The search of new efficient scintillators can be simplified by the application of ion beams. The beam flux density of the order magnitude 1012–1014 ions/cm2sec can be easily achieved. The light yield visualization takes place even for weak scintillators as PbF2 under excitation by such beams. The light response of crystals for ion beams with E0 = 1–2 MeV is not connected with the Cherenkov's radiation. Ion excitation enables one to find the crystals with surface scintillation. Measurement were performed by means of 0.15–2.0 MeV H+, D+ and 4He+ ions with current 0.1–10 nA. Quantitative values of the light yield for the cubic phase of monocrystal PbF2 in random and channeling were obtained. Restrictions of the technique applicability are discussed.

Luminescence from localized states in pure wide-gap crystals is examined from the perspective of modern fast scintillator requirements. Recent advances in the understanding of self-trapped excitons are summarized with regard to excited-state configurations, spectra, and luminescence efficiency. As an example, the fast luminescence of partially quenched type I STEs in RbI offers tuneable-lifetime subnanosecond pulses which could be useful for timing applications. The possible role of hole localization in cross luminescence is discussed. Stokes-shifted luminescence in some pure rare-earth fluoride crystals probably originates from self-trapped excitons of forms to be discussed.

Computational methods for modeling the optical properties of substitutional transition-metal impurities in insulating solids, potentially applicable to some scintillator and phosphor materials, are reviewed. Methods considered include crystal-field and semi-empirical ligand-field models; SCF-Xα-SW, SCF-RHF-LCAO, SCF-UHF-LCAO and CI ab initio methods: and ICECAP and HADESR embedded cluster methods with lattice relaxation. A detailed example of the application of the HADESR method to crystal-field spectra of Cr3+ in halide elpasolites is described. In this method, ab initio molecular-orbital calculations with effective core potentials are performed for selected ionic configurations. Simultaneous relaxation of the cluster and surrounding lattice, with mutual pair. potential interactions, is accomplished by a modified lattice statics program. Calculated properties include pressure-dependent optical transition energies, vibration frequencies and radiationless transition rates.

Fully relativistic ab initio calculations have been performed on energy levels of cerium in BaF2, LaF3, YAP and YAG. Also nonrelativistic calculations were done on cerium in LSO. 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 BaF2 as well as in the low symmetry crystals LaF3, YAP, YAG and LSO.

Lanthanum phosphate activated by cerium and terbium is a well known lamp phosphor which is used for its green emission. In this paper, we have investigated the electronic structure and associated optical properties of lanthanum phosphate doped with cerium using the self consistent field scattered wave molecular orbital cluster approach. Relativistic corrections are included for the lanthanum and cerium centered clusters. In spite of the small magnitude of crystal field splitting, a significant admixture of the f-like orbitals with the ligand orbitals is observed for cerium. The optical transitions are calculated using the transition state formalism. The absorption peaks due to electronic transition from the f-like states to d-like states of cerium are found to be 271 rim, 259 nm, 245 nm, 208 nm and 194 nm which are in good agreement with the experiment. The nature of the band gap transition in lanthanum phosphate is also discussed.

We have performed quantum electronic ab initio calculations of clusters of alkali halides, both pure and doped. In order to estimate the pure crystal energy gap and the dopant excitation, we have considered a central cation and four shells of ions surrounding it. We have studied KC1, Nal, NaCl and KI, both pure and with a Tl+ ion replacing thecentral alkali cation. Encouraging results are presented for absorption and emission.

Models are discussed of the first stage of the scintillation mechanism in an inorganic scintillator: the primary interaction and the thermalization process. The model predictionsare compared with experimental results, an extension of the theory is made and conclusions are drawn concerning the best description of the first stage.

The results of computer simulation of the fast stages of energy relaxation in insulators after the VUV or XUV photon absorption are presented. The simulation involves two stages:the inelastic scattering of excitations with production of secondary excitations and thermalization through phonon emission. The main attention is focused on the spatial distribution functions of excitations, i.e. one-particle and two-particle distribution functions.The latter Function determines the energy transfer at final stages of energy relaxation and is important for different quenching processes and the acceleration of the luminescence decay. The Monte-Carlo simulation was carried out for BaF2 crystal for photon energies from 20 eV to 100 eV. The simulation shows that the two-particle distribution functions and thus the kinetics depend on the energy of excitation.

The electronic structures of pure BaF2 crystal and lanthanum doped BaF2 crystal have been calculated in a self-consistent molecular-cluster model. The cluster is embedded in the crystal lattice and the entire system treatediteratively in the Hartree-Fock-Slater local-density theory. As lanthanum doped BaF2 is concerned, the obtained results revealed that the F1–i which is introduced by the lanthanum may contribute to the suppression ofthe slow component in the scintillation light of BaF2 crystal.

The linear combination of atomic orbital-molecular orbital embedded-cluster is used to investigate the electronic structure and binding energies of small oxygen impurity clustersin the local density theory. Binding energies for substitutional oxygen ions in BaF2 crystal are studied for its 2–, 1–, and 0 charge states. These are used to assess charge-state stability. The energy eigenvalue spectra and optical transitions are obtained by transition state calculations for the oxygen impurities. These results of optical transition calculations obtained are very good and provide a useful interpretation of the ultraviolet-absorption experiments.

Local-density-functional calculations have been performed to study the electronic structure and charge-transfer transitions of RE3+ —O2–(RE=Eu and Tm) associates in barium fluoride crystal. These systemsare simulated by small clusters which are surrounded by over 2000 point charges. It is found that the presence of oxygen in the lattice strongly influences the optical propertiesof RE3+ ions. The charge transfer transitions of RE3+O2– and RE ions with ligand F— ions derived from the embedded cluster are equal to 5.1 eV and 6.2 eV. The energy gap derived from the HFS model with REOBa2F6 clusters embedded in the crystal is 9.8 eV,which is near the experimental results.

The mechanism of scintillations in crystals on the base of Gd3+ - Ce3+ transfer have been developed.

Results from recent radiation damage studies in high quality BaF2 and CeF3 crystals are presented. Optical absorption and electron paramagnetic resonance (EPR) techniques are used to identify specific radiation damage mechanisms. Specific attention is given to the role of oxygen and hydrogen in the room temperature damage of BaF2. Also, Mn2+ ions are shown to change valence state in BaF2during room temperature irradiation. Numerous optical absorption bands are created in CeF3 during irradiations at low temperature. These bands are associated with electron traps (either F centers or Ce2+ ions) and they thermal anneal below room temperature. An EPR spectrum, assigned to F centers, is observed in low-temperature irradiated CeF3.

Different impurities influence the radiation damage of barium fluoride (BaF2 )crystal in different ways. Stable metal elements, such as alkali elements and alkali earth elements, have no effect on the radiation resistance. Most transtion metal elements areharmful to the radiation resistance. There are three kinds of effects of rare earth elements. Stable trivalence elements, such as Y, La and Lu, have no effect on the radiation resistance, while the elements which changes its valence from +3 to +4 under irradiation, such as Ce and Pr, are harmful. The elements which changes its valence from +3 to +2 underirradiation, such as Sm,Eu,Dy and Yb, are useful in eliminating color centers in visible range. Oxygen and hydroxyl ions which were introduced into the crystal from the raw material of BaF2 or a low vacuum during crystal growing are very harmful to the radiation resistance. The formation of “Complex Color Centers” is considered as the cause of the radiation damage of BaF2 crystal in this paper.

In this paper, we used the Discrete Variational Xα, (DV-Xα) method to calculate the energy level of some rare earth doped BaF2 crystals and indicated that the Eu, Dy and Yb elements will change their valence from +3 to +2 under irradiation and thus change the optical properties of the crystals.The results show that this radiation effect model presented agree well with experiments.

The radiation effect in hydrolyzed BaF2 was investigated through tile changes in their optical absorption and EPR spectra before and after γ-irradiation. The resulti demonstrated that hydroxyl and oxygen can be easily introduced into BaF2by means of a hydrolysis, the most likely modes are OH— ions substituting for fluorine and O2– ions substituting for fluorine associated with charge-compensating fluorine vacancies O2– — F+. Combining with the Hartree-Fock-Slater local-density discrete variational (HFS-Xα-DV) cluster calculation on some possible defects related to hydrogen and oxygen impurities, we propose that the radiation damage observed in hydrolyzedBaF2 can be explained in terms of OH— and O2– — F+ dissociation through a radiolysis.

Recent spectroscopic and radiation damage experiments on a series of CexLa1-xF3 crystals suggest that the scintillation light output is limited by an unusual quenching mechanism, which also plays a major role in minimizing radiation-induced damage. The intensity of the radiation-induced absorptions is a strong function ofthe Ce content x, reaching a maximum for x = 0.03 and a minimum for x = 1. This peculiar dependence appears to be due to the influence of deep-lying Ce levels onboth scintillation mechanism and radiation damage. We suggest that various charge transfer processes can explain many aspects of the performance of CexLa1-xF3 scintillators.

Study of optical properties and radiation damages of Cerium Fluoride crystals doped with Ca, Ba, Sr, La, Nd, Zr and Hf in the wide range of concentrations has been performed withthe goal to obtain high optical transparency of crystals at different cumulative doses under γ-irradiation. Time decay curves and relative light yields of scintillators asa function of doping level were measured using X-ray excitation of samples and single photon counting method.

The optical absorption, light output and effects of radiation damage have been studied ina 2.1 × 2.1 × 2.1 cm3 crystal of YA103:Ce. The light yield produces approximately 2867 photoelectrons per MeV on a photomultiplier tube with abialkali photocathode. The scintillation and light transmission properties of the crystalwere characterized before and after exposure to 1 Mrad of 60 Co gamma rays. Significant radiation induced optical absorption was observed near the optical band edge ofthe crystal, along with an extremely intense phosphorescence after irradiation. Partial spontaneous recovery from the damage occurred at room temperature, and it was possible to induce additional recovery by optical bleaching.

The optical absorption, light output and effects of radiation damage have been studied ina 2.2 × 2.2 × 12.8 cm3 crystal of lead tungstate. The crystal exhibits a longitudinal attenuation length which varies from roughly 13 cm to 1.5 m over the region where the scintillation emission occurs (400-800 nm). The light yield produces approximately 10 photoelectrons per MeV on a photomultiplier tube with a bialkali photocathode. The scintillation and light transmission properties of the crystal were characterized before and after exposure to 1 Mrad of 60Co gamma rays. Significant radiation induced optical absorption was observed in this crystal, along with a considerable amount of phosphorescence after irradiation. Partial spontaneous recovery from the damage occurred at room temperature, and it was possible to induce additional recovery by optical bleaching.