Microscopy Flashcards
What are Photons, Rays, Beams and Pencils?
- 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.
What is the difference between absorption and reflection?
- 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 .
What is the main performance limiter for circular lenses?
- 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)
What is the ‘rayleigh criterion’ for spatial resolution?
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.
What is resolution, depth of focus and field of view?
- 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.
What is Secondary Emission from SEM?
- 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.
What is Backscatter Emission from SEM?
- 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
What is X-Ray Emission?
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.
What does standard SEM require?
- 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.
What does environmental SEM require?
Environmental SEM requires a non-conductive sample without coating.
What is the process of Transmission Electron Microscopy?
- 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.
What does the electron-accelerating voltage control of the SEM?
- 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
What does the objective-lens current control in SEM?
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.
How is SEM-EDX used in the analysis of GSR?
- 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.
How are GSR particles classified?
- 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.
