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Photoluminescence and thermoluminescence study of KCaSO4Cl doped with Dy and Ce synthesized by acid distillation method
Journal of Luminescence 145 (2014) 299–306
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Photoluminescence and thermoluminescence study of KCaSO4Cl doped with Dy and Ce synthesized by acid distillation method Bhushan P. Kore a, N.S. Dhoble b, S.P. Lochab c, S.J. Dhoble a,n a
Department of Physics, RTM Nagpur University, Nagpur 440033, India Department of Chemistry, Sevadal Mahila Mahavidyalaya, Nagpur 440009, India c Inter-University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 April 2013 Received in revised form 21 June 2013 Accepted 10 July 2013 Available online 29 July 2013
Photoluminescence and thermoluminescence properties of KCaSO4Cl doped with dysprosium and cerium have been studied. Dy/Ce doped KCaSO4Cl phosphors were synthesized by the acid distillation method. The samples were characterized by XRD, SEM, PL and TL for structural, morphological and luminescence studies. The SEM image analysis of KCaSO4Cl phosphor shows nearly spherical particles with diameter varying between 3–10 μm. In the present host Dy3+ emission at 482 and 573 nm is observed due to 4 F9/2-6H15/2 and 4F9/2-6H13/2 transition, respectively, whereas the PL emission spectra of KCaSO4Cl:Ce phosphor shows two luminescence bands at 307 nm and 326 nm and are attributed to the allowed inter configurational transitions from the 5d-level to the 2F5/2 and 2F7/2 levels of Ce3+ ion. Effect of annealing on the structure of the glow curve is investigated for KCaSO4Cl:Dy phosphors. Thermoluminescence linearity has been studied for 0.1–9000 Gy dose of gamma rays. Linear behavior over a large dose range between 0.1 Gy and 170 Gy was found. In addition to this trap parameters of KCaSO4Cl:Dy were studied using computerized glow curve deconvolution. & 2013 Elsevier B.V. All rights reserved.
Keywords: Acid distillation Luminescence Glow curve Deconvolution Dosimetry
1. Introduction Luminescent materials have sparked significant attention for their practical and promising applications in display devices and medical applications [1–4]. In particular, trivalent dysprosium doped phosphors have been investigated extensively because of the intense visible and wide range emission from 400 nm to 700 nm applicable for various lighting devices [5–8]. The rare earth Dy3+ ions have two dominant emission bands in the blue and yellow region. The blue emission (470–500 nm) is due to 4F9/2 -6H15/2 transition and the yellow emission (560–600 nm) is due to 4F9/2-6H13/2 transition [6,9]. The yellow emission of Dy3+ is especially hypersensitive to the local environment. By modifying the crystal structure, the intensity ratio IYellow/IBlue can be controlled and resultant color of the phosphors can be tuned. By adjusting the yellow to blue intensity ratio it is possible to achieve proximate white light emission [8]. The position of the excited 5d levels of the trivalent lanthanides in inorganic host matrix relative to the 4fn ground state configuration is essential for many luminescence properties of phosphors. Position of these 5d bands relative to 4f levels is also crucial for many applications of luminescent inorganic materials [10].
n
Corresponding author. Tel.: +91 9822710204. E-mail address: [email protected] (S.J. Dhoble).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.07.030
The positions of these 5d levels depend strongly on the crystalline environment and may change from few to tens of thousands cm 1, from compound to compound [11–13]. In scintillation crystals, Ce3+ is of interest as activator ion. Ce3+ usually shows a fast dipole allowed luminescence with typically 107 50 ns decay time [14]. Since the 4f–5d transitions of Ce3+ are parity allowed and its luminescence decay constant is usually several tens of nanoseconds. Because of this fast decay characteristic, Ce3+ activated luminescent materials are important scintillators to detect X/γ rays or thermal neutrons [15–17]. Thermoluminescence (TL) is a very common method used for estimations of doses of ionizing radiations. It has been found that the intensity and area of TL glow peaks are proportional to the received dose and this is the basis for using TL phosphors in dosimetry of ionizing radiations [18]. Various TL investigations have shown that defect centers play a crucial role in TL since, the release of holes/electrons from defect centers at characteristic traps initiate the luminescence process in the material. The demand for dosimetry of ionizing radiations is growing day by day because of its utility in all branches of science where ionizing radiation is used and variety of medical applications including radiation therapy, diagnostic radiology, and radiotherapy mailed dosimetry [19–21]. Thermoluminescence properties of sulfate based phosphors activated by different rare earths have been the subject of intense research for many years. These studies have resulted in a variety of applications [22,23]. Those TL materials
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based on CaSO4 have been widely applied for medical and environmental dosimetry [24–26]. This fact has attracted the interest for studying the TL response of KCaSO4Cl. The thermoluminescence properties of some new sulfate based materials were investigated by different researchers [27,28]. The study of the luminescence as a function of the temperature, the so called glow curve, is used to determine the trapping parameters. Investigation of glow curves is widely used to extract more information about kinetics in thermoluminescence materials. The typical glow curve contains one or more glow peaks. Each peak gives information about only trap level occupied by charge carriers, in the TL material. The analysis of the glow peak is done by an empirical method which depends upon a parameter called the order of kinetics [29,30]. In order to obtain more detailed information we have to fit the experimental glow curve data to a particular TL model which best describe the TL as a function of these parameters [31,32]. In the study of relatively deep trap defect state in various materials as well as in TL dating, a detailed analysis of TL glow curve is crucial. This paper describes the preparation procedure and dosimetric characteristics of KCaSO4Cl microcrystalline phosphor. The formation of the material was confirmed by the X-ray diffraction (XRD). The morphology of the material was studied by scanning electron microscope (SEM). Its TL response to γ-rays of 60Co has been studied in more details. The TL glow curves of microcrystalline phosphor have been deconvoluted to separate various glow peaks using GCD functions suggested by Kitis et al. [33]. Trapping parameters, dose response, fading and reusability of the microcrystalline KCaSO4Cl:Dy phosphor also have been investigated in this study.
2. Synthesis All the samples investigated in this paper were prepared by following the method described by Yamashita et al. [34] and Dixon
et al. [35]. For the preparation of KCaSO4Cl phosphors all starting materials used were of analytical grade. Samples of KCaSO4Cl doped with rare earths were prepared by dissolving CaSO4, KCl and rare earths in hot sulfuric acid (the excess acid) at about 300 1C for 20 h to form small crystals. During acid evaporation (distillation) the acid vapors were subject to condensation. After cooling, the samples were repeatedly washed with double distilled water in order to remove the traces of acid and unwanted impurities if any incorporated during the synthesis then filtered and dried in hot air oven at 80 1C. No further additional heat treatments were given to samples. In this way the phosphors were prepared and made ready for further characterizations. The XRD technique was used in order to confirm the formation of desired product. The phase structure were characterized by X-ray diffraction (XRD) pattern using a PANalytical diffractometer with Cu Kα radiation (λ ¼1.5405 Å) operating at 40 kV, 30 mA. The morphology of the powders was studied by scanning electron microscopy (SEM) using a JEOL JSM 6380 A microscope. The photoluminescence (PL) emission spectra of the samples were recorded by using a RF-5301PC SHIMADZU Spectrofluorophotometer. Emission and excitation spectra were recorded using a spectral slit width of 1.5 nm. For TL studies, samples were exposed to gamma rays from a 60CO source at room temperature at the rate of 0.58 kGy/hr. The TL glow curves were recorded with the help of Nucleonix 1009 TL reader.
3. Results and discussion 3.1. XRD XRD study was carried out in order to confirm the formation of desired compound synthesized by acid distillation method. The XRD pattern of the KCaSO4Cl compared with that of the constituents phases and from comparison it is concluded that the phosphor is successfully prepared. The XRD pattern shown in Fig. 1 is in accordance with the XRD pattern reported in [36], in which the KCaSO4Cl was prepared by solid state reaction method. 3.2. SEM study
Fig. 1. X-ray diffraction pattern of KCaSO4Cl host matrix.
In order to obtain information about the surface morphology of present phosphor SEM analysis was carried out. The typical morphological images are presented in Fig. 2. From the micrograph it could be seen that particles of KCaSO4Cl have shapes like pyramids, cubes and some of them are of arbitrary shape, with fine surfaces. SEM photographs reveal that the phosphor powder is composed of irregular structured particles and the grains are asymmetrical shaped. An average crystallite size of crystallites was found to be in 5–20 μm range which is considered very favorable for thermoluminescence characteristics of a phosphor material [37].
Fig. 2. SEM images of KCaSO4Cl prepared by acid distillation method.
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3.3. Photoluminescence studies 3.3.1. Photoluminescence study of KCaSO4Cl:Dy phosphor Luminescence of Dy3+ has attracted much attention because of its white light emission. The photoluminescence spectra of Dy3+ doped KCaSO4Cl sample under excitation at 350 nm and emission spectra monitored at 572 nm is shown in Figs. 3 and 4 respectively. The excitation spectrum in the range 250–400 nm consisting of several peaks, came from the ground state of 6H15/2 to the excited states of 4f9 electronic configurations of the Dy3+ ions, which located at 325 nm (6H15/2-4M17/2), 350 nm (6H15/2-6P7/2), 364 nm(6H15/2-4I11/2), 389 nm (6H15/2-4I13/2) respectively [38]. Among all these excitation bands, the band at 350 nm possessed the maximum intensity that is due to 6H15/2-6P7/2. Thereby, the emission spectra were monitored at 350 nm in order to optimize Dy3+ luminescence. As shown in Fig. 4, the emission spectrum of Dy3+ has two groups of emissions located at 482 nm and 573 nm, which correspond to the transitions from 4F9/2-6H15/2 (blue), 4 F9/2-6H13/2 (yellow), respectively. Fig. 5 illustrates the energy levels of Dy3+ ion, which consist of five emission wavelengths at 482 nm, 576 nm, 660 nm, 755 nm and 840 nm. Among these emission, the 4F9/2-6H13/2 emission belongs to hypersensitive transition with ΔJ ¼2, which is strongly influenced by outside environments of Dy3+. When the Dy3+ is located at a lowsymmetry local site (without an inversion center), the emission from 4F9/2-6H13/2 transition is often dominated in emission spectra [38]. It was found from literatures that the 4F9/2-6H15/2 magnetic dipole transition was prominent when Dy3+ was located
Fig. 5. Energy level diagram of Dy3+ ion.
Fig. 6. Variation in the PL intensity as a function of the Dy3+ ion concentrations. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. Excitation spectrum of KCaSO4Cl:Dy phosphor monitored at λemi ¼ 482 nm.
Fig. 4. Emission spectrum of KCaSO4Cl:Dy phosphor at λext ¼ 350 nm. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
at high symmetry sites, while the 4F9/2-6H13/2 electric dipole transition was stronger when Dy3+ was located at low-symmetry sites. In this sample, the blue emission (4F9/2-6H15/2) at 482 nm was stronger than the yellow emission (4F9/2-6H13/2) at 573 nm, which demonstrate that the Dy3+ ions occupied a high-symmetry site in this KCaSO4Cl host. Fig. 6 indicates the emission intensity increased with the increase of Dy3+ concentration, and reached a maximum value for 0.2 mol% of Dy and then decreased with increasing Dy3+ ions due to the concentration quenching. In addition, the position of emission peaks was not changed for different Dy3+ concentration.
3.3.2. Photoluminescence study of KCaSO4Cl:Ce phosphor It is well-known that the rare-earth ion Ce3+ shows the 4f25d transitions resulting in a broad band emission in the UV to visible range. In the excited state, the 4f-shell is empty and there is only single 5d-electron remaining which interact with the crystalline environment. In the ground state, Ce3+ ion has the [Xe] 4f1 configuration, which results in only two 4f1 energy levels, namely 2 F5/2 and 2F7/2, shown in Fig. 10. The 4f–5d transitions corresponding to optical absorption and fluorescence of Ce3+ in crystals are parity and spin allowed so that lifetimes of the fluorescence are in the range of 10–60 ns [39]. The peak positions of the Ce3+ 4f-5d
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absorption and Ce3+ 5d-4 f emission bands in a compound vary when 5d electron of Ce interacts with crystal field which in turn depends on many factors like the type of ligands surrounding the Ce3+ ion, the Ce3+ to ligand distances, and the point symmetry [40]. Due to these interactions, the spectral properties of the Ce3+ ion will differ from one host to another. When Ce3+ is introduced in a host lattice the average energy of the 5d configuration is lowered and the 2D3/2 and 2D5/2 states are further split by the crystalline environment. Depending on the site symmetry of Ce3+ at the most five distinct 5d states may form [41]. Under photon excitation the Ce3+ ion in KCaSO4Cl shows a strong luminescence in the ultra-violet as well as in red region. Fig. 7 shows the optical excitation spectra of KCaSO4Cl:Ce two main bands located at 252 and 293 nm attributed to 4f-5d transitions of Ce3+. When deconvolution of the excitation spectra of KCaSO4Cl: Ce was carried out it is observed that the excitation spectra consist of five distinct peaks, as shown in Fig. 8. From the excitation spectra it can be easily seen that this broad nature is actually composed of several (5) sub bands [42] peaking at 218, 237, 252, 260 and 293 nm, shown in Figs. 8 and 9. Emission spectra of KCaSO4Cl:Ce with different Ce concentrations are presented in Fig. 10. It is well-known that the Ce3+ emissions consist of double band corresponding to the splitting between the 2F5/2 ground state and the 2F7/2 state due to the spin–orbit coupling and this splitting is about 2000 cm 1. Two luminescence bands at 307 and 326 nm are observed in emission spectra of KCaSO4Cl:Ce, as shown in Fig. 9 which can be attributed to the allowed inter configurational transitions from the 5d level to 2F5/2 and 2F7/2 levels of Ce3+ and the splitting between these two levels is found to be 1793 cm 1.
Fig. 9. Emission spectrum of KCaSO4Cl:Ce phosphor at λext ¼293 nm.
Fig. 10. Energy level diagram of Ce3+ ion.
Fig. 7. Excitation spectrum of KCaSO4Cl:Ce phosphor monitored at λemi ¼326 nm.
Generation of a broad red emission with good color purity can be obtained by means of rare earth ions such as Eu2+ and Mn2+ [43–45]. In addition to this, Ce3+ doped host matrix can also be used as red-emitting phosphors but application of Ce3+ have been rarely found and studied. As a result of this information on their applications is mostly unavailable. As discussed above Ce3+ emission usually consists of a broad band due to the parity allowed transition between the lowest crystal field components of the 5d excited state and the 4f ground state, and it often varies in color from ultraviolet to yellow, depending on the different types of host lattices. Along with UV emission, the emission in the red region is also observed which is due to f–f transitions [46,47]. The unusual red emission from Ce3+ can be found in a host lattice with strong covalency or a large Stokes shift [47]. As the concentration of Ce3+ ion increases the corresponding intensity of all peaks increases linearly and maximum intensity is observed for higher concentration (5 mol%) of Ce3+ ion. This indicates KCaSO4Cl host matrix can accommodate higher concentration of cerium. 3.4. Thermoluminescence study
Fig. 8. Gaussian fitted excitation spectrum of KCaSO4Cl:Ce phosphor.
3.4.1. Thermoluminescence study of KCaSO4Cl:Dy phosphor Fig. 11 shows thermoluminescence (TL) glow curves of microcrystalline KCa(SO4)Cl:Dy pristine sample and Fig. 12 shows TL glow curves of samples annealed at 773 K and both exposed to γ-radiation from 60Co source at room temperature. It could be observed from the figures that the pristine sample shows single glow peak of arbitrary shape with a hump at lower temperature side whereas sample annealed at 773 K have two unresolved peaks at around 422 K and 540 K, respectively. An annealing procedure was utilized to improve the performance of TL material, before the irradiation [48]. It is well known that the heat
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The occurrence of concentration quenching in TL is in accordance with the photoluminescence study as discussed in Section 3.3.1. In KCa(SO4)Cl:Dy phosphors, Dy3+ probably incorporated substitutionally for an alkaline earth cation such as Ca2+ and acting as an emission center. An electron–hole recombination takes place at or near the Dy3+ center which leads exciting Dy3+ to emit light. The host lattice has very little effect on the emission wavelengths of the rare earth; however it does affect the trapping center significantly [35]. Results presented here demonstrate that the sensitivity of prepared KCa(SO4)Cl:Dy phosphor is about 1.22 times more than that of high sensitive CaSO4:Dy TL phosphor.
Fig. 11. TL glow curve of KCaSO4Cl:Dy phosphor (at gamma ray exposure of 5 Gy).
Fig. 12. TL glow curve of KCaSO4Cl:Dy phosphor (at gamma ray exposure of 5 Gy) annealed at 773 K.
treatment before the irradiation generally re-establishes the defect equilibrium that exists in the material and heating at elevated temperature can also empty the deep traps which are produced during synthesis. Heat treatment is essential to avoid the influence of these defects on the TL sensitivity of a given peak as they can act as competitors [49]. There is a very drastic change that can be seen in the shape of the glow curves of pristine sample and annealed sample, as observed from Figs. 11 and 12. This change in the glow curve structures cannot occur just because of the change in the particle size on annealing. The change in nature of the glow curve might also be due to impurity ion getting well dispersed in the host material [27]. Fig. 12 shows the TL glow curves of the annealed KCaSO4Cl:Dy sample. It could be observed from this figure that in microcrystalline KCaSO4Cl powder sample there are two prominent peaks, one around 422 K and the other one at around 540 K. It has also been observed that with increasing the concentration of dopant corresponding peak heights/sensitivity of the both, low temperature (422 K) peak and high temperature (540 K) peaks go on increasing. The maximum intensity was observed for 0.2 mol% concentration of Dy after that the intensity goes on decreasing for higher concentration of Dy. This could be explained by fact that with increasing the concentration of Dy in the KCaSO4Cl, population of defects in sample increases which makes more interaction of defects among themselves results in decrease in TL intensity.
3.4.2. Thermoluminescence study KCaSO4Cl:Ce phosphor The TL glow curves of KCaSO4Cl:Ce for a dose of 5 Gy is shown in Fig. 13. All the glow curves show similar structure, but differ in intensity. The glow curves have a single high temperature peak around 427 K. With increase in the concentration of Ce in KCaSO4Cl:Ce there is decrease in the intensity of peaks is observed which is in contradiction to the photoluminescence studies of KCaSO4Cl:Ce in which maximum intensity observed for maximum (5 mol%) concentration of Ce. This might be due to the different mechanism involved in between these two phenomenons. KCaSO4Cl:Dy shows the maximum TL sensitivity therefore it is chosen for the further study. 3.4.3. Analysis of glow curves by GCCD curve fitting and trapping parameters The structure of the insulating solids or semiconducting solid comprises of point defects, naturally occurring or artificially created. These defects form electronic states in the forbidden band and have great importance in understanding the thermoluminescence. TL investigations have also shown that defect centers play a crucial role in TL analysis. The formation and the stability of the defect centers depend on the method of preparation of phosphors and the activators [50–52]. TL strongly depends on the host material, the type of activator, radiation induced defect centers, dose and type of ionizing radiation [53–56]. Dosimetric characteristics of TL materials are mainly depending on kinetic parameters. Kinetic parameters quantitatively describe the trapping-emitting centers responsible for the TL emission. Therefore, determination of the kinetic parameters is an active area of research for better understanding of TL process. There are various methods for evaluating the trapping parameters [57]. When the single glow peak is present, the methods such as peak shape, various heating rates and initial rise methods are suitable methods to determine
Fig. 13. TL glow curve of KCaSO4Cl:Ce phosphor.
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trapping parameters. When nature of the glow peak is complex, then either we have to first isolate each individual peak from the others using partial thermal annealing treatment or to make a complete glow curve analysis using deconvolution [50,58–60]. Therefore, in this study Chen's peak shape method along with the deconvolution has been used to analyze the glow curves of KCaSO4Cl:Dy. The deconvolution procedure is a purely mathematical instrument which is used for analysis of complex glow curve structures. This tool is helpful for getting approximate idea about the possible number of trapping states involved in TL as well as trapping parameters. For kinetic and trap depth analysis TL glow curves were recorded at a heating rate of 5 K/s. For this samples were exposed to a low dose of 5 Gy. The glow curve deconvolution (GCCD) curve fitting in KCaSO4Cl:Dy microcrystalline material was done using glow curve deconvolution (GCD) function. The measured glow curves were analyzed to resolve the individual peaks, assuming first order, second order and general order kinetics. The mathematical expression used in this study for the TL phenomena is derived from Kitis et al. [33] and is given by the following functions, For first order, " ! E TT m T2 IðTÞ ¼ I m exp 1 þ 2 kT Tm Tm E TT m 2kT 2kT m 1 exp kT E Tm E For second order, E TT m IðTÞ ¼ 4I m exp kT Tm ! " #2 2 T 2kT E TT m 2kT m exp þ1þ 1 E kT T m E T 2m For general order, E TT m ðb=b1Þ exp I ¼ Imb kT T m " #ðb=ðb1ÞÞ 2kT T 2 E TT m 2kT m þ 1 þ ðb1Þ ðb1Þ 1 exp E kT T m E T 2m Where, I(T) is the TL intensity at temperature T (K), Im—the maximum peak intensity, E—the activation energy (eV) and k—the Boltzmann constant.
cδ ¼ 0:976 þ 7:3ðm0:42Þ
bδ ¼ 0
cω ¼ 2:52 þ 10:2 ðm0:42Þ bω ¼ 1; with m¼ 0.42 for the case of first-order TL glow peaks, and m ¼0.52 for the case of second-order peaks [62]. The theoretical peak was generated using these parameters and was separated from the experimental glow curve. Activation energy (E) was again calculated using the same set of equations. This procedure was continuously repeated for all the TL peaks till a theoretical glow curve which is obtained by convolution best fitted with experimental one. The trapping parameters obtained from the deconvoluted glow peaks are tabulated in Table 1. It can be observed from Table 1 that the kinetic order is larger than 1 for all 3 peaks, and it is very close to the second order kinetics for second peak. This means, in principle that a retrapping effect should be present [63]. Fig. 14 shows the deconvoluted curves and the theoretical curve fitted with the experimental curve after convolution of three fitted peaks. It could be seen from Fig. 14 that there is a very good curve fitting between simulated glow curve and the experimental one. The figure of merit (FOM) of fitting has also been determined using the formula, FOM ¼ Σj i
TLexp TLfit j Σi TLfit
where, TLexp and TLfit represent the experimental TL intensity data and the values of the fitting functions, respectively. Here summation extends over all the available experimental data points. The calculated FOM percentage value is 0.24 which shows good fitting. The activation energies of various traps are very much different. The different types of traps might have altered releasing and retrapping probabilities. Hence, there is change in frequency factor for second and third peak. Table 1 Kinetic parameters using the GCD function. Sample name
Peak Peak temperature Tm (K)
Order of kinetic (b)
Activation energy E (eV)
Frequency factor S (s 1)
KCa (SO4)Cl: Dy
1 2 3
1.6 1.9 1.77
0.5866 0.6768 1.6
2.259 106 2.922 106 8.03 1014
418.57 518.1 578.77
3.5. Glow curve shape methods The order of kinetics and activation energy of the isolated peak was found using Chen's set of empirical formulae. Practically it is found that the form factor μg is independent of the activation energy, Ea and strongly depends on the order of kinetics. To determine the order of kinetics (other than first or second order), use of the correlation between order of kinetics (b) and the form factor (μg) given by Chen was made [61]. Activation energy (E) was calculated by using Chen's equations, which gives the trap depth in terms of τ, δ, ω. A general formula for E was given by, E ¼ cγ ðkT 2m =γÞ–bγ ð2kT m Þ where, γ is τ, δ, or ω are the constants cγ and bγ for the three equations (τ, δ, or ω). Chen's method does not require knowledge of the kinetic order, which is found by using the symmetry factor m from the peak shape. The values of cγ and bγ are summarized as below cτ ¼ 1:510 þ 3:0ðm0:42Þ bτ ¼ 1:58 þ 4:2ðm0:42Þ
Fig. 14. KCa(SO4)Cl:Dy exposed to gamma ray dose of 5 Gy.
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Fig. 15. TL response of microcrystalline KCa(SO4)Cl:Dy exposed to γ-rays in the dose range 10–1000 Gy. Fig. 16. Fading plot of microcrystalline KCa(SO4)Cl:Dy.
3.6. Linearity, fading and reusability The use of the TL dosimetry to estimate the original given dose in the irradiated product could be an interesting improvement of this technique. The relative TL intensity was plotted as a function of γ-ray irradiation dose for the KCa(SO4)Cl:Dy phosphor sample in Fig.15. The TL dose dependence curve was observed to be almost linear in the studied dose range from 0.1 Gy to 170 Gy. It was found that the phosphor exhibited a linear response over a range of 0.1–170 Gy. The KCa(SO4)Cl:Dy material found to obey supralinear behavior in the dose range from 0.1 Gy to 48 Gy then becomes linear and above 170 Gy it saturates. This is very remarkable, from the application point of view; the microcrystalline phosphor could be used for estimation of such very high doses. A reliable dosimetric study of a TL material should be based on a good knowledge of the kinetics parameters. For example, the loss of the dosimetric information stored in the material, after irradiation, is strongly dependent on the position of the trapping levels within the forbidden gap. TL investigations have also shown that defect centers play a crucial role in TL analysis. The formation and the stability of the defect centers depend on the method of preparation of phosphors and the activators [52,55]. For fading study of microcrystalline material, a sample of an irradiated material was stored for few days without taking any precautions to shield it from light, humidity and other environmental factors. In this study the glow curves were recorded for a period of around 45 days. It can be seen in Fig. 16 that there was about 15% fading up to a period of 25 days after which it became constant indicating no severe fading. The reusability of the synthesized phosphor was tested by 5 successive cycles of annealing, irradiation and readout of the same sample. For being a good dosimetric material the sensitivity of a sample does not change after several cycles of exposures and readouts. This fact was taken into consideration and the microcrystalline KCa(SO4)Cl:Dy was tested for its reusability. A 5 mg mass of annealed sample was taken to give a gamma dose of 5 Gy and to record the TL glow curve. The remaining sample was again annealed at 773 K. A second exposure of the same dose of 5 Gy was then given and the glow curve was recorded. 5 such cycles of exposures and glow curve recordings were performed. As shown in Fig. 17, no significant change in the TL intensity was observed which illustrates good reusability of material, which is very important property of TL material in terms of dosimetric point of view. The five distinct bands are attributed to the f–d transitions of Ce3+ in the host lattice. When a Ce3+ ion enters exclusively one
Fig. 17. Reusability of microcrystalline KCa(SO4)Cl:Dy phosphor.
specific site, its 5d state will be split into 2–5 different components depending on the site symmetry. With C1 site symmetry five distinct 4f-5d excitation bands are expected for Ce3+ in a specific site [64]. Thus this observation seems to suggest that the occurrence of five f–d excitation bands is due to C1 site symmetry of Ce3+ in the host lattice. As could be seen from Figs. 11 and 12, the glow curve structure of pristine and samples annealed at 773 K are very much different. This change in the shape of glow curve after annealing is attributed to the well dispersion of activator into the host matrix and elimination of unwanted defects which are produce during synthesis. The glow curve deconvolution result shows that the main glow curve consists of 3 different peaks which are attributable to three different types of traps. The variation in intensity of the deconvoluted peaks clearly indicates that, the numbers of traps responsible for each peak are not in the same proportion this is due to changes in recombination center population by γ-rays during irradiation. The increase in TL intensity with increasing dose of γ-rays indicates that the number of traps inside the material are increasing which results in rise in intensity. At higher dose levels the distance between nearest neighbor track decreases and probability of electron escaping from the track and reaching neighboring track increases which in turn enhances the TL sensitivity. For further rise in dose causes decrease in the distances between neighboring tracks. Beyond this dose causes traps
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to merge and overlap which do not contribute to additional TL. This is due to the full occupancy of the available trap/luminescence centers. This is true in the present case of KCaSO4Cl:Dy too. However the process is not as simple and more study required on the interaction among defects. 4. Conclusion The polycrystalline powder samples of Dy/Ce doped KCaSO4Cl phosphor were prepared by an acid distillation method. The micro-crystallites posses nearly pyramidal morphology as evident from their SEM micrographs having diameter between 1 μm and 10 μm. The size of crystallites between this range is favorable for TL dosimetry. The characteristic emission of Dy and Ce is observed which confirms the presence of dopant in their stable valence state. The study presented that the KCaSO4Cl:Dy3+ could be efficiently excited by near-ultraviolet (NUV) light in the region of 300–400 nm and the emission peaks located at 482 nm (blue) and 573 nm (yellow). Five excitation bands due to 4f–5d transitions of Ce3+ in the host lattice were observed whereas only two emission bands of Ce3+ were found in UV as well as red region. The unusual red emission from Ce3+ can be found in a host lattice with strong covalency or a large Stokes shift. Furthermore presence of characteristic emission of Dy and Ce indicates the existence of these two activators in host lattice, in their stable valence state. From thermoluminescence studies it is observed that the sensitivity of KCaSO4Cl:Dy microcrystalline material is found to be 1.22 times more sensitive than CaSO4:Dy commercial phosphor. The annealing temperature affect is more predominantly seen in KCaSO4Cl:Dy sample from its TL characteristics. In case of pristine sample the TL glow curve structure is very much different than that of the annealed one. The TL sensitivity of the annealed sample was found to be more than that of pristine sample. Moreover, the trapping parameters of KCaSO4Cl:Dy sample were calculated by using CGCD program and good agreement is seen between the experimental glow curve and simulated glow curve. Thus, easy method of preparation, good sensitivity, linear response over a wide range of exposure, good reusability are some of the noble features of the presented phosphor which will make it useful for its applications in radiation dosimetry. Acknowledgment Authors are grateful to the Board of Research in Nuclear Sciences (BRNS), the Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out this work under research project (sanctioned letter no 2011/37P/10/BRNS/144). References [1] C.R. Ronda, J. Alloys Compd. 225 (1995) 534. [2] B. Kazan, Displays (1985) 85. [3] M. K1osowski, L. Czopyk, K. Kisielewicz, D. Kabat, P. Olko, M.P.R. Waligorski, Rad. Meas. 45 (2010) 719. [4] T. Kron, Radiat. Prot. Dosim. 85 (1999) 333. [5] Chang-Hong Han Choi, Chong-Hong Kim, Pyun, Sung-Jin Kim, J. Lumin. 82 (1999) 25. [6] Jinyong Kuang, Yingliang Liu, Jianxian Zhang, J. Solid State Chem. 179 (2006) 266. [7] Enrico Cavalli, Enrico Bovero, Alessandro Belletti, J. Phys.: Condens. Matter 14 (2002) 5221. [8] Panli You, Guangfu Yin, Xianchun Chen, Bo Yue, Zhongbing Huang, Xiaoming Liao, Yadong Yao, Opt. Mater. 33 (2011) 1808. [9] Daniela Parisi, Alessandra Toncelli, Mauro Tonelli, Enrico Cavalli, Enrico Bovero, Alessandro Belletti, J. Phys.: Condens. Matter 17 (2005) 2783. [10] Naoto Hirosaki Rong-Jun Xie, Sci. Technol. Adv. Mater. 8 (2007) 588. [11] P. Dorenbos, Physi. Rev. B 62 (2000) 15640.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and thermoluminescence study of KCaSO4Cl doped with Dy and Ce synthesized by acid distillation method Bhushan P. Kore a, N.S. Dhoble b, S.P. Lochab c, S.J. Dhoble a,n a
Department of Physics, RTM Nagpur University, Nagpur 440033, India Department of Chemistry, Sevadal Mahila Mahavidyalaya, Nagpur 440009, India c Inter-University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 April 2013 Received in revised form 21 June 2013 Accepted 10 July 2013 Available online 29 July 2013
Photoluminescence and thermoluminescence properties of KCaSO4Cl doped with dysprosium and cerium have been studied. Dy/Ce doped KCaSO4Cl phosphors were synthesized by the acid distillation method. The samples were characterized by XRD, SEM, PL and TL for structural, morphological and luminescence studies. The SEM image analysis of KCaSO4Cl phosphor shows nearly spherical particles with diameter varying between 3–10 μm. In the present host Dy3+ emission at 482 and 573 nm is observed due to 4 F9/2-6H15/2 and 4F9/2-6H13/2 transition, respectively, whereas the PL emission spectra of KCaSO4Cl:Ce phosphor shows two luminescence bands at 307 nm and 326 nm and are attributed to the allowed inter configurational transitions from the 5d-level to the 2F5/2 and 2F7/2 levels of Ce3+ ion. Effect of annealing on the structure of the glow curve is investigated for KCaSO4Cl:Dy phosphors. Thermoluminescence linearity has been studied for 0.1–9000 Gy dose of gamma rays. Linear behavior over a large dose range between 0.1 Gy and 170 Gy was found. In addition to this trap parameters of KCaSO4Cl:Dy were studied using computerized glow curve deconvolution. & 2013 Elsevier B.V. All rights reserved.
Keywords: Acid distillation Luminescence Glow curve Deconvolution Dosimetry
1. Introduction Luminescent materials have sparked significant attention for their practical and promising applications in display devices and medical applications [1–4]. In particular, trivalent dysprosium doped phosphors have been investigated extensively because of the intense visible and wide range emission from 400 nm to 700 nm applicable for various lighting devices [5–8]. The rare earth Dy3+ ions have two dominant emission bands in the blue and yellow region. The blue emission (470–500 nm) is due to 4F9/2 -6H15/2 transition and the yellow emission (560–600 nm) is due to 4F9/2-6H13/2 transition [6,9]. The yellow emission of Dy3+ is especially hypersensitive to the local environment. By modifying the crystal structure, the intensity ratio IYellow/IBlue can be controlled and resultant color of the phosphors can be tuned. By adjusting the yellow to blue intensity ratio it is possible to achieve proximate white light emission [8]. The position of the excited 5d levels of the trivalent lanthanides in inorganic host matrix relative to the 4fn ground state configuration is essential for many luminescence properties of phosphors. Position of these 5d bands relative to 4f levels is also crucial for many applications of luminescent inorganic materials [10].
n
Corresponding author. Tel.: +91 9822710204. E-mail address: [email protected] (S.J. Dhoble).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.07.030
The positions of these 5d levels depend strongly on the crystalline environment and may change from few to tens of thousands cm 1, from compound to compound [11–13]. In scintillation crystals, Ce3+ is of interest as activator ion. Ce3+ usually shows a fast dipole allowed luminescence with typically 107 50 ns decay time [14]. Since the 4f–5d transitions of Ce3+ are parity allowed and its luminescence decay constant is usually several tens of nanoseconds. Because of this fast decay characteristic, Ce3+ activated luminescent materials are important scintillators to detect X/γ rays or thermal neutrons [15–17]. Thermoluminescence (TL) is a very common method used for estimations of doses of ionizing radiations. It has been found that the intensity and area of TL glow peaks are proportional to the received dose and this is the basis for using TL phosphors in dosimetry of ionizing radiations [18]. Various TL investigations have shown that defect centers play a crucial role in TL since, the release of holes/electrons from defect centers at characteristic traps initiate the luminescence process in the material. The demand for dosimetry of ionizing radiations is growing day by day because of its utility in all branches of science where ionizing radiation is used and variety of medical applications including radiation therapy, diagnostic radiology, and radiotherapy mailed dosimetry [19–21]. Thermoluminescence properties of sulfate based phosphors activated by different rare earths have been the subject of intense research for many years. These studies have resulted in a variety of applications [22,23]. Those TL materials
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based on CaSO4 have been widely applied for medical and environmental dosimetry [24–26]. This fact has attracted the interest for studying the TL response of KCaSO4Cl. The thermoluminescence properties of some new sulfate based materials were investigated by different researchers [27,28]. The study of the luminescence as a function of the temperature, the so called glow curve, is used to determine the trapping parameters. Investigation of glow curves is widely used to extract more information about kinetics in thermoluminescence materials. The typical glow curve contains one or more glow peaks. Each peak gives information about only trap level occupied by charge carriers, in the TL material. The analysis of the glow peak is done by an empirical method which depends upon a parameter called the order of kinetics [29,30]. In order to obtain more detailed information we have to fit the experimental glow curve data to a particular TL model which best describe the TL as a function of these parameters [31,32]. In the study of relatively deep trap defect state in various materials as well as in TL dating, a detailed analysis of TL glow curve is crucial. This paper describes the preparation procedure and dosimetric characteristics of KCaSO4Cl microcrystalline phosphor. The formation of the material was confirmed by the X-ray diffraction (XRD). The morphology of the material was studied by scanning electron microscope (SEM). Its TL response to γ-rays of 60Co has been studied in more details. The TL glow curves of microcrystalline phosphor have been deconvoluted to separate various glow peaks using GCD functions suggested by Kitis et al. [33]. Trapping parameters, dose response, fading and reusability of the microcrystalline KCaSO4Cl:Dy phosphor also have been investigated in this study.
2. Synthesis All the samples investigated in this paper were prepared by following the method described by Yamashita et al. [34] and Dixon
et al. [35]. For the preparation of KCaSO4Cl phosphors all starting materials used were of analytical grade. Samples of KCaSO4Cl doped with rare earths were prepared by dissolving CaSO4, KCl and rare earths in hot sulfuric acid (the excess acid) at about 300 1C for 20 h to form small crystals. During acid evaporation (distillation) the acid vapors were subject to condensation. After cooling, the samples were repeatedly washed with double distilled water in order to remove the traces of acid and unwanted impurities if any incorporated during the synthesis then filtered and dried in hot air oven at 80 1C. No further additional heat treatments were given to samples. In this way the phosphors were prepared and made ready for further characterizations. The XRD technique was used in order to confirm the formation of desired product. The phase structure were characterized by X-ray diffraction (XRD) pattern using a PANalytical diffractometer with Cu Kα radiation (λ ¼1.5405 Å) operating at 40 kV, 30 mA. The morphology of the powders was studied by scanning electron microscopy (SEM) using a JEOL JSM 6380 A microscope. The photoluminescence (PL) emission spectra of the samples were recorded by using a RF-5301PC SHIMADZU Spectrofluorophotometer. Emission and excitation spectra were recorded using a spectral slit width of 1.5 nm. For TL studies, samples were exposed to gamma rays from a 60CO source at room temperature at the rate of 0.58 kGy/hr. The TL glow curves were recorded with the help of Nucleonix 1009 TL reader.
3. Results and discussion 3.1. XRD XRD study was carried out in order to confirm the formation of desired compound synthesized by acid distillation method. The XRD pattern of the KCaSO4Cl compared with that of the constituents phases and from comparison it is concluded that the phosphor is successfully prepared. The XRD pattern shown in Fig. 1 is in accordance with the XRD pattern reported in [36], in which the KCaSO4Cl was prepared by solid state reaction method. 3.2. SEM study
Fig. 1. X-ray diffraction pattern of KCaSO4Cl host matrix.
In order to obtain information about the surface morphology of present phosphor SEM analysis was carried out. The typical morphological images are presented in Fig. 2. From the micrograph it could be seen that particles of KCaSO4Cl have shapes like pyramids, cubes and some of them are of arbitrary shape, with fine surfaces. SEM photographs reveal that the phosphor powder is composed of irregular structured particles and the grains are asymmetrical shaped. An average crystallite size of crystallites was found to be in 5–20 μm range which is considered very favorable for thermoluminescence characteristics of a phosphor material [37].
Fig. 2. SEM images of KCaSO4Cl prepared by acid distillation method.
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3.3. Photoluminescence studies 3.3.1. Photoluminescence study of KCaSO4Cl:Dy phosphor Luminescence of Dy3+ has attracted much attention because of its white light emission. The photoluminescence spectra of Dy3+ doped KCaSO4Cl sample under excitation at 350 nm and emission spectra monitored at 572 nm is shown in Figs. 3 and 4 respectively. The excitation spectrum in the range 250–400 nm consisting of several peaks, came from the ground state of 6H15/2 to the excited states of 4f9 electronic configurations of the Dy3+ ions, which located at 325 nm (6H15/2-4M17/2), 350 nm (6H15/2-6P7/2), 364 nm(6H15/2-4I11/2), 389 nm (6H15/2-4I13/2) respectively [38]. Among all these excitation bands, the band at 350 nm possessed the maximum intensity that is due to 6H15/2-6P7/2. Thereby, the emission spectra were monitored at 350 nm in order to optimize Dy3+ luminescence. As shown in Fig. 4, the emission spectrum of Dy3+ has two groups of emissions located at 482 nm and 573 nm, which correspond to the transitions from 4F9/2-6H15/2 (blue), 4 F9/2-6H13/2 (yellow), respectively. Fig. 5 illustrates the energy levels of Dy3+ ion, which consist of five emission wavelengths at 482 nm, 576 nm, 660 nm, 755 nm and 840 nm. Among these emission, the 4F9/2-6H13/2 emission belongs to hypersensitive transition with ΔJ ¼2, which is strongly influenced by outside environments of Dy3+. When the Dy3+ is located at a lowsymmetry local site (without an inversion center), the emission from 4F9/2-6H13/2 transition is often dominated in emission spectra [38]. It was found from literatures that the 4F9/2-6H15/2 magnetic dipole transition was prominent when Dy3+ was located
Fig. 5. Energy level diagram of Dy3+ ion.
Fig. 6. Variation in the PL intensity as a function of the Dy3+ ion concentrations. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. Excitation spectrum of KCaSO4Cl:Dy phosphor monitored at λemi ¼ 482 nm.
Fig. 4. Emission spectrum of KCaSO4Cl:Dy phosphor at λext ¼ 350 nm. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
at high symmetry sites, while the 4F9/2-6H13/2 electric dipole transition was stronger when Dy3+ was located at low-symmetry sites. In this sample, the blue emission (4F9/2-6H15/2) at 482 nm was stronger than the yellow emission (4F9/2-6H13/2) at 573 nm, which demonstrate that the Dy3+ ions occupied a high-symmetry site in this KCaSO4Cl host. Fig. 6 indicates the emission intensity increased with the increase of Dy3+ concentration, and reached a maximum value for 0.2 mol% of Dy and then decreased with increasing Dy3+ ions due to the concentration quenching. In addition, the position of emission peaks was not changed for different Dy3+ concentration.
3.3.2. Photoluminescence study of KCaSO4Cl:Ce phosphor It is well-known that the rare-earth ion Ce3+ shows the 4f25d transitions resulting in a broad band emission in the UV to visible range. In the excited state, the 4f-shell is empty and there is only single 5d-electron remaining which interact with the crystalline environment. In the ground state, Ce3+ ion has the [Xe] 4f1 configuration, which results in only two 4f1 energy levels, namely 2 F5/2 and 2F7/2, shown in Fig. 10. The 4f–5d transitions corresponding to optical absorption and fluorescence of Ce3+ in crystals are parity and spin allowed so that lifetimes of the fluorescence are in the range of 10–60 ns [39]. The peak positions of the Ce3+ 4f-5d
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absorption and Ce3+ 5d-4 f emission bands in a compound vary when 5d electron of Ce interacts with crystal field which in turn depends on many factors like the type of ligands surrounding the Ce3+ ion, the Ce3+ to ligand distances, and the point symmetry [40]. Due to these interactions, the spectral properties of the Ce3+ ion will differ from one host to another. When Ce3+ is introduced in a host lattice the average energy of the 5d configuration is lowered and the 2D3/2 and 2D5/2 states are further split by the crystalline environment. Depending on the site symmetry of Ce3+ at the most five distinct 5d states may form [41]. Under photon excitation the Ce3+ ion in KCaSO4Cl shows a strong luminescence in the ultra-violet as well as in red region. Fig. 7 shows the optical excitation spectra of KCaSO4Cl:Ce two main bands located at 252 and 293 nm attributed to 4f-5d transitions of Ce3+. When deconvolution of the excitation spectra of KCaSO4Cl: Ce was carried out it is observed that the excitation spectra consist of five distinct peaks, as shown in Fig. 8. From the excitation spectra it can be easily seen that this broad nature is actually composed of several (5) sub bands [42] peaking at 218, 237, 252, 260 and 293 nm, shown in Figs. 8 and 9. Emission spectra of KCaSO4Cl:Ce with different Ce concentrations are presented in Fig. 10. It is well-known that the Ce3+ emissions consist of double band corresponding to the splitting between the 2F5/2 ground state and the 2F7/2 state due to the spin–orbit coupling and this splitting is about 2000 cm 1. Two luminescence bands at 307 and 326 nm are observed in emission spectra of KCaSO4Cl:Ce, as shown in Fig. 9 which can be attributed to the allowed inter configurational transitions from the 5d level to 2F5/2 and 2F7/2 levels of Ce3+ and the splitting between these two levels is found to be 1793 cm 1.
Fig. 9. Emission spectrum of KCaSO4Cl:Ce phosphor at λext ¼293 nm.
Fig. 10. Energy level diagram of Ce3+ ion.
Fig. 7. Excitation spectrum of KCaSO4Cl:Ce phosphor monitored at λemi ¼326 nm.
Generation of a broad red emission with good color purity can be obtained by means of rare earth ions such as Eu2+ and Mn2+ [43–45]. In addition to this, Ce3+ doped host matrix can also be used as red-emitting phosphors but application of Ce3+ have been rarely found and studied. As a result of this information on their applications is mostly unavailable. As discussed above Ce3+ emission usually consists of a broad band due to the parity allowed transition between the lowest crystal field components of the 5d excited state and the 4f ground state, and it often varies in color from ultraviolet to yellow, depending on the different types of host lattices. Along with UV emission, the emission in the red region is also observed which is due to f–f transitions [46,47]. The unusual red emission from Ce3+ can be found in a host lattice with strong covalency or a large Stokes shift [47]. As the concentration of Ce3+ ion increases the corresponding intensity of all peaks increases linearly and maximum intensity is observed for higher concentration (5 mol%) of Ce3+ ion. This indicates KCaSO4Cl host matrix can accommodate higher concentration of cerium. 3.4. Thermoluminescence study
Fig. 8. Gaussian fitted excitation spectrum of KCaSO4Cl:Ce phosphor.
3.4.1. Thermoluminescence study of KCaSO4Cl:Dy phosphor Fig. 11 shows thermoluminescence (TL) glow curves of microcrystalline KCa(SO4)Cl:Dy pristine sample and Fig. 12 shows TL glow curves of samples annealed at 773 K and both exposed to γ-radiation from 60Co source at room temperature. It could be observed from the figures that the pristine sample shows single glow peak of arbitrary shape with a hump at lower temperature side whereas sample annealed at 773 K have two unresolved peaks at around 422 K and 540 K, respectively. An annealing procedure was utilized to improve the performance of TL material, before the irradiation [48]. It is well known that the heat
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The occurrence of concentration quenching in TL is in accordance with the photoluminescence study as discussed in Section 3.3.1. In KCa(SO4)Cl:Dy phosphors, Dy3+ probably incorporated substitutionally for an alkaline earth cation such as Ca2+ and acting as an emission center. An electron–hole recombination takes place at or near the Dy3+ center which leads exciting Dy3+ to emit light. The host lattice has very little effect on the emission wavelengths of the rare earth; however it does affect the trapping center significantly [35]. Results presented here demonstrate that the sensitivity of prepared KCa(SO4)Cl:Dy phosphor is about 1.22 times more than that of high sensitive CaSO4:Dy TL phosphor.
Fig. 11. TL glow curve of KCaSO4Cl:Dy phosphor (at gamma ray exposure of 5 Gy).
Fig. 12. TL glow curve of KCaSO4Cl:Dy phosphor (at gamma ray exposure of 5 Gy) annealed at 773 K.
treatment before the irradiation generally re-establishes the defect equilibrium that exists in the material and heating at elevated temperature can also empty the deep traps which are produced during synthesis. Heat treatment is essential to avoid the influence of these defects on the TL sensitivity of a given peak as they can act as competitors [49]. There is a very drastic change that can be seen in the shape of the glow curves of pristine sample and annealed sample, as observed from Figs. 11 and 12. This change in the glow curve structures cannot occur just because of the change in the particle size on annealing. The change in nature of the glow curve might also be due to impurity ion getting well dispersed in the host material [27]. Fig. 12 shows the TL glow curves of the annealed KCaSO4Cl:Dy sample. It could be observed from this figure that in microcrystalline KCaSO4Cl powder sample there are two prominent peaks, one around 422 K and the other one at around 540 K. It has also been observed that with increasing the concentration of dopant corresponding peak heights/sensitivity of the both, low temperature (422 K) peak and high temperature (540 K) peaks go on increasing. The maximum intensity was observed for 0.2 mol% concentration of Dy after that the intensity goes on decreasing for higher concentration of Dy. This could be explained by fact that with increasing the concentration of Dy in the KCaSO4Cl, population of defects in sample increases which makes more interaction of defects among themselves results in decrease in TL intensity.
3.4.2. Thermoluminescence study KCaSO4Cl:Ce phosphor The TL glow curves of KCaSO4Cl:Ce for a dose of 5 Gy is shown in Fig. 13. All the glow curves show similar structure, but differ in intensity. The glow curves have a single high temperature peak around 427 K. With increase in the concentration of Ce in KCaSO4Cl:Ce there is decrease in the intensity of peaks is observed which is in contradiction to the photoluminescence studies of KCaSO4Cl:Ce in which maximum intensity observed for maximum (5 mol%) concentration of Ce. This might be due to the different mechanism involved in between these two phenomenons. KCaSO4Cl:Dy shows the maximum TL sensitivity therefore it is chosen for the further study. 3.4.3. Analysis of glow curves by GCCD curve fitting and trapping parameters The structure of the insulating solids or semiconducting solid comprises of point defects, naturally occurring or artificially created. These defects form electronic states in the forbidden band and have great importance in understanding the thermoluminescence. TL investigations have also shown that defect centers play a crucial role in TL analysis. The formation and the stability of the defect centers depend on the method of preparation of phosphors and the activators [50–52]. TL strongly depends on the host material, the type of activator, radiation induced defect centers, dose and type of ionizing radiation [53–56]. Dosimetric characteristics of TL materials are mainly depending on kinetic parameters. Kinetic parameters quantitatively describe the trapping-emitting centers responsible for the TL emission. Therefore, determination of the kinetic parameters is an active area of research for better understanding of TL process. There are various methods for evaluating the trapping parameters [57]. When the single glow peak is present, the methods such as peak shape, various heating rates and initial rise methods are suitable methods to determine
Fig. 13. TL glow curve of KCaSO4Cl:Ce phosphor.
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trapping parameters. When nature of the glow peak is complex, then either we have to first isolate each individual peak from the others using partial thermal annealing treatment or to make a complete glow curve analysis using deconvolution [50,58–60]. Therefore, in this study Chen's peak shape method along with the deconvolution has been used to analyze the glow curves of KCaSO4Cl:Dy. The deconvolution procedure is a purely mathematical instrument which is used for analysis of complex glow curve structures. This tool is helpful for getting approximate idea about the possible number of trapping states involved in TL as well as trapping parameters. For kinetic and trap depth analysis TL glow curves were recorded at a heating rate of 5 K/s. For this samples were exposed to a low dose of 5 Gy. The glow curve deconvolution (GCCD) curve fitting in KCaSO4Cl:Dy microcrystalline material was done using glow curve deconvolution (GCD) function. The measured glow curves were analyzed to resolve the individual peaks, assuming first order, second order and general order kinetics. The mathematical expression used in this study for the TL phenomena is derived from Kitis et al. [33] and is given by the following functions, For first order, " ! E TT m T2 IðTÞ ¼ I m exp 1 þ 2 kT Tm Tm E TT m 2kT 2kT m 1 exp kT E Tm E For second order, E TT m IðTÞ ¼ 4I m exp kT Tm ! " #2 2 T 2kT E TT m 2kT m exp þ1þ 1 E kT T m E T 2m For general order, E TT m ðb=b1Þ exp I ¼ Imb kT T m " #ðb=ðb1ÞÞ 2kT T 2 E TT m 2kT m þ 1 þ ðb1Þ ðb1Þ 1 exp E kT T m E T 2m Where, I(T) is the TL intensity at temperature T (K), Im—the maximum peak intensity, E—the activation energy (eV) and k—the Boltzmann constant.
cδ ¼ 0:976 þ 7:3ðm0:42Þ
bδ ¼ 0
cω ¼ 2:52 þ 10:2 ðm0:42Þ bω ¼ 1; with m¼ 0.42 for the case of first-order TL glow peaks, and m ¼0.52 for the case of second-order peaks [62]. The theoretical peak was generated using these parameters and was separated from the experimental glow curve. Activation energy (E) was again calculated using the same set of equations. This procedure was continuously repeated for all the TL peaks till a theoretical glow curve which is obtained by convolution best fitted with experimental one. The trapping parameters obtained from the deconvoluted glow peaks are tabulated in Table 1. It can be observed from Table 1 that the kinetic order is larger than 1 for all 3 peaks, and it is very close to the second order kinetics for second peak. This means, in principle that a retrapping effect should be present [63]. Fig. 14 shows the deconvoluted curves and the theoretical curve fitted with the experimental curve after convolution of three fitted peaks. It could be seen from Fig. 14 that there is a very good curve fitting between simulated glow curve and the experimental one. The figure of merit (FOM) of fitting has also been determined using the formula, FOM ¼ Σj i
TLexp TLfit j Σi TLfit
where, TLexp and TLfit represent the experimental TL intensity data and the values of the fitting functions, respectively. Here summation extends over all the available experimental data points. The calculated FOM percentage value is 0.24 which shows good fitting. The activation energies of various traps are very much different. The different types of traps might have altered releasing and retrapping probabilities. Hence, there is change in frequency factor for second and third peak. Table 1 Kinetic parameters using the GCD function. Sample name
Peak Peak temperature Tm (K)
Order of kinetic (b)
Activation energy E (eV)
Frequency factor S (s 1)
KCa (SO4)Cl: Dy
1 2 3
1.6 1.9 1.77
0.5866 0.6768 1.6
2.259 106 2.922 106 8.03 1014
418.57 518.1 578.77
3.5. Glow curve shape methods The order of kinetics and activation energy of the isolated peak was found using Chen's set of empirical formulae. Practically it is found that the form factor μg is independent of the activation energy, Ea and strongly depends on the order of kinetics. To determine the order of kinetics (other than first or second order), use of the correlation between order of kinetics (b) and the form factor (μg) given by Chen was made [61]. Activation energy (E) was calculated by using Chen's equations, which gives the trap depth in terms of τ, δ, ω. A general formula for E was given by, E ¼ cγ ðkT 2m =γÞ–bγ ð2kT m Þ where, γ is τ, δ, or ω are the constants cγ and bγ for the three equations (τ, δ, or ω). Chen's method does not require knowledge of the kinetic order, which is found by using the symmetry factor m from the peak shape. The values of cγ and bγ are summarized as below cτ ¼ 1:510 þ 3:0ðm0:42Þ bτ ¼ 1:58 þ 4:2ðm0:42Þ
Fig. 14. KCa(SO4)Cl:Dy exposed to gamma ray dose of 5 Gy.
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Fig. 15. TL response of microcrystalline KCa(SO4)Cl:Dy exposed to γ-rays in the dose range 10–1000 Gy. Fig. 16. Fading plot of microcrystalline KCa(SO4)Cl:Dy.
3.6. Linearity, fading and reusability The use of the TL dosimetry to estimate the original given dose in the irradiated product could be an interesting improvement of this technique. The relative TL intensity was plotted as a function of γ-ray irradiation dose for the KCa(SO4)Cl:Dy phosphor sample in Fig.15. The TL dose dependence curve was observed to be almost linear in the studied dose range from 0.1 Gy to 170 Gy. It was found that the phosphor exhibited a linear response over a range of 0.1–170 Gy. The KCa(SO4)Cl:Dy material found to obey supralinear behavior in the dose range from 0.1 Gy to 48 Gy then becomes linear and above 170 Gy it saturates. This is very remarkable, from the application point of view; the microcrystalline phosphor could be used for estimation of such very high doses. A reliable dosimetric study of a TL material should be based on a good knowledge of the kinetics parameters. For example, the loss of the dosimetric information stored in the material, after irradiation, is strongly dependent on the position of the trapping levels within the forbidden gap. TL investigations have also shown that defect centers play a crucial role in TL analysis. The formation and the stability of the defect centers depend on the method of preparation of phosphors and the activators [52,55]. For fading study of microcrystalline material, a sample of an irradiated material was stored for few days without taking any precautions to shield it from light, humidity and other environmental factors. In this study the glow curves were recorded for a period of around 45 days. It can be seen in Fig. 16 that there was about 15% fading up to a period of 25 days after which it became constant indicating no severe fading. The reusability of the synthesized phosphor was tested by 5 successive cycles of annealing, irradiation and readout of the same sample. For being a good dosimetric material the sensitivity of a sample does not change after several cycles of exposures and readouts. This fact was taken into consideration and the microcrystalline KCa(SO4)Cl:Dy was tested for its reusability. A 5 mg mass of annealed sample was taken to give a gamma dose of 5 Gy and to record the TL glow curve. The remaining sample was again annealed at 773 K. A second exposure of the same dose of 5 Gy was then given and the glow curve was recorded. 5 such cycles of exposures and glow curve recordings were performed. As shown in Fig. 17, no significant change in the TL intensity was observed which illustrates good reusability of material, which is very important property of TL material in terms of dosimetric point of view. The five distinct bands are attributed to the f–d transitions of Ce3+ in the host lattice. When a Ce3+ ion enters exclusively one
Fig. 17. Reusability of microcrystalline KCa(SO4)Cl:Dy phosphor.
specific site, its 5d state will be split into 2–5 different components depending on the site symmetry. With C1 site symmetry five distinct 4f-5d excitation bands are expected for Ce3+ in a specific site [64]. Thus this observation seems to suggest that the occurrence of five f–d excitation bands is due to C1 site symmetry of Ce3+ in the host lattice. As could be seen from Figs. 11 and 12, the glow curve structure of pristine and samples annealed at 773 K are very much different. This change in the shape of glow curve after annealing is attributed to the well dispersion of activator into the host matrix and elimination of unwanted defects which are produce during synthesis. The glow curve deconvolution result shows that the main glow curve consists of 3 different peaks which are attributable to three different types of traps. The variation in intensity of the deconvoluted peaks clearly indicates that, the numbers of traps responsible for each peak are not in the same proportion this is due to changes in recombination center population by γ-rays during irradiation. The increase in TL intensity with increasing dose of γ-rays indicates that the number of traps inside the material are increasing which results in rise in intensity. At higher dose levels the distance between nearest neighbor track decreases and probability of electron escaping from the track and reaching neighboring track increases which in turn enhances the TL sensitivity. For further rise in dose causes decrease in the distances between neighboring tracks. Beyond this dose causes traps
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to merge and overlap which do not contribute to additional TL. This is due to the full occupancy of the available trap/luminescence centers. This is true in the present case of KCaSO4Cl:Dy too. However the process is not as simple and more study required on the interaction among defects. 4. Conclusion The polycrystalline powder samples of Dy/Ce doped KCaSO4Cl phosphor were prepared by an acid distillation method. The micro-crystallites posses nearly pyramidal morphology as evident from their SEM micrographs having diameter between 1 μm and 10 μm. The size of crystallites between this range is favorable for TL dosimetry. The characteristic emission of Dy and Ce is observed which confirms the presence of dopant in their stable valence state. The study presented that the KCaSO4Cl:Dy3+ could be efficiently excited by near-ultraviolet (NUV) light in the region of 300–400 nm and the emission peaks located at 482 nm (blue) and 573 nm (yellow). Five excitation bands due to 4f–5d transitions of Ce3+ in the host lattice were observed whereas only two emission bands of Ce3+ were found in UV as well as red region. The unusual red emission from Ce3+ can be found in a host lattice with strong covalency or a large Stokes shift. Furthermore presence of characteristic emission of Dy and Ce indicates the existence of these two activators in host lattice, in their stable valence state. From thermoluminescence studies it is observed that the sensitivity of KCaSO4Cl:Dy microcrystalline material is found to be 1.22 times more sensitive than CaSO4:Dy commercial phosphor. The annealing temperature affect is more predominantly seen in KCaSO4Cl:Dy sample from its TL characteristics. In case of pristine sample the TL glow curve structure is very much different than that of the annealed one. The TL sensitivity of the annealed sample was found to be more than that of pristine sample. Moreover, the trapping parameters of KCaSO4Cl:Dy sample were calculated by using CGCD program and good agreement is seen between the experimental glow curve and simulated glow curve. Thus, easy method of preparation, good sensitivity, linear response over a wide range of exposure, good reusability are some of the noble features of the presented phosphor which will make it useful for its applications in radiation dosimetry. Acknowledgment Authors are grateful to the Board of Research in Nuclear Sciences (BRNS), the Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out this work under research project (sanctioned letter no 2011/37P/10/BRNS/144). References [1] C.R. Ronda, J. Alloys Compd. 225 (1995) 534. [2] B. Kazan, Displays (1985) 85. [3] M. K1osowski, L. Czopyk, K. Kisielewicz, D. Kabat, P. Olko, M.P.R. Waligorski, Rad. Meas. 45 (2010) 719. [4] T. Kron, Radiat. Prot. Dosim. 85 (1999) 333. [5] Chang-Hong Han Choi, Chong-Hong Kim, Pyun, Sung-Jin Kim, J. Lumin. 82 (1999) 25. [6] Jinyong Kuang, Yingliang Liu, Jianxian Zhang, J. Solid State Chem. 179 (2006) 266. [7] Enrico Cavalli, Enrico Bovero, Alessandro Belletti, J. Phys.: Condens. Matter 14 (2002) 5221. [8] Panli You, Guangfu Yin, Xianchun Chen, Bo Yue, Zhongbing Huang, Xiaoming Liao, Yadong Yao, Opt. Mater. 33 (2011) 1808. [9] Daniela Parisi, Alessandra Toncelli, Mauro Tonelli, Enrico Cavalli, Enrico Bovero, Alessandro Belletti, J. Phys.: Condens. Matter 17 (2005) 2783. [10] Naoto Hirosaki Rong-Jun Xie, Sci. Technol. Adv. Mater. 8 (2007) 588. [11] P. Dorenbos, Physi. Rev. B 62 (2000) 15640.
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