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Auger Spectroscopy


Figure 4 : Auger electron energy spectrum of a presolar silicate grain from the carbonaceous chondrite Miller Range 07687: (a) direct spectrum with Auger intensities superimposed on a background of secondary and backscattered electrons; (b) derivative spectrum from which the Auger intensities for each element can be determined from the peak-to-peak heights.


information about the location of elements within and around the grains of interest ( Figure 5 ).


A key feature of our Physical Electronics (PHI) 700 Scanning Auger Nanoprobe is that it has electron optics with coaxial primary and secondary electron paths ( Figure 1b ). T is is accomplished through the use of a rotationally symmetrical “cylindrical mirror analyzer,” positioned around the primary electron beam column, for measuring the emitted electron energies. T is is an important similarity to the NanoSIMS, which also has coaxial primary and secondary ion beams ( Figure 1 ). Because SIMS is a sputtering technique, the measurement of geologically complex samples, like the meteorites we analyze, results in topographical variations because diff erent phases sputter at diff erent rates. Since our Auger instrument has coaxial electron optics, like the NanoSIMS, both instruments view the sample from the same relative point of view without geometrical


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distortions. T is allows the precise alignment of images from the two instruments, which is necessary to obtain correlated isotopic and elemental information on these very small presolar grains, most of which are less than 500 nm in diameter. Image alignment is accomplished by comparing secondary electron images from the two instruments using custom-written soſt ware. As noted above, sample charging is an important issue in the analysis of geological samples. Several factors in our application of Auger spectroscopy have fortuitously combined to mitigate sample charging issues in most of our analyses. Because the grains we are measuring are very small, less than the electron penetration range at typical primary beam energies [ 5 ], most of the electrical charge is actually deposited in the more conductive C-rich regions outside the grains of interest [ 2 ]. Most of the samples in which presolar grains are found contain enough carbonaceous matter to act as a conductor in these samples. In addition, the thin


www.microscopy-today.com • 2018 March


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