Ionisation detectors Flashcards
What are the features needed for the wall/entrance window of an ionisation detector?
Electrically conductive
Strong and easy to machine
Hydrophobic (for water tank use)
Minimise spectral perturbations - same density, Z
What are the common materials for the wall of an ionisation chamber?
Graphite, tissue equivalent plastic (A1SL), PMMA (Roos), Mylar foil (NACP)
What are the key features of the central collecting electrode?
Electrically conductive
Rigid to mitigate mechanical stress
Diameter ~1mm
What effects does the material of the central collecting electrode have on the spectrum?
High Z overresponds to photons <1MV
High density affects the attenuation and scattering of the photons
What are the common materials for the cntral collecting electrode?
Aluminium - most common as weighted atomic number of Al/graphite is a good match for air/water
Graphite
Conducting plastics
What are the purposes of the guard rings?
To prevent leakage from collector electrode, minimise scatter, define a uniform field of the collecting volume of parallel plate chambers
What are perturbation factors and what are the main 4 for an ionisation chamber?
They are the aspects of the chamber which impact the electron fluence of a beam, as they deviate from the uniform medium of interest relative to water They are displacement cavity wall central electrode
What is the cause of the displacement perturbation factor?
The chamber introduces a non-uniformity so it reduces attenuation, resulting in a higher signal at depth.
This varies with beam quality and cavity dimensions
Apply a shift to correct
What are the shifts to account for the displacement perturbation factor?
Cylindrical = -0.6xinner radius of cavity
Parallel plate = inner surface of the top window
What is the cause of the cavity perturbation factor?
The air cavity reduces scatter so there is greater signal downstream
How significant is the cavity perturbation factor?
For parallel plate chambers the cavity perturbation factor is mitigated as the collection diameter»_space;electrode separation and they have wide guard rings relative to the electrode separation
It has a minimal effect on photons after Dmax
How significant is the wall perturbation factor?
Important for photons with a strong energy dependence
<0.5% for elctrons
How the wall perturbation factor calculated as it is a complex inter-relationship between several parameters?
Monte-Carlo modelling
What causes the central electrode perturbation factor?
The significant variation from the reference medium
How much does the central electrode perturbation factor affect the readings?
Negligable for graphite, 0.4-0.8% for aluminium
It can be incorporated into Pwall and will offset the impact
<0.2% for electrons
What is general ion recombination affected by?
Dose rate as the number of tracks per unit volume increases the probability
Transit time
It increases as pulse interval approaches collection times
What is ion recombination?
When ions recombine into neutral particles before they are measured. The can be from the same track, primary, or different tracks, general. General is important when considering photons and electrons
What voltage should be applied to a chamber to prevent fion?
Most centres apply Vmax for saturation, but this can lead to charge multiplication whilst still requiring an fion correction
NPL applies V = 200V for NE2611 cylindrical chambers and 100V for parallel plate chambers
What is the Boag equation for fion?
fion = (1/u)ln(1+u), u = mu . d . S^2/V, whre S = (a-b).(((a+b)/(a-b)).(ln(a/b)/2))^0.5 a = rdius of outer electrode b = radius of gas space c = radius of central electrode r = radius of chamber
What are the equations for field strength for parallel plate and cylindrical chambers?
Parallel plate: X(r) = V/(rln(a/b))
Cylindrical: X(r) = V/S
How can fion be determined?
Jaffe plot - for new chamber
Dual voltage technique - routine
Boag - need to measure DPP and electrode separation - only if no alternative
What is a Jaffe plot?
Extrapolate 1/M vs 1/V for a series of measurements to ensure that its response is linear
What value should V1/V2 have for the dual voltage technique? What is the equation for fion?
3-4
fion = (M1/M2 - 1)(V1/V2 - 1)^-1 +1
What error does ion recombination introduce for common chambers?
- 5% NE2611 (250V)
- 2% Semiflex 0.125cc (250V)
- 5% (300V) - 3.5% (100V) NACP-02
Does fion increase or decrease for FFF beams?
Increase to 1-2%. Anything higher indicates an electrical fault
What is the source of polarity effects?
The reading varies when the polarity is reversed for low energy electrons as a result of charge balance differences on electrodes of asymmetric design
How is fpol calculated?
fpol = (Q+ + Q-)/2Q, should be < V0
What are the causes of leakage?
Intrinsic - impurities permit charge to cross insulation
Radiation Induced - transient electric field in chamber components other than the cavity
Mechanical - pinched cables and loose connectors
What cause stem effects?
Scatter and attenuation from the beam passing through the cable lead to an increase in detected signal
How is leakage and scatter assessed
Scatter is assessed by irradiating a dummy stem and a ratio taken
Leakage is assessed via an orthogonal pair of irradiations in a rectangular field
What energies are stem effects significant for and what is the maximum effect?
> 2MV
10%
What is the cause of directional dependence of an ionisation chamber?
The cavity will have geometrical variations and stem effects
The central electrode can be bent, causing major directional dependence on the long axis
Important to know about for scanning measurements
What are the typical sensitivities of different types of chambers?
Farmers: 20nC/Gy
Parallel plate: 5-10nC/Gy
Small farmers (0.125cc): 3.3nC/Gy
Mini farmers (0.03cc): 0.8nC/Gy
Why is it important to know the sensitivity of chambers?
Ned to match sensitivity to the capabilities of the electrometer
High resolution needs small cavity so small sensitivity
Stem and leakage effects more significant at low sensitivity
Why is it important to allow chambers to settle properly?
Can cause unpredictable errors and it is avoidable
How do you ensure the chamber is settled?
Keep taking repeat reading until consistent measurements, can take over 30 minutes
How may chamber construction affect Initial/Equilibrium ratio (IER)?
Radiation induced conductivity in insulation material.
High field gradients in insulator near active volume.
Impurities in construction materials
How are Strontium checks performed and what are they for?
Place chamber in jig, with a strontium source - has a known decay
Apply fTp and decay corrections
Checks for leakage and the consistency of reading of a chamber
