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Crystal growth, Nd distribution and luminescence properties of (Na0.425+xLu0.575−x−yNdy)F2.15−2x single crystals
Journal of Crystal Growth 318 (2011) 791–795
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth, Nd distribution and luminescence properties of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals Yuki Furuya a,n, Hidehiko Tanaka a, Kentaro Fukuda a,b, Noriaki Kawaguchi a,b, Yuui Yokota a, Takayuki Yanagida a, Valery Chani a, Martin Nikl c, Akira Yoshikawa a,d a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Research & Development Division, Tokuyama., Co. Ltd., ICR-building, Minamiyoshinari, Aoba-ku, Sendai, Japan c Institute of Physics, the Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic d New Industry Creation Hatchery Center (NICHe), 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b
a r t i c l e in f o
abstract
Available online 18 November 2010
(Na0.425 + xLu0.575 x yNdy)F2.15 2x (x ¼ 0, y ¼0, 0.001, 0.01, 0.05, 0.1, 0.2; x ¼ 7 0.01, 0.02, 0.05, y ¼0.05) single crystals were grown from the melt using the precise atmosphere control type Micro-Pulling-Down (m-PD) method to examine their potential as a new VUV scintillators. The grown crystals were singlephase materials with fluorite type structure (Fm 3m, Z¼ 4) as confirmed by XRD and had good crystallinity. The distribution of the crystal constituents in axial and radial directions was evaluated, and only negligible non-uniformity along growth axis was detected. The crystals demonstrated 70–90% transmittance above 176–188 nm wavelength and Nd3 + 5d–4f luminescence (when exited by X-ray) observed around 185 nm. Both 5d–4f absorption and emission peaks shifted to longer wavelength region. The radioluminiscence measurements under 5.5 MeV a-ray excitation (241Am) demonstrated the light yield of 35–90 [Ph/5.5 MeV-a] and the decay time of 5.5–7.5 ns. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Radiation A2. Single crystal growth B1. Inorganic compound B2. Scintillater materials B3. Scintillators
1. Introduction Recently, numbers of scintillating materials have been developed for medical and industrial applications, such as X-ray CT, PET, or SPECT. 5d–4f Nd3 + luminescence (parity and spin allowed transition) in vacuum–ultraviolet (VUV) region is proposed for practical application of gas proportional counters as a light detectors [1]. Development of VUV scintillator is important because VUV emission (at wavelength shorter than 200 nm) is necessary to excite the gas. Such new type detectors were considered and evaluated previously. However the attempt was unsuccessful, because both light yield of VUV scintillator and the quantum efficiency of the photo-detector were low. Therefore, it is interesting to examine new fluoride host materials for VUV scintillators, which have higher light yield suitable for the gas counters. Only the fluoride crystals that have wide band gap can be considered as candidates for the host crystal for VUV luminescence. (Na0.5 + xR0.5 x)F2 2x (R¼Y, Pr–Lu) solid solutions have disordered modified fluorite type structure (space group Fm-3m), in which cations (Na + , R3 + ) are randomly distributed within their common lattice sites [2]. These materials are transparent from IR to VUV. Therefore, they are appropriate for number of optical
applications including radiation output windows, phosphors, lasers, and scintillators for short wavelength region. Especially, (Na0.425 + xLu0.575 x)F2.15 2x is a promising VUV scintillator because of its high density 6.2 g/cm3 and large effective atomic number Zeff ¼64. According to the phase diagram, it melts congruently when composition ratio is LuF3:NaF¼57.5:42.5[3]. In addition, optimization of Na/Lu ratio may improve VUV scintillating performance. The (Na0.5 + xR0.5 x)F2 2x (R ¼Y, Gd–Lu) crystals melt congruently, and their congruent composition is shifted to greater Na content from Gd to Lu [3]. Distribution of Nd dopant in (Na0.425 + xLu0.575 x)F2.15 2x, is also not well understood. In this report, (Na0.425 + xLu0.575 x yNdy)F2.15 2x (x¼0, y¼0, 0.001, 0.01, 0.05, 0.1, 0.2) crystals growth by Micro-Pulling-Down (m-PD) method [4] are discussed. The properties of these crystals were evaluated considering their possible application as VUV scintillators. The crystals with y¼0.05 and x¼ 70.01, 0.02, 0.05 were also examined with intension to achieve highest emission output.
2. Experiment 2.1. Crystal growth and X-ray characterization
n
Corresponding author. Tel.: + 81 22 217 5167; fax: + 81 22 217 5102. E-mail address: [email protected] (Y. Furuya).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.048
(Na0.425 + xLu0.575 x yNdy)F2.15 2x (x¼ 70.01, 0.02, 0.05, y¼0.05 and x¼0, y¼0, 0.001, 0.01, 0.05, 0.1, 0.2) single crystals
792
Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
were grown by the Micro-Pulling-Down (m-PD) method with precise atmosphere control [4]. Starting materials were prepared from 99.99% pure NdF3, NaF, and LuF3 powders produced by Stella Chemifa Corporation. Every mixture was loaded into a graphite crucible that was then installed into the growth chamber. The chamber was evacuated up to 10 2 Pa. Thereafter, the crucible was heated up to 400 1C and kept for about 1 h at this temperature in order to remove oxygen traces caused by moisture at the surface of the raw materials and adsorbents on the chamber surface. During this baking procedure, the chamber was further evacuated down to 10 4 Pa. After the baking, the chamber was filled with high purity Ar (99.999%) and CF4 (99.999%) gases (Ar:CF4 ¼9:1) until ambient pressure. Then, the crucible was heated up to the melting of (Na0.425 + xLu0.575 x yNdy)F2.15 2x (about 940 1C). Thin Pt rod was used as the seed. It was first inserted into the channel (orifice) positioned at the bottom of the crucible until its wetting with the melt. Thereafter the seed was pulled down with constant rate of 0.12 mm/min. The melt solidification interface was kept constant, and this required reducing of the power supplied to the crucible for 0.5–2% before the end of the growth. The phase composition of the obtained crystals was confirmed by the X-ray powder diffraction (XRD) using RINT-2000 (Rigaku) at room temperature. The crystallinity of the grown crystal was measured by X-ray rocking curve (XRC) method using Advanced Thin Film X-ray System, Rigaku (ATX) with an extra high resolution diffractometer (Rigaku) attached to a multilayer X-ray mirror. The XRC profiles were examined with a 4-bounce Ge(2 2 0) channel-cut monochromator+ 2-bounce Ge(2 2 0) channel-cut analyzer.
3. Result and discussion 3.1. Crystal growth and X-ray characterization Two series of (Na0.425 + xLu0.575 x yNdy)F2.15 2x (x ¼ 70.01, 0.02, 0.05, y¼0.05 and x ¼0, y¼0, 0.001, 0.01, 0.05, 0.1, 0.2) single crystals were grown by the m-PD method. The crystals were transparent, crackless, and had no visible inclusions (Fig. 1). The XRD patterns of all the grown crystals have been taken, and all the crystals were single-phase materials with CaF2-type structure (Fm-3m, Z ¼4). The lattice parameters were ranged from ˚ and the lattice constants increased almost a¼5.44 to a¼5.53 A, linearly with Nd content as it was expected (Fig.2). X-ray rocking
2.2. Chemical composition To study spatial distribution of Nd in the grown crystals, chemical composition was measured by electron microprobe analysis (EPMA) with JXA-8621MX (JEOL). ZAF correction was also made, where Z stands for the atomic number, A is the absorption correction factor, and F is the fluorescence correction factor. The measurements were performed on cross-sectioned samples both along the growth axis (step ¼500 mm) and in radial directions (step ¼50 mm).
Fig. 1. View of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals grown by the m-PD method. (x ¼ 0, (a) y ¼0, (b) y¼ 0.001, (c) y¼ 0.01, (d) y¼ 0.05, (e) y¼ 0.1, (f) y¼0.2 and y¼0.05, d + 5. x ¼0.05, d + 2. x ¼0.02, d + 1. x ¼ 0.01, d 1. x ¼ 0.01, d 2. x¼ 0.02, d 5. x¼ 0.05).
2.3. Optical properties
2.4. Scintillation responses To perform the measurements, the crystals were optically coupled with PMT (R8778, Hamamatu, 1300 V bias) by optical grease (Krytox 16350, DuPont). To determine the light yield, the energy spectrum (pulse height spectrum) was recorded under 5.5 MeV a-ray excitation (241Am source), using a shaping amplifier with a shaping time of 0.5 ms (ORTEC 572) and a multichannel analyzer (MCA, Amptec 8000 A). At the same time, the decay time was also examined using an oscilloscope TDS3052B (Tektronix, Inc.).
5.54
5.52 Lattice constant / Å
After EPMA, the crystals were cut and polished to the dimensions of 1 2 10 mm3. The transmittance and radioluminescence spectra in the VUV region were measured using a spectrometer equipped with KV-201 monochromator (Bunkoh-Keiki) in pure nitrogen gas. Deuterium lamp and X-ray tube with Mo anode were used as a light and X-ray sources, respectively. The output signals were detected with photomultiplier tubes (PMT) Hamamatsu R6380 and R374.
5.5
5.48
5.46
5.44 0
5
10 15 Nd concentration / %
20
25
Fig. 2. Lattice constant of the grown (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals vs. Nd content.
Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
curves of the selected crystals were also examined. o scan was performed for the reflection from the (2 2 0) plane at 2y ¼47.0411. The full-width at half-maximum (FWHM) was measured to be 74.2 arcsec for x¼0 and y¼0.05 sample. This demonstrates that the m-PD grown crystals had the crystallinity that is comparable to that of commercially available optical single crystals, and their structure had sufficient quality for the examination of optical properties. The FWHM of the other crystals were also low and ranged from 68 to 105 arcsec, except 227.5 arcsec for x ¼0 and y¼0.2 crystal. To evaluate the distribution of Na, Lu, and Nd, EPMA measurements were performed using samples cut from the crystal with y¼0.05 both along the growth axis (Fig.3) and in the radial directions (middle part of crystals). Exponential fitting was applied and the value of effective segregation coefficients (keff) for Na, Lu, and Nd in (Na0.425 + xLu0.575 x yNdy)F2.15 2x were calculated using Scheil–Pfann expression CS ¼ keff ð1g=100Þkeff 1 , C0
ð1Þ
793
where C0 and Cs represent the ion concentration in the starting melt and in the crystal at the solidification fraction g, respectively. The as calculated segregation coefficients ranged between 0.98 and 1.02 (Table 1). These values were very close to unity, and therefore could not be considered as actual ones because the accuracy of EPMA is not sufficient to distinguish these small variations of composition. This is also demonstrated in Fig. 4 as negligible tendencies in nonuniform distribution of all constituents. These results could be probably helpful when more detailed studies of segregation will be performed. As a final conclusion, the segregation of all constituents in radial direction was not definitely observed. As for non-uniform distribution in radial direction, it was not detected.
3.2. Optical properties Fig. 4 illustrates the transmittance spectra of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals with fixed Na/Lu ratio (x ¼0). The crystals had 70–90% transmittance in the wavelength range exceeding 176–188 nm depending on their composition. The absorption peak wavelength was shifted to longer wavelength region when content of Nd in the crystals increased. Weak absorption due to Nd3 + 4d–4f transition was also observed around 200, 250, and 295 nm. Fig. 5 presents the transmittance spectra of (Na0.425 + x Lu0.575 x yNdy)F2.15 2x single crystals with fixed Nd content (y ¼0.05). All the crystals had optical absorption edge at 180 nm and about 80% transmittance at wavelengths exceeding 190 nm. No clear effect of Na content on transmittance was observed.
Table1 Calculated keff in (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals with y ¼0.05 (for reference only). Growth axis
keff (Na + ) keff (Lu3 + ) keff (Nd3 + )
Fig. 3. Cation distribution along growth axis of (Na0.425 + xLu0.575 x yNdy)F2.15 + 2x (y¼ 0.05, x¼ 0.05): (a) Nd (J) and (b) Lu (J) and Na(m). The nominal values represent content of the constituent in the melt.
Radial
Na 5%
Na 2%
Na 1%
Na+ 1%
Na +2%
Na+ 5%
1.02 0.98 0.98
1.02 0.98 0.98
1.02 0.98 0.98
0.98 1.02 1.02
0.98 1.02 1
0.98 1.02 1
1 1 1
Fig. 4. Transmittance spectra of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (x¼ 0, (a) y¼0, (b) y¼0.001, (c) y¼0.01, (d) y¼ 0.05, (e) y¼ 0.1, (f) y¼ 0.2).
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Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
700
Transmittance, T /%
100
80
680
60
660
(d+5) (d+2) (d+1) (d) (d-1) (d-2) (d-5)
(d+5) (d+2) 40
640
(d+1) (d) 620
(d-1)
20
(d-2) (d-5) 0 175
180
185
190
195
600 200
wavelength, / nm Fig. 5. Transmittance spectra of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (y¼0.05, d+ 5. x¼ 0.05, d +2. x¼ 0.02, d + 1. x ¼0.01, d. x ¼0, d 1. x¼ 0.01, d 2. x¼ 0.02, d 5. x¼ 0.05).
170
175
180
185
190
195
200
Fig. 7. X-ray excited luminescence spectra of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (y¼0.05, d + 5. x ¼0.05, d+ 2. x¼ 0.02, d +1. x¼ 0.01, d. x¼ 0, d 1. x¼ 0.01, d 2. x¼ 0.02, d 5. x¼ 0.05).
Fig. 6. X-ray excited luminescence spectra (X-ray tube with Mo anode, 35 kV, 60 mA) of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (x ¼0, (a) y ¼0, (b) y¼0.001, (c) y¼0.01, (d) y¼ 0.05, (e) y ¼0.1, (f) y¼0.2).
Fig. 6 demonstrates X-ray excited luminescence spectra of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals (x ¼0) with Nd3 + 5d–4f emissions observed around 185, 235, and 265 nm. According to the Dieke diagram and the results of [5–7], the origins of 185, 235, and 265 nm peaks were estimated as 4f25d to 4f3(4IJ), 4f25d to 4f3(3FJ), and 4f25d to 4f3(4GJ), respectively. The wavelength of the luminescent peaks was shifted to longer wavelength region when content of Nd in the crystals increased. This peak shift is most probably caused by the shift of strong absorption due to Nd3 + 5d–4f transition. This idea can be confirmed by the following observation: the peaks centered at 235 and 265 nm wavelengths did not shift as much as that centered at 185 nm. The intensity of the luminescence at 235 and 265 nm increased when Nd content increased from 1% to 5% Nd (y ¼0.01, 0.05), and decreased when the doping exceeded 5%. Fig. 7 shows X-ray excited luminescence spectra of (Na0.425 + x Lu0.575 x yNdy)F2.15 2x crystals with fixed Nd content (y¼0.05). The Nd3 + 5d–4f emissions were observed around 185 nm. Also, the peaks of greater luminescence intensity had shorter wavelength than that of low intensity ones. The peak shift in the crystals with the same Nd content (Fig.7) is considered to be caused
Fig. 8. a-ray (241Am) excited pulse height spectra of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals. Y-axis (counts/channel) represents detection efficiency, and X-axis (channel) represents light yield.
by shift of strong absorptions due to Nd3 + 5d–4f transition. In fact, high transparent samples have high luminescent intensity.
3.3. Scintillation responses In order to estimate the light yield, the measurements of a-ray (241Am) exited pulse height spectra of the crystals was performed (Fig.8). The peak channels of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals (x ¼0) were 23.4, 35.6, and 30 for y¼0.001, 0.01, and 0.05 samples, respectively. The results were fitted by a single Gaussian function. y¼0.1 and 0.2 samples did not demonstrate clear peaks. Light yield of the samples can be evaluated by simply comparing the channel observed for the samples with that reported for the 8%Nd:LaF3 (Channel¼67, 100 [Ph/5.5 MeV-a]) [8]. When compared with 8% Nd:LaF3, the light yield of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals were evaluated to be 35, 53, and 45 [Ph/5.5 MeV-a] for y¼0.001, 0.01, and 0.05,
Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
Fig. 9. a-ray (241Am) exited decay curve of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystal (x ¼0, y¼ 0.05). The fitting curve is also shown.
795
Their chemical uniformity was evaluated, and only negligible segregation along growth axis was detected, and no segregation in radial direction was observed. The crystals demonstrated 70–90% transmittance in VUV wavelength region starting from 176 to 188 nm. The absorption peak was shifted to direction of longer wavelength range when Nd content in the crystals increased. X-ray exited Nd3 + 5d–4f luminescence, observed around 185 nm, was also shifted to longer wavelength region with increase in of Nd content. By changing Na/Lu ratio in the y¼0.05 crystals, little improvement of transmittance and emission intensity were observed for the low Na content samples. The light yield of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x was evaluated to be approximately 90 [Ph/5.5 MeV-a] for the sample with x¼ 0.05 and y¼0.05. The decay time of the crystals were also evaluated to be around 5.5–7.5 ns under 5.5 MeV a-ray excitation. These results indicate that (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals are not appropriate for VUV scintillator as compared with Nd3 + :LaF3 due to their low light yield. However, there is a chance that better performance of these relatively flexible crystals can be demonstrated through further optimization of Na/Lu ratio and Nd content. References
respectively. For the best sample (x¼ 0.05, y¼0.05), the peak channel was 60, i.e the light yield was approximately 90 [Ph/ 5.5 MeV-a]. The results on decay time of the (Na0.425 + x Lu0.575 x yNdy)F2.15 2x crystals are presented in Fig. 9. The decay time profiles were fitted by one component exponential function, and the decay time was in the range of 5.5–7.5 ns.
4. Summary The (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals were grown from the melt by the m-PD method. They had acceptable crystallinity as it follows from the rocking curve measurements.
[1] A. Ochi, T. Nagayoshi, S. Koishi, T. Tanim-ori, T. Nagae, M. Nakamura, Nucl. Instrum. Meth. A 471 (2001) 264–267. [2] A.A. Blistanov, P. Chernov, D.N. Karimov, T.V. Ouvarova, J. Cryst. Growth 237– 239 (2002) 899–903. [3] B.P. Sobolev, T.H.E. RARE, Earth Trifluorides Part1, Institut d’Estudis Catalans, Barcelona, 2001, pp. 210–211. [4] A. Yoshikawa, T. Satonaga, K. Kamada, H. Sato, M. Nikl, N. Solovieva, T. Fukuda, J. Cryst. Growth 270 (2004) 427–432. [5] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, Wiley Interscience, New York, 1968. [6] P. Dorenbos, C.W.E. van Eijk, R.W. Hollander, P. Schotanus, IEEE Trans. Nucl. Sci. 37 (2) . [7] D. Lo, V.N. Makhov, N.M. Khaidukov, J.C. Krupa, J.Y. Gesland, J. Lumin. 106 (2004) 15–20. [8] H. Sekiya, Proceedings of the 34th ICHEP, Philadelphia, 2008.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth, Nd distribution and luminescence properties of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals Yuki Furuya a,n, Hidehiko Tanaka a, Kentaro Fukuda a,b, Noriaki Kawaguchi a,b, Yuui Yokota a, Takayuki Yanagida a, Valery Chani a, Martin Nikl c, Akira Yoshikawa a,d a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Research & Development Division, Tokuyama., Co. Ltd., ICR-building, Minamiyoshinari, Aoba-ku, Sendai, Japan c Institute of Physics, the Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic d New Industry Creation Hatchery Center (NICHe), 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b
a r t i c l e in f o
abstract
Available online 18 November 2010
(Na0.425 + xLu0.575 x yNdy)F2.15 2x (x ¼ 0, y ¼0, 0.001, 0.01, 0.05, 0.1, 0.2; x ¼ 7 0.01, 0.02, 0.05, y ¼0.05) single crystals were grown from the melt using the precise atmosphere control type Micro-Pulling-Down (m-PD) method to examine their potential as a new VUV scintillators. The grown crystals were singlephase materials with fluorite type structure (Fm 3m, Z¼ 4) as confirmed by XRD and had good crystallinity. The distribution of the crystal constituents in axial and radial directions was evaluated, and only negligible non-uniformity along growth axis was detected. The crystals demonstrated 70–90% transmittance above 176–188 nm wavelength and Nd3 + 5d–4f luminescence (when exited by X-ray) observed around 185 nm. Both 5d–4f absorption and emission peaks shifted to longer wavelength region. The radioluminiscence measurements under 5.5 MeV a-ray excitation (241Am) demonstrated the light yield of 35–90 [Ph/5.5 MeV-a] and the decay time of 5.5–7.5 ns. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Radiation A2. Single crystal growth B1. Inorganic compound B2. Scintillater materials B3. Scintillators
1. Introduction Recently, numbers of scintillating materials have been developed for medical and industrial applications, such as X-ray CT, PET, or SPECT. 5d–4f Nd3 + luminescence (parity and spin allowed transition) in vacuum–ultraviolet (VUV) region is proposed for practical application of gas proportional counters as a light detectors [1]. Development of VUV scintillator is important because VUV emission (at wavelength shorter than 200 nm) is necessary to excite the gas. Such new type detectors were considered and evaluated previously. However the attempt was unsuccessful, because both light yield of VUV scintillator and the quantum efficiency of the photo-detector were low. Therefore, it is interesting to examine new fluoride host materials for VUV scintillators, which have higher light yield suitable for the gas counters. Only the fluoride crystals that have wide band gap can be considered as candidates for the host crystal for VUV luminescence. (Na0.5 + xR0.5 x)F2 2x (R¼Y, Pr–Lu) solid solutions have disordered modified fluorite type structure (space group Fm-3m), in which cations (Na + , R3 + ) are randomly distributed within their common lattice sites [2]. These materials are transparent from IR to VUV. Therefore, they are appropriate for number of optical
applications including radiation output windows, phosphors, lasers, and scintillators for short wavelength region. Especially, (Na0.425 + xLu0.575 x)F2.15 2x is a promising VUV scintillator because of its high density 6.2 g/cm3 and large effective atomic number Zeff ¼64. According to the phase diagram, it melts congruently when composition ratio is LuF3:NaF¼57.5:42.5[3]. In addition, optimization of Na/Lu ratio may improve VUV scintillating performance. The (Na0.5 + xR0.5 x)F2 2x (R ¼Y, Gd–Lu) crystals melt congruently, and their congruent composition is shifted to greater Na content from Gd to Lu [3]. Distribution of Nd dopant in (Na0.425 + xLu0.575 x)F2.15 2x, is also not well understood. In this report, (Na0.425 + xLu0.575 x yNdy)F2.15 2x (x¼0, y¼0, 0.001, 0.01, 0.05, 0.1, 0.2) crystals growth by Micro-Pulling-Down (m-PD) method [4] are discussed. The properties of these crystals were evaluated considering their possible application as VUV scintillators. The crystals with y¼0.05 and x¼ 70.01, 0.02, 0.05 were also examined with intension to achieve highest emission output.
2. Experiment 2.1. Crystal growth and X-ray characterization
n
Corresponding author. Tel.: + 81 22 217 5167; fax: + 81 22 217 5102. E-mail address: [email protected] (Y. Furuya).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.048
(Na0.425 + xLu0.575 x yNdy)F2.15 2x (x¼ 70.01, 0.02, 0.05, y¼0.05 and x¼0, y¼0, 0.001, 0.01, 0.05, 0.1, 0.2) single crystals
792
Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
were grown by the Micro-Pulling-Down (m-PD) method with precise atmosphere control [4]. Starting materials were prepared from 99.99% pure NdF3, NaF, and LuF3 powders produced by Stella Chemifa Corporation. Every mixture was loaded into a graphite crucible that was then installed into the growth chamber. The chamber was evacuated up to 10 2 Pa. Thereafter, the crucible was heated up to 400 1C and kept for about 1 h at this temperature in order to remove oxygen traces caused by moisture at the surface of the raw materials and adsorbents on the chamber surface. During this baking procedure, the chamber was further evacuated down to 10 4 Pa. After the baking, the chamber was filled with high purity Ar (99.999%) and CF4 (99.999%) gases (Ar:CF4 ¼9:1) until ambient pressure. Then, the crucible was heated up to the melting of (Na0.425 + xLu0.575 x yNdy)F2.15 2x (about 940 1C). Thin Pt rod was used as the seed. It was first inserted into the channel (orifice) positioned at the bottom of the crucible until its wetting with the melt. Thereafter the seed was pulled down with constant rate of 0.12 mm/min. The melt solidification interface was kept constant, and this required reducing of the power supplied to the crucible for 0.5–2% before the end of the growth. The phase composition of the obtained crystals was confirmed by the X-ray powder diffraction (XRD) using RINT-2000 (Rigaku) at room temperature. The crystallinity of the grown crystal was measured by X-ray rocking curve (XRC) method using Advanced Thin Film X-ray System, Rigaku (ATX) with an extra high resolution diffractometer (Rigaku) attached to a multilayer X-ray mirror. The XRC profiles were examined with a 4-bounce Ge(2 2 0) channel-cut monochromator+ 2-bounce Ge(2 2 0) channel-cut analyzer.
3. Result and discussion 3.1. Crystal growth and X-ray characterization Two series of (Na0.425 + xLu0.575 x yNdy)F2.15 2x (x ¼ 70.01, 0.02, 0.05, y¼0.05 and x ¼0, y¼0, 0.001, 0.01, 0.05, 0.1, 0.2) single crystals were grown by the m-PD method. The crystals were transparent, crackless, and had no visible inclusions (Fig. 1). The XRD patterns of all the grown crystals have been taken, and all the crystals were single-phase materials with CaF2-type structure (Fm-3m, Z ¼4). The lattice parameters were ranged from ˚ and the lattice constants increased almost a¼5.44 to a¼5.53 A, linearly with Nd content as it was expected (Fig.2). X-ray rocking
2.2. Chemical composition To study spatial distribution of Nd in the grown crystals, chemical composition was measured by electron microprobe analysis (EPMA) with JXA-8621MX (JEOL). ZAF correction was also made, where Z stands for the atomic number, A is the absorption correction factor, and F is the fluorescence correction factor. The measurements were performed on cross-sectioned samples both along the growth axis (step ¼500 mm) and in radial directions (step ¼50 mm).
Fig. 1. View of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals grown by the m-PD method. (x ¼ 0, (a) y ¼0, (b) y¼ 0.001, (c) y¼ 0.01, (d) y¼ 0.05, (e) y¼ 0.1, (f) y¼0.2 and y¼0.05, d + 5. x ¼0.05, d + 2. x ¼0.02, d + 1. x ¼ 0.01, d 1. x ¼ 0.01, d 2. x¼ 0.02, d 5. x¼ 0.05).
2.3. Optical properties
2.4. Scintillation responses To perform the measurements, the crystals were optically coupled with PMT (R8778, Hamamatu, 1300 V bias) by optical grease (Krytox 16350, DuPont). To determine the light yield, the energy spectrum (pulse height spectrum) was recorded under 5.5 MeV a-ray excitation (241Am source), using a shaping amplifier with a shaping time of 0.5 ms (ORTEC 572) and a multichannel analyzer (MCA, Amptec 8000 A). At the same time, the decay time was also examined using an oscilloscope TDS3052B (Tektronix, Inc.).
5.54
5.52 Lattice constant / Å
After EPMA, the crystals were cut and polished to the dimensions of 1 2 10 mm3. The transmittance and radioluminescence spectra in the VUV region were measured using a spectrometer equipped with KV-201 monochromator (Bunkoh-Keiki) in pure nitrogen gas. Deuterium lamp and X-ray tube with Mo anode were used as a light and X-ray sources, respectively. The output signals were detected with photomultiplier tubes (PMT) Hamamatsu R6380 and R374.
5.5
5.48
5.46
5.44 0
5
10 15 Nd concentration / %
20
25
Fig. 2. Lattice constant of the grown (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals vs. Nd content.
Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
curves of the selected crystals were also examined. o scan was performed for the reflection from the (2 2 0) plane at 2y ¼47.0411. The full-width at half-maximum (FWHM) was measured to be 74.2 arcsec for x¼0 and y¼0.05 sample. This demonstrates that the m-PD grown crystals had the crystallinity that is comparable to that of commercially available optical single crystals, and their structure had sufficient quality for the examination of optical properties. The FWHM of the other crystals were also low and ranged from 68 to 105 arcsec, except 227.5 arcsec for x ¼0 and y¼0.2 crystal. To evaluate the distribution of Na, Lu, and Nd, EPMA measurements were performed using samples cut from the crystal with y¼0.05 both along the growth axis (Fig.3) and in the radial directions (middle part of crystals). Exponential fitting was applied and the value of effective segregation coefficients (keff) for Na, Lu, and Nd in (Na0.425 + xLu0.575 x yNdy)F2.15 2x were calculated using Scheil–Pfann expression CS ¼ keff ð1g=100Þkeff 1 , C0
ð1Þ
793
where C0 and Cs represent the ion concentration in the starting melt and in the crystal at the solidification fraction g, respectively. The as calculated segregation coefficients ranged between 0.98 and 1.02 (Table 1). These values were very close to unity, and therefore could not be considered as actual ones because the accuracy of EPMA is not sufficient to distinguish these small variations of composition. This is also demonstrated in Fig. 4 as negligible tendencies in nonuniform distribution of all constituents. These results could be probably helpful when more detailed studies of segregation will be performed. As a final conclusion, the segregation of all constituents in radial direction was not definitely observed. As for non-uniform distribution in radial direction, it was not detected.
3.2. Optical properties Fig. 4 illustrates the transmittance spectra of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals with fixed Na/Lu ratio (x ¼0). The crystals had 70–90% transmittance in the wavelength range exceeding 176–188 nm depending on their composition. The absorption peak wavelength was shifted to longer wavelength region when content of Nd in the crystals increased. Weak absorption due to Nd3 + 4d–4f transition was also observed around 200, 250, and 295 nm. Fig. 5 presents the transmittance spectra of (Na0.425 + x Lu0.575 x yNdy)F2.15 2x single crystals with fixed Nd content (y ¼0.05). All the crystals had optical absorption edge at 180 nm and about 80% transmittance at wavelengths exceeding 190 nm. No clear effect of Na content on transmittance was observed.
Table1 Calculated keff in (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals with y ¼0.05 (for reference only). Growth axis
keff (Na + ) keff (Lu3 + ) keff (Nd3 + )
Fig. 3. Cation distribution along growth axis of (Na0.425 + xLu0.575 x yNdy)F2.15 + 2x (y¼ 0.05, x¼ 0.05): (a) Nd (J) and (b) Lu (J) and Na(m). The nominal values represent content of the constituent in the melt.
Radial
Na 5%
Na 2%
Na 1%
Na+ 1%
Na +2%
Na+ 5%
1.02 0.98 0.98
1.02 0.98 0.98
1.02 0.98 0.98
0.98 1.02 1.02
0.98 1.02 1
0.98 1.02 1
1 1 1
Fig. 4. Transmittance spectra of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (x¼ 0, (a) y¼0, (b) y¼0.001, (c) y¼0.01, (d) y¼ 0.05, (e) y¼ 0.1, (f) y¼ 0.2).
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Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
700
Transmittance, T /%
100
80
680
60
660
(d+5) (d+2) (d+1) (d) (d-1) (d-2) (d-5)
(d+5) (d+2) 40
640
(d+1) (d) 620
(d-1)
20
(d-2) (d-5) 0 175
180
185
190
195
600 200
wavelength, / nm Fig. 5. Transmittance spectra of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (y¼0.05, d+ 5. x¼ 0.05, d +2. x¼ 0.02, d + 1. x ¼0.01, d. x ¼0, d 1. x¼ 0.01, d 2. x¼ 0.02, d 5. x¼ 0.05).
170
175
180
185
190
195
200
Fig. 7. X-ray excited luminescence spectra of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (y¼0.05, d + 5. x ¼0.05, d+ 2. x¼ 0.02, d +1. x¼ 0.01, d. x¼ 0, d 1. x¼ 0.01, d 2. x¼ 0.02, d 5. x¼ 0.05).
Fig. 6. X-ray excited luminescence spectra (X-ray tube with Mo anode, 35 kV, 60 mA) of (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals (x ¼0, (a) y ¼0, (b) y¼0.001, (c) y¼0.01, (d) y¼ 0.05, (e) y ¼0.1, (f) y¼0.2).
Fig. 6 demonstrates X-ray excited luminescence spectra of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals (x ¼0) with Nd3 + 5d–4f emissions observed around 185, 235, and 265 nm. According to the Dieke diagram and the results of [5–7], the origins of 185, 235, and 265 nm peaks were estimated as 4f25d to 4f3(4IJ), 4f25d to 4f3(3FJ), and 4f25d to 4f3(4GJ), respectively. The wavelength of the luminescent peaks was shifted to longer wavelength region when content of Nd in the crystals increased. This peak shift is most probably caused by the shift of strong absorption due to Nd3 + 5d–4f transition. This idea can be confirmed by the following observation: the peaks centered at 235 and 265 nm wavelengths did not shift as much as that centered at 185 nm. The intensity of the luminescence at 235 and 265 nm increased when Nd content increased from 1% to 5% Nd (y ¼0.01, 0.05), and decreased when the doping exceeded 5%. Fig. 7 shows X-ray excited luminescence spectra of (Na0.425 + x Lu0.575 x yNdy)F2.15 2x crystals with fixed Nd content (y¼0.05). The Nd3 + 5d–4f emissions were observed around 185 nm. Also, the peaks of greater luminescence intensity had shorter wavelength than that of low intensity ones. The peak shift in the crystals with the same Nd content (Fig.7) is considered to be caused
Fig. 8. a-ray (241Am) excited pulse height spectra of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals. Y-axis (counts/channel) represents detection efficiency, and X-axis (channel) represents light yield.
by shift of strong absorptions due to Nd3 + 5d–4f transition. In fact, high transparent samples have high luminescent intensity.
3.3. Scintillation responses In order to estimate the light yield, the measurements of a-ray (241Am) exited pulse height spectra of the crystals was performed (Fig.8). The peak channels of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals (x ¼0) were 23.4, 35.6, and 30 for y¼0.001, 0.01, and 0.05 samples, respectively. The results were fitted by a single Gaussian function. y¼0.1 and 0.2 samples did not demonstrate clear peaks. Light yield of the samples can be evaluated by simply comparing the channel observed for the samples with that reported for the 8%Nd:LaF3 (Channel¼67, 100 [Ph/5.5 MeV-a]) [8]. When compared with 8% Nd:LaF3, the light yield of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals were evaluated to be 35, 53, and 45 [Ph/5.5 MeV-a] for y¼0.001, 0.01, and 0.05,
Y. Furuya et al. / Journal of Crystal Growth 318 (2011) 791–795
Fig. 9. a-ray (241Am) exited decay curve of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystal (x ¼0, y¼ 0.05). The fitting curve is also shown.
795
Their chemical uniformity was evaluated, and only negligible segregation along growth axis was detected, and no segregation in radial direction was observed. The crystals demonstrated 70–90% transmittance in VUV wavelength region starting from 176 to 188 nm. The absorption peak was shifted to direction of longer wavelength range when Nd content in the crystals increased. X-ray exited Nd3 + 5d–4f luminescence, observed around 185 nm, was also shifted to longer wavelength region with increase in of Nd content. By changing Na/Lu ratio in the y¼0.05 crystals, little improvement of transmittance and emission intensity were observed for the low Na content samples. The light yield of the (Na0.425 + xLu0.575 x yNdy)F2.15 2x was evaluated to be approximately 90 [Ph/5.5 MeV-a] for the sample with x¼ 0.05 and y¼0.05. The decay time of the crystals were also evaluated to be around 5.5–7.5 ns under 5.5 MeV a-ray excitation. These results indicate that (Na0.425 + xLu0.575 x yNdy)F2.15 2x crystals are not appropriate for VUV scintillator as compared with Nd3 + :LaF3 due to their low light yield. However, there is a chance that better performance of these relatively flexible crystals can be demonstrated through further optimization of Na/Lu ratio and Nd content. References
respectively. For the best sample (x¼ 0.05, y¼0.05), the peak channel was 60, i.e the light yield was approximately 90 [Ph/ 5.5 MeV-a]. The results on decay time of the (Na0.425 + x Lu0.575 x yNdy)F2.15 2x crystals are presented in Fig. 9. The decay time profiles were fitted by one component exponential function, and the decay time was in the range of 5.5–7.5 ns.
4. Summary The (Na0.425 + xLu0.575 x yNdy)F2.15 2x single crystals were grown from the melt by the m-PD method. They had acceptable crystallinity as it follows from the rocking curve measurements.
[1] A. Ochi, T. Nagayoshi, S. Koishi, T. Tanim-ori, T. Nagae, M. Nakamura, Nucl. Instrum. Meth. A 471 (2001) 264–267. [2] A.A. Blistanov, P. Chernov, D.N. Karimov, T.V. Ouvarova, J. Cryst. Growth 237– 239 (2002) 899–903. [3] B.P. Sobolev, T.H.E. RARE, Earth Trifluorides Part1, Institut d’Estudis Catalans, Barcelona, 2001, pp. 210–211. [4] A. Yoshikawa, T. Satonaga, K. Kamada, H. Sato, M. Nikl, N. Solovieva, T. Fukuda, J. Cryst. Growth 270 (2004) 427–432. [5] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, Wiley Interscience, New York, 1968. [6] P. Dorenbos, C.W.E. van Eijk, R.W. Hollander, P. Schotanus, IEEE Trans. Nucl. Sci. 37 (2) . [7] D. Lo, V.N. Makhov, N.M. Khaidukov, J.C. Krupa, J.Y. Gesland, J. Lumin. 106 (2004) 15–20. [8] H. Sekiya, Proceedings of the 34th ICHEP, Philadelphia, 2008.