C, post-IR OSL and post-IR IRSL225 �C signals,respectively. For the IRSL50 �C and post-IR OSL signals, the corre-sponding traps are dominated by Trap A, and for the post-IRIRSL225 �C signal, the traps dominated by Trap B. The traps associ-ated with relatively high temperature TL are believed to be morestable compared to those with relatively low temperature TL.Therefore, the ratios of Traps A to B can indicate the stability of thecorresponding optical signals. The ratios suggest that the thermalstability of these signals are post-IR IRSL225 �C > post-IROSL> IRSL50 �C, this is consistent with the behaviors of these signalsshown in Fig. 3. The implication for dating is that elevated

0.0

0.3

0.6

0.9

1.2IRSL50°C

0.0

0.3

0.6

0.9

1.2Post-IR OSL

Opt

ical

sig

nal r

educ

tion

(I),

%/°C

200 250 300 350 400 450 5000.0

0.3

0.6

0.9

1.2Post-IR IRSL 225°C

Temperature, °C

�C (a), post-IR OSL (c) and post-IR IRSL225 �C(e) signals, respectively. The data wereto increasing annealing temperature were also plotted as the function of annealinghe signal depletion caused by the 0.1 s stimulations were corrected for using the factorsout without annealing.

Fig. 6. Curve fitting results for the pulse annealing data of the IRSL50 �C (a), post-IR OSL(b) and post-IR IRSL225 �C (c) signals obtained using a heating rate b of 3 �C/s in Fig. 5b,d and f.

11.2

12.0

12.8

13.6Trap A

1/Tm, K-1, x10-3

ln(T

2 m/β

)

y=18224x-20.26

IRSL50°CTrap B

y=19576x-19.51

y=18148x-20.87

Post-IR OSL

11.2

12.0

12.8

13.6

y=19929x-21.42

1.6 1.7 1.8 1.9

11.2

12.0

12.8

13.6

y=20887x-22.11

Post-IR IRSL225°C

a

b

c

Fig. 7. Arrhenius plots for Traps A and B for the IRSL50 �C, post-IR OSL, and post-IRIRSL225 �C signals without deconvolution. The Tm values were obtained from curvefitting results of Fig. 5b, d and f.

Table 5Trap parameters of IRSL, post-IR OSL and post-IR IRSL225 �C signals.

Signal E (eV) S (s�1) s (ka) at20 �C

TL peakposition (�C)

IRSL50 �C (Trap A) 1.57 � 0.14 1.1 � 1013 2.8 � 103 310IRSL50 �C (Trap B) 1.69 � 0.13 5.6 � 1012 5.5 � 105 365Post-IR OSL (Trap A) 1.57 � 0.15 2.1 � 1013 1.2 � 103 295Post-IR OSL (Trap B) 1.72 � 0.13 4.0 � 1013 2.7 � 105 340Post-IR IRSL225 �C 1.80 � 0.14 8.4 � 1013 3.3 � 106 355

X. Fu et al. / Radiation Measurements 47 (2012) 210e218216

temperature post-IR IRSL signals from K-feldspar should be pref-erentially used with respect to thermal stability. A reduced anom-alous fading rate for the post-IR OSL and post-IR IRSL signals(Thomsen et al., 2008) may be explained by a higher Trap Bcomponent that may have a lower fading rate (Li and Li, 2011).

The trap parameters of the different components of the IRSL50 �C,post-IR OSL and post-IR IRSL225 �C signals were calculated based onthe method of Li et al. (1997). Assuming first order kinetics, therelationship between thermal parameters of luminescence trapsand linear heating rates employed can be expressed as:

ln�T2m=b

�¼ ðE=kTmÞ � lnðSk=EÞ (1)

Where b is the linear heating rate and Tm is the temperature atwhich the reduction rate of the luminescence signals reaches theirmaximumvalues. E, S and k are the trap depth, frequency factor andBoltzmann constant, respectively. The values of Tm at variousheating rates for Traps A and B were determined by the position ofthe fitted peaks (Figs. 5 and 6). E and S values were obtained byplotting lnðT2

m=bÞ versus 1/Tm (Fig. 7). The lifetime s was thuscalculated using the following equation (Aitken, 1985):

s ¼ S�1expðE=kTÞ (2)

The corresponding TL peak position T * was obtained using theequation (Aitken, 1985)

E=kT*exp�E=kT*

�¼ sT*=b (3)

All kinetic parameters obtained are listed in Table 5. It is notedthat for the post-IR IRSL225 �C signal, only the trap parameters for

the whole 330 �C peak were calculated, because Tm for Traps A andB cannot be precisely determined by fitting thewide peak. It shouldalso be noted that because in such calculation the depth of exitedstate and band-tail state are not taken into consideration, the trapdepth obtained may be a lower limit value. It is shown that thecalculated trap depth of the post-IR IRSL225 �C signal is larger thanthat of the post-IR OSL signal, and both are larger than that of theIRSL50 �C signal. The lifetimes of Traps A and B for IRSL50 �C signalwere calculated to be 2.8 � 103 and 5.5 � 105 ka at 20 �C, respec-tively, and for the post-IR OSL signal they are 1.2 � 103 and2.7 � 105 ka, respectively. These lifetime values are smaller thanthat (3.3 � 106 ka) of the traps for the post-IR IRSL225 �C signal. Thecorresponding TL peaks calculated for these signals are broadlyconsistent with those observed in Fig. 2f. The results for theIRSL50 �C signal are similar to that of Li and Tso (1997). The differ-ence in the trap parameters of the post-IR IRSL signals between this

X. Fu et al. / Radiation Measurements 47 (2012) 210e218 217

study and the results reported by Li and Li (2011) may be due todifferent samples. It should be noted that the post-IR IR100 �C andpost-IR IR150 �C stimulations before post-IR IRSL225 �C stimulationduring post-IR IRSL225 �C measurements in this experiment wasomitted (Table 4), resulting in the post-IR IRSL100 �C and post-IRIRSL150 �C signals accumulation on the post-IR IRSL225 �C signal.

3.3.2. Interpretation using single-trap modelAs discussed in Section 3.1.2, in the single-trap model of Jain and

Ankjærgaard (2011), preheating reduces the luminescence effi-ciency by filling the available holes, the hole concentration is thusa controlling factor for the shapes of pulse annealing curves. Thepulse annealing curves for the IRSL50 �C and post-IR IRSL signals canbe interpreted as a reflection of the stability of electron-hole pairs,rather than a trap depth. Therefore, in this model, the trap depthsobtained for the IRSL50 �C and post-IR IRSL signals in Table 5 are thusconsidered to be their apparent values. In the viewof the single trapmodel, a more complex numerical model for simulation, whichtakes into consideration the energy states and the probability ofrecombination, may be used to analyze those pulse annealing dataand get credible trap parameters in the future. As discussed inSection 3.1.2, since the blue simulation is a direct ionization process,the post-IR OSL signal can be a mixture of signals originating fromtraps with different depths, the post-IR OSL signal is not directlyrelevant to the single trap model.

Figs. 5 and 6 also indicate that the elevated temperature post-IRIRSL signal is more thermally stable than the IRSL50 �C and post-IROSL signals. The luminescence properties of the elevated temper-ature post-IR IRSL signals, such as higher thermal stability andlower anomalous fading rate (Thomsen et al., 2008; Buylaert et al.,2009; Li and Li, 2011), make these signals have intrinsic potentialadvantages for dating old samples over other luminescence signalsfrom K-feldspar.

4. Conclusions

The museum sample of K-feldspar (microcline) exhibits two TLpeaks at w140 and w330 �C and accordingly the TL can be dividedinto the low and high temperature TL. Corresponding to the lowtemperature TL, the OSL and post-IR OSL signals appear to be moreassociated with the low temperature TL than the IRSL�50 �C signal.Corresponding to the high temperature TL, the post-IR IRSLsignals, especially for the high temperature post-IR IRSL signal, aremore thermally stable than the IRSL50 �C and post-IR OSL signals.The data of IRSL and post-IR IRSL signals in this study can beinterpreted with the single-trap and multiple-trap models. Furtherinvestigations to test those models are needed. The high temper-ature post-IR IRSL225 �C signal is shown to be relatively hard to bebleached by blue-light or even simulated sunlight compared withthe IRSL50 �C and post-IR OSL signals. The results imply thatelevated temperature post-IR IRSL signals can be preferentiallyapplied in optical dating over other signals from K-feldspar, butthe bleaching of the signals prior to burial should be considered indating sediments. This requires further investigations on moresediment samples.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (NSFC, No. 40871009). Discussions withSheng-Hua Li and Bo Li are greatly appreciated. We also thank twoanonymous reviewers for their valuable comments and construc-tive suggestions, which significantly improved the quality of themanuscript.

Appendix. Supplementary data

Supplementary data related to this article can be found online atdoi:10.1016/j.radmeas.2011.12.007.

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