A new type of luminescence mechanism in large band-gap insulators: Proposal for fast scintillation materials
Luminescence components of BaF 2 (180-230 nm), CsF (220-500 nm), CsCl (200-300 nm), CsBr (220-280 nm), and RbF (200-450 nm) have been investigated at room temperature by using synchrotron radiation as a light source. These luminescence components have excitation thresholds at the energy differences from the outermost Ba 2+ 5p, or M +mp ( m = 4 and 5 for M = Rb and Cs, respectively) core state to the conduction band, and are attributeted to the radiative decay of electrons in the X -np halogen valence bands ( n = 2, 3, and 4 for X = F, Cl and Br, respectively) to the outermost-core hole states. Single bunch operation of synchrotron radiation has been used to determine the lifetime of the above luminescence. The measured lifetimes in ns are: BaF 2: 0.88±0.02; CsF: 2.9±0.1; CsCl: 0.88±0.07; CsBr: 0.07±0.03; and RbF: 1.3±0.1 From the above results, it is proposed that insulators having large band-gap energy compared to the energy difference between the valence band and the outermost-core state are candidates for fast scintillation materials, for example, CsCl, CsBr, RbF, KF, BaCl 2, BaBr 2, and BaI 2.