13. Effect of Radiation of Matter Flashcards
What force will charged particles passing through a material predominantly interact with?
Interact with the nuclei and electrons via the EM force
Which particles will get transferred the most momentum in an interaction in a nucleus, and why?
Momentum will be transferred to the electrons from the atoms as they are the lightest particles
What are ionization energy losses?
Energy transferred in a collision which liberate electrons from atoms
- Need a lot of collisions for an ion to lose all its energy
What are ionisation energy losses proportional to when not considering very low or very high energies?
1/v^2
How is the large ion motion affected when considering ionization collisions?
Direction is mostly unaffected due to the large mass
- Range depends on the charge of the ion due to its larger density and energy losses
Describe the graph for the energy loss and the range of a radiation particle
Constant gradient, then big slope to the (Bragg) peak then a very sharp drop off as it comes to rest
- Heavier ions have a narrower peak
How is the trajectory different for electron-electron collisions
Radiation can scatter easier by large anges
- Trajectory doesn’t follow a straight path
How is the penetration different for an electron as opposed to an alpha particle
Has much lower energy losses
- Range will be much higher e.g 1800 micrometres in aluminium and an alpha particle is 3
Why can electrons cause Bremmstrahlung radiation?
As they are charged and light, and significant changes in energy or large energy collisions can cause a large acceleration
- Causes emission of radiation
Describe the relationship between ionization energy losses, energy and Bremmstrahlung energy losses
Ionization losses go down as energy goes up
Bremmstrahlung energy losses go up as energy goes up
Why is the electron range not as well defined as for ions
It is lighter and can undergo large scattering due to its low mass, so the trajectory is not linear
What are the three processes in which gamma rays interact with matter?
- Photoelectric effect
- Compton scattering
- Pair production
Describe the photoelectric effect
Photon energy is fully aborbed by an atomic electron which is liberated with KE = Photon E - BE
What is X-ray fluoresence?
If the photoelectron is emitted from an inner shell, the atom will deexcite, replacing the inner shell electron with one from the outer shell
Describe the absorption coefficient graph against gamma ray energy for the photoelectric effect
Starts high with some sharp edges, then exp. decay
- Sharp edges due to shell model
What type of electrons does the photoelectric effect interact with and why?
Only bound electrons
- Cannot conserve energy and momentum with free electrons
- As photon energy goes up, absorption prob. decreases as it sees the electron more like a free one
Define Compton scattering
When a photon scatters from a free electron
- The result is a photon with lower energy E’_photon and the electron gains KE = E_photon - E’_photon
Define pair production
One photon creates an electron positron pair
E_photon = KE_e + KE_e+ + 2mc^2
Where must pair production occur in order to conserve energy and momentum?
Near the atomic nucleus
Describe the absorption coefficient graph against gamma ray energy for the compton scattering and pair production
Compton: broad negative parabola
Pair: Starts at a significant x axis intercept, then starts a steep parabola `
Describe the dominant effect for the absorption coefficient at low, medium and high gamma ray energies
Low energy: photoelectric
Medium energy: Compton
High: Pair production
What are the two processes for neutron interactions
Moderation and neutron capture
What is moderation?
Elastic scattering in which the neutron loses energy
- Dominant for most materials
What is neutron capture?
Absorption of a neutron creating a compound nucleus
- Energy released in form of gamma rays, or charged particles which are used in detection due to their large cross section at low neutron energies
How is neutron capure used in detection?
Thermalise neutrons, and then convert them to charged particles via neutron capture
- Detected
