Ion-recombination correction factor ksat for spherical ion chambers irradiated by continuous photon beams

• Published in: Physics in medicine & biology Vol. 41; no. 6; pp. 1025 - 1036
• Main Authors: PIERMATTEI, A, AZARIO, L, ARCOVITO, G, TONI, M. P
• Format: Journal Article
• Language: English
• Published: Bristol Institute of Physics 01.06.1996
• Subjects: Applied radiobiology (equipment, dosimetry...); Biological and medical sciences; Biological effects of radiation; Biometry; Biophysical Phenomena; Biophysics; Brachytherapy - instrumentation; Brachytherapy - statistics & numerical data; Fundamental and applied biological sciences. Psychology; Humans; Models, Theoretical; Photons; Radiometry - instrumentation; Radiometry - statistics & numerical data; Radiotherapy Planning, Computer-Assisted; Tissues, organs and organisms biophysics; Photon irradiation; Continuous wave; Treatment; Bias voltage; Ionization chamber; Correction factor; Ionic current; Dosimetry; Ion ion recombination; Radiotherapy; Electrical characteristic
• ISSN: 0031-9155; 1361-6560
• DOI: 10.1088/0031-9155/41/6/006

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.