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Optical and scintillation properties of LiCaAlF6:Eu crystal
Journal of Luminescence 102–103 (2003) 815–818
Optical and scintillation properties of LiCaAlF6:Eu crystal N.V. Shirana,*, A.V. Gektina, S.V. Neichevaa, V.A. Kornienkoa, K. Shimamurab, N. Ishinoseb b
a Institute for Single Crystals, 60 Lenin Avenue, Kharkov 61001, Ukraine Kagami Memorial Laboratory of Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku, Tokyo 169-0051, Japan
Abstract Absorption, photo- and radioluminescence spectra have been investigated for as-grown and irradiated LiCaAlF6:Eu crystal. Eu2+ ions markedly over lap. Irradiation induces Eu3+-Eu2+ transition and F-centers formation. Color centers destruction in the process of crystal annealing is accompanied by thermostimulated luminescence (peaks at 340, 420 and 490 K) with the typical for Eu2+ ion emission (370 nm). Scintillation light output is defined by the thickness of sample, Eu2+ concentration and contribution of factors, controlling Eu3+!Eu2+ transformation in LiCaAlF6:Eu. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Scintillator; Energy storage; Color center; Thermostimulated luminescence
1. Introduction As it has been shown recently [1], Eu-doped LiCaAlF6 (LiCAF) crystals exhibit luminescence (maximum at 370 nm and decay time B1 ms) associated with 4f65d-4f7 transition in Eu2+ ions, substituting Ca2+ ones. These crystals may be used as effective detectors for registration of Xray, b- and g-radiation as well as thermal neutrons. The light yield was estimated to be 30% of NaI:Tl. Spectrometric scintillation parameters as well as absorption and luminescence properties of LiCAF:Eu crystal are similar to that of well-known CaF2:Eu scintillator. Shallow and ultradeep traps were found in Ce3+-doped LiCAF scintillators [2]. Exactly these *Corresponding author. Tel.: +380-572-308-367; fax: +380572-321-082. E-mail address: [email protected] (N.V. Shiran).
traps lead to the energy storage and, as a result, to the afterglow appearance and light output decreasing in LiCAF:Ce irradiated at RT. Europium ions, in contrary to cerium, enter generally in bivalent state into the most alkali and alkali-earth halide crystals. Eu2+ ions are arranged in Ca2+sites in LiCAF lattice and require no compensation. This fact suggests of lack of deep traps in Eu-doped LiCAF. The aim of the present study was to investigate various activator emission and color centers in LiCAF:Eu crystals.
2. Experimental Eu-doped crystals were grown by Czochralski technique in CF4 atmosphere [3]. EuF3 concentration in starting material was 0.5 mol%. Methods of absorption, excitation, emission and
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00647-6
N.V. Shiran et al. / Journal of Luminescence 102–103 (2003) 815–818
816
thermoluminescence were described elsewhere [4]. Spectrometric characteristics were determined using the set ups represented in Ref. [1]. Intensity (arb. un.)
Absorption, excitation and luminescence spectra of LiCAF:Eu crystal are shown in Fig. 1. Bands at 202, 255 nm and in the range of 290–350 nm, characteristic for transitions in Eu2+ ion, are revealed in absorption spectrum. The luminescence excited directly in activator absorption bands consists of a single Gaussian-shaped narrow band with a maximum at 370 nm and a half-width B0.14 eV. Emission and absorption bands overlap, i.e. reabsorption takes place. Main X-ray excited luminescence coincides completely with the same ones observed at photoexcitation. As was found earlier ([1, Fig. 3]), along with the main band at 370 nm some weak emission lines are revealed at X-ray excitation in the long-wavelength region, with the most pronounced one at 594 nm connected with Eu3+ions. The latter points to the presence of an activator ion not only in the bivalent state but also in trivalent state. The intensity of the main radioluminescence (at 370 nm) is strongly temperature dependent (Figs. 2 and 3). The yield of Eu2+ emission decreases with
1
20
2
10
3
(A)
0 1
(B)
10 2
0
3
300 350 Wavelength (nm)
250
400
Fig. 2. X-ray excited luminescence of Eu-doped (A) and pure (B) LiCAF crystals at 163 K (1), 174 K (2) and 206 K (3).
3
Yield (arb. un.)
3. Results
30
20 1
10 2
0 1.0
5
1
2
3
0.5
0
0 200
160
Intensity (arb. un.)
Absorption (cm–1)
10
300 Wavelength (nm)
400
Fig. 1. Absorption (1), excitation (2) and photoluminescence (3) spectra of LiCAF:Eu crystal at room temperature.
200 240 Temperature (K)
Fig. 3. Temperature dependence of STE yield (280 nm) in pure (1) and Eu-doped (2) LiCAF; Eu2+ luminescence (370 nm) in LiCAF:Eu (3).
the temperature rise from 150 K to RT. The UV emission (at 280 nm, half-width near 0.67 eV) connected with the self-trapped excitons (STE) is revealed in pure and doped LiCAF below 200 K. The intensity of this intrinsic emission is far less in Eu-containing crystal than in pure one. Radiation damage study has shown that absorption and luminescence spectra change negligibly if irradiation dose is getting lower (o10 Gy).
N.V. Shiran et al. / Journal of Luminescence 102–103 (2003) 815–818
–1
∆KF (cm )
Induced absorption (cm–1)
3
2
3 2 1
1
0
2
2
0
1
2
3
2
Dose (10 Gy)
1 1
0 200
300 Wavelength (nm)
400
Fig. 4. X-ray induced absorption in pure (1) and Eu-doped (2) LiCAF. Inset: dose dependence of F-center accumulation in pure (1) and Eu-doped (2) crystals.
3
(A)
40
1
2
20
1 10
0
(B)
1 1`
0.5
2`
0 300
400 500 600 Temperature (K)
0 5
TSL-intensity (arb. un.)
30
2 Absorption (cm–1)
Absorption bands connected with F-centers appear at 260 and 267 nm, in pure and doped samples accordingly (Fig. 4). F-center accumulation efficiency is significantly higher in Eu-containing sample. It should be noted that Eu2+- and F-absorption bands increased simultaneously in LiCAF:Eu crystal at irradiation. Afterglow emission was not found at RT immediately after high-dose irradiation in Eudoped LiCAF crystal in contrary to the Ce-doped LiCAF. But the intensity of X-ray excited Eu2+and Eu3+ -luminescence becomes significantly reduced in irradiated sample. Annealing of preliminary irradiated crystals leads to re-establishing of their transmission. This process is accompanied by thermostimulated luminescence (TSL) (Fig. 5). Absorption bands induced in the region of Eu2+ f–d transitions disappear near 340 K, whereas F-color centers destruct abruptly at temperatures higher than 420 K in LiCAF:Eu. F-band annihilates gradually up to 650 K in pure samples. The glow curve peak positions (340, 420 and 490 K) are similar in undoped and Eu-contained samples, but TSL intensity is negligible in pure crystal and much higher in doped one. Thermoluminescence of LiCAF:Eu appears as the band at 370 nm typical for Eu2+-ion.
817
2.5 0 700
Fig. 5. Temperature dependence of F-center absorption band (1,10 ) and glow curves (2,20 ) of Eu-doped (A) and pure (B) LiCAF crystals.
4. Discussion Obtained results show that the overlapping of Eu2+ absorption and emission bands takes place in LiCAF:Eu (Fig. 1). Therefore, the intensity of Eu2+ ions luminescence significantly depends on the thickness of the crystal and Eu-concentration. The presence of Eu3+ in parallel with Eu2+ ions was estimated by the characteristic line-type ‘‘red’’ emission existence in as grown LiCAF:Eu crystal. Excess charge of Eu3+ has to be compensated by lithium vacancy or/and by (Li+) Ca-defect. Irradiation leads to Eu2+-absorption increase. It is supposedly connected with electron capture at Eu3+ ion, i.e. Eu3+-Eu2+ transformation. It results in the decrease of Eu3+ ion and increase of Eu2+ ion concentrations. The arising of Eu2+ ion amount defines the higher reabsorption and, as a result, the reduction of Eu2+ emission yield. The radioluminescence connected with Eu2+ ion quenches with temperature rise from 150 K to RT. Intrinsic UV emission of STE appears below 200 K. The intensity of STE luminescence in Eucontaining crystal is much below than in pure one. It may be connected with Vk center trapping near Eu2+ ion and/or reabsorption of STE emission and Eu2+ absorption bands.
818
N.V. Shiran et al. / Journal of Luminescence 102–103 (2003) 815–818
F-center accumulation in LiCAF:Eu is more efficient than in pure crystal (Fig. 4). Considerable distortion of lattice and, as a result, the creation of additional vacancies in activated crystal may be explained by size difference of Eu2+ and Ca2+ ( accordingly). Holes may be ions (1.31and 1.14 A, localized at or near Eu2+ ions and lithium vacancy. F-centers may be trapped near (Eu3+– V Li) dipoles with Fz like center formation. The hole (captured near Eu2+) delocalization and recombination with electron of F-center may explain the abrupt F-center destroying in Eudoped sample (in contrary to the gradual process in pure one) (Fig. 5). Study of TSL shows that Eu-ion introduction into LiCAF does not lead to the new carrier traps generation (Fig. 5). Increasing of glow curve intensity and thermoemission with characteristic spectrum (370 nm) points to the existence of effective radiative recombination Eu2+-centers in LiCAF:Eu. As was above mentioned, LiCAF:Eu and CaF2:Eu crystal demonstrate several common properties: high scintillation efficiency, low afterglow, Eu2+ ion emission and absorption, overlapping, etc. The ultrafast (o30 ps) rise-on stage of X-ray excited pulse for CaF2:Eu, observed in Ref. [5] allowed to suggest that the energy transfer connects with sequential hole–electron recombination in this crystal. Similarity of properties indicates the possibility of this mechanism in LiCAF:Eu as well. But special spectral-kinetics investigations are needed to provide support for this assumption.
5. Conclusions Studies of absorption, luminescence, scintillation properties and radiation damage of LiCAF:Eu crystal showed that this material would be an effective radiation detector. The main emission band at 370 nm is connected with Eu2+ ion substituted Ca2+ ones. In addition, Eu3+ ions were found in a minor amount. High-dose irradiation leads to the Eu3+-Eu2+ transformation. As a result the intensity of Eu2+ ion emission is reduced due to enhancement of reabsorption. In this connection it should be noted that the scintillation efficiency of LiCAF:Eu crystal is defined not only by thickness of the sample and Eu2+ ion concentration but also by Eu3+ ion content and contribution of factors controlling Eu3+!Eu2+ transformations.
References [1] N.V. Shiran, A.V. Gektin, S.V. Neicheva, E.P. Sysoeva, E.V. Sysoeva, K. Shimamura, A. Bensalah, T. Satonaga, Funct. Mater. 8 (2001) 732. [2] A.V. Gektin, N.V. Shiran, S.V. Neicheva, K. Shimamura, A. Bensalah, T. Fukuda, Nucl. Instrum. Methods 486A (2002) 274. [3] K. Shimamura, A. Bensalah, H. Sato, V. Sudesh, H. Machida, Cryst. Res. Technol. 36 (2001) 801. [4] N. Shiran, A. Gektin, I. Krasovitskaya, Radiat. Meas. 29 (1998) 337. [5] M.I. Weber, S.E. Derenzo, W.W. Mosses, J. Lumin. 87–89 (2000) 830.
Optical and scintillation properties of LiCaAlF6:Eu crystal N.V. Shirana,*, A.V. Gektina, S.V. Neichevaa, V.A. Kornienkoa, K. Shimamurab, N. Ishinoseb b
a Institute for Single Crystals, 60 Lenin Avenue, Kharkov 61001, Ukraine Kagami Memorial Laboratory of Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku, Tokyo 169-0051, Japan
Abstract Absorption, photo- and radioluminescence spectra have been investigated for as-grown and irradiated LiCaAlF6:Eu crystal. Eu2+ ions markedly over lap. Irradiation induces Eu3+-Eu2+ transition and F-centers formation. Color centers destruction in the process of crystal annealing is accompanied by thermostimulated luminescence (peaks at 340, 420 and 490 K) with the typical for Eu2+ ion emission (370 nm). Scintillation light output is defined by the thickness of sample, Eu2+ concentration and contribution of factors, controlling Eu3+!Eu2+ transformation in LiCaAlF6:Eu. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Scintillator; Energy storage; Color center; Thermostimulated luminescence
1. Introduction As it has been shown recently [1], Eu-doped LiCaAlF6 (LiCAF) crystals exhibit luminescence (maximum at 370 nm and decay time B1 ms) associated with 4f65d-4f7 transition in Eu2+ ions, substituting Ca2+ ones. These crystals may be used as effective detectors for registration of Xray, b- and g-radiation as well as thermal neutrons. The light yield was estimated to be 30% of NaI:Tl. Spectrometric scintillation parameters as well as absorption and luminescence properties of LiCAF:Eu crystal are similar to that of well-known CaF2:Eu scintillator. Shallow and ultradeep traps were found in Ce3+-doped LiCAF scintillators [2]. Exactly these *Corresponding author. Tel.: +380-572-308-367; fax: +380572-321-082. E-mail address: [email protected] (N.V. Shiran).
traps lead to the energy storage and, as a result, to the afterglow appearance and light output decreasing in LiCAF:Ce irradiated at RT. Europium ions, in contrary to cerium, enter generally in bivalent state into the most alkali and alkali-earth halide crystals. Eu2+ ions are arranged in Ca2+sites in LiCAF lattice and require no compensation. This fact suggests of lack of deep traps in Eu-doped LiCAF. The aim of the present study was to investigate various activator emission and color centers in LiCAF:Eu crystals.
2. Experimental Eu-doped crystals were grown by Czochralski technique in CF4 atmosphere [3]. EuF3 concentration in starting material was 0.5 mol%. Methods of absorption, excitation, emission and
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2313(02)00647-6
N.V. Shiran et al. / Journal of Luminescence 102–103 (2003) 815–818
816
thermoluminescence were described elsewhere [4]. Spectrometric characteristics were determined using the set ups represented in Ref. [1]. Intensity (arb. un.)
Absorption, excitation and luminescence spectra of LiCAF:Eu crystal are shown in Fig. 1. Bands at 202, 255 nm and in the range of 290–350 nm, characteristic for transitions in Eu2+ ion, are revealed in absorption spectrum. The luminescence excited directly in activator absorption bands consists of a single Gaussian-shaped narrow band with a maximum at 370 nm and a half-width B0.14 eV. Emission and absorption bands overlap, i.e. reabsorption takes place. Main X-ray excited luminescence coincides completely with the same ones observed at photoexcitation. As was found earlier ([1, Fig. 3]), along with the main band at 370 nm some weak emission lines are revealed at X-ray excitation in the long-wavelength region, with the most pronounced one at 594 nm connected with Eu3+ions. The latter points to the presence of an activator ion not only in the bivalent state but also in trivalent state. The intensity of the main radioluminescence (at 370 nm) is strongly temperature dependent (Figs. 2 and 3). The yield of Eu2+ emission decreases with
1
20
2
10
3
(A)
0 1
(B)
10 2
0
3
300 350 Wavelength (nm)
250
400
Fig. 2. X-ray excited luminescence of Eu-doped (A) and pure (B) LiCAF crystals at 163 K (1), 174 K (2) and 206 K (3).
3
Yield (arb. un.)
3. Results
30
20 1
10 2
0 1.0
5
1
2
3
0.5
0
0 200
160
Intensity (arb. un.)
Absorption (cm–1)
10
300 Wavelength (nm)
400
Fig. 1. Absorption (1), excitation (2) and photoluminescence (3) spectra of LiCAF:Eu crystal at room temperature.
200 240 Temperature (K)
Fig. 3. Temperature dependence of STE yield (280 nm) in pure (1) and Eu-doped (2) LiCAF; Eu2+ luminescence (370 nm) in LiCAF:Eu (3).
the temperature rise from 150 K to RT. The UV emission (at 280 nm, half-width near 0.67 eV) connected with the self-trapped excitons (STE) is revealed in pure and doped LiCAF below 200 K. The intensity of this intrinsic emission is far less in Eu-containing crystal than in pure one. Radiation damage study has shown that absorption and luminescence spectra change negligibly if irradiation dose is getting lower (o10 Gy).
N.V. Shiran et al. / Journal of Luminescence 102–103 (2003) 815–818
–1
∆KF (cm )
Induced absorption (cm–1)
3
2
3 2 1
1
0
2
2
0
1
2
3
2
Dose (10 Gy)
1 1
0 200
300 Wavelength (nm)
400
Fig. 4. X-ray induced absorption in pure (1) and Eu-doped (2) LiCAF. Inset: dose dependence of F-center accumulation in pure (1) and Eu-doped (2) crystals.
3
(A)
40
1
2
20
1 10
0
(B)
1 1`
0.5
2`
0 300
400 500 600 Temperature (K)
0 5
TSL-intensity (arb. un.)
30
2 Absorption (cm–1)
Absorption bands connected with F-centers appear at 260 and 267 nm, in pure and doped samples accordingly (Fig. 4). F-center accumulation efficiency is significantly higher in Eu-containing sample. It should be noted that Eu2+- and F-absorption bands increased simultaneously in LiCAF:Eu crystal at irradiation. Afterglow emission was not found at RT immediately after high-dose irradiation in Eudoped LiCAF crystal in contrary to the Ce-doped LiCAF. But the intensity of X-ray excited Eu2+and Eu3+ -luminescence becomes significantly reduced in irradiated sample. Annealing of preliminary irradiated crystals leads to re-establishing of their transmission. This process is accompanied by thermostimulated luminescence (TSL) (Fig. 5). Absorption bands induced in the region of Eu2+ f–d transitions disappear near 340 K, whereas F-color centers destruct abruptly at temperatures higher than 420 K in LiCAF:Eu. F-band annihilates gradually up to 650 K in pure samples. The glow curve peak positions (340, 420 and 490 K) are similar in undoped and Eu-contained samples, but TSL intensity is negligible in pure crystal and much higher in doped one. Thermoluminescence of LiCAF:Eu appears as the band at 370 nm typical for Eu2+-ion.
817
2.5 0 700
Fig. 5. Temperature dependence of F-center absorption band (1,10 ) and glow curves (2,20 ) of Eu-doped (A) and pure (B) LiCAF crystals.
4. Discussion Obtained results show that the overlapping of Eu2+ absorption and emission bands takes place in LiCAF:Eu (Fig. 1). Therefore, the intensity of Eu2+ ions luminescence significantly depends on the thickness of the crystal and Eu-concentration. The presence of Eu3+ in parallel with Eu2+ ions was estimated by the characteristic line-type ‘‘red’’ emission existence in as grown LiCAF:Eu crystal. Excess charge of Eu3+ has to be compensated by lithium vacancy or/and by (Li+) Ca-defect. Irradiation leads to Eu2+-absorption increase. It is supposedly connected with electron capture at Eu3+ ion, i.e. Eu3+-Eu2+ transformation. It results in the decrease of Eu3+ ion and increase of Eu2+ ion concentrations. The arising of Eu2+ ion amount defines the higher reabsorption and, as a result, the reduction of Eu2+ emission yield. The radioluminescence connected with Eu2+ ion quenches with temperature rise from 150 K to RT. Intrinsic UV emission of STE appears below 200 K. The intensity of STE luminescence in Eucontaining crystal is much below than in pure one. It may be connected with Vk center trapping near Eu2+ ion and/or reabsorption of STE emission and Eu2+ absorption bands.
818
N.V. Shiran et al. / Journal of Luminescence 102–103 (2003) 815–818
F-center accumulation in LiCAF:Eu is more efficient than in pure crystal (Fig. 4). Considerable distortion of lattice and, as a result, the creation of additional vacancies in activated crystal may be explained by size difference of Eu2+ and Ca2+ ( accordingly). Holes may be ions (1.31and 1.14 A, localized at or near Eu2+ ions and lithium vacancy. F-centers may be trapped near (Eu3+– V Li) dipoles with Fz like center formation. The hole (captured near Eu2+) delocalization and recombination with electron of F-center may explain the abrupt F-center destroying in Eudoped sample (in contrary to the gradual process in pure one) (Fig. 5). Study of TSL shows that Eu-ion introduction into LiCAF does not lead to the new carrier traps generation (Fig. 5). Increasing of glow curve intensity and thermoemission with characteristic spectrum (370 nm) points to the existence of effective radiative recombination Eu2+-centers in LiCAF:Eu. As was above mentioned, LiCAF:Eu and CaF2:Eu crystal demonstrate several common properties: high scintillation efficiency, low afterglow, Eu2+ ion emission and absorption, overlapping, etc. The ultrafast (o30 ps) rise-on stage of X-ray excited pulse for CaF2:Eu, observed in Ref. [5] allowed to suggest that the energy transfer connects with sequential hole–electron recombination in this crystal. Similarity of properties indicates the possibility of this mechanism in LiCAF:Eu as well. But special spectral-kinetics investigations are needed to provide support for this assumption.
5. Conclusions Studies of absorption, luminescence, scintillation properties and radiation damage of LiCAF:Eu crystal showed that this material would be an effective radiation detector. The main emission band at 370 nm is connected with Eu2+ ion substituted Ca2+ ones. In addition, Eu3+ ions were found in a minor amount. High-dose irradiation leads to the Eu3+-Eu2+ transformation. As a result the intensity of Eu2+ ion emission is reduced due to enhancement of reabsorption. In this connection it should be noted that the scintillation efficiency of LiCAF:Eu crystal is defined not only by thickness of the sample and Eu2+ ion concentration but also by Eu3+ ion content and contribution of factors controlling Eu3+!Eu2+ transformations.
References [1] N.V. Shiran, A.V. Gektin, S.V. Neicheva, E.P. Sysoeva, E.V. Sysoeva, K. Shimamura, A. Bensalah, T. Satonaga, Funct. Mater. 8 (2001) 732. [2] A.V. Gektin, N.V. Shiran, S.V. Neicheva, K. Shimamura, A. Bensalah, T. Fukuda, Nucl. Instrum. Methods 486A (2002) 274. [3] K. Shimamura, A. Bensalah, H. Sato, V. Sudesh, H. Machida, Cryst. Res. Technol. 36 (2001) 801. [4] N. Shiran, A. Gektin, I. Krasovitskaya, Radiat. Meas. 29 (1998) 337. [5] M.I. Weber, S.E. Derenzo, W.W. Mosses, J. Lumin. 87–89 (2000) 830.