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Dose and dose rate dependence of time-resolved OSL from Korean paleosol quartz
Radiation Measurements 46 (2011) 1518e1521
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Dose and dose rate dependence of time-resolved OSL from Korean paleosol quartz M.J. Kim a, J.H. Choi b, D.G. Hong a, * a b
Department of Physics, Kangwon National University, Chuncheon, Gangwon-Do 200-701, South Korea Geochronology Team, Korea Basic Science Institute, Daejeon 305-333, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2010 Received in revised form 4 March 2011 Accepted 9 April 2011
Using a newly developed small X-ray irradiator equipped with a mini X-ray generator (Varian VF-50J) and TR-OSL measurement system attached to an MCS (ORTEC MCS-PCI), the influence of dose and dose rate of X-ray radiation on TR-OSL was investigated in paleosol quartz from the Chungdang-dong paleolithic site, Korea. All lifetimes were calculated by a curve-fitting method using a single stretchedexponential function with b ¼ 0.86. Physical characteristics, natural lifetime, thermal quenching activation energy DE, and thermal assistance activation energy Ea were firstly determined. From the results of the investigation of dependence on dose and dose rate of X-ray radiation using fully bleached samples, the lifetime decreased significantly to about 20 ms; when the X-ray dose was increased to 300 Gy at a dose rate of 0.5 Gy/s. Also, despite different radiation dose rates, at a dose of 50 Gy both lifetime and related intensity were independent of dose rate change. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: TR-OSL Dose dependence Dose rate dependence X-ray irradiation
1. Introduction Time-resolved OSL (TR-OSL) has many obvious advantages in the field of optical dating as well as physical research (Chithambo, 2007a). Firstly, a high signal-to-background ratio is achieved since the luminescence after the light pulse is measured with photomultiplier noise only. Secondly, TR-OSL has been widely used to study the luminescence lifetime properties of quartz and feldspar and to investigate the physical characteristics of charge recombination based on the effect of thermal treatment. Thirdly, the TR-OSL signal is very useful in optical dating of a mixed sample because the quartz signal is successfully isolated against feldspar contamination by using the difference of lifetime between quartz and feldspar. The aim of this work is to study TR-OSL dependence on dose and dose rate of X-ray radiation from Korean paleosol quartz samples. Using newly developed equipment for X-ray irradiation and TR-OSL measurement, we firstly measured the natural TR-OSL signal to evaluate luminescence lifetime and to determine the kinetic variables based on temperature dependency. The influence of lifetime on ionizing radiation was investigated through increased doses and different dose rates of X-ray radiation, respectively. 2. Experimental equipment Two experimental systems for irradiating a sample with X-rays and measuring TR-OSL were newly developed to investigate the * Corresponding author. Tel.: þ82 33 250 8473; fax: þ82 33 257 9689. E-mail address: [email protected] (D.G. Hong). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.03.022
influence of dose and dose rate, respectively, on lifetime (Kim, 2010; Kim et al., in press). A small X-ray irradiator was designed as a stand-alone type and mainly comprised of a Varian VF-50J mini X-ray generator (50 W max. at 50 kV and 1 mA), Pb collimator, delay shutter, and Al absorber. The shutter was additionally delayed during the initial 4 s to prevent the unstable X-ray beam from irradiating onto the sample. Apart from this, the X-ray energy spectrum consisted of both low- and high-energy components, as found from an X-ray spectrometer (Amptek, Co., USA). The Al absorber, of thickness 300 mm, was located below the delay shutter to completely remove the lowenergy peak at about 9 keV, because this has no capability of full penetration through a monolayer of coarse quartz grains (Andersen et al., 2003). X-ray dose rates increased linearly to 0.8 Gy/s with increasing emission current of the mini X-ray generator. The instrument used in TR-OSL measurement was modified from the KBSI OSL system (Choi et al., 2006). In the OSL system, a sample heater was manufactured by using Kanthal wire and its temperature was kept constant at any level up to 300 C. It was employed with a set of two 470 nm blue power LEDs (LXHL-PB02) as optical stimulation source, combined with a 3-mm-thick Schott GG420 long-pass filter and VIS liquid light guide (VIS-LLG, Newport #77631). Luminescence was detected between 255 and 405 nm (FWHM) by a photomultiplier tube (bialkali EMI 9235QB) through a 7.5-mm-thick U340 band-pass filter. For TR-OSL measurement, a multichannel scaler (ORTEC MCSPCI) and a LED pulse power driver were added as shown in Fig. 1. A pulse signal generated from a general purpose Counter/Timer
M.J. Kim et al. / Radiation Measurements 46 (2011) 1518e1521
port in a DAQ board (NI PCI 6250) was introduced into the LED pulse power driver directly connected with the blue power LEDs and the trigger-signal port of the MCS, respectively, and the start of pulse stimulation was synchronized with that of a sweep from the MCS. Once the sample was stimulated at a pulse width defined by the dynamic range and duty ratio in the DAQ, the MCS simultaneously began to acquire photon counts per dwell time measured sequentially both during the pulse and after the pulse. After repeating the measurements during a number of sweeps, the TR-OSL spectrum was finally obtained as a plot of cumulative photon counts against time for the dynamic range selected.
1519
LED ON
LED OFF
TR-OSL intensity (a. u.)
3000
2000
Build-up signal Quartz contribution Feldspar contribution
1000
3. Sample and measurement
0 0 2 4 6 8 10
50
100
150
200
250
Three well-bleached soil samples, designated as CHD1, CHD2, and CHD3, from the top layer at the Chungdang-dong paleolithic site in Korea were collected (Kim, 2010). The samples, grain size 125e250 mm, were treated with HCl and H2O2 to remove carbonates and organics and then treated with concentrated HF for 1 h to reduce feldspars and etch the quartz. In spite of these robust chemical treatments, feldspar contamination was found from infrared stimulated luminescence (IRSL) measured by using some aliquots of each resultant paleosol quartz sample. Using the newly developed instruments, the paleosol quartz sample was stimulated with the optimal pulse condition of a pulse width of 10 ms and dynamic range of 250 ms, and TR-OSL was recorded at a dwell time of 1 ms and with 100,000 sweeps (Kim, 2010). There was no preheating after X-ray irradiation, because the phosphorescence present after irradiation is not correlated in time with the pulse signal and hence does not affect luminescence lifetime (Chithambo, 2007b). The curve fitting for the lifetime calculation was based on the MarquardteLevenberg minimization method and was performed by using the commercial software SigmaPlot (SPSS Inc., USA).
under natural dose. As shown in Fig. 2, it was composed of a buildup signal during the pulse (LED ON) and a decreased TR-OSL signal after the light pulse (LED OFF). The initial portion in the TR-OSL spectrum was extremely decreased, and this turned out to be the influence of feldspar contamination, which was related to the very short lifetime (Denby et al., 2006) and was previously found through chemical treatments. All lifetimes were obtained from the pure quartz TR-OSL spectra, except for the initial 2 channels using the curve-fitting method. It was known that a stretched-exponential function was in better agreement with TR-OSL, rather than a simple exponential function (Chithambo, 2005). Therefore, lifetime was evaluated by fitting a stretched-exponential function in addition to photomultiplier noise only:
4. Results and discussion
i h IðtÞ ¼ I0 exp ðt=sÞb þ Bg
4.1. Physical characteristics of TR-OSL Before investigating the lifetime dependences on dose and dose rate, the physical characteristics of the paleosol quartz samples were first determined. Luminescence was measured at 120 C
Time (μs) Fig. 2. A typical TR-OSL signal measured from natural paleosol quartz at 120 C.
where s is the lifetime of the paleosol quartz, b is a characteristic parameter related to the distribution of lifetimes, and I0 is the initial intensity at time t ¼ 0. As shown in the inset of Fig. 3, the optimum value of b for the best fit was defined as 0.86 from the minimum of the figure of merit (FOM). This was calculated from the natural TR-
PM tube EMI 9235QB
2 blue power LED
Pre-amplifier
LXHL-PB02
MSA-0404
Multichannel scaler Pulse power 6.5 DCV, 4A
Personal computer
TR-OSL
ORTEC MCS-PCI Trigger
LED pulse power driver d i
TTL
(1)
DAQ board
Kanthal heater
NI PCI 6250
DCV, 2A 10 DCV
Fig. 1. A schematic diagram of the TR-OSL measurement system.
1520
M.J. Kim et al. / Radiation Measurements 46 (2011) 1518e1521
1500
FOM (%)
TR-OSL intensity (a. u.)
3.5
1200 900
Table 1 Physical characteristics obtained from TR-OSL measurements on natural paleosol quartz samples collected at the Chungdang-dong paleolithic site, Korea.
Beta = 0.86
3.0
Lifetime (ms, 1s SE)
Thermal quenching activation energy (eV, 1s SE)
Thermal assistance activation energy (eV, 1s SE)
CHD1 CHD2 CHD3
31.62 0.28 30.22 0.26 33.34 0.58
0.75 0.16 0.68 0.14 0.70 0.13
0.017 0.003
2.0 1.5
0.6
0.8
1.0
1.2
Beta
600
0.030 0.018
Total
300
SEF
0
Bg 0
50
100
150
200
250
Time (μs) Fig. 3. An example of curve fitting to the natural TR-OSL signal using a single stretched-exponential function with b ¼ 0.86. The inset shows how to define the optimum b value.
OSL signal of paleosol quartz against the value of b in the range 0.6e1.2 after fixing the value of the lifetime as that of natural paleosol quartz determined from curve fitting with a single exponential function. Fig. 3 presents a typical result from curve fitting of the natural TR-OSL signals using the stretched-exponential function with b ¼ 0.86. The resultant lifetime was accepted where the value of the FOM was below 2.5%, and the lifetimes of natural paleosol quartz samples CHD1, 2, 3 were finally evaluated as 31.62 0.28 ms, 30.22 0.28 ms, and 33.34 0.58 ms, respectively. Two important kinetic parameters, thermal quenching activation energy DE and thermal assistance activation energy Ea, were determined from the temperature dependences of the lifetime and intensity of the natural TR-OSL signal, respectively. From the MotteSeitz configurational coordinate model, the characteristic functions were derived as below:
sðTÞ ¼
Sample
2.5
srad
1 þ CexpðDE=kTÞ
(2)
300 Gy, with 50 Gy intervals, were firstly employed for each set of 3 aliquots from the bleached paleosol quartz samples, respectively. The irradiating conditions of the mini X-ray generator were such that the high voltage was 50 kV and the emission current was 0.7 mA, which was identical with a dose rate of 0.5 Gy/s. TR-OSL was then measured at 120 C without preheating, and the lifetime was determined from curve fitting using the stretched-exponential function with b ¼ 0.86. Fig. 4 shows the dependence of the mean lifetime calculated from each set of 3 aliquots on X-ray dose. Lifetimes at 50 Gy are similar to those of natural samples, and lifetimes decreased to about 20 ms as the X-ray dose was increased to 300 Gy, as seen by a solid line of best fit through data points in Fig. 4. The results shown in Fig. 4 can be explained using the previously suggested energy band model (Chithambo, 2008). In the model, the emission of luminescence involves a non-radiative centre and three luminescence centres, LS, LL, and LH, with which distinct lifetimes sS, sL, and sH, where sH > sL > sS, can be associated. Also, it is assumed that LS has the highest hole-capture probability during irradiation, followed by LL and then LH. With an increase of X-ray dose, the production of electronehole pairs is also increased. At low doses, although LS has the highest hole-capture probability, the concentration of holes in these is much lower than that in LL or LH. This means that the contribution of sS to the resultant lifetimes is negligible. On the other hand, at high doses, the concentration of LS holes is increased and the contribution of sS becomes more significant. Thus, increasing X-ray radiation enhances the dominance of the shorter-lifetime hole centre, and this is seen as a decrease in TR-OSL lifetimes. 4.3. Dose rate dependence of TR-OSL lifetime
(3)
where srad is the radiative lifetime at temperature absolute zero, C is a scaling parameter, k is Boltzmann’s constant, and I0 is the initial value of luminescence intensity (Chithambo, 2007b). These functions were applied to the mean lifetimes and intensities of the natural TR-OSL signals measured from each set of 3 aliquots. The resultant kinetic parameters of all paleosol quartz samples agreed well with each other within the error range, except that the thermal assistance activation energy of sample CHD2 could not be calculated. The physical characteristics of the natural TR-OSL signals from the paleosol quartz samples are summarized in Table 1. 4.2. Dose dependence of TR-OSL lifetime All paleosol quartz samples were fully bleached by using a solar simulator (SOL2: Honle/500S, UV technology) equipped with a UV lamp for sunlight simulation for 10 min prior to investigating the dependences of TR-OSL lifetime on the dose and dose rate of the Xray radiation. For the purpose of examination of the dependence on the dose of X-ray radiation, increased radiation doses in the range from 50 to
For the purpose of evaluation of the dependence on the dose rate of X-ray radiation, doses of 50 Gy with different radiation dose
36
CHD1 CHD2 CHD3 Curve fitting
32
Lifetime (μs, 1σ SE)
I expð EB =kTÞ IðTÞ ¼ 0 1 þ CexpðDE=kTÞ
28
24
20
16
50
100
150
200
250
300
X-ray dose (Gy) Fig. 4. Dependence of lifetime on increasing doses of X-ray radiation.
40
Norm. intensity
M.J. Kim et al. / Radiation Measurements 46 (2011) 1518e1521
CHD1
Lifetime (μs, 1σ SE)
CHD2 CHD3
36
1.25 1.00 0.75 0.0
32
0.2
0.4
0.6
0.8
28
24 0.0
0.2
0.4
0.6
0.8
Xray doserate (Gy/s) Fig. 5. Influence on lifetime of different dose rates of X-ray radiation. The inset shows the independence of total TR-OSL intensity.
rates, 0.02, 0.05, 0.1, 0.3, 0.5, and 0.7 Gy/s, were firstly employed for each set of 3 aliquots from the bleached paleosol quartz samples, respectively. The irradiating conditions of the mini X-ray generator were such that the high voltage was 50 kV and the emission current was changed from 0.025 to 1 mA. The measurement of TR-OSL and the calculation of lifetimes were carried out in the same way as described in the previous section. Fig. 5 shows the influence of X-ray dose rate on the mean lifetime calculated from each set of 3 aliquots. Not only are all lifetimes independent of X-ray dose rates, but they are also similar to those of natural samples. This means that the production rate of electronehole pairs and hole-capture probability related to a nonradiative centre and three luminescence centres, LS, LL, and LH, during irradiation are not affected by the given dose rates. From this result, the total intensity of TR-OSL must be maintained at the same level, and this is verified from the inset of Fig. 5. Although there has been much previous research regarding the dependence of luminescence response on dose rate, the results do not converge on a theoretical model, even in some cases with the same host material, such as quartz (Chen et al., 1981; Mondragon et al., 1988; Halliburton et al., 1993). Therefore, there is a need for more research under various conditions of X-ray radiation in the future. 5. Conclusions The influence of dose and dose rate of X-ray radiation on TR-OSL was investigated in paleosol quartz from the Chungdang-dong paleolithic site, Korea. For this study, a small X-ray irradiator
1521
equipped with a mini X-ray generator (Varian VF-50J) and a TR-OSL measurement system attached to an MCS (ORTEC MCS-PCI) were newly developed, respectively. All lifetimes were calculated by a curve-fitting method using a single stretched-exponential function with b ¼ 0.86. Physical characteristics, natural lifetime, thermal quenching activation energy DE, and thermal assistance activation energy Ea, were first determined from the natural paleosol quartz samples and these agreed well with each other. From the results of investigation of dependence on dose and dose rate of X-ray radiation using fully bleached samples, lifetime decreased significantly to about 20 ms as the X-ray dose was increased to 300 Gy at a dose rate of 0.5 Gy/s. Also, despite different radiation dose rates at a dose of 50 Gy both lifetime and related intensity were independent of dose rate change. These features were explained with reference to a modified energy band model using the relation with the production of electronehole pairs and hole-capture probability. Acknowledgement This work was financially supported by funding through Korea Basic Science Institute (Grant No. F31604). References Andersen, C.E., Botter-Jensen, L., Murray, A.S., 2003. A mini X-ray generator as an alternative to a 90Sr/90Y beta source in luminescence dating. Radiat. Meas. 37, 557e561. Chen, R., McKeever, S.W.S., Durrani, S.A., 1981. Solution of the kinetic equations governing trap filling. Consequences concerning dose dependence and doserate effects. Phys. Rev. B. 24, 4931e4944. Chithambo, M.L., 2005. Towards models for analysis of time-resolved luminescence spectra from quartz. Appl. Radiat. Isotopes 62, 941e942. Chithambo, M.L., 2007a. The analysis of time-resolved optically stimulated luminescence: I. Theoretical considerations. J. Phys. D. Appl. Phys. 40, 1874e1879. Chithambo, M.L., 2007b. The analysis of time-resolved optically stimulated luminescence: II. Computer simulations and experimental results. J. Phys. D. Appl. Phys. 40, 1880e1889. Chithambo, M.L., 2008. The dependence of luminescence lifetimes on additive irradiation in natural sedimentary quartz: sands from Santa Elina. Brazil. Phys. Stat. Sol 5, 630e633. Choi, J.H., et al., 2006. Manufacturing and Development of Luminescence Measurement System Equipped with X-ray Generator (KBSI-2006-1232-01310703). Denby, P.M., Botter-Jensen, L., Murray, A.S., Thomsen, K.J., Moska, P., 2006. Application of pulsed OSL to the separation of the luminescence components from a mixed quartz/feldspar sample. Radiat. Meas. 41, 774e779. Halliburton, L.E., Hofstaetter, A., Scharmann, A., Scripsick, M.P., Edwards, G.J., 1993. Dose-rate dependence in the production of point defects in quartz. Appl. Radiat. Isotopes 44, 273e277. Kim, M.J., 2010. Physical Characteristics of POSL and Age Determination for Quartz from Korean Paleosol. PhD Thesis. Kangwon National University, Korea. Kim, M.J., Nam, K.G., Jang, Y.S., Kim, S.Y. Characteristics and application of a small X-ray radiation source for luminescence dating. GeumGang Archaeology, in press. Mondragon, M.A., Chen, C.Y., Halliburton, L.E., 1988. Observation of a dose-rate dependence in the production of point defects in quartz. J. Appl. Phys. 63, 4937e4941.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Dose and dose rate dependence of time-resolved OSL from Korean paleosol quartz M.J. Kim a, J.H. Choi b, D.G. Hong a, * a b
Department of Physics, Kangwon National University, Chuncheon, Gangwon-Do 200-701, South Korea Geochronology Team, Korea Basic Science Institute, Daejeon 305-333, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2010 Received in revised form 4 March 2011 Accepted 9 April 2011
Using a newly developed small X-ray irradiator equipped with a mini X-ray generator (Varian VF-50J) and TR-OSL measurement system attached to an MCS (ORTEC MCS-PCI), the influence of dose and dose rate of X-ray radiation on TR-OSL was investigated in paleosol quartz from the Chungdang-dong paleolithic site, Korea. All lifetimes were calculated by a curve-fitting method using a single stretchedexponential function with b ¼ 0.86. Physical characteristics, natural lifetime, thermal quenching activation energy DE, and thermal assistance activation energy Ea were firstly determined. From the results of the investigation of dependence on dose and dose rate of X-ray radiation using fully bleached samples, the lifetime decreased significantly to about 20 ms; when the X-ray dose was increased to 300 Gy at a dose rate of 0.5 Gy/s. Also, despite different radiation dose rates, at a dose of 50 Gy both lifetime and related intensity were independent of dose rate change. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: TR-OSL Dose dependence Dose rate dependence X-ray irradiation
1. Introduction Time-resolved OSL (TR-OSL) has many obvious advantages in the field of optical dating as well as physical research (Chithambo, 2007a). Firstly, a high signal-to-background ratio is achieved since the luminescence after the light pulse is measured with photomultiplier noise only. Secondly, TR-OSL has been widely used to study the luminescence lifetime properties of quartz and feldspar and to investigate the physical characteristics of charge recombination based on the effect of thermal treatment. Thirdly, the TR-OSL signal is very useful in optical dating of a mixed sample because the quartz signal is successfully isolated against feldspar contamination by using the difference of lifetime between quartz and feldspar. The aim of this work is to study TR-OSL dependence on dose and dose rate of X-ray radiation from Korean paleosol quartz samples. Using newly developed equipment for X-ray irradiation and TR-OSL measurement, we firstly measured the natural TR-OSL signal to evaluate luminescence lifetime and to determine the kinetic variables based on temperature dependency. The influence of lifetime on ionizing radiation was investigated through increased doses and different dose rates of X-ray radiation, respectively. 2. Experimental equipment Two experimental systems for irradiating a sample with X-rays and measuring TR-OSL were newly developed to investigate the * Corresponding author. Tel.: þ82 33 250 8473; fax: þ82 33 257 9689. E-mail address: [email protected] (D.G. Hong). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.03.022
influence of dose and dose rate, respectively, on lifetime (Kim, 2010; Kim et al., in press). A small X-ray irradiator was designed as a stand-alone type and mainly comprised of a Varian VF-50J mini X-ray generator (50 W max. at 50 kV and 1 mA), Pb collimator, delay shutter, and Al absorber. The shutter was additionally delayed during the initial 4 s to prevent the unstable X-ray beam from irradiating onto the sample. Apart from this, the X-ray energy spectrum consisted of both low- and high-energy components, as found from an X-ray spectrometer (Amptek, Co., USA). The Al absorber, of thickness 300 mm, was located below the delay shutter to completely remove the lowenergy peak at about 9 keV, because this has no capability of full penetration through a monolayer of coarse quartz grains (Andersen et al., 2003). X-ray dose rates increased linearly to 0.8 Gy/s with increasing emission current of the mini X-ray generator. The instrument used in TR-OSL measurement was modified from the KBSI OSL system (Choi et al., 2006). In the OSL system, a sample heater was manufactured by using Kanthal wire and its temperature was kept constant at any level up to 300 C. It was employed with a set of two 470 nm blue power LEDs (LXHL-PB02) as optical stimulation source, combined with a 3-mm-thick Schott GG420 long-pass filter and VIS liquid light guide (VIS-LLG, Newport #77631). Luminescence was detected between 255 and 405 nm (FWHM) by a photomultiplier tube (bialkali EMI 9235QB) through a 7.5-mm-thick U340 band-pass filter. For TR-OSL measurement, a multichannel scaler (ORTEC MCSPCI) and a LED pulse power driver were added as shown in Fig. 1. A pulse signal generated from a general purpose Counter/Timer
M.J. Kim et al. / Radiation Measurements 46 (2011) 1518e1521
port in a DAQ board (NI PCI 6250) was introduced into the LED pulse power driver directly connected with the blue power LEDs and the trigger-signal port of the MCS, respectively, and the start of pulse stimulation was synchronized with that of a sweep from the MCS. Once the sample was stimulated at a pulse width defined by the dynamic range and duty ratio in the DAQ, the MCS simultaneously began to acquire photon counts per dwell time measured sequentially both during the pulse and after the pulse. After repeating the measurements during a number of sweeps, the TR-OSL spectrum was finally obtained as a plot of cumulative photon counts against time for the dynamic range selected.
1519
LED ON
LED OFF
TR-OSL intensity (a. u.)
3000
2000
Build-up signal Quartz contribution Feldspar contribution
1000
3. Sample and measurement
0 0 2 4 6 8 10
50
100
150
200
250
Three well-bleached soil samples, designated as CHD1, CHD2, and CHD3, from the top layer at the Chungdang-dong paleolithic site in Korea were collected (Kim, 2010). The samples, grain size 125e250 mm, were treated with HCl and H2O2 to remove carbonates and organics and then treated with concentrated HF for 1 h to reduce feldspars and etch the quartz. In spite of these robust chemical treatments, feldspar contamination was found from infrared stimulated luminescence (IRSL) measured by using some aliquots of each resultant paleosol quartz sample. Using the newly developed instruments, the paleosol quartz sample was stimulated with the optimal pulse condition of a pulse width of 10 ms and dynamic range of 250 ms, and TR-OSL was recorded at a dwell time of 1 ms and with 100,000 sweeps (Kim, 2010). There was no preheating after X-ray irradiation, because the phosphorescence present after irradiation is not correlated in time with the pulse signal and hence does not affect luminescence lifetime (Chithambo, 2007b). The curve fitting for the lifetime calculation was based on the MarquardteLevenberg minimization method and was performed by using the commercial software SigmaPlot (SPSS Inc., USA).
under natural dose. As shown in Fig. 2, it was composed of a buildup signal during the pulse (LED ON) and a decreased TR-OSL signal after the light pulse (LED OFF). The initial portion in the TR-OSL spectrum was extremely decreased, and this turned out to be the influence of feldspar contamination, which was related to the very short lifetime (Denby et al., 2006) and was previously found through chemical treatments. All lifetimes were obtained from the pure quartz TR-OSL spectra, except for the initial 2 channels using the curve-fitting method. It was known that a stretched-exponential function was in better agreement with TR-OSL, rather than a simple exponential function (Chithambo, 2005). Therefore, lifetime was evaluated by fitting a stretched-exponential function in addition to photomultiplier noise only:
4. Results and discussion
i h IðtÞ ¼ I0 exp ðt=sÞb þ Bg
4.1. Physical characteristics of TR-OSL Before investigating the lifetime dependences on dose and dose rate, the physical characteristics of the paleosol quartz samples were first determined. Luminescence was measured at 120 C
Time (μs) Fig. 2. A typical TR-OSL signal measured from natural paleosol quartz at 120 C.
where s is the lifetime of the paleosol quartz, b is a characteristic parameter related to the distribution of lifetimes, and I0 is the initial intensity at time t ¼ 0. As shown in the inset of Fig. 3, the optimum value of b for the best fit was defined as 0.86 from the minimum of the figure of merit (FOM). This was calculated from the natural TR-
PM tube EMI 9235QB
2 blue power LED
Pre-amplifier
LXHL-PB02
MSA-0404
Multichannel scaler Pulse power 6.5 DCV, 4A
Personal computer
TR-OSL
ORTEC MCS-PCI Trigger
LED pulse power driver d i
TTL
(1)
DAQ board
Kanthal heater
NI PCI 6250
DCV, 2A 10 DCV
Fig. 1. A schematic diagram of the TR-OSL measurement system.
1520
M.J. Kim et al. / Radiation Measurements 46 (2011) 1518e1521
1500
FOM (%)
TR-OSL intensity (a. u.)
3.5
1200 900
Table 1 Physical characteristics obtained from TR-OSL measurements on natural paleosol quartz samples collected at the Chungdang-dong paleolithic site, Korea.
Beta = 0.86
3.0
Lifetime (ms, 1s SE)
Thermal quenching activation energy (eV, 1s SE)
Thermal assistance activation energy (eV, 1s SE)
CHD1 CHD2 CHD3
31.62 0.28 30.22 0.26 33.34 0.58
0.75 0.16 0.68 0.14 0.70 0.13
0.017 0.003
2.0 1.5
0.6
0.8
1.0
1.2
Beta
600
0.030 0.018
Total
300
SEF
0
Bg 0
50
100
150
200
250
Time (μs) Fig. 3. An example of curve fitting to the natural TR-OSL signal using a single stretched-exponential function with b ¼ 0.86. The inset shows how to define the optimum b value.
OSL signal of paleosol quartz against the value of b in the range 0.6e1.2 after fixing the value of the lifetime as that of natural paleosol quartz determined from curve fitting with a single exponential function. Fig. 3 presents a typical result from curve fitting of the natural TR-OSL signals using the stretched-exponential function with b ¼ 0.86. The resultant lifetime was accepted where the value of the FOM was below 2.5%, and the lifetimes of natural paleosol quartz samples CHD1, 2, 3 were finally evaluated as 31.62 0.28 ms, 30.22 0.28 ms, and 33.34 0.58 ms, respectively. Two important kinetic parameters, thermal quenching activation energy DE and thermal assistance activation energy Ea, were determined from the temperature dependences of the lifetime and intensity of the natural TR-OSL signal, respectively. From the MotteSeitz configurational coordinate model, the characteristic functions were derived as below:
sðTÞ ¼
Sample
2.5
srad
1 þ CexpðDE=kTÞ
(2)
300 Gy, with 50 Gy intervals, were firstly employed for each set of 3 aliquots from the bleached paleosol quartz samples, respectively. The irradiating conditions of the mini X-ray generator were such that the high voltage was 50 kV and the emission current was 0.7 mA, which was identical with a dose rate of 0.5 Gy/s. TR-OSL was then measured at 120 C without preheating, and the lifetime was determined from curve fitting using the stretched-exponential function with b ¼ 0.86. Fig. 4 shows the dependence of the mean lifetime calculated from each set of 3 aliquots on X-ray dose. Lifetimes at 50 Gy are similar to those of natural samples, and lifetimes decreased to about 20 ms as the X-ray dose was increased to 300 Gy, as seen by a solid line of best fit through data points in Fig. 4. The results shown in Fig. 4 can be explained using the previously suggested energy band model (Chithambo, 2008). In the model, the emission of luminescence involves a non-radiative centre and three luminescence centres, LS, LL, and LH, with which distinct lifetimes sS, sL, and sH, where sH > sL > sS, can be associated. Also, it is assumed that LS has the highest hole-capture probability during irradiation, followed by LL and then LH. With an increase of X-ray dose, the production of electronehole pairs is also increased. At low doses, although LS has the highest hole-capture probability, the concentration of holes in these is much lower than that in LL or LH. This means that the contribution of sS to the resultant lifetimes is negligible. On the other hand, at high doses, the concentration of LS holes is increased and the contribution of sS becomes more significant. Thus, increasing X-ray radiation enhances the dominance of the shorter-lifetime hole centre, and this is seen as a decrease in TR-OSL lifetimes. 4.3. Dose rate dependence of TR-OSL lifetime
(3)
where srad is the radiative lifetime at temperature absolute zero, C is a scaling parameter, k is Boltzmann’s constant, and I0 is the initial value of luminescence intensity (Chithambo, 2007b). These functions were applied to the mean lifetimes and intensities of the natural TR-OSL signals measured from each set of 3 aliquots. The resultant kinetic parameters of all paleosol quartz samples agreed well with each other within the error range, except that the thermal assistance activation energy of sample CHD2 could not be calculated. The physical characteristics of the natural TR-OSL signals from the paleosol quartz samples are summarized in Table 1. 4.2. Dose dependence of TR-OSL lifetime All paleosol quartz samples were fully bleached by using a solar simulator (SOL2: Honle/500S, UV technology) equipped with a UV lamp for sunlight simulation for 10 min prior to investigating the dependences of TR-OSL lifetime on the dose and dose rate of the Xray radiation. For the purpose of examination of the dependence on the dose of X-ray radiation, increased radiation doses in the range from 50 to
For the purpose of evaluation of the dependence on the dose rate of X-ray radiation, doses of 50 Gy with different radiation dose
36
CHD1 CHD2 CHD3 Curve fitting
32
Lifetime (μs, 1σ SE)
I expð EB =kTÞ IðTÞ ¼ 0 1 þ CexpðDE=kTÞ
28
24
20
16
50
100
150
200
250
300
X-ray dose (Gy) Fig. 4. Dependence of lifetime on increasing doses of X-ray radiation.
40
Norm. intensity
M.J. Kim et al. / Radiation Measurements 46 (2011) 1518e1521
CHD1
Lifetime (μs, 1σ SE)
CHD2 CHD3
36
1.25 1.00 0.75 0.0
32
0.2
0.4
0.6
0.8
28
24 0.0
0.2
0.4
0.6
0.8
Xray doserate (Gy/s) Fig. 5. Influence on lifetime of different dose rates of X-ray radiation. The inset shows the independence of total TR-OSL intensity.
rates, 0.02, 0.05, 0.1, 0.3, 0.5, and 0.7 Gy/s, were firstly employed for each set of 3 aliquots from the bleached paleosol quartz samples, respectively. The irradiating conditions of the mini X-ray generator were such that the high voltage was 50 kV and the emission current was changed from 0.025 to 1 mA. The measurement of TR-OSL and the calculation of lifetimes were carried out in the same way as described in the previous section. Fig. 5 shows the influence of X-ray dose rate on the mean lifetime calculated from each set of 3 aliquots. Not only are all lifetimes independent of X-ray dose rates, but they are also similar to those of natural samples. This means that the production rate of electronehole pairs and hole-capture probability related to a nonradiative centre and three luminescence centres, LS, LL, and LH, during irradiation are not affected by the given dose rates. From this result, the total intensity of TR-OSL must be maintained at the same level, and this is verified from the inset of Fig. 5. Although there has been much previous research regarding the dependence of luminescence response on dose rate, the results do not converge on a theoretical model, even in some cases with the same host material, such as quartz (Chen et al., 1981; Mondragon et al., 1988; Halliburton et al., 1993). Therefore, there is a need for more research under various conditions of X-ray radiation in the future. 5. Conclusions The influence of dose and dose rate of X-ray radiation on TR-OSL was investigated in paleosol quartz from the Chungdang-dong paleolithic site, Korea. For this study, a small X-ray irradiator
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equipped with a mini X-ray generator (Varian VF-50J) and a TR-OSL measurement system attached to an MCS (ORTEC MCS-PCI) were newly developed, respectively. All lifetimes were calculated by a curve-fitting method using a single stretched-exponential function with b ¼ 0.86. Physical characteristics, natural lifetime, thermal quenching activation energy DE, and thermal assistance activation energy Ea, were first determined from the natural paleosol quartz samples and these agreed well with each other. From the results of investigation of dependence on dose and dose rate of X-ray radiation using fully bleached samples, lifetime decreased significantly to about 20 ms as the X-ray dose was increased to 300 Gy at a dose rate of 0.5 Gy/s. Also, despite different radiation dose rates at a dose of 50 Gy both lifetime and related intensity were independent of dose rate change. These features were explained with reference to a modified energy band model using the relation with the production of electronehole pairs and hole-capture probability. Acknowledgement This work was financially supported by funding through Korea Basic Science Institute (Grant No. F31604). References Andersen, C.E., Botter-Jensen, L., Murray, A.S., 2003. A mini X-ray generator as an alternative to a 90Sr/90Y beta source in luminescence dating. Radiat. Meas. 37, 557e561. Chen, R., McKeever, S.W.S., Durrani, S.A., 1981. Solution of the kinetic equations governing trap filling. Consequences concerning dose dependence and doserate effects. Phys. Rev. B. 24, 4931e4944. Chithambo, M.L., 2005. Towards models for analysis of time-resolved luminescence spectra from quartz. Appl. Radiat. Isotopes 62, 941e942. Chithambo, M.L., 2007a. The analysis of time-resolved optically stimulated luminescence: I. Theoretical considerations. J. Phys. D. Appl. Phys. 40, 1874e1879. Chithambo, M.L., 2007b. The analysis of time-resolved optically stimulated luminescence: II. Computer simulations and experimental results. J. Phys. D. Appl. Phys. 40, 1880e1889. Chithambo, M.L., 2008. The dependence of luminescence lifetimes on additive irradiation in natural sedimentary quartz: sands from Santa Elina. Brazil. Phys. Stat. Sol 5, 630e633. Choi, J.H., et al., 2006. Manufacturing and Development of Luminescence Measurement System Equipped with X-ray Generator (KBSI-2006-1232-01310703). Denby, P.M., Botter-Jensen, L., Murray, A.S., Thomsen, K.J., Moska, P., 2006. Application of pulsed OSL to the separation of the luminescence components from a mixed quartz/feldspar sample. Radiat. Meas. 41, 774e779. Halliburton, L.E., Hofstaetter, A., Scharmann, A., Scripsick, M.P., Edwards, G.J., 1993. Dose-rate dependence in the production of point defects in quartz. Appl. Radiat. Isotopes 44, 273e277. Kim, M.J., 2010. Physical Characteristics of POSL and Age Determination for Quartz from Korean Paleosol. PhD Thesis. Kangwon National University, Korea. Kim, M.J., Nam, K.G., Jang, Y.S., Kim, S.Y. Characteristics and application of a small X-ray radiation source for luminescence dating. GeumGang Archaeology, in press. Mondragon, M.A., Chen, C.Y., Halliburton, L.E., 1988. Observation of a dose-rate dependence in the production of point defects in quartz. J. Appl. Phys. 63, 4937e4941.