Performance of large lanthanum bromide scintillators

摘要

Cerium-doped lanthanum bromide, LaBr_3(Ce), scintillator possesses several outstanding properties that make it an attractive choice for security, medical, and geophysical radiation detection applications. Among these properties are good density (5.1 g/cc), excellent energy resolution (～3% full-width at half-maximum (FWHM) at E_γ = 662 keV), brightness (> 65,000 photon/MeV), and speed, (τ_d<20ns). Unfortunately, the development of many prospective devices using this scintillator has been hampered by the lack of large crystals (> 100 cc). The anisotropic thermal expansion exhibited by this material makes it difficult to grow large ingots due to the build up of internal stresses as it cools, causing fracturing. Recently, Saint-Gobain Crystals has achieved successful growths of large unfractured ingots, from which large detectors have been assembled (> 150 cc). The outstanding properties seen in small pieces are retained up to at least 155 cc (the largest assembled into a single detector thus far). A cylindrical LaBr_3(Ce = 5%) crystal with dimensions of diameter = 51mm, and length = 76mm achieves energy resolution of 3.1% FWHM at 662 keV, and brightness of 165% of NaI with good uniformity throughout the crystal. Scintillation light yield and energy resolution have been examined as a function of crystal size and γ-ray energy. Spatial mapping of a large crystal was examined and shown to be uniform. Large crystals enable accurate measurements of the intrinsic γ-ray background from ~(138)La (0.09% nat. abun., γ-ray emission at 1436 and 789 keV). This background is shown to scale appropriately in size with theoretical calculations.