MRI: Acquisition of a field emission electron microprobe for Caltech Division of Geological and Planetary Sciences

This Major Research Instrumentation (MRI) award will enable the purchase, installation, and commissioning of a state-of-the-art instrument for chemical analysis of natural and engineered solid materials down to the nanometer (one billionth of a meter) scale. The instrument will be available to all science and engineering programs across the Caltech campus, NASA’s Jet Propulsion Laboratory, as well as to outside users.

The instrument may be operated remotely and will be made available via the Remotely Accessible Instruments in Nanotechnology (RAIN) consortium for free access by student researchers and classes at minority-serving institutions nationwide ranging from community and technical colleges to high schools and even elementary schools. The electron microprobe is a basic tool of solid Earth geology and geochemistry as well as related fields such as meteoritics and planetary sample return, environmental microbiology, material science and nanotechnology.

The advanced electron source included in this instrument provides a very bright, focused beam that enables imaging at high spatial resolution and excitation of characteristic X-rays from a very small analytical volume. Each chemical element emits X-rays at particular wavelengths; the count rate of X-ray photons emitted by a sample at these characteristic wavelengths is proportional to the concentration of that element in the sample.

The electron probe automates the job of separating X-rays by wavelength, counting the number of X-ray photons emitted at particular wavelengths, and comparing the count rate in an unknown material to that in standard materials. This allows the electron microprobe to detect when an element is present above a low detection limit and to determine the abundance of each element in a sample with about 1% relative precision.

Initial applications will include characterization of tiny mineral grains in meteorites that date back to (or even precede) the origin of the Solar system, experimental samples that reproduce conditions in the deep Earth or during collisions in the asteroid belt, and functional materials for batteries and energy generation.

The key capabilities that the new instrument will bring are the field emission electron source, the Si-drift detector energy dispersive X-ray spectrometer, and the high-resolution cathodoluminescence sensor. While nanoscale imaging resolution, essential for targeting analyses and ensuring sample homogeneity, is straightforward, optimizing the spatial resolution of quantitative analysis without sacrificing accuracy or precision, requires special care.

The new instrument will support a study of new approaches to unsupported thin specimen analysis as a path to high-resolution analysis. Challenges to be overcome include holding unsupported specimens in the beam path, updated software that accounts accurately for thin samples, and an extensive campaign of verification against well-characterized standards.

Turning to more of the applications of the new instrument and its improved imaging and analysis capabilities, researchers will focus on several goals. These include: (1) analysis of experimental samples to probe diffusion at short length scales, fine-grained multiphase assemblages, and synthesized starting materials whose homogeneity at the nano-scale is essential information; (2) analysis of terrestrial igneous, metamorphic, and sedimentary rocks including basaltic glasses that may contain nano-inclusions, zircons for geochronology, and redox-sensitive tracers with unknown hosting phases; (3) analysis of meteorites and new nano-minerals including refractory inclusions, high-pressure shock-induced phases, non-destructive bulk analysis, and bio-synthetic single-domain magnetic crystals; and (4) analysis of microbiological specimens from the environment and culture, with single-cell resolution of the distribution of key macro- and micro-nutrient elements.

This award was co-funded by the MRI program and the Instrumentation and Facilities program in the Earth Science Division.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.