Saturation current and collection efficiency for ionization chambers in pulsed beams

Area of Science:

• Medical Physics
• Radiation Dosimetry

Background:

• Ionization chambers are crucial for measuring radiation dose.
• Accurate determination of saturation current and collection efficiency is vital for reliable dosimetry, especially with pulsed radiation beams.
• Existing methods may overestimate saturation current due to unaddressed physical phenomena.

Purpose of the Study:

• To investigate the discrepancy between measured and predicted ionization chamber currents in the near-saturation region under pulsed megavoltage beams.
• To develop and validate a more accurate model for calculating collection efficiency that accounts for excess current phenomena.

Main Methods:

• Experimental measurements of ionization chamber current versus polarizing voltage in the extreme near-saturation region.
• Analysis of excess current attributed to charge multiplication and stem effects.
• Development of a semi-empirical model incorporating an exponential term into Boag's equation.

Main Results:

• A faster-than-linear rise in chamber current was observed in the near-saturation region.
• The conventional two-voltage technique may overestimate saturation current by up to 0.7% for standard field sizes.
• The developed semi-empirical model accurately reflects experimental data, including recombination, charge multiplication, and stem effects.

Conclusions:

• Excess current in ionization chambers under pulsed beams is significant and requires specific modeling.
• The proposed semi-empirical model provides a more accurate method for determining collection efficiency in pulsed radiation fields.
• This improved dosimetry approach enhances the reliability of measurements in medical and radiation physics applications.