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7.5.1

IonizationIonization

If sufficient energy is transferred in an interaction of radiation with matter, ion pairs are formed. This ionization is caused by an electron absorbing sufficient energy that its energy then exceeds the binding energy of its atom and becomes free. It leaves behind a positively charged atom resulting in an ion pair. The amount of ionization produced is proportional to the energy delivered to the medium and so measurement of the amount of ionization can be the basis of not only a radiation detector but also of dose measurement. Ionization chambers usually measure charge created in air.

7.5.2

_{Principles of operationPrinciples of operation}

7.5.2.1 The need for applied voltage

In order to measure the number of ions produced they must be collected. This is done simply by attracting them to an electrode of opposite charge and counting them in a device called an electrometer. In travelling through the medium, the ions produced will be attracted to any ions of opposite charge in the vicinity and there will be a tendency to recombine. If recombination occurs the ionization event cannot be detected and hence will not contribute t o the measurement and the dose will be underestimated. To minimize the chance of recombination, the potential difference (voltage) between the electrodes must be sufficient to collect the electrons quickly. The collection of electrons constitutes a current through the electrometer which can be measured by a current meter. As the voltage is increased, the number of electrons collected (i.e. pre- vented from recombining) increases until all of the available electrons are being collected. Care must be taken to minimize the leakage of charge collected from the chamber assembly by use of appropriate insulation materials.

7.5.3

Types of ionization chambersTypes of ionization chambers

7.5.3.1 Free air ionization chamber

The free air chamber is a primary standard designed to determine exposure for pho- ton energies up to around 300kV. Free air chambers are maintained by national stand- ards laboratories and used to calibrate reference standards, which in turn are used to calibrate the routine equipment used in the clinic (see later). The free air chamber is designed to comply as closely as possible with the definition of exposure and the con- ditions to satisfy charge particle equilibrium and is shown schematically in Fig. 7.4 and the UK National Physical Laboratory’s free air chamber is shown in Fig. 7.5. It consists of an air filled metal box with an aperture allowing a well defined area (A) of X-rays to pass through the chamber so that it only interacts with air within the chamber. Ions created

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when the X-rays interact with the air are attracted toward two high voltage electrodes placed on opposite sides of the box. The electrode separation is sufficient to ensure that electrons emanating in the shaded region lose all their energy before they reach the electrodes i.e. they are completely stopped in air. Electronic equilibrium is established as long as the shaded volume is sufficiently far inside the box that full electron build up has been achieved. This means that electrons produced in but travelling out of the dotted region and not collected by the electrodes are compensated for by electrons entering this region from outside and subsequently collected by the electrodes. The total electron charge (Q) is measured and the mass of air calculated

A E Electric field lines

Collimator Beam

Primary interaction volume

B P₂ P1 C D S − + Fig. 7.4

Fig. 7.4 Schematic diagram of a free air ionization chamber. The guard plates outside the AB plate exclude any signal from the region where the electric field lines are bowed to ensure that the volume from which ionization is collected is known.

Fig. 7.5

Fig. 7.5 The free air ionization chamber at the UK’s National Physical Laboratory. Note the round beam entry window in the steel casing. Reproduced with permission from the National Physics Laboratory.

IONIZATION CHAMBER DEVICES 75

from the air density (ρ) and dimensions of the shaded volume. Several small correction factors are applied to account for such things as X-ray attenuation in air between the entrance aperture and shaded collection region, the presence of any water vapour, scattered radiation entering the chamber from outside, ionization from bremsstrahlung, not all electrons being collected by the positive electrode etc. Although basic construction is similar the medium energy free air chamber is substantially larger (1m3 _{) in order to}

satisfy the definition of exposure and to ensure electronic equilibrium is reached at the measurement volume.

As the beam energy increases, the separation of the plates must increase to prevent electrons reaching the plate before giving up all of their energy.

7.5.3.2 Thimble chambers

More useful in the clinic is a relatively small ionization chamber, usually thimble shaped, which encompasses a typically 0.1 cm3 _{–1.0 cm}3 _{cavity within which ionization}

occurs (the active volume) and is collected between the chamber’s axial electrode and its conducting walls. Such a chamber is shown in Fig. 7.6 . The walls are close to tissue or air equivalence using a low-Z material such as graphite or conducting plastic. The central electrode is usually aluminium. This has a relatively high atomic number but the effective atomic number of the detector as a whole is similar to air. The commonly used Farmer type thimble chamber has an external diameter of 7mm and a 1mm Aluminium electrode and a 0.5mm graphite external wall making the gap between the electrodes 2.5mm. In practice 200V is used to guarantee collection of virtually all ions created. The thimble is one of the key components in a dosimetry system as it accurately defines the cavity volume in which ionization of the contained air mass occurs as well as forming one of the electrodes. The material the thimble is made from is important in determining the properties of the chamber. The standard Farmer type cavity chamber is useable across the full range of radiotherapy energies from around 50kV to 25MV.

The theoretical basis on which the cavity type chamber can be used in the kV energy range (to 300kV) is quite distinct to that in the MV energy range. At kV energies the thimble material is assumed to be air equivalent, whereas in the MV energy range it is assumed to be water equivalent. A detailed description is beyond the scope of this chapter, but a brief overview is given as follows for completeness.

Insulators

Outer conducting wall (electrode)

Collecting electrode

Guard ring A

Fig. 7.6

Fig. 7.6 Example of a simple thimble ionization chamber. The conducting wall is shown and axial central aluminium electrode. The wall and electrode are separated by an insulator.

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For the free air chamber described above, if it were possible to compress the air around the shaded measurement volume so that electronic equilibrium still holds by say a factor of 1000 without affecting the shaded volume, the electron ranges in this compressed volume would also be reduced by a factor of 1000. If the electron fluence, i.e. the number, direction and energy of electrons leaving this compressed volume were identical to that in air then the conditions of charged particle equilibrium would be satisfied. Suitable thimble material (graphite, conducting nylon) having similar properties to ‘very dense’ air would then enable exposure measurement by simulating a free air chamber.

7.5.3.3 Parallel plate chambers

One disadvantage of the thimble chamber is its inability to pinpoint the position of a dose measurement in a high dose gradient radiation field. Plane parallel chambers allow the measuring point to be much better defined in space and to have a finer measure- ment resolution in one dimension. A parallel plate chamber consists of two conducting plate walls only one of which usually is the beam entrance wall as shown in Fig. 7.7. Parallel plate chambers are recommended for measurements in electron beams and surface and build up dose measurements in photon beams.

7.5.4

ElectrometerElectrometer

7.5.4.1 Principles of operation

In order to convert ionization produced in a chamber into an indication of exposure, the ionization must be collected and measured. To quantify the rate of exposure, the rate of flow of charge (i.e. current) through the circuit should be measured with an ammeter. To quantify the total exposure, all of the charge produced in the cavity (which, when moving, constitutes the current) needs to be collected and measured. This is done by storing the charge in a capacitor until the end of the exposure and then reading the potential difference across it. The current produced by a 0.6cm3 _{ionization chamber}

is typically 10–9 _{A or, for small volume pinpoint ionisation chambers, as low as 10}–12 _A.

Electrometers may also be used to assess dose rate, rather than dose.

Collecting electrode Guard ring Open to atmosphere Insulator Fig. 7.7

Fig. 7.7 Plane parallel chambers. The measuring point is typically immediately inside the upper front face electrode. The guard ring defines the collecting volume.

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