Particle Interactions with Matter (LET and RBE) Flashcards
What is collisional energy loss?
Negatively charged electron experiencing repulsive Coulomb force resulting interaction and energy lost from the incident electron.
It’s also a form of dose deposition such as linear energy transfer (LET)
What is another type of energy loss other than collisional energy loss?
Radiative energy loss
What is radiative energy loss?
any charged particle undergoing acceleration near traveling a nucleus produces radiation in the form of photon which is also called “bremsstrahlung” photons.
this is also the way we produce x-ray or radiation is through the radiative energy loss.
What formalism is developed to describe the radiative energy loss that comes from the bremsstrahlung process?
S rad (Radiative Stopping Power)
What are the two types of energy loss mechanisms and their examples?
- Collisional (via Coulomb) –> This is the way you deposit dose ~ LET
- Radiative (via acceleration) –> Bremsstrahlung (This is the way we provide X-ray)
Stopping power is made up for two types of energy loss, what are they?
Total stopping power (S tot = S coll (stopping power from collisional energy loss) + S rad (stopping power from radiative energy loss)
Sometimes S coll (stopping power from collisional energy loss) is also called
electronic stopping power since it involves the electrons in the atom
S rad (stopping power from radiative energy loss) is also called
nuclear stopping power since it involves an interaction with the nucleus
What is the quantity that S coll (collisional stopping power = electronic stopping power) is closely related to?
Linear energy transfer (LET), which is a description of energy deposited locally and is useful b/c it is related to biological effects operating at the level of the cell
What two parameters depend on radiative stopping power (S rad)?
Mass and Z
Which type of stopping power or energy loss is associated with Bremsstrahlung process?
S rad, or radiative stopping power or radiative energy loss
What is the S rad (radiative stopping power) equation and what is the implication between proton vs electron?
S rad = k x (Z/mass)^2
radiative stopping power (S rad) is proportional to the square of the atomic number, Z and to the inverse square of mass
For proton beams, the radiative stopping power will be relatively small whereas for electron, it will be large
Also, the radiative stopping power will be higher in high Z materials
Unlike radiative stopping power that depends on Z and mass, the collisional stopping power depends on
the number of electrons per unit gram in the material (higher the Z the electrons per unit gram goes down)
Is collisional stopping power high or low in high Z material?
low, because high Z material has lower electron density
draw the graph of stopping power vs energy of water and lead for electrons
For heavy particle such as proton, almost all of the stopping power is
collisional loss or radiative loss?
Collisional loss as radiative loss for heavy charged particle are very small
draw the graph of stopping power of proton in water (stopping power vs energy)
Explain the relationship of the stopping power and the energy of the particle? and explain the reason.
Figure 7.2.2
The stopping power of the proton increases as the energy of the particle decreases and this is b/c as the speed (or energy) decreases the loss of energy is higher
what parameter is most relevant to explaining the path of charged particle?
Mass
Therefore, electron which has a very small mass, does not travel in straght line unlike heavy particles such as proton and carbon
What is LET (linear energy transfer)?
Why is it useful?
What is collisional stopping power?
A quantity that describes the energy deposited locally near the point of interaction of a particle.
It is useful b/c it can be tied to biological effects operating at the level of the cell.
LET is equal to the collisional stopping power, S coll
LET depends on
types of radiation (X-ray vs light charged particle vs heavier charged particle
LET is much higher for more massive charged particles (up to 100 > KeV/um)
LET is high-LET modality b/c
the products they produce (protons and various nuclei) are high-LET
LET has a strong impact on the biological effectiveness of radiation modalities and the metric used to quantify this is
relative biological effectiveness (RBE)
How is RBE defined?
RBE is defined as the ratio of doses two types of radiation that achieve the same biological effect
Describes the relationship between LET and RBE
As LET increases RBE also increases up to approximately 100 keV/um where the RBE can be 3 or more.
LET is relatively low for
X-rays, gamma rays, and electrons (< few keV/um)
LET is much higher for more
massive charged particles (up to > 100 keV/um)
this is due to a z^2 dependence for collisional stopping power (per eq. 7.3 on page 67)
Neutrons are a high-LET modality b/c
the products they produce which are protons and various nuclei are high-LET
