LiF - A non-destructive method for raw material identification

Laser-induced fluorescence (LiF) provides a spectroscopic technique for identification of rare earth elements (REE) that may be applied to natural environments. The method uses the material-specific luminescence emissions that are induced by laser-stimulation of a certain wavelength. The distinct emission lines of REE make them well suited for the development of a LiF-based exploration technique. However, the natural variability demands for comprehensive investigations of the spectral characteristics of REE and effects from matrix minerals.
To identify those spectral information that are robust and unequivocal, we analyse spectra of REE standards measured in different matrix minerals including phosphates and fluorides. We use variable laser wavelengths from UV (325 nm) and a detection range from 340 nm to 1080 nm. First results show spectral characteristics that sort REE in three groups due to: no distinct emission lines, absorption features, distinct luminescence emission lines. Measured in different matrix minerals, we determine shifts for some of the spectral features and some disappear or decline in intensity. Preliminary tests using wavelengths longer than UV aim at a more selective stimulation of REE emissions and at reducing the unspecific emission of all luminescent components of a sample. Results are promising but highlight that optimized stimulation wavelength may be REE-specific. We additionally investigated natural rocks with a well-characterized REE content. First results show that LiF is able to reproduce spectral characteristics of REE in natural rocks, but also outline the need to reduce matrix mineral emission by selective stimulation and delayed detection windows.