Cathodoluminescence as a Forensic Tool

Cathodoluminescence (CL) is the emission of visible or near visible light from a sample that has been bombarded by an electron beam. CL emission is often characteristic of the geological environment of formation of the mineral and many of the most ubiquitous minerals (e.g., quartz, feldspar, and carbonate minerals) are cathodoluminescent. Since CL is observed in many materials routinely encountered as trace evidence, the variation in luminescence for a particular mineral can perhaps be used to discriminate among samples from different sources or, in certain cases, provide information about the provenance of a sample.

Prior research has demonstrated that cold cathode CL with light microscopy provides a relatively fast method to screen soil samples through visual identification of luminescent minerals and to determine if multiple populations of a given mineral type exist. In addition to visual observation, high-resolution CL spectroscopy can offer more detailed information about specific activators (e.g. defects and trace elements responsible for luminescence) in a given mineral. For example, in feldspar minerals, the chemical composition can be estimated on the basis of the Mn²⁺ and Fe³⁺ emission bands. In heavy minerals such as zircon, monazite, and apatite, rare earth element activators, typically present at 1-500 ppm, can be identified and quantified with high resolution spectroscopy. Together, visual and spectroscopic examination of mineral components can be combined to provide a variety of information about soil and sand samples that complement more traditional analytical techniques.

This presentation will provide an introduction to the principles and practice of CL in forensic geology with a specific focus on the spectroscopic information that can be obtained from geological samples. A specific example demonstrating the applicability of the technique to concrete masonry units will be presented with discussion about the potential and limitations of CL in cases of comparison, authentication, and geographic sourcing.