Enhanced thermoluminescence properties of CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphor

Abstract

The application of thermoluminescence technique in radiation dosimetry spans field of health physics, biological and geological sciences and personnel monitoring; this led to the search for new compositions with desirable dosimetric properties. In the present work, CaSrAl₂SiO₇:Tb³⁺ and CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphors were prepared by solid-state reaction method and their TL properties were studied in detail. The comparison of their TL results showed that co-doping of Ce³⁺ ions enhanced TL response of CaSrAl₂SiO₇:Tb³⁺ phosphors; this was also verified from measurement of TL emission spectra of the samples. Optimized glow curves were analysed and TL parameters were extracted from Chen’s method. Co-doped CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphor is found to be useful in dosimetric application.

1 Introduction

When an insulating or a superconducting material is exposed to any kind of ionizing radiation, deposited energy is stored in the defect sites and colour centres of the crystal lattice. Due to action of heat energy, a fraction of this stored energy released and emitted as visible light which is called thermoluminescence [1, 2]. Nowadays application of various radiations such as ultraviolet, X-rays, β-rays, γ-rays in the different fields like medical, industrial, agriculture, etc., is increasing [3]. Thermoluminescence (TL) is one of the techniques used in radiation dosimetry [4]. Thermoluminescent materials are more investigated in on-going researches, because they found to have increasing application in thermoluminescence dosimeters. Thermoluminescent dosimeters necessarily have linearity of TL response with exposed radiation dose.

CaSrAl₂SiO₇ is one of the members of melilite group; these melilites that are basically silicate-based materials showed their use in TL dosimetry. In this work, thermoluminescence properties of CaSrAl₂SiO₇:Tb³⁺ phosphor are investigated and also observed that co-doping of Ce³⁺ ions enhanced TL of CaSrAl₂SiO₇:Tb³⁺ phosphor. The effect of different Tb³⁺ concentration in both single and co-doped samples on the TL glow curves has been recorded and glow curve is analysed by computerized glow curve deconvolution method. To the best of our knowledge, there is no earlier report on thermoluminescence investigation on CaSrAl₂SiO₇:Tb³⁺ and CaSrAl₂SiO₇: Ce³⁺,Tb³⁺ phosphors.

2 Synthesis and experimental

Phosphors for the study were prepared by solid-state reaction method at high temperature using analytical reagent (AR) grade raw chemicals with more than 99% accuracy. Calcium carbonate, strontium carbonate, aluminium oxide, silicon di-oxide, ceric oxide and terbium oxide were the starting materials. The amounts of the raw chemicals were calculated according to stoichiometric ratio in CaSrAl₂SiO₇:Ce³⁺ (0.5 mol%),Tb³⁺ (y mol%), where y = 1.0, 3.0, 5.0, 7.0, 10.0 mol%. Required quantities of raw materials were mixed homogeneously using agate mortar and pestle for 3 h. The resultant mixtures were fired at high temperature 1300 °C for 5 h. It was then slowly cooled to room temperature inside the closed furnace. The powders so formed were collected by crushing the prepared samples. Now the samples were ready for all other characterization studies.

Phase of synthesized phosphor was confirmed by X-ray diffraction (XRD) technique using Cu-Kα radiation with the help of D2 phaser Bruker diffractometer. Thermoluminescence investigation of phosphor was performed by TLD reader, Nucleonix TL 1009I with constant heating rate of 5 °C/s. As sample must be exposed to any kind of radiation before measurement of TL, UV cabinet is used, where the samples were exposed to UV radiation of 254 nm. Band pass filters of different wavelengths have been used to record TL emission spectrum. To record TL emission spectrum, the processes used in recording TL glow curves were repeated; here the sample that is placed in canthal strip was covered with band pass filter. The band pass filter allows the signals of selected range of wavelengths; hence in TL emission spectrum, characteristic emission peaks were observed.

3 Results and discussion

3.1 XRD investigation

To verify the preparation of phosphors, they were analysed by XRD tool that reveals the phase composition. Figure 1 shows the XRD patterns of Tb³⁺ singly doped and Ce³⁺,Tb³⁺ co-doped CaSrAl₂SiO₇ phosphors along with JCPDS file. It is clear that phosphors have good crystallization with no any impurity phase, as experimental diffraction patterns well matched with JCPDS file 26-0327 of CaSrAl₂SiO₇. Hence, the as-prepared phosphors have tetragonal crystallographic structure whose space group is P$\overline{4}$2₁ m [5].

Fig. 1

Diffraction patterns of CaSrAl₂SiO₇:Tb³⁺ and CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphors

3.2 Thermoluminescence investigation

Thermoluminescence (TL) is light emission due to moderate heating of a solid, previously exposed to ionizing radiation. The absorption of the ionizing radiation (UV, X-, β- or γ-rays, high-energy particles) creates traps, and fills them (or those already exist) with electrons and/or holes even at low temperatures. The subsequent heating migrates the electrons and/or holes in the lattice, until they fall into other traps (or recombine) with a consequent photon emission [6]. Thermoluminescent emission from TL materials is very sensitive to the amount and nature of dopant element and radiation effect. So in the present research work, we used Ce³⁺ and Tb³⁺ as single and co-dopant ions with their various concentrations; after that TL measurement was performed for each sample with optimized dopant and co-dopant concentration with various ultraviolet (UV) exposure time and then TL kinetic parameters were extracted from the Chen’s peak shape method.

3.2.1 Thermoluminescence of CaSrAl₂SiO₇:Ce³⁺

Let us first focus on the investigated thermoluminescence properties of CaSrAl₂SiO₇:Ce³⁺ phosphor; this was already discussed in our previous work in which Ce³⁺ concentration was varied as 0.1, 0.3, 0.5, 1.0, 2.0, 3.0 and 4.0 mol% [7]. The optimized glow curve was obtained for 0.5 mol% Ce³⁺-doped CaSrAl₂SiO₇ phosphor. This optimized phosphor showed an increment in TL intensity up to 35 min of UV irradiation time. As we further increased UV exposure time, saturation in TL intensity was found.

3.2.2 Thermoluminescence of CaSrAl₂SiO₇:Tb³⁺

At the second attempt, TL properties of CaSrAl₂SiO₇:Tb³⁺ phosphor was seen and this report is dealing here for the first time. Figure 2 represents a series of Tb³⁺ concentration (1.0, 3.0, 5.0, 7.0, 10.0 mol%)-dependent TL glow curves of CaSrAl₂SiO₇:Tb³⁺ samples. TL peak position is independent of the concentration of Tb³⁺ element and exhibits no shift with change in doping concentration. The only variation obtained in the TL peak intensity while increasing Tb³⁺ concentration. It can be understood from the inset of Fig. 2 that TL intensity of sample first increased with rise in Tb³⁺ concentration, attained a highest value at 5.0 mol% of Tb³⁺ ions and then decreased with further increase in Tb³⁺ concentration; this behaviour arises from concentration quenching process after a particular Tb³⁺ concentration. As we increase impurity concentration, there is an increase in the number of defects/traps which in turn implies a growth in the density of charge carriers being trapped upon irradiation. Therefore the initial rise in the TL peak intensity or area of the glow curves. Furthermore, on being thermally stimulated, these charge carriers release from traps which in turn recombine with their counterparts at the recombination centre and yield diverse TL glow peaks with elevated height [8].

Fig. 2

Concentration-dependent TL glow curves of CaSrAl₂SiO₇:yTb³⁺ phosphors with 10-min UV exposure time

As thermoluminescence properties are greatly affected by exposure time, TL property of CaSrAl₂SiO₇:Tb³⁺ (5.0 mol%) phosphor was measured with different UV exposure time. For variable UV exposure time, some of the selected TL glow curves of CaSrAl₂SiO₇:Tb³⁺ (5.0 mol%) are represented in Fig. 3. Spectral behaviour was same with increase in length of UV exposure time but peak TL intensity or glow curve area altered with increase in exposure time. The growth in TL response was observed as exposure time was boosted and after 60 min of UV exposure TL response diminished with more added exposure time. With increasing irradiation dose more and more trapping centres or luminescent centres responsible for the TL glow peaks are getting filled. Upon thermal stimulation, these traps liberate their charge carriers and they get recombine with their counterparts, giving rise to different glow peaks. When all the trapping centres that are subjected to desired TL emission get filled, the saturation or decrease in the TL intensity starts appearing [8,9,10].

Fig. 3

Effect of UV exposure time on the TL glow curve of CaSrAl₂SiO₇:Tb³⁺ (5.0 mol%) phosphor

3.2.3 Thermoluminescence of CaSrAl₂SiO₇:Ce³⁺,Tb³⁺

Thermoluminescence experiments were performed on the series of CaSrAl₂SiO₇:xCe³⁺ and CaSrAl₂SiO₇:yTb³⁺ phosphors that the results point out optimal concentration of Ce³⁺ as x = 0.5 mol% and of Tb³⁺ as y = 5.0 mol%. In the present work, co-doping of Ce³⁺ and Tb³⁺ was also performed in CaSrAl₂SiO₇ host and TL properties of this co-doped sample were also investigated and compared with TL of singly doped samples. Co-doping was performed with fixed concentration of Ce³⁺ (x = 0.5 mol%) and variable concentration of Tb³⁺ (y = 1.0, 3.0, 5.0, 7.0, 10.0 mol%).

Figure 4 shows TL glow curves of CaSrAl₂SiO₇:Ce³⁺ (x = 0.5 mol%), Tb³⁺ (y = 1.0, 3.0, 5.0, 7.0, 10.0 mol%) phosphors with fixed UV irradiation time. Glow curves for all samples had similar shape but with change in glow curve area. Concentration quenching occurred at 5.0 mol% of Tb³⁺ ions (see inset of Fig. 4). Like Ce³⁺ and Tb³⁺ singly doped samples, impact of various UV irradiation time on the TL glow curve of optimized co-doped CaSrAl₂SiO₇:Ce³⁺ (0.5 mol%),Tb³⁺ (5.0 mol%) sample was seen and this result is depicted in Fig. 5. It can be clearly seen from Fig. 5 that TL intensity of optimized co-doped phosphor increased up to 100 min of UV exposure and then decreased.

Fig. 4

Tb³⁺ concentration-dependent TL glow curves of CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphor with 30-min UV exposure time

Fig. 5

Effect of UV exposure time on the TL glow curve of CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphor

3.2.4 Evaluation of TL parameters

To determine TL parameters, broad glow curves of 60-min UV exposed CaSrAl₂SiO₇:Tb³⁺ (5.0 mol%) and 100-min UV exposed CaSrAl₂SiO₇:Ce³⁺ (0.5 mol%),Tb³⁺ (5.0 mol%) samples were first applied to computerized glow curve deconvolution (CGCD) method. Both samples were found to have four overlapping peaks in their parent glow curves. The position of constituent peaks is shown in Fig. 6. TL parameters like geometrical shape factor (μ_g), order of kinetics (b), frequency factor (S) and activation energy (E) were extracted using Chen’s peak shape method. The calculated TL parameters of Tb³⁺-doped and Ce³⁺,Tb³⁺ co-doped CaSrAl₂SiO₇ phosphors are listed in Table 1.

Fig. 6

Representation of peak deconvolution on parent glow curve of a CaSrAl₂SiO₇:Tb³⁺ and b CaSrAl₂SiO₇: Ce³⁺,Tb³⁺ phosphors

Table 1 Calculated TL parameters

3.2.5 TL emission spectra

Figure 7 represents comparison of TL emission spectra of CaSrAl₂SiO₇:Ce³⁺, CaSrAl₂SiO₇:Tb³⁺ and CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphors. Emission spectra were recorded using interference band pass filters of various wavelengths from 400 to 700 nm. Emission spectra of Ce³⁺-doped sample was already discussed in our previous article [7], here it is shown for comparison only. The three samples have characteristic emission peaks of respective dopant and co-dopant ions. CaSrAl₂SiO₇:Tb³⁺ sample has emission peaks at around 480, 545 and 590 nm these are due to characteristic transition ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅ and ⁵D₄ → ⁷F₄ of Tb³⁺ ions [11]. CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ sample has characteristic peak of Ce³⁺ ions at around 420 nm, due to 5d (⁵D_3/2) → 4f (²F_5/2) [12, 13]and characteristic peaks of Tb³⁺ ions situated at around 480, 545 and 600 nm; these are due to above-mentioned transitions of Tb³⁺ ions.

Fig. 7

TL Emission spectra of CaSrAl₂SiO₇:Ce³⁺, CaSrAl₂SiO₇:Tb³⁺ and CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphors

Figure 8 shows the comparative TL glow curves of the three optimized phosphors. Ce³⁺,Tb³⁺ co-doped phosphor has most intense TL properties among all three. Also from Fig. 7 it is clearly visible that TL emission of cerium ions is decreased, while TL emission of terbium ions is increased in co-doped sample; hence co-doping of small amount of Ce³⁺ ions, i.e., 0.5 mol%, may enhance TL emission of Tb³⁺ ions in CaSrAl₂SiO₇ host.

Fig. 8

Comparative TL glow curves of CaSrAl₂SiO₇:Ce³⁺, CaSrAl₂SiO₇:Tb³⁺ and CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphors

Present study concludes that among the three, Ce³⁺,Tb³⁺ co-doped CaSrAl₂SiO₇ phosphor has almost linear increment in total TL intensity with respect to UV exposure time up to 100 min, which is very large range as compared to other UV irradiated silicate, aluminate and aluminosilicate-based phosphors such as Ca₂Al₂SiO₇:Ce³⁺,Tb³⁺ [14]; Ca₂Al₂SiO₇:Ce³⁺[15]; BaMgAl₁₀O₁₇:Ce³⁺ [16]. Hence CaSrAl₂SiO₇:Ce³⁺,Tb³⁺ phosphor may be most appropriately useful for UV dosimeter application.

4 Conclusions

CaSrAl₂SiO₇:yTb³⁺ and CaSrAl₂SiO₇:Ce³⁺,yTb³⁺ phosphors were prepared by solid-state reaction method. Detail comparative TL study of singly doped and co-doped samples was investigated. Concentration-dependent TL glow curves of CaSrAl₂SiO₇:yTb³⁺ phosphor and CaSrAl₂SiO₇:Ce³⁺,yTb³⁺ phosphors were measured and found that 5.0 mol % is optimized concentration of Tb³⁺ ions in both samples. Tb³⁺ singly doped sample showed linear increment in TL intensity up to 60 min, while Ce³⁺,Tb³⁺ co-doped sample showed this linearity up to 100 min, also TL properties of co-doped sample got enhanced with inclusion of 0.5 mol% of Ce³⁺ ions, this was also verified from TL emission spectra. Hence the optimized CaSrAl₂SiO₇:Ce³⁺(0.5 mol%),Tb³⁺(5.0 mol%) phosphor may be a potential candidate for UV dosimeter application.