ENERGY-TRANSFER PROCESSES INVOLVING DIFFERENT LUMINESCENCE-CENTERS IN BAF2CE

The luminescence of cerium-doped barium fluoride crystals has been investigated as a function of wavelength and time. For excitation uv light, x-rays and gamma rays were used. The well known cross luminescence (CL) near 200 nm and self-trapped exciton luminescence (STE) near 300 nm, which are due to excitations of the host lattice, are observed at low cerium concentrations. For increasing cerium concentrations, the CL and sm emission is gradually replaced with cerium emission. Luminescence due to at least three different cerium related centres was found. The decay of the luminescence under gamma excitation is non-exponential. At low cerium concentrations, its duration is much longer than the decay time of the excited level of the predominant cerium centre. At higher cerium concentrations, the luminescence decays faster. These observations are explained in terms of an energy transfer model. The cerium luminescence due to energy transfer from CL and STE centres was calculated using the parameters obtained from experiment. Comparing the calculations to the observed photon outputs and decay curves leads to an interpretation of the luminescence mechanism in BaF2:Ce. Apart from radiative and dipole-dipole energy transfer from CL and STE centres to cerium centres, transfer from the STE to unknown centres also plays a role, as well as direct excitation of cerium centres by free or trapped electrons and holes.