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

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 kQ (for photons) and kE (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 kQ and kE, 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 kQ have a relative standard measurement uncertainty of 1 %.

At PTB's clinical linacs, kQ 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 kQ values for chambers of the type Opens external link in new windowPTW 30016 compared to the values stated in IAEA TRS-398.

kQ values determined experimentally at PTB compared to the values stated in IAEA TRS-398. The experimentally determined values exhibit a lower measurement uncertainty.

The values of the correction factor kQ 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.