Enhanced energy dispersive inelastic X-ray Scattering for advanced environmental monitoring

One of the useful characteristics for evaluating the chemical speciation of a given element is to use the dependence of Resonant Inelastic X-ray Scattering (RIXS) on the electronic configuration of its environment. The remarkable amount of information provided by this methodology, combined with advances in high-intensity X-ray sources such as synchrotrons or FELs, has driven significant growth in RIXS in recent years. However, transferring this technique to conventional laboratories, while desirable, represents a major instrumentation challenge due to its low probability of occurrence, which becomes even more critical in specific applications such as studying heavy metal contamination. Indeed, in these studies, contaminant concentrations can be low, requiring maximum efficiency in sample excitation.

In the present work, we analyzed the possibility of using an X-ray tube as an excitation source for RIXS in the energy-dispersive configuration (EDIXS). To increase the efficiency of the experimental setup, an X-ray tube with an anode made of the same material as the element under study was used, combined with a monochromator that selected the characteristic Kβ line of the source. This line generally has a slightly lower energy but is very close to the K-edge of the element that produces it, thereby inducing resonant inelastic scattering with high probability.

The proposed configuration was implemented to analyze copper, using a monochromator with a large acceptance solid angle (12.5° aperture) based on a logarithmic spiral profile covered with 18 pieces of flat Si(100) crystal. The manufacturing and implementation processes of this monochromator, specifically for a copper X-ray tube, are described in detail in this work. The proposed EDIXS methodology, implemented with this device, was applied to a set of inorganic copper compounds to evaluate sensitivity to the chemical environment, achieving results similar to those previously reported using synchrotron radiation. Finally, the methodology was tested for monitoring the chemical state of copper absorbed by aquatic plants in a phytoremediation system, yielding satisfactory results in agreement with previous predictions. The proposed methodology offers a viable alternative for implementing RIXS in conventional laboratories, with sufficient sensitivity to analyze the chemical state of elements at concentrations as low as 1 % w/w, typically found in environmental contamination monitoring.