Crystal Growth and Scintillation Properties of CeF 3
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ABSTRACT Large single crystals of CeF 3 , 40 mm in diameter up to 140 mm in length, have been grown using the vertical Bridgman technique. Their optical and scintillation properties and their radiation hardness have been measured.
INTRODUCTION A few years ago Cerium fluoride (CeF 3 ) has been identified as a potentially interesting scintillator for high energy physics 11, 2]. During the last years this material has been intensively studied and it appears now as a very good candidate for high resolution electromagnetic calorimeters [3]. Indeed CeF3 exhibits a high density (6.16 g/cm 3 ), small radiation length (1.68 cm) and Moliere radius (2.6 cm) and a fast luminescence with decay times lower than 30 ns and no slow components [3, 4, 5]. Moreover it appeared to be highly resistant to ionizing radiations [6]. CeF 3 single crystals are generally grown using the Bridgman-Stockbarger technique under fluorine atmosphere [8, 9, 10, 1 l. Some scavengers, i.e. PbF 2 , can also be added to the melt to avoid oxygen incorporation [8]. The optical and scintillation properties of large CeF 3 single crystals are reported in this paper. Their radiation hardness has also been studied.
RESULTS Single crystals of CeF 3 , 40 mm in diameter up to 140 mm in length, have been grown using the classical vertical Bridgman technique. The starting materials were CeF3 powders prepared by fluorination of pure Cerium nitrate from different producers. An inert atmosphere and graphite crucibles were used. During the growth process, the crucible was lowered at a low rate (1.5 mm/h) in order to prevent cracks in crystals. Nevertheless large crystals are usually highly strained and can easily crack upon cooling. Five samples of different sizes have been measured at CERN by P. Lecoq. They were cut from crystals grown under various experimental conditions and from different raw materials. The characteristics of the crystals (size, optical absorption band edge and the light yields, decay times and radiation hardness under gamma ray excitation) are reported in Table 1. The light yield strongly depends on the size, the geometry and the wrapping of the tested sample. The values measured on our crystals are in good agreement with the results given in [3] for CeF3 crystals of similar sizes, i.e. about 55% of the light yield of BGO. 105 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
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The transmission spectra of these five samples are compared on Figure 1. Since the thicknesses of the samples are different, one must consider the absorption coefficient to compare the position 1 of the absorption band edge. Then, the optical band edge for an absorption coefficient of 10 mvaries from 287 to 299 nm depending on the growth conditions and on the purity of the raw materials [Table 1]. The position of this band-edge seems to be correlated with the light yield and the radiation hardness of
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