Welcome to
the University of Wisconsin-Madison
Department of Geoscience
Eugene Cameron EPMA Lab
Soft X-Ray EPMA Research

Update November 17, 2013

Traditionally, EPMA has been done at 15-20 keV, for a variety of reasons, principal ones being: For the most part, the electron source was the hair-pin W filament, whose spot size on the sample, at typical probe currents (20-30 nA) would be in the 0.5-1 micron diameter range. So the smallest typical 'normal' (eye of the beholder) object (say a feldspar crystal) might be in the 2-3 micron diameter range. Forget about a 1 um exsolution lamella in a pyroxene or ilmenite.

With the emergence of the Field Emission Electron Probe (JEOL in 2003 and CAMECA in 2011) and the ability to shrink the electron beam diameter by a factor of 5-10, it has now become possible to consider EPMA of sub-micron size features. This depends upon several factors, a combination of (1) the beam diameter (itself a combination of the keV and nA being used), (2) the incident electrons' scattering distance (lateral and depth) in the specimen, and (3) the edge energy required to excite a particular X-ray line.

Since 2011, we have been investigating low voltage (5 keV) EPMA of iron silicides, resulting from the discovery by UW Madison researchers of micron and sub-micron iron silicides in an impacted lunar feldspar grain. Phil Gopon's Ph.D. research focuses upon both this lunar iron silicide as well as the application of low voltage EPMA to terrestrial samples. Phil's lunar iron silicide EPMA research has now been published (2013) in Microscopy and Microanalysis. He reports on the benefit of utilizing an otherwise ignored iron L X-ray line, the LL line, which is not involved with chemical bonding AND is far away from the L3 edge (the nemesis of the La X-ray line), both which make use of the Fe La line next to impossible for EPMA. (Fe Lb is not much better).

Other current low voltage research projects which we are involved with:

A recent paper by Merlet and Llovet with a formulation for x-ray analytical resolution that combines both the electron beam diameter and the x-ray range (lateral and depth)

Peter McSwiggen has an online tutorial on "Characterization of sub-micrometer features with FE-EPMA" His Figure 4 shows that the optimal keV to produce the smallest analytical volume is in the 5-8 keV range.

Most of the low voltage EPMA work going on is happening in Europe, and the recent EMAS (European Microbeam Analysis Society) conference in Porto, Portugal, features many talks and posters dealing with low voltage EPMA, FE EPMA and critical issues such as carbon contamination. We will post a list of those talks and posters....

We thank and acknowledge colleagues who have assisted our ongoing low voltage EPMA research, including Xavier Llovet (U of Barcelona) and Peter Sobol (UW-Madison), and particularly those in making use of their Field Emission microprobes available to us: Bill Horn at Exxon-Mobil (JXA-8530), David Snoeybous, Michel Outrequin, Chrystel Hombourger (CAMECA, SXFive-FE), Silvia Richter and Philippe Pinard (RWTH Aachen, JXA-8530) and Henny Cathey (ASU, JXA-8530). We will be joining the FE ranks in late 2014 with an SXFive-FE.

Some older publications references low voltage x-ray lines (e.g. M lines of REE)

For more information, contact:

John Fournelle
Dept of Geology and Geophysics
University of Wisconsin-Madison
1215 W. Dayton St
Madison, WI 53706
(608) 262-7964 (office) 265-4798 (lab) 262-0693 (fax)  
johnf*at*geology dot wisc dot edu

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