Imaging
Available Signals
Five signal types are available for imaging
on our electron microprobe: secondary electrons, backscattered electrons, x-ray
intensity, absorbed current, and cathodoluminescence. Descriptions of the
available signals are given below, along with examples of images and their
applications for most signal types.
Secondary
Electron Imaging (SEI).
Secondary electrons are low energy electrons emitted from very near the sample
surface. This signal provides an image of the sample topography, and hence,
external morphology. This can be applied not only to simple characterization of
a sample material but also to a variety of other applications including
component failure analysis or the determination of chemical stability of
materials indicated by growth or dissolution features.
This image of natural cuprite
(Cu2O) was acquired at 74x magnification, which is near the 50x
lower limit of magnification for our instrument. It shows the general acicular
to fibrous nature of the material seen at low magnification.
This image of the same cuprite
was acquired at 500x magnification, which is near the limit of resolution by
typical far-field optical microscopy. It shows the reticulated habit of grains
that appear prismatic at lower magnification.
This image of the same cuprite
was acquired at 3000x magnification, which exceeds the resolution of typical
optical microscopy. It shows that grains appearing smooth at lower
magnification are coated by fine, dendritic to radial
skeletal growth.
Backscattered Electron
Imaging (BSEI). Backscattered
electrons have higher energies than secondaries, and
are produced when electrons from the primary beam are "bounced" back
out of the sample by elastic collisions with atoms. The number of electrons a
given atom will backscatter is proportional to its mean atomic number.
Materials composed of larger, heavier atoms will backscatter more electrons, producing brighter gray tones in the images than
less dense materials (differences in average atomic mass of 0.1 amu can be resolved). Backscattered electrons thus produce
an image that is related to material composition, providing both spatial and
chemical information. This signal is especially useful for characterizing
fine-grained multicomponent materials (first two
images, below) and for documenting chemical heterogeneity in single coherent
phases (third image, below).
This is a low magnification BSE image of mortar from
the brickwork of the Sarkeys energy center. It shows
the general structure of the mortar including fine aggregate comprised of
quartz (Qtz) and orthoclase (Or) sand grains plus minor hematite (Fe2O3:
white, near center of image), set in a matrix of Ca- and Al-silicates. Note the
heterogeneity within the matrix, especially with respect to the porosity
gradient from the more massive high-Ca cement at the left to the more variable
and hydrous Si-rich cements at the right.
This image is a detail from near the of the
center of the previous image, and shows heterogeneity within the Si-rich cement
defined by sequential formation of (1) early ferruginous di-calcium
aluminate, followed by (2) di-calcium silicate, and
finally very hydrous low-calcium silicates (3).
This image is of two banded tourmaline crystals with
quartz (Qtz) and orthoclase (Kfs) from a granitic rock. Banding in the
tourmaline, nominally Na(Fe,Mg)3Al6(BO3)2Si8O18(OH,F)2,
is due exclusively to variations in the Fe:Mg ratio
(the Fe-rich zones are lighter), as the molecular fractions of other components
in these crystals are essentially constant.
X-Ray Imaging
Elemental
Distributions. X-ray intensity images document the distribution of
selected elements in a material. Therefore, they can show the chemical basis
for intensity variations observed by BSEI. X-ray images can be acquired with
either or both the WDS and EDXA detectors. Images can be calibrated to produce quantitative
elemental distribution maps, either by automated process during acquisition or
manually after acquisition if the concentrations for two points of different
intensity are known. Image output can be in color or black/white, and color
schemes for the images can be user-defined. In the following example of a
metallurgical slag, brighter colors indicate higher intensities (red = max,
black = min). The BSE image (upper left) shows the complexity of the slag in
which the four most abundant phases, in order of decreasing average atomic mass
are: (1) metallic Fe (white in BSEI: note the high intensity in the Fe Ka
image, and no intensity in the O Ka image); (2) skeletal, apparently cubic (Cr,
Mn)-oxide (light gray in BSEI: note the high intensities in the Cr Ka image,
and low intensity (blue) in the O Ka image); (3) interstitial
Cr-aluminosilicate that is probably quenched to glass (dark gray in BSEI: note
low intensity in Cr Ka image, but moderate intensity in Si Ka and high
intensity in Al Ka images) and (4) silica (SiO2) that is likely
quartz (black in BSEI, high red intensities in the Si Ka and O Ka images.
Resolution of
phases with similar electron backscattering cross-sections. In
some cases, different phases have very similar average atomic masses, and may
be difficult or impossible to distinguish by BSEI. In such cases, even subtle
differences in composition between the phases may permit their discrimination
by x-ray imaging.
The example below
shows three images of an experimental product from a high tempeature
study of crystal growth from granitic melt. The left BSEI shows patchy
intergrowth of potassic alkali feldspar ((K,Na)AlSi3O8): white) with
albite feldspar (NaAlSi3O8: medium gray) and quartz (SiO2:
medium gray) in melt that was quenched to glass (darker gray, lower right
corner of image). Because albite and quartz have very similar electron
backscattering cross sections, they are often difficult to distinguish by BSEI.
This distinction is easily made, however, by the use of x-ray imaging. The
center image is of Na distribution: the red areas are richest in Na and, hence,
are albite. The right image shows Si distribution; the red areas are quartz
which is the phase having the highest Si content in this system.
Absorbed Current Imaging
Absorbed current images show differences in
electrical conductivity within a sample. As such, the signal can be used to
examine variations in composition or structure, especially in conductive
materials like metal alloys and electrical components like this component
socket (solder fillet) in a printed circuit board.
Image size: 1580 x 1580 mm
Cathodoluminescence Imaging
Cathodoluminescence (CL) is the emission of
energy in the form of light in the UV to near-IR (including the visible
wavelengths) produced by many materials when bombarded by high energy sources,
which is an electron beam in our instrument. For many or most such substances,
the color and intensity of the cathodoluminescent signal are very sensitive to
trace element chemistry and/or defects in the mineral lattice. This makes the
CL signal very useful for distinguishing zoning in crystals that may not be
resolved by optical microscopy or BSEI. This can be applied to growth history
and kinetics for many minerals or synthetic compounds, especially as a guide
for selecting points for quantitative analysis of minor to trace level
components by WDS.
The following CL image of a U-rich zircon
crystal in epoxy was acquired in about 1.5 minutes using a 5 nA
beam current. Note that uranium quenches
cathodoluminescence in zircon, causing weak luminescence in this grain (many zones
show CL intensity comparable to or weaker than the surrounding epoxy).
Image
size: 287 x 287 mm
Imaging Methods
Images, especially those utilizing electron
or absorbed current signals, are typically viewed in live-time. Live-time imaging
is accomplished by digital signal capture and display on 15" monitors (two
channels are available) much like those of standard SVGA computer displays. The
rate of beam scanning can be varied from TV rate to 4 seconds per frame, and
signal acquisition time can be averaged in the range of 200 - 10,000 mS.
Live-time images can be captured directly to digital files of a variety of
formats providing great flexibility, exportability, and opportunities for image
enhancement and analysis.
Digital image acquisition is accomplished
using the SAMx® MaxView and PCHiMax
imaging systems. These packages support resolutions from 64 to 2048 pixels on a
side (rectangular and square arrays are available), acquisition by beam or
stage rastering, full independent control of up to 15
x-ray signals (five from WDS plus ten from EDXA) and two other input channels
(electron, absorbed current, and/or cathodoluminescence signals) with user
selectable dwell times. Set-up for x-ray imaging can be done using quantitative
analytical files; by also acquiring background intensities, images can take the
form of quantitative element distribution maps.
Image Enhancement,
Analysis, and Output
A variety of software packages are available
for simple viewing and enhancement of digital images. Among these the SAMx® PCHiMax, and Media Cybernetics Image Pro Plus® run as
off-line solutions, support intensity and spatial filtering, arithmetic
operators, and text annotation. Image Pro Plus® has significantly greater
capabilities for text annotation and image manipulation, and thus most
enhancement and editing is performed with this package using standard (TIFF,
JPEG, GIF, BMP) file formats.
Quantitative image analysis is performed using
the Media Cybernetics Image Pro Plus® package. In addition to image enhancement
capabilities, this package supports a wide array of features for deriving
quantitative information from images. Among those features are included: manual
and semi-automated object measurement (size, intensity, shape, orientation),
classification, and counting, with result analysis and statistics exportable to
spreadsheets; spatial and intensity calibrations with corrections for
non-linearity; spatial, logical, arithmetic, and background correction
operations; false coloration of gray-tone images, true 24-bit color processing
of RGB images, and color-based image segmentation; and capabilities for
simultaneously working with multiple images. When used in concert with logical
(Boolean) and arithmetic operators, the latter feature can be extremely useful
for producing phase distribution maps in complex multi-component systems from
combinations of x-ray intensity and backscattered electron images.
Image output can be either to hardcopy or
digital file (a large variety of file formats are supported including TIFF,
JPEG, GIF, and many others).