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OSL studies of alkali fluoroperovskite single crystals for radiation dosimetry
Optical Materials 58 (2016) 497e503
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
OSL studies of alkali fluoroperovskite single crystals for radiation dosimetry D. Joseph Daniel a, A. Raja a, U. Madhusoodanan b, *, O. Annalakshmi b, P. Ramasamy a a b
SSN Research Centre, SSN College of Engineering, Kalavakkam, 603 110, Tamil Nadu, India Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2016 Received in revised form 7 June 2016 Accepted 12 June 2016
This paper presents a preliminary investigation of the optically stimulated luminescence (OSL) of alkali fluoroperovskite single crystals for radiation dosimetry. The perovskite-like KMgF3, NaMgF3 and LiBaF3 polycrystalline compounds doped with rare earths (Eu2þ and Ce3þ) were synthesized by standard solid state reaction technique. Phase purity of the synthesized compounds was analyzed by powder X-ray diffraction technique. Single crystals of these compounds have been grown from melt by using vertical Bridgman-Stockbarger method. The Linearly Modulated OSL and Continuous Wave OSL measurements were performed in these alkali fluorides using blue light stimulation. Thermal bleaching experiments have shown that OSL signals originate from traps which are unstable near 200 C, thus proving the suitability of the signals for dosimetric purposes. Optical bleaching measurements were also performed for these fluoride samples. OSL dose response was studied as a function of dose which was found to increase with beta dose. © 2016 Published by Elsevier B.V.
Keywords: Fluorides Crystal growth X-ray diffraction Luminescence
1. Introduction The use of optically stimulated luminescence (OSL) for personnel dosimetry was first suggested several decades ago by Antonov-Romanovskii et al. [1]. The OSL is of increasing interest during the last years as a procedure for radiation dosimetry alternative to TL [2]. This OSL is finding applications in a wide variety of radiation dosimetry fields, including personnel monitoring, environmental monitoring, retrospective dosimetry, and space dosimetry [3]. Crystals with perovskite structure have extensive practical applications and are convenient models for investigating the optical properties of rare earth metal impurity ions. Fluoroperovskites have the general chemical formula ABX3, where A and B stand for an alkali metal and an alkaline earth metal respectively. X represents halide ion. Generally, these crystals are characterized by high band gap energy (greater than 5 eV) [4], possess high thermal, mechanical durability and are less hygroscopic. Such compounds recently received full attention in view of their possible use as radiation detectors, if doped with suitable activators [5e7].
* Corresponding author. E-mail address: [email protected] (U. Madhusoodanan). http://dx.doi.org/10.1016/j.optmat.2016.06.019 0925-3467/© 2016 Published by Elsevier B.V.
The objective of the present study is to investigate the OSL properties of (Eu2þ and Ce3þ) Co-doped fluorides compounds (KMgF3, NaMgF3 and LiBaF3) prepared by vertical BridgmanStockbarger technique as potential OSL dosimeters. KMgF3 and LiBaF3 belongs to a family of fluoroperovskites with cubic crystal system of lattice constant a ¼ 3.978 Å and a ¼ 3.988 Å respectively [8,9]. NaMgF3 belongs to orthorhombic crystal system with the unit cell dimensions of (a ¼ 5.363 Å; b ¼ 5.503 Å; c ¼ 7.676 Å) [10]. The samples were studied for their dosimetric properties in terms of the TL and OSL response to irradiation with different beta radiation doses, such as the linear response to beta exposure and fading behaviors. 2. Experimental Polycrystalline compound of EuF3 and CeF3 (0.2e0.5 mol %) codoped KMgF3, NaMgF3 and LiBaF3 were synthesized by using conventional solid state reaction method. single crystal of these compounds were grown by resistively heated vertical BridgmanStockbarger technique in graphite crucibles under vacuum atmosphere (106 mbar). The furnace was adjusted using a Eurotherm (model 2404) fine temperature controller with an accuracy of ±1 C. Sealed quartz tube carrying charge was translated with a translation rate of (~3 mm/h) across an axial temperature gradient
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of (~10 C/cm). The solid-liquid interface was located in the gradient zone. The temperature of the melting zone was controlled to slightly higher than the melting point of the crystal. Powder Xray diffraction technique was used to check the phase purity of the material. STOE diffractometer was used to collect the X-ray diffraction data by employing Cu Ka radiation. The OSL measurements of the sample were taken using a RISØ TL/OSL-DA-20 automatic reader with an integrated 90Sr/90Y beta source and under nitrogen atmosphere. In order to investigate the effect of optical bleaching on the LM-OSL signal all fluoride samples (KMgF3, NaMgF3 and LiBaF3) were irradiated with 6 Gy b-dose and bleached with blue LED light (470 nm) for different durations in the range from 1 to 150 s. About Solver: The Solver is an Excel Add-in, a software program that could be found in the Tools menu; it can be installed by checking the Solver Add-in following the path: Tools Add-in Refs. [14,15]. Solver is a general-purpose optimisation package that is used in order to find a maximum, minimum or specified value of the target cell. The Solver code is a product of Frontline Systems Inc. The Solver can be used to minimise the sum of squares of residuals (differences between yobsd and ycalc) and thus perform a least-square fitting. It can be used to perform either linear or non-linear least-square fitting. The Solver uses the Generalised Reduced Gradient non-linear optimisation code. For each of the changing cells, the Solver evaluates the partial derivative of the objective function (the target cell) with respect to the changing cell ai, by means of the finite difference method. The Solver uses a matrix including the partial derivatives to determine the gradient of the response surface and thus decides how to change the values of the changing cells in order to approach the desired solution.
LiBaF3) and all the (h k l) planes were well indexed. The PXRD results indicate that the obtained samples are in single phase. No impurity lines are observed. 3.2. Linearly Modulated OSL (LM e OSL) Bulur proposed an innovative Linearly Modulated (LM) method of measuring optically stimulated luminescence (OSL) which is now widely known as LM-OSL [11]. The LM-OSL signal is observed by linearly increasing the stimulation power of the light source during measurement, and it has many advantages over continuous wave OSL (CW-OSL) measured with constant stimulation power. By ramping the stimulation power from zero to a particular value (usually the maximum power of the light source), the OSL signal appears as a series of peaks; each peak represents a component of the OSL signal with a particular physical parameter, namely the photoionization cross-section. The LM-OSL signal allows more effective and accurate characterization of each OSL component than the CW-OSL signal, and thus, LM-OSL can be used as an essential tool for understanding processes giving rise to OSL and improvement of the optical dating procedure based on measurement of CW-OSL. These OSL signals arise from radiative recombination of charge carriers, de-trapped from all optically sensitive traps. Deconvolution of OSL signal is necessary to get information of individual traps. LM-OSL was measured using 100s stimulation time and 90% of LEDs power. The LM-OSL curves were fitted using Add-in utility e Solver for Microsoft Excel with three components using first order kinetic equation [12,13]. The shape of single LM-OSL peak is characterized by:
IðtÞ ¼ 1:6487Im 3. Results and discussions 3.1. Powder X-Ray diffraction In order to identify the obtained phase, powder X-ray diffraction analysis was carried out for all the synthesized compounds in the 2q range 20e70 . Fig. 1 shows the powder XRD pattern of synthesized pure and Eu2þ and Ce3þ co-doped KMgF3 compounds. The obtained diffraction peaks were compared with corresponding JCPDS card of each compound (PDF Card No.86-2480 for KMgF3, PDF Card No. 13-0303 for NaMgF3 and PDF Card No. 180715 for
Fig. 1. Powder XRD patterns.
t t 2 exp 2 tm 2tm
! (1)
where Im e maximum peak intensity (a.u.), tm e time corresponding to peak maximum (s). Deconvolution: The blue LED LM-OSL curve shapes analyzed
Fig. 2. LM-OSL signal from KMgF3 sample showing the components by curve fitting.
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
499
and resolved into their individual components are presented in Figs. 2e4 for all the fluoride samples KMgF3 NaMgF3 and LiBaF3 respectively. As seen from the figures the LM-OSL measurement does not resolve any peak like structure, indeed a broad curve is observed (similar to the one previously published by Bulur et al., [18]). For each component the time of peak maximum (tmax) was established. Using this parameter the detrapping probability (b) can be calculated from the expression:
tmax
rffiffiffi T ¼ b
(2)
where T is total illumination time. Method of calculation of photoionization cross section (s) was described by Choi [16]. Photoionization cross section is related to detrapping probability and maximum stimulation photon flux (J):
b ¼ Js
(3)
For a monochromatic stimulation light, detrapping probability is proportional to the photoionization cross-section s and the light intensity f on the sample (b ¼ sf) (Table 1). Such a measurement scheme has proven to be useful in distinguishing individual OSL components in samples exhibiting more than one OSL signal with different photoionization cross-sections [17,18]. For stimulation wavelength 470 nm and power density 50 mW cm2 used in the experiment the calculated photon flux was 1.1 1017 s1 cm2. Using above parameters, the photoionization cross sections for all components were established. Photoionization cross-section (s): The photoionization crosssection, a parameter in OSL phenomenon is of major interest. For OSL to occur, it is first necessary for an optical absorption to occur that stimulates charge carriers out of a trap, or optically ionizes them. The probability of photo ionization (P), of any given trap is simply the product of the incident light intensity and the probability that the trap will ionize by stimulation with a photon of this
Fig. 4. LM-OSL signal from LiBaF3 sample showing the components by curve fitting.
Table 1 Calculated photoionization cross section. Materials
Components (tmax)
b (s1)
s (cm2)
Relative s
KMgF3
10.0 70.0
0.7000 0.0142
5.9000 1018 1.2100 1019
1.00 0.02
NaMgF3
9.0 29.0 40.0 75.0
1.2340 0.1189 0.0625 0.0177
1.1218 1.0720 0.5636 0.1545
1017 1018 1018 1018
1.00 0.10 0.05 0.01
LiBaF3
11.5 11.6 55.0
0.7561 0.7431 0.0330
0.6873 1017 0.6755 1017 0.3000 1018
1.00 0.98 0.04
light. This is described by the following equation:
sðEÞ ¼
P ðprobability optical ionizationÞ
4 ðnumber of particles per unit areaÞ
(4)
where f is the optical stimulation intensity and s(E) is the photoionization cross-section for interaction of the trap with an incident photon. E is the energy of the individual photons of incident light. Since the photoionization cross-section for a trap is a function of the photon energy of the incident light, the probability of trap ionization depends on the photon energy. A material produces OSL, the optical ionization (P) will be proportional to the OSL output and is therefore an important parameter to understand. The energy (E) of one photon is given by
E ¼ hn
Fig. 3. LM-OSL signal from NaMgF3 sample showing the components by curve fitting.
where h is Planck’s constant, 6.63 1034 J s ¼ 6.63 1034 W s2, and n is frequency, it can be written as n ¼ c/l. Where c is the speed of light and l is wavelength. For light with wavelength (l) 470 nm, n ¼ (3 108/ 470 109) s1. The power of the LED used in this experiment was 50 mW cm2
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Thus, the energy of one photon produced by a blue-LED (470 nm) is calculated to be 4.21019 W s. The number of photons per unit time per unit area is 1.18 1017 s1 cm2 (0.05 W cm2/4.2 1019 W s). 3.3. Thermal decay of LM-OSL Thermal stability of a signal considered for radiation dosimetry is a very important criterion. In this work this was checked to ensure that the thermal stability of the signal is high enough. For this purpose, LM-OSL curves from a 12 Gy irradiated sample were obtained for preheat temperatures ranging from 100 to 250 C. Annealing in the temperature region (100e250 C) causes significant depletion to these components in all fluoride samples. As seen from the graph Fig. 5, the LM-OSL signal of KMgF3 sample is stable up to approximately 200 C and becomes unstable around 250 C, proving the suitability of the signal for dosimetry purposes. This KMgF3 has a very bright TL signal appearing around 250 C which is very sensitive to light. The thermal decay of LM-OSL signal of NaMgF3 sample is shown in Fig. 6. It clearly depicts the decrease in the intensity around 150e175 C which relates the first peak of its TL glow curve around 130 C. Further decrease in LM-OSL signals around 250 C relates to the second TL peak of around 200 C. In the case of LiBaF3 samples LM-OSL thermal decay is shown in Fig. 7. The sharp peak in the initial stages is identified as fast decaying component and also it is related to the low temperature peaks in the TL glow curve of the LiBaF3 samples. This LiBaF3 has three peak systems in TL glow curve (shown in Fig. 8). The initial decrease around 120 C in LM-OSL corresponds to the low temperature TL peak and it was stable up to 200 C. This similar behaviour of some separate OSL components could be simply attributed to the facts that first order model is not a good approximation and the signal of some components originates probably from one trap.
Fig. 6. Effect of pre-annealing on the OSL signal from NaMgF3 samples.
3.4. Optical bleaching of the OSL signal Figs. 8e10 show the LM-OSL curves measured from a 6 Gy irradiated fluoride sample for various bleaching times (1e150 s) by blue LED illumination. It is observed that all the peak maxima of the LM-OSL curves shift to longer times as the signal is bleached. The peak maxima (tmax) values were found to be monotonically increasing with the bleaching time showing no clearly visible plateau regions.
Fig. 5. Effect of pre-annealing on the OSL signal from KMgF3 samples.
Fig. 7. Effect of pre-annealing on the OSL signal from LiBaF3 samples.
Fig. 8. TL glow curve of (b e dose 6 Gy) irradiated LiBaF3: Eu2þ, Ce3þ at heating rate of 5 C/s.
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
Fig. 9. Bleaching of the OSL signal from KMgF3 samples.
501
Fig. 11. Bleaching of the OSL signal from LiBaF3 samples.
Fig. 10. Bleaching of the OSL signal from NaMgF3 samples. Fig. 12. CW-OSL signals from KMgF3 sample showing the components by curve fitting.
3.5. Continuous wave OSL (CW e OSL) CW-OSL is the most widely used stimulation method in dosimetry and luminescence dating due to its simplicity and frequently satisfactory performance. The original work of Huntley et al., used continuous stimulation of the sample with a constant light intensity [19]. The simplest OSL readout approach consists of stimulating the detector with light of constant intensity, a modality known as continuous-wave OSL (CW-OSL). The OSL stimulation spectra of the co-doped fluoride samples were measured after b-irradiation (dose 6 Gy). The typical OSL decay curves recorded at continuous wave (CW) stimulation with blue laser line (l ¼ 470 nm) is shown in Figs. 11e13 for KMgF3, NaMgF3 and LiBaF3 respectively. The shape of the OSL decays varied considerably from sample to sample. The same process explains the rapid decay of the light emission in the first ~5 s of the OSL curve of the irradiated sample. The decay is characterized by a fast initial decay followed by a slower decay, which is an evidence of the contribution of trapping centers with different photo ionization cross-sections. As it is seen from the figures the CW-OSL is a monotonous exponential-like decay. However, the signal is not purely single exponential but can be approximated by two first-order
Fig. 13. CW-OSL signals from NaMgF3 sample showing the components by curve fitting.
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Table 2 Fitted parameters for the exponential decay components of the CW-OS. Materials
Curves (i)
KMgF3
Curve Curve Curve Curve Curve Curve Curve
NaMgF3
LiBaF3
e e e e e e e
1 2 1 2 3 1 2
ti (s)
Ai
1.20 70.0 3.0 4.10 29.0 1.0 2.58
1.85 1.10 0.42 1.38 1.00 9.00 4.04
(arb.units)
Area (%)
FOM (%)
3.50 90.80 3.60 16.70 72.70 21.40 38.30
0.001% 0.005%
0.001%
exponential decay functions, which can be written as
IOSL ¼ I1 exp½ t=t1 þ I2 exp½ t=t2
(5)
where IOSL is the total intensity and I1, I2 stand for the initial intensities of exponentially decaying fast and slow components of OSL curve, respectively. The t1 and t2 are attenuation constant of exponentially decaying fast and slow components of OSL curve respectively. The detrapping rates, ti of these samples (KMgF3, NaMgF3 and LiBaF3) were determined using curve fitting. As seen from the figures the residue is smaller than 1% of the maximum except for the very initial data points. The fitted parameters are listed in Table 2.
Fig. 15. Responses in function of the dose for KMgF3 samples.
3.6. Dose response Figs. 14e16 correspond to different beta doses of KMgF3, NaMgF3 and LiBaF3 samples respectively. Their dose dependence can clearly be seen. The total luminescence counts increase as dose increases as expected in the luminescence technique and the increasing dose distorts the shape of the peak. The reason for the initial increase in luminescence intensity was thermally metastable traps. In this simple model, the rate of decay of the OSL curve depends only on s and f, but not on n0, which means that the shape of the OSL curve does not change with the dose (see Figures) (Fig. 17). On the other hand, the total area under the OSL curve depends only on n0, but not on s or f, because,
Z∞
Z∞ IOSL ðtÞdt∝
0
n0 sfesft dt ¼ n0
(6)
0
Fig. 14. CW-OSL signals from LiBaF3 sample showing the components by curve fitting.
Fig. 16. Responses in function of the for NaMgF3 samples.
Fig. 17. Responses in function of the for LiBaF3 samples.
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
where n0 is the detrapping probability. This means that the total area under the OSL curve, or the total number of photons emitted, is not affected by changes in the stimulation intensity. The above observations have practical implications for OSL radiation dosimetry.
503
Acknowledgement This work was financially supported by IGCAR, Department of Atomic Energy, Government of India. References
4. Summary The preliminary study of OSL properties of fluoride compounds has been performed. Optically Stimulated Luminescence from fluoride samples was studied using the LM-OSL and CW-OSL techniques. It was observed that the LM-OSL curve can be described by the sum of two first-order components each with different thermal stability and bleaching rate. The signal depletion during preheat is correlated with TL glow curves. The correct identification and characterization of these components need a more detailed investigation. Recognizing more than one component in the OSL signal has been made possible by the linearly modulating stimulation OSL measurement. Linear modulation provides an important new approach in the analysis of OSL signals, particularly for the study of the characteristics of the minor components in a stimulation curve. A dependence of photoionization cross section (s) on wavelength was calculated for all the mixed fluorides for the fast and medium components. From the CW-OSL measurements the detrapping rates, ti, of each fluoride compounds were determined using curve fitting.
[1] V. Antonov-Romanovskii, I.F. Keirum-Marcus, M.S. Poroshina, Z.A. Trapeznikova, in: Conference of the Academy of Sciences of the USSR on the Peace full Uses of Atomic Energy. USAEC, Moscow. Report AEC-tr-2435, vol. 239, 1956, p. 1. [2] S.W.S. McKeever, Nucl. Instrum. Methods Phys. Res. B 184 (2001) 29e54. [3] L. Botter-Jensen, C.E. Andersen, G.A.T. Duller, A.S. Murray, Radiat. Meas. 37 (2003) 535e541. [4] T. Nishimatsu, et al., Jpn. J. Appl. Phys. Part 2 (41) (2002) L365eL367. [5] A.V. Gektin, I.M. Krasovitskaya, N.V. Shiran, Radiat. Meas. 29 (1998) 337e340. [6] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dosim. 82 (1999) 151e152. [7] A.V. Gektin, J. Lumin 87e89 (2000) 1283e1285. [8] Banwari Lal, et al., Mater Chem. Phys. 83 (2004) 158e168. [9] S. Mahlik, et al., J. Phys. Conden. Matter 21 (2009) 235603e235611. [10] A. Ocafrain, J. Cryst. Growth 166 (1996) 414e418. [11] E. Bulur, Radiat. Meas. 26 (1996) 701e709. [12] D. Afouxenidis, G.S. Polymeris, N.C. Tsirliganis, G. Kitis, Radiat. Prot. Dosim. 149 (2012) 363e370. [13] G. Kitis, V. Pagonis, Radiat. Meas. 43 (2008) 373e741. [14] E.J. Billo, Excel for Scientists and Engineers, Wiley, 2007. [15] D. Fylstra, L. Lasdon, L. Watson, A. Waren, Interfaces 28 (1998) 29e55. [16] J.H. Choi, G.A.T. Duller, A.G. Wintle, Anc. TL 24 (1) (2006). [17] E. Bulur, L. Bøtter-Jensen, A.S. Murray, Radiat. Meas. 32 (2000) 407e411. [18] E. Bulur, L. Bøtter-Jensen, A.S. Murray, Beam interactions with materials and atoms, Nucl. Instrum. Methods Phys. Res. B 179 (2001) 151e159. [19] D.J. Huntley, D.I. Godfrey-Smith, M.L.W. Thewalt, Nature 313 (1985) 105e107.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
OSL studies of alkali fluoroperovskite single crystals for radiation dosimetry D. Joseph Daniel a, A. Raja a, U. Madhusoodanan b, *, O. Annalakshmi b, P. Ramasamy a a b
SSN Research Centre, SSN College of Engineering, Kalavakkam, 603 110, Tamil Nadu, India Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2016 Received in revised form 7 June 2016 Accepted 12 June 2016
This paper presents a preliminary investigation of the optically stimulated luminescence (OSL) of alkali fluoroperovskite single crystals for radiation dosimetry. The perovskite-like KMgF3, NaMgF3 and LiBaF3 polycrystalline compounds doped with rare earths (Eu2þ and Ce3þ) were synthesized by standard solid state reaction technique. Phase purity of the synthesized compounds was analyzed by powder X-ray diffraction technique. Single crystals of these compounds have been grown from melt by using vertical Bridgman-Stockbarger method. The Linearly Modulated OSL and Continuous Wave OSL measurements were performed in these alkali fluorides using blue light stimulation. Thermal bleaching experiments have shown that OSL signals originate from traps which are unstable near 200 C, thus proving the suitability of the signals for dosimetric purposes. Optical bleaching measurements were also performed for these fluoride samples. OSL dose response was studied as a function of dose which was found to increase with beta dose. © 2016 Published by Elsevier B.V.
Keywords: Fluorides Crystal growth X-ray diffraction Luminescence
1. Introduction The use of optically stimulated luminescence (OSL) for personnel dosimetry was first suggested several decades ago by Antonov-Romanovskii et al. [1]. The OSL is of increasing interest during the last years as a procedure for radiation dosimetry alternative to TL [2]. This OSL is finding applications in a wide variety of radiation dosimetry fields, including personnel monitoring, environmental monitoring, retrospective dosimetry, and space dosimetry [3]. Crystals with perovskite structure have extensive practical applications and are convenient models for investigating the optical properties of rare earth metal impurity ions. Fluoroperovskites have the general chemical formula ABX3, where A and B stand for an alkali metal and an alkaline earth metal respectively. X represents halide ion. Generally, these crystals are characterized by high band gap energy (greater than 5 eV) [4], possess high thermal, mechanical durability and are less hygroscopic. Such compounds recently received full attention in view of their possible use as radiation detectors, if doped with suitable activators [5e7].
* Corresponding author. E-mail address: [email protected] (U. Madhusoodanan). http://dx.doi.org/10.1016/j.optmat.2016.06.019 0925-3467/© 2016 Published by Elsevier B.V.
The objective of the present study is to investigate the OSL properties of (Eu2þ and Ce3þ) Co-doped fluorides compounds (KMgF3, NaMgF3 and LiBaF3) prepared by vertical BridgmanStockbarger technique as potential OSL dosimeters. KMgF3 and LiBaF3 belongs to a family of fluoroperovskites with cubic crystal system of lattice constant a ¼ 3.978 Å and a ¼ 3.988 Å respectively [8,9]. NaMgF3 belongs to orthorhombic crystal system with the unit cell dimensions of (a ¼ 5.363 Å; b ¼ 5.503 Å; c ¼ 7.676 Å) [10]. The samples were studied for their dosimetric properties in terms of the TL and OSL response to irradiation with different beta radiation doses, such as the linear response to beta exposure and fading behaviors. 2. Experimental Polycrystalline compound of EuF3 and CeF3 (0.2e0.5 mol %) codoped KMgF3, NaMgF3 and LiBaF3 were synthesized by using conventional solid state reaction method. single crystal of these compounds were grown by resistively heated vertical BridgmanStockbarger technique in graphite crucibles under vacuum atmosphere (106 mbar). The furnace was adjusted using a Eurotherm (model 2404) fine temperature controller with an accuracy of ±1 C. Sealed quartz tube carrying charge was translated with a translation rate of (~3 mm/h) across an axial temperature gradient
498
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
of (~10 C/cm). The solid-liquid interface was located in the gradient zone. The temperature of the melting zone was controlled to slightly higher than the melting point of the crystal. Powder Xray diffraction technique was used to check the phase purity of the material. STOE diffractometer was used to collect the X-ray diffraction data by employing Cu Ka radiation. The OSL measurements of the sample were taken using a RISØ TL/OSL-DA-20 automatic reader with an integrated 90Sr/90Y beta source and under nitrogen atmosphere. In order to investigate the effect of optical bleaching on the LM-OSL signal all fluoride samples (KMgF3, NaMgF3 and LiBaF3) were irradiated with 6 Gy b-dose and bleached with blue LED light (470 nm) for different durations in the range from 1 to 150 s. About Solver: The Solver is an Excel Add-in, a software program that could be found in the Tools menu; it can be installed by checking the Solver Add-in following the path: Tools Add-in Refs. [14,15]. Solver is a general-purpose optimisation package that is used in order to find a maximum, minimum or specified value of the target cell. The Solver code is a product of Frontline Systems Inc. The Solver can be used to minimise the sum of squares of residuals (differences between yobsd and ycalc) and thus perform a least-square fitting. It can be used to perform either linear or non-linear least-square fitting. The Solver uses the Generalised Reduced Gradient non-linear optimisation code. For each of the changing cells, the Solver evaluates the partial derivative of the objective function (the target cell) with respect to the changing cell ai, by means of the finite difference method. The Solver uses a matrix including the partial derivatives to determine the gradient of the response surface and thus decides how to change the values of the changing cells in order to approach the desired solution.
LiBaF3) and all the (h k l) planes were well indexed. The PXRD results indicate that the obtained samples are in single phase. No impurity lines are observed. 3.2. Linearly Modulated OSL (LM e OSL) Bulur proposed an innovative Linearly Modulated (LM) method of measuring optically stimulated luminescence (OSL) which is now widely known as LM-OSL [11]. The LM-OSL signal is observed by linearly increasing the stimulation power of the light source during measurement, and it has many advantages over continuous wave OSL (CW-OSL) measured with constant stimulation power. By ramping the stimulation power from zero to a particular value (usually the maximum power of the light source), the OSL signal appears as a series of peaks; each peak represents a component of the OSL signal with a particular physical parameter, namely the photoionization cross-section. The LM-OSL signal allows more effective and accurate characterization of each OSL component than the CW-OSL signal, and thus, LM-OSL can be used as an essential tool for understanding processes giving rise to OSL and improvement of the optical dating procedure based on measurement of CW-OSL. These OSL signals arise from radiative recombination of charge carriers, de-trapped from all optically sensitive traps. Deconvolution of OSL signal is necessary to get information of individual traps. LM-OSL was measured using 100s stimulation time and 90% of LEDs power. The LM-OSL curves were fitted using Add-in utility e Solver for Microsoft Excel with three components using first order kinetic equation [12,13]. The shape of single LM-OSL peak is characterized by:
IðtÞ ¼ 1:6487Im 3. Results and discussions 3.1. Powder X-Ray diffraction In order to identify the obtained phase, powder X-ray diffraction analysis was carried out for all the synthesized compounds in the 2q range 20e70 . Fig. 1 shows the powder XRD pattern of synthesized pure and Eu2þ and Ce3þ co-doped KMgF3 compounds. The obtained diffraction peaks were compared with corresponding JCPDS card of each compound (PDF Card No.86-2480 for KMgF3, PDF Card No. 13-0303 for NaMgF3 and PDF Card No. 180715 for
Fig. 1. Powder XRD patterns.
t t 2 exp 2 tm 2tm
! (1)
where Im e maximum peak intensity (a.u.), tm e time corresponding to peak maximum (s). Deconvolution: The blue LED LM-OSL curve shapes analyzed
Fig. 2. LM-OSL signal from KMgF3 sample showing the components by curve fitting.
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
499
and resolved into their individual components are presented in Figs. 2e4 for all the fluoride samples KMgF3 NaMgF3 and LiBaF3 respectively. As seen from the figures the LM-OSL measurement does not resolve any peak like structure, indeed a broad curve is observed (similar to the one previously published by Bulur et al., [18]). For each component the time of peak maximum (tmax) was established. Using this parameter the detrapping probability (b) can be calculated from the expression:
tmax
rffiffiffi T ¼ b
(2)
where T is total illumination time. Method of calculation of photoionization cross section (s) was described by Choi [16]. Photoionization cross section is related to detrapping probability and maximum stimulation photon flux (J):
b ¼ Js
(3)
For a monochromatic stimulation light, detrapping probability is proportional to the photoionization cross-section s and the light intensity f on the sample (b ¼ sf) (Table 1). Such a measurement scheme has proven to be useful in distinguishing individual OSL components in samples exhibiting more than one OSL signal with different photoionization cross-sections [17,18]. For stimulation wavelength 470 nm and power density 50 mW cm2 used in the experiment the calculated photon flux was 1.1 1017 s1 cm2. Using above parameters, the photoionization cross sections for all components were established. Photoionization cross-section (s): The photoionization crosssection, a parameter in OSL phenomenon is of major interest. For OSL to occur, it is first necessary for an optical absorption to occur that stimulates charge carriers out of a trap, or optically ionizes them. The probability of photo ionization (P), of any given trap is simply the product of the incident light intensity and the probability that the trap will ionize by stimulation with a photon of this
Fig. 4. LM-OSL signal from LiBaF3 sample showing the components by curve fitting.
Table 1 Calculated photoionization cross section. Materials
Components (tmax)
b (s1)
s (cm2)
Relative s
KMgF3
10.0 70.0
0.7000 0.0142
5.9000 1018 1.2100 1019
1.00 0.02
NaMgF3
9.0 29.0 40.0 75.0
1.2340 0.1189 0.0625 0.0177
1.1218 1.0720 0.5636 0.1545
1017 1018 1018 1018
1.00 0.10 0.05 0.01
LiBaF3
11.5 11.6 55.0
0.7561 0.7431 0.0330
0.6873 1017 0.6755 1017 0.3000 1018
1.00 0.98 0.04
light. This is described by the following equation:
sðEÞ ¼
P ðprobability optical ionizationÞ
4 ðnumber of particles per unit areaÞ
(4)
where f is the optical stimulation intensity and s(E) is the photoionization cross-section for interaction of the trap with an incident photon. E is the energy of the individual photons of incident light. Since the photoionization cross-section for a trap is a function of the photon energy of the incident light, the probability of trap ionization depends on the photon energy. A material produces OSL, the optical ionization (P) will be proportional to the OSL output and is therefore an important parameter to understand. The energy (E) of one photon is given by
E ¼ hn
Fig. 3. LM-OSL signal from NaMgF3 sample showing the components by curve fitting.
where h is Planck’s constant, 6.63 1034 J s ¼ 6.63 1034 W s2, and n is frequency, it can be written as n ¼ c/l. Where c is the speed of light and l is wavelength. For light with wavelength (l) 470 nm, n ¼ (3 108/ 470 109) s1. The power of the LED used in this experiment was 50 mW cm2
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Thus, the energy of one photon produced by a blue-LED (470 nm) is calculated to be 4.21019 W s. The number of photons per unit time per unit area is 1.18 1017 s1 cm2 (0.05 W cm2/4.2 1019 W s). 3.3. Thermal decay of LM-OSL Thermal stability of a signal considered for radiation dosimetry is a very important criterion. In this work this was checked to ensure that the thermal stability of the signal is high enough. For this purpose, LM-OSL curves from a 12 Gy irradiated sample were obtained for preheat temperatures ranging from 100 to 250 C. Annealing in the temperature region (100e250 C) causes significant depletion to these components in all fluoride samples. As seen from the graph Fig. 5, the LM-OSL signal of KMgF3 sample is stable up to approximately 200 C and becomes unstable around 250 C, proving the suitability of the signal for dosimetry purposes. This KMgF3 has a very bright TL signal appearing around 250 C which is very sensitive to light. The thermal decay of LM-OSL signal of NaMgF3 sample is shown in Fig. 6. It clearly depicts the decrease in the intensity around 150e175 C which relates the first peak of its TL glow curve around 130 C. Further decrease in LM-OSL signals around 250 C relates to the second TL peak of around 200 C. In the case of LiBaF3 samples LM-OSL thermal decay is shown in Fig. 7. The sharp peak in the initial stages is identified as fast decaying component and also it is related to the low temperature peaks in the TL glow curve of the LiBaF3 samples. This LiBaF3 has three peak systems in TL glow curve (shown in Fig. 8). The initial decrease around 120 C in LM-OSL corresponds to the low temperature TL peak and it was stable up to 200 C. This similar behaviour of some separate OSL components could be simply attributed to the facts that first order model is not a good approximation and the signal of some components originates probably from one trap.
Fig. 6. Effect of pre-annealing on the OSL signal from NaMgF3 samples.
3.4. Optical bleaching of the OSL signal Figs. 8e10 show the LM-OSL curves measured from a 6 Gy irradiated fluoride sample for various bleaching times (1e150 s) by blue LED illumination. It is observed that all the peak maxima of the LM-OSL curves shift to longer times as the signal is bleached. The peak maxima (tmax) values were found to be monotonically increasing with the bleaching time showing no clearly visible plateau regions.
Fig. 5. Effect of pre-annealing on the OSL signal from KMgF3 samples.
Fig. 7. Effect of pre-annealing on the OSL signal from LiBaF3 samples.
Fig. 8. TL glow curve of (b e dose 6 Gy) irradiated LiBaF3: Eu2þ, Ce3þ at heating rate of 5 C/s.
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
Fig. 9. Bleaching of the OSL signal from KMgF3 samples.
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Fig. 11. Bleaching of the OSL signal from LiBaF3 samples.
Fig. 10. Bleaching of the OSL signal from NaMgF3 samples. Fig. 12. CW-OSL signals from KMgF3 sample showing the components by curve fitting.
3.5. Continuous wave OSL (CW e OSL) CW-OSL is the most widely used stimulation method in dosimetry and luminescence dating due to its simplicity and frequently satisfactory performance. The original work of Huntley et al., used continuous stimulation of the sample with a constant light intensity [19]. The simplest OSL readout approach consists of stimulating the detector with light of constant intensity, a modality known as continuous-wave OSL (CW-OSL). The OSL stimulation spectra of the co-doped fluoride samples were measured after b-irradiation (dose 6 Gy). The typical OSL decay curves recorded at continuous wave (CW) stimulation with blue laser line (l ¼ 470 nm) is shown in Figs. 11e13 for KMgF3, NaMgF3 and LiBaF3 respectively. The shape of the OSL decays varied considerably from sample to sample. The same process explains the rapid decay of the light emission in the first ~5 s of the OSL curve of the irradiated sample. The decay is characterized by a fast initial decay followed by a slower decay, which is an evidence of the contribution of trapping centers with different photo ionization cross-sections. As it is seen from the figures the CW-OSL is a monotonous exponential-like decay. However, the signal is not purely single exponential but can be approximated by two first-order
Fig. 13. CW-OSL signals from NaMgF3 sample showing the components by curve fitting.
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Table 2 Fitted parameters for the exponential decay components of the CW-OS. Materials
Curves (i)
KMgF3
Curve Curve Curve Curve Curve Curve Curve
NaMgF3
LiBaF3
e e e e e e e
1 2 1 2 3 1 2
ti (s)
Ai
1.20 70.0 3.0 4.10 29.0 1.0 2.58
1.85 1.10 0.42 1.38 1.00 9.00 4.04
(arb.units)
Area (%)
FOM (%)
3.50 90.80 3.60 16.70 72.70 21.40 38.30
0.001% 0.005%
0.001%
exponential decay functions, which can be written as
IOSL ¼ I1 exp½ t=t1 þ I2 exp½ t=t2
(5)
where IOSL is the total intensity and I1, I2 stand for the initial intensities of exponentially decaying fast and slow components of OSL curve, respectively. The t1 and t2 are attenuation constant of exponentially decaying fast and slow components of OSL curve respectively. The detrapping rates, ti of these samples (KMgF3, NaMgF3 and LiBaF3) were determined using curve fitting. As seen from the figures the residue is smaller than 1% of the maximum except for the very initial data points. The fitted parameters are listed in Table 2.
Fig. 15. Responses in function of the dose for KMgF3 samples.
3.6. Dose response Figs. 14e16 correspond to different beta doses of KMgF3, NaMgF3 and LiBaF3 samples respectively. Their dose dependence can clearly be seen. The total luminescence counts increase as dose increases as expected in the luminescence technique and the increasing dose distorts the shape of the peak. The reason for the initial increase in luminescence intensity was thermally metastable traps. In this simple model, the rate of decay of the OSL curve depends only on s and f, but not on n0, which means that the shape of the OSL curve does not change with the dose (see Figures) (Fig. 17). On the other hand, the total area under the OSL curve depends only on n0, but not on s or f, because,
Z∞
Z∞ IOSL ðtÞdt∝
0
n0 sfesft dt ¼ n0
(6)
0
Fig. 14. CW-OSL signals from LiBaF3 sample showing the components by curve fitting.
Fig. 16. Responses in function of the for NaMgF3 samples.
Fig. 17. Responses in function of the for LiBaF3 samples.
D.J. Daniel et al. / Optical Materials 58 (2016) 497e503
where n0 is the detrapping probability. This means that the total area under the OSL curve, or the total number of photons emitted, is not affected by changes in the stimulation intensity. The above observations have practical implications for OSL radiation dosimetry.
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Acknowledgement This work was financially supported by IGCAR, Department of Atomic Energy, Government of India. References
4. Summary The preliminary study of OSL properties of fluoride compounds has been performed. Optically Stimulated Luminescence from fluoride samples was studied using the LM-OSL and CW-OSL techniques. It was observed that the LM-OSL curve can be described by the sum of two first-order components each with different thermal stability and bleaching rate. The signal depletion during preheat is correlated with TL glow curves. The correct identification and characterization of these components need a more detailed investigation. Recognizing more than one component in the OSL signal has been made possible by the linearly modulating stimulation OSL measurement. Linear modulation provides an important new approach in the analysis of OSL signals, particularly for the study of the characteristics of the minor components in a stimulation curve. A dependence of photoionization cross section (s) on wavelength was calculated for all the mixed fluorides for the fast and medium components. From the CW-OSL measurements the detrapping rates, ti, of each fluoride compounds were determined using curve fitting.
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