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Initial recombination in a parallel-plate ionization chamber exposed to heavy ions
1Therapeutic Beam Assessment Office, Research Center of Charged Particle Therapy, National Institute of Radiological Sciences, Chiba, Japan. kanai@nirs.go.jp
Physics in Medicine and Biology
|December 30, 1998
Summary
This study quantifies columnar recombination in ionization chambers using heavy-ion beams. Results show a strong linear energy transfer (LET) dependence, crucial for accurate absorbed dose determination in radiation therapy.
Area of Science:
- Medical Physics
- Radiation Detection and Measurement
Background:
- Accurate absorbed dose determination is critical for heavy-ion radiation therapy and biological experiments.
- Ionization chambers are standard detectors, but their response can be affected by columnar recombination in high-energy particle fields.
Purpose of the Study:
- To investigate the linear energy transfer (LET) dependence of columnar recombination in parallel-plate ionization chambers.
- To determine correction factors for accurate absorbed dose measurements in heavy-ion fields.
- To analyze the track radius parameter (b) in Jaffe theory based on experimental data.
Main Methods:
- Ion collection efficiency was measured using carbon, neon, argon, and iron beams at various energies (e.g., 135 MeV/u, 95 MeV/u, 90 MeV/u).
- Measurements were conducted in different gases, including air, carbon dioxide, argon, and tissue-equivalent gas.
- Jaffe theory was applied to analyze columnar recombination and determine the track radius parameter (b).
Main Results:
- A linear relationship between the inverse of ionization charge ratio and inverse electric field strength was observed, consistent with Jaffe theory.
- The gradient of this relationship increased with increasing LET, indicating a strong LET dependence of columnar recombination in air and carbon dioxide.
- This LET dependence was not observed in tissue-equivalent gas, nitrogen, or argon.
- The calculated track radius parameter (b) was found to be within 0.001-0.005 cm, with an increasing trend correlating with LET.
Conclusions:
- Columnar recombination shows a significant LET dependence, particularly in certain gases like air and CO2.
- Correction for initial recombination effects is essential for obtaining accurate depth-dose distributions in heavy-ion beams.
- The study refines the understanding of Jaffe theory parameters for heavy-ion dosimetry, improving dose measurement accuracy in radiation therapy.