In situ 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.

Nonproportional response of LaBr3:Ce and LaCl3:Ce scintillators to synchrotron x-ray irradiation.

The nonproportional scintillation response of LaBr(3) doped with 5% Ce(3+) and of LaCl(3) doped with 10% Ce(3+) was measured using highly monochromatic synchrotron irradiation. To estimate the photon response, pulse height spectra at many finely spaced energy values between 9 and 100 keV were measured. The experiment was carried out at the X-1 beamline at the Hamburger Synhrotronstrahlungslabor (HASYLAB) synchrotron radiation facility in Hamburg, Germany. Special attention was paid to the x-ray fluorescence escape peaks as they provide us with additional information about photon response in the range 1.2-14.5 keV for LaBr(3):Ce and 2.0-11.6 keV for LaCl(3):Ce. A rapid variation of the photon response curve is observed near the lanthanum K-electron binding energy for both scintillators. A dense sampling of data was performed around this energy and those data are used to apply a method, which we call K-dip spectroscopy. This method allows us to derive the electron response curves of LaBr(3):Ce and LaCl(3):Ce down to energies as low as 0.1 keV.