Search

The short-wavelength sub-nanosecond luminescence in BaF 2 material, discovered in the early 80s, gave a new direction to research into fast scintillators and phosphors. In contrast to 'typical' luminescence occurring within the forbidden band gap, the new type of emission was due to transitions between the upper core and valence bands. Thus it was named core-valence luminescence (CVL); the terms cross-luminescence and Auger-free luminescence have also been used. Given its unusual nature, this new luminescence process has attracted considerable interest in the field of condensed matter physics. During the sequent decades (till the end of the previous century) CVL has been experimentally observed in several dozen compounds and the conditions for detecting CVL have been established. Interest in this kind of luminescence increased in recent years due to the emergence of new methods of crystal synthesis, the development of high-speed solid-state photodetectors, and the construction of unique modern set-ups using synchrotron radiation. This paper analyzes the numerous studies of CVL, obtained mainly over the past decade. Experimental and theoretical data on this topic are considered in detail. Special attention is given to new results on scintillation characteristics of BaF 2 in the form of single crystals, ceramics, nanoparticles, and composites. The features of CVL in binary and ternary compounds are discussed. Some Cs- and Ba-based ternary halides show relatively high light yields and represent promising fast scintillators. The prospects of using materials with CVL as fast scintillators for time-of-flight positron emission tomography and other applications are deliberated. • Overview of recent theoretical and experimental findings on Core Valence luminescence. • Most attractive CVL crystals for ultrafast scintillator applications analyzed. • Prospects for future studies and underexplored CVL materials listed. [ABSTRACT FROM AUTHOR]

In this study, we have synthesized scintillation materials based on Ce-doped Cs 2 ZnCl 4 crystals. The light yield was enhanced by up to 20% by doping Cs 2 ZnCl 4 with Ce 3+ ions. In the scintillation time profiles, fast components exhibited decay time constants on the order of nanoseconds, which was ascribed to Auger-free luminescence (AFL). The light yield of the AFL component decreased at 10 mol% Ce 3+ concentration, which is mainly attributed to the reabsorption of AFL photons inside the crystals by Ce 3+ ions, as seen in the scintillation spectra. Long components had decay time constants of approximately 30 ns. In addition, at 10 mol% Ce 3+ concentration, a prominent band appeared at approximately 500 nm in the scintillation spectrum, which was not observed in the photoluminescence spectra. The long components in the scintillation time profiles and the 500 nm band in the scintillation spectra were tentatively attributed to self-trapped excitons perturbed by Ce 3+ ions. [ABSTRACT FROM AUTHOR]

We synthesized a fast scintillation material based on a ternary halide crystal, CsCaCl 3 , which is known to exhibit Auger-free luminescence (AFL) under core-level excitation. We characterized its luminescence properties using vacuum ultraviolet (VUV) light for excitation. We observed a broad AFL band centered at approximately 300 nm for the core hole excitation, with a decay time of 2.3 ns. In addition to AFL, we observed several luminescence bands for interband and sub-band-gap excitation. The scintillation light yield was estimated to be 410 photons/MeV. We observed a fast scintillation component due to the AFL having a decay time constant of 2.3 ns. This fast component accounts for 58% of the total scintillation. These results indicate that a fast scintillation material was successfully fabricated, employing AFL as the luminescence mechanism based on a ternary halide crystal. [ABSTRACT FROM AUTHOR]

Abstract: Nd concentration dependence on the scintillation properties of Nd x Ba1− x F2+ x (x =0.001–0.5) crystals were investigated. From X-ray induced emission spectra, the intensity from an intrinsic luminescence of BaF2 including core-valence luminescence and self-trapped-exciton (STE) emission were decreased with an increase of Nd concentration. Decay times under 241Am α-ray excitation were evaluated by the Photomultiplier Tube (PMT) Hamamatsu R8778 that was sensitive across the vacuum ultra-violet (VUV) and visible wavelength range. The average decay times indicated that Nd doped BaF2 with lower Nd concentration has obviously slower decay times than that of samples with higher Nd concentration because the decay time of STE emission was slower than that of the core–valence luminescence in BaF2 and the emission of 5d–4f transition on Nd3+. The light yield of fast Nd3+ 5d–4f luminescence was quite lower than STE emission of BaF2. [ABSTRACT FROM AUTHOR]

The state of the art of core–valence luminescence (CVL) in a number of halides (BaF2, CsCl, LiBaF3, etc.) is analysed. The following items are discussed: common properties of CVL, a suppression of slow emission component in BaF2, an effect of secondary excitation density on intensity and decay profile of the emission, a neutron–gamma discrimination, spectrum simulations, impurity-associated CVL, an origin of slow- and long-wavelength CVL components, a lattice relaxation, vibrations of lattice environment and phonon broadening of the emission bands.Experimental results on impurity-associated CVL of Rb1-xCsxBr (x<0.2) are presented and discussed. It is shown that the impurity-associated luminescence gives us a convenient way for the improvement of light output and variation of the emission wavelength of the scintillators. [Copyright &y& Elsevier]

Fast scintillation materials based on ternary halide mixed crystals, Cs(Ca 1−x Mg x)Cl 3 , were synthesized. The luminescence at the defects or impurity sites within the band-gap excitation were observed. Upon X-ray excitation, bands corresponding to the intrinsic Auger-free luminescence (AFL) for CsCaCl 3 and CsMgCl 3 were observed, and the wavelength of the mixed crystals bands was similar to that of CsCaCl 3. Cs(Ca 1−x Mg x)Cl 3 exhibited a short decay time, which is typical for AFL in CsCl-based compounds, and CsMgCl 3 exhibited the shortest decay time of 1.1 ns and the smallest proportion of the first component (65%). On the other hand, CsCaCl 3 exhibited the longest decay time constant of 2.0 ns and the largest proportion of the first component (92%). The light yields of the fast components for CsCaCl 3 were larger than that of BaF 2 , whereas the light yields of the mixed crystals were lower than those of CsCaCl 3 and CsMgCl 3. The proportion of Cs/Mg in the mixed crystals was expected to affect the decay time constants and light yields, but changes in these properties did not vary much with Cs/Mg ratio. • Auger-free luminescence band was clearly observed under X-ray excitation for Cs(Ca 1−x Mg x)Cl 3. • Cs(Ca 1−x Mg x)Cl 3 exhibited short scintillation decay time constants of less than 2.0 ns. • The fast component light yields of CsCaCl 3 was larger than that of BaF 2 [ABSTRACT FROM AUTHOR]