Luminescence is the emission of light when a material is excited. Many common minerals will produce luminescence when excited by many different stimuli. For example fluorite glows under UV excitation (a blacklight) in a process called photoluminescence. Light caused by electron bombardment is called cathodoluminescence. X-rays and heat can cause radioluminescence and thermoluminescence to name just a few typical measurement techniques. These luminescent properties of minerals have been studied extensively as a clue to the mineral trace element composition, provenance, and the environment in which they were formed. However, while measuring the UV-Vis light luminescence of minerals is straightforward, there are several features that make the data less quantitative than desired. For example, for cathodoluminescence, the light output depends critically on the electron flux striking the sample, the interplay between the number of luminescent activator elements and luminescent quenching elements within the mineral matrix, the orientation of the mineral crystal towards the electron beam, and the irradiation damage experienced during the measurement. These complications have led to only limited use of cathodoluminescence outside purely geochemical provenance studies.
This research project has three different goals, each designed to make luminescent signatures of minerals more valuable for separating and identifying common minerals that show up as forensic trace evidence.
The first goal is to calibrate and use a new cathodoluminescence system at Hope College to characterize many replicate samples of feldspars, zircons and carbonates. Students will be involved in the sample identification and preparation, data acquisition and analysis of the cathodoluminescence, and compiling the results into database to ascertain the effect of each of the uncertainties in the measurement techniques listed above. Together with elemental identification of trace elements within each mineral structure, these data would be the first step in the creation of a valuable reference library of forensic signatures in the cathodoluminescence spectra.
A second aspect to this project is to generate split samples of certain highly-reproducible cathodoluminescent minerals, where one fraction of the sample will be irradiated with a high neutron flux to determine if there is a measurable cathodoluminescent signature generated by the neutron bombardment. Students will be involved in experimental design, sample selection and the data acquisition and analysis of UV-Vis spectra before and after neutron irradiation. The interpretation of these results will show whether luminescence could be used as a nuclear forensics tool to measure the presence of special nuclear material near to common minerals found in building materials.
The third part of this project is to compare the cathodoluminescent signatures of the minerals to luminescence spectra obtained from the same minerals analyzed by ion beam-induced luminescence. This technique uses accelerated protons from the Hope College Ion Beam Analysis Laboratory to produce the UV-Vis luminescence in the same minerals used in the other two parts of this project. For some minerals we have shown that these two measurement methods produce virtually identical spectra. However, for an entire class of feldspar minerals we observe a shift in luminescent peaks that has not been reported in the literature to date. Understanding the geochemical reasons for this experimental observation will require many more replicate measurements of the same sample by both techniques to identify the causes unambiguously. Students will be involved in the accelerator operation, data acquisition and analysis of the luminescence spectra from both techniques.
All three aspects of this research project will result in publishable material in the fields of nuclear methods development, forensic science and geochemistry. |
