Gas scattering leads to direct contributions to the spectrum from the environmental gas, as well as remote generation of x rays by electrons scattered out of the focussed beam.
This paper will consider the special aspects of x-ray spectrometry and microanalysis performed in the VPSEM-ESEM, especially the impact of gas scattering on spectrum quality, methods of specimen preparation to minimize the effects of gas scattering, practical aspects of qualitative and quantitative x-ray microanalysis, and prospects for future improvements in this area.
Gas scattering of the primary beam is the single most important difference between performing x-ray spectrometry with the conventional low pressure (i.e., high vacuum) SEM and with the elevated pressure (low vacuum) VPSEM-ESEM instruments.
The inevitable gas scattering, both elastic and inelastic, of a fraction of the primary beam electrons has a significant and frequently severe impact on both qualitative and quantitative Si-EDS x-ray microanalysis in the VPSEM-ESEM.
2.2 Primary Beam Gas Scattering: Remote Excitation of X Rays
While a large fraction, 50 % or more, of the beam current can be transferred from the focused beam to the skirt due to gas scattering, the electron current density (A/[cm.sup.2]) at any point in the skirt is much lower than that in the focussed beam.
The linear behavior vs pressure is eventually lost at high pressures because gas scattering takes so much current out of the beam striking the alloy wire that the peak intensities of the alloy components begin to decrease significantly.
4 reveals that, depending on the degree of gas scattering, the pathological peak due to the contribution to the composite spectrum of the remote scattering onto the surrounding matrix can have the appearance of a trace, minor, or major constituent, Fig.
Both elastic and inelastic gas scattering effects can be recognized in the same spectrum.
Several strategies have been developed for dealing with gas scattering and the inevitable excitation of portions of the specimen remote from that being interrogated by the direct beam .
3.1 Selection of Instrumental Parameters to Minimize Gas Scattering