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high-performance x-ray detection in a new analytical electron microscopeX-ray detection by energy-dispersive spectrometry in the analytical electron microscope (AEM) is often limited by low collected X-ray intensity (P), modest peak-to-background (P/B) ratios, and limitations on total counting time (tau) due to specimen drift and contamination. A new AFM has been designed with maximization of P. P/B, and tau as the primary considerations. Maximization of P has been accomplished by employing a field-emission electron gun, X-ray detectors with high collection angles, high-speed beam blanking to allow only one photon into the detector at a time, and simultaneous collection from two detectors. P/B has been maximized by reducing extraneous background signals generated at the specimen holder, the polepieces and the detector collimator. The maximum practical tau has been increased by reducing specimen contamination and employing electronic drift correction. Performance improvments have been measured using the NIST standard Cr thin film. The 0-3 steradian solid angle of X-ray collection is the highest value available. The beam blanking scheme for X-ray detection provides 3-4 times greater throughput of X-rays at high count rates into a recorded spectrum than normal systems employing pulse-pileup rejection circuits. Simultaneous X-ray collection from two detectors allows the highest X-ray intensity yet recorded to be collected from the NIST Cr thin film. The measured P/B of 6300 is the highest level recorded for an AEM. In addition to collected X-ray intensity (cps/nA) and P/B measured on the standard Cr film, the product of these can be used as a figure-of-merit to evaluate instruments. Estimated minimum mass fraction (MMF) for Cr measured on the standard NIST Cr thin film is also proposed as a figure-of-merit for comparing X-ray detection in AEMs. Determinations here of the MMF of Cr detectable show at least a threefold improvement over previous instruments.
Document ID
19970014269
Document Type
Reprint (Version printed in journal)
Authors
Lyman, C. E.
(Lehigh Univ. Bethlehem, PA United States)
Goldstein, J. I.
(Massachusetts Univ. Amherst, MA United States)
Williams, D. B.
(Lehigh Univ. Bethlehem, PA United States)
Ackland, D. W.
(Lehigh Univ. Bethlehem, PA United States)
vonHarrach, S.
(Fisons Instruments/VG Microscopes West Sussex, United Kingdom)
Nicholls, A. W.
(Fisons Instruments/VG Microscopes West Sussex, United Kingdom)
Statham, P. J.
(Oxford Instruments Buckinghamshire, United Kingdom)
Date Acquired
August 17, 2013
Publication Date
November 1, 1994
Publication Information
Publication: Journal of Microscopy
Volume: 176
Issue: Pt 2
Subject Category
Inorganic and Physical Chemistry
Report/Patent Number
NASA-CR-203728
NAS 1.26:203728
Funding Number(s)
CONTRACT_GRANT: DE-FGO2-86ER-45269
CONTRACT_GRANT: NAG9-45
Distribution Limits
Public
Copyright
Public Use Permitted.
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