OPTICAL SPECTROSCOPY AND SCINTILLATION MECHANISMS OF CE(X)LA(1-X)F3

In this paper we present spectroscopic and scintillation studies of mixed cerium lanthanum trifluoride crystals CexLa1-xF3. A scintillation mechanism is proposed in which the light output of the CexLa1-xF3 scintillator is determined by three processes: a direct excitation of Ce3+ ions by secondary electrons and x rays, an ionization of Ce3+ 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 Ce3+ 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 Ce3+ 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 gamma 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 Ce2+ provides electron traps, competing for electrons with holes localized on F2- and Ce4+ ions. Therefore, mostly one process, namely the direct excitation of Ce3+ ions by secondary electrons and photons, contributes to the light output of CeF3. 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.