Chamber Factors

Creation of charge

kV Photons

Low beam energies are able to undergo predominantly photoelectric interactions to create ions and electrons with very short mean free paths before recombining or depositing energy. Hence for ions to be collected at the central electrode (assuming negative bias) and give signal, the ions are generated in or near the air gap surrounding the electrode. Signal is therefore collected predominantly from interactions in the chamber wall itself.

MV Electrons

For electron beams, the mean free path is on the order of centimetres, and so chamber measured charge will be a result of electrons that exhaust their path length within the chambers sensitive volume as well as ions created by electrons in close proximity. Hence charge is a result of ions created outside and inside the chamber wall.

MV Photons

High energy photons can deposit large amounts of energy creating ions with long mean free paths. The ions leading to charge collection are therefore from outside the chamber wall.

Polarisation Correction Factor kpol

There is a difference in the charge collected from a positively biased and a negatively biased chamber. This is due to the measured charge actually being the result of both the current from the beam interactions plus or minus the current influenced by the chamber’s bias. It is a charge balance effect occuring within the central eletrode.

A simple way to estimate the polarisation effect is to take a measurement at each chamber polarity (given suitable time for the chamber to equilibrate between polarities), and average the values using $\dfrac{M_+ - M_-}{2}$ to take into account the sign of the readings. Mavg/M should be ±2% [Metcalfe] or 5% [Kahn].

The region where polarity perturbation is greatest is in the build up, where electrons are not in equilibrium and so CEN will produce the largest underestimate (typically 2% ?)

Polarisation effects are energy dependant. At higher photon energies, the effect should be less prominent, due to the increased velocity behind the charges resulting in less perturbation. However for electrons, this is doesn’t apply (High E electron beams become lower E beams at depths [spectrum change with depth] and so they still are prone to extraneous interactions). They are also depth and field size dependant.

Recombination Correction Factor ks

This is a result of recombination within the sensitive volume, hence detected signal is underestimating the real signal. It is a factor of the potential across the chamber. If the chamber is operated below the saturation voltage, some charges are lost due to recombination. Three types of recombination are:
1. General Recombination (collisions of different tracks) dominant
2. Initial Recombination (collisions from charges created on the same track)
3. Ionic Diffusion (diffusion against the potential).

Recombination effects differ depending on the type of radiation (continuous i.e. Co-60, pulsed beams or older style raster scanned beams). Pulsed beams have higher flux than continuous beams, so recombination will be greater as the chance of recombination interactions will be greater.

This also means dose rate can affect recombination effects, depending on how the linac creates higher dose rates. For instance, Varian fit more pulses per second to increase DR which doesn’t change recomination, but Siemens increase the pulse height to increase DR, which increases the general type recombination.

The ‘two-voltage technique’ is commonly used to determine the recombination correction factor. Chamber signal is recorded at two voltages, the operating voltage and a lower voltage (half or a third of operating voltage). For pulsed beams, the collection efficiency can then be determined from the empirical quadratic fit of

\begin{align} f = a_0 + a_1(\dfrac{M_{op}}{M_{low}}) + a_2(\dfrac{M_{op}}{M_{low}})^2 \end{align}

using tabulated ai values, which are dependent on the ratio of potentials used. For continuous beams, there is another equation.

In the saturation region, typical values for the recombination factor are less than 1% (correction of 0.99), with 3% (0.97) being time for a new chamber. Values of >1 are not realistic in the saturation region, but could possibly show an amplification effect if potential is on the high side of saturation region (Above the saturation voltage region, there is charge multiplication/amplification due to excessive potential).

Chamber Perturbation Corrections

When the chamber is calibrated by a standards laboratory, there are several perturbation corrections applied.
Pcav corrects for any changes in electron fluence due to the camber being present.
Pcel corrects for the chamber response due to the central electrode being non-equivalent to the surrounding medium
Pwall corrects for the chamber response due to the walls being non-equivalent to the surrounding medium.
Pdis corrects for difference in fluence across the chamber volume between the actual chamber placement and the effective placement. Basically unity for photons, but matters for high electron energies. Used when chamber centre is at zref. Part of kQ,Qo. Not needed if peff used.
Peff corrects for effective point of measurement. Only for electrons with cylindrical chambers in 398, everything in 277

The ionization chamber should ideally be tissue equivalent. However, the farmer chamber is made of materials whose density is not close to tissue.
The graphite cap has a lower density than tissue, and so the central electrode is made of the higher atomic number aluminium to make an effective Z close to tissue when these two are averaged. This minimises the overall perturbation factor.

The air cavity also affects the effective point of measurement, due to its relatively low Z.

How is an electrometer value obtained?

The charge value displayed on an electrometer is the result of several steps of processing from a chamber to the display.

  • Initially, a charged particle interaction occurs on the ion chamber electrode. The particle causing the interaction results from other interactions and where these originate is another matter.
  • The interaction causes a charge to travel the dosimeter and electrometer circuit. The charge is summed with all other charges to produce a current, which is amplified by the electrometer (op amp).
  • The amplified current (analogue signal) is passed through to an ADC which thresholds the current and converts it to a binary value.
  • The binary value is fed into the display circuitry resulting in a digital value displayed.

Extra-Cameral Current

Extra-cameral current is any current caused by charges collected outside of a chamber’s sensitive volume, for example from an irradiated cable.

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