Recombination in liquid-filled ionization chambers beyond the Boag limit

Area of Science:

• Medical Physics
• Radiation Detection and Measurement

Background:

• Liquid-filled ionization chambers (LICs) are susceptible to charge carrier recombination, particularly under pulsed irradiation.
• Existing saturation correction methods, like the Boag equation, assume complete charge collection between pulses, which is often not met in clinical settings due to short pulse repetition frequencies.

Purpose of the Study:

• To experimentally and numerically characterize charge collection efficiencies in LICs beyond the Boag limit, specifically addressing scenarios with overlapping charge carriers from different pulses.
• To evaluate the applicability of existing saturation correction methods in the presence of charge overlap.

Main Methods:

• Investigated recombination in a LIC array by varying dose-per-pulse, pulse repetition frequency, and polarization voltage.
• Conducted measurements using a Truebeam Linac (FF and FFF modalities) and employed the two-dose-rate method combined with a reference chamber for efficiency determination.
• Utilized numerical simulations to complement experimental findings on collection efficiencies.

Main Results:

• Charge overlap significantly increases recombination in LICs.
• The functional dependence of collection efficiencies on dose-per-pulse remains linear in the near-saturation region, even with overlap.
• The dependence of collection efficiencies on polarization voltage deviates from the Boag equation predictions in the overlapping scenario.

Conclusions:

• The two-dose-rate method can be adapted for saturation corrections in LICs with charge overlap, provided measurements are made at the same pulse repetition frequency.
• Correction methods relying on the voltage-dependence of the Boag equation (e.g., three-voltage method) are not suitable for overlapping charge carrier situations in LICs.