KA 3 Flashcards
What is KERMA? what type of particles does it relate to?
KERMA is the kinetic energy released/transferred from indirectly ionizing radiation (photons, neutrons) to charged particles (electrons= directly ionizing radiation) through various photon interactions: compton, photoelectric, PP, scatter.
What two interactions do charged particles transfer energy to a medium?
excitation and ionization
What is the premise of Bragg gray cavity theory?
Bragg gray cavity theory allows us to determine dose to a medium by measurement of dose deposited in an air-filled cavity (ion chamber) placed within that medium. Scaled by ratio of stopping powers
What are the two conditions required of Bragg Gray theory?
- cavity size/thickness is much smaller than the range of charged particles such that the cavity does not perturb partial fluence
- absorbed dose deposited in the cavity is deposited solely by charged particles crossing it (i.e. not from photon interactions inside the cavity that liberate electrons)
According to van Dyk textbook Chp 19.4, above what energy can Bragg Gray theory be typically applied?
300 keV and above
state why typical air-filled chambers cannot be considered as Bragg-Gray cavities for low and medium kV energy photon radiation?
Due to the low energy of the secondary electrons, those generated outside the cavity are not energetic enough to enter the cavity. Hence, most secondary electrons are generated within the cavity.
This doesn’t satisfy Gragg gray theory conditions.
Given Bragg Bray conditions do not apply for dosimetry of low and medium energy x-rays, what conditions are required?
Charge particle equilibrium describes the situation where the number and energy of charged particles that are liberated in a volume and leave the volume equals the energy and number of particles liberated elsewhere that enter the the volume.
When CPE exists, what two terms can be approximately equated?
Absorbed dose= collisional kerma
This is important as we can equate a measurable quantity, absorbed dose, to a quantity we can calculate, kerma. This allows us to relate the absorbed dose in one media to a dose in a different media based on u/p ratios.
What condition is required such that a dose calibration factor, Nk, can be used for a chamber?
A chamber is at a point where electronic equilibrium (CPE) exists for a particular beam quality
Define stopping power
Stopping power=energy loss by electrons per unit path length of a material
What does Spencer-attix cavity theory take into account that bragg-gray doesn’t?
Considers delta electrons. Photon interactions with matter= electrons, these electrons can liberate other electrons which may have enough energy to leave the cavity. This reduces the energy absorbed in the cavity and requires modification of the stopping powers in the gas.
https://jocwhite.wordpress.com/wp-content/uploads/2017/03/chapter-10-cavity-theory.pdf
What needs to be considered when converting electron PDI to PDD?
Stopping power ratio of water:air which varies as a function of depth for electron beams
Why for photon beams, does PDI=PDD?
In photon beams, the stopping power (energy loss by electrons per unit path length of a material) ratio air to water is independent of depth. This is true beyond dmax.
What is k_(Q,Q0) an abbreviation for? In general, what does k_Q do?
k_(Q,Q0)=N_(D,w,Q)/N_(D,w,Q0)
i.e. ratio of the absorbed dose to water calibration coefficients
Correction of a measurement performed in different beam qualities/energies
What are the units of N_d,w? Hence what is the equation?
mGy/nC. Equation N_dw= D_w,Q/M_q
where D=dose to water in beam quality Q and M is the measurement in nC
What are the 3 key purposes of a guard ring in PPC design?
- create a uniform electric field between electrodes that helps to define the collection volume more accurately
- ensures that scattered secondary electronsfrom the side walls do not contribute to the measured signal
- prevent leakage current
In TRS 398, what is minimum ratio of a PPC sensitive volume diameter to the cavity height?
5x
In TRS 398, what is minimum width of the guard ring ring as a ratio of the cavity height?
guard ring=1.5 x cavity height
Define what an influence quantity is
Quantities that are not the subject of a measurement yet influence the measurement
List 4 influence quantities in ionisation chamber dosimetry.
- kTP: Temperature pressure: a vented chamber means the mass of the air changes which hence alters the number of air molecules that have the chance to be ionised
- kpol: Polarity: under the same irradiation conditions, opposite polarity (+ and -) of a chamber can produce a different signal.
- ks: recombination. Ion recombination as a function of electric field strength (potential difference).
- electrometer calibration factor: systematic factor. Unity correction if chamber and electrometer are calibrated together.
Describe what mass absorption coefficient refers to?
The mass absorption coefficient, u/p, is a measure of how much energy is removed from a beam in a cross-section per unit mass. Units: cm^2/g. E.g. at low energies, the difference in mass absorption coefficient is significant between low and high Z materials. I.e. material like lead is going to remove more energy from the beam than low Z material. This is why high-Z materials are used for shielding when photoelectric interactions dominate.
u/p total has contributions from compton, PP, photoelectric, etc.
What design feature of the PPC contributes to the polarity effect?
Mass of plastic at the rear of the chamber.
Radiation interactions with this rear plastic liberate electrons that leave the backside of the chamber. Loss of electrons = positive current. So if the collecting electrode is negative, the current is decreased. If the collecting electrode is positive, the current increases. Source: C. Fox lecture (TEAP Series)
What is the purpose of the air hole/dimple at the rear of PPC?
To make the chamber more symmetrical to minimise polarity effects.
Define extracameral effect. What can it impact?
Extracameral current is a result of ionization that is collected from outside the designated collecting volume from irradiated cable. A major cause of the polarity effects include Compton current and extracameral current.
List the four perturbation factors
- Pwall: accounts for non-water equivalence of chamber wall
- Pdisplacement: accounts for the replacement of a volume of water with the detector air cavity. Specific to cylindrical chamber. An alternative method to account for this is using EPOM. Doesnt apply to PPC as the EPOM is the front surface of the cavity.
- Pcentral_electrode: accounts for the metal central electrode (cylindrical chamber)
- pcavity: corrects for differences of in-scattering electrons. Electron fluence inside cavity differs from the measuring medium in the absence of the cavity.
State the purpose of perturbation factors
In reality, measurement conditions deviate from ideal Bragg-Gray detector conditions. Hence, perturbation correction factors are applied to account for these deviation.
How are perturbation factors accounted for in absolute dosimetry?
Accounted for in K_q equation provided in the calibration certificate
Why are stopping power ratios used in MV dosimetry only and not kV?
stopping power =energy loss by electrons per unit path length of a material
For Bragg Gray cavity conditions, liberated electrons created outside the cavity in the medium, strictly cross the cavity. Cavity samples the electron fluence. Hence the chamber is measuring the number of ion pairs created by electrons crossing the cavity which can be related to the energy lost by these electrons. Hence, Stopping power is relevant.
Why are mass attenuation/mass absorption (u/p) ratios used in kV dosimetry only?
u/p describes the fraction of photons removed (transferred) from an x-ray beam per unit mass. Units: g^-1. Transfer of kinetic energy is to charged particles.
For kV beams, liberated electrons deposit energy locally due to being low energy.
For a measuring cavity in a low X-ray beam, electrons liberated in the medium are not energetic enough to enter the cavity (attenuated) and hence only electrons liberated within the cavity contribute to the charge measurement. This scenario deviates from Bragg Cavity theory and hence according to the theory, stopping power ratios cannot be used. Hence u/p is used and is relevant to convert air kerma to absorbed dose in water.
What is the definition for exposure?
Exposure if a measure of the charge produced in air due to photon interactions with the air.
Measured in C/kg
What does level1b audit involve and what is it checking?
reference dosimetry, involving independent secondary standard measurement in water tank (1D or 3D).
What is the fail tolerance for photon and electrons outputs in a level1b audit?
2% photons
3% electrons
what does the correction factor of Pion account for?
corrects for ionisation recombination losses that occur at the time of measurement
what does the correction factor of P_dis account for?
Accounts for the effect of replacing a volume of material (e.g.water) with the detector cavity (air) when the reference point of the chamber is taken to be the chamber central. Used as an alternative to the EPOM.
what does the correction factor of Pwall account for?
corrects for the effect of the difference between the chamber wall material and the phantom material (e.g. water).
The overall perturbation factor, P_Q, of a chamber is the product of four perturbation correction factors. List the 4 perturbation factors
Pcav
Pcel
Pdis
Pwall
Why do perturbation factors such as pdis, pwall, etc need to be considered? overall, what do they account for?
Provide corrections for when ideal Bragg-gray detector conditions are not met
what does the correction factor Pcel account for?
cel=central electrode
corrects the response of the chamber due to effect of central electrode material.
what does the correction factor Pcav account for?
corrects for the response of an ionisation chamber for the effects related to the air cavity, largely the in-scattering of electrons, produces a difference in electron fluence in the presence of air vs. medium of the material.
Why does Pdis not apply to plane parallel ion chambers?
Pdis corrects for the replacement of a volume of the phantom with the cavity of the chamber when the reference point of the chamber is taken to be at the chamber centre. For PPC, the reference point is NOT the chamber central but at the entry window surface.
How are perturbation effects accounted for in absolute dosimetry?
Perturbation effects related to the chamber cavity are inherently accounted for in the K_Q factor.
