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Electron Microscope. Magnification and Resolution., Summaries of Medical Physics

Medical University VarnaMedical Physics

The wave properties of particles, the De-Broglie wavelength of charged particles, and the components of an electron microscope. It also discusses the generation of the electron beam, accelerating voltage, vacuum, specimens, magnification system, and visualisation of the image. a detailed explanation of the working principles of a transmission electron microscope (TEM) and its parts. It also explains the magnification formula for a TEM. useful for students studying physics, biology, and engineering.

Typology: Summaries

2020/2021

Available from 02/01/2022

daniel-raj
daniel-raj 🇧🇬

11 documents

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18. Electron Microscope:
* Wave properties of particles: All elementary particles have a wave like
nature; wave of de-Broglie. The wavelength of de-Broglie depends on
the mass m and velocity v of the particle;
λ = h/mv
* De-Broglie wavelength of charged particles: particles with charge q
can be accelerated by electric fields (Voltage U) to change their De-
Broglie wavelength;
mv2/2 = qU
* At 100 kV accelerating voltage λ = 4 pm. This wavelength is about
100000 times shorter than the average wavelength of visible light
(500nm). Image resolution can be greatly improved if de-Broglie waves
are used for observation instead of visible light.
* EM: a beam of electron is directed towards the specimen. The image
is due to the different scattering or reflection of the accelerated
electrons by different parts of the specimen.
Types: TEM (Transmission, scanning, reflection)
SEM
REM
STEM
*Generation of the electron beam: A heated tungsten filament emits
electrons – thermionic emission.
* Accelerating Voltage: Can be as high as 106 V. For biological
specimens 100 – 120 kV is used. The specimen can be damaged by
electrons at higher voltages.
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18. Electron Microscope:

  • Wave properties of particles: All elementary particles have a wave like nature; wave of de-Broglie. The wavelength of de-Broglie depends on the mass m and velocity v of the particle; λ = h/mv
  • De-Broglie wavelength of charged particles: particles with charge q can be accelerated by electric fields (Voltage U) to change their De- Broglie wavelength; mv^2 /2 = qU
  • At 100 kV accelerating voltage λ = 4 pm. This wavelength is about 100000 times shorter than the average wavelength of visible light (500nm). Image resolution can be greatly improved if de-Broglie waves are used for observation instead of visible light.
  • EM: a beam of electron is directed towards the specimen. The image is due to the different scattering or reflection of the accelerated electrons by different parts of the specimen. Types: TEM (Transmission, scanning, reflection) SEM REM STEM *Generation of the electron beam: A heated tungsten filament emits electrons – thermionic emission.
  • Accelerating Voltage: Can be as high as 10^6 V. For biological specimens 100 – 120 kV is used. The specimen can be damaged by electrons at higher voltages.
  • Vacuum: The electrons are accelerated in vacuum. This reduces the energy loss due to scattering by gas atoms. All the electrons will have the same energy which improves the image quality.
  • Specimens: Vacuum causes rapid evaporation and/or deformation of the specimen. The specimen is prepared by Chemical processing, freezing, dehydration, embedding with a resin, ultrathin slicing (40mm) with a microtome and staining by heavy metals.
  • Magnification system: Electron lenses use electric and magnetic fields to change the direction of the electrons (to focus the beam). The different types are electrostatic, magnetic and electromagnetic. The focal length can be adjusted electrically.
  • Visualisation of the image:
  • Fluorescent screen
  • Photographic film
  • CCD camera and monitor
  • TEM parts:
  • Electron gun
  • Condenser lens
  • objective lens
  • Intermediate image
  • Projection lens
  • Fluorescent screen
  • Magnification: WTEM = L^2 / 4fofp
  • fo: focal length of the objective lens
  • fp: focal length of the projection lens
  • L: distance between the adjacent foci of the objective and projection lenses