Give+a+brief+description+about+secondary+detector+and+backscatter+detector

-- Secondary Electrons and Detection - secondary detector Secondary electrons are specimen electrons that obtain energy by inelastic collisions with beam electrons. They are defined as electrons emitted from the specimen with energy less than 50ev. Secondary electrons are predominantly produced by the interactions between energetic beam electrons and weakly bonded conduction-band electrons in metals or the valence electrons of insulators and semiconductors. There is a great difference between the amounts of energy contained by beam electrons compared to the specimen electrons and because of this, only a small amount of kinetic energy can be transferred to the secondary electrons.

An electron detector is used with the SEM to convert the radiation of interest into an electrical signal for manipulation and display by signal processing electronics, which to you and me is much like a television. Most SEM's are equipped with an Everhart-Thornley (E-T) detector. It works in the following manner: The scintillated material is struck by an energetic electron. This collision produces photons which are conducted by total internal reflection in a light guide to a photomultiplier. These photons are now in the form of light so they can pass through a vacuum environment and a quartz glass window. The photon is then converted back into an electron current where a positive bias can attract the electrons and collect them so that they will be detected. . . . Backscatter Electrons and Detection - backscatter detector 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.

As the name implies, elastic scattering results in little 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.
 * __Elastic__**

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.

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.
 * __Inelastic__**

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 http://mse.iastate.edu/microscopy/backscatter.html