Physics Lecture 10: Photon Interactions

Pair Production

• The energy transferred to pair production can be calculated using the equation: E=2m0c2+KEe+KEpE = 2m_0c^2 + KE_e + KE_pE=2m0​c2+KEe​+KEp​, where m0m_0m0​ is the rest mass of an electron, ccc is the speed of light, KEeKE_eKEe​ is the kinetic energy of the electron, and KEpKE_pKEp​ is the kinetic energy of the positron.

Z-dependence

• The interaction that exhibits a Z4Z^4Z4 dependence is the photoelectric effect.

Screening Correction

• The screening correction in photon interactions accounts for the shielding of the nucleus by the surrounding electrons. This correction is especially relevant for high-energy photons.

Screening and appa_{pp}app​

• When there is complete screening, the appa_{pp}app​ value reaches its maximum, signifying full shielding of the nucleus.

Pair Production Cross-section

• Increasing photon energy, within the range of pair production, increases the cross-section of the interaction.

Logarithmic Energy Dependence

• The interaction that exhibits a Z2Z^2Z2 dependence alongside a logarithmic energy dependence is the Compton scattering.

Photon Energy Dependence

• The interaction that is primarily affected by photon energy, decreasing as 1/hv31/hv^31/hv3 is the photoelectric effect.

Z and Logarithm of Energy

• The photon interaction that exhibits dependence on both ZZZ and the logarithm of energy is pair production.

Photoelectric Effect and Atomic Number

• As atomic number (Z) increases, the probability of the photoelectric effect (PE) also increases.

Photoelectric Effect and Photon Energy

• As photon energy (E) increases, the probability of the photoelectric effect decreases.

Atomic Attenuation Coefficient Cliffs

• The "cliffs" observed on graphs of the atomic attenuation coefficient for the photoelectric effect around absorption edges indicate the binding energy of electrons in a specific shell. When photon energy exceeds the binding energy, the photoelectric effect becomes much more probable.

Mass Attenuation Coefficient and Z

• The mass attenuation coefficient generally increases proportionally to the atomic number (Z), although the exact relationship can be more complex.

Photoelectric Effect and Electron Emission

• The electron produced during the photoelectric effect is a photoelectron, and its emission is characterized by being energetic and its ejection from the atom.

Atomic Attenuation Coefficient Regions

• The three distinct regions in the atomic attenuation coefficient for the photoelectric effect are defined by:
  • Low energy region: Photoelectric effect dominates
  • Intermediate energy region: Compton scattering dominates
  • High energy region: Pair production dominates

Photoelectric Atomic Cross-section and Energy

• At low energies, the photoelectric atomic cross-section is inversely proportional to the fourth power of photon energy.

Compton Scattering Mass Energy Transfer Coefficient

• During the Compton scattering process, the mass energy transfer coefficient generally decreases as photon energy increases.

Primary Charged Particle

• A primary charged particle used in medical physics is an electron.

511 keV Photons

• The interaction that results in the release of two 511 keV photons is positron annihilation.

Lead Interaction Energy Level

• For calculations involving the mass energy absorption coefficient in lead, the relevant photon energy is 1 MeV.

Ionization Responsible Particle

• In medical physics, the charged particle interaction primarily responsible for ionization is the electron interaction.

Auger Effect Secondary Particle

• During the Auger effect, the secondary charged particle produced is the Auger electron.

Heavy Ion Therapy

• Particle therapy using heavy ions, such as Carbon-6, is called hadron therapy.

Positron-Electron Interaction

• When a positron interacts with an electron, annihilation occurs, resulting in the production of two 511 keV photons.

Photon Absorption by Lead

• The absorption of a 2 MeV photon by lead is primarily attributed to pair production.

Photoelectric Effect Outcome

• The primary result of the photoelectric effect is the emission of a photoelectron.

Photon-Electron Interaction

• The photon interaction involving the production of both a photon and an electron is Compton scattering.

Pair Production Energy Dependence

• The energy dependency of the pair production interaction can be described as increasing with photon energy above a threshold (1.022 MeV).

Positron Annihilation

• The interaction where a positron annihilates with a resting electron is specifically called positron annihilation.

Compton Recoil Electron Energy

• For photon energies between 10 keV and 3 MeV, kinetic energy of a Compton recoil electron increases with increasing photon energy.

Energy Transfer in Compton Scattering

• The primary effect of energy transfer to the medium in Compton scattering is the production of heat.

Complete Energy Transfer

• The interaction that results in complete energy transfer from the photon to the medium is the photoelectric effect.

Photoelectron Average Kinetic Energy

• The average kinetic energy of a photoelectron depends on the binding energy of the electron in the atomic shell, which can be influenced by the specific atom.

Photon Effect on Electron (Thompson Scattering)

• According to Thompson scattering, an incoming photon causes an electron to oscillate, which then emits radiation. The photon itself is not absorbed.

Photon Energy Increase in Compton Effect

• As photon energy increases in the Compton effect, the Compton scattering angle decreases, meaning the scattered photon is more likely to be deflected in the forward direction.

Klein-Nishina Coefficient

• The Klein-Nishina Coefficient in the Compton effect quantifies the probability of Compton scattering for a given photon energy and angle.

Compton Total Cross-section at Low Energies

• At low energies, the total Compton cross-section is proportional to the square of the electron's classical radius.

Compton Scattering Angles

• At low energies, side scattering is more prevalent compared to back and forward scattering in the Compton effect.

Compton Effect Electron Assumption

• The analysis of the Compton effect assumes that electrons are free and stationary.

Low Photon Energy Impact

• Compared to higher energies, low photon energy results in a smaller energy transfer to the electron, with a larger scattering angle.

Compton Atomic Cross-section Dependence

• The Compton atomic cross-section depends on the atomic number (Z) and the photon energy.

Photoelectric Effect Energy Dependence at High Energy

• At high photon energy (hv), the energy dependence of the photoelectric effect follows a trend of decreasing with increasing photon energy.

Characteristic X-Ray Emission

• When only characteristic x-rays are emitted during the photoelectric effect, the total energy transferred is the difference between the incident photon energy and the binding energy of the electron.

Photoelectric Atomic Number Dependence

• The photoelectric effect exhibits an atomic number (Z) dependence, characterized by an increase in probability with increasing Z.

Auger Electron Emission

• If only Auger electrons were emitted during the photoelectric effect, hypothetically, the total energy transferred would be equal to the difference between the incident photon energy and the binding energy of the electron, minus the energy associated with the Auger electron.

Energy Transferred with Auger Electrons

• The energy transferred to charged particles if only Auger electrons are emitted is equal to the energy of the Auger electron.

Total Energy Transfer in Photoelectric Effect

• The total energy transferred during the photoelectric effect is determined by adding the energy of the photoelectron and the characteristic x-rays that are emitted.

No Characteristic X-ray Emission

• If no characteristic x-rays are produced during the photoelectric effect, all the energy of the incident photon is transferred to the photoelectron.

Mass Attenuation Coefficient

• In the photoelectric effect, the term 'mass attenuation coefficient' refers to the probability per unit mass that a photon will interact via the photoelectric effect.

This lecture focuses on various interactions between photons and charged particles. It reviews key concepts such as the photoelectric effect, pair production, and attenuation coefficients, along with important equations and diagrams. Students will deepen their understanding of photon behavior and related phenomena in physics.