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Development of new scintillator materials is a continuous effort, which recently has been focused on materials with higher stopping power. Higher stopping power can be achieved if the compositions include elements such as Tl (Z=81) or Lu (Z=71), as the compounds gain higher densities and effective atomic numbers. In context of medical imaging this translates into high detection efficiency (count rates), therefore, better image quality (statistics, thinner films) or lower irradiation doses to patients in addition to lowering of cost. Many known scintillator hosts, commercial or in research stages, are alkali metal halides (Cs, K, Rb). Often these monovalent ions can be replaced with monovalent Tl. Since Tl has a higher atomic number than for example Cs (55), this increases the stopping power of modified compounds. A good example of an enhanced host is Ce doped Tl2LaCl5 (5.2g/cm3), that mirrors less dense Ce doped K2LaCl5 (2.89g/cm3). Tl substation also increased the luminosity to >60,000 ph/MeV, as it often leads to a reduction in the bandgap. Another example is the dual mode (gamma/neutron) Ce doped Cs2LiYCl6 scintillator (density 3.31g/cm3). Substitution creates Ce doped Tl2LiYCl6 with density of 4.5g/cm3, with much better stopping power and 20% higher light yield. Binary Tl-compounds are also of interest, although mostly they are semiconductors. Notable example of a scintillator is double doped TlCl with Be, I. This scintillator offers fast Cherenkov emission topped off with scintillation signal for achieving better energy resolution. Another family of interesting and dense compositions is based on Lu2O3 ceramics. Lu2O3 is one of the densest hosts (9.2g/cm3) available offering high stopping power. Lu2O3 doped with Eu3+ is known to be a high luminosity scintillator, however, this emission is very slow (1 to 3ms), which limits its utility. On the other hand, ultra-fast, 1ns, scintillation can be achieved with the Yb3+ doping that can be used for timing or high count-rate applications. However, while fast, Yb3+ doped Lu2O3 has very low luminosity. Recently, we have shown a middle ground performance, with Lu2O3 doped with La3+. This composition generates scintillation with 1,000ns decay time and up to 20,000 ph/MeV luminosity. Moreover, the material demonstrates very good energy resolution.
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