### Article

## Air filled ionization chambers and their response to high LET radiation

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Published: | September 24, 2009 |
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### Outline

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**Background:** Air filled ionization chambers (ICs) are widely used for absolute dosimetry, not only in photon beams but also in beams of heavy charged particles. Within the IC, electron hole pairs are generated by the energy deposition originating from incoming radiation. High-LET particles create a high density of charge carriers in the core of particle tracks. As a consequence an increased (so called "initial") recombination of the charge carriers takes place, besides of the general (volume) recombination described by the Boag theory.

A theory for a sub-type of initial recombination ("columnar" recombination) is the Jaffe theory, which was developed in 1913 by Jaffe. He solved a differential equation by applying several simplifications and approximations such as a Gaussian shaped track whose width serves as free parameter. These simplifications and the use of an simplified charge carrier distribution are leading to discrepancies between theory and experiments.

**Material and methods: **We solved the fundamental differential equation presented by Jaffe numerically, taking into account both diffusion and recombination terms and realistic models of the initial charge carrier distribution developed by track structure theory. More specifically, we solved the equation for the geometrical setup of the Bragg-peak IC, which is a plane parallel IC with a 2 mm spacing between the electrodes. The sensitive volume of the IC is located in a thermoplastic housing of several mm thickness.

**Results:** We compare the experimental results of the collection efficiency of the Bragg peak IC to both the Jaffe theory and to our numerical solution of the diffusion recombination equation. Fitting a Jaffe curve to the measured collection efficiency resulted in values comparable to the literature. Calculations assuming radial dose distributions coming from track structure require long computation times, caused by the high spatial resolution and the subsequent requirements to temporal resolution.

**Conclusion:** Our numerical solution of the diffusion recombination assuming a Gaussian beam shape is relatively well described by the Jaffe theory. Additionally, preliminary results show that that the calculated response does not depend on the core radius of the radial dose distribution.

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### References

- 1.
- Jaffe GC. On the theory of columnar recombination. Annal Phys. 1913;42:303-344
- 2.
- Kania T, et al. Initial recombination in a parallel-plate ionization chamber exposed to heavy ions. Phys Med Biol. 1998;43:3549-58.
- 3.
- Elsaesser T, et al. Impact of track structure on biological treatment planning in ion radiotherapy. New J Phys. 2008;10:075005.