Examining the influence of humidity on reference ionization chamber performance

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Ionization Chamber Technology

Background:

• International dosimetry protocols correct for air density but often neglect humidity's effect on ionization chambers.
• Limited experimental data exists on humidity's impact on modern reference-class ionization chambers.
• Farmer-type ionization chambers are widely used in clinical dosimetry.

Purpose of the Study:

• To experimentally investigate the effect of relative humidity on Farmer-type ionization chambers.
• To assess humidity's influence on ionization measurements using Co-60 and Sr-90 radiation beams.
• To determine if humidity variations cause hysteresis or chamber-type-dependent effects.

Main Methods:

• Measurements were conducted in an environmental cabinet with controlled relative humidity (8%–98%).
• Two radiation sources were used: an external Co-60 beam and an internal Sr-90 check source.
• Four Farmer-type ionization chamber models (IBA FC65-G, IBA FC65-P, PTW 30013, Exradin A19) were tested for reproducibility and hysteresis.

Main Results:

• Co-60 beam measurements showed minimal variation (<0.1%) above 10% RH, consistent with existing data.
• Sr-90 measurements revealed no humidity dependence within experimental uncertainty (0.05%).
• Excellent repeatability was observed across humidity cycles, with no hysteresis or chamber-type dependency.

Conclusions:

• Humidity does not significantly affect the performance of the investigated Farmer-type ionization chambers.
• These findings support the current practice in dosimetry protocols of not correcting for humidity within typical operational limits.
• The study provides experimental validation for the negligible impact of humidity on these specific ionization chambers.