Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse
| Autor: | C. Caporali, Maria Pimpinella, A Petrucci, R F Laitano, A.S. Guerra |
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| Rok vydání: | 2006 |
| Předmět: |
Ions
Free electron model Models Statistical Materials science Drift velocity Radiological and Ultrasound Technology Radiotherapy Planning Computer-Assisted Time constant Electrons Radiotherapy Dosage Electron Models Theoretical Radiation Dosage Ion Radiotherapy High-Energy Electromagnetic Fields Absorbed dose Ionization Calibration Humans Dosimetry Radiology Nuclear Medicine and imaging Particle Accelerators Atomic physics Radiometry |
| Zdroj: | Physics in Medicine and Biology. 51:6419-6436 |
| ISSN: | 1361-6560 0031-9155 |
| DOI: | 10.1088/0031-9155/51/24/009 |
| Popis: | The correction for charge recombination was determined for different plane-parallel ionization chambers exposed to clinical electron beams with low and high dose per pulse, respectively. The electron energy was nearly the same (about 7 and 9 MeV) for any of the beams used. Boag's two-voltage analysis (TVA) was used to determine the correction for ion losses, k(s), relevant to each chamber considered. The presence of free electrons in the air of the chamber cavity was accounted for in determining k(s) by TVA. The determination of k(s) was made on the basis of the models for ion recombination proposed in past years by Boag, Hochhäuser and Balk to account for the presence of free electrons. The absorbed dose measurements in both low-dose-per-pulse (less than 0.3 mGy per pulse) and high-dose-per-pulse (20-120 mGy per pulse range) electron beams were compared with ferrous sulphate chemical dosimetry, a method independent of the dose per pulse. The results of the comparison support the conclusion that one of the models is more adequate to correct for ion recombination, even in high-dose-per-pulse conditions, provided that the fraction of free electrons is properly assessed. In this respect the drift velocity and the time constant for attachment of electrons in the air of the chamber cavity are rather critical parameters because of their dependence on chamber dimensions and operational conditions. Finally, a determination of the factor k(s) was also made by zero extrapolation of the 1/Q versus 1/V saturation curves, leading to the conclusion that this method does not provide consistent results in high-dose-per-pulse beams. |
| Databáze: | OpenAIRE |
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