Study of the PTW microLion chamber temperature dependence

Area of Science:

• Medical Physics
• Radiation Detection

Background:

• Liquid ionization chambers are increasingly used in radiotherapy.
• Temperature dependence of liquid ionization chambers requires further investigation, unlike well-established air ionization chambers.

Purpose of the Study:

• To investigate the temperature dependence of a liquid ionization chamber's signal.
• To compare the temperature effect in liquid chambers with air chambers and model it using Onsager theory.

Main Methods:

• Measured the temperature dependence of a PTW microLion isooctane-filled ionization chamber.
• Tested across a ~±10 °C range around 20 °C for three voltages and two beam qualities ((60)Co and 50 kV x-rays).

Main Results:

• The liquid ionization chamber signal showed a positive linear dependence on temperature (~0.24% K⁻¹ at 800 V with (60)Co).
• This temperature dependence has a different nature and opposite sign compared to air ionization chambers.
• Onsager theory successfully modeled the (60)Co results but showed inconsistencies for 50 kV x-rays.

Conclusions:

• Liquid ionization chambers exhibit a significant, positive linear temperature dependence, comparable in magnitude but opposite in sign to air chambers.
• Onsager theory provides a good model for the temperature effect in liquid chambers under certain conditions.
• Further research is needed to refine models for specific radiation qualities like low-energy x-rays.