Chpt 1+2: Radiation Generation
What is the Poynting vector
S=ExH. It gives the source of the radiation field. In a static electric or magnetic field, S=0, therefore no radiation is produced
What is true about the mass of a moving object
it always gets heavier
define and give examples for relative vs. absolute quantities
Relative quantities depend on the observer while absolute ones do not. Rest mass and the speed of light in a vacuum are absolute quantities.
What is the radiation energy transfer fraction (g)
This is the energy that is transferred via bremsstrahlung radiation. It needs to be subtracted from the energy absorption because its energy is not transferred to particles locally.
What is dose measurement in medical physics and health physics based on
It is based on the experimental measurement by dosimeters and calculation based on the detailed understanding of the physics of radiation interaction with matter
A charged particle with a constant speed can produce radiation in what two cases
1. in a medium with refractive index n, an electron can radiate to produce Cherenkov radiation.
2. circular motion due to non-zero acceleration
What is the wave function in the general schrodinger equation
The wave function gives the probability of finding the particle at a given moment in time and at a given location x
How does Raleigh scattering differ from Thompson scattering?
- Thompson involves a free electron, or an electron where the external driving frequency is much higher than the intrinsic oscillation frequency
- Raleigh scattering has the electron modeled as bound to the nucleus with a spring
- Rayleigh scattering does not alter the wavelength of the photon scattered, whereas Thomson scattering does
What is the difference between TERMA and KERMA
TERMA- the total energy released per unit mass so it is the energy fluence times the mass attenuation coefficient
KERMA- the kinetic energy released per unit mass so it is the energy fluences times the mass energy transfer coefficient
If you irradiate a small cavity that can be filled with different materials, the ratio of the Dose to material 1 and Dose to material 2 is proportional to the ratio of the stopping power of material 1 and the stopping power of material 2
How do heavy charged particles mainly lose energy
collisional energy transfer (not radiative loss)
define a conductor using the words conduction band and quantum energy states
A conductor has a large number of electrons in the conduction band and a large number of empty quantum energy states in the conduction band. Under an applied electric field, the electrons can form an electrical current with low resistivity
What is R_CSDA
range in continuous slowing down approximation. this gives the mean path length along a particle's trajectory (how far it travels in matter before losing all of its kinetic energy)
How are dose and exposure related
Under CPE, dose is equal to the mean energy needed to ionize a charged particle divided by e times the exposure
What are the two conditions required for Bragg-Gray cavity theory to be used
1. Bragg-Gray condition- The cavity used does not perturb the incident fluence across the cavity. This means the cavity must be very small or the density of the medium in the cavity is very low
2. deposited dose is assumed to be entirely deposited by the charged particles passing through the cavity
How is energy transfer involved in the three branches of medical physics (imaging, therapy, and health physics)
imaging- the amount of energy transfer determines the signal level in the information encoding and decoding process
therapy- the amount of energy transfer determines treatment effects
health physics- the amount of energy transfer determines how much potential harm is caused to an individual
How is the finite well problem different from the free electron in a finite conductor
The probability of the electron being at 0 and L in the finite well problem in 0, whereas, it is possible in the finite conductor problem
What is the difference between collisional stopping power and radiation stopping power
Collisional stopping power is from interactions between the charge particle and orbital electrons of the absorber; radiative stopping power is from interactions between the charged particles and the nucleus of the absorber
What are the conditions for radiation equilibrium (x4)
1. atomic compositions are uniformly distributed in a medium
2. Homogeneous density of atoms; atoms per unit volume is the same everywhere in the volume
3. radioactive sources are uniformly distributed
4. there is no external electric field or magnetic field to disturb the charge distribution
What is Spencer-Attix cavity theory
It is a Bragg-Gray theory but with a low energy cutoff, normally 10keV. The energy cutoff is necessary because low-energy delta rays are absorbed locally, which violates the Bragg-Gray condition.
How can we find the electric and magnetic fields given a charge distribution and current distribution
1. specify that it is a bounded space
2. take the curl of both ampere and faraday's laws to get second-order partial differential equations that can be solved with appropriate boundary conditions
How does beta + decay occur
beta + decay: proton goes to neutron, positron and electron neutrino
An isolated proton cannot decay into a neutron because a neutron is heavier. The proton must take binding energy to convert into a neutron. This is possible because it costs more energy to hold together more protons in the nucleus. When there is a mass difference between mother and daughter atoms that is greater than 2 m_e c^2 (1.022 MeV), a portion of binding energy can be invested into the process.
What are the interaction types for Coulomb interaction between charged particles and orbital electrons of absorber atoms (X4)
1. elastic scattering- no energy loss but changes in direction of motion
2. soft inelastic scattering- impact parameter b is larger than the size of atom a. Some energy is transferred to the electrons nad some electrons might be knocked out to become delta rays
3. hard inelastic scattering- when impact parameter b is comparable with the size of atom a; also results in energy loss to electrons
4. in flight annihilation- positrons annihilate with electrons to result in nuclear pair production or triplet production
When calculating the energy transfer fraction for Compton scattering and pair production, do you need to include the fluorescence yield? Why.
NO.
Compton justification- Compton scattering occurs for outer shell orbital electrons, se we can immediately argue that the fluorescence yield, if any, should be extremely low and can be safely ignored
Pair production justification- the fluorescence yield is much lower than the 2me^2 in the numerator and the threshold energy for the event to occur, so you can safely neglect the fluorescence yield term
What is the premise of the Burlin cavity theory
When a cavity is very large, secondary electrons generated likely won't escape, so photons can be the dose messenger. measure the change in the number of photons across the cavity. done with the ratio of the mass absorption coefficients.
When a cavity is very small, the ratio of the collisional stopping powers can be used because secondary electrons crossing the cavity will be the dose messenger.
Burlin cavity theory interpolates between the above scenerios