When the electron beam
strikes the sample some of the electrons will interact with the nucleus of the
atom in much the same way a space craft will interact with the gravity of a
planet. The negatively-charged electron will be attracted to the positive
nucleus but if the angle is just right instead of being captured by the
"gravitational pull" of the nucleus it will circle the nucleus and come back out
of the sample without slowing down. These electrons are called backscattered
electrons because they come back out of the sample. Because they are moving so
fast, they travel in straight lines. In order to form an image with BSE
(backscattered electrons), a detector is placed in their path. When they hit the
detector a signal is produced which is used to form the TV image. All the elements have different sized nuclei. As the size of the atom nucleus increases, the number of BSE increases. Thus, BSE can be used to get an image that showed the different elements present in a sample.
 | This image shows the inside of the sample chamber. 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. |
| 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 . |  |