Title of Presentation: Surface Elemental Analysis with 1 mm Spatial Resolution for Samples in Ambient Atmosphere Using the AEXS Instrument
Primary (Corresponding) Author: Jaroslava Z. Wilcox
Organization of Primary Author: NASA Jet Propulsion Laboratory
Co-Authors: Eduardo Urgiles, Risaku Toda, and Joy Crisp
Abstract: Atmospheric Electron X-ray Spectrometer (AEXS) is a miniature instrument based on the excitation of XRF spectra from samples in planetary atmospheres in situ using a focused electron beam. Unlike in SEM, the samples are not drawn into the vacuum of the electron column due to the use of a thin electron transmissive membrane that isolates the vacuum within the AEXS electron column. The spectra are analyzed using an energy-dispersive detector to determine surface elemental abundance for the irradiated spots with high-to-medium spatial resolution, enabling to assess sample heterogeneity.
AEXS has been demonstrated including characterization of the effect of the membrane on XRF spectra and developing a stand-alone 20keV probe sealed with a 500nm thick SiN membrane. The probe requires no active pumping, has been used for performing elemental analysis of NIST and USGS traceable standards with a good agreement with the certified composition for samples in up to about 90 Torr-cm thick atmospheres. The spatial resolution has been tested by resolving 1 mm size mineral grains for a Gabbro (Norite) sample, a big improvement in XRF instrument capabilities flown on previous NASA missions. The present effort addresses integration of the microprobe with a high voltage power supply, including a dielectric housing necessitated by low electrical strength at Mars atmosphere pressure. When implemented on a mobile platform, AEXS would be able to determine elemental composition of freshly exposed rock, as a part of a payload that would also include a visual light camera capable of imaging the area being analyzed. Though widely used in the laboratory SEM, electron excitation has not been previously used due to the difficulty of generating electron beams in ambient atmospheres. To decrease power consumption and mass, the thermionic emitters should be replaced with CNT based field-emitters, greatly simplifying power supply architecture.