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The melt growth of large LuAP single crystals for PET scanners
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172 www.elsevier.com/locate/nima
The melt growth of large LuAP single crystals for PET scanners Ashot Petrosyana,, Karine Ovanesyana, Grigory Shirinyana, Tatyana Butaevaa, Marina Derdzyana, Christian Pedrinib, Christophe Dujardinb, Nicolas Garnierb, Irina Kamenskikhb,c a
Laboratory of Crystal Growth of Luminescent Materials, Institute for Physical Research, Armenian National Academy of Science, Ashtarak-2, Ashtarak-2378410, Armenia b Laboratoire de Physico-Chimie des Materaux Luminescents, Universite Lyon1, Campus de La Doua, 69622 Villeurbanne, France c Synchrotron Radiation Laboratory, Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119899, Russia Crystal Clear Collaboration Available online 21 August 2004
Abstract Performance properties of LuAP, a material of highly promising potential for future PET scanners, are presented, as they relate to crystal growth and composition. The light yield measured in 2 2 10 mm3 elements with 0.4–0.5% Ce and cut from large size crystals (100 mm long and 15 mm in diameter) grown by the Bridgman technique is improved to 40% LSO. The ratio between light yield measured in vertical and horizontal arrangements in the best crystals is near 90%. The role of chemical purity in respect to divalent impurities is studied. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.-h; 61.72-s; 29.40-m Keywords: Lutetium; Perovskite; Cerium; Bridgman; Impurities; Light yield
1. Introduction The Crystal Clear Collaboration is constructing small animal positron emission tomography ClearPET scanners, which will offer better sensitivity and resolution, than what is available in existing Corresponding author. Tel.: +374-1-288150; fax: +374-3-
231172. E-mail address: [email protected] (A. Petrosyan).
systems [1]. Future developments are foreseen to include brain and PEM scanners, as well as full body systems. At present, LSO/LuYAP phoswich matrix is under construction based on LuYAP of Lu0.7Y0.3AlO3:Ce composition. However, LuAP (LuAlO3:Ce) with its unsurpassed decay remains the most desirable material for PET scanners. The first single-phase LuAP crystals grown by the Bridgman method were made available in the middle of 1990s [2,3]. Since then, a broad spectrum
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.259
ARTICLE IN PRESS A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
of compositions has been grown to establish composition-dependent properties [4,5]. Reliable and convenient set ups for comparable and rapid scintillation measurements were developed. A parallel effort to improve the quality resulted in various useful approaches to obtain the required crystals. The present study gives the status of Bridgman LuAP based on characterization of several dozens of single crystals; it is intended to provide an insight into some important properties, such as light yield or self-absorption, as they relate to crystal growth and composition.
2. Crystals and experimental methods LuAP single crystals were grown from the melt using the Bridgman technique as described previously in Ref. [3]. Lutetium oxide was from NeoChem (Moscow) with total residual rareearths about 10 ppm with major input from Er (5 ppm) and Yb (3 ppm). Among the divalent impurities the major input is from Ca (5 ppm). For aluminum oxide, sapphire crackle was used coming from traditional Bridgman or Verneuil production. Single crystals were grown in diameters 10–15 mm with typical boule length 60–100 mm. Optical microscopy, X-ray, optical absorption and UV-irradiation techniques were applied for characterization of crystals. The Ce concentration was estimated by optical methods using calibration dependences based on analytical measurements. 2 2 10 mm3 elements polished on all sides were fabricated for scintillation light yield measurements in LPCML using a specially designed roundabout with identical clusters for individual elements and providing for a high relative accuracy of measurements. The elements were coated with Teflon tape and coupled in the vertical position to XP2020Q photomultiplier. Excitation from 137Cs-gamma source and shaping time of 2 ms were used [6]. The light yield was determined by comparing the photopeak position to that of a reference LSO sample polished to the same dimensions (the light
169
yield of LSO is generally considered to be 25,000 photons/MeV).
3. Results and discussion 3.1. Crystal defects Defects present in as-grown LuAP crystals can be grouped into several types. The typical macroscopic defects are solid or gaseous inclusions, twin boundaries and cracks; their density is controlled by appropriate selection of the growth rate, temperature gradients and the cooling rate. The second type of defects is color centers, which degrade optical transmission in the range of Ce3+ emission. They reflect the occurrence of traps commonly observed as glow peaks in thermoluminescence spectra and which are capable of interfering with energy transfer to Ce3+ ions. Finally, non-uniform Ce3+ distribution (k ¼ 0:17 [3]) results in concentration gradients over the length of elements. In longitudinally cut 10 mm long elements the concentration varies from one end to another due to normal freeze distribution; in transversally cut elements the concentration is lower in the center and higher at the ends, which is due to a non-planar shape of the solid–liquid interface. In elements cut from large crystals the measured concentration gradients are about 5%. 3.2. Orientation Analysis of the resulting crystal orientations has shown that it is impossible to single out one direction of preferential growth. A large variety of orientations occurs, e.g. /1 1 6S, /1 1 3S, /2 0 4S, etc. A parameter uniting the variety of measured crystallographic directions is that for all of them, the interplane distance is small and lies in the 1.156–1.634 A˚ range. Knowledge of crystal orientation is important in many cases and, in particular, in measurements of Ce3+ concentration by optical means, since the line intensity is orientation dependent (Fig. 1). For these measurements three 0.15 mm thick plates were cut from a large oriented sample parallel to its faces and corresponding to the major crystal-
ARTICLE IN PRESS A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
170
120
45
// b
100
40
// c
35
80
Light Yield % LSO
Absorption coefficient, cm-1
// a
60
40
30
25
20
2002
20
2003 Large boule
15
0 200
240
280
320 λ, nm
360
400 10 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55
Fig. 1. Absorption spectra measured in LuYAP (Lu=80%) plates oriented along the major crystallographic axes.
Ce concentration (at.%)
Fig. 2. Light yield in various LuAP series as a function of Ce concentration.
lographic directions—a, b and c. As seen from the figure, the background absorption also slightly varies as a function of crystal orientation.
60 55 50
3.3. Scintillation light yield The scintillation light yield measured in the vertical geometry in several series of 2 2 10 mm3 LuAP as a function of Ce concentration is given in Fig. 2. The slope for the series cut from large crystals is steeper suggesting a higher crystal quality. The highest light yield measured in elements cut from large boules is near 40%LSO. Corresponding figures for comparable size LuAP grown by the Czochralski method were reported in Ref. [7]. The light yield in the vertical arrangement measured from eighteen 2 2 10 mm elements cut transversally from a single large LuAP boule is shown in Fig. 3. The gradual increase of the light in this series reflects the fact that the Ce3+ ion concentration increases along the length of the boule due to the normal freeze distribution. The ratio between vertical and horizontal geometry in the best crystals is about 90% (60–70 % in the most crystals). (Fig. 4), which indicates a high optical quality and only a weak self-absorption. For elements cut from small size crystals, the earlier reported ratio was 30% for
Light Yield, % LSO
45
#546 - LuAP: Ce 2x2x10mm3 vertical arrangement
40 35 30 25 20 15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Crystal number
Fig. 3. Light yield measured in a series of 2 2 10 mm3 LuAP elements cut from a single boule.
LuAP and 56% for LuYAP [8]. The thickness dependence of the light output is generally attributed to the presence of a broad band background absorption in the range, where Ce3+ absorption bands are located and which is a result of Ce doping. The underlying absorption is present in LuAP crystals grown by both the Czochralski [9] and Bridgman [6] techniques and may extend to
ARTICLE IN PRESS A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
171
1.8
400 1.6
LuAP: Ca(20 ppm)
LuAP # 448 1 - horizontal position 2 - vertical position
1.4
350 300 Counts
Norm. counts
1.2 1.0 0.8
250 200
0.6
150
0.4
100
2
1
0.2
LuAP
50
0.0 500
1000
0 200
Channel No
Fig. 4. Energy spectra for LuAP measured in the vertical and horizontal geometry.
400 nm thus overlapping the Ce3+ emission. The light yield of large crystals is therefore affected by self-absorption of the Ce3+ emission, as the distance covered by the emitted photons increases.
3.4. The role of chemical purity It is clear that while an overall chemical purity is desirable, not all of unintentional impurities are capable of damaging the scintillation. Divalent impurities have been considered as detrimental to perovskite-type materials, especially those doped with Pr3+ or Ce3+. Their function would include distortions of the lattice, but also increasing the concentration of anion vacancies (or potential Fcenters), or stimulating Ce4+ states. In LuAP intentionally co-doped with calcium in ppm amounts, the underlying absorption drastically increases, as compared to that in LuAP. Calcium co-doped LuAP exhibits no photopeak, when measured under identical conditions with a similar size LuAP (2 2 10 mm) (Fig. 5). Rare-earth ions, which can be stabilized in divalent state (Yb, Eu), are known to be capable of quenching Ce3+ scintillation in some crystals, and therefore they may also be detrimental to LuAP.
300
400
500
600
700
Chanel No Fig. 5. Energy spectra for LuAP and calcium co-doped LuAP single crystals.
4. Conclusions Large size, optical quality LuAP crystals have been grown by the Bridgman technique. The light yield measured in 2 2 10 mm3 elements in vertical geometry is near 40% LSO. The ratio between vertical and horizontal geometry in the best crystals is about 90%. Residual calcium in ppm amounts is capable of degrading the scintillation.
Acknowledgements This work is carried out in the frame of the Crystal Clear Collaboration and supported by ISTC. References [1] E. Auffray, P. Bruyndonckx, O. Devroede, A. Fedorov, et al., Nucl. Instr. and Meth. A 527 (2004) 171. [2] C. Dujardin, C. Pedrini, D. Bouttet, W. Verweij, et al., in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT’95, Delft University Press, Delft, 1996, pp. 336–339. [3] A.G. Petrosyan, C. Pedrini, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT’95, Delft University Press, Delft, 1996, pp. 498–501.
ARTICLE IN PRESS 172
A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
[4] A.G. Petrosyan, G.O. Shirinyan, K.L. Ovanesyan, C. Pedrini, et al., Nucl. Instr. and Meth. A 486 (2002) 74. [5] A.G. Petrosyan, K.L. Ovanesyan, G.O. Shirinyan, M.V. Derzyan, et al., Radiat. Eff. Def. Solids 157 (2002) 943. [6] C. Dujardin, C. Pedrini, W. Blanc, J.C. Gacon, et al., J. Phys.: Condens. Matter 10 (1998) 3061.
[7] W.W. Moses, S.E. Derenzo, A. Fyodorov, M. Korzhik, et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 275. [8] C. Kuntner, H. Aniger, E. Auffray, J. Glodo, et al., Nucl. Instr. and Meth. A 486 (2002) 176. [9] A. Lempicki, M.H. Randles, D. Wisniewski, M. Balcerzyk, et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 280.
Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172 www.elsevier.com/locate/nima
The melt growth of large LuAP single crystals for PET scanners Ashot Petrosyana,, Karine Ovanesyana, Grigory Shirinyana, Tatyana Butaevaa, Marina Derdzyana, Christian Pedrinib, Christophe Dujardinb, Nicolas Garnierb, Irina Kamenskikhb,c a
Laboratory of Crystal Growth of Luminescent Materials, Institute for Physical Research, Armenian National Academy of Science, Ashtarak-2, Ashtarak-2378410, Armenia b Laboratoire de Physico-Chimie des Materaux Luminescents, Universite Lyon1, Campus de La Doua, 69622 Villeurbanne, France c Synchrotron Radiation Laboratory, Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119899, Russia Crystal Clear Collaboration Available online 21 August 2004
Abstract Performance properties of LuAP, a material of highly promising potential for future PET scanners, are presented, as they relate to crystal growth and composition. The light yield measured in 2 2 10 mm3 elements with 0.4–0.5% Ce and cut from large size crystals (100 mm long and 15 mm in diameter) grown by the Bridgman technique is improved to 40% LSO. The ratio between light yield measured in vertical and horizontal arrangements in the best crystals is near 90%. The role of chemical purity in respect to divalent impurities is studied. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.-h; 61.72-s; 29.40-m Keywords: Lutetium; Perovskite; Cerium; Bridgman; Impurities; Light yield
1. Introduction The Crystal Clear Collaboration is constructing small animal positron emission tomography ClearPET scanners, which will offer better sensitivity and resolution, than what is available in existing Corresponding author. Tel.: +374-1-288150; fax: +374-3-
231172. E-mail address: [email protected] (A. Petrosyan).
systems [1]. Future developments are foreseen to include brain and PEM scanners, as well as full body systems. At present, LSO/LuYAP phoswich matrix is under construction based on LuYAP of Lu0.7Y0.3AlO3:Ce composition. However, LuAP (LuAlO3:Ce) with its unsurpassed decay remains the most desirable material for PET scanners. The first single-phase LuAP crystals grown by the Bridgman method were made available in the middle of 1990s [2,3]. Since then, a broad spectrum
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.259
ARTICLE IN PRESS A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
of compositions has been grown to establish composition-dependent properties [4,5]. Reliable and convenient set ups for comparable and rapid scintillation measurements were developed. A parallel effort to improve the quality resulted in various useful approaches to obtain the required crystals. The present study gives the status of Bridgman LuAP based on characterization of several dozens of single crystals; it is intended to provide an insight into some important properties, such as light yield or self-absorption, as they relate to crystal growth and composition.
2. Crystals and experimental methods LuAP single crystals were grown from the melt using the Bridgman technique as described previously in Ref. [3]. Lutetium oxide was from NeoChem (Moscow) with total residual rareearths about 10 ppm with major input from Er (5 ppm) and Yb (3 ppm). Among the divalent impurities the major input is from Ca (5 ppm). For aluminum oxide, sapphire crackle was used coming from traditional Bridgman or Verneuil production. Single crystals were grown in diameters 10–15 mm with typical boule length 60–100 mm. Optical microscopy, X-ray, optical absorption and UV-irradiation techniques were applied for characterization of crystals. The Ce concentration was estimated by optical methods using calibration dependences based on analytical measurements. 2 2 10 mm3 elements polished on all sides were fabricated for scintillation light yield measurements in LPCML using a specially designed roundabout with identical clusters for individual elements and providing for a high relative accuracy of measurements. The elements were coated with Teflon tape and coupled in the vertical position to XP2020Q photomultiplier. Excitation from 137Cs-gamma source and shaping time of 2 ms were used [6]. The light yield was determined by comparing the photopeak position to that of a reference LSO sample polished to the same dimensions (the light
169
yield of LSO is generally considered to be 25,000 photons/MeV).
3. Results and discussion 3.1. Crystal defects Defects present in as-grown LuAP crystals can be grouped into several types. The typical macroscopic defects are solid or gaseous inclusions, twin boundaries and cracks; their density is controlled by appropriate selection of the growth rate, temperature gradients and the cooling rate. The second type of defects is color centers, which degrade optical transmission in the range of Ce3+ emission. They reflect the occurrence of traps commonly observed as glow peaks in thermoluminescence spectra and which are capable of interfering with energy transfer to Ce3+ ions. Finally, non-uniform Ce3+ distribution (k ¼ 0:17 [3]) results in concentration gradients over the length of elements. In longitudinally cut 10 mm long elements the concentration varies from one end to another due to normal freeze distribution; in transversally cut elements the concentration is lower in the center and higher at the ends, which is due to a non-planar shape of the solid–liquid interface. In elements cut from large crystals the measured concentration gradients are about 5%. 3.2. Orientation Analysis of the resulting crystal orientations has shown that it is impossible to single out one direction of preferential growth. A large variety of orientations occurs, e.g. /1 1 6S, /1 1 3S, /2 0 4S, etc. A parameter uniting the variety of measured crystallographic directions is that for all of them, the interplane distance is small and lies in the 1.156–1.634 A˚ range. Knowledge of crystal orientation is important in many cases and, in particular, in measurements of Ce3+ concentration by optical means, since the line intensity is orientation dependent (Fig. 1). For these measurements three 0.15 mm thick plates were cut from a large oriented sample parallel to its faces and corresponding to the major crystal-
ARTICLE IN PRESS A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
170
120
45
// b
100
40
// c
35
80
Light Yield % LSO
Absorption coefficient, cm-1
// a
60
40
30
25
20
2002
20
2003 Large boule
15
0 200
240
280
320 λ, nm
360
400 10 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55
Fig. 1. Absorption spectra measured in LuYAP (Lu=80%) plates oriented along the major crystallographic axes.
Ce concentration (at.%)
Fig. 2. Light yield in various LuAP series as a function of Ce concentration.
lographic directions—a, b and c. As seen from the figure, the background absorption also slightly varies as a function of crystal orientation.
60 55 50
3.3. Scintillation light yield The scintillation light yield measured in the vertical geometry in several series of 2 2 10 mm3 LuAP as a function of Ce concentration is given in Fig. 2. The slope for the series cut from large crystals is steeper suggesting a higher crystal quality. The highest light yield measured in elements cut from large boules is near 40%LSO. Corresponding figures for comparable size LuAP grown by the Czochralski method were reported in Ref. [7]. The light yield in the vertical arrangement measured from eighteen 2 2 10 mm elements cut transversally from a single large LuAP boule is shown in Fig. 3. The gradual increase of the light in this series reflects the fact that the Ce3+ ion concentration increases along the length of the boule due to the normal freeze distribution. The ratio between vertical and horizontal geometry in the best crystals is about 90% (60–70 % in the most crystals). (Fig. 4), which indicates a high optical quality and only a weak self-absorption. For elements cut from small size crystals, the earlier reported ratio was 30% for
Light Yield, % LSO
45
#546 - LuAP: Ce 2x2x10mm3 vertical arrangement
40 35 30 25 20 15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Crystal number
Fig. 3. Light yield measured in a series of 2 2 10 mm3 LuAP elements cut from a single boule.
LuAP and 56% for LuYAP [8]. The thickness dependence of the light output is generally attributed to the presence of a broad band background absorption in the range, where Ce3+ absorption bands are located and which is a result of Ce doping. The underlying absorption is present in LuAP crystals grown by both the Czochralski [9] and Bridgman [6] techniques and may extend to
ARTICLE IN PRESS A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
171
1.8
400 1.6
LuAP: Ca(20 ppm)
LuAP # 448 1 - horizontal position 2 - vertical position
1.4
350 300 Counts
Norm. counts
1.2 1.0 0.8
250 200
0.6
150
0.4
100
2
1
0.2
LuAP
50
0.0 500
1000
0 200
Channel No
Fig. 4. Energy spectra for LuAP measured in the vertical and horizontal geometry.
400 nm thus overlapping the Ce3+ emission. The light yield of large crystals is therefore affected by self-absorption of the Ce3+ emission, as the distance covered by the emitted photons increases.
3.4. The role of chemical purity It is clear that while an overall chemical purity is desirable, not all of unintentional impurities are capable of damaging the scintillation. Divalent impurities have been considered as detrimental to perovskite-type materials, especially those doped with Pr3+ or Ce3+. Their function would include distortions of the lattice, but also increasing the concentration of anion vacancies (or potential Fcenters), or stimulating Ce4+ states. In LuAP intentionally co-doped with calcium in ppm amounts, the underlying absorption drastically increases, as compared to that in LuAP. Calcium co-doped LuAP exhibits no photopeak, when measured under identical conditions with a similar size LuAP (2 2 10 mm) (Fig. 5). Rare-earth ions, which can be stabilized in divalent state (Yb, Eu), are known to be capable of quenching Ce3+ scintillation in some crystals, and therefore they may also be detrimental to LuAP.
300
400
500
600
700
Chanel No Fig. 5. Energy spectra for LuAP and calcium co-doped LuAP single crystals.
4. Conclusions Large size, optical quality LuAP crystals have been grown by the Bridgman technique. The light yield measured in 2 2 10 mm3 elements in vertical geometry is near 40% LSO. The ratio between vertical and horizontal geometry in the best crystals is about 90%. Residual calcium in ppm amounts is capable of degrading the scintillation.
Acknowledgements This work is carried out in the frame of the Crystal Clear Collaboration and supported by ISTC. References [1] E. Auffray, P. Bruyndonckx, O. Devroede, A. Fedorov, et al., Nucl. Instr. and Meth. A 527 (2004) 171. [2] C. Dujardin, C. Pedrini, D. Bouttet, W. Verweij, et al., in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT’95, Delft University Press, Delft, 1996, pp. 336–339. [3] A.G. Petrosyan, C. Pedrini, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT’95, Delft University Press, Delft, 1996, pp. 498–501.
ARTICLE IN PRESS 172
A. Petrosyan et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 168–172
[4] A.G. Petrosyan, G.O. Shirinyan, K.L. Ovanesyan, C. Pedrini, et al., Nucl. Instr. and Meth. A 486 (2002) 74. [5] A.G. Petrosyan, K.L. Ovanesyan, G.O. Shirinyan, M.V. Derzyan, et al., Radiat. Eff. Def. Solids 157 (2002) 943. [6] C. Dujardin, C. Pedrini, W. Blanc, J.C. Gacon, et al., J. Phys.: Condens. Matter 10 (1998) 3061.
[7] W.W. Moses, S.E. Derenzo, A. Fyodorov, M. Korzhik, et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 275. [8] C. Kuntner, H. Aniger, E. Auffray, J. Glodo, et al., Nucl. Instr. and Meth. A 486 (2002) 176. [9] A. Lempicki, M.H. Randles, D. Wisniewski, M. Balcerzyk, et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 280.