Eu2+ activated Na21Mg(SO4)10Cl3 phosphors

Journal of Luminescence 143 (2013) 337–342

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Photoluminescence and thermoluminescence properties of Dy3+/Eu2+ activated Na21Mg(SO4)10Cl3 phosphors Bhushan P. Kore a, N.S. Dhoble b, K. Park 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 Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 16 December 2012 Received in revised form 20 April 2013 Accepted 26 April 2013 Available online 17 May 2013

In the present work luminescence properties of rare earth (RE) doped Na21Mg(SO4)10Cl3 were studied. Modified solid state method was employed to synthesize the phosphors. The influence of RE (RE ¼Dy and Eu) doping on the luminescence properties of as prepared phosphor were investigated in detail. PL emission spectra of the Na21Mg(SO4)10Cl3:Dy phosphor exhibits the characteristic emission of Dy. The characteristic Dy3+ emission in the form of peaks around 482 and 576 nm corresponding to transitions 4 F9/2-6H15/2 and 4F9/2-6H13/2 was seen when excited by excitation wavelength 351 nm. However, interesting thermoluminescence results are observed in case of Dy as well as Eu doped Na21Mg(SO4)10Cl3. The TL glow curves for Na21Mg(SO4)10Cl3:Dy exhibit broad peak composed of three overlapping peaks, these peaks were deconvoluted using deconvolution program. The peaks at different temperatures indicate that different sets of traps are being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). The peaks observed were due to formation of trap levels by γ-rays irradiation and subsequently activation of traps on thermal stimulation. The trapping parameters for both the samples were calculated using Chen's peak shape method and reported in this paper. & 2013 Elsevier B.V. All rights reserved.

Keywords: Phosphors Thermoluminescence (TL) Photoluminescence (PL) Deconvolution Na21Mg(SO4)10Cl3

1. Introduction Thermoluminescence (TL) is a very important technique due to its applications in various fields such as radiation therapy, dosimetry, geology, space research and other research related areas [1–4]. Studies on radiation induced defects in insulating and semiconducting materials have been interesting over the last few decades [5]. Several materials such as LiF:Ti, Mg and α-Al2O3:C, CaSO4:Dy due to their excellent thermoluminescent properties such as high TL efficiency, dose response, thermal stability, high sensitivity and reproducibility, are now commonly used as thermoluminescent dosimeters (TLD) in a great diversity fields of applications. The main applications of these materials are in radiation dosimetry, for personnel and environmental monitoring [6,7]. Many sensitive synthetic materials are developed for fulfilling the above mentioned properties [8,9]. Different preparative methods [10,11] and thermoluminescent properties of several materials have been studied so far [12] and it is found that mixed alkali/ alkaline sulfate constitute a class of thermoluminescence phosphors with good performances, especially when doped with appropriate activators [13]. Sulfate based TL materials are synthesized and studied because of their well desired characteristics like a high temperature

n

Corresponding author. 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.2013.04.053

glow peak, linear response with ionizing radiation exposure, negligible fading and an easy methods of preparation [14]. There are several thermoluminescent materials such as CaSO4:Eu, Ag, K2Ca2(SO4)3:Eu, KMgSO4Cl doped with Dy, Ce and Mn etc. of which almost all has been studied for improvement in the thermoluminescence characteristics and the trapping parameters [15–17]. The study of the luminescence as a function of the temperature, the so called glow curve, is used to determine the trapping parameters and its integral is proportional to the radiation dose absorbed by the irradiated sample. The position, shape and intensities of the glow peaks are related to the properties of traps responsible for the TL. The shape and position of the resultant TL glow curves can be analyzed to extract information about the various parameters of the trapping process such as activation energy which is the thermal energy required to liberate the trapped electrons and holes, frequency factor, trap depth, trapping and retrapping rates etc. A popular method of analyzing a TL glow curve in order to ascertain the kinetic parameters E, s, and b is by considering the shape or geometrical properties of the peak. TL glow peaks corresponding to second-order kinetics are characterized by an almost symmetrical shape, whereas first-order peaks are asymmetrical. Grossweiner was the first to use the shape of the glow peak to calculate the trap depth E [18]. In this paper, the kinetic parameters of Rare Earth (RE)-doped Na21Mg(SO4)10Cl3 phosphor, synthesized by the modified solid state diffusion technique, are reported. In irradiated phosphor

B.P. Kore et al. / Journal of Luminescence 143 (2013) 337–342

Na21Mg(SO4)10Cl3:Dy peak consisting of three overlapping (unresolved) peaks was observed whereas in Na21Mg(SO4)10Cl3:Eu single peak peaking at 136.5 1C was observed. For the first time the kinetic parameters of these materials were calculated by peak shape method and the results are presented in this paper.

2. Experimental The samples Na21Mg(SO4)10Cl3 (pure); Na21Mg(SO4)10Cl3:Dy and Na21Mg(SO4)10Cl3:Eu were prepared by a modified solid state diffusion method. While preparing the samples, the constituents Na2SO4(Loba, 99% pure), NaCl(Loba, 99% pure), MgSO4(Loba, 99% pure), Dy2O3(Merck 99.9% pure) and Eu2O3(Merck 99.9% pure) were taken in a stoichiometric ratio and crushed in a mortar pestle for 1 h. Then this material was heated at 350 1C for 3 h; after 3 h heating the material was again crushed for an hour and finally heated at 650 1C for 18 h resulting in the compounds of Na21Mg (SO4)10Cl3:Dy and Na21Mg(SO4)10Cl3:Eu in powder form according to the following chemical reaction.

Na2SO4+3NaCl+MgSO4-Na21Mg(SO4)10Cl3 The samples were then slowly cooled at room temperature, at cooling rate of 0.5 1C/min. The resultant polycrystalline material was crushed to fine powder in a mortar pestle, the resultant powder formed was used for further study. SEM micrographs were obtained using a HITACHI S-4800 scanning electron microscope. The SEM micrographs were taken at 5000 V accelerating voltage, 8300 μm working distance, 7800 nA emission current, at high lens mode with fast scan speed and gray scale color mode. The prepared host lattice was characterized for their phase purity and crystallinity by X-ray powder diffraction (XRD) using a X'pert-PRO PANalytical diffractometer (Cu-Kα radiation) at a scanning step of 0.001, in the 2θ range from 10 to 801. The photoluminescence (PL) emission spectra of the samples were recorded using a Fluorescence spectrometer (Shimadzu, RF 5301 PC). Excitation and emission spectra were recorded using a spectral slit width of 1.5 nm. For TL studies, samples were exposed to gamma rays from a 60 Co source at room temperature at the rate of 0.58 kGy/hr. After the desired exposure, TL glow curves were recorded with the help of Nucleonix 1009I TL reader, at a heating rate of 5 1C s−1. All the measurements were carried out in an open atmosphere. The Nucleonix 1009I TL reader consists of photomultiplier tube (931B), DC Amplifier, IR filters and milivolt recorder. For TL measurement, each time 5 mg of phosphor is used which is in powder form, having particle size as specified in Section (3.2). For comparison TL glow curve of standard thermoluminescence dosimeter (TLD) CaSO4:Dy was recorded, under identical conditions. For measurement of dose response and fading three aliquot of the same sample were used for taking each measurement. Therefore, single point in these plots corresponds to average of three readings.

3. Results and discussion 3.1. XRD study Fig. 1 shows the XRD patterns from pure Na21Mg(SO4)10Cl3 powder. X-ray diffraction pattern indicates the presence of crystalline Na21Mg(SO4)10Cl3 host lattices. The XRD-pattern of the as prepared phosphor powder shows good agreement with standard ICDD file no. 41-1473.The final product was formed in

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Experimental ICDD 41-1473

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2 Theta Fig. 1. X-ray diffraction pattern of the Na21Mg(SO4)10Cl3 host lattice.

homogeneous form, the XRD pattern of Na21Mg(SO4)10Cl3 did not show the presence of the phases of starting materials like Na2SO4, NaCl, MgSO4 and other likely phases which indicates the formation of the desired compound. The mineralogical name of Na21Mg(SO4)10Cl3 is D'ansite. Na21Mg(SO4)10Cl3 has a cubic crystal structure with space group of I-43m and the cell parameters are a¼ b¼c ¼15.95 Ǻ, V ¼4029.55, and Z¼4. It has been found that the crystal structure of Na21Mg(SO4)10Cl3 is Isometric of class Hextetrahedral type. Point group: −43m. As tetrahedral {211} crystals, modified by {211} and{110}[19]. 3.2. SEM study The morphology of the Na21Mg(SO4)10Cl3 phosphor was analyzed using SEM as shown in Fig. 2.The SEM micrographs in (a) and (b) shows the agglomerated particles of oval shapes, whereas from micrographs (c) and (d) the spherical agglomerated particles can be observed. From SEM observation the estimation of particle size is uncertain since the particles are agglomerated; approximately the particle size varies from 0.2 μm to 0.6 μm. 3.3. PL Studies 3.3.1. PL study of Na21Mg(SO4)10Cl3:Dy A series of Na21Mg(SO4)10Cl3:Dy samples has been synthesized with Dy concentration ranging from 0.05 to 1 mol%. Fig. 3 shows the excitation spectrum in the range 250–400 nm consisting of four peaks, arising due to (6H15/2-4M17/2), (6H15/2-6P7/2), (6H15/2-4I11/2) and (6H15/2-4I13/2) transitions which are located at 325 nm, 351 nm, 365 nm, 388 nm respectively. The photoluminescence emission spectra of Dy3+ doped Na21Mg(SO4)10Cl3:Dy sample under excitation at 351 nm is shown in Fig. 4. The emission spectrum of Dy3+ has two groups of emissions located at 482 and 576 nm, which correspond to the transitions of 4F9/2-6H15/2 (blue), 4F9/2-6H13/2 (yellow) respectively. Among the two emission peaks, the 4F9/2-6H13/2 emission belongs to hypersensitive transition with ΔJ ¼2, which is strongly influenced by outside environments of Dy3+ [20]. In the excitation spectrum of 1 mol% Dy3+ doped Na21Mg(SO4)10Cl3, the peaks which range from 250 to 400 nm are due to 4f–4f transitions of Dy3+ [21]. For the lower concentration of Dy there is no photoluminescence observed. 3.3.2. PL study of Na21Mg(SO4)10Cl3:Eu The PL spectra of Na21Mg(SO4)10Cl3:Eu (x¼0.2 mol%, 0.5 mol%, and 1 mol%) phosphors are presented in Fig. 5, monitored at 370 nm (as shown in inset of Fig. 5.). It can be seen that the phosphors exhibit a broad blue emission band with a peak at around 450 nm, which is corresponding to the 5d-4f allowed transition of Eu2+. From the spectra it is clear that the PL intensity increases with increasing Eu2+

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Fig. 2. SEM micrographs of Na21Mg(SO4)10Cl3 phosphor.

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Fig. 3. Excitation spectra of Na21Mg(SO4)10Cl3:Dy monitored at 482 nm.

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Fig. 5. Emission spectra of Na21Mg(SO4)10Cl3:Eu at λex ¼ 370 nm.

concentration and reaches the maximum at x¼ 1 mol% concentration. We suggest that Eu2+ ions will preferably substitute Mg2+ sites in Na21Mg(SO4)10Cl3, since the ions' charge, radius of activator ions and cations in the host are in close proximity.

7

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Wavelength (nm) Fig. 4. Emission spectra of Na21Mg(SO4)10Cl3:Dy at λex ¼351 nm.

3.4. TL Studies 3.4.1. Na21Mg(SO4)10Cl3:Dy Thermoluminescence is a very common and simple technique used for estimation of doses of high-energy ionizing radiations absorbed by materials. As-prepared Na21Mg(SO4)10Cl3 samples, did not show any thermoluminescence response. However, samples irradiated with gamma rays shows good TL response. Fig. 6 shows a glow curve for the sample exposed by gamma rays. The TL glow curves of Na21Mg(SO4)10Cl3:Dy compound show unresolved glow peak, consisting of two peaks, indicating that three types of traps are being activated within the particular temperature range with its own value of activation energy (E) and frequency factor (s). The

B.P. Kore et al. / Journal of Luminescence 143 (2013) 337–342

a - Pure b - 0.05mol% c - 0.1mol% d - 0.2mol% e - 0.5mol% f - 1mol% g - CaSO 4:Dy

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Temperature (°C) Fig. 6. TL glow curve of Na21Mg(SO4)10Cl3:Dy phosphor (at gamma ray exposure of 6 Gy).

shape of the glow curve remains almost the same for different concentrations of Dy3+ but the height of the glow peak is raising. The sensitivity of main glow peak is raising. with increase of concentrations of doped Dy3+ ion in Na21Mg(SO4)10Cl3. The increase in glow peak sensitivity is linear upto 1 mol% concentration of Dy3+ (the maximum concentration used in this study) and this will be favorable for TL studies. The increase in the intensities of the glow peaks with increase of dopant concentration can be understood by the fact that more and more defects were created. Therefore, we can conclude that the distributions of traps produced by the irradiation of gamma-ray can be altered greatly by the change in the concentrations of Dy3+ ion doped in Na21Mg (SO4)10Cl3 phosphor. Studies on the TL glow curves of Na21Mg (SO4)10Cl3 samples doped individually with different rare earth impurities show that the peak temperature and the activation energies of the glow peaks are depend on the type of activator present in phosphor. In comparison to dosimetric peak of CaSO4: Dy (220 1C), Na21Mg(SO4)10Cl3:Dy is found to be 1.23 times more sensitive. 3.4.2. Na21Mg(SO4)10Cl3:Eu Fig. 7 shows the glow curve for a sample of Na21Mg(SO4)10Cl3: Eu, exposed to 6 Gy dose of γ-rays. For the investigation of the variation in glow curves on dopant concentration, the TL signals for different Eu concentrations were measured and are represented in Fig. 7. The Eu concentrations in Na21Mg(SO4)10Cl3:Eu used were 0.05, 0.1, 0.2, 0.5 and 1 mol%. The maximum TL intensity is observed for 0.1 mol% concentration of Eu, peaking at 136.5 1C and further increase in Eu concentration ceases the luminescence; this is due to concentration quenching. The TL glow curves of Na21Mg(SO4)10Cl3:Eu compound show single glow peak peaking at 136.5 1C indicating that only one set of traps are being activated within the particular temperature range having its own value of activation energy (E) and frequency factor (s). In comparison to dosimetric peak of CaSO4:Dy our prepared material that is Na21Mg (SO4)10Cl3:Eu is found to be 4 times less sensitive and more sensitivity than CaSO4:Dy is observed for Na21Mg(SO4)10Cl3:Dy phosphor. 3.5. Linearity, fading and reusability The TL intensity of glow peak increases linearly with variation in the dose from 100 mGy to 10 Gy and is shown in Fig. 8. The increase in the intensities of the glow peaks with increase of

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Temperature (°C) Fig. 7. TL glow curve of Na21Mg(SO4)10Cl3:Eu phosphor (at gamma ray exposure of 6 Gy).

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Dose (In mGy) Fig. 8. TL response of microcrystalline Na21Mg(SO4)10Cl3:Dy exposed to γ-rays in the dose range 100 mGy–12 Gy.

radiation dose suggests that more and more traps, responsible for these glow peaks, were getting filled with the increase of radiation dose and subsequently these traps releases the charge carriers on thermal stimulation, to finally recombine with their counterparts, thus giving rise to glow peaks of different intensities. The TL glow curves show good response upto 10 Gy and above this exposure the TL peak intensity is going in saturation stage. The prepared phosphor Na21Mg(SO4)10Cl3:Dy is useful for TL dosimetry upto 8 Gy dose of gamma exposure. In order to make samples useful in radiation dosimetry their TL should be stable and should not die away upon storage after exposure to ionizing radiations. The present material was stored for a few days without taking any protection to shield it from light and humidity and it was found that glow peak was reasonably stable as shown in Fig. 9. The fading observed was about 8 to 10% during a period of 30 days indicating no severe fading. Reusability is one of the most useful property that sample should posses in order to find a place in any application. If the sensitivity of a sample does not change after several cycles of exposures and readouts then it is termed as a phosphor with good dosimetric characteristics. For studying the reusability of the sample, sample was given exposure of 6 Gy of γ-rays and TL glow curve was recorded. Several such cycles of exposures and glow curve recordings were executed. The plot between TL readout

B.P. Kore et al. / Journal of Luminescence 143 (2013) 337–342

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Fig. 11. Tm–Tstop plot for thermoluminescence glow curve of Na21Mg(SO4)10Cl3:Dy (Dy ¼1 mol%) phosphor.

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In the present case, at a time 18 samples were exposed to the γ dose of 5 Gy and each sample was quenched at different Tstop temperatures from 100 1C to 270 1C then the TL curves were recorded with a linear heating rate of 5 1C s−1. In this sample, the rise from one plateau region to another is fairly sharp, as observed in Fig. 11. The plot indicates the presence of only two flat regions as the Tstop is increased, however this increase is not monotonic and two jumps are observed, indicating the presence of 2 peaks. On the basis of the results of this analysis it seems apparent that the TL glow curve is due to a complex trap structure, namely a discrete trap distribution. In addition to Tm–Tstop method, all the glow curves were also analyzed by CGCD method to obtain the number of glow peaks. This method has become very popular to obtain the number of glow peaks in the complex glow curves and their kinetic parameters.

Fig. 10. Reusability of microcrystalline Na21Mg(SO4)10Cl3:Dy.

cycles and TL sensitivities is shown in Fig. 10. No significant change in the sensitivity of the glow peak was observed, as observed in Fig. 10. Several methods [22–25] for determining kinetic parameters of TL phosphors require the values of Tm, T1 and T2. The determination of trapping parameters from the thermoluminescence glow curves of Na21Mg(SO4)10Cl3:Dy and Na21Mg(SO4)10Cl3:Eu is given below. 3.6. Analysis of TL glow curve and calculations of kinetic parameters 3.6.1. Tm–Tstop procedure for determining peak positions The broad nature of the recorded glow curves may be attributed to the overlap of various peaks having a continuous distribution of their trap depths. It is well known that the broad TL peaks may be resolved using glow curve deconvolution (GCD) mathematical functions based upon various TL models. In this study to extract the information about positions of various constituent glow peaks the Tm–Tstop method was employed. Tm–Tstop method helps to separate the overlapping peaks and to determine Tm value for each of them. The flat regions in the Tm–Tstop plot indicate position of the peaks in complex glow curve. The method consists in heating at a linear rate a pre-irradiated sample, to some temperature Tstop. The sample is then quenched to room temperature and then reheated in order to record the entire remaining glow curve. The process is repeated several times with same quenched/pre-irradiated sample at different Tstop values.

3.6.2. Glow curve deconvolution (CGCD) For determination of nature of TL process computerized glow curve fitting methods have been used and these methods were found to be very helpful in understanding advances in TL mechanism. In this study the glow curve convolution deconvolution (GCCD) curve fitting in Na21Mg(SO4)10Cl3:Dy material was done. The order of kinetics and activation energy of the isolated peak were found using Chen's set of empirical formulae [26,27]. To determine the general 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 [28,29]. Once E and b are known, s can be evaluated by Chen and Kirsh [30]. In first-order kinetics it is assumed that there is no such retrapping of charges whereas in second-order kinetics retrapping of charges occurs [28]. Trap depth or the thermal activation energy (E) was again calculated using the set of equations given by Chen. This procedure was repeated for all the TL peaks till a theoretical glow curve was obtained by their convolution to overlap with experimental glow curve. Some authors have reported evaluation of kinetic parameters using Chen's peak method applied directly to the peaks, which were deconvoluted using the origin 6.1 software without using any Glow Curve Deconvolution (GCD) function [31,32]. Fig. 12 shows the experimental glow curve for Na21Mg(SO4)10Cl3: Dy (Dy¼1 mol% doped) at heating rate of 5 1C s−1, which has been deconvoluted into two peaks using GCD function. Tm–Tstop method confirmed that experimental glow curve consisting of only two peaks which attributes to two types of traps. Fig. 12 illustrates close resemblance between experimental and theoretical glow curve

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Temperature (°C) Fig. 12. Deconvoluted glow curve of Na21Mg(SO4)10Cl3:Dy.

Table 1 Kinetic parameters using the GCD function. Sample

Na21Mg (SO4)10Cl3:Dy Na21Mg (SO4)10Cl3:Eu

complex nature of glow curve in Na21Mg(SO4)10Cl3:Dy is resolved by deconvolution and these deconvoluted glow curves are attributed to two types of traps. The Tm–Tstop procedure indicates that the position of dosimetric peak shifts slightly toward the high temperature side with increasing Tstop. These results imply that the dosimetric peak of Na21Mg(SO4)10Cl3:Dy after γ-irradiation can be best described as a superposition of glow peaks. Table 1 gives the values of trapping parameters of glow peaks of Na21Mg (SO4)10Cl3:Dy and Na21Mg(SO4)10Cl3:Eu phosphors calculated by Chen's peak shape methods. The γ dose response of this peak is linear in the dose range 100 mGy–8 Gy. The post-irradiation fading of this peak at room temperature is also less than 10% in one month. At present days, there is a great demand of the dosimetric phosphors which exhibit simple and sharp glow curves. The compound Na21Mg(SO4)10Cl3:Eu has been found to have simple and sharp glow peak and moreover it can be prepared very easily. Na21Mg(SO4)10Cl3:Dy has more sensitivity than CaSO4:Dy. Further work is in progress to clarify details of the defect within the material and glow curve structure.

Acknowledgement

Peak no

Tm(1C) Symmetry factor (μg)

Order of kinetics

Activation Energy, E (eV)

Frequency factor s (s−1)

1 2

151.4 192.5

0.5032 0.4908

1.9 1.8

0.9767 1.2292

1.2494  1011 6.39  1012

1

136.5

0.52

2

0.7181

6.245  108

obtained by GCD function. The position of respective peaks, trap parameters and order of kinetics are shown in Table 1. When the trapped electrons make transition to the conduction band by the thermal energy, they have two kinds of chances to make transition toward lower energy side. One is the retrapping process returning to the same kind of traps and another is the recombination with the hole accompanied by the emission of TL light. If the probability of being re-trapped is negligible then probability of rapid recombination process increases and glow curve has a narrow peak shape. Instead, if the retrapping dominates, the recombination with the holes is suppressed and the glow curve has a wide peak. These two descriptions are called the first order kinetics and the second order kinetics phenomena. Between these two types, the general order kinetics is introduced for providing a proper analytic continuation from the discrete two types of kinetics. The isolated peaks were analyzed with the help of Chen's peak shape method [33] to evaluate the peak parameters, using the values of the parameters Tm, T1 and T2 from the deconvoluted TL glow curves, as shown in Fig. 12. The geometrical form factor for the phosphor Na21Mg(SO4)10Cl3:Dy is calculated to be equal to 0.5032 for first peak at 151.4 1C, 0.4908 for second peak at 192.5 1C and 0.4838 indicating general-order kinetics having b values 1.9 and 1.8 respectively. For Na21Mg(SO4)10Cl3:Eu the peak obey second order kinetics having μg ¼0.52 which illustrates retrapping of charges in the phosphor [23]. Trapping parameters of all the peaks are summarized in Table 1.

Authors (BPK, NSD and SJD) are grateful to the Board of Research in Nuclear Sciences (BRNS), 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] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

4. Conclusion [29]

Comparison with data concerning undoped and Dy-doped Na21Mg(SO4)10Cl3 allows for identification of the contributions of impurities to the glow curves. The TL glow curve of Na21Mg (SO4)10Cl3:Dy shows a complex structure of glow curve whereas in case of Na21Mg(SO4)10Cl3:Eu simple glow curve is observed. This

[30] [31] [32] [33]

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