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La-doped PbWO4 scintillating crystals grown in large ingots
Nuclear Instruments and Methods in Physics Research A 414 (1998) 325—331
La-doped PbWO scintillating crystals grown in 4 large ingots Kazuhiko Hara!, Mitsuru Ishii", Masaaki Kobayashi#,*, Martin Nikl$, Hideaki Takano!, Masashi Tanaka!, Kiyokazu Tanji", Yoshiyuki Usuki% ! Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan " SIT, Shonan Institute of Technology, Fujisawa 251-0046, Japan # High Energy Accelerator Research Organization (KEK), Tsukuba-Shi, Ibaraki-ken, 305-0801, Japan $ Institute of Physics, Academy of Sciences of Czech Republic, Cukrovarnicka 10, 16200 Prague, Czech Republic % Furukawa Co., Kamiyoshima, Yoshima, Iwaki 970-1153, Japan Received 1 April 1998
Abstract We achieved a significant increase in the ingot size of La-doped PbWO single crystals to 65 mm in diameter and 4 300 mm in length by using the Czochralski technique and three times of recrystallization. Four blocks each with a typical size of 20]20]230 mm3 were taken from an ingot. The optical as well as the scintillation characteristics of the large-size crystals, including the radiation hardness, were found to be similar to or even better than those of smaller crystals. The radiation-induced absorption coefficient k was as small as 0.5—1 m~1 at 107 rad of 60Co c-ray irradiation. ( 1998 *3 Elsevier Science B.V. All rights reserved.
1. Introduction In our previous papers [1—3], we reported that La doping improves dramatically the characteristics of lead tungstate (PbWO or PWO) scintillat4 ing crystals including optical transmittance, decay time and radiation hardness. The obtained improvements can be regarded as a significant breakthrough to push this material to a new stage for practical use in c-ray detectors at high energies.
* Corresponding author. Tel.: #81 298 64 5427; fax: #81 298 64 7831; e-mail: [email protected].
Then, we turned our efforts to increasing the ingot size so that four blocks each with a typical size of 2]2]23 cm3 could be cut from an ingot. The increase in the ingot size enables the reduction of the cost per crystal block. The ingot size, which was originally 3.5 cm (diameter)]8 cm (length), has been successfully increased in two steps, first to 4 cm]25 cm and then to 6.5 cm]30 cm. We evaluated the scintillation characteristics, including radiation hardness, of the large ingots in order to verify that the characteristics should not be degraded by increasing the ingot size but could be improved. The aim of the present paper is to describe the result of the above-mentioned evaluation.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 3 4 - 2
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2. Test samples The test samples are listed in Table 1. Two blocks (PWO24-1 and -2) each with a size of 2]2]9 cm3 were taken from an ingot (PWO24) of 4 cm (diameter) ]25 cm (length) on the seed and the tail sides, respectively. Four blocks each with a size of 2]2]23 cm3 were cut from an ingot (PWO25) of 6.5 cm (diameter)]30 cm (length). Three of them (PWO25-1, -2 and -3) were tested in the present study. Two small test pieces, PWO25U and PWO25L, were also cut from the ingot PWO25 near the seed and the tail ends, respectively. Both ingots (PWO24 and PWO25) were grown in the same way as before [4], i.e. by three times of recrystallization in air in the Czochralski method using a platinum crucible of 19 cm (diameter)] 16 cm (depth). The nominal purities of raw materials (PbO and WO ) were 99.99%. A small amount 3 of La was added to the melt in the third recrystallization process with a concentration of 135 atomic ppm (at. ppm). Table 2 gives a typical result of the element analysis made for both the seed and the tail ends of the ingot PWO25 by using a glow discharge mass spectrometer (GDMS) or an inductioncoupled plasma spectrometer (ICP). The La concentration in the PWO25 ingot ranged from 220 (near the seed end at the solidification fraction g"0.01) to 140 (near the bottom end at g"0.4) at. ppm, giving a segregation coefficient of 1.1—1.2. The La concentration in the PWO24 ingot was 262 at. ppm at g"0.15 and 230 at. ppm at g"0.24, giving
a segregation coefficient of 2.5—2.7. The larger concentration of La in PWO24 than in PWO25 is discussed later in Section 3.5 in relation to the higher radiation hardness obtained in the former than the latter crystal.
3. Results 3.1. Transmission Transmission spectra were measured with a spectrophotometer (Hitachi 220 or U-3400). The transmission spectra of PWO25-1 across the length (23 cm) and the thickness (2 cm) are compared in Fig. 1. From the difference between the two spectra, the internal attenuation coefficient a can be deduced according to a formula ¹"¹ exp(!az), 0 where ¹ and z denote the transmittance and the longitudinal distance, respectively. The magnitude of a was approximately 0.014—0.016 cm~1 at the peak emission wavelength 420 nm of PWO in all the samples (PWO25-1,-2,-3, PWO24-1, -2). 3.2. Excitation-emission spectra We measured photoluminescence from the crystal surface by using a fluorescence spectrophotometer (Hitachi F4500). The emission and excitation peaks sit at around 415 and 315 nm, respectively, in both large ingots (see Table 1); this
Table 1 List of the tested La-doped PbWO samples. j and j denote the excitation and emission peak wavelengths, respectively, and LY the 4 %9 %. scintillation light intensity for 60Co c-rays (see text) Sample name
Size (cm)
Ingot
j /j %9 %. (nm)
LY p.e./MeV
Notes
PWO24-1 PWO24-2 PWO25-1 PWO25-2 PWO25-3 PWO25U PWO25L
2]2]9 2]2]9 2]2]23 2]2]23 2]2]23 1]1]2 1]1]2
PWO24 PWO24 PWO25 PWO25 PWO25 PWO25 PWO25
315/415 315/415 — — — 315/415 315/415
11.1! — 6.0! — 7.2 31 30
Seed-side half of the ingot Tail-side half of the ingot
!Measured about 20 days after the irradiation by 107 rad.
Seed end of the ingot Tail end of the ingot
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Table 2 A GDMS result of the ingot PWO25 measured close to the seed and the tail ends. The impurities are given in units of at. ppm. Element
Seed end
Tail end
Li Be B Na Mg Si S Cl K Ti Cr Mn Ni Co Cu Zn Ge Se Rb Nb Mo Cd Sb La! Ce Te Pt Bi
(0.001 (0.001 0.08 0.43 0.06 0.80 0.12 0.39 0.28 0.13 0.008 0.06 0.11 (0.003 0.02 0.03 (0.003 0.34 (0.001 (0.006 (0.02 (0.05 0.02 223! (0.04 (0.003 (0.01 (0.002
(0.002 (0.002 (0.003 0.32 0.05 0.95 0.18 0.29 0.43 0.13 0.11 0.12 0.09 (0.003 0.02 0.02 (0.004 0.41 (0.001 (0.007 (0.01 (0.02 (0.03 144! (0.03 0.02 1.41" (0.001
!The La concentration obtained from the ICP analysis is given. The obtained values for La were about three times larger than those obtained by the GDMS. We take the values obtained by the ICP analysis because the calibration (the effective sample volume used for the analysis, corrections on the efficiency, etc.) is better controlled in the ICP as far as La is concerned. "The large Pt concentration must come from the Pt crucible.
result was the same as in smaller ingots [5]. No sizable differences were seen between the seed and the tail ends.
3.3. Scintillation light intensity We measured the scintillation light intensity produced by 60Co c-rays in the small samples (PWO25U and PWO25L) by mounting each
Fig. 1. Comparison of the transmission spectra of PWO25-1 across the length of 23 cm (longitudinal) and the width of 2 cm (transversal).
crystal on a 2 in photomultiplier tube (PMT, Hamamatsu R2259) with a bialkali photocathode and a silica window. The pulse height was analyzed by a PHA (LeCroy qVt) in the charge mode within a 1 ls gate. The 1.25 MeV peak, the average of the 1.17 and 1.33 MeV photoelectron peaks, was seen on a broad background in the pulse height spectrum. From a comparison of the peak channel and the width of the peak, we found that the light intensity obtained in the present samples was similar to those in the samples cut from the smaller ingots described in Ref. [5]. More precise absolute measurement was carried out by comparing the position of the 1.25 MeV peak with the single photoelectron (p.e.) peak. The latter peak was created by employing an LED pulser mounted at a large distance from the PMT. The crystal face of 2]2 cm2 (1]2 cm2 for the small samples PWO25U and PWO25L) was coupled to the PMT and a 60Co source was placed on the opposite face. As seen in Table 1, the light intensity was about 31 p.e./MeV in the small sample (PWO25L), 11 p.e./MeV in the 9 cm long sample PWO24-1 (after 107 rad irradiation) and 7 p.e./MeV in the 23 cm long sample PWO25-3. The obtained light intensities seem to be similar to those reported in a recent paper by the Crystal Clear Collaboration [6], although direct comparison is difficult due to different measuring
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conditions; in the measurement described in [6], the crystals are tapered so as to make the light collection more efficient, and the c-ray source was placed closer to the PMT (at 2.5 cm from the PMT). 3.4. Decay time We measured the decay time spectra within 1 ls using a conventional single photoelectron technique. The start pulse for the time measurement was taken from a small Gd SiO : Ce (GSO : Ce) 2 5 scintillator, which was mounted on a 2 in PMT (Hamamatsu R329 with a bialkali photocathode) and set close to a 60Co c-ray source. The stop pulse was the single photoelectron signal of the PbWO : La crystal mounted on another fast 2 in. 4 PMT (Hamamatsu H3177) with a bialkali photocathode and a silica window. The decay time spectrum was registered with a time to pulse height converter (LeCroy qVt). The obtained spectrum was similar to that given in Ref. [1] and fitted with two exponentials superimposed on a constant background. The obtained result was similar to that obtained in the samples cut from the smaller PWO : La ingots described in Ref. [1]; q"5.1 ns (for 93% of the total intensity) and q"31 ns (7%) in PWO24-1, and q"3.9 ns (84%) and q"15 ns (16%) in PWO25L. As described in Ref. [1], two exponentials were enough to give a good fit for the La-doped crystals, instead of three exponentials necessary for undoped crystals. 3.5. Radiation damages Radiation damages were measured by irradiating the samples with 60Co c-rays at Japan Atomic Energy Research Institute (JAERI). Five cycles of irradiation followed by a measurement of transmission and light intensity were carried out to cover the accumulated dose from 103 to 107 rad (10 to 105 Gy)1 by increasing it ten times per cycle. The irradiation period was 13, 10, 9, 33 min and 4 h for 103, 104, 105, 106, and 107 rad, respectively. The 11 Roentgen roughly corresponds to 1 rad in 2 cm thick PbWO at the energies of 60Co c-rays (1.25 MeV on the aver4 age).
Fig. 2. Sketch of the setup for irradiating the long PWO samples. 60Co sources in thin pellets were arranged in a cylindrical shape. The dose profile (I ) along the crystal length is also c shown. The distance D was adjusted to give the required dose rate. Each sample was rotated by 180° along the vertical axis at the half-time of the total irradiation period to achieve uniform irradiation (see text).
dose rate was adjusted by mounting the samples at different distances from the calibrated 60Co source. We employed a setup as sketched in Fig. 2, where the dose profile along the crystal length is shown. The samples were rotated by 180° at the half time of each irradiation period in order to make the irradiation uniform along the crystal thickness (2 cm). The measurement of the damages was begun just after the interruption of each cycle of irradiation, and completed in 30 min. Two different quantities were measured. One was the transmittance of the light from a NaI : Tl-241Am and a GSO : Ce241Am light pulsers2 [7] through the length (9 or 23 cm) of the PWO crystals (see Fig. 3a). An additional NaI : Tl-241Am light pulser was mounted directly on the PMT window in order to monitor the gain of the PMT. The other was the scintillation intensity in the PWO crystal for 60Co c-rays (see Fig. 3b). For a c-ray entering the PWO crystal, coincidence was required between the signals of the
2For the NaI : Tl-241Am light pulser, see [7]. The GSO : Ce241Am light pulser was newly constructed by replacing the NaI : Tl crystal in the NaI : Tl-241Am light pulser with a small GSO : Ce crystal (5 mm in diameter and 1 mm in thickness), and by removing the glass window. The FWHM energy resolution of the 5.49 MeV 241Am alphas in the GSO : Ce-241Am light pulser was 13% and their GEE (gamma equivalent energy) was 1.15 MeV.
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Fig. 3. Sketch of the measuring setups for radiation damages (see text). (a) Left: measurement of the transmission of light from a constant amplitude light pulser (NaI : Tl-241Am and GSO : Ce-241Am). (b) Right: measurement of the scintillation light intensity for 60Co c-rays. G.G.: gate generator.
two PMTs (PMT1 and PMT2) viewing the sample from both ends. The discrimination levels of both PMTs were set at a level slightly smaller than the single p.e. signal. The single p.e. signal as a calibration source was created by injecting weak light of an LED through optical fibers (see Fig. 3b). The result of the transmission measurement using the NaI : Tl light pulser is presented in Fig. 4a. It is to be noted that the emission of NaI : Tl has a peak at &420 nm similarly to that of PWO. The transmittance was degraded by about 38% across 23 cm in PWO25-1 (by about 12% across 8.6 cm in PWO24-1) after the irradiation by 107 rad. The corresponding result of the measurement of scintillation intensity is given in Fig. 4b. As the accumulated dose increased, the reduction in the light intensity was qualitatively similar to that in the transmittance but quantitatively more significant. The light intensity decreased by about 60% in PWO25-1 (about 30% in PWO24-1). The degradation in the light intensity was by a factor of about 2—2.5 larger than in the transmittance for both PWO25-1 and PWO24-1. This difference may be attributed to the difference in the effective path lengths of light. In the transmission measurement, the effective path length is approximately equal to the geometrical length (23 cm in PWO25-1, and
Fig. 4. Radiation damage and its recovery in four samples PWO24-1,2 and PWO25-1,2 obtained from (a) the transmission measurement using a NaI : Tl-241Am light pulser in the setup sketched in Fig. 3a, and (b) the scintillation intensity measurement in the setup sketched in Fig. 3b.
8.6 cm in PWO24-1). In the light intensity measurement, however, a simulation showed that the effective path length is about twice the geometrical one, because the scintillation light emitted isotropically from the interaction point of the injected c-ray has to travel longer paths due to the reflection at the walls. Recovery of the radiation damages was also measured after the irradiation by 107 rad. In the
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transmission measurement, the recovery occurred to some extent, i.e. by 10%, in the 23 cm long sample PWO25-1 (6% in the 9 cm long one PWO24-1) rather quickly within an hour. Further recovery since then was small and within the systematic error (in p) of about 1.5%. In the light intensity measurement, a part of the observed fast recovery could be attributed to the temperature rise due to the irradiation. After the correction for the temperature rise according to the temperature coefficient of !1.9%/°C [8], the fast recovery was within the systematic measurement error of $5%. From the result of the transmission measurement given in Fig. 4a, we calculated the induced absorption coefficient k defined as *3 k "(1/d) ln(¹ /¹), (1) *3 0 where d is the thickness across which the transmittance ¹ (before irradiation) and ¹ (after irradia0 tion) were measured. The result is given in Fig. 5; k at 107 rad was 1.6—2.0 m~1 in PWO25 and *3 1.2—1.4 m~1 in PWO24. More precise measurement of the radiation-induced absorption with a spectrophotometer was
Fig. 5. Radiation-induced absorption coefficient k in four sam*3 ples (PWO24-1,2 and PWO25-1,2) versus the accumulated dose, derived from the result given in Fig. 4a. The recovery in 138 h after 107 rad irradiation is also shown for PWO24-1 and PWO25-1. The obtained k is an effective value for the emission *3 of NaI : Tl, which has a peak at &420 nm similar to the emission of PWO.
carried out 20 days after the 107 rad irradiation. The result is given in Fig. 6a for PWO25-1 and in 6b for PWO24-1. Fig. 7 gives the spectra of k ver*3 sus wavelength in different samples. The k spec*3 trum of each sample has two maxima at 380 and 540 nm. The k at 420 nm, the emission peak *3 wavelength of PWO, was 0.7—1.1 m~1 in PWO25 and 0.3—0.4 m~1 in PWO24. This result indicates that PWO24 may be radiation harder than PWO25. A possible reason for this is that the La concentration in PWO25 was smaller than in PWO24 (see Section 2). During the final (i.e. third) recrystallization of PWO25, the computer which controlled the crystal growth happened to stop. The melt was heated again and the crystallization was restarted from the beginning. As a result, evaporation of La was larger during the growth of PWO25 than that of PWO24, reducing the La concentration in PWO25 compared with PWO24.
Fig. 6. Radiation-induced degradation in the transmittance across the length of (a) 23 cm in PWO25-1 and (b) 8.6 cm in PWO24-1.
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large crystals than before on the smaller crystals. The radiation-induced absorption coefficient at the emission peak wavelength 420 nm was as small as 0.3—1.1 m~1 depending on the samples in 20 days after 107 rad irradiation, showing an excellent radiation hardness. Recovery occurred quickly in an hour to some extent, while further slower recovery was not large.
Acknowledgements Fig. 7. Radiation-induced absorption coefficient k versus *3 wave- length in four samples (PWO24-1,-2 and PWO25-1,-2) measured 20 days after the 107 rad irradiation.
4. Summary and discussions We summarize the obtained results as follows: 1. The ingot size of La-doped PbWO single crys4 tals was significantly increased to 6.5 cm (in diameter)]30 cm (in length). The same growing technique as before [3], i.e. the three times recrystallization in air by the Czochralski method, was used. Rectangular crystal blocks, with a typical size of 2]2]23 cm3 per block, were cut from the large ingots and evaluated with respect to the scintillation characteristics including the radiation hardness. 2. The optical and scintillation characteristics of the large ingots, including transmittance, excitation-emission spectra, decay time, and scintillation light intensity, were similar to those obtained previously in the smaller PWO : La ingots [1,2]. 3. The radiation damages due to 60Co c-rays were measured with a higher precision on the present
The authors would like to express their deep thanks to H. Ishibashi (Hitachi Chem. Co.) for help in using the spectrophotometer (U-3400 equipped with a large sample chamber) and helpful discussions, and to H. Nagayama (JAERI) for precise irradiation. One of the authors (MK) is deeply thankful to H. Sugawara, S. Iwata, S. Yamada and K. Nakamura of KEK for supporting the present work. The present work is partly supported by a Grant-in-Aid from the Japanese Ministry of Education, Science and Culture.
References [1] M. Kobayashi et al., Nucl. Instr. Meth. A 399 (1997) 261. [2] M. Kobayashi et al., Nucl. Instr. Meth. A 404 (1998) 149. [3] S. Baccaro et al., Phys. Stat. Sol. a 160 (1997) R5. [4] Y. Usuki et al., Proc. Workshop on Scintillating Crystals, KEK, 1997, p. 207. [5] M. Kobayashi et al., Nucl. Instr. Meth. A 373 (1996) 333. [6] E. Auffray et al., Improvement of several properties of lead tungstate crystals with different doping ions, CMS Note 97/54, CERN, Nucl. Instr. Meth. to be published. [7] M. Kobayashi et al., Nucl. Instr. Meth. 224 (1984) 318. [8] G.A. Alexeev et al., Nucl. Instr. Meth. A 364 (1995) 307.
La-doped PbWO scintillating crystals grown in 4 large ingots Kazuhiko Hara!, Mitsuru Ishii", Masaaki Kobayashi#,*, Martin Nikl$, Hideaki Takano!, Masashi Tanaka!, Kiyokazu Tanji", Yoshiyuki Usuki% ! Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan " SIT, Shonan Institute of Technology, Fujisawa 251-0046, Japan # High Energy Accelerator Research Organization (KEK), Tsukuba-Shi, Ibaraki-ken, 305-0801, Japan $ Institute of Physics, Academy of Sciences of Czech Republic, Cukrovarnicka 10, 16200 Prague, Czech Republic % Furukawa Co., Kamiyoshima, Yoshima, Iwaki 970-1153, Japan Received 1 April 1998
Abstract We achieved a significant increase in the ingot size of La-doped PbWO single crystals to 65 mm in diameter and 4 300 mm in length by using the Czochralski technique and three times of recrystallization. Four blocks each with a typical size of 20]20]230 mm3 were taken from an ingot. The optical as well as the scintillation characteristics of the large-size crystals, including the radiation hardness, were found to be similar to or even better than those of smaller crystals. The radiation-induced absorption coefficient k was as small as 0.5—1 m~1 at 107 rad of 60Co c-ray irradiation. ( 1998 *3 Elsevier Science B.V. All rights reserved.
1. Introduction In our previous papers [1—3], we reported that La doping improves dramatically the characteristics of lead tungstate (PbWO or PWO) scintillat4 ing crystals including optical transmittance, decay time and radiation hardness. The obtained improvements can be regarded as a significant breakthrough to push this material to a new stage for practical use in c-ray detectors at high energies.
* Corresponding author. Tel.: #81 298 64 5427; fax: #81 298 64 7831; e-mail: [email protected].
Then, we turned our efforts to increasing the ingot size so that four blocks each with a typical size of 2]2]23 cm3 could be cut from an ingot. The increase in the ingot size enables the reduction of the cost per crystal block. The ingot size, which was originally 3.5 cm (diameter)]8 cm (length), has been successfully increased in two steps, first to 4 cm]25 cm and then to 6.5 cm]30 cm. We evaluated the scintillation characteristics, including radiation hardness, of the large ingots in order to verify that the characteristics should not be degraded by increasing the ingot size but could be improved. The aim of the present paper is to describe the result of the above-mentioned evaluation.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 3 4 - 2
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2. Test samples The test samples are listed in Table 1. Two blocks (PWO24-1 and -2) each with a size of 2]2]9 cm3 were taken from an ingot (PWO24) of 4 cm (diameter) ]25 cm (length) on the seed and the tail sides, respectively. Four blocks each with a size of 2]2]23 cm3 were cut from an ingot (PWO25) of 6.5 cm (diameter)]30 cm (length). Three of them (PWO25-1, -2 and -3) were tested in the present study. Two small test pieces, PWO25U and PWO25L, were also cut from the ingot PWO25 near the seed and the tail ends, respectively. Both ingots (PWO24 and PWO25) were grown in the same way as before [4], i.e. by three times of recrystallization in air in the Czochralski method using a platinum crucible of 19 cm (diameter)] 16 cm (depth). The nominal purities of raw materials (PbO and WO ) were 99.99%. A small amount 3 of La was added to the melt in the third recrystallization process with a concentration of 135 atomic ppm (at. ppm). Table 2 gives a typical result of the element analysis made for both the seed and the tail ends of the ingot PWO25 by using a glow discharge mass spectrometer (GDMS) or an inductioncoupled plasma spectrometer (ICP). The La concentration in the PWO25 ingot ranged from 220 (near the seed end at the solidification fraction g"0.01) to 140 (near the bottom end at g"0.4) at. ppm, giving a segregation coefficient of 1.1—1.2. The La concentration in the PWO24 ingot was 262 at. ppm at g"0.15 and 230 at. ppm at g"0.24, giving
a segregation coefficient of 2.5—2.7. The larger concentration of La in PWO24 than in PWO25 is discussed later in Section 3.5 in relation to the higher radiation hardness obtained in the former than the latter crystal.
3. Results 3.1. Transmission Transmission spectra were measured with a spectrophotometer (Hitachi 220 or U-3400). The transmission spectra of PWO25-1 across the length (23 cm) and the thickness (2 cm) are compared in Fig. 1. From the difference between the two spectra, the internal attenuation coefficient a can be deduced according to a formula ¹"¹ exp(!az), 0 where ¹ and z denote the transmittance and the longitudinal distance, respectively. The magnitude of a was approximately 0.014—0.016 cm~1 at the peak emission wavelength 420 nm of PWO in all the samples (PWO25-1,-2,-3, PWO24-1, -2). 3.2. Excitation-emission spectra We measured photoluminescence from the crystal surface by using a fluorescence spectrophotometer (Hitachi F4500). The emission and excitation peaks sit at around 415 and 315 nm, respectively, in both large ingots (see Table 1); this
Table 1 List of the tested La-doped PbWO samples. j and j denote the excitation and emission peak wavelengths, respectively, and LY the 4 %9 %. scintillation light intensity for 60Co c-rays (see text) Sample name
Size (cm)
Ingot
j /j %9 %. (nm)
LY p.e./MeV
Notes
PWO24-1 PWO24-2 PWO25-1 PWO25-2 PWO25-3 PWO25U PWO25L
2]2]9 2]2]9 2]2]23 2]2]23 2]2]23 1]1]2 1]1]2
PWO24 PWO24 PWO25 PWO25 PWO25 PWO25 PWO25
315/415 315/415 — — — 315/415 315/415
11.1! — 6.0! — 7.2 31 30
Seed-side half of the ingot Tail-side half of the ingot
!Measured about 20 days after the irradiation by 107 rad.
Seed end of the ingot Tail end of the ingot
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Table 2 A GDMS result of the ingot PWO25 measured close to the seed and the tail ends. The impurities are given in units of at. ppm. Element
Seed end
Tail end
Li Be B Na Mg Si S Cl K Ti Cr Mn Ni Co Cu Zn Ge Se Rb Nb Mo Cd Sb La! Ce Te Pt Bi
(0.001 (0.001 0.08 0.43 0.06 0.80 0.12 0.39 0.28 0.13 0.008 0.06 0.11 (0.003 0.02 0.03 (0.003 0.34 (0.001 (0.006 (0.02 (0.05 0.02 223! (0.04 (0.003 (0.01 (0.002
(0.002 (0.002 (0.003 0.32 0.05 0.95 0.18 0.29 0.43 0.13 0.11 0.12 0.09 (0.003 0.02 0.02 (0.004 0.41 (0.001 (0.007 (0.01 (0.02 (0.03 144! (0.03 0.02 1.41" (0.001
!The La concentration obtained from the ICP analysis is given. The obtained values for La were about three times larger than those obtained by the GDMS. We take the values obtained by the ICP analysis because the calibration (the effective sample volume used for the analysis, corrections on the efficiency, etc.) is better controlled in the ICP as far as La is concerned. "The large Pt concentration must come from the Pt crucible.
result was the same as in smaller ingots [5]. No sizable differences were seen between the seed and the tail ends.
3.3. Scintillation light intensity We measured the scintillation light intensity produced by 60Co c-rays in the small samples (PWO25U and PWO25L) by mounting each
Fig. 1. Comparison of the transmission spectra of PWO25-1 across the length of 23 cm (longitudinal) and the width of 2 cm (transversal).
crystal on a 2 in photomultiplier tube (PMT, Hamamatsu R2259) with a bialkali photocathode and a silica window. The pulse height was analyzed by a PHA (LeCroy qVt) in the charge mode within a 1 ls gate. The 1.25 MeV peak, the average of the 1.17 and 1.33 MeV photoelectron peaks, was seen on a broad background in the pulse height spectrum. From a comparison of the peak channel and the width of the peak, we found that the light intensity obtained in the present samples was similar to those in the samples cut from the smaller ingots described in Ref. [5]. More precise absolute measurement was carried out by comparing the position of the 1.25 MeV peak with the single photoelectron (p.e.) peak. The latter peak was created by employing an LED pulser mounted at a large distance from the PMT. The crystal face of 2]2 cm2 (1]2 cm2 for the small samples PWO25U and PWO25L) was coupled to the PMT and a 60Co source was placed on the opposite face. As seen in Table 1, the light intensity was about 31 p.e./MeV in the small sample (PWO25L), 11 p.e./MeV in the 9 cm long sample PWO24-1 (after 107 rad irradiation) and 7 p.e./MeV in the 23 cm long sample PWO25-3. The obtained light intensities seem to be similar to those reported in a recent paper by the Crystal Clear Collaboration [6], although direct comparison is difficult due to different measuring
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conditions; in the measurement described in [6], the crystals are tapered so as to make the light collection more efficient, and the c-ray source was placed closer to the PMT (at 2.5 cm from the PMT). 3.4. Decay time We measured the decay time spectra within 1 ls using a conventional single photoelectron technique. The start pulse for the time measurement was taken from a small Gd SiO : Ce (GSO : Ce) 2 5 scintillator, which was mounted on a 2 in PMT (Hamamatsu R329 with a bialkali photocathode) and set close to a 60Co c-ray source. The stop pulse was the single photoelectron signal of the PbWO : La crystal mounted on another fast 2 in. 4 PMT (Hamamatsu H3177) with a bialkali photocathode and a silica window. The decay time spectrum was registered with a time to pulse height converter (LeCroy qVt). The obtained spectrum was similar to that given in Ref. [1] and fitted with two exponentials superimposed on a constant background. The obtained result was similar to that obtained in the samples cut from the smaller PWO : La ingots described in Ref. [1]; q"5.1 ns (for 93% of the total intensity) and q"31 ns (7%) in PWO24-1, and q"3.9 ns (84%) and q"15 ns (16%) in PWO25L. As described in Ref. [1], two exponentials were enough to give a good fit for the La-doped crystals, instead of three exponentials necessary for undoped crystals. 3.5. Radiation damages Radiation damages were measured by irradiating the samples with 60Co c-rays at Japan Atomic Energy Research Institute (JAERI). Five cycles of irradiation followed by a measurement of transmission and light intensity were carried out to cover the accumulated dose from 103 to 107 rad (10 to 105 Gy)1 by increasing it ten times per cycle. The irradiation period was 13, 10, 9, 33 min and 4 h for 103, 104, 105, 106, and 107 rad, respectively. The 11 Roentgen roughly corresponds to 1 rad in 2 cm thick PbWO at the energies of 60Co c-rays (1.25 MeV on the aver4 age).
Fig. 2. Sketch of the setup for irradiating the long PWO samples. 60Co sources in thin pellets were arranged in a cylindrical shape. The dose profile (I ) along the crystal length is also c shown. The distance D was adjusted to give the required dose rate. Each sample was rotated by 180° along the vertical axis at the half-time of the total irradiation period to achieve uniform irradiation (see text).
dose rate was adjusted by mounting the samples at different distances from the calibrated 60Co source. We employed a setup as sketched in Fig. 2, where the dose profile along the crystal length is shown. The samples were rotated by 180° at the half time of each irradiation period in order to make the irradiation uniform along the crystal thickness (2 cm). The measurement of the damages was begun just after the interruption of each cycle of irradiation, and completed in 30 min. Two different quantities were measured. One was the transmittance of the light from a NaI : Tl-241Am and a GSO : Ce241Am light pulsers2 [7] through the length (9 or 23 cm) of the PWO crystals (see Fig. 3a). An additional NaI : Tl-241Am light pulser was mounted directly on the PMT window in order to monitor the gain of the PMT. The other was the scintillation intensity in the PWO crystal for 60Co c-rays (see Fig. 3b). For a c-ray entering the PWO crystal, coincidence was required between the signals of the
2For the NaI : Tl-241Am light pulser, see [7]. The GSO : Ce241Am light pulser was newly constructed by replacing the NaI : Tl crystal in the NaI : Tl-241Am light pulser with a small GSO : Ce crystal (5 mm in diameter and 1 mm in thickness), and by removing the glass window. The FWHM energy resolution of the 5.49 MeV 241Am alphas in the GSO : Ce-241Am light pulser was 13% and their GEE (gamma equivalent energy) was 1.15 MeV.
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Fig. 3. Sketch of the measuring setups for radiation damages (see text). (a) Left: measurement of the transmission of light from a constant amplitude light pulser (NaI : Tl-241Am and GSO : Ce-241Am). (b) Right: measurement of the scintillation light intensity for 60Co c-rays. G.G.: gate generator.
two PMTs (PMT1 and PMT2) viewing the sample from both ends. The discrimination levels of both PMTs were set at a level slightly smaller than the single p.e. signal. The single p.e. signal as a calibration source was created by injecting weak light of an LED through optical fibers (see Fig. 3b). The result of the transmission measurement using the NaI : Tl light pulser is presented in Fig. 4a. It is to be noted that the emission of NaI : Tl has a peak at &420 nm similarly to that of PWO. The transmittance was degraded by about 38% across 23 cm in PWO25-1 (by about 12% across 8.6 cm in PWO24-1) after the irradiation by 107 rad. The corresponding result of the measurement of scintillation intensity is given in Fig. 4b. As the accumulated dose increased, the reduction in the light intensity was qualitatively similar to that in the transmittance but quantitatively more significant. The light intensity decreased by about 60% in PWO25-1 (about 30% in PWO24-1). The degradation in the light intensity was by a factor of about 2—2.5 larger than in the transmittance for both PWO25-1 and PWO24-1. This difference may be attributed to the difference in the effective path lengths of light. In the transmission measurement, the effective path length is approximately equal to the geometrical length (23 cm in PWO25-1, and
Fig. 4. Radiation damage and its recovery in four samples PWO24-1,2 and PWO25-1,2 obtained from (a) the transmission measurement using a NaI : Tl-241Am light pulser in the setup sketched in Fig. 3a, and (b) the scintillation intensity measurement in the setup sketched in Fig. 3b.
8.6 cm in PWO24-1). In the light intensity measurement, however, a simulation showed that the effective path length is about twice the geometrical one, because the scintillation light emitted isotropically from the interaction point of the injected c-ray has to travel longer paths due to the reflection at the walls. Recovery of the radiation damages was also measured after the irradiation by 107 rad. In the
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transmission measurement, the recovery occurred to some extent, i.e. by 10%, in the 23 cm long sample PWO25-1 (6% in the 9 cm long one PWO24-1) rather quickly within an hour. Further recovery since then was small and within the systematic error (in p) of about 1.5%. In the light intensity measurement, a part of the observed fast recovery could be attributed to the temperature rise due to the irradiation. After the correction for the temperature rise according to the temperature coefficient of !1.9%/°C [8], the fast recovery was within the systematic measurement error of $5%. From the result of the transmission measurement given in Fig. 4a, we calculated the induced absorption coefficient k defined as *3 k "(1/d) ln(¹ /¹), (1) *3 0 where d is the thickness across which the transmittance ¹ (before irradiation) and ¹ (after irradia0 tion) were measured. The result is given in Fig. 5; k at 107 rad was 1.6—2.0 m~1 in PWO25 and *3 1.2—1.4 m~1 in PWO24. More precise measurement of the radiation-induced absorption with a spectrophotometer was
Fig. 5. Radiation-induced absorption coefficient k in four sam*3 ples (PWO24-1,2 and PWO25-1,2) versus the accumulated dose, derived from the result given in Fig. 4a. The recovery in 138 h after 107 rad irradiation is also shown for PWO24-1 and PWO25-1. The obtained k is an effective value for the emission *3 of NaI : Tl, which has a peak at &420 nm similar to the emission of PWO.
carried out 20 days after the 107 rad irradiation. The result is given in Fig. 6a for PWO25-1 and in 6b for PWO24-1. Fig. 7 gives the spectra of k ver*3 sus wavelength in different samples. The k spec*3 trum of each sample has two maxima at 380 and 540 nm. The k at 420 nm, the emission peak *3 wavelength of PWO, was 0.7—1.1 m~1 in PWO25 and 0.3—0.4 m~1 in PWO24. This result indicates that PWO24 may be radiation harder than PWO25. A possible reason for this is that the La concentration in PWO25 was smaller than in PWO24 (see Section 2). During the final (i.e. third) recrystallization of PWO25, the computer which controlled the crystal growth happened to stop. The melt was heated again and the crystallization was restarted from the beginning. As a result, evaporation of La was larger during the growth of PWO25 than that of PWO24, reducing the La concentration in PWO25 compared with PWO24.
Fig. 6. Radiation-induced degradation in the transmittance across the length of (a) 23 cm in PWO25-1 and (b) 8.6 cm in PWO24-1.
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large crystals than before on the smaller crystals. The radiation-induced absorption coefficient at the emission peak wavelength 420 nm was as small as 0.3—1.1 m~1 depending on the samples in 20 days after 107 rad irradiation, showing an excellent radiation hardness. Recovery occurred quickly in an hour to some extent, while further slower recovery was not large.
Acknowledgements Fig. 7. Radiation-induced absorption coefficient k versus *3 wave- length in four samples (PWO24-1,-2 and PWO25-1,-2) measured 20 days after the 107 rad irradiation.
4. Summary and discussions We summarize the obtained results as follows: 1. The ingot size of La-doped PbWO single crys4 tals was significantly increased to 6.5 cm (in diameter)]30 cm (in length). The same growing technique as before [3], i.e. the three times recrystallization in air by the Czochralski method, was used. Rectangular crystal blocks, with a typical size of 2]2]23 cm3 per block, were cut from the large ingots and evaluated with respect to the scintillation characteristics including the radiation hardness. 2. The optical and scintillation characteristics of the large ingots, including transmittance, excitation-emission spectra, decay time, and scintillation light intensity, were similar to those obtained previously in the smaller PWO : La ingots [1,2]. 3. The radiation damages due to 60Co c-rays were measured with a higher precision on the present
The authors would like to express their deep thanks to H. Ishibashi (Hitachi Chem. Co.) for help in using the spectrophotometer (U-3400 equipped with a large sample chamber) and helpful discussions, and to H. Nagayama (JAERI) for precise irradiation. One of the authors (MK) is deeply thankful to H. Sugawara, S. Iwata, S. Yamada and K. Nakamura of KEK for supporting the present work. The present work is partly supported by a Grant-in-Aid from the Japanese Ministry of Education, Science and Culture.
References [1] M. Kobayashi et al., Nucl. Instr. Meth. A 399 (1997) 261. [2] M. Kobayashi et al., Nucl. Instr. Meth. A 404 (1998) 149. [3] S. Baccaro et al., Phys. Stat. Sol. a 160 (1997) R5. [4] Y. Usuki et al., Proc. Workshop on Scintillating Crystals, KEK, 1997, p. 207. [5] M. Kobayashi et al., Nucl. Instr. Meth. A 373 (1996) 333. [6] E. Auffray et al., Improvement of several properties of lead tungstate crystals with different doping ions, CMS Note 97/54, CERN, Nucl. Instr. Meth. to be published. [7] M. Kobayashi et al., Nucl. Instr. Meth. 224 (1984) 318. [8] G.A. Alexeev et al., Nucl. Instr. Meth. A 364 (1995) 307.