Development of the new functional
The fluoride single crystals have a wide transparency range from the VUV to the infrared region. Therefore, they can be used in various applications such as the window material for lithography, the host material for the short wavelength laser, host material for the eye-safe laser, and scintillation materials. However, an advanced atmosphere control is indispensable for the fluoride single crystal growth as lot of them are hygroscopic and deliquescent. This is an important point for the fluoride single crystal production.
Specific single-crystal materials are usually grown by Czochralski method（Cz） and Bridgman method（BG). However, these methods are not so effective in the search for new materials
because relatively long time is required to enable the growth of large single crystals with reasonable quality. For this reason the research of the functional fluoride single crystals can hardly be performed using these methods.
We have significantly improved the speed of the search for the fluoride single crystal by introducing the micro-pulling down (μ-PD) method which was successfully adapted to the growth of fluoride single-crystals in our laboratory. With this technique the crystal growth was accelerated by 1 to 2 orders of magnitude when compared with the previously mentioned methods(Cz method and BG method, etc.). The technological points in the fluoride single-crystal production that still need to be refined are the deliquescence, the impurity control, the vapor pressure, etc.
The μ-PD device for the fluoride single crystal production is designed in a similar way as the μ-PD device for the oxide material but it is improved and an advanced atmosphere control is enabled. The improvement consists in the four following points: (1)Installation of oil diffusion pump to make a high vacuum. (2)Change of the crucible material and the heat insulator from quartz to a fluorine material. (3)Change of the window material from the fused silica to CaF2. (4)Use of gas scavenger. Moreover, we successfully adapted the machine for the shape-controlled crystal growth which was the next step in the method development. (rod board corner) It is important to know the wettability character of the crucible material and the fluoride to control the shape. For instance, the cuboidal crystal in the figure is obtained by designing the shape of the die of the crucible. With this approach we also managed to grow a single-crystal board of 10mm in width and 1mm in thickness (see the figure below).
We are now able to grow many kinds of fluoride single-crystals, for example CaF2，BaF2，CeF3，PrF3，NdF3，BaMgF4，BaLiF3, etc. by this method.
Recently, we have been focusing on various scintillator crystals that might find their applications in gamma / x-ray radiation detection, i.e. high density fluoride crystals were produced and their scintillation characteristics were evaluated.
Using the modified micro-pulling-down method the Ce:PrF3 single crystals were grown and the evaluation of their scintillation characteristics was published. Figure shows the luminescence spectra of the Ce:PrF3 for different (0, 0.1, 0.5, 1, 3mol%) Ce concentrations. The short-wavelength luminescence near 290nm originates from the Ce 4f-5d transition and its intensity increases with increasing Ce concentration. Moreover, 400nm luminescence (with a lifetime of 600ns or less) originates from Pr 4f-4f transition and decreases with the Ce concentration. This shows that the energy transfer from PrF3 host matrix to the Ce3+ activator ion occurs. Figure shows the 300nm photoluminescence decay curve of the 3mol% Ce:PrF3 under the UV excitation. The fast course of the curve is characterized by the decay time of 20ns. The way of increasing the luminescence intensity using the energy transfer becomes a new direction in the future scintillator development although the practical use of the Ce:PrF3 is difficult. It is a very interesting phenomenon although still being only the academic topic.
On the other hand, the fluoride scintillators are considered also for the neutron detection. The neutron detection can be used in the field of security, the medical treatment, and the research and development, etc. With increasing requirements in the mentioned fields the development of more efficient scintillation materials is needed. High sensitivity to neutrons and low sensitivity to gamma radiation are the necessary features. Neutron-sensitive materials should contain elements with sufficiently high neutron-capture crossection such as 6Li or 10B. The resulting nuclear reaction will yield the alpha and/or other charged particles. In the beginning we focused on LiCaAlF6(LiCAF) doped with Ce and evaluated its scintillation characteristics. The Figure shows a pulse hight spectrum after the α rays excitation. (In the material the alpha rays induce similar processes as the nuclear reactions of neutrons with the mentioned nuclei.) The high sensitivity to the α rays was found while the sensitivity to gamma rays was low. Simulateneous excitation by α rays and gamma rays showed a good alpha/gamma discrimination capability of the material. Moreover, when Ce:BaLiF3 (whose sensitivity to gamma rays is high) is used, the peak of the α rays cannot be detected. The discrimination of the α rays and the γ rays is an important feature of the neutron scintillator.
As mentioned above, fluoride single-crystals are successfully developed in this laboratory using the modified fluoride μ-PD method, and the new functional fluoride single-crystals are searched for.