<i>In situ</i> diagnostics of the crystal-growth process through neutron imaging:application to scintillators

Neutrons are known to be unique probes in situations where other types of radiation fail to penetrate samples and their surrounding structures. In this paper it is demonstrated how thermal and cold neutron radiography can provide time-resolved imaging of materials while they are being processed (e.g. while growing single crystals). The processing equipment, in this case furnaces, and the scintillator materials are opaque to conventional X-ray interrogation techniques. The distribution of the europium activator within a BaBrCl:Eu scintillator (0.1 and 0.5% nominal doping concentrations per mole) is studied in situ during the melting and solidification processes with a temporal resolution of 5-7 s. The strong tendency of the Eu dopant to segregate during the solidification process is observed in repeated cycles, with Eu forming clusters on multiple length scales (only for clusters larger than ∼50 µm, as limited by the resolution of the present experiments). It is also demonstrated that the dopant concentration can be quantified even for very low concentration levels (∼0.1%) in 10 mm thick samples. The interface between the solid and liquid phases can also be imaged, provided there is a sufficient change in concentration of one of the elements with a sufficient neutron attenuation cross section. Tomographic imaging of the BaBrCl:0.1%Eu sample reveals a strong correlation between crystal fractures and Eu-deficient clusters. The results of these experiments demonstrate the unique capabilities of neutron imaging for in situ diagnostics and the optimization of crystal-growth procedures

Thermal quenching of Eu2+ emission in Ca- and Sr-Ga2S4 in relation with VRBE schemes

Structural and optical properties of MGa2S4 (M = Mg, Zn, Ca, Sr, Ba) compounds have been compared, and the vacuum referred binding energy (VRBE) schemes were constructed for the lanthanide ions in the iso-structural compounds CaGa2S4 and SrGa2S4 employing literature data. The VRBE of an electron in the 5d excited state of Eu2+ was found at 0.75 and 0.97 eV below the bottom of the conduction band (CB) in CaGa2S4:Eu and SrGa2S4:Eu, respectively. Such differences explains the unexpected higher thermal quenching temperature reported for Eu2+-doped SrGa2S4 (T50% = ∼475 K) compared to Eu2+-doped CaGa2S4 (T50% = 400 K) The significantly lower VRBE at the CB-bottom in CaGa2S4 versus SrGa2S4 may be explained by the shorter Ga-S bond lengths in SrGa2S4

Photoluminescence and excited states dynamics in PbWO4:Pr3+ crystals

Luminescence and photo-thermally stimulated defects creation processes are studied for a Pr3+-doped PbWO4 crystal at 4.2-400 K under excitation in the band-to-band, exciton, and charge-transfer transitions regions, as well as in the Pr3+-related absorption bands. Emission spectra of Pr3+ centers depend on the excitation energy, indicating the presence of Pr3+ centers of two types. The origin of these centers is discussed. The 2.03-2.06 eV emission, arising from the D-1(2) -> H-3(4) transitions of Pr3+ ions, is found to be effectively excited in a broad intense absorption band peaking at 4.2 K at 3.92 eV. By analogy with some other Pe(3+)-doped compounds, this band is suggested to arise from an electron transfer from an impurity Pr3+ ion to the crystal lattice W6+ or Pb2+ ions. The dynamics of the Pr3+-related excited states is clarified. In the PbWO4:Pr crystal studied, the concentration of single oxygen and lead vacancies as traps for electrons and holes is found to be negligible