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Optical and scintillation property of Ce, Ho and Eu-doped PbF2
Radiation Measurements 55 (2013) 120e123
Contents lists available at SciVerse ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Optical and scintillation property of Ce, Ho and Eu-doped PbF2 Shunsuke Kurosawa a, b, *, Yuui Yokota a, Takayuki Yanagida b, Akira Yoshikawa a, b a b
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan
h i g h l i g h t s PbF2 crystal has a heavy density and short radiation length. Luminescent properties of Ce, Eu and Ho doped PbF2 crystals were investigated. Ce-doped PbF2 had very small reaction in radio- and photo luminescence. Eu and Ho doped PbF2 had some peaks from Eu3þ and Ho3þ, respectively. 0.1-mol% Eu and Ho:PbF2 had 5.5-MeV alpha-ray peaks in pulse height spectra.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2012 Received in revised form 23 February 2013 Accepted 11 March 2013
PbF2 crystal has favorable properties such as a high density and short radiation length, while the scintillation light output is too small to obtain a photo-absorption peak under gamma ray excitation. In this paper, we report scintillation response for 0.1 or 0.5-mol% Ce, Eu and Ho doped PbF2 crystals grown by an annealing method under 5.5-MeV alpha ray excitation. Although Ce doped PbF2 crystals did not show intense photo- and radio-luminescence, Eu and Ho doped ones showed several peaks excited under UV and 5.5-MeV alpha ray excitation, respectively. Additionally, full-absorption peaks in the pulse height spectra of 0.1-mol% Eu and Ho doped PbF2 were ascribed to 5.5-MeV alpha-rays. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Scintillator PbF2 High density materials Radiation response
1. Introduction Scintillators are used in various applications such as medical imaging, high energy physics and astronomy. Using a scintillator with a short radiation length or high stopping power, a compact scintillator detector can be developed. The compact size is required in positron emission tomography camera, because the small crystal size improves the position resolution. PbF2 has a high density (7.77 g/cm3), short radiation length (0.93 g/cm3), and high transmittance in the ultra violet region (Kozma et al., 2002). Although PbF2 is used as a Cherenkov radiator for electromagnetic calorimeter (Dally and Hofstadter, 1968a, 1968b; Anderson et al., 1990; Kozma et al., 2002), the light output of scintillation light is very small (Anderson et al., 1994).
* Corresponding author. Present address: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. Tel.: þ81 22 217 2214; fax: þ81 22 217 2217. E-mail address: [email protected] (S. Kurosawa). 1350-4487/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2013.03.005
In order to obtain a larger scintillation light output, PbF2 crystals doped with some dopants were studied (Mao et al., 2009). 0.15 mol % Gd-doped PbF2 showed weak scintillation light excited by a 60Co source and its intensity was double of that of the Cherenkov light (Woody et al., 1996). However, pulse height spectra of PbF2 with other dopants excited by gamma or alpha rays have not been reported so far. Ce and Eu is expected to show 5d-4f transition which has a relatively fast decay time of less than approximately 0.1 and 2 msec for Ce3þ and Eu2þ, respectively. These dopants are often used in various scintillators. In addition, Ho3þ-doped laser materials have a strong emission peak at approximately 550 nm (Malinowski et al., 2004), and the emission wavelength matches with the sensitivity of a Si-avalanche photodiode (APD). The maximum quantum efficiency of the Si-APD is approximately 80% around 550 nm, while that of a photomultiplier is w40% at approximately 400 nm. That is why the scintillation detector with the Si-APD is expected to be more efficient than the photomultiplier-based one, if the scintillation emission is peaking above 500 nm. Therefore, we measured the pulse height spectra of PbF2 with several dopants (Ce, Ho, Eu) under 5.5-MeV alpha ray excitation.
S. Kurosawa et al. / Radiation Measurements 55 (2013) 120e123
121
Fig. 3. Powder X-ray diffraction patterns of Eu:PbF2, and closed circles denote the bPbF2 phase (X-ray diffraction powder catalogue card 06-0251).
Fig. 1. Schematic view of an annealing method.
Samples with different concentrations (0.1 and 0.5 mol%) of the dopants were tested with a photomultiplier as a first step. 2. Materials and experimental methods For RE:PbF2 (RE ¼ Ce, Eu, and Ho), the stoichiometric mixture of the PbF2, CeF3, EuF3 and HoF3 powders (all of 99.99% purity) was used, and concentrations of all the dopants were either 0.1 or 0.5 mol%. Theses crystals were grown by an annealing method described in Fig. 1 (Kurosawa et al., 2012). First, the growth chamber was evacuated, and the crucible was heated up to w300 C. During this baking procedure, the chamber was further evacuated down to 104. After baking, a mixture of more than 99.99995-vol.% purity Ar and CF4 gases in a pressure ratio of 9:1 at 1 atm (sealed) was introduced into the chamber. The sample powders in the graphite crucible were then heated with a heating rate of approximately 450 C/h until reaching the melting temperature. Then the temperature was kept at approximately 900 C for 30 min, and the crucible was cooled at a rate of approximately 300 C/h down to room temperature. In order to check the phase of the obtained crystals, powder Xray diffraction analysis was performed from 20 to 80 using a diffractometer (RIGAKU, RINT2000). The X-ray source was CuKa line with an accelerating voltage of 40 kV, and tube current of 40 mA. We investigated some optical properties of the samples after cutting and polishing; (i) Transmittance was measured with spectrophotometer (JASCO, V-530) in the wavelength range of 190e
Fig. 2. Photograph of PbF2 crystals, grown by the annealing method, doped with 1-mol % Ce, Eu, and Ho for (a)-(c), respectively.
900 nm, (ii) the radio-luminescence spectra at room temperature were measured with a spectrofluorometer (Edinburgh Instruments FLS920) using 5.5-MeV alpha rays (241Am) as the excitation source, (iii) Under X-ray irradiation, the radio-luminescence spectra also measured with the X-ray generator (Rigaku, RINT-2000) and a CCD spectrometer camera (Andor, DU-20-OE), and (iv) Photoluminescence spectra (emission and excitation) were measured with the same spectrofluorometer as the radio-luminescence. To obtain the pulse height spectra, we measured samples under alpha ray excitation from an 241Am source using a photomultiplier (Hamamatsu, R7600) which was operated at a high voltage of 800 V.
Fig. 4. Transmittance of PbF2 crystals doped with Ce (i), Eu (ii) and Ho (iii), and (a) and (b) denote a dopant concentration of 0.1 and 0.5 mol%, respectively. (iv) denotes pure PbF2.
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Fig. 5. 5.5-MeV alpha-ray-excited luminescence spectra of PbF2 crystals doped with 0.5-mol% Eu (i), and Ho (ii).
3. Results and discussion PbF2 crystals doped with Ce, Eu and Ho were successfully grown by annealing method (Fig. 2). Powder X-ray diffraction patterns for all the samples demonstrated peaks consistent with the expected PbF2 phase, which possesses cubic crystal system with space group Fm3m as shown in Fig. 3. After polishing, all the samples had a thickness of 1 mm. Transmittance of the samples with 0.1 mol% dopants were w70% above 350 nm, while 0.5-mol% Ce or Eu doped PbF2 had a worse transmittance of approximately 60% (Fig. 4). However, Ho:PbF2 had almost the same transmittance of 70%.
Fig. 6. X-ray-excited luminescence spectra of PbF2 crystals doped with 0.5-mol% Ce (a), Eu (b-i), and Ho (b-ii).
Fig. 7. Photo-luminescence spectra of PbF2 crystals doped with 0.5-mol% Eu (i), and Ho (ii).
The radio-luminescence spectrum of PbF2 doped with 0.5-mol% Ce under 5.5-MeV alpha ray excitation had too weak luminescence to detect a peak, while that of Eu:PbF2 and Ho:PbF2 had some peaks which can be ascribed to the 5D0,1,2,3 / 7Fj (j ¼ 0, 1, 2, 3) transitions of Eu3þ, and 5S2 / 5I8 transitions of Ho3þ(Fig. 5), respectively. On the other hand, the radio-luminescence spectrum of Ce:PbF2 excited by X-rays showed a peak regardless (5d / 4f emission) low emission intensity under alpha ray excitation, because X-ray intensity from the generator was larger than the alpha-ray intensity from an 241Am source (Fig. 6). Thus, the measurement was faster and easier with the X-ray generator. Although the peaks in the radio-luminescence spectra of Eu:PbF2 and Ho:PbF2 under X-ray irradiation had the same position as in the alpha-ray-excited radio-luminescence spectra, the intensity ratios between the peaks were different due to the different sensitivities of the monochromator throughput (a photomultiplier for Fig. 5 and a CCD camera for Fig. 6) as a function of wavelength. Especially, some peaks were clearly detected above 650 nm in Fig. 6, while they were not found in Fig. 5, since the photomultiplier has lower sensitivity than the CCD camera above 650 nm. Fig. 7 shows photo-luminescence spectra of Eu:PbF2 and Ho:PbF2. The peak positions corresponded to those in the radioluminescence spectra as shown in Fig. 5, while no peak was found in the spectrum for Ce:PbF2. Here, these spectra were excited at 444 and 393-nm for Eu and Ho-doped sample, respectively. The results for 0.1-mol% doped samples were quite similar to those of 0.5-mol% samples. The pulse height spectra of Eu:PbF2 and Ho:PbF2 under alpha rays excitation from an 241Am source are shown in Fig. 8. Although the light outputs were small (less than 500 photons/5.5-MeV alpha
Fig. 8. Pulse height spectra irradiated with 5.5-MeV alpha rays from an
241
Am source.
S. Kurosawa et al. / Radiation Measurements 55 (2013) 120e123
rays), 5.5-MeV alpha-ray peaks were found in the spectra of 0.1-mol % Eu and Ho doped PbF2. 4. Conclusions We have investigated optical and scintillation properties of 0.1 or 0.5-mol% Ce, Eu and Ho doped PbF2 crystals. Although the emission intensities of Ce-doped crystals irradiated with X-rays, alpha rays or UV photons were too small, the X-ray radioluminescence spectrum showed a 5d-4f emission peak due to the high intensity of the X-ray excitation. On the other hand, Eu and Ho doped PbF2 crystals had several emission peaks in the spectra. Additionally, in the pulse-height spectra for the 0.1-mol% Eu and Ho doped PbF2 crystals the 5.5-MeV alpha-ray peaks were found. Acknowledgments This work is supported by Japan Society for the Promotion of Science Research Fellowships for Young Scientists (S. Kurosawa), the funding program for next generation world-leading researchers, Japan society for promotion of science, SENTAN of Japan Science and Technology Agency and the Association for the Progress of New Chemical Technology. In addition, we would like to
123
thank Mr. Yoshihiko Nakamura in Institute of Multidisciplinary Research for Advanced Materials (IMRAM)/Tohoku University, and Mr. Hiroshi Uemura, Ms. Keiko Toguchi and Ms. Megumi Sasaki in IMR/Tohoku University for their supports. References Anderson, D.F., Kobayashi, M., Woody, C.L., Yoshimura, Y., 1990. Lead fluoride: an ultra-compact Cherenkov radiator for Em Calorimetry. Nucl. Instr. Meth. Phys. Res. A 290, 385e389. Anderson, D.F., Kierstead, J.A., Lecoq, P., Stoll, S., Woody, C.L., 1994. A search for scintillation in doped and orthorhombic lead fluoride. Nucl. Instr. Meth. Phys. Res. A 342, 473e476. Dally, E.B., Hofstadter, R., 1968a. High energy g-Ray detector with good resolution. Rev. Sci. Instr. 39, 658e659. Dally, E.B., Hofstadter, R., 1968b. A lead fluoride Cerenkov Shower Counter. IEEE Trans. Nucl. Sci. 15, 76e81. Kozma, P., Bajgar, R., Kozma Jr., P., 2002. Radiation resistivity of PbF2 crystals. Nucl. Instr. Meth. Phys. Res. A 484, 149e152. Kurosawa, S., Yanagida, T., Yokota, Y., Yoshikawa, A., 2012. Crystal growth and scintillation properties of fluoride scintillators. IEEE Trans. Nucl. Sci. 59, 2173e2176. Mao R., Zhang L., Member, IEEE, Zhu R-Y., 2009. Search for scintillation in doped lead fluoride crystals. Nuclear science Symposium Conference Record (NSS/ MIC), 2009 IEEE, pp. 2182e2186. Malinowski, M., Kaczkana, M., Wnukb, A., Szuflinska, M., 2004. Emission from the high lying excited states of Ho3þ ions in YAP and YAG crystals. J. Luminescence 106, 269e279. Woody, C.L., Stoll, S.P., Kierstead, J.A., 1996. Observation of fast scintillation light in a PbF2:Gd crystal. IEEE Trans. Nucl. Sci. 43, 1303e1305.
Contents lists available at SciVerse ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Optical and scintillation property of Ce, Ho and Eu-doped PbF2 Shunsuke Kurosawa a, b, *, Yuui Yokota a, Takayuki Yanagida b, Akira Yoshikawa a, b a b
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan
h i g h l i g h t s PbF2 crystal has a heavy density and short radiation length. Luminescent properties of Ce, Eu and Ho doped PbF2 crystals were investigated. Ce-doped PbF2 had very small reaction in radio- and photo luminescence. Eu and Ho doped PbF2 had some peaks from Eu3þ and Ho3þ, respectively. 0.1-mol% Eu and Ho:PbF2 had 5.5-MeV alpha-ray peaks in pulse height spectra.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2012 Received in revised form 23 February 2013 Accepted 11 March 2013
PbF2 crystal has favorable properties such as a high density and short radiation length, while the scintillation light output is too small to obtain a photo-absorption peak under gamma ray excitation. In this paper, we report scintillation response for 0.1 or 0.5-mol% Ce, Eu and Ho doped PbF2 crystals grown by an annealing method under 5.5-MeV alpha ray excitation. Although Ce doped PbF2 crystals did not show intense photo- and radio-luminescence, Eu and Ho doped ones showed several peaks excited under UV and 5.5-MeV alpha ray excitation, respectively. Additionally, full-absorption peaks in the pulse height spectra of 0.1-mol% Eu and Ho doped PbF2 were ascribed to 5.5-MeV alpha-rays. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Scintillator PbF2 High density materials Radiation response
1. Introduction Scintillators are used in various applications such as medical imaging, high energy physics and astronomy. Using a scintillator with a short radiation length or high stopping power, a compact scintillator detector can be developed. The compact size is required in positron emission tomography camera, because the small crystal size improves the position resolution. PbF2 has a high density (7.77 g/cm3), short radiation length (0.93 g/cm3), and high transmittance in the ultra violet region (Kozma et al., 2002). Although PbF2 is used as a Cherenkov radiator for electromagnetic calorimeter (Dally and Hofstadter, 1968a, 1968b; Anderson et al., 1990; Kozma et al., 2002), the light output of scintillation light is very small (Anderson et al., 1994).
* Corresponding author. Present address: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. Tel.: þ81 22 217 2214; fax: þ81 22 217 2217. E-mail address: [email protected] (S. Kurosawa). 1350-4487/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2013.03.005
In order to obtain a larger scintillation light output, PbF2 crystals doped with some dopants were studied (Mao et al., 2009). 0.15 mol % Gd-doped PbF2 showed weak scintillation light excited by a 60Co source and its intensity was double of that of the Cherenkov light (Woody et al., 1996). However, pulse height spectra of PbF2 with other dopants excited by gamma or alpha rays have not been reported so far. Ce and Eu is expected to show 5d-4f transition which has a relatively fast decay time of less than approximately 0.1 and 2 msec for Ce3þ and Eu2þ, respectively. These dopants are often used in various scintillators. In addition, Ho3þ-doped laser materials have a strong emission peak at approximately 550 nm (Malinowski et al., 2004), and the emission wavelength matches with the sensitivity of a Si-avalanche photodiode (APD). The maximum quantum efficiency of the Si-APD is approximately 80% around 550 nm, while that of a photomultiplier is w40% at approximately 400 nm. That is why the scintillation detector with the Si-APD is expected to be more efficient than the photomultiplier-based one, if the scintillation emission is peaking above 500 nm. Therefore, we measured the pulse height spectra of PbF2 with several dopants (Ce, Ho, Eu) under 5.5-MeV alpha ray excitation.
S. Kurosawa et al. / Radiation Measurements 55 (2013) 120e123
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Fig. 3. Powder X-ray diffraction patterns of Eu:PbF2, and closed circles denote the bPbF2 phase (X-ray diffraction powder catalogue card 06-0251).
Fig. 1. Schematic view of an annealing method.
Samples with different concentrations (0.1 and 0.5 mol%) of the dopants were tested with a photomultiplier as a first step. 2. Materials and experimental methods For RE:PbF2 (RE ¼ Ce, Eu, and Ho), the stoichiometric mixture of the PbF2, CeF3, EuF3 and HoF3 powders (all of 99.99% purity) was used, and concentrations of all the dopants were either 0.1 or 0.5 mol%. Theses crystals were grown by an annealing method described in Fig. 1 (Kurosawa et al., 2012). First, the growth chamber was evacuated, and the crucible was heated up to w300 C. During this baking procedure, the chamber was further evacuated down to 104. After baking, a mixture of more than 99.99995-vol.% purity Ar and CF4 gases in a pressure ratio of 9:1 at 1 atm (sealed) was introduced into the chamber. The sample powders in the graphite crucible were then heated with a heating rate of approximately 450 C/h until reaching the melting temperature. Then the temperature was kept at approximately 900 C for 30 min, and the crucible was cooled at a rate of approximately 300 C/h down to room temperature. In order to check the phase of the obtained crystals, powder Xray diffraction analysis was performed from 20 to 80 using a diffractometer (RIGAKU, RINT2000). The X-ray source was CuKa line with an accelerating voltage of 40 kV, and tube current of 40 mA. We investigated some optical properties of the samples after cutting and polishing; (i) Transmittance was measured with spectrophotometer (JASCO, V-530) in the wavelength range of 190e
Fig. 2. Photograph of PbF2 crystals, grown by the annealing method, doped with 1-mol % Ce, Eu, and Ho for (a)-(c), respectively.
900 nm, (ii) the radio-luminescence spectra at room temperature were measured with a spectrofluorometer (Edinburgh Instruments FLS920) using 5.5-MeV alpha rays (241Am) as the excitation source, (iii) Under X-ray irradiation, the radio-luminescence spectra also measured with the X-ray generator (Rigaku, RINT-2000) and a CCD spectrometer camera (Andor, DU-20-OE), and (iv) Photoluminescence spectra (emission and excitation) were measured with the same spectrofluorometer as the radio-luminescence. To obtain the pulse height spectra, we measured samples under alpha ray excitation from an 241Am source using a photomultiplier (Hamamatsu, R7600) which was operated at a high voltage of 800 V.
Fig. 4. Transmittance of PbF2 crystals doped with Ce (i), Eu (ii) and Ho (iii), and (a) and (b) denote a dopant concentration of 0.1 and 0.5 mol%, respectively. (iv) denotes pure PbF2.
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S. Kurosawa et al. / Radiation Measurements 55 (2013) 120e123
Fig. 5. 5.5-MeV alpha-ray-excited luminescence spectra of PbF2 crystals doped with 0.5-mol% Eu (i), and Ho (ii).
3. Results and discussion PbF2 crystals doped with Ce, Eu and Ho were successfully grown by annealing method (Fig. 2). Powder X-ray diffraction patterns for all the samples demonstrated peaks consistent with the expected PbF2 phase, which possesses cubic crystal system with space group Fm3m as shown in Fig. 3. After polishing, all the samples had a thickness of 1 mm. Transmittance of the samples with 0.1 mol% dopants were w70% above 350 nm, while 0.5-mol% Ce or Eu doped PbF2 had a worse transmittance of approximately 60% (Fig. 4). However, Ho:PbF2 had almost the same transmittance of 70%.
Fig. 6. X-ray-excited luminescence spectra of PbF2 crystals doped with 0.5-mol% Ce (a), Eu (b-i), and Ho (b-ii).
Fig. 7. Photo-luminescence spectra of PbF2 crystals doped with 0.5-mol% Eu (i), and Ho (ii).
The radio-luminescence spectrum of PbF2 doped with 0.5-mol% Ce under 5.5-MeV alpha ray excitation had too weak luminescence to detect a peak, while that of Eu:PbF2 and Ho:PbF2 had some peaks which can be ascribed to the 5D0,1,2,3 / 7Fj (j ¼ 0, 1, 2, 3) transitions of Eu3þ, and 5S2 / 5I8 transitions of Ho3þ(Fig. 5), respectively. On the other hand, the radio-luminescence spectrum of Ce:PbF2 excited by X-rays showed a peak regardless (5d / 4f emission) low emission intensity under alpha ray excitation, because X-ray intensity from the generator was larger than the alpha-ray intensity from an 241Am source (Fig. 6). Thus, the measurement was faster and easier with the X-ray generator. Although the peaks in the radio-luminescence spectra of Eu:PbF2 and Ho:PbF2 under X-ray irradiation had the same position as in the alpha-ray-excited radio-luminescence spectra, the intensity ratios between the peaks were different due to the different sensitivities of the monochromator throughput (a photomultiplier for Fig. 5 and a CCD camera for Fig. 6) as a function of wavelength. Especially, some peaks were clearly detected above 650 nm in Fig. 6, while they were not found in Fig. 5, since the photomultiplier has lower sensitivity than the CCD camera above 650 nm. Fig. 7 shows photo-luminescence spectra of Eu:PbF2 and Ho:PbF2. The peak positions corresponded to those in the radioluminescence spectra as shown in Fig. 5, while no peak was found in the spectrum for Ce:PbF2. Here, these spectra were excited at 444 and 393-nm for Eu and Ho-doped sample, respectively. The results for 0.1-mol% doped samples were quite similar to those of 0.5-mol% samples. The pulse height spectra of Eu:PbF2 and Ho:PbF2 under alpha rays excitation from an 241Am source are shown in Fig. 8. Although the light outputs were small (less than 500 photons/5.5-MeV alpha
Fig. 8. Pulse height spectra irradiated with 5.5-MeV alpha rays from an
241
Am source.
S. Kurosawa et al. / Radiation Measurements 55 (2013) 120e123
rays), 5.5-MeV alpha-ray peaks were found in the spectra of 0.1-mol % Eu and Ho doped PbF2. 4. Conclusions We have investigated optical and scintillation properties of 0.1 or 0.5-mol% Ce, Eu and Ho doped PbF2 crystals. Although the emission intensities of Ce-doped crystals irradiated with X-rays, alpha rays or UV photons were too small, the X-ray radioluminescence spectrum showed a 5d-4f emission peak due to the high intensity of the X-ray excitation. On the other hand, Eu and Ho doped PbF2 crystals had several emission peaks in the spectra. Additionally, in the pulse-height spectra for the 0.1-mol% Eu and Ho doped PbF2 crystals the 5.5-MeV alpha-ray peaks were found. Acknowledgments This work is supported by Japan Society for the Promotion of Science Research Fellowships for Young Scientists (S. Kurosawa), the funding program for next generation world-leading researchers, Japan society for promotion of science, SENTAN of Japan Science and Technology Agency and the Association for the Progress of New Chemical Technology. In addition, we would like to
123
thank Mr. Yoshihiko Nakamura in Institute of Multidisciplinary Research for Advanced Materials (IMRAM)/Tohoku University, and Mr. Hiroshi Uemura, Ms. Keiko Toguchi and Ms. Megumi Sasaki in IMR/Tohoku University for their supports. References Anderson, D.F., Kobayashi, M., Woody, C.L., Yoshimura, Y., 1990. Lead fluoride: an ultra-compact Cherenkov radiator for Em Calorimetry. Nucl. Instr. Meth. Phys. Res. A 290, 385e389. Anderson, D.F., Kierstead, J.A., Lecoq, P., Stoll, S., Woody, C.L., 1994. A search for scintillation in doped and orthorhombic lead fluoride. Nucl. Instr. Meth. Phys. Res. A 342, 473e476. Dally, E.B., Hofstadter, R., 1968a. High energy g-Ray detector with good resolution. Rev. Sci. Instr. 39, 658e659. Dally, E.B., Hofstadter, R., 1968b. A lead fluoride Cerenkov Shower Counter. IEEE Trans. Nucl. Sci. 15, 76e81. Kozma, P., Bajgar, R., Kozma Jr., P., 2002. Radiation resistivity of PbF2 crystals. Nucl. Instr. Meth. Phys. Res. A 484, 149e152. Kurosawa, S., Yanagida, T., Yokota, Y., Yoshikawa, A., 2012. Crystal growth and scintillation properties of fluoride scintillators. IEEE Trans. Nucl. Sci. 59, 2173e2176. Mao R., Zhang L., Member, IEEE, Zhu R-Y., 2009. Search for scintillation in doped lead fluoride crystals. Nuclear science Symposium Conference Record (NSS/ MIC), 2009 IEEE, pp. 2182e2186. Malinowski, M., Kaczkana, M., Wnukb, A., Szuflinska, M., 2004. Emission from the high lying excited states of Ho3þ ions in YAP and YAG crystals. J. Luminescence 106, 269e279. Woody, C.L., Stoll, S.P., Kierstead, J.A., 1996. Observation of fast scintillation light in a PbF2:Gd crystal. IEEE Trans. Nucl. Sci. 43, 1303e1305.