EM Spectrum - Wave Wonders
- EM radiation: Oscillating, perpendicular electric (E) & magnetic (B) fields. Travels at speed of light ($c$).
- $c \approx 3 \times 10^8 \text{ m/s}$ in vacuum.
- Wave-particle duality (photons).
- Energy: $E = h\nu = hc/\lambda$
- $h$: Planck's constant
- $\lambda$: Wavelength, $\nu$: Frequency
- Energy: $E = h\nu = hc/\lambda$
- Spectrum (↓$\lambda$, ↑E, ↑$\nu$):
📌 Roman Men Invented Very Unusual X-ray Guns
- Radio → Microwave → IR → Visible → UV → X-ray → Gamma ray
- Radio → Microwave → IR → Visible → UV → X-ray → Gamma ray
- Key Properties:
- Photons: no charge, no mass.
- Unaffected by E/B fields.
- Vary in $\lambda, \nu, E$.
⭐ X-rays originate from electron shell transitions, while Gamma rays originate from nuclear decay, regardless of their energy levels.
X-ray Production - Beam Basics
- Source: Thermionic emission from heated Tungsten filament (~2200°C) releases e⁻.
- Acceleration: High voltage (kVp) accelerates e⁻ to anode (Tungsten target).
- Tube Voltage (kVp): 25-150 kVp (diagnostic). Controls X-ray quality (penetrability) & max energy.
- Tube Current (mA): Controls X-ray quantity (photon number).
- Target Interactions:
- Bremsstrahlung: ~80-90%. e⁻ brakes near nucleus, emits X-ray. Continuous spectrum.
- Characteristic: Incident e⁻ ejects inner shell e⁻; outer e⁻ fills vacancy, emits specific energy X-ray. Discrete.
- Beam: Quantity (Intensity $I \propto mA \cdot (kVp)^2 / d^2$), Quality (↑kVp, ↑filtration).
⭐ High-Yield Fact: ~99% of incident electron kinetic energy converts to heat at the anode; only ~1% becomes X-rays.
EM Interactions - Photon Phate
| Interaction | Energy (Dominance) | Z Dependence | Key Outcome(s) | Relevance |
|---|---|---|---|---|
| Coherent (Rayleigh) | < 10 keV | $\propto Z^2$ | Photon scatter, no E loss | Minor haze, negligible dose |
| Photoelectric (PEA) | Low E (20-100 keV) | $\propto Z^3/E^3$ | Photon absorbed, e⁻ out, char. X-ray | High contrast (bone), ↑dose |
| Compton (CS) | Mid E (0.1-10 MeV) | $\propto \rho_e$, Z-indep. | Photon scatter (↓E), e⁻ out | Soft tissue contrast, scatter, staff hazard |
| Pair Production (PP) | > 1.022 MeV (Threshold) | $\propto Z^2$ | e⁻/e⁺ pair; Annihilation (2x 0.511 MeV) | PET, high-E RT |
⭐ Compton scattering is the most probable interaction between x-rays and soft tissue over a significant portion of the diagnostic energy range.
Radiation Quantities - Unit Roundup
| Quantity | Definition | SI Unit (Symbol) | Trad. Unit (Symbol) | Conversion |
|---|---|---|---|---|
| Exposure (X) | Ionization in air (X/γ rays) | C/kg | Roentgen (R) | 1 R = $2.58 \times 10^{-4}$ C/kg |
| Absorbed Dose (D) | Energy absorbed/mass | Gray (Gy) | rad | 1 Gy = 100 rad |
| Equivalent Dose (H) | Absorbed dose $\times$ radiation type ($w_R$) | Sievert (Sv) | rem | 1 Sv = 100 rem |
| Effective Dose (E) | Equivalent dose $\times$ tissue sens. ($w_T$) | Sievert (Sv) | rem | 1 Sv = 100 rem |
⭐ For X-rays, γ-rays, and beta particles, the radiation weighting factor is 1. Thus, for these radiations, Absorbed Dose in Gray is numerically equal to Equivalent Dose in Sievert.
High‑Yield Points - ⚡ Biggest Takeaways
- Electromagnetic radiation (EMR) travels at the speed of light (approx. 300 million m/s in vacuum).
- Photon energy is directly proportional to frequency, and inversely proportional to wavelength.
- X-rays (from electron shells) and gamma rays (from nucleus) are key ionizing EMRs in medicine.
- Inverse Square Law: EMR intensity decreases rapidly with increasing distance from the source.
- Shorter wavelength EMR possesses higher energy and greater penetration capabilities.
- EMR exhibits wave-particle duality, behaving as both waves and discrete particles (photons).
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