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Ionoluminescence and formation of color centers in α-Al2O3 single crystals under proton irradiation
Radiation Measurements 45 (2010) 362–364
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Ionoluminescence and formation of color centers in a-Al2O3 single crystals under proton irradiation A.V. Kruzhalov, I.I. Milman, O.V. Ryabukhin, I.G. Revkov, E.N. Litovchenko* Ural State Technical University – UPI, Physics and Technology, 19 Mira Street, 620002 Ekaterinburg, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2009 Received in revised form 4 September 2009 Accepted 9 November 2009
This paper describes results of experimental studies on radiation defects in nominally pure single crystals of corundum in two initial states: a-Al2O3 with an unperturbed lattice and a-Al2O3:C with a high concentration of anion vacancies. Defects were identified from optical absorption spectra, ionoluminescence, pulsed cathodoluminescence and photoluminescence spectra. It is shown that mostly color centers of the F- and Fþ-types are formed in the a-Al2O3 lattice under irradiation with 5,7 MeV protons. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Luminescence Aluminium oxide F-center
1. Introduction The monocrystalline alumina is characterized by a whole set of extremely useful properties, namely the high values of mechanical strength, melting temperature, thermal conductivity, radiation resistance, chemical inertness, dielectric strength and optical transparency within the wide wavelength range. Due to these features a-Al2O3-based materials are still the matter of intense research aimed to discover their possible prospects in electronics and nuclear technology, power optics and quantum devices. Luminescent features of such crystals are of special interest due to their application in creation of nuclear radiation detectors, which are based on the phenomena of thermostimulated (TL) (Chen and McKeever, 1997), optically stimulated (OSL) (Botter-Jensen et al., 2003) and radioluminescence (RL) (Damkjaer et al., 2008). It is worthy of note that nominally pure crystals with initially undisturbed structure have poor luminescent properties. The introduction of color centers or defects into the crystal lattice is used to obtain luminescence rise up to the level sufficient for practical application in TL dosimetry. Usually F-centers (oxygen vacancy with two trapped electrons) and Fþ-centers (oxygen vacancy with single electron). Depending on the defect introduction method one 2þ can create also F2-centers, Fþ 2 -centers and F2 -centers (double oxygen vacancy with four, three and two trapped electrons respectively).
The TLD-500K (a-Al2O3:C) thermoluminescence dosimeters, being based on the anion-defective alumina monocrystals, have already wide spread among the Russia and abroad as well. Anion sublattice defects are created here by thermochemical treatment or, alternatively, by crystal growth under the reducing vacuum environment with the presence of graphite. There are numerous examples of using the OSL method abroad in the personal dosimetry, environmental monitoring and space research (Kulharni et al., 2007), and for the detection of neutrons (Mittani et al., 2007) and heavy charged particles (McKeever et al., 2007). Special attention has been devoted to the OSL dosimetry of high-energy protons (Edmund et al., 2007) in anticancer therapy, where it is important to perform local measurements using smallsize detectors in vivo, including real-time diagnostics (Akselrod et al., 2006). In this connection there is a new necessity for further extension of our conceptions concerning radiation defects formation mechanisms taking place in crystals under low and medium energy charged particles irradiation, as well as our knowledge of their resulting luminescence properties and practical application. The present work comprises our recent experimental results concerning the creation of radiation defects and optically active color centers of a-Al2O3 lattice under the proton beam bombardment. Defect types were identified using optical spectroscopy methods.
2. Experimental techniques * Corresponding author. E-mail address: [email protected] (E.N. Litovchenko). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.11.018
The samples under study were optically transparent monocrystals of artificial corundum with initially undisturbed structure
A.V. Kruzhalov et al. / Radiation Measurements 45 (2010) 362–364
3. Results and discussion On the Fig. 1 one can see the real-time proton-induced luminescence (IPL) spectrum of a-Al2O3 crystals and its dose dependency. Analogous spectrum for a-Al2O3:C samples is given for comparison (curve 5). As it is seen from the figure, intensity maxima are located near 420 nm and correspond to well studied luminescence of F-centers in a-Al2O3:C crystals. We assume that significant increase in IPL intensity during the irradiation reflects the dynamics of F-type vacancy defects accumulation in the crystal lattice. This supposition agrees well with optical absorption data for a-Al2O3 crystals measured before (Fig. 2, curve 1) and after irradiation (curve 2). Radiation-induced a-Al2O3 optical absorption spectra are identical to the a-Al2O3:C crystals ones. The 205 nm OA band observed for irradiated crystals corresponds to F-centers while 230 nm and 255 nm bands respond to Fþ-centers. Thus, under chosen conditions the dominant type of radiation defects in corundum lattice is the oxygen vacancy with corresponding color centers of F and Fþ type. Unlike the samples with initially undisturbed structure, the intensity of IPL for F-centers luminescence band of a-Al2O3:C
2,5 2,0
Absorption, a.u.
and standard TLD-500K detectors as well. All samples had follow size: height – 1 mm and a diameter – 5 mm. According to optical absorption data the concentration of F- and Fþ-centers of a-Al2O3:C crystals were 1023–1024 m3 and 1021–1022 m3 respectively. The exposure of unoriented samples was performed at room temperatures using the 5,7 MeV protons with fluence varied from 1010 to 1016 cm2 taken out from USTU-UPI R-7 cyclotron research channel through 50 mm titanium foil. With the aim of distant and real-time luminescence spectra acquisition the lateral face of the samples was optically connected with the 15 m fiber cable which transferred the signal to photorecording device. The same device was used for pulse cathodoluminescence (PCL) measurements of irradiated samples within the 350–800 nm spectral range. The PCL excitation was performed by 150 keV electron beam with 2 ns half-maximum pulse duration and the current value of 1 kA. The identification of radiation-induced color centers was carried out using optical absorption spectra measured on Helios spectrophotometer. Proton penetration depths and damage profiles in the a-Al2O3 lattice were estimated with help of SRIM 2008.03.142 software.
363
1,5 1,0 2
0,5 1
0,0
200
220
240
260
λ, nm
280
300
320
Fig. 2. The optical absorption spectra for a-Al2O3 crystals. 1 – initial; 2 – after proton irradiation with fluence 5,7 1013 cm2.
crystals (Fig. 1, curve 5) rapidly became stabilized on the constant level and hardly changed with dose accumulation. Third figure shows the PCL spectra for a-Al2O3 samples, recorded from the irradiated (curve 1) and from the opposite untreated (curve 2) sides. It contains also unirradiated a-Al2O3:C samples spectrum (curve 3). The comparison of the first two curves shows that proton beam irradiation leads to significant rise of 420 nm luminescence due to F-band. However, besides the F-band, one more maximum at 550 nm appears in the third curve, corresponding to F2þ 2 -centers. Therefore thermochemical coloration creates, in addition to simple point defects, complex defect configurations. Complex PCL, IPL and OA spectra (Figs. 1–3) analysis demonstrates the presence of similar color centers in all crystals studied, in spite of difference in the nature of vacancies and their introduction technologies. During irradiation of a-Al2O3 samples the Frenkel pairs (a vacancy together with interstitial oxygen) are formed while in case of thermochemical coloration the oxygen deficiency in anion sublattice is created. The second difference is the spatial distribution of defects. Being uniformly spread among the volume of a-Al2O3:C samples, they demonstrate nonuniform distribution along the proton propagation path in a-Al2O3. According to SRIM software calculations, the track length for 5,7 MeV protons is about 150 nm. The maximum vacancies concentration at such depths is by an order of magnitude higher than average value for the rest of the volume.
160 400
120 100 80
5
60 3
40 20 0 350
PCL, a.u.
IL intensity, a.u.
140
1
400
4
1
200 3
100
2
450 λ, nm
300
500
550
Fig. 1. The real-time proton-induced luminescence spectra for a-Al2O3 crystals (1, 2, 3, 4) and a-Al2O3:C crystals (5) in its dependency on proton fluence. 1 – 6 1010 cm2; 2 – 1,8 1012 cm2; 3 – 9 1012 cm2; 4 – 5,7 1013 cm2; 5 – 5,7 1013 cm2.
0 350
2
400
450 500 λ, nm
550
600
Fig. 3. The pulse cathodoluminescence spectra for a-Al2O3 crystals after proton irradiation with fluence 5,7 1013 cm2 (1, 2) and unirradiated a-Al2O3:C crystal (3). 1 – PCL registered from irradiated face; 2 – from the opposite side.
364
A.V. Kruzhalov et al. / Radiation Measurements 45 (2010) 362–364
4. Conclusion
2,4
Absorption, a.u.
2,2 2,0 1,8
2
1
1,6 1,4 1,2
200
220
240
260 λ, nm
280
300
320
Fig. 4. Optical absorption spectra for a-Al2O3:C crystals: 1- initial samples; 2 – irradiated with fluence 1016 cm2.
The commensurability of OA bands intensities for F- and Fþcenters in irradiated a-Al2O3 and corresponding maxima for a-Al2O3:C (Fig. 2, curve 2 and Fig. 4, curve 1) may be the evidence of oxygen vacancies numbers comparability for radiation damaged layer (20% of a-Al2O3 volume) and for whole a-Al2O3:C sample. The latter fact finds its proof in the results of F-centers PCL intensities comparison for both types of crystals (curve 1 and curve 3 Fig. 3), if one assumes that the F-centers PCL bands intensities are proportional to their concentration and that the volumes excited (about 60 mm in depth for 150 KeV electrons, as given by empirical equation) are equal. It is clearly seen that the 420 nm luminescence band intensity for crystals excited from the proton-irradiated face is even higher than corresponding intensity for a-Al2O3:C.
The dose dependency of IPL at F-centers band under 5,7 MeV protons excitation of a-Al2O3 monocrystals was investigated. It was shown that IPL intensity growth is accompanied with accumulation of oxygen vacancies and corresponding F- and Fþ-color centers in the crystal lattice. For thermochemically colored crystals IPL almost does not demonstrate rise, reaching the stationary level which is proportional to proton beam current density. This feature can find its application in charged particles diagnostics systems. It was shown by means of PCL that F-centers concentration in thin surface layer of irradiated crystal may exceed the average value for thermochemically colored crystals. References Akselrod, M.S., Botter-Jensen, L., McKeever, S.W.S., 2006. Optical Stimulated Luminescence and Its Use in Madical Dosimetry. Radiat. Meas. 41, 78–99 pp. Botter-Jensen, L., McKeever, S.W.S., Wintle, A.G., 2003. Optically Stimulated Luminescence Dosimetry. Elsevier, Amsterdam, 356 pp. Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. Word Scientific Publishing Co, Singapore, 560 pp. Damkjaer, S.M.S., Andersen, C.E., Aznar, M.C., 2008. Improved Real-Time Dosimetry Using The Radioluminescence Signal from Al2O3:C. Radiat. Meas. 43, 893–897 pp. Edmund, J.M., Andersen, C.E., Greilich, S., et al., 2007. Optically Stimulated Luminescence from Al2O3:C Irradiated with 10-60 MeV Protons. Nucl. Instrum. Methods. Phys. Res. Sect. A 580, 466–468 pp. Kulkarni, M.S., Mishra, D.R., Sharma, D.N., 2007. Versatile Integrated System for Thermoluminescence and Optically Stimulated Luminescence Measurement. Nucl. Instrum. Methods. Phys. Res. Sect. B 262, 348–356 pp. Mittani, J.C.R., Silva, A.A.R., Vanhavere, F., et al., 2007. Investigation of Neutron Converters for Production of Optically Stimulated Luminescence (OSL) Neutron Dosimeters Using Al2O3:C. Nucl. Instrum. Methods. Phys. Res. Sect. B 260, 663–671 pp. McKeever, S.W.S., Benton, E.R., Gaza, R., et al., 2007. Book of Abstract 15th Intern. Conf. on Solid State Dosimetry, Delft, The Netherlands, July 8–13. 35 pp.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Ionoluminescence and formation of color centers in a-Al2O3 single crystals under proton irradiation A.V. Kruzhalov, I.I. Milman, O.V. Ryabukhin, I.G. Revkov, E.N. Litovchenko* Ural State Technical University – UPI, Physics and Technology, 19 Mira Street, 620002 Ekaterinburg, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2009 Received in revised form 4 September 2009 Accepted 9 November 2009
This paper describes results of experimental studies on radiation defects in nominally pure single crystals of corundum in two initial states: a-Al2O3 with an unperturbed lattice and a-Al2O3:C with a high concentration of anion vacancies. Defects were identified from optical absorption spectra, ionoluminescence, pulsed cathodoluminescence and photoluminescence spectra. It is shown that mostly color centers of the F- and Fþ-types are formed in the a-Al2O3 lattice under irradiation with 5,7 MeV protons. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Luminescence Aluminium oxide F-center
1. Introduction The monocrystalline alumina is characterized by a whole set of extremely useful properties, namely the high values of mechanical strength, melting temperature, thermal conductivity, radiation resistance, chemical inertness, dielectric strength and optical transparency within the wide wavelength range. Due to these features a-Al2O3-based materials are still the matter of intense research aimed to discover their possible prospects in electronics and nuclear technology, power optics and quantum devices. Luminescent features of such crystals are of special interest due to their application in creation of nuclear radiation detectors, which are based on the phenomena of thermostimulated (TL) (Chen and McKeever, 1997), optically stimulated (OSL) (Botter-Jensen et al., 2003) and radioluminescence (RL) (Damkjaer et al., 2008). It is worthy of note that nominally pure crystals with initially undisturbed structure have poor luminescent properties. The introduction of color centers or defects into the crystal lattice is used to obtain luminescence rise up to the level sufficient for practical application in TL dosimetry. Usually F-centers (oxygen vacancy with two trapped electrons) and Fþ-centers (oxygen vacancy with single electron). Depending on the defect introduction method one 2þ can create also F2-centers, Fþ 2 -centers and F2 -centers (double oxygen vacancy with four, three and two trapped electrons respectively).
The TLD-500K (a-Al2O3:C) thermoluminescence dosimeters, being based on the anion-defective alumina monocrystals, have already wide spread among the Russia and abroad as well. Anion sublattice defects are created here by thermochemical treatment or, alternatively, by crystal growth under the reducing vacuum environment with the presence of graphite. There are numerous examples of using the OSL method abroad in the personal dosimetry, environmental monitoring and space research (Kulharni et al., 2007), and for the detection of neutrons (Mittani et al., 2007) and heavy charged particles (McKeever et al., 2007). Special attention has been devoted to the OSL dosimetry of high-energy protons (Edmund et al., 2007) in anticancer therapy, where it is important to perform local measurements using smallsize detectors in vivo, including real-time diagnostics (Akselrod et al., 2006). In this connection there is a new necessity for further extension of our conceptions concerning radiation defects formation mechanisms taking place in crystals under low and medium energy charged particles irradiation, as well as our knowledge of their resulting luminescence properties and practical application. The present work comprises our recent experimental results concerning the creation of radiation defects and optically active color centers of a-Al2O3 lattice under the proton beam bombardment. Defect types were identified using optical spectroscopy methods.
2. Experimental techniques * Corresponding author. E-mail address: [email protected] (E.N. Litovchenko). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.11.018
The samples under study were optically transparent monocrystals of artificial corundum with initially undisturbed structure
A.V. Kruzhalov et al. / Radiation Measurements 45 (2010) 362–364
3. Results and discussion On the Fig. 1 one can see the real-time proton-induced luminescence (IPL) spectrum of a-Al2O3 crystals and its dose dependency. Analogous spectrum for a-Al2O3:C samples is given for comparison (curve 5). As it is seen from the figure, intensity maxima are located near 420 nm and correspond to well studied luminescence of F-centers in a-Al2O3:C crystals. We assume that significant increase in IPL intensity during the irradiation reflects the dynamics of F-type vacancy defects accumulation in the crystal lattice. This supposition agrees well with optical absorption data for a-Al2O3 crystals measured before (Fig. 2, curve 1) and after irradiation (curve 2). Radiation-induced a-Al2O3 optical absorption spectra are identical to the a-Al2O3:C crystals ones. The 205 nm OA band observed for irradiated crystals corresponds to F-centers while 230 nm and 255 nm bands respond to Fþ-centers. Thus, under chosen conditions the dominant type of radiation defects in corundum lattice is the oxygen vacancy with corresponding color centers of F and Fþ type. Unlike the samples with initially undisturbed structure, the intensity of IPL for F-centers luminescence band of a-Al2O3:C
2,5 2,0
Absorption, a.u.
and standard TLD-500K detectors as well. All samples had follow size: height – 1 mm and a diameter – 5 mm. According to optical absorption data the concentration of F- and Fþ-centers of a-Al2O3:C crystals were 1023–1024 m3 and 1021–1022 m3 respectively. The exposure of unoriented samples was performed at room temperatures using the 5,7 MeV protons with fluence varied from 1010 to 1016 cm2 taken out from USTU-UPI R-7 cyclotron research channel through 50 mm titanium foil. With the aim of distant and real-time luminescence spectra acquisition the lateral face of the samples was optically connected with the 15 m fiber cable which transferred the signal to photorecording device. The same device was used for pulse cathodoluminescence (PCL) measurements of irradiated samples within the 350–800 nm spectral range. The PCL excitation was performed by 150 keV electron beam with 2 ns half-maximum pulse duration and the current value of 1 kA. The identification of radiation-induced color centers was carried out using optical absorption spectra measured on Helios spectrophotometer. Proton penetration depths and damage profiles in the a-Al2O3 lattice were estimated with help of SRIM 2008.03.142 software.
363
1,5 1,0 2
0,5 1
0,0
200
220
240
260
λ, nm
280
300
320
Fig. 2. The optical absorption spectra for a-Al2O3 crystals. 1 – initial; 2 – after proton irradiation with fluence 5,7 1013 cm2.
crystals (Fig. 1, curve 5) rapidly became stabilized on the constant level and hardly changed with dose accumulation. Third figure shows the PCL spectra for a-Al2O3 samples, recorded from the irradiated (curve 1) and from the opposite untreated (curve 2) sides. It contains also unirradiated a-Al2O3:C samples spectrum (curve 3). The comparison of the first two curves shows that proton beam irradiation leads to significant rise of 420 nm luminescence due to F-band. However, besides the F-band, one more maximum at 550 nm appears in the third curve, corresponding to F2þ 2 -centers. Therefore thermochemical coloration creates, in addition to simple point defects, complex defect configurations. Complex PCL, IPL and OA spectra (Figs. 1–3) analysis demonstrates the presence of similar color centers in all crystals studied, in spite of difference in the nature of vacancies and their introduction technologies. During irradiation of a-Al2O3 samples the Frenkel pairs (a vacancy together with interstitial oxygen) are formed while in case of thermochemical coloration the oxygen deficiency in anion sublattice is created. The second difference is the spatial distribution of defects. Being uniformly spread among the volume of a-Al2O3:C samples, they demonstrate nonuniform distribution along the proton propagation path in a-Al2O3. According to SRIM software calculations, the track length for 5,7 MeV protons is about 150 nm. The maximum vacancies concentration at such depths is by an order of magnitude higher than average value for the rest of the volume.
160 400
120 100 80
5
60 3
40 20 0 350
PCL, a.u.
IL intensity, a.u.
140
1
400
4
1
200 3
100
2
450 λ, nm
300
500
550
Fig. 1. The real-time proton-induced luminescence spectra for a-Al2O3 crystals (1, 2, 3, 4) and a-Al2O3:C crystals (5) in its dependency on proton fluence. 1 – 6 1010 cm2; 2 – 1,8 1012 cm2; 3 – 9 1012 cm2; 4 – 5,7 1013 cm2; 5 – 5,7 1013 cm2.
0 350
2
400
450 500 λ, nm
550
600
Fig. 3. The pulse cathodoluminescence spectra for a-Al2O3 crystals after proton irradiation with fluence 5,7 1013 cm2 (1, 2) and unirradiated a-Al2O3:C crystal (3). 1 – PCL registered from irradiated face; 2 – from the opposite side.
364
A.V. Kruzhalov et al. / Radiation Measurements 45 (2010) 362–364
4. Conclusion
2,4
Absorption, a.u.
2,2 2,0 1,8
2
1
1,6 1,4 1,2
200
220
240
260 λ, nm
280
300
320
Fig. 4. Optical absorption spectra for a-Al2O3:C crystals: 1- initial samples; 2 – irradiated with fluence 1016 cm2.
The commensurability of OA bands intensities for F- and Fþcenters in irradiated a-Al2O3 and corresponding maxima for a-Al2O3:C (Fig. 2, curve 2 and Fig. 4, curve 1) may be the evidence of oxygen vacancies numbers comparability for radiation damaged layer (20% of a-Al2O3 volume) and for whole a-Al2O3:C sample. The latter fact finds its proof in the results of F-centers PCL intensities comparison for both types of crystals (curve 1 and curve 3 Fig. 3), if one assumes that the F-centers PCL bands intensities are proportional to their concentration and that the volumes excited (about 60 mm in depth for 150 KeV electrons, as given by empirical equation) are equal. It is clearly seen that the 420 nm luminescence band intensity for crystals excited from the proton-irradiated face is even higher than corresponding intensity for a-Al2O3:C.
The dose dependency of IPL at F-centers band under 5,7 MeV protons excitation of a-Al2O3 monocrystals was investigated. It was shown that IPL intensity growth is accompanied with accumulation of oxygen vacancies and corresponding F- and Fþ-color centers in the crystal lattice. For thermochemically colored crystals IPL almost does not demonstrate rise, reaching the stationary level which is proportional to proton beam current density. This feature can find its application in charged particles diagnostics systems. It was shown by means of PCL that F-centers concentration in thin surface layer of irradiated crystal may exceed the average value for thermochemically colored crystals. References Akselrod, M.S., Botter-Jensen, L., McKeever, S.W.S., 2006. Optical Stimulated Luminescence and Its Use in Madical Dosimetry. Radiat. Meas. 41, 78–99 pp. Botter-Jensen, L., McKeever, S.W.S., Wintle, A.G., 2003. Optically Stimulated Luminescence Dosimetry. Elsevier, Amsterdam, 356 pp. Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. Word Scientific Publishing Co, Singapore, 560 pp. Damkjaer, S.M.S., Andersen, C.E., Aznar, M.C., 2008. Improved Real-Time Dosimetry Using The Radioluminescence Signal from Al2O3:C. Radiat. Meas. 43, 893–897 pp. Edmund, J.M., Andersen, C.E., Greilich, S., et al., 2007. Optically Stimulated Luminescence from Al2O3:C Irradiated with 10-60 MeV Protons. Nucl. Instrum. Methods. Phys. Res. Sect. A 580, 466–468 pp. Kulkarni, M.S., Mishra, D.R., Sharma, D.N., 2007. Versatile Integrated System for Thermoluminescence and Optically Stimulated Luminescence Measurement. Nucl. Instrum. Methods. Phys. Res. Sect. B 262, 348–356 pp. Mittani, J.C.R., Silva, A.A.R., Vanhavere, F., et al., 2007. Investigation of Neutron Converters for Production of Optically Stimulated Luminescence (OSL) Neutron Dosimeters Using Al2O3:C. Nucl. Instrum. Methods. Phys. Res. Sect. B 260, 663–671 pp. McKeever, S.W.S., Benton, E.R., Gaza, R., et al., 2007. Book of Abstract 15th Intern. Conf. on Solid State Dosimetry, Delft, The Netherlands, July 8–13. 35 pp.