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Growth and uniformity improvement of large-size PbWO4 crystal with PbF2 doping
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475 www.elsevier.com/locate/nima
Growth and uniformity improvement of large-size PbWO4 crystal with PbF2 doping Chongzhi Yea,b,, Weidong Xianga, Jingying Liaob a
College of Chemistry and Materials Science, Wenzhou University, Wenzhou 325035, People’s Republic of China Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China
b
Received 8 March 2008; received in revised form 2 April 2008; accepted 7 April 2008 Available online 12 April 2008
Abstract PbF2-doped lead tungstate (PbWO4) scintillation crystals have been grown by the modified vertical Bridgman method along /0 0 1S orientation. Large-size PbWO4 crystals up to 200 mm in length and 24 24 mm2 in section have been obtained reproducibly. The uniformity of the crystal block has been investigated by means of X-ray diffraction (XRD), longitudinal transmittance, X-ray excited luminescence and light yield. The results showed that Bridgman-grown PbWO4 crystals have good optical and scintillation uniformity. r 2008 Elsevier B.V. All rights reserved. Keywords: Lead tungstate; Bridgman technique; Scintillation; Uniformity; Doping
1. Introduction PbWO4 (PWO) single crystals were intensively studied in the last 10 years because of their use as scintillator detectors at LHC experiments at CERN [1]. The large and successful effort invested on the development of lead tungstate crystals has led several other experiments such as BTeV, the ALICE PHOS calorimeter and The Photon Ball to choose this crystal for their detector [2]. One of the serious remaining drawbacks of this material consists in its low light yield, which is about 40 times lower than that of the Bi4Ge3O12 (BGO) [3]. Due to the welldeveloped crystal growth technology, relatively low production cost and excellent scintillating properties of this material, it is worth searching for some ways to increase the light yield of PWO, which would make it suitable for other application communities such as positron emission tomography (PET), which is becoming an increasingly important medical diagnosis for cancers and other diseases.
Corresponding author at: College of Chemistry and Materials Science, Wenzhou University, Wenzhou 325035, People’s Republic of China, Tel.: +86 577 86689641. E-mail address: [email protected] (C. Ye).
0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.04.007
Several attempts have been carried out to increase PWO light yield by an additional appropriate doping [4–6] or annealing treatment [7]. Mo ions are one of the most familiar ions doped in PWO to modify its luminescence [8]. However, small additions of Mo atoms produce a shift of the position of maximum X-ray luminescence from 420 to 500 nm and amplify the very slow component of decay. PWO co-doped by Mo and Nb (1530 and 500 ppm) could give the best luminescence intensity comparable with that of BGO crystals under the same conditions [9]. Co-doping with La or Y [8,10] can suppress such slow decay while the light yield improvement is only about 2.2 times with respect to undoped PWO. A higher light yield was achieved in Mo, Cd and Sb [11] triply doped crystals, but also induced slow luminescence in tens of ms. F ions are one of the most important ions doping in PWO to increase the light yield and improve the transparence in the long-wave region [12,13]. A factor of about 2.5 increase of light yield within 200 ns was achieved compared with undoped PWO samples. Moreover, F ion doping did not change the position of maximum X-ray luminescence in 420 nm [13]. These results reveal that PWO:F crystals might have potential use in PET materialS, and even in optoelectronic applicationS. In the present work, large-size PWO crystals with PbF2 doping have been
ARTICLE IN PRESS C. Ye et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475
473
grown by the vertical Bridgman method. A series of block crystals were cut from a 200 mm length PWO crystal sample and the uniformity of optical properties of largesize PWO:F crystals was characterized. 2. Crystal growth and sample preparation
Fig. 2. X-ray diffraction of PbF2-doped and undoped PWO crystals.
80 70 60 Transmittance (%)
PWO crystals doped with F were grown by the modified vertical Bridgman method. The raw materials were 5 N pure PbO and WO3 powders, and were mixed in stoichiometric ratio. The charge mixture is first melted at about 1150 1C, which is 30 1C above the melting point of pure PWO in a platinum crucible to ensure complete homogeneity, and then poured into a designed thin platinum crucible to form polycrystalline PWO grogs for crystal growth. PbF2 was introduced into the charge mixture by the initial concentration of 1000 at ppm. In order to prevent contamination from the external atmosphere and suppress component deviation from stoichiometric composition due to evaporation in air atmosphere, the designed thin platinum crucible was sealed almost completely. The details of schematic of the furnace and crystal growth technology can be seen in Ref. [14]. Crystal ingots with a size of 25 25 200 mm3 were obtained, and a sequence of block crystal samples was cut from it for uniformity measurement (Fig. 1). All these samples from S1 to S5 are of sizes 24 24 20 mm3, with six faces optically polished. All samples are transparent, colorless and without visible defects such as cracking, inclusions, scattering centers and growth striation. The crystals were annealed at 200 1C for 10 h before optical and scintillation characters were investigated. Samples in X-ray diffraction (XRD) experiments were crushed and ballmilled near sample S3.
S1 S2 S3 S4 S5
50 40 30 20 10 0 300
350
400
450
500
550
600
650
700
Wavelength (nm)
3. Experiments and results
Fig. 3. The optical transmission spectra of samples S1–S5.
3.1. X-ray diffraction Powder XRD data were collected at room temperature on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry (flat graphite monochromator, scintillation counter) using CuKa radiation. The sample was mounted on a standard flat-plate aluminum sample holder; powder diffraction data were recorded in the 2y range of 20–801. c Axis
Top
Seed
20 mm 200 mm Fig. 1. The illustration of five block samples cut from a large-size PWO:F ingot.
Fig. 2 shows the XRD of PbF2-doped and undoped PWO crystal. The XRD patterns of the doped PWO crystal matched very well with that of the undoped sample, which means that PbF2 doping did not change the crystal structure. 3.2. Transmittance The longitudinal transmittance of each block was measured using a SUIMADZU UV-2501(PC)S spectrometer whose accuracy achieved 70.002 abs (Fig. 3). The transmission spectra of samples S1–S5 along the c-axis direction are shown in Fig. 2. From Fig. 2, it can be seen that the transmission spectra of all samples are almost superposed into one curve, except sample S5 for which the transmittance at the 320–700 nm wavelength range was smaller than the other samples. The transmittance of sample S5 was 10% smaller than sample S1 at the peak emission wavelength 420 nm of PWO crystal.
ARTICLE IN PRESS C. Ye et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475
474
3.3. X-ray excited luminescence
50 45 40 Light yield (p.e/MeV)
X-ray excited luminescence (XEL) spectra were measured by using a FluorMain X-ray excited spectrometer (W anticathode target, 80 kV, 4 mA). The luminescence spectra were obtained by 44 W plate grating monochromator and Hamamatsu R928-28 photomultiplier. Fig. 4 shows the XEL spectra of all the samples with approximately the same luminescence at peak wavelength near 425 nm; no significant differences were seen in the luminescence spectra. The blue luminescence of PWO crystal at about 425 nm is attributed to self-trapped excited blue luminescence [15]. And the luminescent intensity of S5 is lower than other samples.
35 30 25 20 15
S1 S2 S4 S5 non-doped PWO
10 5 0 0
100 200 300 400 500 600 700 800 900 1000 1100
3.4. Light yield
Time (ns)
The light yield at room temperature for PWO:F blocks were measured using a Hamamatsu photomultiplier tube (R2059PMT), which has a bialkali photocathode and fused silica window. A collimated gamma ray from 137Cs source was used to excite the sample. The PWO:F crystals were coupled to PMT on the surface of /0 0 1S with Dow Corning 200 fluid, all other faces were wrapped with two layers of Tyvek paper to increase the collected light. The measured results of all blocks are shown in Fig. 5. As can be seen, the light yields of samples S1–S4 are almost same at every time gate point, but the light yield of sample S5 is less than others by above 20%. From the results of above experiments of measuring optical transmittance, XEL and light yield for all samples, it can be concluded that large-size PWO:F crystals grown with the Bridgman method have excellent uniformity of optical properties except the top part. One reason for poor quality of the top part is the evaporation of PbO; during the crystal growth process, the evaporation rate of PbO is faster than that of WO3, which results in large amount of lead vacancies especially in the top part of as-grown PWO
50 S1 S2 S3 S4 S5
Intensity (a.u.)
40
Fig. 5. The light yield of samples S1–S5 and pure PWO in different time gates.
ingots. This deviation will surely damage the performance of PWO crystals; another reason may be contamination elements accumulated in the top part. During crystal growth, some impurities in materials with effective segregation coefficient smaller than 1 such as K+, Na+, Ca2+, Si4+, etc. may concentrate at the top part of PWO:F ingots, which also jeopardize the optical properties. To obtain large-size PWO:F crystals with excellent uniformity of optical properties, purifying raw material, doping and growing ingots may be good measurements. 4. Conclusions Large-size PbWO4 single crystals with PbF2 doping were grown by the modified Bridgman method. The concentration of dopant, PbF2, in the melt was 1000 at ppm. The crystal structure, longitudinal transmittance, XEL and light yield have been investigated. The results showed that Bridgman-grown PbWO4 crystals were of good uniformity in photoluminescence, transmittance and light yield. Acknowledgment The financial support by the Science Technology Foundation of Wenzhou (Grant No. H20060017) is appreciated.
30
20
References
10
0 300
S3
350
400 450 500 Wavelength (nm)
550
Fig. 4. X-ray excited luminescence spectra of samples S1–S5.
600
[1] ECAL CMS Technical Design Report Electromagnetic Calorimeter, 14 December 1997. [2] P. Lecoq, Nucl. Instr. and Meth. A 537 (2005) 15. [3] A.A. Annenkov, M.V. Korzhik, P. Lecoq, Nucl. Instr. Meth. A 490 (2002) 30. [4] A. Annenkov, et al., IEEE Trans. Nucl. Sci. NS-47 (2000) 1311. [5] M. Nikl, P. Boha´cek, E. Miho´kova´, et al., Radiat. Meas. 33 (2001) 705.
ARTICLE IN PRESS C. Ye et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475 [6] M. Kobayashi, Y. Usuki, M. Ishii, M. Nikl, Nucl. Instr. and Meth. A 486 (2002) 170. [7] M. Kobayashi, Y. Usuki, M. Ishii, M. Itoh, Nucl. Instr. and Meth. A 537 (2005) 312. [8] A. Annenkov, A. Borisevitch, A. Hofstaetter, et al., Nucl. Instr. and Meth. A 450 (2000) 71. [9] M. Nikl, P. Bohacek, E. Mihokova, A. Vedda, et al., Radiat. Eff. Defects Solids 157 (2002) 937.
475
[10] M. Nikl, et al., Phys. Status Solidi (A) 182 (2000) R3. [11] M. Kobayashi, et al., Nucl. Instr. and Meth. A 540 (2005) 381. [12] X. Liu, G. Hu, X. Feng, Y. Huang, Y. Zhang, Phys. Status Solidi A 190 (2002) R1. [13] C. Ye, J. Liao, P. Shao, J. Xie, Nucl. Instr. and Meth. A 566 (2006) 757. [14] P.Z. Yang, et al., J. Cryst. Growth 236 (2002) 589. [15] A.N. Belsky, et al., Chem. Phys. Lett. 243 (1995) 552.
Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475 www.elsevier.com/locate/nima
Growth and uniformity improvement of large-size PbWO4 crystal with PbF2 doping Chongzhi Yea,b,, Weidong Xianga, Jingying Liaob a
College of Chemistry and Materials Science, Wenzhou University, Wenzhou 325035, People’s Republic of China Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China
b
Received 8 March 2008; received in revised form 2 April 2008; accepted 7 April 2008 Available online 12 April 2008
Abstract PbF2-doped lead tungstate (PbWO4) scintillation crystals have been grown by the modified vertical Bridgman method along /0 0 1S orientation. Large-size PbWO4 crystals up to 200 mm in length and 24 24 mm2 in section have been obtained reproducibly. The uniformity of the crystal block has been investigated by means of X-ray diffraction (XRD), longitudinal transmittance, X-ray excited luminescence and light yield. The results showed that Bridgman-grown PbWO4 crystals have good optical and scintillation uniformity. r 2008 Elsevier B.V. All rights reserved. Keywords: Lead tungstate; Bridgman technique; Scintillation; Uniformity; Doping
1. Introduction PbWO4 (PWO) single crystals were intensively studied in the last 10 years because of their use as scintillator detectors at LHC experiments at CERN [1]. The large and successful effort invested on the development of lead tungstate crystals has led several other experiments such as BTeV, the ALICE PHOS calorimeter and The Photon Ball to choose this crystal for their detector [2]. One of the serious remaining drawbacks of this material consists in its low light yield, which is about 40 times lower than that of the Bi4Ge3O12 (BGO) [3]. Due to the welldeveloped crystal growth technology, relatively low production cost and excellent scintillating properties of this material, it is worth searching for some ways to increase the light yield of PWO, which would make it suitable for other application communities such as positron emission tomography (PET), which is becoming an increasingly important medical diagnosis for cancers and other diseases.
Corresponding author at: College of Chemistry and Materials Science, Wenzhou University, Wenzhou 325035, People’s Republic of China, Tel.: +86 577 86689641. E-mail address: [email protected] (C. Ye).
0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.04.007
Several attempts have been carried out to increase PWO light yield by an additional appropriate doping [4–6] or annealing treatment [7]. Mo ions are one of the most familiar ions doped in PWO to modify its luminescence [8]. However, small additions of Mo atoms produce a shift of the position of maximum X-ray luminescence from 420 to 500 nm and amplify the very slow component of decay. PWO co-doped by Mo and Nb (1530 and 500 ppm) could give the best luminescence intensity comparable with that of BGO crystals under the same conditions [9]. Co-doping with La or Y [8,10] can suppress such slow decay while the light yield improvement is only about 2.2 times with respect to undoped PWO. A higher light yield was achieved in Mo, Cd and Sb [11] triply doped crystals, but also induced slow luminescence in tens of ms. F ions are one of the most important ions doping in PWO to increase the light yield and improve the transparence in the long-wave region [12,13]. A factor of about 2.5 increase of light yield within 200 ns was achieved compared with undoped PWO samples. Moreover, F ion doping did not change the position of maximum X-ray luminescence in 420 nm [13]. These results reveal that PWO:F crystals might have potential use in PET materialS, and even in optoelectronic applicationS. In the present work, large-size PWO crystals with PbF2 doping have been
ARTICLE IN PRESS C. Ye et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475
473
grown by the vertical Bridgman method. A series of block crystals were cut from a 200 mm length PWO crystal sample and the uniformity of optical properties of largesize PWO:F crystals was characterized. 2. Crystal growth and sample preparation
Fig. 2. X-ray diffraction of PbF2-doped and undoped PWO crystals.
80 70 60 Transmittance (%)
PWO crystals doped with F were grown by the modified vertical Bridgman method. The raw materials were 5 N pure PbO and WO3 powders, and were mixed in stoichiometric ratio. The charge mixture is first melted at about 1150 1C, which is 30 1C above the melting point of pure PWO in a platinum crucible to ensure complete homogeneity, and then poured into a designed thin platinum crucible to form polycrystalline PWO grogs for crystal growth. PbF2 was introduced into the charge mixture by the initial concentration of 1000 at ppm. In order to prevent contamination from the external atmosphere and suppress component deviation from stoichiometric composition due to evaporation in air atmosphere, the designed thin platinum crucible was sealed almost completely. The details of schematic of the furnace and crystal growth technology can be seen in Ref. [14]. Crystal ingots with a size of 25 25 200 mm3 were obtained, and a sequence of block crystal samples was cut from it for uniformity measurement (Fig. 1). All these samples from S1 to S5 are of sizes 24 24 20 mm3, with six faces optically polished. All samples are transparent, colorless and without visible defects such as cracking, inclusions, scattering centers and growth striation. The crystals were annealed at 200 1C for 10 h before optical and scintillation characters were investigated. Samples in X-ray diffraction (XRD) experiments were crushed and ballmilled near sample S3.
S1 S2 S3 S4 S5
50 40 30 20 10 0 300
350
400
450
500
550
600
650
700
Wavelength (nm)
3. Experiments and results
Fig. 3. The optical transmission spectra of samples S1–S5.
3.1. X-ray diffraction Powder XRD data were collected at room temperature on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry (flat graphite monochromator, scintillation counter) using CuKa radiation. The sample was mounted on a standard flat-plate aluminum sample holder; powder diffraction data were recorded in the 2y range of 20–801. c Axis
Top
Seed
20 mm 200 mm Fig. 1. The illustration of five block samples cut from a large-size PWO:F ingot.
Fig. 2 shows the XRD of PbF2-doped and undoped PWO crystal. The XRD patterns of the doped PWO crystal matched very well with that of the undoped sample, which means that PbF2 doping did not change the crystal structure. 3.2. Transmittance The longitudinal transmittance of each block was measured using a SUIMADZU UV-2501(PC)S spectrometer whose accuracy achieved 70.002 abs (Fig. 3). The transmission spectra of samples S1–S5 along the c-axis direction are shown in Fig. 2. From Fig. 2, it can be seen that the transmission spectra of all samples are almost superposed into one curve, except sample S5 for which the transmittance at the 320–700 nm wavelength range was smaller than the other samples. The transmittance of sample S5 was 10% smaller than sample S1 at the peak emission wavelength 420 nm of PWO crystal.
ARTICLE IN PRESS C. Ye et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475
474
3.3. X-ray excited luminescence
50 45 40 Light yield (p.e/MeV)
X-ray excited luminescence (XEL) spectra were measured by using a FluorMain X-ray excited spectrometer (W anticathode target, 80 kV, 4 mA). The luminescence spectra were obtained by 44 W plate grating monochromator and Hamamatsu R928-28 photomultiplier. Fig. 4 shows the XEL spectra of all the samples with approximately the same luminescence at peak wavelength near 425 nm; no significant differences were seen in the luminescence spectra. The blue luminescence of PWO crystal at about 425 nm is attributed to self-trapped excited blue luminescence [15]. And the luminescent intensity of S5 is lower than other samples.
35 30 25 20 15
S1 S2 S4 S5 non-doped PWO
10 5 0 0
100 200 300 400 500 600 700 800 900 1000 1100
3.4. Light yield
Time (ns)
The light yield at room temperature for PWO:F blocks were measured using a Hamamatsu photomultiplier tube (R2059PMT), which has a bialkali photocathode and fused silica window. A collimated gamma ray from 137Cs source was used to excite the sample. The PWO:F crystals were coupled to PMT on the surface of /0 0 1S with Dow Corning 200 fluid, all other faces were wrapped with two layers of Tyvek paper to increase the collected light. The measured results of all blocks are shown in Fig. 5. As can be seen, the light yields of samples S1–S4 are almost same at every time gate point, but the light yield of sample S5 is less than others by above 20%. From the results of above experiments of measuring optical transmittance, XEL and light yield for all samples, it can be concluded that large-size PWO:F crystals grown with the Bridgman method have excellent uniformity of optical properties except the top part. One reason for poor quality of the top part is the evaporation of PbO; during the crystal growth process, the evaporation rate of PbO is faster than that of WO3, which results in large amount of lead vacancies especially in the top part of as-grown PWO
50 S1 S2 S3 S4 S5
Intensity (a.u.)
40
Fig. 5. The light yield of samples S1–S5 and pure PWO in different time gates.
ingots. This deviation will surely damage the performance of PWO crystals; another reason may be contamination elements accumulated in the top part. During crystal growth, some impurities in materials with effective segregation coefficient smaller than 1 such as K+, Na+, Ca2+, Si4+, etc. may concentrate at the top part of PWO:F ingots, which also jeopardize the optical properties. To obtain large-size PWO:F crystals with excellent uniformity of optical properties, purifying raw material, doping and growing ingots may be good measurements. 4. Conclusions Large-size PbWO4 single crystals with PbF2 doping were grown by the modified Bridgman method. The concentration of dopant, PbF2, in the melt was 1000 at ppm. The crystal structure, longitudinal transmittance, XEL and light yield have been investigated. The results showed that Bridgman-grown PbWO4 crystals were of good uniformity in photoluminescence, transmittance and light yield. Acknowledgment The financial support by the Science Technology Foundation of Wenzhou (Grant No. H20060017) is appreciated.
30
20
References
10
0 300
S3
350
400 450 500 Wavelength (nm)
550
Fig. 4. X-ray excited luminescence spectra of samples S1–S5.
600
[1] ECAL CMS Technical Design Report Electromagnetic Calorimeter, 14 December 1997. [2] P. Lecoq, Nucl. Instr. and Meth. A 537 (2005) 15. [3] A.A. Annenkov, M.V. Korzhik, P. Lecoq, Nucl. Instr. Meth. A 490 (2002) 30. [4] A. Annenkov, et al., IEEE Trans. Nucl. Sci. NS-47 (2000) 1311. [5] M. Nikl, P. Boha´cek, E. Miho´kova´, et al., Radiat. Meas. 33 (2001) 705.
ARTICLE IN PRESS C. Ye et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 472–475 [6] M. Kobayashi, Y. Usuki, M. Ishii, M. Nikl, Nucl. Instr. and Meth. A 486 (2002) 170. [7] M. Kobayashi, Y. Usuki, M. Ishii, M. Itoh, Nucl. Instr. and Meth. A 537 (2005) 312. [8] A. Annenkov, A. Borisevitch, A. Hofstaetter, et al., Nucl. Instr. and Meth. A 450 (2000) 71. [9] M. Nikl, P. Bohacek, E. Mihokova, A. Vedda, et al., Radiat. Eff. Defects Solids 157 (2002) 937.
475
[10] M. Nikl, et al., Phys. Status Solidi (A) 182 (2000) R3. [11] M. Kobayashi, et al., Nucl. Instr. and Meth. A 540 (2005) 381. [12] X. Liu, G. Hu, X. Feng, Y. Huang, Y. Zhang, Phys. Status Solidi A 190 (2002) R1. [13] C. Ye, J. Liao, P. Shao, J. Xie, Nucl. Instr. and Meth. A 566 (2006) 757. [14] P.Z. Yang, et al., J. Cryst. Growth 236 (2002) 589. [15] A.N. Belsky, et al., Chem. Phys. Lett. 243 (1995) 552.