Trapping parameters determination and modeling of the thermoluminescence process in K2GdF5:Dy3+

Optik 127 (2016) 3959–3963

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Trapping parameters determination and modeling of the thermoluminescence process in K2 GdF5 :Dy3+ Ahmed Kadari a,b,∗ , Nicholas M. Khaidukov c , Rabah Mostefa b , E.C. Silva d , Luiz O. Faria e a

Department of Chemistry, Faculty of Sciences of the Matter, Ibn Khaldoun University of Tiaret, BP P 78 Zaaroura, Tiaret, Algeria Department of Physics, Electronic Microscopy and Materials Sciences Laboratory (EMMSL), B.P.1505 El M’Naouar, Oran, Algeria Institute of General and Inorganic Chemistry, Leninskii Prospect 31, 119991 Moscow, Russia d Depto. de Engenharia Nuclear (DEN/UFMG-MG), Av. Antônio Carlos 6627, 31270-970 Belo Horizonte, MG, Brazil e Centro de Desenvolvimento da Tecnologia Nuclear, Av. Antonio Carlos 6627, C.P.941, 30161-970 Belo Horizonte, MG, Brazil b c

a r t i c l e

i n f o

Article history: Received 20 November 2015 Accepted 11 January 2016 Keywords: Thermoluminescence Trapping parameters Model Dose

a b s t r a c t K2 GdF5 :Dy crystals show excellent TL output for fast and thermal neutrons. An additional thermoluminescent peak, as yet unidentified, is revealed in TL of K2 GdF5 doped with different dysprosium Dy3+ concentrations, after exposing to 12.1 mSv of gamma radiation. In 2013, Silva et al., have studied the thermoluminescent responses of K2 GdF5 : Dy3+ exposed to photon and neutron radiation fields. The results presented in this paper show that the introduction of the dysprosium Dy3+ induced the appearing of an additional TL peak at 234 ◦ C (507 K). Particularly, for K2 GdF5 crystals doped exactly with 5.0 at.% Dy3+ , the TL output reaches its maximum. In the first part of this paper, the glow curves of the pure and doped K2 GdF5 after gamma radiation were analyzed and the kinetics parameters (trap depth (E) and frequency factor (s)) were determined by using the Chen’s peak shape method. The appearance of the additional peak has been explained and attributed to the energy transfer that occurs from Gd3+ to Dy3+ . The mechanisms of the charge transfer have been studied carefully based on the excitation spectra of the Dy3+ doped K2 GdF5 crystal at the wavelengths 575 nm and 485 nm, respectively. In the second part of this paper; the thermoluminescence properties of both the pure K2 GdF5 and Dy3+ doped K2 GdF5 have been investigated using the 3T1R and 4T1R models, respectively. © 2016 Elsevier GmbH. All rights reserved.

1. Introduction Thermoluminescence (TL) is a powerful technique to investigate the effect of pre-irradiation heat treatment, impurities and defects in the crystals. Thermoluminescent dosimetry has become a current dosimetric tool in photon, electron and neutron beams for dose determination. In view of their excellent dosimetric properties, they are commonly used for dose determination in “in vivo” dosimetry, as well as in phantoms simulating patient treatments. In this context, ultra-sensitive thermoluminescent dosimeters (TLD) based on alkali fluorides and alkaline-earth fluorides, in particular, those doped with rare earth ions, e.g. CaF2 :Dy3+ (TLD-200) and CaF2 :Tm3+ (TLD-300), CaF2 :Cu (TLD-451) have been investigated and extensively utilized for dosimetry of ionizing radiation [1,2].

∗ Corresponding author at: Faculty of Sciences of the Matter, Ibn Khaldoun University of Tiaret, Department of Chemistry, BP P 78 Zaaroura, Tiaret, Algeria. Tel.: +213 775955627. E-mail addresses: [email protected], [email protected] (A. Kadari). http://dx.doi.org/10.1016/j.ijleo.2016.01.097 0030-4026/© 2016 Elsevier GmbH. All rights reserved.

Recently, K2 YF5 crystals singly doped with rare earth ions (RE), e.g. Ce3+ , Tb3+ , Dy3+ or Tm3+ , have been shown to be attractive TL materials for detection and discrimination of different types of radiation [3]. It is to be noted that metals do not exhibit TL while all insulators and semiconductors are expected to exhibit TL. This is because of the presence of trapping levels within the band gap (Eg ) of these materials. The forbidden band of the pure K2 YF5 crystal is 6.55 eV and with the impurity is 5.16 eV [4]. The theoretical calculations were performed on a crystal model for the compound consisting of a ˚ symmetry group Pna21 (33) cubic cell of 10.791, 6.607 and 7.263 A. It contains, for the K2 YF5 [5] pure system, 4 yttrium, 8 potassium and 19 fluorine atoms. The introduction of rare earths (RE) impurities in a material makes changes in the physical properties of this material [6–9] and can often induces dramatic changes in the optical, electrical, and magnetic properties [10,11]. K2 GdF5 crystals doped with Dy3+ have been proposed for TL dosimetry. Their optical properties have been investigated by several authors [12,13]. In particular, the thermoluminescent properties of isostructural K2 YF5 and K2 GdF5 crystals doped with different concentrations of Tb3+ ions in response to ␣, ␤, gamma and X-ray irradiation are well reported in the literature [14–18]. However, it was only recently

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that Silva et al. [19] have studied for the first time the thermoluminescent responses of K2 GdF5 :Dy3+ exposed to neutron radiation fields, in view of the high proportion of 155 Gd and 157 Gd isotopes in the natural Gd, i.e., 14.7% and 15.7%, with high neutron cross section. Due to the unavoidable presence of gamma rays in neutron beams, the detector response to these beams requires a detailed study in order to determine the sensitivity for each component. Thus, in that study, the thermoluminescence responses of undoped and Dy3+ doped K2 GdF5 crystals, synthesized under hydrothermal conditions, exposed to gamma radiation fields were investigated. The TL curves were fitted by four glow peaks centered at 153, 185, 216 and 234 ◦ C, the latter being present only in K2 Gd0.95 Dy0.05 F5 crystals. Particularly, in this Dy3+ concentration, the K2 GdF5 crystals have presented a high TL response for thermal neutrons, comparable to that of 6 LiF:Mg,Ti (TLD-600) irradiated at the same conditions. In this context, in order to improve our understanding about the energy parameters and the mechanisms behind the thermoluminescent properties of the K2 Gd0.95 Dy0.05 F5 crystals, this paper provides a detailed kinetic study of the thermoluminescent glow peaks using a refined peak fitting approach. Additionally, the appearance of the high temperature TL glow peak at 234 ◦ C (507 K) has been attributed to the energy transfer that occurs from Gd3+ to Dy3+ . In the second part of this paper; the thermoluminescence properties of both the pure K2 GdF5 and Dy3+ doped K2 GdF5 have been investigated using the 3T1R and 4T1R models, respectively.

Fig. 1. Glow curve deconvolution of 12.1 mSv of gamma irradiated K2 GdF5 crystal.

2. Experiment K2 GdF5 single crystals doped with 0.2, 1.0, 5.0 and 10.0 at.% Dy3+ ions as well as undoped K2 GdF5 crystals up to 1 cm3 in size were grown by a direct temperature-gradient method as a result of the reaction of potassium fluoride aqueous solutions with appropriate mixtures of 99.99% pure rare earth oxides under hydrothermal conditions [20]. Polished crystalline samples with thickness of about 1 mm were utilized for the TL measurements. The examined samples were exposed at room temperature (RT) to gamma rays with photon energy of 662 keV from a 137 Cs gamma source, with delivered personal equivalent doses Hp (10) (in Sv) measured by ionization chambers calibrated at the secondary standard laboratory LCS-IRD/CNEN, which in turn is traceable to an International Atomic Energy Agency (IAEA) primary standard laboratory, as well as to X-rays with effective energies of 33.3, 41.1 and 52.5 keV. The measurements of TL glow curves were performed with a Harshaw–Bicron 3500 TLD reader operating with a linear temperature profile over the range from 50 up to 300 ◦ C in the resistive mode by using a heating rate of 10 ◦ C/s and reading cycles of 35 s. The detailed experimental steps have been given in the previously published paper by Silva et al. [19]. 2.1. Results and discussion The experimental TL glow curves of un-doped K2 GdF5 and 5.0 at.% Dy3+ doped K2 GdF5 crystals exposed to 12.1 mSv of gamma radiation are shown in Figs. 1 and 2. Also, new deconvolutions process of the experimental curves has been presented in these Figures. All samples showed dominant glow peak at 185 ◦ C at a heating rate of 10 ◦ C/s. For increasing amount of Dy3+ concentration, the intensity of the 185 ◦ C peak decreases, except for the sample with 5.0 at.% Dy3+ , as shown in Table 1. The decrease of the relative TL intensity of the 213 ◦ C peak is probably due to the formation of complex dopants aggregates that offer on alternative non radiative pathways for Dy3+ relaxations due to ion–ion interaction. The appearance of more than one peak indicates the existence of different kinds of trapping centers, capable to capture electrons during the gamma

Fig. 2. Glow curve deconvolution of 12.1 mSv of gamma irradiated 5.0 at.% Dy3+ doped K2 GdF5 crystal.

irradiation. Comparing the TL glow curve of the undoped crystal with the Dy3+ doped K2 YF5 crystal, we see that an additional glow peak around 234 ◦ C can be proposed. 2.2. Traps parameters determination In order to estimate the kinetic parameters that include activation energy (E), order of kinetics (b) and frequency factor (s). We note here that there are many published paper about the deconvolutions and the analysis of the thermoluminescence glow curves [21–23]. In the present study, the obtained glow curves were deconvoluted using the glow curve deconvolution program available in origin 7.5 software which is shown in Figs. 1 and 2. Such parameters have been determined. First, the overlapped peaks were separated to single glow peaks, and then the analysis of the set of individual glow peaks was done using the Chen’s peak shape method [24]. 2.3. Theory The Chen’s peak shape [24] is a popular method of analyzing TL glow curves in order to ascertain the kinetic parameters E, s, and b by considering the shape or geometrical properties of the peak. TL glow peaks corresponding to second-order kinetics are Table 1 TL output for the peak centered at 185 ◦ C for increasing concentrations of Dy3+ . % Dy

TL output peak at 185 ◦ C (a.u.)

0.0

0.2

1.0

5.0

10.0

54.5

28.6

18.2

46.4

15.4

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Table 2 Kinetics parameters of different peaks in TL glow curves obtained for K2 GdF5 doped with different Dy3+ concentrations after exposing to 12.1 mSv of gamma radiation. Chen’s peak shape (PS) method Samples

Peaks

Tm (K)

b (g )

E (eV)

K2 GdF5

1 2 3 1 2 3 4

426 458 481 426 458 489 507

1 (0.43) 1 (0.45) 1 (0.46) 1 (0.42) 1 (0.43) 1 (0.40) 1 (0.46)

1.231 1.300 1.422 0.878 1.078 1.597 1.711

K2 GdF5 :Dy3+ (5.0 at.%)

characterized by an almost symmetrical shape, whereas first-order peaks are asymmetrical. The trap depth using this method is given by the following equations:



E˛ = c˛

2 kTm ˛



− b˛ (2kTm ) ,

(1)

where ˛ = , ı and ω. Here Tm is the peak temperature at the maximum, T1 and T2 are, respectively, the temperatures on either side of Tm , corresponding to half intensity.  = Tm − T1 is the half-width at the low temperature side of the peak, ı = T2 − Tm is the half-width toward the fall-off side of the glow peak, ω = T2 − T1 is the total half-width, C = 1.51 + 3.0(g − 0.42), b = 1.58 + 4.2(g − 0.42), Cı = 0.976 + 7.3(g − 0.42), b␦ = 0, Cω = 2.52 + 10.2(g − 0.42), bω = 1. The form factor (symmetry factor; g ) is given by: g =

T2 − Tm T2 − T1

(2)

The frequency factor (s) is given by the following relationship: s=

ˇE 2 kTm



exp

E 2 kTm



1 + (b − 1)

2kTm E

−1

,

(3)

where ˇ is the linear heating rate and k is the Boltzmann constant (k = 8.6 × 10−5 eV K−1 ). Table 2 shows the calculated trapping parameters for the deconvoluted peaks of the K2 GdF5 after exposing to 12.1 mSv of gamma radiation at different concentration of Dy3+ . It is seen in Table 2 that all peaks show the first order of kinetics. The activation energy varies from 0.231 to 0.422 eV for the undoped sample, and from 0.878 to 1.711 eV for the Dy3+ doped sample. 3. Physical explanation of the appearance of the 234 ◦ C TL glow peak The individual additional thermoluminescent peak is shown in Fig. 3. The appearance of this TL glow peak at 234 ◦ C (507 K) for the

Fig. 3. Individual additional thermoluminescence glow peak at 234 ◦ C in 5.0 at.% Dy3+ :K2 GdF5 sample.

s (s−1 ) 1.31 × 1014 6.63 × 1013 2.58 × 1014 6.31 × 109 1.99 × 1011 1.01 × 1016 3.57 × 1016

n0 3.22 × 106 1.94 × 107 1.48 × 107 5.93 × 106 1.55 × 107 7.56 × 106 9.63 × 106

5.0 at.% Dy3+ :K2 GdF5 sample indicates a probable change in the trap structure of K2 GdF5 with the doping concentration of dysprosium Dy3+ impurity. Van Do et al. [12] have showed that the energy transfer occurs from Gd3+ to Dy3+ , resulting in the additional intense excitation UV-narrow bands for the luminescence of the Dy3+ ions. We can see from the UV excitation spectra of the Dy3+ emission at the wavelengths 575 nm and 485 nm of the K2 GdF5 :Dy3+ crystals [12], that two narrow lines peaking at 254 nm and 273 nm are observed in the UV-side, which correspond to the transitions from the ground state 8 S7/2 of Gd3+ ion to its excited states 6 DJ (J = 7/2, 9/2) and 6 I11/2 , respectively [25]. These lines are not observed in the excitation and absorption spectra of the Dy3+ doped samples without Gd3+ component [26,27]. This implies that energy transfer from Gd3+ to Dy3+ ions has occurred in this crystal. Consequently, the luminescence of Dy3+ ions in K2 GdF5 could be strongly excited by the additional light with 254 nm and 273 nm wavelengths. In the K2 GdF5 :Dy3+ matrix, we suggest that Dy3+ ions can efficiently capture holes and serve simultaneously as recombination and luminescence centers. The electrons are trapped at some intrinsic traps of different types, e.g. at anion vacancies forming F centers, and accordingly, the observed TL glow peaks corresponds to the release of electrons from these traps. The detrapped electrons during thermal stimulation recombine with the holes at the (Dy3+ )+ centers resulting (Dy3+ )* ions, which then emit their specific luminescence. Generally, it is assumed that above the room temperature F centers are mobile and disappear due to recombination with completely defects. Thus, the defects responsible for high temperature glow peaks should be some complexes or aggregates with participation of F centers.

4. Phenomenological model The simulation of the thermoluminescence process in the pure K2 GdF5 and Dy3+ doped K2 GdF5 has been performed by using a three and four electrons trapping levels and one kind of recombination center. Fig. 4(a) and (b) shows a brief description of the energies levels diagrams of ours proposed models in this paper, for simulating the three and four thermoluminescence glow curves of the pure K2 GdF5 and Dy3+ doped K2 GdF5 , respectively. In this part we use the following parameters: Ni is the concentration of electron traps or hole centers (cm−3 ); si , the frequency factor (s−1 ); Ei , the electron trap depth below the conduction band or hole center energy levels above the valence band (eV); Ai , the valence band to trap transition probability coefficient (cm3 s−1 ) and, the conduction band to hole center transition probability coefficients (cm3 s−1 ). The parameters nc and nv represent the instantaneous concentrations of electrons and holes in the conduction and valence band, respectively. M and m (cm−3 ) are the concentration of hole centers and trapped holes, respectively. The set of simultaneous differential equations governing the process during excitation is: dni = nc (Ni − ni ) Ai , dt

(4)

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Table 3 Sets of trapping parameters used in the simulation presented in this paper. Samples

Levels

Parameters Ni (cm−3 )

Ei (eV)

si (s−1 )

Ai (cm3 s−1 )

Bi (cm3 s−1 )

K2 GdF5

1 (426 K) 2 (458 K) 3 (481 K) L-centre

3.91 × 108 3.50 × 109 1.98 × 108 1.0 × 1010

1.193 1.280 1.422 2.000

1.31 × 1014 1.20 × 1014 2.58 × 1014 2.00 × 1010

8.0 × 10−7 1.5 × 10−6 5.0 × 10−9 1.0 × 10−8

0 0 0 1.95 × 10−5

K2 GdF5 :Dy3+ (5.0 at.%)

1 (426 K) 2 (458 K) 3 (489 K) 4 (507 K) L-centre

3.91 × 108 3.5 × 109 1.98 × 108 6.35 × 108 1.0 × 1010

0.880 1.080 1.600 1.710 2.00

6.60 × 109 9.75 × 1012 1.50 × 1016 3.57 × 1016 2.00 × 1010

8.0 × 10−7 1.5 × 10−6 1.1 × 10−7 1.0 × 10−8 1.0 × 10−8

0 0 0 0 8.0 × 10−6

here (i = 1,. . .,3 for the pure K2 GdF5 ), or For (i = 1,. . .,4 for the Dy3+ doped K2 GdF5 )

here (i = 1,. . .,3 for the pure K2 GdF5 ), or For (i = 1,. . .,4 for the Dy3+ doped K2 GdF5 )

dm = nv (M − m)B − Am mnc , dt

(5)

dm = −Am mnc , dt

dnv = X − B(M − m)nv , dt

(6)

dm dnv  dni dnc = + − dt dt dt dt

(7)

dnc dm  dni = − . dt dt dt

(9)

4

(10)

i=1

3

i=1

4 doped K2 GdF5 we use ˙i−1 For the The governing equations for the heating stage are:

Dy3+



E dni = nc (Ni − ni ) Ai − ni si exp − i dt kB T



,

4 For the Dy3+ doped K2 GdF5 we use ˙i−1 In both cases the thermoluminescence intensity is given by the following expression:

I(T ) = nc m.B.

(8)

Fig. 4. Description of the thermoluminescence mechanism in (a) pure K2 GdF5 and (b) Dy3+ doped K2 GdF5 .

(11)

4.1. Numerical results The relevant sets of differentials equations (Eqs. (1)–(8)) were solved using the Matlab odes23 solver. The simulated TL glows

Fig. 5. Comparison between the experimental (dot line) and the sumilated (open circle) TL glow curves of: (a) the pure K2 GdF5 and (b) the Dy3+ doped K2 GdF5 sample. The set of trapping parameters used in to running the programs are reported in Table 3.

A. Kadari et al. / Optik 127 (2016) 3959–3963

curves (open circle) using the proposed model and the experimental TL glows curves (dot line) for the pure K2 GdF5 and Dy3+ doped K2 GdF5 samples are shown in Fig. 5(a) and (b), respectively. These calculated TL glows curves were obtained by using our proposed models. The simulated curves obtained using our proposed models were in good agreement with the experimental glow curves of pure K2 GdF5 and Dy3+ doped K2 GdF5 . The sets of parameters used in this simulation are given in Table 3. 5. Conclusion In this work a detailed kinetics study has been presented. The sets of trapping parameters have been evaluated using the Chen’s peak shape PS method (these parameters are the kinetics order b the activation energies E and frequencies factors s). The additional high temperature TL glow peak at 234 ◦ C has been attributed to the energy transfer that occurs from Gd3+ to Dy3+ . The mechanisms of this charge transfer have been studied carefully based on the excitation spectra of the Dy3+ doped K2 GdF5 crystal at the wavelengths 575 nm and 485 nm, presented in the previously published studies. We can also conclude that our presented models describe fairly well the thermoluminescence mechanisms in the pure K2 GdF5 and the Dy3+ doped K2 GdF5 . Acknowledgements This work has been partially supported by the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Fundac¸ão de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and Comissão Nacional de Energia Nuclear (CNEN) and the Russian Foundation for Basic Research (RFBR Grants 10-02-91167 and 10-03-90305). References [1] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, USA, 1985, pp. 374.

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