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UV-dosimetric material based on KMgF3 perovskite
PII:
Radiation Measurements Vol. 29, No. 3±4, pp. 337±340, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1350-4487(98)00054-7 1350-4487/98 $19.00 + 0.00
UV-DOSIMETRIC MATERIAL BASED ON KMGF3 PEROVSKITE A. V. GEKTIN, I. M. KRASOVITSKAYA and N. V. SHIRAN Institute for Single Crystals, 60 Lenin Ave., 310001, Kharkov, Ukraine
1
AbstractÐIt is shown that KMgF3(Eu) crystals eciently store energy from 250±320 nm UV irradiation (the wavelength range most dangerous for health). The composition of anion and cation dopant determines both the intensity and the spectra of the thermostimulated luminescence. The mechanism of energy storage resulting from UV irradiation with energy lower than the band gap is proposed. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION The deterioration of the atmospheric ozone layer has resulted in a growing interest for techniques and materials which provide dosimetry of the ultraviolet (UV) irradiation in the 250±320 nm range. This wavelength region is the most dangerous for health (Herlihy et al., 1994). UV sensitive materials (Nagpal and Kathuria, 1981; Aguirre et al., 1991; Hsu et al., 1993) are usually not selective dosimeters, since their excitation spectrum includes wavelengths outside the dangerous UV range. In some materials, the induced UV storage is bleached by visible light. KMgF3(Eu) crystals eciently store ionizing energy (Furetta et al., 1990; Shiran et al., 1995; Gektin et al., 1997). In spite of the fact that the band gap (Eg) in such ¯uoroperovskites as KMgF3, NaMgF3, RbMgF3 is large (011 eV), the thermostimulated luminescence (TSL) was excited by the energy lower than Eg (Kristianpoller and Trieman, 1983). This indicates that an energy transfer mechanism is not required for electron transition through the conduction band. This means that ABX3 type materials are sensitive to irradiation with energies smaller than Eg. The goal of the present study is to determine the sensitivity of KMgF3 crystals to UV irradiation and to verify the mechanisms of UV storage.
2. EXPERIMENT AND RESULTS Pure and Eu-doped KMgF3 crystals were grown by vertical Bridgman method in a reactive atmosphere in graphite crucibles using specially prepared stoichiometric material. Four types of crystals were studied: as-grown, subjected to high-temperature annealing in oxygen atmosphere, EuF3-doped and Eu2O3-doped. Optical absorption spectra of initial and UVexposed samples were recorded by UV-VIS spec337
trometer M-40. UV irradiation was performed using a Hg-lamp (10ÿ5 W cmÿ2) with 254 nm ®lter at room temperature. Thermoluminescence were measured at constant heating rate of 0.288C sÿ1. TSL spectra were recorded by double monochromatic luminescence spectrometer SDL type with photomultiplier PMT-100. Oxygen-enriched KMgF3 crystals have an absorption band in the 230 nm region which is our previously proposed explanation of O2ÿ ions (Shiran et al., 1995; Gektin et al., 1997). Intense absorption bands in the 240±280 nm range correspond to transitions from the main to excited states in Eu2+ ions (Fig. 1). Absorption spectra of the UV irradiated samples do not considerably dier from that of unirradiated samples. Only after a prolonged exposure (more than 4 h) do weak absorption bands with maxima at 193 and 330 nm appear. Using the sensitive TSL method allows us to notice the eect of the UV irradiation. Figure 2 shows that the lowest TSL intensity is observed in the samples that have no noticeable bands in the UV range, whereas in the samples that contain oxygen impurity the storage eciency is substantially higher. An additional peak at Tm=5608C appears. Europium oxide (not ¯uoride) doping stimulates the appearance of the intense peak at 3958C, together with the general increase of TSL intensity. The high-temperature TSL peaks (with Tm>4008C), as well as in the case of g-irradiation, are conditioned by O2ÿ impurity and have the spectral emission corresponding to radiative transitions in such ions. The emission of the samples with O2ÿ ions has complex spectrum consisting of overlapping bands in the 400±600 nm range. Doping by Eu2+ results in the appearance of the additional Eu2+ luminescence at 359 nm (f±f transition) (Garsia et al., 1988). The intensity and spectral composition of the TL of UV exposed crystals depends on the Eu2O3 concentration (Figs 3 and 4). For Eu2O3 concentration
A. V. GEKTIN et al.
338
Fig. 1. Absorption spectra of KMgF3(Eu) crystals grown from the melt containing (1) 0.01%, (2) 0.1%, (3) 1.5% of Eu2O3.
less than <0.1%, color centers luminescence (F2, 590 nm; F3, 460 nm), as well as oxygen (550 nm) and Eu2+ ions (359 nm) emissions, are observed in the thermoluminescence. At higher Eu2+ concentrations practically all the stored energy is emitted through the 359 nm line in all TSL peaks, except the highest temperature one (Tm=5608C). In this peak, along with the 359 nm emission of Eu2+ ions, emission characteristic of the competing oxygen luminescence can be observed. Figure 4 shows the dependence of various TSL peak intensities on the Eu2O3 concentration. The intensity of the highest TSL peak (3958C) grows quasilinearly with concentration of Eu2O3 up to 1% in the melt.
3. DISCUSSION The eciency of UV storage in KMgF3(Eu) is determined by optical absorption of Eu2+ ions in
Fig. 2. TSL curves or UV irradiated (10 min) KMgF3 crystals: pure (1), oxygen enriched (2), doped by equal concentration of Eu (1 at%) in form of EuF3 (3) and Eu2O3 (4).
Fig. 3. Thermoluminescence spectra of UV irradiated (10 min) KMgF3(Eu2O3) crystals with concentration of Eu2O3 in the melt: 1, 0.01%, T < 3508C; 2, 0.01%, T>3508C; 3, 1.5%, 20±6008C.
the UV range. This region coincides with the most dangerous wavelength range (l < 320 nm) for health. While the excitation wavelength increases from 260 to 320 nm, the maximum permissible dose for UV irradiation grows too. It is noteworthy that the energy stored in KMgF3(Eu) can not be bleached by visible light. One can suppose that absorption in the UV range (200±230 nm) of nominally pure KMgF3 crystals occurs in centers whose excitation (or ionization) leads to ®lling of the available traps by charge carriers, or to the creation of such traps. The latter defects are not the vacancies since their absorption bands are situated in the near UV region, e.g. their energies are half the KMgF3 energy gap (Eg311.5 eV). The appearance of additional absorption after high-temperature anneal-
Fig. 4. Dependence of TSL peak intensities on the concentration of Eu2O3 in KMgF3. UV irradiation, 10 min, 208C. Emission maximum is 359 nm. 2208C (1), 3958C (2) and 5608C (3).
UV-DOSIMETRIC MATERIAL BASED ON KMgF3 PEROVSKITE ing of samples in atmospheric air indicates that they are enriched by oxygen anions. However, as one can see in Fig. 2, curve 2, when the oxygen content is increased, the TSL intensity grows, and additional peaks appear in the high-temperature range (T>4508C). The energy storage during g-irradiation and its release during heating can be understood in the framework of the classic description of the TSL process (McKeever et al., 1985), i.e. the model of thermally activated release of electrons from traps and the electron transition through the conduction band to luminescence centers. The situation in the case of UV irradiation is much more complicated, since the excitation energy equals practically one half Eg. That is why we shall consider this case in more detail. The energy storage mechanism in KMgF3 crystals, with traces of oxygen, during UV irradiation can be proposed as follows. During the UV exposure, O2ÿ ions are excited, with further ionization and tunnel transition of electrons to adjacent anion vacancies with the formation of F centers and Oÿ ions. Further heating releases electrons. Then electrons again are trapped by Oÿ ions, creating excited (O2ÿ)*. (O2ÿ)* relaxation leads to the 550 nm emission. The hole center Oÿ absorbs at 330 nm and is stable up to 3508C. Thus, the oxygen center is both a trap and an emission center simultaneously. The mechanism of energy storage and TSL in Eu-doped KMgF3 crystals diers from the above mentioned. In this case, the emission center is the Eu2+ ion. The mechanism of UV storage in KMgF3(Eu2O3) crystals is based on the electron transition from the UV-excited oxygen ion to the Eu2+ ion. A probable scheme of energy storage and release under the UV irradiation of KMgF3(Eu2O3) can be represented as follows: UV
T
Eu2 . . . O2ÿ ÿ ÿ4 ÿ Eu Oÿ ÿ ÿ4 ÿ Eu2 . . . O2ÿ 4 4Eu2 . . . O
2ÿ
hv 359 nm hv 550 nm:
1
The process diers from the one occurring in KMgF3(O) crystals: here, under the UV irradiation, the electron does not move from the O2ÿ center to the anion vacancy (v+a), but to an adjacent Eu2+ ion. Thus, the Eu2+ transforms to the Eu+ resulting in the appearance of a TSL peak at Tm=3958C. Heating the UV-exposed sample to higher temperatures empties the electron trap, and the inverse transition can occur: Eu 4 Eu2 4Eu
2
hv 359 nm:
2
The process is accompanied by the appearance of a characteristic Eu2+ emission line at 359 nm.
339
4. CONCLUSION It has been shown that the UV sensitivity of KMgF3 is mainly determined by the dopant. Oxygen doping causes the appearance of deep traps (>4508C) and emission at lm=550 nm. When the crystals are doped by Eu2+, the electron transition from O2ÿ centers to Eu2+ results in the formation of Eu+ centers which are stable up to 3958C. The competition between the admixture luminescence centers in KMgF3(O,Eu) crystals determines the spectral composition of the emission of the TSL peaks: a 550 nm band when only O2ÿ is available, a line at 359 nm corresponding to the luminescence of Eu2+, or both emissions in the Eu2O3 doped crystal. It has been shown that KMgF3(Eu2O3) crystals with the optimal dopant concentration are prospective materials for UV dosimetry and that they possess the following properties: . ecient energy storage during the UV irradiation in the 240±300 nm range; . intense high temperature TSL peak (3958C) (no fading at room temperature); . intensity of the main TSL peak grows proportional to the increase of UV exposure; . easily detectable TSL emission (at 359 nm); . high transparency of UV-irradiated samples in the emission range; . store energy is not bleached by visible light.
REFERENCES Aguirre de Career, A., Lifante, G., Cusso, F. and Jaque, F. (1991) Eu-doped alkali halides as a selective UVdosimeter material in actinic region. Applied Physics Letters 58, 1825±1831. Furetta, C., Bacci, C., Rispoli, B. and Scacco, A. (1990) Luminescence and dosimetric performances of KMgF3 crystals doped with metal impurity ions. Radiative Protection Dosimetry 33, 107±110. Garsia, J. M., Sibley, W. and Hunt, C. (1988) Optical properties of Eu2+, Eu3+ and Mn2+ in cubic perovskite crystals. J. Luminescence 42, 35±40. Gektin, A. V., Krasovitskaya, I. M. and Shiran, N. V. (1997) High temperature thermoluminescence of KMgF3 based crystals. J. Luminescence 72±74, 664± 666. Herlihy, E., Gies, P. H. and Roy, L. R. (1994) Personal dosimetry of solar UV-radiation for dierent outdoor activities. Photochemistry and Photobiology 60, 288± 294. Hsu, P. S., Wang, C. D., Weng, P. S. and Li, S. H. (1993) Ultraviolet response of CaF2 ±Tm phosphor with double temperature treatment. Radiation Protection Dosimetry 47, 235±238. Kristianpoller, N. and Trieman, B. (1983) Irradiation eects in perovskite-type ¯uoride. Radiation Eects 72, 201±208. McKeever, S. W. S., Rhodes, J. E. and Mathur, V. K. (1985) Numerical solution to the rate equation governing in simultaneous release of electron and hole during thermoluminescence and isothermal decay. Physical Review 32B, 3835±3843.
340
A. V. GEKTIN et al.
Nagpal, J. S. and Kathuria, V. K. (1981) Some of studies of thermoluminescence MgF2 phosphores. The International Journal Applied Radiation and Isotopes 32, 147±151.
Shiran, N. V., Gektin, A. V., Komar, V. K., Krasovitskaya, I. M. and Shlyakhturov, V. V. (1995) Thermoluminescence of KMgF3(RE) crystals. Radiation Measurements 24, 435±437.
Radiation Measurements Vol. 29, No. 3±4, pp. 337±340, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1350-4487(98)00054-7 1350-4487/98 $19.00 + 0.00
UV-DOSIMETRIC MATERIAL BASED ON KMGF3 PEROVSKITE A. V. GEKTIN, I. M. KRASOVITSKAYA and N. V. SHIRAN Institute for Single Crystals, 60 Lenin Ave., 310001, Kharkov, Ukraine
1
AbstractÐIt is shown that KMgF3(Eu) crystals eciently store energy from 250±320 nm UV irradiation (the wavelength range most dangerous for health). The composition of anion and cation dopant determines both the intensity and the spectra of the thermostimulated luminescence. The mechanism of energy storage resulting from UV irradiation with energy lower than the band gap is proposed. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION The deterioration of the atmospheric ozone layer has resulted in a growing interest for techniques and materials which provide dosimetry of the ultraviolet (UV) irradiation in the 250±320 nm range. This wavelength region is the most dangerous for health (Herlihy et al., 1994). UV sensitive materials (Nagpal and Kathuria, 1981; Aguirre et al., 1991; Hsu et al., 1993) are usually not selective dosimeters, since their excitation spectrum includes wavelengths outside the dangerous UV range. In some materials, the induced UV storage is bleached by visible light. KMgF3(Eu) crystals eciently store ionizing energy (Furetta et al., 1990; Shiran et al., 1995; Gektin et al., 1997). In spite of the fact that the band gap (Eg) in such ¯uoroperovskites as KMgF3, NaMgF3, RbMgF3 is large (011 eV), the thermostimulated luminescence (TSL) was excited by the energy lower than Eg (Kristianpoller and Trieman, 1983). This indicates that an energy transfer mechanism is not required for electron transition through the conduction band. This means that ABX3 type materials are sensitive to irradiation with energies smaller than Eg. The goal of the present study is to determine the sensitivity of KMgF3 crystals to UV irradiation and to verify the mechanisms of UV storage.
2. EXPERIMENT AND RESULTS Pure and Eu-doped KMgF3 crystals were grown by vertical Bridgman method in a reactive atmosphere in graphite crucibles using specially prepared stoichiometric material. Four types of crystals were studied: as-grown, subjected to high-temperature annealing in oxygen atmosphere, EuF3-doped and Eu2O3-doped. Optical absorption spectra of initial and UVexposed samples were recorded by UV-VIS spec337
trometer M-40. UV irradiation was performed using a Hg-lamp (10ÿ5 W cmÿ2) with 254 nm ®lter at room temperature. Thermoluminescence were measured at constant heating rate of 0.288C sÿ1. TSL spectra were recorded by double monochromatic luminescence spectrometer SDL type with photomultiplier PMT-100. Oxygen-enriched KMgF3 crystals have an absorption band in the 230 nm region which is our previously proposed explanation of O2ÿ ions (Shiran et al., 1995; Gektin et al., 1997). Intense absorption bands in the 240±280 nm range correspond to transitions from the main to excited states in Eu2+ ions (Fig. 1). Absorption spectra of the UV irradiated samples do not considerably dier from that of unirradiated samples. Only after a prolonged exposure (more than 4 h) do weak absorption bands with maxima at 193 and 330 nm appear. Using the sensitive TSL method allows us to notice the eect of the UV irradiation. Figure 2 shows that the lowest TSL intensity is observed in the samples that have no noticeable bands in the UV range, whereas in the samples that contain oxygen impurity the storage eciency is substantially higher. An additional peak at Tm=5608C appears. Europium oxide (not ¯uoride) doping stimulates the appearance of the intense peak at 3958C, together with the general increase of TSL intensity. The high-temperature TSL peaks (with Tm>4008C), as well as in the case of g-irradiation, are conditioned by O2ÿ impurity and have the spectral emission corresponding to radiative transitions in such ions. The emission of the samples with O2ÿ ions has complex spectrum consisting of overlapping bands in the 400±600 nm range. Doping by Eu2+ results in the appearance of the additional Eu2+ luminescence at 359 nm (f±f transition) (Garsia et al., 1988). The intensity and spectral composition of the TL of UV exposed crystals depends on the Eu2O3 concentration (Figs 3 and 4). For Eu2O3 concentration
A. V. GEKTIN et al.
338
Fig. 1. Absorption spectra of KMgF3(Eu) crystals grown from the melt containing (1) 0.01%, (2) 0.1%, (3) 1.5% of Eu2O3.
less than <0.1%, color centers luminescence (F2, 590 nm; F3, 460 nm), as well as oxygen (550 nm) and Eu2+ ions (359 nm) emissions, are observed in the thermoluminescence. At higher Eu2+ concentrations practically all the stored energy is emitted through the 359 nm line in all TSL peaks, except the highest temperature one (Tm=5608C). In this peak, along with the 359 nm emission of Eu2+ ions, emission characteristic of the competing oxygen luminescence can be observed. Figure 4 shows the dependence of various TSL peak intensities on the Eu2O3 concentration. The intensity of the highest TSL peak (3958C) grows quasilinearly with concentration of Eu2O3 up to 1% in the melt.
3. DISCUSSION The eciency of UV storage in KMgF3(Eu) is determined by optical absorption of Eu2+ ions in
Fig. 2. TSL curves or UV irradiated (10 min) KMgF3 crystals: pure (1), oxygen enriched (2), doped by equal concentration of Eu (1 at%) in form of EuF3 (3) and Eu2O3 (4).
Fig. 3. Thermoluminescence spectra of UV irradiated (10 min) KMgF3(Eu2O3) crystals with concentration of Eu2O3 in the melt: 1, 0.01%, T < 3508C; 2, 0.01%, T>3508C; 3, 1.5%, 20±6008C.
the UV range. This region coincides with the most dangerous wavelength range (l < 320 nm) for health. While the excitation wavelength increases from 260 to 320 nm, the maximum permissible dose for UV irradiation grows too. It is noteworthy that the energy stored in KMgF3(Eu) can not be bleached by visible light. One can suppose that absorption in the UV range (200±230 nm) of nominally pure KMgF3 crystals occurs in centers whose excitation (or ionization) leads to ®lling of the available traps by charge carriers, or to the creation of such traps. The latter defects are not the vacancies since their absorption bands are situated in the near UV region, e.g. their energies are half the KMgF3 energy gap (Eg311.5 eV). The appearance of additional absorption after high-temperature anneal-
Fig. 4. Dependence of TSL peak intensities on the concentration of Eu2O3 in KMgF3. UV irradiation, 10 min, 208C. Emission maximum is 359 nm. 2208C (1), 3958C (2) and 5608C (3).
UV-DOSIMETRIC MATERIAL BASED ON KMgF3 PEROVSKITE ing of samples in atmospheric air indicates that they are enriched by oxygen anions. However, as one can see in Fig. 2, curve 2, when the oxygen content is increased, the TSL intensity grows, and additional peaks appear in the high-temperature range (T>4508C). The energy storage during g-irradiation and its release during heating can be understood in the framework of the classic description of the TSL process (McKeever et al., 1985), i.e. the model of thermally activated release of electrons from traps and the electron transition through the conduction band to luminescence centers. The situation in the case of UV irradiation is much more complicated, since the excitation energy equals practically one half Eg. That is why we shall consider this case in more detail. The energy storage mechanism in KMgF3 crystals, with traces of oxygen, during UV irradiation can be proposed as follows. During the UV exposure, O2ÿ ions are excited, with further ionization and tunnel transition of electrons to adjacent anion vacancies with the formation of F centers and Oÿ ions. Further heating releases electrons. Then electrons again are trapped by Oÿ ions, creating excited (O2ÿ)*. (O2ÿ)* relaxation leads to the 550 nm emission. The hole center Oÿ absorbs at 330 nm and is stable up to 3508C. Thus, the oxygen center is both a trap and an emission center simultaneously. The mechanism of energy storage and TSL in Eu-doped KMgF3 crystals diers from the above mentioned. In this case, the emission center is the Eu2+ ion. The mechanism of UV storage in KMgF3(Eu2O3) crystals is based on the electron transition from the UV-excited oxygen ion to the Eu2+ ion. A probable scheme of energy storage and release under the UV irradiation of KMgF3(Eu2O3) can be represented as follows: UV
T
Eu2 . . . O2ÿ ÿ ÿ4 ÿ Eu Oÿ ÿ ÿ4 ÿ Eu2 . . . O2ÿ 4 4Eu2 . . . O
2ÿ
hv 359 nm hv 550 nm:
1
The process diers from the one occurring in KMgF3(O) crystals: here, under the UV irradiation, the electron does not move from the O2ÿ center to the anion vacancy (v+a), but to an adjacent Eu2+ ion. Thus, the Eu2+ transforms to the Eu+ resulting in the appearance of a TSL peak at Tm=3958C. Heating the UV-exposed sample to higher temperatures empties the electron trap, and the inverse transition can occur: Eu 4 Eu2 4Eu
2
hv 359 nm:
2
The process is accompanied by the appearance of a characteristic Eu2+ emission line at 359 nm.
339
4. CONCLUSION It has been shown that the UV sensitivity of KMgF3 is mainly determined by the dopant. Oxygen doping causes the appearance of deep traps (>4508C) and emission at lm=550 nm. When the crystals are doped by Eu2+, the electron transition from O2ÿ centers to Eu2+ results in the formation of Eu+ centers which are stable up to 3958C. The competition between the admixture luminescence centers in KMgF3(O,Eu) crystals determines the spectral composition of the emission of the TSL peaks: a 550 nm band when only O2ÿ is available, a line at 359 nm corresponding to the luminescence of Eu2+, or both emissions in the Eu2O3 doped crystal. It has been shown that KMgF3(Eu2O3) crystals with the optimal dopant concentration are prospective materials for UV dosimetry and that they possess the following properties: . ecient energy storage during the UV irradiation in the 240±300 nm range; . intense high temperature TSL peak (3958C) (no fading at room temperature); . intensity of the main TSL peak grows proportional to the increase of UV exposure; . easily detectable TSL emission (at 359 nm); . high transparency of UV-irradiated samples in the emission range; . store energy is not bleached by visible light.
REFERENCES Aguirre de Career, A., Lifante, G., Cusso, F. and Jaque, F. (1991) Eu-doped alkali halides as a selective UVdosimeter material in actinic region. Applied Physics Letters 58, 1825±1831. Furetta, C., Bacci, C., Rispoli, B. and Scacco, A. (1990) Luminescence and dosimetric performances of KMgF3 crystals doped with metal impurity ions. Radiative Protection Dosimetry 33, 107±110. Garsia, J. M., Sibley, W. and Hunt, C. (1988) Optical properties of Eu2+, Eu3+ and Mn2+ in cubic perovskite crystals. J. Luminescence 42, 35±40. Gektin, A. V., Krasovitskaya, I. M. and Shiran, N. V. (1997) High temperature thermoluminescence of KMgF3 based crystals. J. Luminescence 72±74, 664± 666. Herlihy, E., Gies, P. H. and Roy, L. R. (1994) Personal dosimetry of solar UV-radiation for dierent outdoor activities. Photochemistry and Photobiology 60, 288± 294. Hsu, P. S., Wang, C. D., Weng, P. S. and Li, S. H. (1993) Ultraviolet response of CaF2 ±Tm phosphor with double temperature treatment. Radiation Protection Dosimetry 47, 235±238. Kristianpoller, N. and Trieman, B. (1983) Irradiation eects in perovskite-type ¯uoride. Radiation Eects 72, 201±208. McKeever, S. W. S., Rhodes, J. E. and Mathur, V. K. (1985) Numerical solution to the rate equation governing in simultaneous release of electron and hole during thermoluminescence and isothermal decay. Physical Review 32B, 3835±3843.
340
A. V. GEKTIN et al.
Nagpal, J. S. and Kathuria, V. K. (1981) Some of studies of thermoluminescence MgF2 phosphores. The International Journal Applied Radiation and Isotopes 32, 147±151.
Shiran, N. V., Gektin, A. V., Komar, V. K., Krasovitskaya, I. M. and Shlyakhturov, V. V. (1995) Thermoluminescence of KMgF3(RE) crystals. Radiation Measurements 24, 435±437.