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Chamber-to-chamber variation of the energy dependence of the response of ionization chambers

29.09.2008

For 10 ionization chambers of the type PTW 30013 and 10 of the type PTW 31013, individual responses in the high-energy photon radiation fields of PTB’s linear accelerator were determined relative to the response in the 60Co radiation field. The variation in the obtained kQ factors of all chambers amounts to less than 0.3 % (related to the mean value of all kQ factors). The tests carried out offer some potential for further improvement of the data contained in the DIN 6800-2 standard.

Introduction
In accordance with the DIN 6800-2 standard [1], the varying response of an ionization chamber during calibration in the 60Co radiation field and during measurement in the high-energy photon radiation field is corrected by using the correction factor kQ. This correction factor depends both on the radiation quality of the photon radiation and on the structure and the dimensions of the ionization chamber used.

The values of the correction factor kQ stated in the DIN 6800-2 standard were largely derived from model calculations and Monte Carlo simulations, which were based on the nominal structure of the respective type of ionization chamber. Possible chamber-specific variations (e.g. as a result of production tolerances) were not taken into account here.

The objective of the tests carried out was to experimentally determine the variation in the individual kQ factors for individual chambers of the types PTW 30013 and PTW 31013.

Approach
PTB Braunschweig has several secondary standard ionization chambers for which individual kQ values were determined by direct comparison with the primary standard for the unit of the absorbed dose to water (see [2]). By using these secondary standard chambers, the individual kQ factors of the respective 10 ionization chambers of the types PTW 30013 and PTW 31013 in the 8 MV and 16 MV photon fields of the linear accelerator of the PTB Braunschweig were determined.

For this purpose, all chambers were first calibrated in PTB’s 60Co reference radiation field; after that, the ratios of the chamber readings to be investigated and several secondary standard ionization chambers were determined under reference conditions in the high-energy photon radiation fields. Under the assumption that the dose rate measured with the secondary standard ionization chamber and the chamber under test in the same photon radiation field is the same, the beam quality correction factor kQ of the chamber under test was determined.

Results
The following table contains a summary of the experimentally obtained kQ values; the figure shows the kQ values obtained for each individual chamber for the two photon energies:

Chamber type
PTW 30013
PTW 31013
Nominal acceleration voltage8 MV16 MV8 MV16 MV
Beam quality index Q0.7160.7620.7160.762
Mean value of all individual kQ values0.97610.95870.97480.9592
Relative degree of fluctuation of the individualkQ values (related to the respective mean value)0.21 %0.20 %0.29 %0.23 %
Relative standard measurement uncertainty0.28 %0.28 %0.28 %0.28 %

Discussion
The tests presented here show that the chamber-to-chamber variation of the kQ values amounts – at least for the chamber types PTW 30013 and PTW 31013 tested here – to - at most - approx. 0.3 %. Thus, experimentally determined chamber-type-specific kQ values can be stated with a relative standard measurement uncertainty of 0.28 %. This measurement uncertainty, in which the model-specific variation of the kQ values has already been taken into account, is clearly lower than the measurement uncertainty of 1.0 % used so far in the DIN 6800-2 standard. The tests carried out thus offer some potential for further improvement of the data contained in the DIN 6800-2 standard.

Figure : Experimentally determined kQ values for the chamber types PTW 30013 and PTW 31013 in the 8 MV and 16 MV photon radiation fields of the PTB linear accelerator.

Literature

  1. DIN Deutsches Institut für Normung e.V.: DIN 6800-2:
    Dose Measurement Procedures in accordance with the Probe Method for Photon- and Electron Radiation – Part 2: Dosimetry of High-energy Photon- and Electron Radiation with Ionization Chambers.
    Berlin, March 2008
  2. A. Krauss, R.-P. Kapsch:
    Calorimetric determination of kQ factors for NE 2561 and NE 2571 ionization chambers in 5 cm x 5 cm and 10 cm x 10 cm radiotherapy beams of 8 MV and 16 MV photons.
    Phys. Med. Biol. 52 (2007), 6243-6259