Lecture 5 Flashcards
Define the photoelectric effect
When a photon with a given energy, greater than the work function (ionisation energy), hits an atom and ionises it. The photon is absorbed and a photoelectron is produced with kinetic energy equal to the difference between the photon energy and work function.
What type of particle is absorbed in the photoelectric effect in health physics?
A gamma (or x-ray) photon
Define Compton scatter
An incident photon with a given energy interacts with an electron of an atom which is scattered so that a photon and an electron are emitted. This is a perfectly elastic collision so the photon does not hit the nucleus.
Compton scatter occurs at a _________ energy than the photoelectric effect.
Greater
What is the equation for the energy of the scattered gamma photon in Compton scatter?
E’ = final energy
E = initial energy
mₑ = electron mass
c = speed of light
θ = scatter angle
What is the equation for the energy of the scattered (recoil) electron in Compton scatter?
E’ₑ = final electron energy
Eᵧ = initial gamma energy
E’ᵧ = final gamma energy
mₑ = electron mass
c = speed of light
θ = scatter angle
How can the energy equations of the gamma photon and the electron be found for Compton scatter?
Using conservation of energy and momentum rules.
Describe a Compton scattering diagram
Describe the Compton scattering graph for energy change with change in scatter angle
Define pair production
The opposite of annihilation between a positron and an electron. A positron and electron can be spontaneously created if a gamma photon has a large enough energy.
Where does pair production occur?
It occurs near a nucleus but does not ionise the atoms.
What is the equation for the total kinetic energy of pair production?
KE = Eᵧ - 2 * 511keV
KE = total kinetic energy of positron and electron
Eᵧ = energy of gamma photon
What are the three ways that gamma radiation interacts with matter?
- Photoelectric effect
- Compton scattering
- Pair production
What is the attenuation of x-rays and gamma rays?
The amount of absorption and scattering of x-ray or gamma-ray energy when the rays are directed into an object. This occurs because some of the photons interact with the particles in the matter.
What is the shape of the attenuation curve for x-rays and gamma rays?
Exponential
Define Beer’s law
The formula that relates the intensity of a beam of photons to the distance moved.
What is the differential equation for Beer’s law?
I(x) = intensity
x = distance
µ(x) = linear attenuation coefficient
What is the equation for Beer’s law?
I(x) = intensity
x = distance
I₀ = initial intensity
µ(x) = linear attenuation coefficient
What is the mass attenuation coefficient?
It is the linear attenuation coefficient divided by the material density. It is usually expressed in cm²/g.
What is the equation for the mass attenuation coefficient?
C = mass attenuation coefficient
µ = linear attenuation coefficient
ρ = material density
What is the linear attenuation coefficient proportional to?
- Density
- Cube of atomic number
- Inverse cube of photon energy
What type of interaction of radiation with matter dominates for light materials in terms of attenuation?
Compton scattering
What type of interaction of radiation with matter dominates for heavy materials in terms of attenuation?
The photoelectric effect
What is the shape of the mass attenuation v. photon energy graph for soft tissue?
What is a Compton edge?
A feature of a spectrograph that results from Compton scatter in a detector. It looks like a small peak followed by a large drop in energy.
