medphys 2 midterm

Absorbed Dose

Energy deposited per unit mass of medium.

Electromagnetic Radiations

X-rays and gamma rays.

Particulate Radiations

Includes electrons, protons, alpha particles, and heavy charged particles.

Charged Particles

Directly ionizing radiation.

Indirectly Ionizing Radiation

Includes electromagnetic radiations and neutrons.

Non-Ionizing Energy Threshold

E

Average Energy to Produce Ion Pair in Air (W)

33.85 eV.

Specific Ionization (SI)

The number of ion pairs produced per unit track length.

Linear Energy Transfer (LET)

Average loss in energy per unit length of the path of the incident radiation.

LET Equation

LET = SI * W.

Narrow Beam Attenuation Formula

N = N_0 * exp(-mu * x).

Mu (μ)

Linear attenuation coefficient, fraction of photons that interact per unit thickness.

Half Value Layer (HVL)

Thickness that reduces beam intensity to 50%.

HVL Calculation

HVL = ln2 / mu.

TVL Calculation

TVL = ln10 / mu,

HVL for Mono-Energetic Beams

Only mono-energetic beams can be characterized by a single HVL.

Energy Loss Fraction (g)

The fraction 'g' of energy lost, particularly in the MeV range.

Photoelectric Effect

Collision between photon and atom results in ejection of a bound electron.

Photon Energy Relationship

KE = hv - E_b (kinetic energy of ejected electron).

Interaction Probability (photoelectric effect)

Interaction probability = Z³/(hv)³.

Coherent Scattering

No energy is converted to KE; only redirection of the photon occurs.

Compton Scattering

Interaction between photon and electron; some energy is transferred to the electron.

Threshold Energy for Pair Production

1.022 MeV.

Photonuclear Reaction Energy Threshold

Typically 8-10 MeV.

Interactions of Neutrons (KE < 10 MeV)

Elastic collisions with hydrogen nuclei account for most energy deposition.

Elastic and Inelastic Neutron Collisions

KE>10 MeV contributes to energy loss through inelastic scattering.

Thermal Neutrons

Neutrons with E

Neutron Capture Interactions

Most significant in nitrogen and hydrogen in tissue.

Intermediate and Fast Neutron Dominance

Below 100 eV dominated by (n,p) reactions in nitrogen.

Charged Particle Interactions

Characterized by a range, a finite distance calculated from stopping power.

Bragg Peak

A phenomenon where a charged particle transfers more energy at low KE, observed at the end of the track.

Radiative Stopping Power

Energy loss due to bremsstrahlung only observed for electrons.

Mass Collision Stopping Power

Energy loss per unit track length in producing ionization.

Energy Loss Ratio

Approximately E_k * Z / 820.

Memorize Neutron Absorption Cross Section Graph

Graph to understand neutron interactions.

Memorize Percent Depth Dose Graph

Important for radiation therapy planning.

Memorize Stopping Power Graph

Key for understanding energy transfer mechanisms.

Low Mass Effect

Bragg Peak is never observed for electrons due to their low mass.

Soft Collisions

Collisions where energy is minimally transferred.

High Probability Interaction

Photoelectric effect dominates at low energies.

Energy Transfer Mechanism

Slower moving charged particles transfer more energy.

Threshold for Triplet Production

2.044 MeV.

Interaction Probability of Photons

Higher probability near binding energy of electrons.

Photonuclear Interaction Energy

Dependent on atomic number and beam energy.

Elastic Neutron Scattering Contribution

Significant in energy deposition above 100 eV.

Coulomb Force Interactions

Charged particles interact primarily through the Coulomb force.