Ion recombination correction factor in scanned light-ion beams for absolute dose measurement using plane-parallel...

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Particle Therapy

Background:

• Accurate dose determination is critical in light-ion beam radiotherapy.
• Recombination of charges within ionization chambers requires correction factors (k_s) based on international protocols.
• Plane-parallel ionization chambers are commonly used for dosimetry in these beams.

Purpose of the Study:

• To investigate the recombination correction factor (k_s) for IBA PPC40 Roos-type chambers in scanned light-ion beams.
• To compare experimental k_s values with a combined theoretical model incorporating initial and volume recombination.
• To establish practical guidelines for minimizing k_s while avoiding measurement complications.

Main Methods:

• Experimental measurements of k_s were performed using IBA PPC40 Roos-type chambers in scanned proton, helium, carbon, and oxygen beams.
• A theoretical model combining Jaffé's theory (initial recombination) and Boag's theory (volume recombination) was employed.
• Experimental results were compared against the predictions of the combined theoretical model.

Main Results:

• Excellent agreement was observed between experimental and theoretical k_s-values across all investigated light-ion beams.
• The study confirmed that the recombination correction factor (k_s) is significant and cannot be neglected in scanned light-ion beam dosimetry.
• Applying a high voltage to the ionization chamber effectively minimizes k_s, with 300 V found suitable for the tested chamber without introducing charge multiplication issues.

Conclusions:

• The combined theoretical model accurately predicts k_s for scanned light-ion beams.
• High voltage application is an effective strategy to reduce recombination losses in ionization chambers during light-ion beam dosimetry.
• The two-voltage method is not applicable for determining k_s in these scanned beams due to the logarithmic dependence of initial recombination on voltage.