Apertures

Apertures are used to reject the off-axis and off-energy electrons going down the EM column. Aperture is determined by a thin metal strip, called the aperture strip that contains holes of different sizes. The strip is placed in an aperture holder shown in figure 17.3.

Scattering of electrons

We see various objects around us; but how exactly do we see them? How does a light microscope allow us to see a magnified image of a specimen? Why is milk white while water transparent? The answer to all these questions is same: the interaction of light with matter alters one or more properties of the light that it receives. We can see objects around us because they absorb, reflect, or scatter the visible light. A specimen becomes visible only if it brings about changes in the radiation used to visualize it. How do then we image samples using electrons? Electron microscopy is possible because interaction of electrons with matter brings about changes in the electrons or generates new electrons with different energies. A specimen will be transparent to electrons if it does not scatter them and therefore be invisible when analyzed using an electron microscope. Figure 17.4 shows the different processes that result through interaction of electrons with matter.

 

Figure 17.4 Various phenomena that take place during electron interaction with a thin specimen