Question

Please briefly answer all the questions below, and writing those clearer and tidier (It is best to use typing)

This is a problem related to the transmission electron microscopy (TEM)

1. What is the difference between the field-emission and thermionic source TEMs?
2. What is the purpose of the Wehnelt in a thermionic source? Why don’t we need one in a field emission gun(FEG)?
3. Define ‘underfocused’ and ‘overfocused’ in the TEM?
4. What causes spherical aberration and chromatic aberration?

Answer 1

TEM is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image.In the field of optics we have always played with source to increase resolving power as in case of Michelson interferometer first we have used sodium lamp to get interferograms later to increase coherence length we have used laser source. In field emission source TEM we use concept of field emission in which emission of electrons induced by an electrostatic field takes place while in in thermionic source TEM we use concept of thermally induced flow of charge carriers from a surface.

The purpose of the Wehnelt in a thermionic source is to focus and control the electron beam.In thermionic source we use Wehnelt because it works as a lense but in field emission gun we need not require Wehnelt since it uses a single crystal tungsten filament that has a very fine tip .The electrons are not ejected by heating the filament but by applying a very strong electric field that is why more numbers of electrons can be emitted .

If we will not use Wehnelt in thermionic source brightness will be very low so resolving power will be also low.

intensity variation in image is calculated on the basic of contrast .intensity distribution in TEM images varies with defocus. Underfocused term is used for the negative defocus while overfocused term is used for positive defocus.

Spherical aberration arises mainly due to curvature of any lense.When parallel light passes through any lens we get focus shift that is why we get big circle instead of point while in chromatic aberration we get different focus for different wavelength of light .

if we use aperture in our system we can minimize these aberration ,also we can use diffractive optical elements to correct these aberrations.