Characteristic X-Ray
Definition
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 between the two shells and is unique to the element. The K-alpha line of iron sits at 6.40 keV and is the same on every spectrometer in the world.
- Iron K-alpha energy
- 6.40 keV
- Governing principle
- Moseley's law: inter-shell binding energy difference
Common questions
What causes a characteristic X-ray to be emitted?+
A characteristic X-ray is emitted when an outer-shell electron drops down to fill a vacancy in an inner shell that was created by ionization or collision with a primary electron beam. The energy of the photon released depends on the difference in binding energies between the two shells, which is unique to each element.
Why is the energy of a characteristic X-ray the same on all instruments?+
The energy is determined by Moseley's law: it depends only on the element and the specific shells involved in the transition. Because this relationship is a fundamental property of atomic structure, iron's K-alpha line always appears at 6.40 keV regardless of the spectrometer used to measure it.
How are characteristic X-rays used to identify elements in forensic samples?+
Each element produces a unique set of characteristic X-ray energies based on its atomic structure. By measuring the energies of X-rays emitted from a sample, forensic analysts can determine which elements are present, making characteristic X-rays a reliable fingerprint for elemental analysis in techniques like X-ray fluorescence and energy dispersive spectroscopy.
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...
- Back-Scattered Electron (BSE)
- High-energy electron from the primary beam that has bounced elastically off a nucleus and exited the sample. BSE yield increases with atomic...
- Bragg's Law
- nλ = 2d sin(θ). A monochromatic X-ray beam diffracted off the parallel lattice planes of a crystal produces constructive interference only at...
- EDXRF vs WDXRF
- Energy-dispersive XRF reads all photon energies in parallel with a Si(Li) or silicon-drift detector; fast, cheap, lower resolution. Wavelength-dispersive XRF uses a...
- 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...
- 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...
- GSR Pb-Sb-Ba Signature
- The classical elemental signature of inorganic gunshot residue: lead from the bullet core, antimony from the priming compound, barium from the priming...
- ICDD PDF-4
- The Powder Diffraction File maintained by the International Centre for Diffraction Data. A reference database of more than 400,000 crystalline phases that...
- 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;...
- Moseley's Law
- The square root of the characteristic X-ray frequency varies linearly with atomic number Z. Discovered by Henry Moseley in 1913, it is...
- 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...
Explained in these topics
- Electron Microscopy: SEM, TEM and EDSAn X-ray photon emitted when an outer-shell electron fills a vacancy created in an inner shell by the primary electron beam. The photon energy equals the inter...
- X-Ray Spectroscopy: XRF, XRD and X-Ray AbsorptionAn X-ray photon emitted when an outer-shell electron drops into an inner-shell vacancy created by ionisation. Energy is set by the difference between the two s...