Back-Scattered Electron (BSE)
Definition
High-energy electron from the primary beam that has bounced elastically off a nucleus and exited the sample. BSE yield increases with atomic number Z, so a BSE image shows heavy elements bright and light elements dark. This atomic-number contrast is what makes BSE the first detector to turn on when scanning a hand swab for high-Z gunshot residue particles.
- Scattering mechanism
- Elastic backscatter off atomic nuclei
- Image contrast basis
- Atomic number Z (compositional contrast)
- Common forensic use
- Gunshot residue detection on hand swabs
Common questions
What's a back-scattered electron and why does it matter in forensics?+
A back-scattered electron is a primary beam electron that bounced elastically off an atomic nucleus and exited the sample. It matters because BSE yield increases with atomic number Z, so forensic analysts use BSE detectors to find heavy-element particles like gunshot residue on hand swabs. Heavy elements show bright, light elements show dark.
How do BSE images differ from other electron microscopy images?+
BSE images display Z-contrast, a compositional image where brightness directly reflects atomic number. This atomic-number contrast makes BSE ideal for spotting gunshot residue particles and other trace evidence where elemental composition is the key identifier.
Why is BSE the first detector to turn on in a SEM scan for gunshot residue?+
Gunshot residue particles contain high-atomic-number metals like lead, barium, and antimony. BSE imaging shows these elements bright against a dark background of lighter elements in the swab matrix, making residue particles visible and easy to locate during the initial scan.
Related terms
- ASTM E1588
- ASTM International standard practice for gunshot residue analysis by scanning electron microscopy with energy-dispersive spectrometry. Defines particle classification (characteristic, consistent-with, indicative), morphology...
- Secondary Electron (SE)
- Low-energy electron (below 50 eV) ejected from the top few nanometres of the sample when the primary beam knocks a loosely bound...
- Sputter Coating
- Vacuum-deposition technique that lays a thin (5 to 15 nm) conductive layer of gold, gold-palladium or carbon onto a non-conductive sample (polymer,...
- Characteristic X-Ray
- An X-ray photon emitted when an outer-shell electron drops into an inner-shell vacancy created by ionisation. Energy is set by the difference...
- EDS / EDXRF Detector
- Silicon-drift detector mounted at a fixed take-off angle to the sample that reads characteristic X-rays emitted as the primary beam ionises atoms...
- Energy-Dispersive X-Ray Spectroscopy (EDS)
- An X-ray detection method that simultaneously measures the energies of all characteristic X-ray photons emitted from the interaction volume, identifying all elements...
- Environmental SEM (ESEM)
- Variant of conventional SEM that runs the sample chamber at higher pressure (up to several torr of water vapour) instead of the...
- FIB (Focused Ion Beam)
- A technique using a focused gallium ion beam to mill a specimen at a specific site-selected location, producing a TEM-quality cross-section with...
- Interaction Volume
- The pear-shaped region of specimen material within which the primary electron beam deposits energy and generates signals. At 20 kV in a...
- Lead-Free GSR
- Gunshot residue produced by ammunition using non-lead primer formulations (SINTOX, Diazol, or other variants). Does not contain the conventional Pb-Sb-Ba three-element combination;...
- Silicon Drift Detector (SDD)
- The current standard EDS detector type, using a reverse-biased silicon chip cooled by a Peltier element to achieve high count-rate capability (up...
- Ultramicrotome
- A precision instrument for cutting sections of resin-embedded specimens to 60-100 nm thickness using a diamond knife, enabling TEM examination of the...
Explained in these topics
- Electron Microscopy: SEM, TEM and EDSA primary beam electron elastically scattered backward through a large angle by atomic nuclei. Back-scatter yield is proportional to atomic number Z, producing...
- Scanning Electron Microscopy (SEM) with EDXRFHigh-energy electron from the primary beam that has bounced elastically off a nucleus and exited the sample. BSE yield increases with atomic number Z, so a BSE...