Rad Physics - Photon Interactions

Photons travel __________ (speed?)

At the speed of light

What is the speed of light?

3.00 x 10^8 m/s

What does a low intensity beam mean in terms of number of photons?

Low number of photons per square cm

Isotropic definition

emitted evenly in all directions
- Iso = Same

What equation can you use for isotropic interactions?

Inverse Square Law

What is the Inverse Square equation

(Dknown/Dunknown)^2 x Intensity known

With 0.5 distance, the intensity is ____x as intense

4x

Radiation causes what kind of damage?

Biological damage, to DNA

What is Attenuation?

The loss of energy when a beam travels through some material

The change in beam energy intensity depends on ____________ __________ and _______________ _____________ ____________

Beam energy and Attenuating Material Used

What is the linear attenuation coefficient?

The fraction of a beam that is absorbed or scattered per unit thickness of the absorber

What symbol is used for the linear attenuation coefficient

Mu

Linear attenuation coefficient depends on?

Beam energy and material type

What is the most accurate method for specifying x-ray beam quality?

HVL

A hard beam will have a ___________ HVL and ___________ penetrating ability than a soft beam.

high HVL, greater penetrating ability

High HVL = _____________ beam

Harder

What is the HVL equation?

HVL = 0.693/Linear Attenuation Coefficient

Directly Ionizing means?

Delivers energy directly towards matter

Which particles are directly ionizing?

electrons and protons

Indirectly ionizing means?

Energy is transferred to charged particles, THEN is delivered to matter

Which particles are indirectly ionizing?

Neutrons and Photons

As beam energy increases, photon/e- interactions ____________

decrease

What is a photoelectron?

An electron that has been ejected from a nucleus d/t a photon interaction

What are the 3 types of photon/e- interactions?

1. Coherent (Raleigh) Thomson
2. Photoelectric
3. Compton (Incoherent) Scattering

Which photon/e- interaction loses no energy?

Coherent (Raleigh) Thomson

What occurs during Coherent/Raleigh Thomson interactions?

Photons come close to the atom and the energy is absorbed.
- The energy/wavelength of photon emitted = incident photon

Coherent occurs with what energies?

<10KV

What happens during photoelectric interactions?

Photon interacts with inner shell electron. Photon is absorbed COMPLETELY BY Electron. Inner electron is ejected from atom.

Photoelectric interactions cause what?

Characteristic x-rays

Photoelectric occurs with what energies?

<1MV

Which interaction is common in XRT?

Compton Scatter

Wat happens during Compton scatter?

Photon interacts with OUTER shell electron. Electron absorbs some energy and is Ejected.
- Different energy/wavelength of scattered photon vs incident

What are the 2 photon interactions with the nucleus?

1. Pair Production
2. Photonuclear

What occurs during pair production?

Photon interacts with nucleus electric field and DISAPPEARS
- Creates an e-/e+ pair

What is the minimum energy for the incident photon in pair production?

1.02 MV

Pair Production occurs with what energies?

>10MV

What happens during photonuclear interaction?

Photon HITS nucleus and is completely absorbed. Nucleus = excited. Emits Neutrons and Gamma Rays

Photonuclear occurs with what energies?

>10MV AND high Z materials