Polarity effects and apparent ion recombination in microionization chambers

Area of Science:

• Medical Physics
• Radiation Dosimetry

Background:

• Microchambers exhibit anomalous voltage-dependent polarity effects not explained by current correction factors.
• This prevents many commercial microchambers from meeting reference-class ionization chamber standards.

Purpose of the Study:

• To identify the root cause of voltage-dependent polarity effects in microchambers.
• To improve the accuracy and reliability of microchambers for dosimetry.

Main Methods:

• Investigated microchamber prototypes by altering parameters like electrode conductivity and inter-electrode potential difference.
• Utilized COMSOL Multiphysics to simulate electric fields and collecting volumes.
• Compared microchambers to Farmer-type chambers.

Main Results:

• A potential difference between guard and collecting electrodes was identified as the primary cause of voltage-dependent polarity effects.
• Simulations showed altered collecting volumes due to potential differences, leading to polarity effects.
• Smaller collecting volumes exacerbated polarity effects, with microchambers being ten times more sensitive than Farmer-type chambers.

Conclusions:

• The potential difference between electrodes is the main driver of microchamber polarity effects.
• Larger chambers show less relative collecting volume change, explaining the absence of these effects.
• Minimizing inter-electrode potential difference is essential for accurate reference dosimetry with microchambers.