Optical spectroscopy and scintillation mechanisms of ${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{F}}_3$

In this paper we present spectroscopic and scintillation studies of mixed cerium lanthanum trifluoride crystals ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{F}}_3$. A scintillation mechanism is proposed in which the light output of the ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{F}}_3$ scintillator is determined by three processes: a direct excitation of ${\mathrm{Ce}}^{3+}$ ions by secondary electrons and x rays, an ionization of ${\mathrm{Ce}}^{3+}$ ions followed by the capture of electrons and formation of Ce bound excitons and, eventually, a transfer of the energy from the electronic-lattice excitations to ${\mathrm{Ce}}^{3+}$ ions. These three processes occur in various degrees in all inorganic Ce scintillators, and the mixed (Ce,La) trifluorides provide, therefore, an excellent example of their relative importance. The peculiarity of fluorides is that ${\mathrm{Ce}}^{3+}$ ions occur in regular and ‘‘perturbed’’ sites. The lack of a fast energy migration between the Ce ions and, at the same time, an efficient energy transfer to ‘‘perturbed’’ Ce ions lead to nonexponential decays of the Ce emission. Thermal quenching is moderate and radiation trapping can be minimized, and there is no evidence of luminescence concentration quenching. The light output under γ excitation has a maximum value of about 4500 photons per MeV, which is significantly lower than the estimated conversion-limited value of about 25 000 photons per MeV. It is suggested that the stable ${\mathrm{Ce}}^{2+}$ provides electron traps, competing for electrons with holes localized on ${\mathrm{F}}_2^{\mathrm{-}}$ and ${\mathrm{Ce}}^{4+}$ ions. Therefore, mostly one process, namely the direct excitation of ${\mathrm{Ce}}^{3+}$ ions by secondary electrons and photons, contributes to the light output of ${\mathrm{CeF}}_3$. The deomonstrated feasibility of reducing perturbed Ce makes it a strong contender in those applications where high speed, not high light output, is of prime concern.