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Mike Jercinovic - Bethe & Nanogeochronology

Hans Bethe and Nanogeochronology



Mike Jercinovic
University of Massachusettes
Amherst, MA



Electron probe microanalysis (EPMA) evolved from concepts established early in the last century, culminating in Raimond Castaing’s first instrument in 1951. The technique rapidly developed into a method for versatile, efficient chemical microanalysis for major and minor elements, and fundamentally transformed materials characterization. The microprobe has become an indispensible tool in the physical sciences. The new millennium now sees EPMA evolving considerably again as new technology, driven by the needs of scientific inquiry, allowing us to more fully exploit the properties of electrons and X-rays. New instruments are beginning to quantitatively explore spatial dimensions and concentration levels once considered unattainable, including use of the electron probe for in-situ, geochronology, now capable of yielding information from regions below one micron.

The groundwork for these new frontiers lies in the fundamentals of electron interactions with matter first put forth by Hans Bethe. Building on the concepts of scattering introduced in early quantum theory by Niels Bohr, then more rigorously in modern quantum theory by Max Born and Neville Mott, Bethe sought to comprehensively investigate the slowing of charged particles as they pass through matter. Today, much of our practical evaluation of the crucial parameters involved in electron probe microanalysis is owed Bethe’s 1930 epic paper describing the mechanisms of electron energy loss: Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie [Theory of the Passage of Fast Corpuscular Rays through Matter]. Here, Bethe develops quantitative descriptions of elastic and inelastic scattering processes affecting electrons, formalizing the concept of stopping power that we apply, somewhat modified, today. Armed with the computational devices Bethe developed, it became possible to realistically and accurately estimate both electron and X-ray travel through complex solids, giving us the opportunity to predict how a highly focused electron beam of specified energy will be expected to interact with a solid target.

EPMA geochronology involves analysis of high-Z accessory phases. These phases commonly contain significant concentrations of actinides and rare-earths, providing high stopping power and limiting the dimensions of scattering such that, with an appropriately low beam energy and small beam diameter, the analysis can be limited to a sub-micron region. Bethe’s contributions to understanding ionization and overvoltage also allow us to appropriately consider the minimum beam potential for this analysis, and to evaluate the expected influence of fluorescence at a distance. With new technology providing very small beam diameters even at high current and relatively low voltage (8-15 kV), we have now demonstrated nanoscale geochronology in rocks from Northern Canada. Hans Bethe’s contributions are seminal, and are in no small part responsible for the analytical capabilities we seek to continually improve upon.

Alternative presentations:
In-situ Trace Element Analysis in
           Complex, Multiphase Materials by EPMA

Improved Analytical Resolution and Sensitivity in EPMA –
           Some Initial Results from the Ultrachron Development Project

Mike Jercinovic is an Associate Professor in the Department of Geosciences at the University of Massachusetts, and supervises the Electron Microprobe/SEM Facility in the department. He received his PhD from the University of New Mexico (1988), where he was introduced to microprobe analysis by Klaus Keil. Research at UNM concentrated on alteration processes in natural basaltic glasses and the application of natural analogues to modeling the corrosion of nuclear waste form glass. Mike then supervised the MIT electron microprobe facility for several years, and worked in the private sector in electron microscopy (microelectronic evaluation) before moving on to UMass in 1997. Research at UMass has primarily involved microanalysis applied to tectonic studies, and also includes other diverse geochemical, mineralogical and materials applications. Most recently, Mike’s research focus has largely related to development of new techniques in electron microprobe trace element analysis. This research primarily applies to the analysis of high-Z accessory phases (monazite, xenotime, thorite, etc.) for geochronology, but is also finding relevance in paleoclimate research, and various petrologic applications. This research has led to collaborative development of the new Cameca SX-Ultrachron electron microprobe. This project has included significant new hardware and software dedicated to improving spatial resolution and count precision in electron probe microanalysis.



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