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501: Radiological Physics and Dosimetry
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Taught by: Dr. Chen
Terms in this set (50)
Which of the following will not irradiate?
a) A static charge
b) A moving charge with constant velocity in a vacuum
c) A moving charge with constant velocity in a dielectric medium
d) A moving charge with constant speed travelling on a circle
e) A moving charge with non-zero acceleration
A and B
Which field drops off quicker?
Coulomb or Radiation Field
Coulomb
How can we use perturbation analysis to understand crystal energy band structure?
Find the wave function and energy spectrum of a free electron perturbed by a weak periodic potential (representing the Coulomb forces from a crystal structure). There are two states - one with the lowest potential energy and one with the highest potential energy. This difference in potential yields the energy gap.
How would you figure out the radius of a nucleus?
R = R_0 A^(1/3)
R_0 ~ 1.25 fm
What has a relatively narrow energy band gap and can form a current at temperatures above 0K?
A semiconductor
Give an example of a radiological problem we can use quantum tunneling to understand?
Alpha decay (finding the half-life, probability of transmission, etc.)
Give an example of an application that makes use of beta minus decay? for beta plus decay?
beta minus example: Brachytherapy Co-60
beta plus example: PET and F-18
What is the mean energy transfer fraction? What is the mean energy transfer fraction for Rayleigh interactions? For Compton? for photoelectric? for pair production?
The mean energy transfer fraction is the gained kinetic energy of the charged particles, divided by the incident photon energy.
Rayleigh = 0
Compton = [hv - hv']/[hv]
Photoelectric = [hv - X_PE]/[hv]
Pair Production = [hv - 2m_ec^2]/[hv]
When a free electron exists in an external electric field, how does the electron scatter radiation?
The electron is accelerated by the electric field, and therefore generates its own radiation (which appears as scattered radiation)
Label the sections of the volcano plot below (can't insert picture sorry)
Far left should be photoelectric, middle should be Compton, and far right should be pair production
Why is the sky blue?
The Rayleigh scattering cross section is proportional to 1/lambda^4, so shorter wavelengths (like blue light) scatter more easily
In therapeutic applications, what type of photon interaction dominates?
Compton scattering (check volcano plot)
What happens to energy transferred to charged particles?
1. scattering between electrons and other charged particles
2. radiative energy loss via bremsstrahlung
3. generation of delta-rays/secondary electrons
Fill in the blank: the cross-section of Coulomb scattering is the same in both classical and quantum mechanics up to the validity of the _________________ condition
Born approximation
What's the difference between collisional and radiation stopping power?
Collisional: interaction between charged particles and orbital atoms in the absorber
Radiation: interaction between charged particles and nuclei of matter in the absorber
Which will have a greater collisional stopping power? Lead (Z/A ~ 0.40) or water (Z/A ~ 0.56)?
Water. Because stopping power is proportional to Z/A
What does the mass collision stopping power depend on, both for the incoming particle and the target material?
- Depends on Z_1^2 (where Z_1 is the incoming particle atomic number)
- No mass dependence
- Depends on Z/A (where Z and A are of the target material)
Define radiation yield. How does it change with increasing Z? given a particular Z, how does it change with increasing kinetic energy?
Definition: The fraction of the initial kinetic energy that is released through Bremsstrahlung radiations.
The radiation yield increases with absorber atomic number Z and increases with incident electron kinetic energy (given a particular Z)
What type of charged particle is RCSDA a good approximation for the average range in an absorbing medium?
Heavy charged particles since they don't have a tortuous path like light charged particles
Why do we use LET instead of just collisional stopping power?
LET characterizes how much energy of a charged particle is released in matter locally, which is important for dose estimation. Stopping power may overestimate local energy deposition.
In x-ray imaging, why do we like to use high Z materials for the target?
The total intensity of thick-target radiation is linearly proportional to the atomic number of the target materials. So high Z materials are better for radiation generation
Given the incoming charged particle energy T_0 = 100 keV, what is the maximum photon energy that can be produced via Bremsstrahlung?
hv_max = T_0 = 100 keV
What direction generates strongest radiation power in x-ray (non-relativistic limit)? in Linac (high acceleration, relativistic limit)?
Perpendicular direction for x-ray because its non-relativistic. That's why the x-ray tube is shaped that way.
Forward direction for Linac since its at relativistic speeds
What is the corresponding concept of total mass stopping power of charged particles in photon-matter interactions?
mu_tr/rho: photon energy transferred to electrons per unit distance
What is the corresponding concept of energy absorption coefficient (mu_ab/rho) in the energy transfer processes between a charged particle and a piece of matter?
Mass collisional stopping power S_col
Why is a photon beam referred to as an indirect radiation beam?
The energy transfer processes involved occur in two different physical steps:
i. Transfer energy from photons to charged particles
ii. Release gained kinetic energy of the charged particles via both the collisional and radiative processes.
What is Terma, Kerma, and Collision Kerma?
Terma: Total photon energy released into matter = (mu/rho) x Psi
Kerma: Portion of photon kinetic energy transferred to charged particles = (mu_tr/rho) x Psi
Collision Kerma: Portion of Kerma that is transferred to charged particles via collisional processes = (mu_ab/rho) x Psi
What condition must be in place for us to equate dose and collision kerma?
Charged particle equilibrium (CPE)
When studying dose to a thick foil, what extra considerations do we have to take into account when looking at light charged particles instead of heavy charged particles?
i. Multi-scattering effect: more scattering events = more tortuous path = directional deflection of beam when it exits the foil
ii. Bremsstrahlung radiation must be subtracted to calculate net deposited energy
What is the difference between RE, CPE, and TCPE?
Radiation Equilibrium (RE): Conditions:
i. (R_in)_c = (R_out)_c
ii. (R_in)u = (R_out_u
Charge Particle Equilibrium (CPE): Conditions:
i. (R_in)_c = (R_out)_c
Transient Charged Particle Equilibrium (TCPE): the point at which dose and collision kerma are not the same, but proportional to each other
What is exposure? What is dose? What is kerma? Given an exposure measurement, how can you calculate dose?
Exposure is charged released per unit mass
Dose is energy deposited per unit mass
Kerma is energy released per unit mass
Assuming CPE: D_air = (Wbar/e)_air * X
What is delta-ray equilibrium?
When the energy carried away from the target volume via delta-rays is balanced by the energy carried into the target volume by delta-rays from the surrounding neighboring regions
What length scale is used to classify whether a cavity is small or large? What is the condition for a small cavity? An intermediate? Large?
a. RCSDA
b. Small (d≪R_CSDA)
c. Intermediate (d~R_CSDA)
d. Large (d≫R_CSDA)
When would we want to use Bragg-Gray cavity theory? Burlin? Spencer-Attix?
a. Bragg-Gray: for a small cavity
b. Burlin: for an intermediate cavity
c. Spencer-Attix: when we need to take into account the creation of delta-rays that violate the delta-ray equilibrium in high Z materials
What is the Poynting vector? What is it equal to for non-radiative fields? for radiative?
a. E x H
b. = 0 for non-radiative
c. = non-zero for radiative
Name the major types of photon interaction with matter
a. Rayleigh
b. Compton
c. Photoelectric
d. Pair Production
In Rutherford scattering, what is the effect of changing impact parameter, b? What is the difference between hard and soft collision?
Smaller b = more energy lost
Hard collision: more energy lost for incoming electron (higher Q)
Soft collision: less energy loss (lower Q)
Calculate the electron radius.
Use the fact that the Coulomb energy = rest mass energy
What is the energy threshold for nuclear pair production?
1.022 MeV
Draw a depth-dose plot in water for both photons and electrons. Where is CPE and TCPE? Where is the build-up region? Why does the electron plot drop off sharply?
Can't insert image, so pls look this up
CPE is where Kerma = Dose at dmax
TCPE is past dmax
Build-up region is from depth = 0 to dmax
Collisional losses for electrons are inversely related to velocity (slower electrons = higher energy loss)
What is the approximate energy of the Compton edge? What is the maximum energy that can be imparted to an electron in a Compton interaction? What is the minimum energy?
Compton edge IS the maximum energy transferred to an electron in a compton interaction. = (2hv\epsilon)/(1+2\epsilon)
where \epsilon = hv/m_ec^2
Minimum energy = 0
What is Cherenkov radiation?
Radiation emitted when a charged particle passes through a dielectric medium at a speed v > c/n
What is the energy of the ground state hydrogen atom?
-13.6 eV
How does the atomic cross section depend on Z for Rayleigh scattering? For Compton? For photoelectric? for pair production?
Rayleigh: Z^2
Compton: Z
Photoelectric: Z^4
Pair Production: Z^2
How does the atomic cross section depend on incoming photon energy hv for Rayleigh scattering? For Compton? For photoelectric? for pair production?
Rayleigh: 1/(hv)^2
Compton: Generally decreases with increasing energy , but I've seen also 1/(hv)
Photoelectric: 1/(hv)^3
Pair Production: Generally increases with increasing energy, but I've seen also ln(hv)
What are three general types of external energy sources?
1. photon beam
2. charged particle beam
3. neutron beam
What is a primary electron? a secondary electron?
primary: orbital electrons ionized by external energy sources
secondary: orbital electrons ionized by energetic primary electrons when they move around other atoms
What are the conditions for being able to use Bragg-Gray cavity theory?
1. The presence of the cavity does not perturb the incident fluence across the cavity
2. The deposited dose is assumed to be entirely deposited by the charged particles passing through the cavity
Fill in the blanks: _______ energy delta-rays deposit energy in the target volume at shallow depths. _______ energy delta-rays will pass through the target volume and don't contribute dose to the local target. We can take this into account with the _______________ in the dose calculation.
First blank: low
Second blank: high
Third blank: restricted stopping power
Write Maxwell's equations.
Look up the equations since I can't insert them here.
Easier to remember the differential form:
Gauss' Law
Gauss' Law for Magnetism
Faraday's Law
Ampere's Law
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