Backscatter Detector

How It Works

When an electron beam strikes a sample a large number of signals are generated. One possible signal could be from electrons. The incident electrons that are sent into the sample are scattered in different ways. There are two broad categories to describe electron scattering, namely, elastic and inelastic.

 

Elastic Scattering of Primary Electrons

As the name implies, elastic scattering results in little (<1eV) or no change in energy of the scattered electron, although there is a change in momentum. Since momentum, p=mv, and m doesn’t change, the direction of the velocity vector must change. The angle of scattering can range from 0-180 degrees, with a typical value being about 5 degrees.
Elastic scattering occurs between the negative electron and the positive nucleus. This is essentially Rutherford scattering. Sometimes the angle is such that the electron comes back out of the sample. These are backscattered electrons.
bse_tag
On the far left of the backscatter detector is the lens, in the center is the secondary detector. To collect electrons, the backscatter detector moves under the lens so the electron beam can travel through the hole in its center.
bse_scan This is an image of an aluminum copper alloy formed using backscattered electron imaging. The light area is mostly copper and the dark area is mostly aluminum.

Inelastic Scattering

During inelastic scattering, energy is transferred to the electrons surrounding the atoms and the kinetic energy of the energetic electron involved decreases. A single inelastic event can transfer a various amount of energy from the beam electron ranging from a fraction to many kiloelectron volts. The main processes include phonon excitation, plasmon excitation, secondary electron excitation, continuum X-ray generation, and ionization of inner shells. In all processes of inelastic scattering, energy is lost, though different processes lose energy at varying rates.

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