The possibility of development of new X-ray or gamma ray radiation detection systems based on the vacuum ultraviolet (VUV) scintillators have attracted attention some years ago . One of the candidates for the VUV scintillator can be the Nd-doped materials based on monoclinic BaY2F8. The fast VUV emission around 185 nm with a decay time of several nanoseconds is due to the allowed 5d-4f transition of the Nd3+ ion.
Recently, we tried to improve the scintillation properties by Tm-codoping  or Lu admixture to replace the Y in the BaY2F8 matrix using this method. The Lu admixture increased the material density, which is favorable for increasing the high-energy photon stopping power.
We used the micro-pulling-down method which is a fast and promising crystal growth method suitable for material research and composition screening . The micro-pulling-down setup was modified for the crystal growth of fluorides using carbon hot-zone and argon atmosphere with admixture of CF4 which acted as moisture scavenger gas.
Obviously, it would be interesting to increase the Lu-content as much as possible and preferably reach the BaLu2F8 composition. However, this is very difficult due to BaLu2F8 phase transition from orthorhombic to monoclinic phase occurring around 50°C below the melting point, which is around 945°C. Finally, the monoclinic BaLu2F8 single crystals of reasonable optical quality were grown by the micro-pulling-down method employing the melt-supercooling procedure with the help of LiF flux. From the point of view of luminescence, also the high-temperature orthorhombic modification of BaLu2F8 single crystal might be interesting due to two non-equivalent Lu sites . Better energy transfer from the matrix or between the rare-earth ions can be expected. This modification was prepared by the micro-pulling-down-method using a quenching procedure with specially modified hot-zone. The hot zone was arranged to reach very shallow gradient near the crucible nozzle so that the grown crystal was still kept above the phase transition temperature. Then the crystal was rapidly cooled preventing the phase transition to take place. The micro-pulling-down crystal growth, luminescence and scintillation properties of rare-earth-doped monoclinic and orthorhombic BaLu2F8 VUV scintillation crystals will be compared and discussed. It will be also shown that despite the relatively efficient energy transfer between some rare-earth ions in the orthorhombic BaLu2F8 matrix, the energy transfer from the matrix to the rare-earth ions is hampered due to preferential energy transfer to the lattice defect states.
Another candidate for VUV scintillator can be the ErF3 single crystal. Again, its crystal growth is complicated by a phase transition from hexagonal to tetragonal modification occurring some 20°C below the melting point, which is at 1140°C. However, the melt has a strong tendency for supercooling and in the past some successful attempts of crystal growth have been reported . However, when we tried to prepare the crystal using the micro-pulling-down method, supercooling was not sufficient and phase transition seriously degraded the quality of the crystal, which was not then applicable for any optical characterizations. These difficulties have been overcome with LiF flux to slightly lower the melting point and modified hot-zone to reach extremely steep gradient at the crucible nozzle. Then, high-quality crystals have been prepared. In some cases, Nd3+ ion has been introduced as a luminescence center. In some hosts, the Er3+ 5d-4f emission spectrum coincides with the Nd 4f-5d absorption band and thus the energy could migrate over the Er3+ 5d levels to the Nd3+ ones . Similar mechanism is expected in the ErF3:Nd. This concept is similar to that reported for PrF3:Ce scintillator, where an efficient energy transfer from the Pr3+ 1S0 4f-level to the 5d-levels of Ce3+. On the other hand, we may expect strong energy migration over the Er3+ levels and resulting luminescence quenching. Admixture of other rare-earth-ions might be a solution. Besides the crystal growth, basic photoluminescence and scintillation characteristics of ErF3 admixed with other rare-earth ions will be presented and discussed.
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