WET CHEMICAL SYNTHESIS OF Tb3+ AND Eu3+ DOPED Na2Sr2Mg(BO3)2F2 PHOSPHOR FOR TL DOSIMETER

OZA A.H.¹, DHOBLE S.J.²*, DHOBLE N.S.^3
1Department of Physics, RTM Nagpur University, Nagpur, MS, India.
²Department of Physics, RTM Nagpur University, Nagpur, MS, India.
³Sevadal Mahila Mahavidhyalaya, Sakkardara Square, Nagpur, MS, India.
* Corresponding Author : sjdhoble@rediffmail.com

Received : 28-02-2012     Accepted : 06-03-2012     Published : 15-03-2012
Volume : 3     Issue : 1       Pages : 82 - 84
Int J Knowl Eng 3.1 (2012):82-84

Conflict of Interest : None declared
Acknowledgements/Funding : Authors are grateful to Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out this work under the research project (sanctioned letter No. 2011/37P/10/BRNS/144).

This paper reports the luminescent properties of Tb3+ and Eu3+ doped triple mixed borate (Na2Sr2Mg(BO3)2F2) synthesized by simple wet chemical route. Phosphor has been studied for photoluminescence and thermoluminescence characterizations. PL emission spectra of Na2Sr2Mg(BO3)2F2:Tb3+ shows four peaks showing optimum intensity at 545nm monitored at excitation wavelength 370 nm. The emission spectra exhibit four main bands with the maxima at about 490, 545 (the highest one), 587, and 626 nm, which are due to the transitions from the excited state 5D4 to the ground states 7FJ (J=6, 5, 4, 3) of Tb3+ in the host lattice. PL emission spectra of Eu doped Na2Sr2Mg(BO3)2F2 phosphor shows two well resolved peaks around 593nm and 618nm which are assigned to 5D0→7F1 and 5D0→7F2 transition of Eu3+ ion and a peak around 425nm due to 4f65d1→4f7 transition of Eu2+ in the blue region of visible spectrum. Thermoluminescence glow curves of Na2Sr2Mg(BO3)2F2 phosphor doped with Eu3+ and Tb3+ ions exposed to 0.5 kGy irradiation dose of 60Co are illustrated. TL glow curves consist of only one peak which is fairly symmetrical. Glow peak is observed at 221ºC for Eu3+ doped host and TL intensity is about 2 times less than standard CaSO4:Dy. These properties about the phosphor may prove this a good candidate for TL dosimetry.

Photoluminescence, thermoluminescence, Glow peak, Dosimetry

In many of the applications of luminescent materials, inorganic solids doped with rare earth impurities are used. To understand how rare earth impurities make various applications possible, it is necessary to know the luminescent characteristics of these materials. Basically there are four important parameters viz. excitation type and spectrum, relaxation to emitting state and the decay time, emission intensity and the emission spectrum, which determine the utility of rare earth doped phosphors. All these parameters may further depend on the concentration and temperature. This dependence is equally important in the context of utility of the phosphors in various applications. One of the striking applications of these is radiation dosimeter.
A radiation dosimeter is a device, instrument or system that measures or evaluates, either directly or indirectly, the quantities exposure, kerma, absorbed dose or equivalent dose, or their time derivatives (rates), or related quantities of ionizing radiation. A dosimeter along with its reader is referred to as a dosimetry system. Thermoluminescence dosimeters are gaining more and more attention due to their several advantages over others.Thermoluminescence (TL) is a well known technique that widely used in the dose measurement of ionizing radiations such as UV, X-rays, gamma rays and ion beam. An ideal TL dosimeter phosphor is expected to possess certain features such as: a relatively simple glow curve having ideally a single peak with its temperature over 200ºC, same TL response for all energies of ionizing radiation, high sensitivity that includes both a high efficiency light emission and a low threshold dose, low fading, good linearity of the TL signal in the specific useful range of radiation dose. Many phosphors have been prepared, in this context recently, nanosize TL materials have also been reported by Numan Salah et.al [1] and Sahare et.al [2] .
In this paper we are reporting photoluminescent and Thermoluminescent properties of Tb³⁺ and Eu³⁺ doped Na₂Sr₂Mg(BO₃)₂F₂ Phosphor.

Pure and rare earth doped Na₂Sr₂Mg(BO₃)₂F₂ Phosphor was prepared by a simple wet chemical route.The starting materials used were, Sr(NO₃)₂(Merck), NaF(Himedia), Mg(NO₃)₂6H₂O (Merck), H₃BO₃ (Merck) Eu₂O₃ andTb₂O₃ (SD Fine). The starting materials were dissolved in double distilled water in separate beakers and stirred vigorously till constituents dissolved completely. The individual solutions were added dropwise and turnwise to ensure homogeneity of solution followed by vigorous stirring. The solution was put on stirrer at 80ºC till evaporation of volatile constituents. In this way white crystalline powder of Na₂Sr₂Mg(BO₃)₂F₂ phosphor is prepared.

XRD Pattern
[Fig-1] shows X-ray diffraction pattern of Na₂Sr₂Mg(BO₃)₂F₂. As no JCPDS file of this compound is available therefore it might be considered that our compound has acquired the same phase as it shows. The XRD pattern did not indicate the presence of starting constituents and other probable phases which in turn is an indirect evidence for the complete formation of the required compound.

The photoluminescence spectra of rare earth(Tb,Eu) doped Na₂Sr₂Mg(BO₃)₂F₂ phosphor is (recorded using SHIMADZU RF5301 spectrofluorometer with slit width 1.5 nm) given in figure below. The luminescence behavior of rare earth ions (Dy³⁺, Sm³⁺, Eu³⁺) in magnesium fluoroborate Mg₃BO₃F₃ phosphor were first studied by Liu et al. [2] , later optical characterization of Mg₃BO₃F₃ activated with impurities Dy³⁺ , Gd³⁺ , Eu³⁺ and Pr³⁺ phosphors was done by Van der Voort and Blasse (1991) [3] reported in detail.
Emission spectra of Na₂Sr₂Mg(BO₃)₂F₂:Tb³⁺ shows four peaks showing optimum intensity at 545nm monitored at excitation wavelength 370nm.
The emission spectra exhibit four main bands with the maxima at about 490, 545 (the highest one), 587, and 626 nm, which are due to the transitions from the excited state ⁵D₄ to the ground states ⁷F_J (J=6, 5, 4, 3) of Tb³⁺ in the host lattice. From PL characteristics it can be seen that PL intensity is same for concentrations 0.05m% and 0.1m% and then increase as concentrations are increased further(0.3m%---1m%) shown in [Fig-5] [Fig-5] and [Fig-6] depict excitation and emission spectra of Na₂Sr₂Mg(BO₃)₂F₂:Eu³⁺ phosphor with different concentrations under the excitation 345nm wavelength. Two well resolved peaks are observed around 593nm and 618nm,which are assigned to to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ ion and a peak around 425nm due to ⁴f₆⁵d₁→⁴f₇ transition of Eu²⁺ in the blue region of visible spectrum.

Thermoluminescence characterizations are performed on Thermoluminescence Reader Type TL1009 designed and offered by NUCLEONIX SYSTEMS which is a versatile controller based unit, facilitating the user to subject the TL sample under study to the desired heating profile, to record the digitized TL glow curve. This unit stores both integral value and digitized glow curve into EEPROM memory.
The Z_eff of Na₂Sr₂Mg(BO₃)₂F₂ phosphor is estimated 28.39 and it is very high Z_eff value as comparative CaSO₄:Dy and LiF:Mg,Cu,P well known standard TLD phosphors. Thermoluminescence glow curves of Na₂Sr₂Mg(BO₃)₂F₂ phosphor doped with Eu³⁺ ions and exposed to a 0.5 kGy irradiation dose of 60Co relative to CaSO₄:Dy shown in [Fig-7] . TL glow curves consist of only one peak which is fairly symmetrical. the attachment of fluorides due to their large bandgap, are suitable for impurities to create defect centers [4] . There is a prominent glow peak observed at high temperature around 221ËšC due to high energy traps and it is very useful for thermoluminescence dosimetry (TLD) phosphor characteristics. The glow curve is similar for all four concentrations of Eu only the difference in intensity is occurred. With increasing Eu concentration TL intensity also get enhances and observed maximum for 1 mol%. Comparative TL glow curve show the concentration quenching shown in [Fig-9] . Glow curve of CaSO₄:Dy the phosphor which is widely used in TLD or personal dosimetry of ionizing radiations is also given for the comparison [5] .
[Fig-7] depict TL glow curve of Na₂Sr₂Mg(BO₃)₂F₂:Tb³⁺ phosphor. Single glow curve with very less intensity is observed and promonent peak is seen at around 178ËšC.
The intensity of the prominent glow peak. It is seen that TL intensity of Na₂Sr₂Mg(BO₃)₂F₂:Eu phosphor is 2 times less compared to standard CaSO₄:Dy phosphor.

PL characterization of Na₂Sr₂Mg(BO₃)₂F₂ phosphor doped with Tb shows a strong green emission located at 545 nm which is due to the transitions from the excited state ⁵D₄ to the ground.
States ⁷F₅ indicate that it could be a good green phosphor candidate for creating white light in phosphor converted white LEDs.
TL results of rare earth(RE=Tb,Eu) doped Na₂Sr₂Mg(BO₃)₂F₂ fluoroborate phosphor is estimated in this project report. From the results presented here, it is seen that powders of rare earth(RE=Tb,Eu) activated fluoroborates were prepared in one step using simple wet chemical route. In case of Na₂Sr₂Mg(BO₃)₂F₂:Eu³⁺ phosphor TL intensity is 2 times less compared to standard CaSO₄:Dy TLD phosphor. The compound may prove to be good host for other activators as well for thermoluminescence dosimetry.

Authors are grateful to Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out this work under the research project (sanctioned letter No. 2011/37P/10/BRNS/144).

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	Fig. 1- X-ray powder diffraction pattern of Na2Sr2Mg(BO3)2F2 host
	Fig. 2- Excitation spectrum of Na2Sr2Mg(BO3)2F2:Tb3+
	Fig. 3- Emission spectra of Na2Sr2Mg(BO3)2F2:Tb3+phosphor monitored at 370nm excitation wavelength
	Fig. 4- Concentration of Tb3+ ions vs PL peak intensity graph of Na2Sr2Mg(BO3)2F2:Tb3+ phosphor
	Fig. 5- Excitation spectrum of Na2Sr2Mg(BO3)2F2:Eu3+
	Fig. 6- Emission spectra of Na2Sr2Mg(BO3)2F2:Eu3+
	Fig.7- TL glow curves of Na2Sr2Mg(BO3)2F2:Eu3+ phosphor with different concentration of Eu3+ ions.
	Fig. 8- Comparative TL glow curves of (a) Na2Sr2Mg(BO3)2F2:Eu3+ and (b) CaSO4:Dy phosphors
	Fig. 9- Concentration of Eu3+ ions vs TL peak intensity graph of Na2Sr2Mg(BO3)2F2:Eu3+ phosphor
	Fig. 10- TL glow curves of Na2Sr2Mg(BO3)2F2:Tb3+ phosphor with different concentration of Tb3+ ions