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.
What is an ion pair?
The combination of an electron removed through ionization and the residual positive ion.
In most materials, the amount of energy needed to produce an ion pair (W) is about ___ to ___ eV.
In most materials, the amount of energy needed to produce an ion pair (W) is about 30 to 40 eV.
What is the W-value for air?
33.9 eV
What is excitation?
- In contrast to ionization, insufficient energy is transferred to the orbiting electron to break the electrical binding force, so the electron merely jumps up to a higher atomic energy level rather than leaving the atom entirely.
- In this case, the electron is still bound to the atom so that the electrical neutrality of the atom is not disturbed.
What is bremsstrahlung?
- When a charged particle travels past a nucleus, Coulombic forces cause a deflection in the path of the charged particle.
- The directional change and the speed of the particle is reduced due to the energy loss.
- The energy lost by the particle is emitted as electromagnetic energy in the X-ray region of the electomagnetic spectrum.
What is stopping power (S)?
- Units: MeV cm-1
- The average energy lost by a charged particle per unit distance of travel.
- Analogous to the concept of the attenuation coefficient for gamma rays, and describes how effective the absorber is in removing energy from a beam of charged particles.
What is mass stopping power (S/ρ)?
The stopping power per unit density.
What is specific ionization?
- The average number of ion pairs produced per unit distance of travel of a charged particle.
- Specific ionization = S/W (ion pairs cm-1)
Rule of Thumb: Alpha particles and the skin
Alpha particles up to 7.5 MeV are stopped in the dead layer of normal human skin.
Rule of Thumb: Beta particles in air
Beta particles will penetrate about 4 meters in air per MeV of energy.
Rule of Thumb: Beta particles in soft tissue
Beta particles will penetrate about 0.5 cm in soft tissue per MeV of energy.
Rule of Thumb: Beta particles and the human skin
Beta particles up to 70 keV are stopped in the dead layer of normal human skin.
What is linear energy transfer (LET)?
- Units: MeV cm-1
- The average energy locally deposited in an absorber per unit distance of travel of a charged particle.
- Analogous to the energy absorption coefficient for gamma rays.
What is the chief difference between linear energy transfer (LET) and stopping power (S)?
LET is concerned primarily with the energy left behind in the absorber while S focuses on the energy retained by a charged particle.
Describe
Coherent (Rayleigh) scattering
- All energy of the photon is maintained.
- Photon interacts coherently (uniformly) with orbital electrons, without a change in wavelength.
- Complex models of photon interactions must take into account scattering angles from coherent scattering.
Graph
Probabilities of photon interaction as a graph of Absorber Z vs. Energy of Photon (MeV)
Graph
Photon interactions as a function of attenuation coefficient (cm2 g-1) vs. Energy (MeV)
What condition must exist for absorbed dose to be approximately equal to KERMA?
- Radiative energy losses by liberated charged particles must be negligible ⇒ Eab = Etr
- Eab ⇒ Average energy absorbed per interaction.
- Fluence of indirectly ionizing radiation must be uniform.
- Photon energy cannot be so large that liberated electrons only travel in the forward direction.
- Photon energy must be large enough so their mean free path exceeds the range of liberated electrons.
What is the purpose of a build-up cap on an ionization chamber?
- Assures equilibrium distribution of electrons is present in the wall (at wall/gas interface).
- Ensures dose is produced in the gas.
At what point (depth) in a material is the largest absorbed dose?
The peak would be expected where
- the increase in charged particle population AND
- the increase in absorbed dose
is balanced by
- the decrease in dose associated with the attenuation of the photon beam
In estimating the shielding for scattered X-ray radiation, the reflection coefficient α (albedo factor) can be used. List four parameters that may affect the reflection coefficient.
- Energy of incident photons
- Angle of incidence of photons on the scatterer surface
- Angle of reflection from the scatterer surface
- Material of construction of the scatterer
