03 Interaction of EM ionizing radiation with matter

Introduction to Medical Physics

  • Focus on Interaction of Electromagnetic (E/M) Ionizing Radiation with Matter

  • Presented by Prof. Panagiotis Papagiannis from Medical Physics Lab

Ionizing E/M Radiation

  • Definition: Electromagnetic radiation with energy > 10 keV

  • ** Key Components:**

    • X-rays

    • Gamma rays

  • Energy Conversion: 1 eV = kinetic energy of an electron accelerated by a potential of 1V

    • Example: 10 keV = 103 x 1.6 x 10^-19 Cb V = 1.6 x 10^-16 Joules

E/M Spectrum Characteristics

General Properties by Spectrum Type

  • Radio Waves:

    • Frequency: 10^5 - 3 x 10^10 s^-1

    • Wavelength: 3 km - 0.01 m

    • Energy: 413 peV - 124 μeV

    • Properties: Reflects/absorbed by metals, transparent to biological materials

  • Microwaves:

    • Frequency: Upper end of radio waves spectrum; energy relates to molecular rotation

  • Infrared Radiation:

    • Frequency: 3 x 10^14 to 4.3 x 10^14 s^-1

    • Wavelength: 700 nm - 1 μm

    • Energy: 1.77 eV - 3.1 eV

    • Properties: Absorbed significantly by matter; corresponds to molecular oscillations

  • Visible Light:

    • Frequency: 4.3 x 10^14 to 7.5 x 10^14 s^-1

    • Wavelength: 400 nm - 700 nm

    • Energy: 1.77 eV - 3.1 eV

    • Properties: Strongly absorbed by matter; significant at high temperatures

  • Ultraviolet Radiation:

    • Frequency: 7.5 x 10^14 to 3 x 10^16 s^-1

    • Wavelength: 400 nm - 10 nm

    • Energy: 3.1 eV - 124 eV

    • Properties: Strongly absorbed; does not penetrate matter's surface

  • Soft X-rays and Diagnostic X-rays:

    • Produced through internal electron excitations; significant absorption; causes ionization

  • Gamma rays:

    • Produced by radioactive decay; penetrative; limited absorption; causes ionization

Attenuation of Ionizing E/M Radiation

  • Attenuation Concept: Reduction in intensity as it travels through matter

  • Dependent on:

    • Linear attenuation coefficient (μ)

    • Thickness (x) of material

  • Mathematical Model:

    • Exponential decay represented as N = N0 exp(-μx)

    • I = I0 exp(-μx) where I = intensity, I0 = initial intensity, μ = linear attenuation coefficient

Factors Affecting Attenuation Coefficient (μ)

  • Depends on:

    • Photon energy (E)

    • Atomic number of the material (Z)

    • Density of the material (ρ)

  • Types of Attenuation Coefficients:

    • Linear (μ)

    • Mass (μ/ρ)

Conditions for Exponential Attenuation Law

  • Applies primarily to mono-energetic beams

  • Not applicable for poly-energetic beams

  • Narrow vs Broad Beam Conditions:

    • Narrow beam: exponential law applicable

    • Broad beam: additional factors must be considered

Interaction Mechanisms with Matter

  1. Coherent (Rayleigh) Scattering:

    • No ionization occurs

  2. Photoelectric Effect:

    • Photon absorption causes ionization; emission of a photoelectron

  3. Compton Scattering:

    • Photon scatters off a free electron, causing both ionization and energy transfer

  4. Pair Production:

    • Photon energy transformed into an electron-positron pair above the threshold of 1.02 MeV

Summary of Interactions

  • Interaction Types:

    • Coherent

    • Photoelectric

    • Compton

    • Pair Production

  • Dependence on Energy:

    • Various interactions demonstrate different probabilities based on the photon energy (E) and atomic number (Z)

Suggested Study Materials

  • Refer to "Hendee's Physics of Medical Imaging, 5th Edition" for detailed concepts

  • Online resources and concept check questions available on eclass MEDICEN 54723.

  • Explore works on "Radiation Interaction with matter" and "Compton scattering" at HyperPhysics (available online).

Introduction to Medical Physics

Focus on Interaction of Electromagnetic (E/M) Ionizing Radiation with Matter

Presented by Prof. Panagiotis Papagiannis from Medical Physics Lab

Ionizing E/M Radiation

  • Definition: Electromagnetic radiation with energy greater than 10 keV, capable of causing ionization in matter.

  • Key Components:

    • X-rays: High-energy radiation produced by the interaction of electrons with matter.

    • Gamma rays: Emitted from the nucleus of radioactive atoms, with highly penetrating abilities.

  • Energy Conversion:

    • 1 eV (electron-volt) is the kinetic energy gained by an electron accelerated through a potential difference of 1 volt.

    • Example: 10 keV = 10^3 x 1.6 x 10^-19 Cb V = 1.6 x 10^-16 Joules.

E/M Spectrum Characteristics

General Properties by Spectrum Type

  • Radio Waves:

    • Frequency: Ranges from 10^5 to 3 x 10^10 s^-1

    • Wavelength: Between 3 km and 0.01 m

    • Energy: Ranges from 413 peV to 124 μeV

    • Properties: Reflects off or is absorbed by metals, while being transparent to biological materials.

  • Microwaves:

    • Occupies the upper end of the radio waves spectrum. The energy is closely related to the rotation of molecules, playing a significant role in heating and communication technologies.

  • Infrared Radiation:

    • Frequency: Between 3 x 10^14 and 4.3 x 10^14 s^-1

    • Wavelength: Ranges from 700 nm to 1 μm

    • Energy: Between 1.77 eV and 3.1 eV

    • Properties: It is significantly absorbed by matter, correlating with molecular oscillations, important for thermal imaging and heat technologies.

  • Visible Light:

    • Frequency: Ranges from 4.3 x 10^14 to 7.5 x 10^14 s^-1

    • Wavelength: Between 400 nm and 700 nm

    • Energy: Between 1.77 eV and 3.1 eV

    • Properties: Strongly absorbed by matter, particularly significant at high temperatures; responsible for human vision.

  • Ultraviolet Radiation:

    • Frequency: Ranges from 7.5 x 10^14 to 3 x 10^16 s^-1

    • Wavelength: Between 400 nm and 10 nm

    • Energy: Between 3.1 eV and 124 eV

    • Properties: Strongly absorbed by matter, with limited penetration powers; important for skin reactions and vitamin D synthesis.

  • Soft X-rays and Diagnostic X-rays:

    • Produced through internal electron excitations; exhibit significant absorption which can cause ionization, widely used in medical imaging to diagnose conditions.

  • Gamma rays:

    • Emitted by radioactive decay processes; they possess high penetration abilities and limited absorption, with substantial ionizing potential, often utilized in cancer treatment strategies.

Attenuation of Ionizing E/M Radiation

  • Attenuation Concept: Refers to the reduction in intensity of radiation as it travels through material.

  • Dependent Factors:

    • Linear attenuation coefficient (μ)

    • Thickness (x) of the absorbing material

  • Mathematical Model:

    • Exponential decay is mathematically represented as N = N0 exp(-μx)

    • The intensity I is described by the equation I = I0 exp(-μx), where:

      • I: remaining intensity

      • I0: initial intensity

      • μ: linear attenuation coefficient

Factors Affecting Attenuation Coefficient (μ)

  • The attenuation coefficient is influenced by:

    • Photon energy (E)

    • Atomic number of the material (Z)

    • Density of the material (ρ)

  • Types of Attenuation Coefficients:

    • Linear (μ): Describes how intense the radiation is reduced per unit thickness.

    • Mass (μ/ρ): Takes into account the density of the material being traversed, useful for different material comparisons.

Conditions for Exponential Attenuation Law

  • The exponential attenuation law primarily applies to:

    • Mono-energetic beams: having a single energy level.

    • Narrow vs. Broad Beam Conditions:

      • Narrow beam: where the exponential law is applicable due to minimal scattering effects.

      • Broad beam: requires consideration of additional factors, such as scattering and diffraction effects.

Interaction Mechanisms with Matter

  • Coherent (Rayleigh) Scattering:

    • Interaction where no ionization occurs; the photon changes direction without energy loss.

  • Photoelectric Effect:

    • Involves photon absorption resulting in ionization, leading to the emission of a photoelectron; significant in low energy ranges.

  • Compton Scattering:

    • A photon interacts with a free electron resulting in both ionization and energy transfer; this interaction dominates at intermediate photon energies.

  • Pair Production:

    • Occurs when photon energy exceeds 1.02 MeV, transforming the energy of the photon into an electron-positron pair; significant at high energies.

Summary of Interactions

  • Interaction Types:

    • Coherent

    • Photoelectric

    • Compton

    • Pair Production

  • Dependence on Energy:

    • The probability of different interactions varies based on photon energy (E) and the atomic number (Z) of the interacting material, influencing the applications in medical imaging and radiation therapy.

Suggested Study Materials

  • For detailed concepts, refer to "Hendee's Physics of Medical Imaging, 5th Edition".

  • Access online resources and concept check questions available on eclass MEDICEN 54723.

  • Explore additional works on "Radiation Interaction with Matter" and "Compton Scattering" at HyperPhysics (available online) for comprehensive understanding.