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High-temperature thermoluminescence of KMgF3-based crystals
JOURNAL
OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 72-74 (1997) 664--666
High-temperature
thermoluminescence
A.V. Gektin,
I.M. Krasovitskaya,
of KMgF,-based
crystals
N.V. Shiran
Institutefir Single Crystals, Lenin Ave.. 60, Kharkor, 310 001, Ukraine
Abstract Superhigh-temperature crystals. Thermoemission
KMgF,(Eu203)
thermoluminescence peaks (T > 480’ C, emission at 550 nm) were revealed for KMgF,-based for Eu,03-doped crystals is at 360 nm. It has been shown that energy storage efficiency of
is many times higher than that of LiF(Mg, Ti).
Ker;worrls: Perovskite; Color center; Dosimeter; Thermoluminescence
1. Introduction
2. Results and discussion
KMgF3 crystals color effectively under ionizing irradiation with formation of F, Fz, F3 and F2f centers [ 11. Such a variety of color centers results in their step-by-step annealing beginning from 100°C and up to 450°C [2]. High-temperature peaks (T, > 300°C) are also known to be present in the thermostimulated luminescence (TL). For such peaks, emission spectra depend on the presence of oxygen in crystals and its concentration [2, 31. It was shown earlier that KMgF,(Eu) can be used as dosimeters, but the position of the main TL peak was estimated variously: after Ref. [4], it was at 340°C while according to Ref. [3], at 390°C. The heating rate distinction (20 and 0.28”C s- ‘) contribute additionally to the discrepancy indicated. The purpose of this work is to elucidate the high-temperature TL regularities in KMgF, doped by europium and oxygen.
Growth procedures of KMgFJ crystals were described previously [2,3]. Optical characteristics were measured for three crystal types: as-grown, subjected to a high-temperature annealing in oxygen atmosphere and Eu,O,-doped. TL curves for as-grown and oxygen enriched crystals are shown in Fig. 1. Introduction of 02- ions into the crystal lattice is seen to result in the appearance of an additional peak (567°C). The TL spectrum, being the same for both the crystal types, is a band having a maximum at 550 nm (Fig. 2). Under irradiation of Eu,O,-doped KMgF3, the energy storage is much more effective than in pure ones and is manifested both in absorption spectra and in TL. Therewith, distinctions in the positions of glow-curve peaks and in TL spectra are observed: an additional peak with T, = 390°C appears along with a redistribution in intensities of other peaks (Fig. 3). When the activator content increases from 0.01 to 1.5%, the TL in the 550 nm region disappears, while a line of emission at 360 nm arises, which becomes the prevailing one
*Corresponding author. Tel.: 380(0572) 307981; 380(0572) 320207: e-mail: [email protected].
fax:
0022-23 13/97/$17.00 ,c 1997 Elsevier Science B.V. All rights reserved PII SOO22-2313(96)00231-l
A.V. Gektin et al. 1 Journal of’luminescence
2
400
.2 300 ln gj 200 E 72 100
Temperature,
“C
Fig. I. Glow curves of pure (1) and oxidized (2) KMgF3 crystals after X irradiation (10’ Gy). Heat rate 0.28 C s- ’
400
500 Wavelength,
600 nm
Fig. 2. Luminescence spectra of Eu,O,-doped (I) and oxidized (2) KMgF, crystals in glow peaks higher than 350’ C.
600 ? m .g
400 -
kz E 2
200 -
100
200
_ 300
400
Temperature,
500
600
“C
Fig. 3. Glow curves of KMgF,(Eu,O,) (1) and LiF(Mg, Ti) (2) crystals after isodose y-irradiation (24’Am, 60 keV, 0.03 Gy).
already at 0.1% Eu,O,. To compare the TL efficiency of KMgF3(Eu20,) and of known dosimeter, LiF(Mg, Ti) were subjected to an isodose irradiation under identical conditions. As can be seen from Fig. 3, the light sum emitted is several hundred times larger for KMgF3(Eu20,) than for LiF(Mg, Ti).
72-74 (1997) 664-666
665
Note that high-temperature TL peaks are not associated with the simplest color centers, since the thermal stability of those in KMgFj is lower than 450°C [2]. A significant increase of the light sum stored in doped crystals points to the important part played by admixtures in the high-temperature TL realization. It could be assumed that, similar to the case of alkali halide crystals, a double as e.g. in LiF(Mg, Ti), doping is necessary, where one admixture should act as the chargecarrier trap while another as the recombination center. Experimental data, however, do not confirm that assumption. Consider the case of KMgF, enriched oxygen. Oxygen can be supposed to stabilize electron and hole color centers and the TL at 550 nm (when intrinsic color centers have disappeared completely) is due to transitions in the OzP center. Traces of oxygen-containing impurities are known to arise in alkali and alkali-earth fluoride crystals due to pyrolysis, even inspite of all precautions. Annealing of pure crystals at 700°C in oxygen atmosphere results in an increase of 02- ions concentration due to their diffusion in near-surface layers and causes an intensification of 550 nm luminescence. The O’-Vz dipole can be supposed to be a capture center and a luminescence one simultaneously. The double doping (Eu, 0) is consistent to a greater extent with the traditional scheme of the high-effective storage phosphors development. The doping by Eu203 is a much more effective way to enhance the TL intensity than the use of EuF, [3] as the admixture. In this case, Eu2+ ions act partly as recombination centers, this being confirmed by the appearance of the 360 nm line corresponding to f-f transitions. Along with those, however, an emission band with a maximum at 550 nm occurs at low Eu203 concentration, that is characteristic for KMgF,(O). Comparison of Figs. 1 and 3 points also to an additional peak (532°C) evidencing that the introduction of Eu2+ ions results in the rise of additional traps. Thus, the Eu admixture cannot be considered as the recombination center only, though, no doubt, the energy transfer 02- + Eu2+ takes place.
666
A. V. Gektin et al. / Journal of Luminescence
72-74 (1997) 664-666
3. Conclusion
References
The data obtained evidence that 02-and ELI’+ ions act simultaneously as traps and as recombination centers causing high-temperature TL peaks in KMgF,. To increase intensity of those peaks, simultaneous doping by both admixture is the best way. The dosimetric efficiency of KMgF,(Eu,O,) is many times higher than that of traditional LiF(Mg, Ti).
[l] C. Riley and W. Sibley, Phys. Rev. B 1 (1970) 2789. [2] A. Gektin, V. Komar and N. Shiran, IEEE Trans. Nucl. Sci. 42 (1995) 311. [3] N. Shiran, V. Komar and A. Gektin, Radiat. Meas. 24 (1995) 435. [4] C. Furetta, C. Bacci and B. Rispoli, Rad. Prot. Dosim. 33 (1990) 107.
OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 72-74 (1997) 664--666
High-temperature
thermoluminescence
A.V. Gektin,
I.M. Krasovitskaya,
of KMgF,-based
crystals
N.V. Shiran
Institutefir Single Crystals, Lenin Ave.. 60, Kharkor, 310 001, Ukraine
Abstract Superhigh-temperature crystals. Thermoemission
KMgF,(Eu203)
thermoluminescence peaks (T > 480’ C, emission at 550 nm) were revealed for KMgF,-based for Eu,03-doped crystals is at 360 nm. It has been shown that energy storage efficiency of
is many times higher than that of LiF(Mg, Ti).
Ker;worrls: Perovskite; Color center; Dosimeter; Thermoluminescence
1. Introduction
2. Results and discussion
KMgF3 crystals color effectively under ionizing irradiation with formation of F, Fz, F3 and F2f centers [ 11. Such a variety of color centers results in their step-by-step annealing beginning from 100°C and up to 450°C [2]. High-temperature peaks (T, > 300°C) are also known to be present in the thermostimulated luminescence (TL). For such peaks, emission spectra depend on the presence of oxygen in crystals and its concentration [2, 31. It was shown earlier that KMgF,(Eu) can be used as dosimeters, but the position of the main TL peak was estimated variously: after Ref. [4], it was at 340°C while according to Ref. [3], at 390°C. The heating rate distinction (20 and 0.28”C s- ‘) contribute additionally to the discrepancy indicated. The purpose of this work is to elucidate the high-temperature TL regularities in KMgF, doped by europium and oxygen.
Growth procedures of KMgFJ crystals were described previously [2,3]. Optical characteristics were measured for three crystal types: as-grown, subjected to a high-temperature annealing in oxygen atmosphere and Eu,O,-doped. TL curves for as-grown and oxygen enriched crystals are shown in Fig. 1. Introduction of 02- ions into the crystal lattice is seen to result in the appearance of an additional peak (567°C). The TL spectrum, being the same for both the crystal types, is a band having a maximum at 550 nm (Fig. 2). Under irradiation of Eu,O,-doped KMgF3, the energy storage is much more effective than in pure ones and is manifested both in absorption spectra and in TL. Therewith, distinctions in the positions of glow-curve peaks and in TL spectra are observed: an additional peak with T, = 390°C appears along with a redistribution in intensities of other peaks (Fig. 3). When the activator content increases from 0.01 to 1.5%, the TL in the 550 nm region disappears, while a line of emission at 360 nm arises, which becomes the prevailing one
*Corresponding author. Tel.: 380(0572) 307981; 380(0572) 320207: e-mail: [email protected].
fax:
0022-23 13/97/$17.00 ,c 1997 Elsevier Science B.V. All rights reserved PII SOO22-2313(96)00231-l
A.V. Gektin et al. 1 Journal of’luminescence
2
400
.2 300 ln gj 200 E 72 100
Temperature,
“C
Fig. I. Glow curves of pure (1) and oxidized (2) KMgF3 crystals after X irradiation (10’ Gy). Heat rate 0.28 C s- ’
400
500 Wavelength,
600 nm
Fig. 2. Luminescence spectra of Eu,O,-doped (I) and oxidized (2) KMgF, crystals in glow peaks higher than 350’ C.
600 ? m .g
400 -
kz E 2
200 -
100
200
_ 300
400
Temperature,
500
600
“C
Fig. 3. Glow curves of KMgF,(Eu,O,) (1) and LiF(Mg, Ti) (2) crystals after isodose y-irradiation (24’Am, 60 keV, 0.03 Gy).
already at 0.1% Eu,O,. To compare the TL efficiency of KMgF3(Eu20,) and of known dosimeter, LiF(Mg, Ti) were subjected to an isodose irradiation under identical conditions. As can be seen from Fig. 3, the light sum emitted is several hundred times larger for KMgF3(Eu20,) than for LiF(Mg, Ti).
72-74 (1997) 664-666
665
Note that high-temperature TL peaks are not associated with the simplest color centers, since the thermal stability of those in KMgFj is lower than 450°C [2]. A significant increase of the light sum stored in doped crystals points to the important part played by admixtures in the high-temperature TL realization. It could be assumed that, similar to the case of alkali halide crystals, a double as e.g. in LiF(Mg, Ti), doping is necessary, where one admixture should act as the chargecarrier trap while another as the recombination center. Experimental data, however, do not confirm that assumption. Consider the case of KMgF, enriched oxygen. Oxygen can be supposed to stabilize electron and hole color centers and the TL at 550 nm (when intrinsic color centers have disappeared completely) is due to transitions in the OzP center. Traces of oxygen-containing impurities are known to arise in alkali and alkali-earth fluoride crystals due to pyrolysis, even inspite of all precautions. Annealing of pure crystals at 700°C in oxygen atmosphere results in an increase of 02- ions concentration due to their diffusion in near-surface layers and causes an intensification of 550 nm luminescence. The O’-Vz dipole can be supposed to be a capture center and a luminescence one simultaneously. The double doping (Eu, 0) is consistent to a greater extent with the traditional scheme of the high-effective storage phosphors development. The doping by Eu203 is a much more effective way to enhance the TL intensity than the use of EuF, [3] as the admixture. In this case, Eu2+ ions act partly as recombination centers, this being confirmed by the appearance of the 360 nm line corresponding to f-f transitions. Along with those, however, an emission band with a maximum at 550 nm occurs at low Eu203 concentration, that is characteristic for KMgF,(O). Comparison of Figs. 1 and 3 points also to an additional peak (532°C) evidencing that the introduction of Eu2+ ions results in the rise of additional traps. Thus, the Eu admixture cannot be considered as the recombination center only, though, no doubt, the energy transfer 02- + Eu2+ takes place.
666
A. V. Gektin et al. / Journal of Luminescence
72-74 (1997) 664-666
3. Conclusion
References
The data obtained evidence that 02-and ELI’+ ions act simultaneously as traps and as recombination centers causing high-temperature TL peaks in KMgF,. To increase intensity of those peaks, simultaneous doping by both admixture is the best way. The dosimetric efficiency of KMgF,(Eu,O,) is many times higher than that of traditional LiF(Mg, Ti).
[l] C. Riley and W. Sibley, Phys. Rev. B 1 (1970) 2789. [2] A. Gektin, V. Komar and N. Shiran, IEEE Trans. Nucl. Sci. 42 (1995) 311. [3] N. Shiran, V. Komar and A. Gektin, Radiat. Meas. 24 (1995) 435. [4] C. Furetta, C. Bacci and B. Rispoli, Rad. Prot. Dosim. 33 (1990) 107.