Novel high-stopping power scintillators for medical applications.

Abstract

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 (5.2 g/cm³), that mirrors less dense Ce doped K₂LaCl₅ (2.89 g/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 3.31 g/cm³). Substitution creates Ce doped Tl₂LiYCl₆ with density of 4.5 g/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 (9.2 g/cm³) available offering high stopping power. Lu₂O₃ doped with Eu³⁺ is known to be a high luminosity scintillator, however, this emission is very slow (1-3 ms), which limits its utility. On the other hand, ultra-fast, 1 ns, 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 1,000 ns decay time and up to 20,000 ph/MeV luminosity. Moreover, the material demonstrates very good energy resolution.