Interactions of Radiation with Matter Flashcards
What is the energy of a thermal neutron?
0.025 eV
What is the energy range of a slow neutron?
1 - 10 eV
What is the energy range of a fast neutron?
1 - 20 MeV
What are three slow neutron interactions?
- Radiative Capture
- Charged Particle Emission
- Fission
State the general equation for radiative capture (slow neutron interaction) and give an example (H-1).
State the general equation for charged particle emission (slow neutron interaction) and give an example (B-10).
State the general equation for fission (slow neutron interaction) and give an example (U-235).
____________ is the mechanism responsible for about 80% of fast neutron dose to human tissue.
Elastic scattering is the mechanism responsible for about 80% of fast neutron dose to human tissue.
What are the two fast neutron interactions?
- Elastic scattering
- Inelastic scattering
What is mean free path?
- The average (mean) distance of travel in the medium between interactions.
- Over the distance of one mean free path, each photon in the beam has only a 50% chance of interacting.
Demonstrate elastic and inelastic scattering of fast neutrons.
What is inelastic scattering?
- A fast neutron collides with a target nucleus, and transfer some kinetic energy to the nucleus, raising it to a higher energy level.
- When the nucleus de-excites, it will emit a gamma-ray.
What are three photon interactions?
- Photoelectric Effect
- Compton Scattering
- Pair Production
Describe photoelectric effect.
- The incoming photon transfers its full energy to an inner shell orbital electron, almost always one in the K-shell.
- The energized electron uses the excess energy to escape from the nucleus (i.e., gives up the binding energy).
- The electron escapes the atom with the remaining energy given to it by the photon (photon energy less electron binding energy).
What is the relative probability of photoelectric effect interaction per gram of absorber?
Z3/E3
Describe Compton Scattering.
- The incoming photon transfers a portion of its energy to an orbital electron.
- A lower energy photon (a Compton photon) then leaves, in a different direction, with the remaining energy.
- The Compton electron is ejected from the atom with the energy it receives minus the electron binding energy.
- Any orbital electron which has a binding energy less than about 10% of the photon energy is available to interact by a Compton process.
What is the relative probability of a Compton scattering interaction?
The interaction is independent of atomic number, and decreases with E.
Describe Pair Production.
- In the vicinity of the nucleus of an absorber atom, an incoming photon suddenly disappears, and in its place, appears an electron/positron pair.
- The rest mass for both the electron and positron is 0.511 MeV; therefore, a minimum of 1.022 MeV is required to undergo pair production.
What is the relative probability of Pair Production interaction?
The probability of pair production occurring, per gram of absorber, is directly proportional to the atomic number, Z, and increases with E.
What happens to the positron created by Pair Production?
- The positron eventually collides with an electron due to attraction of opposite charges.
- The two particles annihilate each other and create two photons each carrying 0.511 MeV of energy traveling in exactly opposite directions.
What are the linear and mass energy absorption coefficients?
- The linear energy absorption coefficient (μen) represents the fraction of energy actually removed from photons in the beam per unit distance.
- The mass energy absorption coefficient (μen/ρ) gives the fraction of energy removed per unit density of absorber.
What interaction cofficient is used in shielding calculations? Why?
- Absorption Coefficient
- Because dose is energy deposited per unit mass, calculations using absorption coefficients instead of attenuation cefficients more closely estimate the reduction in dose rate as a result of adding shielding around a photon radiation source.
What are three charged particle energy loss mechanisms?
- Ioniziation
- Excitation
- Bremsstrahlung
What is ionization?
- The complete removal of orbital atomic electrons as a result of the Coulomb force between the charged particle and the orbiting electrons.
- This process removes charge from a neutral atom and so it becomes an ion.

