Backscatter electron images
What is backscatter?
Backscatter is a special way that beam electrons interact with a sample in which the beam electrons are thrown back toward the beam without losing energy.
Imagine dropping a ball onto a hard floor. The force of gravity gives the ball kinetic energy. When the ball hits the ground, it makes a sound (that's some of the energy converted into vibrations in the air and ground), and the atoms in the ball rub against one another and the ground (that's some more energy converted into heat that slightly warms the ball and ground). When the ball flies upward after the collision, it does not go as high as it was originally dropped because it lost energy to sound and heat. This is an example of an inelastic collision.
Backscattering is like what would happen if the ball bounced back to exactly the height from which it was dropped - an elastic collision. In the case of the ball, the ball would then fall again, come back, etc. forever. Backscattered electrons "rebounding" from the sample hit the BSD Backscatter Electron Detector. The BSD only "sees" electrons with exactly the same energy as the original beam.
The bouncing ball is just an analogy. Some people describe the process as being more like a comet orbiting around a star, but to be honest, things on the atomic/subatomic level actually behave in strange quantum mechanical ways. We just imagine analogies like this to envision processes beyond our comprehension - they're not exactly true, but still helpful.
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Why image with backscatter electrons?
Atoms that have more protons in their nucleus are more likely to backscatter beam electrons. Electron detectors just measure the number of electrons hitting the detector. More electrons hitting the detector = brighter spot on the image. The backscatter electron detector takes pictures of samples showing the differences in chemical composition of the things in the sample.
This is a sample of tailings from an abandoned titanium mine. The site is really very inspirational because a fellow is slowly reclaiming a bunch of mine waste into very high quality road-making material. I will take him years, but he will eventually have the site totally back to its natural forest state. I love when people do good things. In this image, bright specks are particles of the mineral ilmenite (FeTiO3) and darker gray samples are feldspar (0.7CaAl2Si2O8•0.3NaAlSi3O8). In regular secondary electron images, all of the grains look the same gray. (7.2 Mb original image)
Backscatter electron image of ilmenite (bright) and feldspar grains (darker grays) from tailings at an abandoned titanium mine showing how BSD images highlight compositional differences in a sample. (7.2 Mb file)
Septarian nodules are round rocks that range from fist-sized to head-sized. They have a cracked surface that looks a lot like a brain. The details of how they form is still a little mysterious, but applying our minds toward solving mysteries is what scientists do!
In this image, the bright white is barite (BaSO4) in a matrix of dark gray quartz (SiO2) and lighter gray calcite (CaCO3).
Granite is a rock formed when underground lava (magma) cools and freezes into interlocking different mineral crystals. When water freezes into ice, that's just one chemical (H2O), so only one kind of crystal forms. Magmas have a more complex chemical composition, so many different kinds of crystals form.
In this backscatter electron image of granite, each type of mineral is a different brightness, depending on the average atomic number in its chemical formula (Zavg ). Magnetite (Fe3O4 → Zavg = 15.7 = white) and ilmenite (FeTiO3 → Zavg = 14.4 = light gray) are surrounded by K-feldspar (KAlSi3O8 → Zavg = 10.6 = dark gray) and both quartz (SiO2 → Zavg = 10.0 = almost black) and albite (NaAlSi3O8 → Zavg = 10.0). Notice how quartz and albite are indistinguishable in backscatter images because the have the same average atomic number. We can use the EDS x-ray analyzer to tell them apart.