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Questions and Answers
What is the primary function of the cathode shield in a thermionic gun?
What is the main difference between a thermionic gun and a field emission gun?
What is the purpose of the second anode in a field emission gun?
What is the material used to make the electromagnetic coils in the electromagnetic lenses?
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How is the magnification of an electromagnetic lens varied?
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What is the main function of the condenser lens in a TEM?
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What is the effect of chromatic aberration in electromagnetic lenses?
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What is the purpose of the projector lens in a TEM?
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What is the effect of astigmatism in electromagnetic lenses?
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Why must the specimen be very thin in a TEM?
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Study Notes
Electron Sources
- There are two types of electron sources: Thermionic gun and Field Emission Gun (FEG)
- Thermionic gun:
- Composed of a tungsten (W) filament, cathode shield (Wehnelt cylinder), and anode plate
- Electrons are emitted when the filament is heated
- Field Emission Gun (FEG):
- Electrons are pulled out of a very fine pointed tungsten tip by a high electric field
- FEG has two anodes: first anode to pull electrons out of the tip and second anode to accelerate electrons to the right potential
- FEG is more expensive and requires ultrahigh vacuum
Electromagnetic Lenses
- Made of electromagnetic coils (copper) that bend the path of electrons
- Used to focus and control the electron beam, ensuring precise placement of electrons
- Magnification of the lens can be varied by controlling the current through the coils
- There are three sets of electromagnetic lenses:
- Condenser lens: doubles as a lens to control spot size and another to control intensity (brightness)
- Objective lens: forms the intermediate magnified image
- Projector (ocular) lens: produces the final further magnified image
Defects of Electromagnetic Lenses
- Chromatic aberration: magnification varies with electron wavelength, causing fringes around objects
- Spherical aberration: magnification differs between the center and edges of the field
- Astigmatism: circular shapes become elliptical in the image, correctable by using a Stigmator
Characteristics of the Electron Beam
- Has a very short wavelength (0.005nm)
- Can be focused by passing through a magnetic field
- Increasing accelerating voltage lowers the wavelength and increases electron speed
- Specimens must be very thin to allow electron penetration and image formation
- Thin sections of the specimen are mounted on copper grids of 3 mm diameter and attached to a holder for examination
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Description
Learn about the three types of electromagnetic lenses in Transmission Electron Microscopy (TEM), including condenser lenses, objective lenses, and projector lenses. Understand their functions and roles in forming magnified images.
