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Radiation quenching of F-center emission in ruby
Nuclear Instruments and Methods in Physics Research B 141 (1998) 384±386
Radiation quenching of F-center emission in ruby V.I. Flerov a
a,*
, A.V. Flerov
b
Nuclear Research Center, Salaspils-1, LV-2169, Latvia b Institute of Physics, Salaspils-1, LV-2169, Latvia
Abstract The only red luminescence intrinsic to chromium ions is observed in ruby crystals with a chromium concentration on the order of 0.01 weight percent. If the ruby has been previously irradiated with high-energy electrons to a ¯uence of 1017 cmÿ2 , F and F emissions appear. These emissions disappear during irradiation by X-rays. The luminescence intensity (I) follows a linear decay on log(I) vs. log(time). The decay kinetics are the same at both room and liquid nitrogen temperatures. Four thermoluminescence (TL) peaks are present for electron-irradiated crystals after UV-exposure, also. These peaks, which have an electronic nature, also disappear under irradiation by X-rays. New evidence supports the idea that the origin of both phenomena is the same. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Ba; 78.55.Hx Keywords: Ruby; Luminescence; Afterglow; Quenching of emission
1. Introduction
2. Experimental procedure
The only red luminescence intrinsic to chromium ions is observed in ruby crystals with a chromium concentration on the order of 0.01 weight percent. If the crystal has been previously subjected to electron radiation, the F and F emissions appear at 3.0 and 3.8 eV, respectively. Four new TL peaks also arise approximately two to three weeks after electron irradiation. The four peaks and the F and F emissions disappear during X-irradiation of the crystal. The aim of this paper is to investigate this phenomenon.
Ruby and sapphire crystals were obtained from GOI (Sankt-Petersburg). Pre-irradiation of the crystals was performed in an electron accelerator with an electron energy of 5 MeV to a ¯uence of 1017 cmÿ2 . UV-radiation was provided by an OI18 illuminator with a mercury±argon high pressure lamp (150 W). All irradiations were performed at room temperature. Thermoluminesence (TL) was measured with a FEU-130 PMT (S-5 response) connected to a K 21 recorder. The luminescence spectra under X-ray excitation were measured in the range of 1.5±4 eV. A copper-anode tube operated at 50 kV, 50 mA was used as the X-ray source. The photoluminescence spectra were measured under excitation with a 500 W super-high
* Corresponding author. Tel.: +371-7336125; fax: +3717901214; e-mail: v¯[email protected].
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 3 1 - 1
V.I. Flerov, A.V. Flerov / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 384±386
385
pressure xenon lamp. The exciting light was passed through a MUM-8 grating monochromator. The emitted ¯uorescence was passed through an interference ®lter and detected with an FEU-39 PMT (S-15 response). 3. Results and discussion Only the chromium emission (near 700 nm or 1.8 eV) is observed in the as-received ruby crystals (Fig. 1, Trace 1). But, after the crystal had been irradiated with high energy electrons, further exposure to X-rays produced new bands (Fig. 1, Trace 2) identi®ed as intrinsic to F and F emissions (3.8 and 3.0 eV, or 330 and 415 nm) [1]. Both emissions disappear completely in approximately one hour during this X-irradiation. The decay in intensity of the F emission band luminescence both at room and liquid nitrogen temperatures (Fig. 2) shows that the decay kinetics do not depend on temperature. The emissions are not restored during annealing but do reappear after storage of the crystal for two or three weeks. It is known [2] that the excited state of F-centers lies either in the conduction band or near its bottom. Therefore, the optical excitation of F-centers by photons with an energy of 6.1 eV creates a strong photoconductivity [3]. Photobleaching of the F absorption band may result from free elec-
Fig. 1. X-ray induced luminescence spectra of ruby crystal (1) as-received sample and (2) after electron irradiation.
Fig. 2. Decay of F-luminescence under X-ray exposure.
trons being captured in traps within the crystal. Electrons in these traps can be released by heating the crystal, overcoming the activation barrier to recombination with the hole centers, creating the TL signal. The ruby crystals containing F centers (those pre-irradiated with electrons) have a TL glow curve as shown in Fig. 3, which is similar to that obtained from reduced samples. As previously described [4], these F centers are formed by self-trapped electrons at F centers. During the X-ray exposure, these peaks also disappear but are restored over time. These phenomena can be explained by the Big Radius State (BRS). It has been shown [5±7] that chromium ions (Cr4 ) can exist in corundum lattices in two states: normal and in the Big Radius State having a size of about 4.2 nm [6]. As a result of X-irradiation, the concentration of the Cr4 BRS state increases. An electron within the sphere
Fig. 3. TL glow curve resulting from a 1 min UV-exposure at 295 K from a ruby crystal previously irradiated with high energy electrons.
386
V.I. Flerov, A.V. Flerov / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 384±386
of in¯uence of a Big Radius State recombines with a hole (Cr4 in the BRS) and cannot participate in other reactions (e.g., be captured in traps or recombine on other luminescence centers that give violet emission). Ions in the Big Radius State at a certain concentration overlap all of the volume of the crystal, blocking all other processes in the crystal. References [1] K.H. Lee, J.H. Crawford, Jr., Phys. Rev. B 19 (1979) 3217.
[2] B.G. Draeger, J.D. Brewer, G.P. Summers, Phys. Rev. B 19 (1979) 1172. [3] B.J. Jeris, J.D. Brewer, G.P. Summers, Phys. Rev. B 24 (1981) 6074. [4] V.I. Flerov, A.V. Flerov, Latvian J. Phys. Technol. Sci. 2 (1997) 33. [5] V.I. Flerov, A.V. Flerov, Radiat. E. Def. Sol. 134 (1995) 239. [6] V.I. Flerov, A.V. Flerov, S.I. Flerov, Radiat. E. Def. Sol. 134 (1995) 371. [7] V.I. Flerov, A.V. Flerov, Latvian J. Phys. Technol. Sci. 4 (1997) 3.
Radiation quenching of F-center emission in ruby V.I. Flerov a
a,*
, A.V. Flerov
b
Nuclear Research Center, Salaspils-1, LV-2169, Latvia b Institute of Physics, Salaspils-1, LV-2169, Latvia
Abstract The only red luminescence intrinsic to chromium ions is observed in ruby crystals with a chromium concentration on the order of 0.01 weight percent. If the ruby has been previously irradiated with high-energy electrons to a ¯uence of 1017 cmÿ2 , F and F emissions appear. These emissions disappear during irradiation by X-rays. The luminescence intensity (I) follows a linear decay on log(I) vs. log(time). The decay kinetics are the same at both room and liquid nitrogen temperatures. Four thermoluminescence (TL) peaks are present for electron-irradiated crystals after UV-exposure, also. These peaks, which have an electronic nature, also disappear under irradiation by X-rays. New evidence supports the idea that the origin of both phenomena is the same. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Ba; 78.55.Hx Keywords: Ruby; Luminescence; Afterglow; Quenching of emission
1. Introduction
2. Experimental procedure
The only red luminescence intrinsic to chromium ions is observed in ruby crystals with a chromium concentration on the order of 0.01 weight percent. If the crystal has been previously subjected to electron radiation, the F and F emissions appear at 3.0 and 3.8 eV, respectively. Four new TL peaks also arise approximately two to three weeks after electron irradiation. The four peaks and the F and F emissions disappear during X-irradiation of the crystal. The aim of this paper is to investigate this phenomenon.
Ruby and sapphire crystals were obtained from GOI (Sankt-Petersburg). Pre-irradiation of the crystals was performed in an electron accelerator with an electron energy of 5 MeV to a ¯uence of 1017 cmÿ2 . UV-radiation was provided by an OI18 illuminator with a mercury±argon high pressure lamp (150 W). All irradiations were performed at room temperature. Thermoluminesence (TL) was measured with a FEU-130 PMT (S-5 response) connected to a K 21 recorder. The luminescence spectra under X-ray excitation were measured in the range of 1.5±4 eV. A copper-anode tube operated at 50 kV, 50 mA was used as the X-ray source. The photoluminescence spectra were measured under excitation with a 500 W super-high
* Corresponding author. Tel.: +371-7336125; fax: +3717901214; e-mail: v¯[email protected].
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 3 1 - 1
V.I. Flerov, A.V. Flerov / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 384±386
385
pressure xenon lamp. The exciting light was passed through a MUM-8 grating monochromator. The emitted ¯uorescence was passed through an interference ®lter and detected with an FEU-39 PMT (S-15 response). 3. Results and discussion Only the chromium emission (near 700 nm or 1.8 eV) is observed in the as-received ruby crystals (Fig. 1, Trace 1). But, after the crystal had been irradiated with high energy electrons, further exposure to X-rays produced new bands (Fig. 1, Trace 2) identi®ed as intrinsic to F and F emissions (3.8 and 3.0 eV, or 330 and 415 nm) [1]. Both emissions disappear completely in approximately one hour during this X-irradiation. The decay in intensity of the F emission band luminescence both at room and liquid nitrogen temperatures (Fig. 2) shows that the decay kinetics do not depend on temperature. The emissions are not restored during annealing but do reappear after storage of the crystal for two or three weeks. It is known [2] that the excited state of F-centers lies either in the conduction band or near its bottom. Therefore, the optical excitation of F-centers by photons with an energy of 6.1 eV creates a strong photoconductivity [3]. Photobleaching of the F absorption band may result from free elec-
Fig. 1. X-ray induced luminescence spectra of ruby crystal (1) as-received sample and (2) after electron irradiation.
Fig. 2. Decay of F-luminescence under X-ray exposure.
trons being captured in traps within the crystal. Electrons in these traps can be released by heating the crystal, overcoming the activation barrier to recombination with the hole centers, creating the TL signal. The ruby crystals containing F centers (those pre-irradiated with electrons) have a TL glow curve as shown in Fig. 3, which is similar to that obtained from reduced samples. As previously described [4], these F centers are formed by self-trapped electrons at F centers. During the X-ray exposure, these peaks also disappear but are restored over time. These phenomena can be explained by the Big Radius State (BRS). It has been shown [5±7] that chromium ions (Cr4 ) can exist in corundum lattices in two states: normal and in the Big Radius State having a size of about 4.2 nm [6]. As a result of X-irradiation, the concentration of the Cr4 BRS state increases. An electron within the sphere
Fig. 3. TL glow curve resulting from a 1 min UV-exposure at 295 K from a ruby crystal previously irradiated with high energy electrons.
386
V.I. Flerov, A.V. Flerov / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 384±386
of in¯uence of a Big Radius State recombines with a hole (Cr4 in the BRS) and cannot participate in other reactions (e.g., be captured in traps or recombine on other luminescence centers that give violet emission). Ions in the Big Radius State at a certain concentration overlap all of the volume of the crystal, blocking all other processes in the crystal. References [1] K.H. Lee, J.H. Crawford, Jr., Phys. Rev. B 19 (1979) 3217.
[2] B.G. Draeger, J.D. Brewer, G.P. Summers, Phys. Rev. B 19 (1979) 1172. [3] B.J. Jeris, J.D. Brewer, G.P. Summers, Phys. Rev. B 24 (1981) 6074. [4] V.I. Flerov, A.V. Flerov, Latvian J. Phys. Technol. Sci. 2 (1997) 33. [5] V.I. Flerov, A.V. Flerov, Radiat. E. Def. Sol. 134 (1995) 239. [6] V.I. Flerov, A.V. Flerov, S.I. Flerov, Radiat. E. Def. Sol. 134 (1995) 371. [7] V.I. Flerov, A.V. Flerov, Latvian J. Phys. Technol. Sci. 4 (1997) 3.