DOI: 10.1021/cg100557e
The LaBr3:Ce Crystal Growth by Self-Seeding Bridgman Technique and Its Scintillation Properties
2010, Vol. 10 4433–4436
Hongsheng Shi,*,† Laishun Qin,† Wenxiang Chai,† Jiayu Guo,† Qinhua Wei,† Guohao Ren,‡ and Kangying Shu† †
College of Materials Science and Engineering, China Jiliang University, Number 258, Xueyuan Road, Jianggan District, Hangzhou City, Zhejiang Province 310018, China, and ‡Shanghai Institute of Ceramics, Chinese Academy of Sciences, Number 215, Chengbei Road, Jiading District, Shanghai 201800, China Received April 27, 2010; Revised Manuscript Received August 20, 2010
ABSTRACT: The LaBr3:Ce crystal is difficult to grow although it has very excellent scintillation properties. In this paper, the LaBr3:Ce crystals have been grown by the self-seeding Bridgman technique, and the L 38 mm 50 mm LaBr3:Ce single crystal ingot has been achieved. It is found that the orientation of the crystal growth has a close relationship with the lowering velocity and the quartz ampule shape. The 1 mm/h lowering velocity is very helpful for the [001] direction to dominate the crystal growth under the certain ampule shape, thus avoiding the crystal crack effectively, and the 0.5 mm/h lowering velocity is helpful for the [100] direction to dominate the crystal growth under the same ampule shape. On this basis, the mechanism of the nuclei competition is discussed. The UV luminescence and the X-ray excited emission spectra of the LaBr3:Ce crystal have also been investigated.
Introduction Most of the crystals are grown by self-seeding Bridgman technique. Especially those crystals, such as the CdZnTe, PbI2, GaAs, CdTe, and AgGaS2 crystal,1-4 which have low melting point and need atmosphere protection. The growth orientation, however, is always random by self-seeding technique, which is one of the disadvantages of this method. The self-seeding Bridgman technique is also the main method for LaX3:Ce (X = Br,Cl) crystal growth.5,6 The LaBr3:Ce and LaCl3:Ce crystals have both excellent scintillation properties, especially the LaBr3:Ce crystal.7-11 The light output of the LaBr3:Ce crystal reaches to 78000ph/KeV with a decay time of 28 ns. The energy resolution of the LaBr3:Ce crystal at 662 KeV is 2.8%. Those scintillation properties are far beyond the NaI:Tl and CsI:Tl crystals that are widely used since the 1950s. Therefore, the LaBr3:Ce crystal attracts lots of attentions since its discovery in 2001.12-15 But the LaBr3:Ce crystal is very difficult to grow. Both of the LaBr3 and CeBr3 are very easy to react with water and oxygen. Both of them have the large thermal expansion anisotropy. The LaBr3 and CeBr3 crystals both belong to the hexagonal system, and the space group is P63/m. In the LaBr3 crystal, a = 7.96 A˚, c = 4.51 A˚. In the CeBr3 crystal, a = 7.95 A˚, c = 4.44 A˚. The LaBr3 and CeBr3 can form the solid solution at the random proportion. But the LaBr3:Ce crystal has a (100) cleavage plane. It is very important to grow the LaBr3:Ce crystal along the [001] direction to avoid the crack.16,17 This is a challenge for the self-seeding Bridgman technique to grow the LaBr3:Ce crystal. In this paper, the LaBr3:Ce crystals have been grown by the self-seeding Bridgman technique, and it is found that the orientation of the LaBr3:Ce crystal growth has a close relationship with the lowering velocity and the ampule shape. The
orientation of the LaBr3:Ce crystal growth can be controlled along the [001] direction, thus avoiding the crack effectively. The mechanism regarding the determination of orientation of the as-grown crystal is discussed. The UV luminescence and the X-ray excited emission spectra of the LaBr3:Ce crystal are also investigated, comparing with that of the CsI:Tl crystal. Experimental Section The starting materials were a mixture of LaBr3 and CeBr3 powders. Both LaBr3 and CeBr3 powders are 4N polycrystalline and anhydrous. The molar ratio of CeBr3 to LaBr3 is 3:97. The materials were charged in the quartz ampule. There was a capillary tip at the bottom of the ampule. Many kinds of ampules have been adopted. The difference of those ampules lies in the capillary tip. The capillary tip of the mostly used ampule has a 3 mm inner diameter and 30 mm length. The capillary tip of the other kinds of ampule has a larger inner diameter. The materials handling process has been done in the glovebox, where the water and oxygen content are both below 1 ppm. After being taken out of the glovebox, the quartz ampule was evacuated and sealed. The quartz ampule was then put in the Bridgman furnace. The Bridgman furnace has the high temperature zone, temperature gradient zone, and low temperature zone. The crystal has been grown in the temperature gradient zone. The vertical temperature gradient is about 30 °C/cm in the temperature gradient zone. After the raw materials were melted and the temperature field reached stability, the quartz ampule was lowered. The lowering velocity was either 0.5 mm/h or 1 mm/h when the crystal growing at the capillary part of the ampule. The lowering velocity is kept at 0.5 mm/h or shift to 0.5 mm/h when the crystal growing at the other part of the ampule.
Results and Discussion
*To whom correspondence should be addressed. Phone: 86-0-13750815074. Fax: 86-571-86835740. E-mail:
[email protected].
It is found that the orientation of the LaBr3:Ce crystal growth has a close relationship with the lowering velocity and the ampule shape. Here the ampule shape means the shape of the capillary tip, and the lowering velocity means the velocity when the crystal is growing at the capillary tip. All the single crystal ingots have been achieved from one kind of ampule of
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Published on Web 09/07/2010
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Figure 1. The L 38 mm 50 mm LaBr3:Ce single crystal ingot.
Figure 2. The 10 10 10 mm3 machined LaBr3:Ce single crystal.
which the capillary tip has a 3 mm inner diameter and 30 mm length. The [100] oriented single crystal ingots were achieved when the lowering velocity was 0.5 mm/h, but the probability to get the single crystal is low. The [001] oriented single crystal ingot was achieved when the lowering velocity was 1 mm/h, and the probability to get the single crystal is very high. The sizes of the largest single crystal ingot reach to L 38 mm 50 mm using the 1 mm/h lowering velocity. This crystal ingot is shown in Figure 1. The machined single crystal with 10 10 10 mm3 size is shown in Figure 2. The mechanism regarding the determination of the orientation of the crystal has been discussed. The UV luminescence and the X-ray excited emission spectra of the LaBr3:Ce crystal were investigated by comparing with the widely used CsI:Tl crystal. Orientation of the Crystal Growth. There are nine coordinate Br- ions around the La3þ ion (the data from ICSD
no. 31581). The Br- ion has only one Wyckoff position in the LaBr3 crystal, but there are two kinds of Br- coordinate ions for the La3þ ion. One kind includes three Br- coordinate ions, of which each has about 3.1568 A˚ distance from the La3þ ion. One kind includes six Br- coordinate ions, of which each has about 3.0946 A˚ distance from the La3þ ion. The structure of the LaBr96- is shown in Figure3. The stacking diagram of the LaBr3 crystal along the c axis is shown in Figure4. It can be observed that there are two parallel Br- layers alongside the (010) plane in Figure4. The Br- ions in one layer belong to the former kind. The Br- ions in another layer belong to the latter kind. The former kind of Br- ions have Coulomb repulsive forces, with the latter kind Br- ions, and the connection between the former kind Brions and the La3þ ion is shielded by the latter kind Br- ions. Therefore, the connection strength between the former kind
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Figure 5. The cleavage planes in the LaBr3:Ce crystal: (a) the cleavage plane vertical to the growth orientation, (b) the cleavage plane parallel to the growth orientation. Figure 3. The structure of the LaBr96- anionic coordination polyhedron.
Figure 4. The stacking diagram of the LaBr3 crystal along the c axis.
Br- ions and the La3þ ion should be weak. As is known, the (010) plane equals to the (100) plane in the LaBr3 crystal. It can explain why the (100) plane is a cleavage plane. There are actually six planes equal to the (100) plane. Those six planes are (100), (010), (110), (100), (010), and (110). They are all parallel to the c axis. The angle between two cleavage planes is either 60° or 120°. The cleavage planes in the real crystal are shown in Figure5. From Figure5, it can clearly be seen that the cleavage planes are very smooth and the angle between two cleavage planes is either 60° or 120°. Another interesting fact is also observed: the cleavage planes are either parallel to the growth orientation or vertical to the growth orientation. Therefore the orientation of the as-grown crystal along the growth direction can be judged by it. When lowering velocity is kept at 0.5 mm/h, the orientation of the as-grown crystal is always [100] and the crystal ingots tend to crack easily. The cleavage planes are shown in Figure5a. When lowering velocity is kept at 1 mm/h, the orientation of the as-grown crystal is always [001] and only a few of the crystal ingots cracked. The cleavage planes are shown in Figure5b. The stresses in the crystal mainly originate from the adhesive forces between the wall of the quartz ampule and the crystal ingot during the cooling stage. When the crystal is grown along the [001] direction, the stresses are born with under the highest symmetry. This is why the [001] direction can help to avoid the crystal crack best. At the initial stage of the crystal growth, multiply oriented crystal nuclei are generated at the bottom of the crucible capillary part. Those nuclei will compete with each other. If one nucleus had dominated the growth process, the single
crystal could be achieved, otherwise the crystal should be polycrystalline. The spontaneously crystallized nuclei tend to have the low-index planes appeared because of Wulff law.18 Therefore the dominating single crystal growth orientation is also a low-index direction. The traditional concept believes that the crystal growth orientation should be random among the low-index directions. The phenomenon observed in the LaBr3:Ce self-seeding crystal growth shows that the orientation of the as-grown crystal should have a close relationship with the lowering velocity. The 1 mm/h lowering velocity greatly helps the [001] orientation to dominate the crystal growth. The 0.5 mm/h lowering velocity slightly helps the [100] orientation to dominate the crystal growth. This orientation is determined by the nucleus which dominates the crystal growth during the nuclei competition stage. Here the competency of the nucleus should include two aspects: one is the capability of the nucleus to grow along the vertical direction, the other is the capability of the nucleus to grow along the horizontal direction. The nucleus which has more capability to develop simultaneously along the vertical and horizontal direction should have more chances to dominate the crystal growth. The driving force for the crystal growth along the horizontal direction should be small. This can partly be deduced from the shape of the top surface of the crystal ingot. In Figure5, it can also be observed that the top surface of the crystal ingot is almost flat. It means the growth interface is almost flat and the horizontal temperature gradient should be small. Therefore, the competency of a nucleus to develop along the vertical direction should be more important, but the competency of the nuclei along the vertical direction is controlled by the lowering velocity. The lowering velocity is like a brake. It controls the maximum growth speed of those nuclei, so the fast lowering velocity will help to show the competency difference of the nuclei to develop along the vertical direction. Because the (100) and (001) planes are singular planes of the LaBr3 crystal and both are vertical, it can be assumed there are two kinds of the nucleus at the initial stage of the crystal growth: one kind of nucleus has [001] direction toward the melt, the other kind nucleus has [100] direction toward the melt. The (100) interplanar distance is a = 7.96 A˚. The (001) interplanar distance is c = 4.51 A˚. The (001) plane should have faster growth speed than the (100) plane, so the former nucleus should grow faster along the vertical direction. The latter nucleus should grow faster along the horizontal direction. The fast lowering velocity should be of great help for the former nucleus to dominate the crystal growth. This should
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Ceramics, Chinese Academy of Science. The PMT model was the Hamamatsu R928. The results are shown in Figure7. There are two emission peaks at 355 nm and 380 nm in the LaBr3:Ce crystal. This results match very well the previous research results on the LaBr3:Ce crystal. Conclusion
Figure 6. The UV luminescence spectra of the LaBr3:Ce and CsI:Tl crystal.
By controlling the velocity at the initial stage of the LaBr3: Ce crystal growth and the quartz ampule shape, the [001] oriented single crystal can be achieved when the LaBr3:Ce crystal grown by the self-seeding Bridgman technique. This makes the self-seeding technique an effective method to grow the large size LaBr3:Ce single crystal. The L 38 mm 50 mm LaBr3:Ce single crystal ingot has been achieved by it. The UV luminescence and the X-ray excited emission spectra of the LaBr3:Ce crystal are investigated by comparing it with the widely used CsI:Tl crystal. Acknowledgment. This work was supported by the NSFC with grant nos. 51002147 and 11075147.
References
Figure 7. The X-ray excited emission spectra of the LaBr3:Ce and CsI:Tl crystal.
be the root cause why the 1 mm/h lowering velocity can always get the (001) oriented single crystal. Also, the low lowering velocity should be helpful for the latter nucleus to dominate the crystal growth, but the difference of the nuclei to develop along the horizontal direction should be small. This should be the reason why the 0.5 mm/h lowering velocity is slightly helpful in obtaining the [100] oriented single crystal. To achieve the single crystal, the shape of the ampule capillary tip is also important. The diameter of the crucible should be thin enough to ensure that few nuclei have been formed. There should also be enough length for the nuclei competition. The capillary tip of the mostly used ampule has 3 mm inner diameter and 30 mm length. Scintillation Properties. The UV luminescence and the X-ray excited emission spectra were investigated by comparing the CsI:Tl crystal with those of the same size. The instrument was the LS50B made by the Perkin-Elmer company. The UV luminescence spectra are shown in Figure6. The X-ray excited emission spectra were investigated by using the self-made instrument by the Shanghai Institute of
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