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Growth problems, thermal expansion and scintillation properties of Ce:Li6Lu(BO3)3 crystals under thermal neutron excitation
Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Growth problems, thermal expansion and scintillation properties of Ce:Li6Lu(BO3)3 crystals under thermal neutron excitation Shangke Pan a,n, Zaiwei Fu b, Dandan Sun a, Guohao Ren a, Yuekun Heng b a b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
Keywords: A2. Czochralski method B1. Borates Scintillation materials Neutron detection
a b s t r a c t The problems of the growth of Ce:Li6Lu(BO3)3 (LLBO) crystal using Czochralski technique have been discussed. The supercooling, cleavage crack and inversion of solid–liquid interface were the main problems to grow high quality LLBO single crystals with fully transparent ingot. The thermal expansion coefficients of LLBO crystal were obtained from HTXRD data: αa ¼23.9 10 6/1C, αb ¼27.8 10 6/1C and αc ¼9.3 10 6/1C. The LLBO crystal with thickness of 1 mm grown from natural raw materials was sensitive to thermal neutrons and insensitive to high energy gamma rays (137Cs). The decay time was statistically recorded to be about 38 ns by the digital oscilloscope. Therefore, LLBO crystal will be a promising fast scintillator for neutron detection. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Inorganic scintillation crystals are usually widely used in the detection of X- or gamma ray radiation. Scintillation crystals containing Li or B elements can be used to detect neutrons for the large cross section of 6Li (n, α)3H reaction with the energy of 4.8 MeV and 10B (n, α) 7Li reaction with the energy of 2.3 MeV [1,2]. Recently, the interest in the detection of thermal neutron has been increasing for the increasing need for neutron detectors in the fields of national security system and neutron scattering equipment, such as radiation portal monitors and spallation neutron sources. The neutron detection crystals of Eu:LiI and Ce: Cs2LiYCl6 can been obtained from the market. But they are very expensive for the sensitivity to water gas and the strictly control of the 6Li isotopes. So crystals containing B elements may be promising cheap scintillators for neutron detection. Our recent work showed that Ce:Li6Lu(BO3)3 (LLBO) crystal is also a fast scintillator with good luminesce efficiency and can also be grown with Czochralski technique[3,4]. Furthermore, compared to LGBO crystal, the absence of Gd3 þ ions makes it possible for natural LLBO crystal without enriched 6Li and 10B isotopes to detect the neutrons with high detection efficiency. Many work have been made on the growth and scintillation properties of Ce:Li6Gd(BO3)3 (LGBO) crystal[5–10]. When growing the LGBO crystal using Czochralski technique, the main problem is the instability of growth interface, in which the solid–liquid interface inversed
n
Corresponding author. Tel.: þ 86 21 69987745; fax: þ 86 21 59927184. E-mail address: [email protected] (S. Pan).
during the growth process and stopped the growth. For LLBO and LGBO crystals are analogues, similar problems may be also met in the investigation of the growth of LLBO using Czochralski technique. In this paper, we will report the growth problems of Ce3 þ -doped Li6Lu(BO3)3 crystal using Czochralski technique. The thermal expansion characterization and scintillation properties under thermal neutron excitation of LLBO crystal will also be reported. 2. Experimental procedure Commercially available Li2CO3, H3BO3, Lu2O3 and CeO2 of 99.99%-up purity were used as initial raw materials, in which CeO2 were used as the dopant hosts to introduce luminescence center of Ce3 þ ions. The Ce:Li6Lu(BO3)3 material for crystal growth were synthesized by means of solid state reaction with two steps. First, Li6Lu(BO3)3 and Li6Ce0.2Lu0.8(BO3)3 materials were sintered under air atmosphere and N2 atmosphere respectively following the solid reaction procedures described in Ref. [8]. Then they were melted together in a Pt or Ir crucible under N2 atmosphere to get the starting materials for crystal growth according to the concentration of Ce ions of 0.5 at% or 2 at% in the melt. The Ce:Li6Lu(BO3)3 crystals were grown by Czochralski technique with the radio frequency heating system. The crystal growth processes was manually controlled by a Eurotherm 818 temperature controller with the accuracy of 0.1 1C under the protection of N2 gas. The typical sketch of Czochralski technique setup was also described in Ref. [8]. During the crystal growth process, the pulling rate was 0.3–0.5 mm/h and the rotation speed was about 3–5 rpm.
0022-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
2
S. Pan et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
The thermal expansion of LLBO crystal was characterized by high temperature X-ray powder diffraction (HTXRD) using 40 kV, 200 mA and the data were collected on a diffractometer (D8 ADVANCE) equipped with CuKα radiation (λ ¼1.54056 Å) at the temperature between 25 1C and 700 1C. The scintillation properties of LLBO crystal with the thickness of 1 mm under neutron excitation measurement setup is shown in Fig. 1. A point neutron source (252Cf) with activity of 5 106 Bq is shielded in a box. The emitted neutrons were thermalized into the thermal range and guided out through a channel and then deposited on the LLBO crystal sample. The LLBO crystal packed by Tyvek films, greased with silicon oil, was coupled with a PMT (XP2020) with the gain of 1 107 at the 2000 V. A lead plate with the 5 cm thickness is fixed in the front of the PMT to shield gamma rays. An additional cadmium plate with the 3 mm thickness is placed between the moderator and lead plate to absorb neutrons to test the background. The 241Am and 137Cs radiation sources were directly placed close to LLBO crystal. The natural radioactivity of 176Lu in LLBO crystal was measured with a CdZnTe gamma-ray spectrometer with the high voltage of 500 V, the amplitude of 192 times, and the shaping time of 100 ns.
Fig. 1. Setup of scintillation properties under the neutron excitation of LLBO crystal.
Fig. 3. The XRD patterns of LLBO crystal at temperatures of 25 1C and 100–700 1C in 100 1C increments and standard PDF card of Li6Gd(BO3)3 (JCPDS 54–1119).
Fig. 2. Problems with supercooling (a), cleavage plane (b), inversion of solid–liquid interface (c) and (d) during the growth processes of LLBO crystal using Czochralski technique.
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
S. Pan et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3. Results and discussion A typical crystal growth process using Czochralski technique should include seed growth, neck growth, shoulder growth, cylindrical growth and tail growth stages. The crystal begins to grow when the seed crystal dips into the melt at the growth temperature. During the seed and neck growth period, the latent heat was mainly released through the neck and seed from the growth interface. The diameters of the seed crystal and neck have great effect on the growth process. In Fig. 2(a) and (b), the crystals were grown by Czochralski technique under the same thermal gradients in an Ir crucible with the dimensions of ϕ 60 mm 40 mm. When the dimensions of the LGBO seed crystals were 3 mm 3 mm and the Table 1 Cell parameters of LLBO crystal at temperatures of 25 1C and 100–700 1C in 100 1C increments. T (1C)
a (Å)
b (Å)
c (Å)
β (1)
V (Å3)
25 100 200 300 400 500 600 700
7.162 7.180 7.198 7.196 7.217 7.272 7.270 7.264
16.163 16.440 16.478 16.569 16.497 16.569 16.523 16.611
6.650 6.641 6.651 6.646 6.655 6.637 6.675 6.702
105.206 105.088 105.153 105.076 105.387 105.034 105.106 105.473
742.78 756.89 761.46 765.12 763.88 772.25 774.09 779.38
Fig. 4. The changes in lattice parameters with temperature of LLBO crystal.
3
diameter of the neck was reduced to about 2 mm, the crystal diameter can only enlarge very gradually until the growth interface was broken into an unstable interface and the supercooled melt couldn't crystallize normally. This made the crystal become opaque (Fig. 2(a)). In Fig. 2(b), the dimension of the LGBO seed crystals were 5 mm 5 mm, and the diameter of the neck was increased to about 4 mm, the crystal diameter increased more quickly, and then the shouldering growth would begin. There was a crack plane across the crystal, which was the (0 1 0) cleavage plane. Fig. 2(c) shows a single crystal ingot grown with the pulling rate of 0.5 mm/h in a Pt crucible with the dimensions of ϕ 80 mm 80 mm. The seeding growth, neck growth and shouldering growth were carried out normally. But the central part was not transparent and the interface was concave towards the melt, which is show in Fig. 2(d). This means the growth interface inversed during the growth process when the crystal length increased to a critical value. So the growth problems of LLBO crystal with Czochralski technique were the supercooling of the melt, cleavage crack and the inversion of the solid–liquid interface. To solve these growth problems, thermal gradient, pulling rate and temperature decreasing rate should be optimized. The coefficients of thermal expansion for LLBO crystal were calculated using HTXRD data. The XRD patterns of LLBO crystal at different temperatures are compared to the standard PDF card of Li6Gd(BO3)3 (JCPDS 54-1119), which are shown in Fig. 3. The XRD peaks of LLBO crystal at different temperatures can be well indexed with monoclinic Li6Gd(BO3)3 and LLBO crystal is also belong to space group P21/c. Then the unit cell parameters at different temperature were obtained from the XRD data, which are list in Table 1. Fig. 4 plots the changes in lattice parameters with temperature for LLBO crystal. According to the formula of the thermal expansion coefficient, αL ¼ ΔL/(L0 ΔT), where ΔL and ΔT are the change of lengthen and temperature respectively, L0 is the initial lengthen. So the thermal expansion coefficients of LLBO crystal along a-, b- and c-axis are: αa ¼23.9 10 6/1C, αb ¼ 27.8 10 6/1C and αc ¼9.3 10 6/1C, respectively. The big difference between the cell parameters axis a-, b-axis and c-axis causes very easy cracks during the crystal cooling and cutting processes. Fig. 5(a) shows the pulse height spectra of LLBO crystal under thermal neutron excitation, and the signal peak with about 600 channels was observed. The pulse height spectra of LLBO crystal under the excitation of low energy gamma rays (241Am) and high energy gamma rays (137Cs) are shown in Fig. 5(b). The full energy peak appeared in the pulse height spectra excited by 241Am with the gamma rays energy of 59.5 keV. When excited by the high energy gamma rays of 137Cs with the energy of 661 keV, the full energy peak couldn't be observed and only X-ray peak of 137Ba (K) appeared. This indicated that LLBO crystal was sensitive to low
Fig. 5. Pulse height spectra of LLBO crystal excited by thermal neutrons (a) and gamma-rays (b).
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
4
S. Pan et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
So LLBO crystal will fulfill the ability of fast response for the next generation neutron detectors. Fig. 7 shows the pulse height spectra of intrinsic gamma-ray in LLBO crystal measured by CdZnTe gamma-ray spectrometer. The back line is the background and the red line is the pulse height spectra where LLBO crystal was used as a radiation source. Four peaks around 600, 900, 2200 and 3300 channels could be observed, which were corresponding to the radioactive of the 176 Lu isotopes in the crystal. The counts of these peaks were relatively low and the 176Lu isotopes in the LLBO crystal will not greatly decrease the discrimination ability of neutron and gamma rays. 4. Conclusion
Fig. 6. The pulses pileups recorded by oscilloscope excited by thermal neutrons.
The problems of supercooling, cleavage crack and inversion of solid–liquid interface are the obstacles to grow high quality LLBO crystal. The thermal expansion coefficients of LLBO crystal were obtained from HTXRD data. The LLBO crystal without enriched 6Li and 10B isotopes was sensitive to thermal neutrons and the signal peak could be observed in the pulse height spectra. The decay time of LLBO crystal was about 38 ns. So the LLBO crystal is a promising fast scintillator for neutron detection. Acknowledgements This work was supported by the National Natural Science foundation of China (No. 51272263) and the State Key Laboratory of Particle Detection and Electronics. References
Fig. 7. The pulse height spectra of intrinsic gamma-ray of
176
Lu in LLBO crystal.
energy γ rays. When excited by high energy gamma rays, few energy can be deposited on the LLBO crystal with the thickness of only 1 mm. This meant that very thin LLBO crystals without enriched 6Li and 10B isotopes can be used to detect thermal neutrons for the ability to discriminate between neutrons and high energy gamma rays. Fig. 6 shows the pulses pileups recorded by oscilloscope excited by thermal neutrons. The decay time can be made a statistics of pulse one by one recorded by the oscilloscope. The decay time under irradiation of thermal neutrons was about 38 ns by using Gauss fit, and it was shorter than that of ZnS: Ag/LiF and lithium glass with the decay time about 1 μs and 70 ns.
[1] V.E. Carel W.E., Nucl. Instrum. Methods Phys. Res., Sect. A 460 (2001) 1–14. [2] V.E. Carel W.E., Radiation measurements, in: Proceedings of the Fifth European Conference on Luminescent Detectors and Transformers of Ionizing Radiation (LUMDETR 2003), 38 (2004) 337–342. [3] F. Yang, S.K. Pan, D.Z. Ding, X.F. Chen, H. Feng, G.H. Ren, J. Cryst. Growth 312 (2010) 2411–2414. [4] Y.K. Heng, R. Jia, Z.W. Fu, G.H. Ren, F. Yang, S.K. Pan, S.L. Li, X.H. Chen, S. Qian, G.M. Huang, in: IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011, pp. 4627–4631. [5] J.P. Chaminade, O. Viraphong, S. Miyazawa, J. Cryst. Growth 237–239 (Part 1) (2002) 864–868. [6] A.N. Shekhovtsov, A.V. Tolmachev, M.F. Dubovik, E.F. Dolzhenkova, T.I. Korshikova, B.V. Grinyov, V.N. Baumer, O.V. Zelenskaya, J. Cryst. Growth 242 (2002) 167–171. [7] F. Yang, S.K. Pan, D.Z. Ding, X.F. Chen, S. Lu, W.D. Zhang, G.H. Ren, J. Alloys Compd. 484 (2009) 837–840. [8] S. Pan, F. Yang, D. Ding, S. Lu, G. Ren, IEEE Trans. Nucl. Sci. 57 (2010) 1300–1303. [9] J.B. Czirr, G.M. MacGillivray, R.R. MacGillivray, P.J. Seddon, Nucl. Instrum. Methods Phys. Res., Sect. A 424 (1999) 15–19. [10] J.P. Chaminade, O. Viraphong, F. Guillen, C. Fouassier, B. Czirr, IEEE Trans. Nucl. Sci. 48 (2001) 1158–1161.
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth problems, thermal expansion and scintillation properties of Ce:Li6Lu(BO3)3 crystals under thermal neutron excitation Shangke Pan a,n, Zaiwei Fu b, Dandan Sun a, Guohao Ren a, Yuekun Heng b a b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
Keywords: A2. Czochralski method B1. Borates Scintillation materials Neutron detection
a b s t r a c t The problems of the growth of Ce:Li6Lu(BO3)3 (LLBO) crystal using Czochralski technique have been discussed. The supercooling, cleavage crack and inversion of solid–liquid interface were the main problems to grow high quality LLBO single crystals with fully transparent ingot. The thermal expansion coefficients of LLBO crystal were obtained from HTXRD data: αa ¼23.9 10 6/1C, αb ¼27.8 10 6/1C and αc ¼9.3 10 6/1C. The LLBO crystal with thickness of 1 mm grown from natural raw materials was sensitive to thermal neutrons and insensitive to high energy gamma rays (137Cs). The decay time was statistically recorded to be about 38 ns by the digital oscilloscope. Therefore, LLBO crystal will be a promising fast scintillator for neutron detection. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Inorganic scintillation crystals are usually widely used in the detection of X- or gamma ray radiation. Scintillation crystals containing Li or B elements can be used to detect neutrons for the large cross section of 6Li (n, α)3H reaction with the energy of 4.8 MeV and 10B (n, α) 7Li reaction with the energy of 2.3 MeV [1,2]. Recently, the interest in the detection of thermal neutron has been increasing for the increasing need for neutron detectors in the fields of national security system and neutron scattering equipment, such as radiation portal monitors and spallation neutron sources. The neutron detection crystals of Eu:LiI and Ce: Cs2LiYCl6 can been obtained from the market. But they are very expensive for the sensitivity to water gas and the strictly control of the 6Li isotopes. So crystals containing B elements may be promising cheap scintillators for neutron detection. Our recent work showed that Ce:Li6Lu(BO3)3 (LLBO) crystal is also a fast scintillator with good luminesce efficiency and can also be grown with Czochralski technique[3,4]. Furthermore, compared to LGBO crystal, the absence of Gd3 þ ions makes it possible for natural LLBO crystal without enriched 6Li and 10B isotopes to detect the neutrons with high detection efficiency. Many work have been made on the growth and scintillation properties of Ce:Li6Gd(BO3)3 (LGBO) crystal[5–10]. When growing the LGBO crystal using Czochralski technique, the main problem is the instability of growth interface, in which the solid–liquid interface inversed
n
Corresponding author. Tel.: þ 86 21 69987745; fax: þ 86 21 59927184. E-mail address: [email protected] (S. Pan).
during the growth process and stopped the growth. For LLBO and LGBO crystals are analogues, similar problems may be also met in the investigation of the growth of LLBO using Czochralski technique. In this paper, we will report the growth problems of Ce3 þ -doped Li6Lu(BO3)3 crystal using Czochralski technique. The thermal expansion characterization and scintillation properties under thermal neutron excitation of LLBO crystal will also be reported. 2. Experimental procedure Commercially available Li2CO3, H3BO3, Lu2O3 and CeO2 of 99.99%-up purity were used as initial raw materials, in which CeO2 were used as the dopant hosts to introduce luminescence center of Ce3 þ ions. The Ce:Li6Lu(BO3)3 material for crystal growth were synthesized by means of solid state reaction with two steps. First, Li6Lu(BO3)3 and Li6Ce0.2Lu0.8(BO3)3 materials were sintered under air atmosphere and N2 atmosphere respectively following the solid reaction procedures described in Ref. [8]. Then they were melted together in a Pt or Ir crucible under N2 atmosphere to get the starting materials for crystal growth according to the concentration of Ce ions of 0.5 at% or 2 at% in the melt. The Ce:Li6Lu(BO3)3 crystals were grown by Czochralski technique with the radio frequency heating system. The crystal growth processes was manually controlled by a Eurotherm 818 temperature controller with the accuracy of 0.1 1C under the protection of N2 gas. The typical sketch of Czochralski technique setup was also described in Ref. [8]. During the crystal growth process, the pulling rate was 0.3–0.5 mm/h and the rotation speed was about 3–5 rpm.
0022-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
2
S. Pan et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
The thermal expansion of LLBO crystal was characterized by high temperature X-ray powder diffraction (HTXRD) using 40 kV, 200 mA and the data were collected on a diffractometer (D8 ADVANCE) equipped with CuKα radiation (λ ¼1.54056 Å) at the temperature between 25 1C and 700 1C. The scintillation properties of LLBO crystal with the thickness of 1 mm under neutron excitation measurement setup is shown in Fig. 1. A point neutron source (252Cf) with activity of 5 106 Bq is shielded in a box. The emitted neutrons were thermalized into the thermal range and guided out through a channel and then deposited on the LLBO crystal sample. The LLBO crystal packed by Tyvek films, greased with silicon oil, was coupled with a PMT (XP2020) with the gain of 1 107 at the 2000 V. A lead plate with the 5 cm thickness is fixed in the front of the PMT to shield gamma rays. An additional cadmium plate with the 3 mm thickness is placed between the moderator and lead plate to absorb neutrons to test the background. The 241Am and 137Cs radiation sources were directly placed close to LLBO crystal. The natural radioactivity of 176Lu in LLBO crystal was measured with a CdZnTe gamma-ray spectrometer with the high voltage of 500 V, the amplitude of 192 times, and the shaping time of 100 ns.
Fig. 1. Setup of scintillation properties under the neutron excitation of LLBO crystal.
Fig. 3. The XRD patterns of LLBO crystal at temperatures of 25 1C and 100–700 1C in 100 1C increments and standard PDF card of Li6Gd(BO3)3 (JCPDS 54–1119).
Fig. 2. Problems with supercooling (a), cleavage plane (b), inversion of solid–liquid interface (c) and (d) during the growth processes of LLBO crystal using Czochralski technique.
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
S. Pan et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3. Results and discussion A typical crystal growth process using Czochralski technique should include seed growth, neck growth, shoulder growth, cylindrical growth and tail growth stages. The crystal begins to grow when the seed crystal dips into the melt at the growth temperature. During the seed and neck growth period, the latent heat was mainly released through the neck and seed from the growth interface. The diameters of the seed crystal and neck have great effect on the growth process. In Fig. 2(a) and (b), the crystals were grown by Czochralski technique under the same thermal gradients in an Ir crucible with the dimensions of ϕ 60 mm 40 mm. When the dimensions of the LGBO seed crystals were 3 mm 3 mm and the Table 1 Cell parameters of LLBO crystal at temperatures of 25 1C and 100–700 1C in 100 1C increments. T (1C)
a (Å)
b (Å)
c (Å)
β (1)
V (Å3)
25 100 200 300 400 500 600 700
7.162 7.180 7.198 7.196 7.217 7.272 7.270 7.264
16.163 16.440 16.478 16.569 16.497 16.569 16.523 16.611
6.650 6.641 6.651 6.646 6.655 6.637 6.675 6.702
105.206 105.088 105.153 105.076 105.387 105.034 105.106 105.473
742.78 756.89 761.46 765.12 763.88 772.25 774.09 779.38
Fig. 4. The changes in lattice parameters with temperature of LLBO crystal.
3
diameter of the neck was reduced to about 2 mm, the crystal diameter can only enlarge very gradually until the growth interface was broken into an unstable interface and the supercooled melt couldn't crystallize normally. This made the crystal become opaque (Fig. 2(a)). In Fig. 2(b), the dimension of the LGBO seed crystals were 5 mm 5 mm, and the diameter of the neck was increased to about 4 mm, the crystal diameter increased more quickly, and then the shouldering growth would begin. There was a crack plane across the crystal, which was the (0 1 0) cleavage plane. Fig. 2(c) shows a single crystal ingot grown with the pulling rate of 0.5 mm/h in a Pt crucible with the dimensions of ϕ 80 mm 80 mm. The seeding growth, neck growth and shouldering growth were carried out normally. But the central part was not transparent and the interface was concave towards the melt, which is show in Fig. 2(d). This means the growth interface inversed during the growth process when the crystal length increased to a critical value. So the growth problems of LLBO crystal with Czochralski technique were the supercooling of the melt, cleavage crack and the inversion of the solid–liquid interface. To solve these growth problems, thermal gradient, pulling rate and temperature decreasing rate should be optimized. The coefficients of thermal expansion for LLBO crystal were calculated using HTXRD data. The XRD patterns of LLBO crystal at different temperatures are compared to the standard PDF card of Li6Gd(BO3)3 (JCPDS 54-1119), which are shown in Fig. 3. The XRD peaks of LLBO crystal at different temperatures can be well indexed with monoclinic Li6Gd(BO3)3 and LLBO crystal is also belong to space group P21/c. Then the unit cell parameters at different temperature were obtained from the XRD data, which are list in Table 1. Fig. 4 plots the changes in lattice parameters with temperature for LLBO crystal. According to the formula of the thermal expansion coefficient, αL ¼ ΔL/(L0 ΔT), where ΔL and ΔT are the change of lengthen and temperature respectively, L0 is the initial lengthen. So the thermal expansion coefficients of LLBO crystal along a-, b- and c-axis are: αa ¼23.9 10 6/1C, αb ¼ 27.8 10 6/1C and αc ¼9.3 10 6/1C, respectively. The big difference between the cell parameters axis a-, b-axis and c-axis causes very easy cracks during the crystal cooling and cutting processes. Fig. 5(a) shows the pulse height spectra of LLBO crystal under thermal neutron excitation, and the signal peak with about 600 channels was observed. The pulse height spectra of LLBO crystal under the excitation of low energy gamma rays (241Am) and high energy gamma rays (137Cs) are shown in Fig. 5(b). The full energy peak appeared in the pulse height spectra excited by 241Am with the gamma rays energy of 59.5 keV. When excited by the high energy gamma rays of 137Cs with the energy of 661 keV, the full energy peak couldn't be observed and only X-ray peak of 137Ba (K) appeared. This indicated that LLBO crystal was sensitive to low
Fig. 5. Pulse height spectra of LLBO crystal excited by thermal neutrons (a) and gamma-rays (b).
Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i
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S. Pan et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
So LLBO crystal will fulfill the ability of fast response for the next generation neutron detectors. Fig. 7 shows the pulse height spectra of intrinsic gamma-ray in LLBO crystal measured by CdZnTe gamma-ray spectrometer. The back line is the background and the red line is the pulse height spectra where LLBO crystal was used as a radiation source. Four peaks around 600, 900, 2200 and 3300 channels could be observed, which were corresponding to the radioactive of the 176 Lu isotopes in the crystal. The counts of these peaks were relatively low and the 176Lu isotopes in the LLBO crystal will not greatly decrease the discrimination ability of neutron and gamma rays. 4. Conclusion
Fig. 6. The pulses pileups recorded by oscilloscope excited by thermal neutrons.
The problems of supercooling, cleavage crack and inversion of solid–liquid interface are the obstacles to grow high quality LLBO crystal. The thermal expansion coefficients of LLBO crystal were obtained from HTXRD data. The LLBO crystal without enriched 6Li and 10B isotopes was sensitive to thermal neutrons and the signal peak could be observed in the pulse height spectra. The decay time of LLBO crystal was about 38 ns. So the LLBO crystal is a promising fast scintillator for neutron detection. Acknowledgements This work was supported by the National Natural Science foundation of China (No. 51272263) and the State Key Laboratory of Particle Detection and Electronics. References
Fig. 7. The pulse height spectra of intrinsic gamma-ray of
176
Lu in LLBO crystal.
energy γ rays. When excited by high energy gamma rays, few energy can be deposited on the LLBO crystal with the thickness of only 1 mm. This meant that very thin LLBO crystals without enriched 6Li and 10B isotopes can be used to detect thermal neutrons for the ability to discriminate between neutrons and high energy gamma rays. Fig. 6 shows the pulses pileups recorded by oscilloscope excited by thermal neutrons. The decay time can be made a statistics of pulse one by one recorded by the oscilloscope. The decay time under irradiation of thermal neutrons was about 38 ns by using Gauss fit, and it was shorter than that of ZnS: Ag/LiF and lithium glass with the decay time about 1 μs and 70 ns.
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Please cite this article as: S. Pan, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.01.023i