J. Andriessen

Ce3+ doped inorganic scintillators

Abstract There is a strong interest in the development of inorganic scintillators with a fast response for applications in high-energy physics, medical diagnostics and a number of other fields. One of the main lines of research concerns luminescence from the allowed 5d-4f transitions of lanthanide ions in appropriately chosen host lattices. Results are presented of research on Ce-doped scintillators.

4fn−15d centroid shift in lanthanides and relation with anion polarizability, covalency, and cation electronegativity

Abstract Data collected on the centroid shift of the 5 d -configuration of Ce 3+ in oxide and fluoride compounds were recently analyzed with a model involving the correlated motion between 5 d -electron and ligand electrons. The correlation effects are proportional to the polarizability of the anion ligands and it leads, like covalency, to lowering of the 5 d -orbital energies. By means of ab initio Hartree–Fock-LCAO calculations including configuration interaction the contribution from covalency and correlated motion to the centroid shift are determined separately for Ce 3+ in various compounds. It will be shown that in fluoride compounds, covalency provides an insignificant contribution. In oxides, polarizability appears to be of comparable importance as covalency.

Experimental and theoretical study of the spectroscopic properties of Ce3+ doped LaCl3 single crystals

Abstract The spectroscopic properties of Ce 3+ in LaCl 3 single crystals were studied at a temperature of 8 K by means of (vacuum) ultra violet excitation between 4 and 20 eV. LaCl 3 :Ce shows an exceptionally small crystal field splitting of the Ce 5d levels and the Stokes shift between absorption and emission is large. Theoretical calculations were performed using a combination of the Hartree–Fock ionic cluster model and the density functional band structure model. Very good agreement with experiment was obtained. In the excited 5d configuration the Ce ion appears to move off-centre by lattice relaxation and the crystal field splitting almost doubles, resulting in the large Stokes shift. A comparison with the properties of the concentrated system CeCl 3 is made. It seems that in this crystal a slight off-centre position already occurs in the ground state 4f configuration.

Luminescence and storage properties of LiYSiO4: Ce

Abstract The prompt luminescence and photostimulated luminescence properties of a LiYSiO4: Ce powder sample are reported and discussed. The feasibility of LiYSiO4: Ce as a (thermal neutron) scintillator or storage phosphor is considered by estimating the absolute scintillation light yield and the PSL yield for a LiYSiO4: Ce single crystal.

Cen+ energy levels in alkaline-earth fluorides and cerium-electron, cerium-hole interactions

The energy levels of three cerium centres in (CaF2, SrF2 and BaF2 were calculated in the framework of the relativistic Dirac-Hartree-Fock formalism. In this study cerium was taken to be in various ionic states and lattice relaxation was included. The centres were the following: the charge-uncompensated cerium centre (Ce) and two charge-compensated cerium centres. The charge-compensating fluorine ion was situated at the (111) next-nearest-neighbour interstitial site (CeFnnn) or at the (100) nearest-neighbour interstitial site (CeFnn). The authors did not work with crystal field parameters, their method being ab initio. The results are compared with a variety of experimental data. Relaxation of the lattice around the cerium centres is crucial for the results. From the location of the energy levels, they conclude that only trivalent and tetravalent cerium centres are stable in the alkaline-earth fluorides. If free electrons are present, only the trivalent cerium centre is stable. The divalent centre without charge compensation is metastable. This is in agreement with experiment, Further, it is found that excitation of trivalent cerium centres by capture of an electron and a hole at the cerium ion is improbable. This agrees with earlier observations of the scintillation decay of Ce3+-doped BaF2.

Electronic structure and transition probabilities in pure and Ce3+ doped BaF2, an explorative study

The energy levels responsible for the cross luminescence in BaF2 are well reproduced with the LCAO cluster approach using Gaussian basis sets. Comparative calculations on CaF2 and SrF2 are, even if the estimated bandwidths are considered, in good agreement with band structure calculations reported in the literature. We will explain quantitatively the appearance of two cross luminescence emission components in BaF2 with a decay time of roughly 1 ns. The work on Ce3+ has shown a substantial reduction of the 5d-4f transition rate due to shielding by the 5p shell. This may be partly responsible for the reported small transition rates measured for this ion in various hosts.

Scintillation properties of possible cross-luminescence materials☆

Abstract Employing X-ray and gamma-ray excitation, we studied the scintillation properties of KMgF 3 , KYF 4 :Rb, K 2 YF 5 , KLuF 4 , RbMgF 3 , KZnF 3 and BaTm 2 F 8 crystals. The first four crystals produce intrinsic cross-luminescence (CL) with a decay time of about 1.5 ns. Intrinsic CL was not observed for the other crystals. Emission spectra, results of decay time measurements, and estimates for the absolute light yield are presented.

Hartree-Fock calculation of BaF2:La systems

Abstract We present the results of an investigation of the electron structures of BaF 2 , LaF 3 and BaF 2 :La crystals. The effects of lattice polarization and relaxation were considered. The calculations were performed using the C rystal 98 code, the molecular statics method and the embedded cluster technique based on the Johnsons scattered wave method.

Calculation of Energy Levels of Cerium in Inorganic Scintillator Crystals

Fully relativistic ab initio calculations have been performed on energy levels of cerium in BaF{sub 2}, LaF{sub 3}, YAP (yttrium aluminum perovskites) and YAG. Also nonrelativistic calculations were done on cerium in LSO (lutetium oxy orthosilicate). The results are in fair agreement with experiment. Both the splitting of the 4f and 5d levels and the 4f-5d energy gap can be explained in the high symmetry crystal BaF{sub 2} as well as in the low symmetry crystals LaF{sub 3}, YAP, YAG and LSO. The more traditional nonrelativistic approach is not capable to handle 4f-5d energy differences but can be used for the description of the 5d state and other properties which do not critically depend on the 4f state. The authors have estimated the position of the cerium levels in the gap of the host materials. So important information for luminescence of cerium centers is obtained.

The centroid shift of the 5d levels of Ce3+ with respect to the 4f levels in ionic crystals, a theoretical investigation

Abstract The centroid shifts of the 5d level of Ce 3+ in BaF 2 , LaAlO 3 and LaCl 3 have been calculated using the ionic cluster approach. By applying configuration interaction as extension of the basic HF-LCAO approach the dynamical polarization contribution to the centroid shift was calculated. This was found to be only successful if basis sets are used optimized for polarization of the anions.