The Air Wall Ionization Chamber

The need to maintain charged particle equilibrium in the sensitive volume in which exposure Is measured, together with a requirement for a practical instrument, comes together in the air-wall chamber. The design rests upon the following principle.

Charged Particle Equilibrium will certainly exist within a small air volume located at the centre of a much larger volume which is uniformly irradiated by a beam of photons, Fig.(7.7).

Fig.(7.7) (a) a large volume of air contains at its centre a small hatched volume, which is surrounded by a shell of thickness R. (b) When R is condensed into a solid shell of thickness r, charged particle equilibrium will still exist throughout the hatched volume. The requirement is that all the secondary electrons which cross the hatched volume, originate in the shell r and that none are generated within the volume.

aluminium anode air volume

graphite cathode ionization current

polarization voltage

Fig.(7.8) A "thimble" ionization chamber. If the cathode material is non-conducting (e.g. an air-equivalent epoxy), its inner wall can be made suitable for use as an electrode by the application of a thin layer of colloidal graphite. The insulator between anode and cathode must be of the highest quality in order to keep the leakage current down to the order of picoamperes or less. The radial field lines mean that the field strength is highest nearest the anode. At a sufficiently high polarization voltage between anode and cathode, the ionization current saturates when all secondary electrons are collected. At lower polarization voltages recombination takes place.

Practical considerations for such a chamber are as follows:

• An ionization chamber for low energy photons, and which has an outer wall and an inner electrode having Zeff approaching that of air (7.78), will approximate to an air-equivalent chamber. Such a practical chamber would therefore be one which had, for example, an outer cathode of graphite (Z=6) and an inner anode of aluminium (Z=13).

• If the wall material is made from a tissue-equivalent plastic, then a thin layer of conducting material must be placed on the inside surface.

• The wall thickness must not be so thick that it attenuates the incoming photon flux so much that the measured dose is lower than it otherwise would be.

• The wall thickness must be thick enough to ensure that all the secondary electrons which cross the air cavity arise only in the wall material and not in the air cavity itself.

• In practice the wall thickness of such a chamber is in the range 400 - 700 jam for photons in the energy range 150 - 300 keV. If the chamber is to be used for higher energies, then an air-equivalent build-up cap is placed around the graphite cathode in order to maintain CPE.

• An ionization chamber therefore measures the dose in the wall material by collecting the charge released into the air volume.

For an air cavity volume of V, air density p and measured ionization current /', the exposure rate is i / (V x p). The dose in the wall is then (7 x W)/(Vx p).

0 0

Post a comment