The Atmospheric Electron X-ray Spectrometer (AEXS) is being developed for performing rapid, nondestructive in situ analyses of the elemental composition of surfaces. The capability of the AEXS to operate in air and to be brought to a surface to be analyzed makes it unnecessary to bring a specimen to a laboratory and prepare the specimen for analysis.

The AEXS belongs to a class of miniature instruments that perform in situ elemental composition analysis by the excitation and analysis of x-ray fluorescence from specimens. Prior techniques include excitation of the x-ray fluorescence by a particles and x rays, both of which entail larger target areas and longer spectrum acquisition times than the electron beam based excitation used in the AEXS. The AEXS is predicted to be able to analyze samples at high spatial resolution determined by its controllable electron-beam spot size. The spectrum acquisition time of the AEXS is expected to be less than a minute. The short spectrum acquisition time results in less energy consumption and makes the AEXS suitable for incorporation into manufacturing, inspection, and quality control processes.

An Atmospheric Electron X-Ray Spectrometer shown in this schematic is designed to be a portable instrument that can be brought to a specimen surface. After operation of about a minute, it can give information on the elemental composition of the specimen surface. The dimensions of the instrument in its final form are expected to be about 4 by 8 by 14 in. (about 10 by 20 by 36 cm).

The figure schematically depicts a laboratory prototype of a portable AEXS instrument. Electrons are generated, accelerated, and focused by a commercially available electron gun within an evacuated enclosure. The vacuum is isolated from the ambient atmosphere by an electron-transparent membrane. The membrane is microfabricated from low-atomic-number materials such as silicon nitride, boron nitride, or diamond. Active pumping of the electron column against leaks and diffusion of air through the membrane can be achieved by an ion pump.

The electron beam passes through the membrane and travels a short distance to the specimen surface. The electrons impinge on the surface, exciting x rays at wavelengths characteristic of surface elements. The x rays are detected by a thermoelectrically cooled silicon positive/intrinsic/negative (PIN) photodiode. The signal from the PIN photodiode is fed into a signal-conditioning amplifier and a multichannel analyzer. The output of the multichannel analyzer is an x-ray spectrum that can be analyzed to determine the elemental composition of the surface.

The instrument is designed to be powered by a battery. The high voltage (30 kV) for the electron gun is generated by a miniature voltage multiplier circuit. Instrument control electronics are required to operate the electron gun, ion pump, x-ray detector, and analyzer. The spectrum from the multichannel analyzer is routed through a serial interface to a computer.

This work was done by Jason Feldman, Thomas George, and Jaroslava Wilcox of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com  under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20463


This Brief includes a Technical Support Package (TSP).
Atmospheric electron X-Ray Spectrometer

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This article first appeared in the August, 2000 issue of Electronics Tech Briefs Magazine.

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