Radiation Production and Characteristics

Fundamentals of Radiation Physics

Introductory Perspective

• Radiologic Technology rests on understanding matter, energy and the electromagnetic spectrum (with x-rays as a subset).
• Precision in physics removes subjective uncertainties; identical measurement methods yield identical results for all observers.
• Successive lectures continually relate back to these foundational measurement principles.

Systems of Measurement

• Physicists value simplicity: only three base (fundamental) measurable quantities exist:
  • Mass
  • Length
  • Time
• All other quantities are derived (secondary) or special.
• International metrology overseen by BIPM (Paris). Standards are periodically re-defined for higher precision.

Base Quantities & Standards

• Mass
  • Defined by the platinum-iridium prototype kilogram kept in Paris (same mass as 1000\,\text{cm}^3 of water at 4\,^{\circ}!\text{C}).
  • Units: kilogram (SI), newton (force/weight), pound (British).
  • Radiation relevance: particulate radiations ((\alpha,\;\beta)) have mass (amu basis); photons (x, (\gamma)) are mass-less but possess mass–energy equivalence.
• Length
  • Historic platinum-iridium meter bar; re-defined 1960 via Kr-86 orange line; currently: distance light travels in 1/299\,792\,458\,\text{s}.
  • Radiography uses: patient thickness (caliper), Source-to-Image-Distance (SID) tailored per anatomy.
• Time
  • From Earth rotation (mean solar day) → tropical year (1956) → cesium-133 atomic clock (1964); accuracy ≈ 1 s in 5000 y.
  • Imaging uses: exposure time, film processing (develop & fix) durations.

Derived (Secondary) Quantities

• Combine bases algebraically.
  • Area = l^2
  • Volume = l^3
  • Density = \dfrac{m}{l^3}
  • Velocity = \dfrac{l}{t}; speed of light c = 3.0\times10^8\,\text{m/s} (x & (\gamma) travel at c).

Special Quantities (Radiologic Science)

• Exposure (x)
• Absorbed dose (D)
• Effective dose (H)
• Radioactivity (A)

Three Systems of Units

• SI adds Ampere, Kelvin, mole, candela.

Mechanics

Core Concepts

• Mechanics = physics of rest (statics) & motion (dynamics).
• Motion descriptors:
  • Velocity: v = \dfrac{d}{t} (vector).
  • Acceleration: a = \dfrac{v}{t}; unit \text{m/s}^2. Constant velocity ⇒ a = 0.
• Speed (scalar) = distance per time.

Newton’s Laws

1. Inertia – a body remains at rest/constant velocity unless acted upon by external force. Inertia ≡ mass.
2. Force – F = ma. Units:
   • 1\,\text{kg·m/s}^2 = 1\,\text{N}
   • 1\,\text{g·cm/s}^2 = 1\,\text{dyne}
   • Pound-force in British.
3. Action–Reaction – forces come in equal & opposite pairs.

Related Quantities

• Weight: W = mg; g=9.8\,\text{m/s}^2 (Earth), 1.6\,\text{m/s}^2 (Moon). Zero gravity ⇒ weightlessness.
• Momentum: p = mv.
• Work: W = Fd; unit joule (J) where 1\,\text{J}=1\,\text{N·m}=1\,\text{kg·m}^2\text{/s}^2.
• Power: P = \dfrac{W}{t}; SI watt (W = J/s); British horsepower.
• Energy Conservation – energy converts forms but total remains constant.
  • Kinetic KE = \dfrac{1}{2}mv^2
  • Potential PE = mgh
• Heat: kinetic energy of molecular motion; 1 calorie raises 1 g H₂O by 1\,^{\circ}!\text{C}. Transfer modes: conduction, convection, radiation; x-ray tube cools mainly by infrared radiation.

Atomic Structure

Historical Models (timeline)

• Greek 4-element concept → Dalton (hook-and-eye) → Thomson (plum pudding) → Rutherford (nuclear) → Bohr (planetary); modern QCD maps >100 sub-particles but radiology focuses on electrons, protons, neutrons.

Fundamental Particles

• Electron: m_e = 9.1\times10^{-31}\,\text{kg}=0.000549\,\text{amu}, charge −1.
• Proton: m_p = 1.673\times10^{-27}\,\text{kg}\approx1\,\text{amu}, charge +1.
• Neutron: m_n = 1.675\times10^{-27}\,\text{kg}, neutral.

Electron Shells & Binding

• Shells K–Q, principal quantum # n; max electrons 2n^2 (Pauli exclusion).
• Outer-shell (valence) e⁻ count equals periodic group; shell number equals period; Octet rule: max 8.
• Binding energy highest at K; designated E_b.

Atomic Nomenclature

• Atomic number Z = protons = electrons (neutral).
• Mass number A = Z + N (nucleons).
• Isotopes (same Z), isobars (same A), isotones (same N), isomers (same Z & A, different energy state).

Fundamental Forces of Nature

• Four forces ranked by strength (strong > electromagnetic > weak > gravity).
  • Strong nuclear – binds nucleons; neutrons add stability.
  • Electromagnetic – binds electrons to nucleus; like charges repel, unlike attract.
  • Weak nuclear – governs beta decay; converts particles (n → p + e⁻ + (\bar\nu_e)).
  • Gravity – weakest (≈10^{38}× weaker than strong); dominant astronomically.

Atomic Bonding

• Covalent – sharing electrons (polar vs non-polar); forms molecules (e.g., carbon chains).
• Ionic – electron transfer; ions (cations +, anions −).
• Metallic – communal sharing; delocalised electrons → good conductivity.

Radioactivity

Radioactive Decay Basics

• Unstable nuclei (radionuclides) reach stability via particle/energy emission.
• Common decay modes:
  • \alpha (He-4)
  • \beta^- / \beta^+
  • \gamma
  • Electron capture
  • Internal conversion
  • Spontaneous fission
  • Isomeric transition
  • Neutron emission

Alpha ((\alpha))

• 2p + 2n, 4\,\text{amu}, +2 charge; high ionisation, short range (<0.1 cm tissue).

Beta ((\beta^- / \beta^+))

• Electrons/positrons, negligible mass, charge ±1; range up to several mm tissue.

Gamma ((\gamma))

• Photon emission; no mass/charge; follows other decays to shed excess energy.

Electron Capture & Internal Conversion

• EC: nucleus captures inner e⁻, p→n + (\nu_e).
• IC: excited nucleus transfers energy to orbital e⁻ → ejection without particle creation.

Spontaneous Fission & Isomeric Transition

• Heavy nuclides (A>~240) split into fragments.
• Metastable states (e.g., ^{99m}_{43}\text{Tc}) decay via \gamma or IC.

Half-Life Concepts

• Physical t_{1/2} – time to reduce activity to 50 % by decay.
• Biological t_{1/2} – removal by physiologic elimination.
• Effective t_{1/2} combines both.
• Decay law: A = A0 e^{-\lambda t} with \lambda = \dfrac{0.693}{t{1/2}}.

Radiation Basics & Sources

Definitions & Modes

• Radiation = energy emitted/transferred through space.
• Ionising (x, (\gamma), (\alpha), (\beta)) vs Non-ionising (ultrasound, MRI RF).
• Ion pair = freed electron (−) + residual positive ion.
• Exposure modes: irradiation vs contamination.

Natural Background (~3 mSv/yr)

• Cosmic rays (↑ with altitude/latitude).
• Terrestrial (uranium/thorium in soil).
• Internal radionuclides (K-40, C-14).
• Radon gas (largest contributor; emits (\alpha)).

Man-Made

• Medical x-rays (largest artificial source).
• Nuclear industry, consumer products, fallout.

Radiation Categories

• Particulate: (\alpha), (\beta).
• Electromagnetic: x-ray, (\gamma).

Comparative Physical Characteristics

Human Exposure Types

1. Background
2. Medical
3. Occupational
4. Public (bystanders).

Radiological Units & Quantities

• Exposure (x): C/kg (SI); roentgen (R). 1\,\text{R}=2.58\times10^{-4}\,\text{C/kg}.
• Absorbed Dose (D): Gray (Gy); 1\,\text{Gy}=100\,\text{rad}.
• Dose Equivalent / Effective Dose (H): Sievert (Sv) where H = D \times \sum w_R; 1\,\text{Sv}=100\,\text{rem}.
• Activity (A): Becquerel (Bq); 1\,\text{Ci}=3.7\times10^{10}\,\text{Bq}.
• Practice unit conversions (μCi→mBq, mSv→μrem, etc.).

Photon Wave–Particle Duality

Sine-Wave Model

• Photons described by frequency f, wavelength \lambda, velocity c.
• Wave equation: c = f\lambda.
• Frequency ↔ wavelength inversely related; amplitude independent.

Quantum Relations

• Planck: E = hf with h = 6.63\times10^{-34}\,\text{J·s}=4.15\times10^{-15}\,\text{eV·s}.
• Relativity: E = mc^2 (mass-energy equivalence; 1 J = 6.24\times10^{18} eV).

Inverse Square Law

• Intensity I \propto \dfrac{1}{d^2}; \dfrac{I1}{I2}=\dfrac{d2^2}{d1^2}.

Duality Statement

• X-ray vs visible photon identical except for energy; both exhibit wave & particle traits.

Historical Perspectives (X-Ray Discovery & Evolution)

1895 – Wilhelm Roentgen

• Accidental observation of barium-platinocyanide fluorescence near Crookes tube.
• Published within weeks; awarded 1901 Nobel Prize; first medical image (wife’s hand).

Technological Milestones

• 1896: Fluorescent intensifying screen (Pupin).
• 1898: Edison’s fluoroscope; withdrew research after assistant Clarence Dally’s fatal injuries.
• 1904: Double-emulsion radiography (Leonard).
• 1907: Snook interupterless transformer (stable high-kV supply).
• 1913: Coolidge hot-cathode vacuum tube (modern tube design) + Bucky stationary grid; Potter-Bucky moving grid 1921.
• 1946-50: Image intensifier adaptation.
• 1960-present: Ultrasound, Gamma camera, CT, PET, MRI, Digital radiography.

Radiation Safety Evolution

• Early frequent burns, epilation, anemia; 1904 first US fatality.
• Protective filters/collimators (Rollins), lead apparel, monitoring devices → modern safe occupation.

Principal X-Ray Properties (Diagnostic Range)

• Invisible, neutral, mass-less, travel at c, straight-line divergent, polyenergetic (kVp defines max energy), cause fluorescence/film exposure, penetrate & attenuate matter, produce secondary radiation, biologically hazardous.

X-Ray Production

Imaging System Components

1. Operating console (kVp, mA, time).
2. High-voltage generator.
3. **