Ion-recombination correction factor ksat for spherical ion chambers irradiated by continuous photon beams.
The large range of reference air kerma rates of brachytherapy sources involves the use of large-volume ionization chambers. When such ionization chambers are used the ion-recombination correction factor ksat has to be determined. In this paper three spherical ion chambers with volumes ranging from 30 to 10(4) cm3 have been irradiated by photons of a 192Ir source to determine the ksat factors. The ionization currents of the ion chambers as a function of the applied voltage and the air kerma rate have been analysed to determine the contribution of the initial and general ion recombination. The ksat values for large-volume ionization chambers obtained by considering the general ion recombination as predominant (Almond's approach) are in disagreement with the results obtained using methods that consider both initial and general ion-recombination contributions (Niatel's approach). Such disagreement can reach 0.7% when high currents are measured for a high-activity source calibration in terms of reference air kerma rate. In this study a new 'two-voltage' method, independent of the voltage ratio given by a dosimetry system, is proposed for practical dosimetry of continuous x- and gamma-radiation beams. In the case where the Almond approach is utilized, the voltage ratio V1/V2 should be less than 2 instead of Almond's limit of V1/V2 < 5.
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Università Cattolica S Cuore, Istituto di Fisica, Roma, Italy.
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Arcovito
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Toni
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MP
D001699:Biometry; D055592:Biophysical Phenomena; D001703:Biophysics; D001918:Brachytherapy / Q000295:instrumentation* / Q000706:statistics & numerical data; D006801:Humans; D008962:Models, Theoretical; D017785:Photons; D011874:Radiometry / Q000295:instrumentation* / Q000706:statistics & numerical data; D011880:Radiotherapy Planning, Computer-Assisted