To investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations.Methods:
Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate factors. Systematic uncertainties in Monte Carlo calculated factors are investigated by varying material properties and chamber dimensions.Results:
Chamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5% were observed in calibration coefficients after a period of eight months although factors were consistent on average within 0.17%. Chamber-to-chamber variations in factors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculated factors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculated factors were − 0.02%, 0.18%, and − 0.16% for 6, 10, and 25 MV beams, respectively.Conclusions:
Excellent agreement is observed on average at the 0.2% level between measured and Monte Carlo calculated factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.
The authors thank Igor Gomola and Frantisek Gabris of IBA, Mark Szczepanski and Christian Pychlau of PTW, and Eric DeWerd and Brian Hooten of Standard Imaging for supplying the chambers and chamber drawings used for this work. The authors thank Ben Spencer of Carleton University for assistance imaging the ion chambers. Work was supported by an OGSST held by B.R. Muir, by NSERC, the CRC program, CFI, and OIT.
II.A. Measurements: Chamber performance and factors
II.B. Monte Carlo calculations: factors and uncertainties
III. RESULTS AND DISCUSSION
III.A. Chamber stabilization
III.B. Leakage currents
III.C. Polarity correction
III.D. Ion recombination
III.D.1. Ion recombination in pulsed beams
III.D.2. Comparison with literature values
III.D.3. Ion recombination in cobalt-60
III.E. Measured and calculated factors
III.F. Chamber-to-chamber variation of measured factors
III.G. Long-term stability
III.H. Systematic uncertainties in calculated factors
III.I. Comparison of measured and calculated values
- Ionization chambers
- Error analysis
- Electric measurements
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