General theory Flashcards
What does stopping power relate to: photons or charged particles?
Charged particles. Stopping power defn: gradual energy loss by an energetic charged particle moving through an absorber.
What is stopping power ?
The gradual energy loss by a energetic charged particles moving through an absorber.
What are the two types of energy losses that contribute to the stopping power?
Radiative (bremsstrahlung) and collisional (orbital electrons interactions via excitation or ionisation)
Stopping power is made up of collisional and radiation components. Which directly contributes to dose?
– Collisional stopping power (Sc) – Energy lost due to collisions with the medium. This directly contributes to dose, as the energy is deposited locally.
Radiative SP is energy lost to radiative processes, including bremsstrahlung. Bremsstrahlung does not usually contribute to dose, as this energy is radiated away.
A component of stopping power is collisional. List the two types of collisions an electron can have with an atomic electron?
(1) inelastic collision: an incident electron transfers some of its energy to an atomic electron to excite it. When this excited atomic electron eventually releases this energy= characteristic x-ray emission
(2) elastic collision: an incident electron transfers some of its energy to an atomic electron, ejecting it from the atom (direct ionisation). Secondary electrons can produce additional ionizations
Source: pg 45 of basic radiotherapy physics and biology textbook
A component of stopping power is radiative. List the two types of collisions an electron can have with a nucleus?
(1) inelastic, where an electron that travels near the nucleus slows down and changes direction= photon emission known as bremsstrahlung x-ray. The process is responsible for the production of MV x-rays in Linac.
(2) elastic: electron bounces off (scatters) from the nucleus, changing direction without energy transfer
remembering the definition for elastic interaction= no loss of Ek of the incident electron as a result of the interaction
What does KERMA stand for? give a description?
Kinetic energy released in media=kerma
KERMA is the energy released from a photon to an electron (transferred energy to electron). Note: these electrons have energy and will not deposit dose locally hence KERMA is not the same as absorbed dose.
what are the two components of Kerma? which component contributes to absorbed dose?
(1) K_collisional, contributes to absorbed dose
(2) K_radiative: electron travels near nucleus= bremsstrahlung emission
What term is used to describe the charge released into a mass of air? i.e. number of ionizations created
exposure/air KERMA
When photons interact with air, ion pair is produced.
Given air kerma is used to describe the number of ionizations produced by photons interacting with air (i.e. charge collection), what is the equation?
X=dQ/dm
where dQ=difference in charge
dm= air mass
What are the 3 key photon interactions with matter?
- photoelectric
- compton
- pair production
what is the energy requirement for a photoelectric interaction to occur?
Incoming energy of the photon is greater than the binding energy of an INNER electron of an atom
Explain a photoelectric interaction? What is the probability of interaction relative to Z and E?
Incoming photon with energy greater than the binding energy of an inner electron of an atom, knocks that electron out from the atom (ionization). A high orbital electron fills the vacancy and a Auger electron (outside shell electron is released) or characteristic x-ray is emitted.
Probability proportional to Z3/E3
what is the most important photon interaction for diagnostic imaging and why?
photoelectric because of the Z3 dependence, bone absorbs A LOT MORE x-rays than soft tissue, and hence showing up clearly on film.
Explain Compton interaction. What is the probability of interaction relative to Z and E?
An incident photon strikes a weakly bound outer shell electron/free, emitting the electron and deflecting the original photon (with different energy).
The probability of Compton interactions is proportional to the number of outer shell electrons available in the medium (i.e. electron density). INDEPENDENT OF Z.
Explain pair production interaction. What is the probability of interaction relative to Z and E?
Pair production occurs at energies above 1.02 MeV (511 keVx2). Dominant interaction above 10 MeV.
Probability proportional to Z2
Process: A photon travels with so much energy that when it hits the electric field of the electron orbitals, it transforms into an electron and positron. The positron eventually finds another direction elsewhere and annihilates it, generating two photons in opposite directions with an exact energy of 511 keV, etc.
Why can diodes be used for electron beam scanning?
Energy independence of mass collision stopping power ratios (Silicon:water) for clinically usable electron beams between 4 and 20 MeV
The response of a diode depends on what parameters?
Temperature, dose rate, energy, potentially angular dependence
How would you verify for MV photon beam scanning, that a diode is suitable to use for measurements in terms of energy response?
compare measured PDD for fixed MV beam (e.g. 6 MV) for a large field (40x40cm2) of a diode to a large volume scanning ion chamber (0.6 cc). Compare PDDs beyond dmax. If the diode curve does not drop off as rapidly as the ion chamber PDD, this is an indication of energy response variations.
For small vs large field beam scanning, how may the scan speed change?
decreased scan speed for small field to account for smaller signal to minimise statistical variations.
what is leakage between two leaves known as?
interleaf leakage
what is transmission through a leaf known as?
intraleaf leakage
At what % of the MTF curve, is the limiting spatial resolution specified?
5%
