APPLICATIONS OF HERFD DETECTORS: AN EXCITING NEW OPPORTUNITY IN ENVIRONMENTAL AND GEOLOGICAL SCIENCES

High-energy resolution fluorescence detectors (HERFDs) have been commissioned at several beamlines and represent an exciting new opportunity to revisit some challenging systems in environmental and geological sciences. HERFD involves the precise alignment of the sample, crystal array, and detector on a Rowland circle to satisfy the diffraction condition for the emission energy of a specific element. HERFDs can most effectively be applied to situations where matrix fluorescence overlaps complicate measuring the targeted element and to the examination of high-Z elements in order to collect more featured and diagnostic spectra. HERFDs are also not as effective at some tasks, such as micro-focused mapping, because a lower fraction of the generated fluorescence signal is measured using an HERFD relative to traditional fluorescence measurements. However, mapping at increased dwell times may be valuable for multiple energy maps when additional energy resolution is needed . Here, we draw from several projects which utilize HERFD technology in different ways to highlight some opportunities and challenges for environmental and geological science applications.

As an example of reducing the impact of matrix fluorescence, we used an HERFD to collect Ge x-ray absorption near-edge spectroscopy (XANES) images, which were characterized by somewhat sharper and more intense edge features than the traditional spectra collected simultaneously. For samples containing hundreds of ppm Ge in a ZnS matrix, the HERFD-XANES edges were characterized by better signal-to-noise ratios and were easier to background subtract because of the lower contribution from the Zn Kb to the measured Ge Ka signal. However, we observed unexplained signal-to-noise issues intermittently in XANES measurements recorded using the HERFD and traditional vortex detector. As an example of the benefits of HERFD for high-Z elements, we examined Pb in a variety of minerals and environmental samples. HERFD spectra are not affected by the core-hole lifetime, the effect responsible for broadening the absorption edges of Pb and other high-Z elements. Thus, HERFD-XANES spectra are significantly more featured and diagnostic than traditional fluorescence XANES. Consequently, the HERFD represents an opportunity to revisit some systems that have previously been challenged by overlaps and broad and poorly featured edges.