Thermoluminescence and glow curves analysis of γ-exposed Eu3+ doped K3Y(PO4)2 nanophosphors

Thermoluminescence and glow curves analysis of γ-exposed Eu3+ doped K3Y(PO4)2 nanophosphors

Accepted Manuscript Title: Thermoluminescence and glow curves analysis of γ-exposed Eu3+ doped K3 Y(PO4 )2 nanophosphors Author: Palvi Gupta A.K. Bedy...

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Accepted Manuscript Title: Thermoluminescence and glow curves analysis of γ-exposed Eu3+ doped K3 Y(PO4 )2 nanophosphors Author: Palvi Gupta A.K. Bedyal Vinay Kumar V.K. Singh Y. Khajuria O.M. Ntwaeaborwa H.C. Swart PII: DOI: Reference:

S0025-5408(15)30096-9 http://dx.doi.org/doi:10.1016/j.materresbull.2015.08.030 MRB 8383

To appear in:

MRB

Received date: Revised date: Accepted date:

13-1-2015 1-7-2015 24-8-2015

Please cite this article as: Palvi Gupta, A.K.Bedyal, Vinay Kumar, V.K.Singh, Y.Khajuria, O.M.Ntwaeaborwa, H.C.Swart, Thermoluminescence and glow curves analysis of rmgamma-exposed Eu3+ doped K3Y(PO4)2 nanophosphors, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Thermoluminescence and glow curves analysis of γ-exposed Eu3+ doped K3Y(PO4)2 nanophosphors Palvi Gupta1, A. K. Bedyal1, Vinay Kumar1,2,3, V. K. Singh1, Y. Khajuria1, O.M. Ntwaeaborwa2 , H.C. Swart2 1

School of Physics, Shri Mata Vaishno Devi University, Katra 182320 (J&K), India Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa

2

Graphical abstract

Highlights:



First time, a detailed comparative study of the glow curves and kinetic parameters was made on K3Y(PO4)2 nanophosphor



Combustion method was employed to synthesize the Eu3+ doped K3Y(PO4)2 nanophosphor.



The nanophosphor exhibited sublinear response suggesting that it is suitable for TL dosimetry.

Abstract Eu 3+ doped K3Y(PO4)2 nanophosphor was synthesized by combustion synthesis using urea as a fuel. The crystal structure and particle morphology of the nanophosphor were investigated by using

X-ray

diffraction

and

transmission

electron

microscopy,

respectively.

A

Thermoluminescence (TL) study was carried out after exposing the samples to gamma radiation. The TL glow curves exhibited a prominent peak at 407 K and a small hump at 478 K. The intensity of the peaks increased with the increase in the dose of the gamma rays (0.01 kGy – 5 kGy). The K3Y(PO4)2: Eu3+ (2.5 mol%) nanophosphor exhibited sublinear TL response to γradiation over a wide range of gamma doses (0.01 kGy −5 kGy). The TLanal program was used 1

to analyze the glow curves of the K3Y(PO4)2 nanophosphor at different doses (0.2 kGy− 5 kGy) and different heating rates (3 K/s−10 K/s). A comparative study was done for kinetic trapping parameters that were determined by the peak shape methods of Chen, Grossweiner and Lushchik. The frequency factors (s) for each glow peak were also calculated. The values of the activation energy (E) obtained by the TLanal program were in good agreement with those obtained by the peak shape methods. The effect of different amount of doses and different heating rates are discussed. Keywords: Luminescence; combustion; thermoluminescence; glow curves; kinetic parameters. 3

Corresponding Author : Email : [email protected]; [email protected] Phone: +91 1991 285699 extn. 2509 Fax: +91-1991-285694

1. Introduction Thermoluminescence (TL) is the light emission occurring during heating of a sample that has been exposed to radiation[1]. In TL, heat radiation is only a stimulant not an excitation agent. TL is an efficient tool used to study the defects/ traps inside a host lattice [2]. This phenomenon involves the radiative recombination of thermally released electrons or holes from their traps, which were created by exposure to ionizing radiation [3]. The light emitted during the TL process is plotted as intensity versus temperature, and it is known as a glow curve. The main objective of TL experiments is to extract data from a series of glow curves and to use these data to calculate values for the various parameters associated with the luminescence processes involved. These parameters include the activation energy (E), frequency factor (s) and order of kinetics (b) [4-5]. Tissue equivalence, reusability, stability, high sensitivity, a simple glow curve structure and dose linearity are the various characteristics of an ideal TL material. TL intensity depends strongly on the material, the type of impurity, radiation induced defect centers, dose and type of ionizing radiation [6-15]. The well-known application of TL is in radiation dosimetery, health physics, biological sciences and radiation protection. The TL process gives an indication as to whether or not a material can be used as a TL dosimeter [1-16] 2

Rare-Earth activated phosphates and oxide phosphors are of interest as they are used in Xray imaging, colour display and fluorescent lamp manufacturing [17-19]. Some of these phosphors have interesting TL properties and they might be useful in TL dosimetry (TLD) applications [20]. Although a large number of organic and inorganic materials exhibits TL emission, only a small fraction of this emission, possesses all the ideal characteristics of a good TLD phosphor [21-22]. Most commonly used and widely studied materials for TLD are sulphates, sulphides and fluorides of alkali and alkaline earth elements. Phosphates and halophosphates of alkali and alkaline earth elements have also been explored for their suitability as TLD materials [23]. For example, TL properties of barium orthophosphate co-doped with Eu and La that act as an efficient X-ray storage phosphor, have been reported [24]. The possible mechanism of Thermally Stimulated Luminescence (TSL) glow was developed by correlating the spectral characteristics and the thermal stability of the radical ions. The TL emission spectrum of the mixed rare earth phosphate powder composed mainly of characteristic line transition of rare earth ions [25]. Phosphate based phosphors are being developed and investigated for radiation dosimetry, lighting and displays [26]. Among the phosphates family, orthophosphates are considered to be important host materials owing to their high chemical stability, low sintering temperature, etc. [27]. However, there are very limited reports in the literature on the TL properties of orthophosphates. Thermoluminescence (TL) properties of calcium phosphate doped with different rare earth ions (Dy3+, Tb 3+, Sm3+) has been extensively studied, because their effective atomic numbers are close to human bones and teeth [28-30]. In 2005 Nakashima et al. [31] gives thermoluminescence mechanism of dysprosium-doped β-tricalcium phosphate phosphor. Gupta et al. [32] studied the photoluminescence and thermoluminescence properties of Tb3+ doped K3Gd(PO4)2 nanophosphor. Recently, we have reported the spectral and surface properties of the K3Y(PO4)2 doped with Eu 3+ nanophosphors [33]. In the present work, we report the TL properties of K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors synthesized by the combustion technique. The glow curves of the K3Y(PO4)2:Eu3+(2.5 mol%) nanophosphors were fitted with the TLanal program, and the results were compared to those obtained by peak shape methods at different doses and heating rates. The crystalline structure of the synthesized phosphor was analyzed by the X-ray diffraction (XRD) technique. 2. Experimental details 3

Potassium yttrium orthophosphate doped with europium nanophosphors were prepared by the combustion method using urea as a fuel. All the reagents, namely potassium nitrate (KNO3), yttrium nitrate hexahydrated (Y(NO3)3.6H2O), ammonium dihydrogen orthophosphate (NH4H2 (PO4)2), urea (NH2CONH2) and europium nitrate (Eu(NO3)3·6H2O) of AR grade were weighed according to the balanced chemical reaction. The ratio of the metal nitrates (oxidizers) to urea (fuel) was calculated maintaining the total oxidizing and reducing valencies of the components at unity, so that the heat liberated during combustion could be maximized. All the reagents in required amounts were dissolved in a few drops of ethanol and thoroughly mixed in an agate mortar resulting in a thick paste. The paste was transferred to an alumina crucible and then to the pre-heated muffle furnace maintained at 550℃. The paste underwent volumetric combustion with the evolution of the large amount of gasses. The reaction took 3-5 minutes to complete. The final white foamy product was cooled to room temperature and was ground gently forming a fine powder. The powder was annealed at 800℃ for 2 h to improve its crystallinity. The balanced chemical equation for the reaction is as follows:

3KNO3 + (1-x) Y (NO3)3.6H2O + 2NH4H2PO4 + 4NH2CONH2 + xEu (NO3)3·6H2O → K3Y1-xEu x(PO4)2 +20 H2O + 4CO2 + 8N2

Powder XRD patterns were recorded using a Bruker D8 Advance diffractometer. The particle morphology was analyzed by transmission electron microscopy (TEM) using a H-7500 (Hitachi Ltd., Tokyo, Japan) operated at 90 kV. The prepared samples were exposed to different doses of gamma rays using the gamma chamber available at the Inter-University Accelerator Centre, New Delhi having Co60 source. After exposure, TL glow curves were recorded on a Harshaw TLD reader (Model 3500) using 5 mg of the sample which was heated at the rate of 5 Ks -1. The TL glow curves for different heating rates ranging from 3−10 Ks-1 were aldo recorded.

3. Results and discussion 3.1 Structure Analysis Figure 1 represents the XRD pattern of K3Y(PO4)2:Eu 3+ (2.5 mol%), together with the stacked plot of the standard pattern of K3Y(PO4)2 (JCPDS file No. 49-0497). The diffraction peaks are consistent with the standard data, which indicates that the prepared phosphor matches well with the pure monoclinic phase of K3Y(PO4)2 orthophosphates with space group P21/m. No 4

diffraction peaks were observed that corresponds to the Eu 3+/any impurity element, which confirms the formation of a single phase K3Y(PO4)2 nanocrystalline phosphor. The inability to detect the Eu3+ ions (radius = 0.947Ǻ) can be attributed to their relatively low concentration. The Eu 3+ ions (ionic radius = 0.947 Ǻ) have most likely occupied the Yttrium (ionic radius =1.04 Ǻ) sites as they have approximately the same ionic radii. The broadening of the XRD diffraction peaks is observed when the crystallite size is lesser than 100 nm. An estimation of the average crystallite size was determined from the most prominent XRD peaks (111, -201, 020, 310) using Debye- Scherrer’s formula [34].

=

.



(1)

where D is the average size of the nanoparticles, λ is the incident wavelength of Cu Kα (1.54 Å) radiation, β is the full-width at half maximum (measured in radians) and θ is the Bragg angle. The average crystallite size of the phosphor was found to be 31 nm confirming that our particles sizes were in the nanoscale . The morphology and the sizes of the nanophosphors were analyzed using a TEM image. Fig. 2 shows the TEM image of the K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors. The diameters of most of the particles were in the range of 25–30 nm. This is in good agreement with the size distributions calculated from the XRD peak broadening.

3.2. TL Study The typical TL glow curves for the prepared K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors exposed to different doses of gamma rays from a Co60 source are shown in figure 3. The TL glow curve exhibits two peaks, peak 1 at 407 K and peak 2 at 478 K. From figure 3, it has been found that with the increase in the dose of the gamma rays (0.01 kGy – 5 kGy) the intensity of peak 1 and peak 2 also was shown to increase, but peak1 was more intense than peak 2. 3.2.1 TL Intensity v/s Dose An essential property of a TL material is that it exhibits a linear relation between TL intensity and absorbed dose. Many TL materials show a nonlinear growth of TL intensity with absorbed dose over certain dose ranges [1]. The dose-response curve can either be linear, sublinear or superlinear, depending on the linearity indexes [16]. Figure 4 shows the variation between the maximum TL intensity and gamma dose of Eu 3+ doped K3Y(PO4)2 phosphor. In order to record the dose-response curve, the phosphor was exposed to different gamma doses at room 5

temperature and the TL intensity of peak 1(407 K) was measured from the glow curve. The response curve in figure 4 exhibits sublinear response in the low dose range (10 Gy – 400 Gy) and a linear response in the high dose range (400 Gy – 5000 Gy). This unusual behavior of the TL response of the nanophosphor can be explained by the track interaction model (TIM) [35-36]. The dose-response curve fitted with a linear function in two parts, the linear expressions are Y (I) = 599.70 + 23.88 D for the range (10 Gy – 400 Gy) and Y (I) = 6867.43+10.91 D for the range (10 Gy – 400 Gy). R2 value is referred to as the goodness of fit. R 2 values range from 0 to 1, with 1 representing a perfect fit between the data and the line drawn through them, and 0 representing no statistical correlation between the data and a line. The values of R2 were 0.98 and 0.99, which means a good fitting between the data and the line drawn. There are several TL materials that exhibit nonlinear dose response over certain doses. The equation for the measured TL intensity is given by Eq. 2: =

,

(2)

where, a is the scaling factor, D is the dose of the radiation and k is a constant. When I is plotted as a function of the dose D on a log–log scale, the above equation yields a straight line with a slope k [37]. The terms superlinear, sublinear and linear dependence are attributed to the conditions when k > 1, k < 1 and k = 1, respectively. Figure 5 shows the dose-response of K3Y(PO4)2:Eu3+ phosphor using log-log scale. By linear fitting, the value of R2 was found to be 0.99, and the slope was found to be 0.724, which is less than 1, i.e. k < 1. From the value of k , it is clear that the present phosphor exhibits a sublinear dependence. The surface to volume ratio in case of nanoparticles is large which results in a higher surface barrier energy of the nanoparticles. At lower doses the energy density is not enough to overcome this barrier and create defects or trapping centers in the nanoparticles and therefore a sublinear response was obtained. The material may be used for dosimetry in the irradiation of foods and other products where the typical dose range for food irradiation is 10-10 4 Gy [39].

3.2.2 Effect of different heating rates on TL intensity The heating rate is a fundamental experimental variable in TL measurements. The TL glow curves of the exposed K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors at different heating rates 3Ks-1, 5Ks-1 and 10 Ks-1 are shown in figure 6. The effects of different heating rates on the K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors for a dose of 5 kGy were studied. From these curves, it has been found that with the increase in the heating rate the peak intensity increased and then 6

decreased. When the heating rate was increased from 3 K/s to 5 K/s, an increase in the peak intensity was observed. However, with further increase in heating rate to 10 K/s, a decrease in the peak intensity was observed with a peak temperature shifting towards the higher temperature. This unusual increase and decrease in the TL intensity can be explained by the fact that at lower heating rates the charge carriers which are responsible for producing the desired luminescence have adequate time to get retrapped at the recombination centre and are not involved in producing luminescence, whereas when the heating rate is high, the phenomena such as thermal quenching of TL intensity due to higher heating rates rises [39]. On the other hand, the shift in the peak temperature with increase in haetion rate from 5-10 K/s is associated with the reason that at low heating rate the sample will be held at a particular temperature for a longer time so that the trap gets empty prior results in a lower peak temperature. But at higher heating rate, the trap does not have enough time to get empty earlier which results in a higher peak temperature.

3.2.3. Analysis of TL glow curve and calculations of trapping parameters For first, second and general order glow curves, Eqs (3), (4) and (5) were proposed by Kittis et al., [40] respectively. For first order    E T  Tm  E T  Tm T 2 I (T )  I m exp 1   2 1    exp    m kT T m Tm  kT T m   

(3)

For second order 2

  E T  Tm   T 2  E T  Tm  I (T )  4 I m exp    2 1    exp   1 m kT T T kT T m m   m   

(4)

and, for general order  b / b 1

 E T  T m I (T )  I m b b / b 1 exp  Tm  kT

    E T  Tm  T2   ( b  1)( 1   ) 2 exp    Zm  Tm Tm     kT 

(5)

where, I(T) is the TL intensity at any temperature T(K), Im is the maximum peak intensity, E is the activation energy in eV (calculated from Chen’s formulae (Eq. 4)), k is the Boltzmann constant (8.6 x 10-5eVK-1), ∆ = 2

/ ,∆

= 2

/ and

= 1 + ( − 1) ∆ .

To determine the kinetic parameters such as the order of kinetics (b), activation energy (E) and frequency factor (s) of each of the deconvoluted glow peaks of the TL materials, the peak shape method, generally known as Chen’s method [41] was used. The other peak shape 7

methods employed in this work were those of Grossweiner [42] and Lushchik [43]. This peak shape method is mainly based on the temperatures T m, T1 and T2, where Tm corresponds to the temperature at the maximum TL intensity while T1 and T2 are the temperatures corresponding to the half of the intensities on either side of the maximum of the glow peak, respectively. In order to determine the kinetic parameters, the following peak shape parameters are to be determined: the total half intensity width

=



low temperature half width

=



, the high temperature half width

=



and the

[44]. The peak shape method was mainly used to

calculate the order of kinetics. The order of kinetics was evaluated from the geometrical factor (

) of the glow peak.

was calculated using Eq. (6) from the known peak shape parameters

and = =



(6)

which includes T1, T2 (the temperatures on the either side of full width at half maximum) and Tm corresponds to the temperature at the maximum TL intensity. The values of geometrical factor (

) for first- and second-order kinetics were 0.42 and 0.52, respectively. Another parameter ( )

proposed by Balarin gives the kinetic order as a function of the parameter

= =



(7)

The Balarin parameter ( ) ranges from 0.7 to 0.8 for the first order kinetics and for the second order kinetics it varies from 1.05 to 1.20 [45]. Commonly in the first order, the process of retrapping is negligible and the trap should be situated very close to the luminescent centre. The features of the second order peak are wider and it is more symmetric than the first order peak. The trap depth or the thermal energy (E) was calculated by using the following set of equations independent of the kinetic order:  kT 2 E  c  m  

   b 2 kT m  





where α stands for , ,  respectively. Cα and b α are obtained using the expressions below: C = 1.51+ 3.0 (g—0.42); b = 1.58+ 4.2 (g—0.42) C= 0.976+ 7.3 (g—0.42); b = 0 C = 2.52+10.2 (g—0.42); b = 1. The general formulae for the peak shape methods (16) are given by: For First order: 8

(8)

= 1.41



= 0.976



(Grossweiner)

(9)

(Lushchik)

(10)

For Second order:

=



(Grossweiner)

(11)

where α =, ,  where C  = 1.68

; C = 1.8313

and C = 3.5217 = 1.71

(Lushchik)

(12)

Frequency factor (s) was calculated by the equation given by Chen and Winer [46] after getting the trap depth (E) and order of kinetics (b)

= exp

[1 + ( − 1)∆ ]

(13)

where β is the linear heating rate. The deconvolution of the glow curves was done by using the TLanal Program given by Chung et al., [47]. Figure 7 shows the deconvoluted curve fitted with the original curve of the K3Y(PO4)2:Eu3+ nanophosphor exposed to 5 kGy gamma-ray at a heating rate of 5 Ks-1. The curve consists of four traps having different energies and kinetic orders. Figure 8 shows the snapshot of the decovoluted glow curve of the K3Y(PO4)2:Eu3+ nanophosphor with a dose of 5 kGy at 5 K/s, using the TLanal program. The figure of merit (FOM) was found to be 1.085, which shows that the experimental and theoretical curves were approximately in good agreement with each other. Trapping parameters of all the peaks at different doses (0.2 kGy −5 kGy) and at different heating rate (3-5 K/s) calculated by using the TLanal program are summarized in Table 1. The symmetric factor (

) and Balarin parameter (γ) calculated for all the peaks at different doses

(0.2 kGy −5 kGy) at different heating rate (3−5 K/s) are listed in Table 2. In Table 2, the values of the activation energy (E) and the frequency factor (s) at different doses (0.2 kGy−5 kGy) are compared for the different peak shape methods (Chen’s, Grossweiner and Lushchik), it was found that the values of activation energy (E) and frequency factor (s) for all the three peak shape methods are approximately the same. Furthermore, the values of the activation energy (E) calculated by the TLanal program (table 1) are also approximately closer to the values as obtained by the different peak shape methods as illustrated in table 2 whereas the values of the frequency factor (s) are a bit different. The order of kinetics (b) for each peak at different heating 9

rates (3 K/s−10 K/s) were determined from the symmetric factor ( and are listed in table 2. The mean value of (

) and Balarin parameter (γ)

) and (γ) for peak 1 at different heating rates (3

K/s -10 K/s) was 0.42 and 0.75, respectively, and these parameters confirm a first order kinetic property. For peaks 2, 3 and 4, the mean value of (

) and (γ) were found to be 0.50 and 1.04;

0.51 and 1.07; 0.52 and 1.1, respectively and it confirms the second order kinetics. At different heating rates (3 K/s−10 K/s), the trapping parameters calculated by different peak shape methods are summarized in table 2 and it was found that the values of the activation energy (E) are approximately the same whereas the values of the frequency factor (s) are slightly different as compared with the TLanal program in table 1. From Table 1 and Table 2, it was found that the maximum glow peaks obeyed the second order kinetics, indicating the occurrence of retrapping phenomena. From Tables 1-2, the value of the frequency factor (s) and activation energy in some traps found to be very large. The occurrence of high values of the activation energy and frequency factor was reported by several authors [48-50]. To elucidate the possibility of high frequency factor and high activation energy, a model of one trap and three recombination centers, one radiative and two non-radiative was proposed [51]. In 2006, another explanation related to occurrence of high frequency factors and high activation energy was given [52], based on the concept of cascade detrapping. The trap parameters calculated by different peak shape methods and TLanal program are approximately in agreement indicating the suitability of the methods used. 4. Conclusions In the present work, Eu 3+ doped K3Y(PO4)2 nanophosphors were successfully synthesized by combustion method. The TL data of K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors irradiated with different gamma doses (0.01 kGy− 5 kGy) were presented. The trap parameters for the product, at a linear heating rate (5K/s) with different doses (0.2 kGy−5 kGy) and at different heating rates (3K/s−10K/s) with a dose of 5 kGy were calculated by using different peak shape methods (Chen’s, Grossweiner and Lushchik) and the results obtained were compared with that obtained by the developed TLanal program. The values of the activation energy (E) obtained by the developed TLanal program were in good agreement with those evaluated by the peak shape methods. The nanophosphor exhibited sublinear response over a wide range of gamma doses, suggesting that it is suitable for dosimetry in the irradiation of foods and other products.

Acknowledgments 10

The authors are thankful to Director, Inter University Accelerator Centre (IUAC), New Delhi for providing gamma exposure facility at Health Physics Lab. This work is financially supported by the BRNS, DAE (Department of Atomic Energy, Govt. of India) under Project reference no. 2012/34/37/BRNS/1035.

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[24] W.J. Schipper, J.J. Hamelink, E.M. Langeveld, G. Blasse,. Trapping of electrons by H+ in the X-ray storage phosphor Ba3(PO4)2:Eu2+, La3+, J. Phys. D Appl. Phys. 26 (1993) 1487–1492. [25] P. Lacconi, M. Junker, B. Guilhot, D. Huguenin, Thermoluminescence of a mixed rare earth phosphate powder La1−x−yCexTb yPO4, Opt. Mater. 17 (2001) 409−414. [26] K.N. Shinde, S.J. Dhoble, N. Brahme, A. Kumar, Combustion synthesis of Sr6AlP5O20:Dy3+ submicron phosphor for high dose TL dosimetry, Radiat. Meas. 46 (2011) 1886–1889. [27] C.C. Lin, Y.S. Tang, S.F. Hu, R.S Liu, KBaPO4:Ln (Ln=Eu, Tb, Sm) phosphors for UV excitable white light-emitting diodes, J. Lumin. 129 (2009) 1682–1684. [28] H. Ohtaki, Y. Fukuda, N. Takeuchi, Thermoluminescence in Calcium Phosphate Doped with Dy2O3, Radiat. Prot. Dosim. 47 (1993)119-122. [29] Y. Fukuda, H. Ohtaki, S. Taniguchi, N. Takeuchi, Thermoluminescence in calcium phosphate doped with samarium, J. Mater. Sci. Lett. 11 (1992) 731 ̶ 732. [30] Y. Fukuda, H. Ohtaki, S. Owaki, Thermoluminescence of Thulium Sensitized by Terbiumin -Phase Ca3 (PO4)2, Phys. Stat. Sol. (a). 144 (1994) K107 ̶ 111. [31] K. Nakashima, M. Takami, M. Ohta, T. Yasue, J. Yamauchi, Thermoluminescence mechanism of dysprosium-doped β-tricalcium phosphate phosphor, J. Lumin. 111 (2005) 113–120. [32] P. Gupta, A.K. Bedyal, V. Kumar, Y. Khajuria, S.P. Lochab, S.S. Pitale, O.M. Ntwaeaborwa, H.C. Swart,. Photoluminescence and thermoluminescence properties of Tb3+ doped K3Gd(PO4)2 nanophosphor. Mater. Res. Bull. 60 (2014) 401 ̶ 411. [33] P. Gupta, A.K. Bedyal, V. Kumar, Y. Khajuria, V. Kumar, E. Coetsee-Hugo, O.M. Ntwaeaborwa, H.C. Swart, Spectral and surface investigations on Eu 3+ doped K3Y(PO4)2 nanophosphor: A promising orange–red phosphor for white lightemitting diodes, Opt. Mater. 36 (2014) 996−1001. [34] B.D. Cullity, Elements of X-ray Diffraction, 2nd Ed., Addison-Wesley, New York, 1956. [35] S. Mahajna, Y.S. Horowtz, The unified interaction model applied to the gamma ray induced supralinearity and sensitization of peak 5 in LiF:Mg,Ti (TLD-100), J. Phys. D: Appl. Phys. 30 (1997) 2603.

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[36] Y.S., Horowtz, Avila, O., Rodrigues M.V., 2001. Theory of heavy charged particle response (efficiency and supralinearity) in TL materials, Nucl. Instrum. Methods B 184, 85. [37] V. Pagonis, G. Kitis, C. Furetta, Numerical and Practical Exercises in thermoluminescence, Springer Science & Business Media, New York, 2006. [38] W.L. McLaughlin, H.M. Khan, W. Warasawas, M. Al-Sheikhly, B.B. Radak, Optical waveguide dosimetry for gamma-radiation in the dose range 10-1 − 10 4 Gy, Radiat. Phys. Chem. 33 (1989) 39–46. [39] S. Bahl, A. Pandey, S.P. Lochab, V.E. Aleynikov, A.G. Molokanov, P. Kumar,. Synthesis and thermoluminescence characteristics of gamma and proton irradiated nanocrystalline MgB4O7: Dy, J. Lumin. 134 (2013) 691–698. [40] G. Kitis, J.M. Gomez-Ros, J.W.N. Tuyn, Thermoluminescence glow-curve deconvolution functions for first, second and general order kinetics, J. Phys. D Appl. Phys. 3 (1998) 2636–2641. [41] R. Chen, Y. Krish, Analysis of Thermally Stimulated Processes, Pergamon Publishing Co. Pte. Ltd, NewYork, 1981. [42] L.I. Grossweiner, A note on the analysis of first order glow curves, J. Appl. Phys. 24 (1953) 1306−7. [43] C.B. Lushchik, The investigation of trapping centres in crystals by the metod of thermal bleaching, Sov. Phys. JETP. 3 (1956) 390−395. [44] G.F.J. Garlick, A.F. Gibson, The Electron Trap Mechanism of Luminescence in Sulphide and Silicate Phosphors, Proc. Phys. Soc. 60 (1948) 574−590. [45] M. Balarin, Direct evaluation of activation energy from half-width of glow peaks and a special nomogram, Phys. Status Solidi A 31 (1975) K111−K114. [46] R. Chen, S.A.A. Winer, Effects of various heating rates on glow curves, J. Appl. Phys. 41 (1970) 5227−5232. [47] K.S. Chung, H.S. Choe, J.I. Lee, J.L. Kim, S.Y. Chang, A computer program for the deconvolution of thermoluminescence glow curves, Radiat. Prot. Dosim. 115 (2005) 343–349. [48] G.C. Taylor, E. Lilley, The analysis of thermoluminescent glow peaks in LiF (TLD100), J. Phys. D. Appl. Phys. 11 (1978) 567 ̶ 581.

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[49] S.G. Gorbics, F.H. Attix, J.A. Pfaff, Temperature stability of CaF2:Mn and LiF(TLD-100) thermoluminescent dosimeters, Int. J. Appl. Radiat. Is. 18 (1967) 625 ̶ 630. [50] R.G. Fairchild, P.L. Mattern, K. Lengweiler, P.W. Levy, Thermoluminescence of LiF TLD-100 dosimeter crystals. IEEE Trans, Nucl. Sci. NS-21( 1974) 366-372. [51] R. Chen,

A. Hag-Yahya, Interpretation of very high activation energies and

frequency factors in TL as being due to competition between centres, Radiat. Prot. Dosim. 6 (1996) 17 ̶ 20. [52] A. Mandowski, Topology-dependent thermoluminescence kinetics, Radiat. Prot. Dosim. 119 (2006) 23 ̶ 28.

15

Figure 1. XRD patterns of (a) JCPDS Card No. 49-0497 and (b) K3Y(PO4)2:Eu3+(2.5 mol%) nanophosphor.

16

Figure 2. TEM image of the K3Y(PO4)2:Eu3+(2.5 mol%) nanophosphor.

17

TL Intensity (a.u.)

6x10

4

5x10

4

4x10

4

3x10

4

2x10

4

1x10

4

3+

K3Y(PO4)2:Eu

0.01 kGy 0.05 kGy 0.1kGy 0.2 kGy 0.4kGy 2 kGy 4 kGy 5 kGy

Peak 1

Peak 2

0 325

350

375

400

425

450

475

500

525

Temperature (K)

Figure 3: TL glow curves of K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors at different doses (0.01 kGy−5 kGy) and heating rate (5Ks-1).

18

4

6x10

4

TL Intensity (a.u.)

5x10

3+

K3Y(PO4)2 :Eu

(b)- 400- 5000 Gy Slope- 10.91

(a)- 10- 400 Gy Slope- 23.88

4

4x10

(b) 2

R  0.99

Original Curve Fitted Curve

4

3x10

4

2x10

4

1x10

(a)

2

R  0.98 0 0

1000

2000

3000

4000

5000

Dose (Gy) Figure 4: Dose-response of K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors.

19

3+

K3Y(PO4)2:Eu

TL Intensity (a.u.)

Linear Fit

10

4

10

3

10

Slope = 0.72 Intercept = 2.086 2 R = 0.99

100

1000

Dose (Gy) Figure 5: Dose-response of the K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors using log-log scale.

20

4

6x10

3 K/s (Peak a) 5 K/s (Peak b) 10 K/s (Peak c)

3+

K3Y(PO4)2:Eu

b

Dose - 5 kGy 4

TL Intensity (a.u.)

5x10

4

4x10

c a

4

3x10

4

2x10

4

1x10

0 325

350

375

400

425

450

475

500

525

Temperature (K) Figure 6: Effect of different heating rates (3 Ks-1 – 10 Ks-1) on TL glow curve of K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphor exposed to5 kGy of gamma rays.

21

Original value Fit Value Peak 1 Peak 2 Peak 3 Peak 4

4

6x10

4

TL Intensity (a.u.)

5x10

4

4x10

1

3

4

3x10

2 4

2x10

4

4

1x10

0 325

350

375

400

425

450

475

500

525

Temperature (K) Figure 7: Deconvoluted curve of the K3Y(PO4)2:Eu3+ nanophosphor exposed to5 kGy of gamma rays at heating rate of 5 Ks-1.

22

Figure 8: Snapshot of deconvoluted glow curve of the K3Y(PO4)2:Eu3+ nanophosphor with a dose of 5 kGy at 5 K/s, using TLanal program.

Table 1: Kinetic parameters of K3Y(PO4)2:Eu3+ (2.5 mol%) nanophosphors using the TLanal program at different doses (0.2 kGy −5 kGy) and at different heating rate (3-5 K/s). Dose (kGy)

Heatinng

Peak

Tm(K)

Activation energy E( eV)

Rate (K)

23

Frequency Factor s (s-1)

0.2

5

1 2 3 4

407 423 434 478

1.10 2.37 2.35 1.61

1.6 x 1013 1.1 x 1024 1.0 x 1023 1.0 x 1012

0.4

5

1 2 3 4

408 428 434 481

1.09 1.88 1.72 1.33

1.1 x 1013 2.2 x 1017 1.4 x 1015 3.1 x 108

2

5

4

5

1 2 3 4 1 2 3 4

403 422 460 483 398 414 431 478

1.21 0.83 0.74 2.10 1.26 2.07 1.64 1.59

5.0 x 1014 2.1 x 10 9 1.6 x 10 2 6.2 x 1016 4.2 x 1015 5.5 x 1019 3.4 x 1013 1.1 x 1011

5

5

5

3

1 2 3 4 1 2 3 4

401 412 433 479 402 424 442 488

1.59 0.96 1.67 1.72 0.98 1.57 0.87 0.75

7.9 x 1014 3.9 x 1011 2.4 x 1014 9.7 x 1012 4.7 x 1011 5.5 x 1012 5.9 x 10 3 1.8 x 10 1

5

5

1 2 3 4

401 412 433 479

1.59 0.96 1.67 1.72

7.9 x 1014 3.9 x 1011 2.4 x 1014 9.7 x 1012

5

10

1 2 3 4

409 427 447 494

1.28 1.98 1.56 1.09

2.3 x 1015 6.4 x 1017 8.8 x 1011 1.1 x 10 5

Table 2: Kinetic parameters of K3Y(PO 4)2:Eu3+ (2.5 mol%) nanophosphors using different peak shape methods at different doses (0.2 kGy −5 kGy) and at different heating rate (3-5 K/s). Dose (kGy) 0.2

Heatinng Rate (K) 5

Peak

1 2 3 4

Order of kinetics (b) γ g 0.41 (1) 0.52 (2) 0.50 (2) 0.51 (2)

0.72 1.09 1 1.05

Chen’s Method E(eV)

s(s-1) 8.8 x 1012 2.1 x 1027 1.0 x 1025 1.4 x 1016

1.07 2.30 2.16 1.57

24

Grossweiner E( eV) 1.06 2.28 2.30 1.61

s(s-1) 6.4 x 1012 1.5x1027 4.6 x 1026 4.2 x 1016

Lushchik E(eV) 1.06 2.19 2.30 1.60

s(s-1) 6.9 x 10 12 1.0 x 10 26 4.9 x 10 26 3.1 x 10 16

0.4

5

1 2 3 4

0.40 (1) 0.50 (2) 0.53 (2) 0.52 (2)

0.66 1 1.13 1.08

1.02 1.79 1.77 1.32

1.7 x 1012 8.4 x 1020 2.2 x 1020 2.3 x 1013

1.07 1.91 1.71 1.32

6.9 x 1012 2.1 x 1022 5.0 x 1019 2.7 x 1013

1.16 1.92 1.62 1.30

1.0 x 10 14 2.9 x10 14 4.3 x 10 18 1.6 x 10 13

2

5

1 2 3 4

0.40 (1) 0.44 (1) 0.53 (2) 0.51 (2)

0.68 0.79 1.13 1.06

1.15 0.90 0.76 2.03

1.2 x 1014 1.7 x 1010 4.3 x 10 7 8.6 x 1020

1.18 0.84 0.74 2.06

2.7 x 1014 3.9 x 10 9 2.7 x 10 7 1.9 x 1021

1.23 0.78 0.74 2.01

1.5 x 1015 6.6 x 108 2.5 x 107 6.0 x 1020

4

5

1 2 3 4

0.42 (1) 0.52 (2) 0.51 (2) 0.50 (2)

0.73 1.08 1.06 1

1.25 2.01 1.61 1.47

4.2 x 1015 2.1 x 1024 3.5 x 1018 1.2 x 1015

1.23 2.01 1.63 1.57

1.9 x 1015 2.2 x 1024 7.4 x 1018 1.5 x 1016

1.20 1.93 1.60 1.60

9.6 x 1014 2.8 x 1023 3.2 x 1018 3.1 x 1016

5

5

1 2 3 4

0.53 (2) 0.41 (1) 0.50 (2) 0.51 (2)

1.14 0.71 1 1.05

1.62 0.93 1.59 1.66

1.5 x 1020 8.3 x 1010 2.1 x 1018 1.4 x 1017

1.56 0.93 1.70 1.70

2.7 x 1019 8.0 x 1010 3.8 x 10 19 4.0 x 10 17

1.47 0.94 1.72 1.68

2.0 x 1018 1.4 x 1011 6.4 x 1019 2.4 x 1017

5

3

1 2 3 4

0.41 (1) 0.50 (2) 0.53 (2) 0.52 (2)

0.70 1 1.17 1.12

0.92 1.44 0.92 0.76

7.2 x 1010 3.5 x 1016 4.9 x 10 9 9.0 x 10 6

0.93 1.53 0.88 0.76

9.9 x 10 10 5.2 x1017 1.5 x 10 9 6.9 x 10 6

0.96 1.55 0.84 0.76

3.1 x 1011 9.5 x 1017 6.2 x 108 7.5 x 106

5

5

1 2 3 4

0.53 (2) 0.41 (1) 0.50 (2) 0.51 (2)

1.14 0.71 1 1.05

0.89 1.64 1.38 1.77

1.5 x 10 11 1.4 x 1020 8.9 x 1015 1.8 x 1018

0.87 1.65 1.47 1.81

6.6 x 1010 2.3 x 1020 1.1 x 1017 4.8 x 1018

0.84 1.61 1.50 1.78

3.1 x 1010 6.8 x 1019 2.2 x 1017 2.4 x 1018

5

10

1 2 3 4

0.44 (1) 0.51 (2) 0.51 (2) 0.53 (2)

0.8 1.07 1.05 1.13

1.62 0.93 1.59 1.66

1.5 x 1020 8.3 x 1010 2.1 x 1018 1.4 x 1017

1.56 0.93 1.70 1.70

2.7 x 1019 8.0 x 1010 3.8 x 10 19 4.0 x 10 17

1.47 0.94 1.72 1.68

2.0 x 1018 1.4 x 1011 6.4 x 1019 2.4 x 1017

25

26