Aj. Wojtowicz et al., OPTICAL SPECTROSCOPY AND SCINTILLATION MECHANISMS OF CE(X)LA(1-X)F3, Physical review. B, Condensed matter, 49(21), 1994, pp. 14880-14895
In this paper we present spectroscopic and scintillation studies of mi
xed cerium lanthanum trifluoride crystals CexLa1-xF3. A scintillation
mechanism is proposed in which the light output of the CexLa1-xF3 scin
tillator is determined by three processes: a direct excitation of Ce3 ions by secondary electrons and x rays, an ionization of Ce3+ ions fo
llowed 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 degre
es in all inorganic Ce scintillators, and the mixed (Ce,La) trifluorid
es provide, therefore, an excellent example of their relative importan
ce. The peculiarity of fluorides is that Ce3+ ions occur in regular an
d ''perturbed'' sites. The lack of a fast energy migration between the
Ce ions and, at the same time, an efficient energy transfer to ''pert
urbed'' Ce ions lead to nonexponential decays of the Ce emission. Ther
mal quenching is moderate and radiation trapping can be minimized, and
there is no evidence of luminescence concentration quenching. The lig
ht output under gamma excitation has a maximum value of about 4500 pho
tons per MeV, which is significantly lower than the estimated conversi
on-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 p
hotons, contributes to the light output of CeF3. The deomonstrated fea
sibility of reducing perturbed Ce makes it a strong contender in those
applications where high speed, not high light output, is of prime con
cern.