PhD Defense: “Rare-Earth and Alkaline-Earth Halides with Scintillation Activators and Co-Dopants Studied by Picosecond Optical Absorption Spectroscopy” Apr. 15, 2019 at 11 AM

Recent discoveries of rare-earth and alkaline-earth halides with scintillation activators and co-dopants showing excellent properties for spectroscopic gamma radiation detection attract a surge in research activity on their scintillation mechanisms. There is still much to learn about excited states in these materials. Understanding behaviors of the free carries and excitons in the first picoseconds are crucial for determining the speed and nonlinearity of response. Questions remain on whether and when the free electrons are trapped on holes, dopants or defects. The nature of interaction and recombination between the photon-excited species are also important. In this thesis, the crucial early evolution of excited populations is studied with picosecond spectroscopy of optical absorption induced by interband excitation.

We identified the self-trapped exciton (STE) absorption bands in LaBr₃:Ce and CeBr3 samples along with a comparative study on the effects of Ce concentration on the STE absorption decay rate. The dominant scintillation mechanism of both LaBr₃:Ce and CeBr₃ is attributed to dipole-dipole energy transfer from the STE to Ce³⁺ dopant ions on the basis of the transient absorption bands. We identified the charge-transfer excitation of excited Ce^3+* ions for the first time. The population rise time of the Ce^3+* excited states in CeBr₃ (~540 fs) is observed to be faster than in LaBr₃:Ce, and reasons are described. We conclude that our picosecond absorption spectroscopy provides a unique method to assist in the improvement of timing resolution by isolating the rise time of population in the emitting state from the rise time of detected scintillation light, aiming for ultrafast time-of-flight detection.

We also studied the effects of interband excitation on undoped BaBrCl and on BaBrCl doped with Eu and/or Au, as measured by picosecond transient absorption spectroscopy. Aside from the identification of STE absorption bands in BaBrCl samples, we concluded that subsequent dipole-dipole energy transfer from STE to Eu is the dominant energy transfer mechanism. Au co-dopant in BaBrCl:Eu has been found to improve the scintillation light yield, and these transient absorption studies support that the mechanism involves suppression of the concentration of pre-existing halide vacancies.