Experimental determination of the response of ionization chambers in high-energy radiation fields

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Ionization dosemeters for the measurement of the absorbed dose to water are usually calibrated under reference conditions in a ⁶⁰Co radiation field. If a dosemeter calibrated in this way is used to measure a dose in a high-energy photon or electron radiation field of another radiation type or energy, then the response change of the dosemeter has to be taken into account. Within the scope of the German standard DIN 6800-2 or the international dosimetry protocol IAEA TRS-398, this is done by applying the beam quality correction factor; this factor is designated as k_Q (for photons) and k_E (for electrons).

The value of this correction factor in a dose measurement depends on the radiation energy at the point of measurement in the water phantom as well as on the set-up, the dimensions and the materials used for the ionization chamber. Since the radiation energy at the point of measurement not only depends on the energy of the radiation inciding on the phantom, but also on the field size, the measurement depth inside the phantom and the distance from the radiation field's axis, also the correction factors k_Q and k_E, respectively, are influenced by these parameters.

For basic dosimetry at clinical linacs, particularly the value of the correction factor is of interest for the reference values of the field size and the depth in the phantom laid down in the standard DIN 6800-2; this value has to be known to allow an accurate dose measurement with the smallest possible uncertainty. The values currently specified in DIN 6800-2 or in the international dosimetry protocol IAEA TRS-398 for the correction factor k_Q have a relative standard measurement uncertainty of 1 %.

At PTB's clinical linacs, k_Q factors were determined experimentally for many types of ionization chambers which are frequently used in practice; this was done by determining the response of the ionization chambers in different photon radiation fields, using the primary standard for the unit of absorbed dose to water (water calorimeter). The relative standard measurement uncertainty of these experimentally determined correction factors amounts to 0.4 % and is thus clearly lower than the values currently specified in the dosimetry protocols. As an example, the figure shows the experimentally determined k_Q values for chambers of the type PTW 30016 compared to the values stated in IAEA TRS-398.

The values of the correction factor k_Q which were determined at PTB experimentally will enter into the next version of the standard DIN 6800-2, which will allow the absorbed dose to water in high-energy photon fields to be measured with a lower measurement uncertainty.