Dosimetry equipment Flashcards
1
Q
List three ways to measure the energy deposited from ionising radiation.
A
- Ionisation: collect ion pairs produced in air.
- Calorimetry: ionising particle shares its energy with many others and eventually ions recombine. Energy ends up as heat.
- Chemical effects: free radicals produced by ionising particles cause chemical changes.
2
Q
What is the definition of exposure and what are the units?
A
- “Total charge of ions (of one sign, so that we know how many ion pairs are produced)… in dry air when all electrons liberated in mass, δm, of air are completely utilised”.
- Unit of Exposure is Charge liberated per unit mass [C/Kg]: i.e. photons interact with the medium to produce electrons. Those electrons deposit their kinetic energy by ionising the air and hence producing ion pairs in the air.
3
Q
What are the complications with measuring exposure?
A
- By definition, want to collect all ions produced by e-s in mass δm.
- But e-s produced inside and outside δm.
- Some ions produced by e-s travelling in are collected.
- Some ions produced by e-s travelling out are lost.
- Want electronic equilibrium to measure exposure.
4
Q
What is electronic equilibrium and what are the dimensional conditions necessary to achieve this?
A
- When no of ions produced by e-s travelling in is equal to no of ions produced by e-s travelling out.
- To achieve electronic equilibrium: minimum dimension between boundaries of δm and surrounding interacting homogeneous medium must be > range of e-s.
- The no of ion pairs collected = the same no as made by all interactions from e-s originating in the collecting volume.
5
Q
Describe how a free-air ionisation chamber works and draw a diagram.
A
- Photon beam passes between parallel plates (high polarising voltage, V).
- X-rays produce electrons which cause ionisation.
- Mass of air in collecting volume, δm, defined by guard plates and beam geometry.
- Ions (not e-s) are collected and measured by circuitry.
- V must be high enough to separate as many +ve and –ve ions before they recombine as possible (impossible to stop all recombination, but the remaining recombination can be quantified).
6
Q
What radiation detector is used at the starting point of the calibration chain for KV x-ray dosimetry?
A
- Free-air ionisation chamber.
- Used at the NPL.
7
Q
What is a primary standard?
A
- A Primary Standard makes an absolute measurement whereas secondary and other reference standard instruments must be calibrated.
- A primary standard measures (or realises) the quantity of interest from first principles.
8
Q
Give two examples of primary standards used at the NPL and which types of radiation they measure.
A
- Free-air ionisation chamber (kV photons).
- Graphite calorimeters (MV photons & electrons).
9
Q
What quantity does the free ionisation chamber measure and give an equation for this quantity.
A
- KERMA.
- i.e. the KE of the electrons produced by photons is used to make ions. These ions are collected.
- KERMA = 33.97[eV]*C/em.
- Since the ion chamber is in equilibrium, KERMA = DOSE (to air).
10
Q
Describe how thimble ionisation chambers are calibrated at the NPL.
A
- NPL KV beam air KERMA measured.
- User thimble ionisation chambers are then irradiated in the same beam and a calibration factor derived for the user’s chamber.
- Secondary standard measurement taken in a reference plane.
11
Q
How do we convert from air KERMA to absorbed dose in water?
A
- Dose to water = dose to air * ratio of mass-energy absorption coefficients for water/air (averaged over photon spectrum in air).
- Dw,z=0 = (M)(Nk)(Bw)[(μen/ρ)w/air]air
- M is reading from users thimble chamber.
- Nk is air KERMA calibration factor.
- Bw is backscatter factor (water).
- [(μen/ρ)w/air]air is the ratio of mass-energy absorption coefficients for water/air.
12
Q
State the differences between a free-air chamber and a thimble chamber.
A
- Free-air chamber:
- Small air cavity, δm + surrounding air volume (dimensions > R).
- Thimble chamber:
- Replace air shell with a solid shell of equivalent Z (e.g. graphite).
- Small air cavity + surrounding air-equivalent wall (dimensions > R).
13
Q
Describe the design of a thimble ionisation chamber and draw a diagram.
A
- Ionisation chamber encloses a small volume of air in thimble-like conducting (graphite) cap with an insulated axial collecting electrode.
- Walls are material with similar atomic number to air, Zair.
- Wall thickness > R in wall (1mm wall = 1m air).
- Can then measure exposure without free-air chamber.
- Dose is proportional to exposure, so these chambers can be calibrated against primary dosimetry standards and used for hospital measurements.
14
Q
Describe how a thimble chamber measures charge.
A
- High polarising voltage between the wall and the central electrode (200V cm-1 gap for saturation).
- Ions produced in the cavity move to one or the other electrode.
- Collected charge is measured using an Electrometer (basically an op-amp).
- Volume is typically 0.6ml.
15
Q
State the differences between large and small ionisation chambers.
A
- Large chambers:
- Very sensitive, but low spatial resolution.
- Used for environmental monitoring.
- Small chambers:
- Good spatial resolution, but small signal.
- Used for fine resolution scanning.
- 0.6 cubic centimeter “Farmer” chamber is the most commonly used chamber for photons in Radiotherapy Physics.
16
Q
What are parallel plate chambers used for and what are the advantages of using them?
A
- Sometimes measure in high dose gradient fields.
- e.g. electron beams, KV beams, build-up region of MV beams.
- Thin in direction of gradient for better resolution.
- Detecting volume still large enough for decent signal.
- Thimble chambers too big to use here, will get contribution from other parts of high gradient field.
17
Q
Describe how a calorimeter works and give an approximate value for the temperature increase from 1Gy.
A
- Absorbed dose = Energy/mass.
- E=mcΔT, therefore:
- Dose = cΔT -> direct measurement of absorbed dose.
- NPL uses graphite calorimeters as Primary Standard (MV & Electrons) and converts to water using ratio of electron densities (photon fluence scaling theorem).
- Water difficult to use directly since it has a large specific heat capacity and difficulties with purity.
- 1Gy produces a ΔT of about 1.5mK in a graphite calorimeter (0.24mK in water).
18
Q
What methods for graphite calorimetry are required to obtain a measurement with acceptable precision?
A
- Need method capable of measuring ΔT = 10^-6K.
- Isolating the core thermally (vacuum).
- Rather than allowing the temperature to rise and attempting to measure it, the NPL have a more accurate method were they keep the temperature constant by reducing the current in a heating circuit in the calorimeter to stop the temperature rising when the radiation beam is turned on. They measure the change in electrical power in their heating circuit and use this to calculate:
- Dose to graphite core = change in (Volts x Amps) x Time beam is on / Mass of graphite core.
- i.e. Dose = (ΔPower*time)/(mass).
19
Q
Describe the structure of a graphite calorimeter and draw a diagram.
A
- Inner graphite core (volume where temperature rise is to be determined).
- Core surrounded by two graphite ‘jackets’ which together present an ~ homogenous graphite phantom.
- Core thermally isolated from outer jackets by a vacuum.
- Temperature rise in core measured using highly sensitive thermistors and converted to dose to graphite.
- The design (jackets) allows the calorimeter to be used in an adiabatic mode i.e. without heat transfer (Dose to graphite core = change in (Volts x Amps) x Time beam is on / Mass of graphite core).
20
Q
Describe how NPL reference chambers and user secondary standard chambers are calibrated at the NPL.
A
- NPL reference chambers:
- Calibrated against calorimeter at different beam qualities.
- Calibration converted from graphite to water using Monte Carlo simulations, measurements and photon fluence scaling theorem.
- User secondary standard chambers:
- Calibrated against NPL reference chambers.
21
Q
What is a secondary standard chamber and what is the procedure for calibration?
A
- High quality dosemeters owned by Hospitals.
- The secondary standards are sent to NPL to be calibrated against the NPL primary standard (kV photons) or, in the case of MV photons, against the NPL reference chambers (where the NPL reference chambers are calibrated against the primary standard calorimeter).
- NPL provides a correction factor to convert dose readings made with the secondary standard into accurate dose measurements.
- This calibration transferred to field instruments via cross-calibration.
- Field instruments used for routine dosimetry.
22
Q
Describe the method for calibrating field instruments.
A
- Field instrument and secondary standard are placed side by side in a large Perspex phantom at a fixed depth and irradiated.
- By irradiating simultaneously at same depth can ensure that they get the same dose.
- Ratio of readings from field instrument and secondary standard taken.
- Repeated twelve times with position of each chamber swapped after each three times and average ratio determined.
23
Q
What is the quality dependent factor and how can it be obtained?
A
- Due to chamber construction, response varies with beam energy spectrum.
- The energy spectrum is quantified by a single number that is referred to as a measure of the beam “quality”.
- The measure of beam quality is the TPR20/10.
- [TPR20/10 is the “Tissue Phantom Ratio” of 20cm reading / 10cm reading. i.e. a ratio of ionisation chamber readings when measured at 2 different depths, everything else kept constant].
- The hospital’s secondary standard chamber is calibrated against the NPL reference chamber (which itself was calibrated against the primary standard calorimeter) at a number of different TPR 20/10 values (quality indices), and a calibration curve produced for the secondary standard chamber.
- ND varies by about 4% across beam qualities for MV photons.
