How X-Rays Are Generated: Electron Deceleration & Target Physics

X-rays are produced when **high-energy electrons are rapidly decelerated** upon striking a metal target. This process, called **bremsstrahlung** ("braking radiation"), occurs in an X-ray tube.

Step-by-Step Physics

1. Electron Emission: Thermionic emission from heated cathode (filament) — typically tungsten.
2. Acceleration: High voltage (kVp: 20–150+ kV) accelerates electrons toward anode.
3. Target Impact: Electrons strike anode target (usually tungsten, sometimes molybdenum/rhodium for mammography).
4. Deceleration & Energy Conversion:
   • Bremsstrahlung (Continuous Spectrum): Electrons deflected by nuclear Coulomb field → kinetic energy → X-ray photons. Max photon energy = tube voltage (kVp).
   • Characteristic Radiation (Line Spectrum): Incident electron ejects inner-shell electron (K-shell); outer-shell electron fills vacancy → emits photon with energy = binding energy difference (e.g., W Kα = 59.3 keV).
5. Beam Formation: X-rays exit through beryllium window; collimated by lead shutters.

Key Parameters Controlling Output

Parameter	Effect on X-Ray Beam
kVp (Peak Kilovoltage)	Controls **penetrating power** (quality) & max photon energy. Higher kVp = more penetrating, more scatter.
mA (Tube Current)	Controls **quantity** of electrons → photon flux (intensity). mAs = mA × time.
Target Material (Z)	High Z (W=74) increases bremsstrahlung efficiency & characteristic energies.
Filtration	Removes low-energy photons (Al/Cu filters) → reduces patient dose, hardens beam.

Applications

• Medical imaging (radiography, CT, fluoroscopy, mammography)
• Security screening (baggage, cargo)
• Industrial radiography (weld inspection, casting integrity)
• Crystallography (XRD)

The line-focus principle (angled anode) creates a small *effective* focal spot for spatial resolution while spreading heat over a larger *actual* area—critical for tube longevity.