Microscopy Flashcards

1
Q

What are Photons, Rays, Beams and Pencils?

A
  • Photons: “particle” of light - smallest amount of light possible
  • Rays: Path of a single photon from a single point on a light source.
  • Pencil: a group of rays divergent from a single point on a light source.
  • Beam: group of pencils originating from all points on a light source.
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2
Q

What is the difference between absorption and reflection?

A
  • Absorption: the photon enters the material but does not exit again. Results in thermal/electrical/chemical changes.
  • Reflection: light ray is turned back into incident material instead of travelling on into the new material.
  • Refraction: the bending of light when it passes from one transparent material to another transparent material where its velocity changes .
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3
Q

What is the main performance limiter for circular lenses?

A
  • Diffraction at a circular aperture is the fundamental limit on performance for circular lenses.
  • Spot size is determined by the diameter of the Airy disk

r = 1.22 x wavelength x (focal length/lens diameter)

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4
Q

What is the ‘rayleigh criterion’ for spatial resolution?

A

Image a: Pattern from a single point.

Image b: Points are resolved when the maximum of a disk overlaps with the minimum of another.

Image c: Fully resolved when there is minima overlap between the two points.

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5
Q

What is resolution, depth of focus and field of view?

A
  • Resolution: ability to distinguish two points on the specimen. Light microscopes reach values near 200 nm.
  • Depth of focus: ability to maintain focus over a range of depths within the specimen: Comparatively low for light microscopes.
  • Field of view: size of specimen that can be imaged at the same time. Comparatively good for light microscopes.
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6
Q

What is Secondary Emission from SEM?

A
  • A secondary electron is an electron from the material, dislodged by the interaction with the probing beam.
  • The electrons have low energy and so need to be in a high vacuum.
  • These are usually emitted from the coating of the specimen (a few nm deep)
  • SE iamges mainly reflect surface topography.
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7
Q

What is Backscatter Emission from SEM?

A
  • A backscattered electron is an electron deflected in the sample.
  • The electrons have higher energy than SE.
  • Depends on the atomic number of material.
  • Backscatter images give compositional information of sample.
  • Emitted from a few microns within the sample.
  • Form lower resolution images as they originate from a larger volume of the sample than SE electrons.
  • BSE show chemical composition
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8
Q

What is X-Ray Emission?

A

X-rays are emitted when dislodged electrons are replaced by an outer shell electron.

Example: a Si L-shell electron replaces a missing K-shell electron.

The emitted X-Ray is of very specific energy.

The spatial resolution of the X-ray is variable and is in order of a micron or more, as they have a much longer path length.

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9
Q

What does standard SEM require?

A
  • Standard SEM requires a conductive sample,.
  • Non-metallic samples such as bugs, plants, fingernails and ceramics, should be coated so they are electrically conductive.
  • Metallic samples can be placed directly into the SEM.
  • Local charge on the specimen can cause a problem as it attracts or repels the incident electrons and deflects the scanning probe, resulting in image distortion or fluctuations in image intensity.
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10
Q

What does environmental SEM require?

A

Environmental SEM requires a non-conductive sample without coating.

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11
Q

What is the process of Transmission Electron Microscopy?

A
  • TEM is where a TEM microscope sends electrons through a sample and detects them on the opposite side.
  • They can resolve features as small as one angstrom.
  • However the sample must be prepared as a slice of material no more than 1000 angstroms thick,
  • Does not work on a small insect but can reveal a virus hiding inside a cell.
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12
Q

What does the electron-accelerating voltage control of the SEM?

A
  • The electron-accelerating voltage determines the kinetic energy E0 of the primary electrons, their penetration depth and the information depth of the BSE image.
  • As SE are generated within a very shallow escape depth below the specimen surface, the high spatial resolution SE image is independent of E0
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13
Q

What does the objective-lens current control in SEM?

A

The objective lens current adjusts the microscope focus and hence image resolution.

Small particles on the specimen offer a convenient feature for focusing - the objective lens current is adjusted until their image is as small and sharp as possible.

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14
Q

How is SEM-EDX used in the analysis of GSR?

A
  • SEM-EDX is used to analyse GSR particles.
  • It looks for the characteristic pattern of Lead, antimony and barium, which are typical for many cartridges.
  • Gunshot residues have two components.
    • Organic (propellant) unburned, partially burned and byproducts.
    • Inorganic: mostly metal from primer and metallic parts of cartridge and weapon itself.
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15
Q

How are GSR particles classified?

A
  • GSR particles are classified according to their elemental content.
  • Only three-component particles containing lead, barium and antimony are unique indicators of primer residue.
  • Two and one-component particles are characteristic of GSR but particles of similar contents may be created in other circumstances.
  • Particles of Fe, Cr, Ni, Cu and Zi are typical of casing, projectile and its jacket as well as the barrel.
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16
Q

How can shooting distance be estimated from GSR?

A
  • GSR particles cool from 1500 oC as it travels away from the gun nozzle.
  • The element with the highest melting point (Ba) will condense first, Sb second, and Pb last.
  • Particles formed near the gun will consist only of a Ba core, those at a intermediate distance will have a Ba core and Sb shell, and those furthers away will have a Ba core, Sb inner shell and Pb outershell.
  • Was possible to accurately estimate firing distance between 20 and 90 cm using Pb and Ba
17
Q

What is the main issue of using SEM-EDX for GSR analysis?

A
  • Some ammunition does not produce unique GSR and lead-free ammunitions are seeing an increase in usage.
  • Cross-contamination in immediate vicinity of the shooting may cast doubt on the analysis.