Correction for volume recombination in liquid ionization chambers at high dose-per-pulse

Purpose: To determine the volume recombination at high dose-per-pulse in liquid ionization chambers (LIC) and to ascertain whether existing calculation methods verified in air-filled chambers may be used to calculate a correction factor.

Methods: Two LICs, one filled with 2,2,4-trimethylpentane (isooctane) the other with tetramethylsilane (TMS), were irradiated in a pulsed, 20 MeV electron beam. Via reference measurements with a Faraday-cup the saturation correction for volume recombination was determined for dose-per-pulse values ranging from about 5 mGy to 1 Gy for both chambers at a pulse duration of 693 ns. In addition, the isooctane-chamber was irradiated with pulses of varying duration, ranging from 5 ps to 10 ms, at a dose-per-pulse of about 76.5 mGy. The dose-per-pulse dependent measurements were compared to calculations based on Boag’s models (with and without a free electron fraction) and the two-dose-rate method. The pulse duration dependent measurements were compared to a numerical calculation that iteratively calculates the charge transport and loss in a 1D model of an ionization chamber.

Results: In TMS only Boag’s model with a free electron fraction is in good agreement with the experimental data. However, in isooctane good agreement is observed between the experimental data, the two-dose-rate method, Boag’s model including a free electron fraction and to a lesser extend also Boag’s model without a free-electron fraction. Furthermore, the pulse duration dependent data for isooctane is well described by the numerical model.

Conclusion: With isooctane as an active medium a LIC could be directly used in a field with high dose-per-pulse utilizing the well established two-dose-rate method to correct for volume recombination. In addition, pulsed fields with variable pulse duration are easily modeled for this medium using a numerical calculation. Other media, as exemplified by the TMS-filled chamber, might require additional considerations, such as including a fraction of free electrons in the consideration of volume recombination.