Angle dependence of signal currents from cylindrical ionisation chambers

Area of Science:

• Medical Physics
• Radiation Detection and Measurement

Background:

• Cylindrical ionization chambers are widely used for measuring gamma ray fields.
• Understanding the angular response of these chambers is crucial for accurate dosimetry.

Purpose of the Study:

• To investigate the angular dependence of signal current in cylindrical ionization chambers.
• To identify the causes of observed signal peaks at specific angles.
• To highlight the implications for measurement and calculation practices.

Main Methods:

• Experimental measurements of signal current from two cylindrical ionization chambers (62.7 cm3 and a smaller one) in 137Cs and 60Co gamma ray fields.
• Varying source-chamber distances (1 m and 2 m) and chamber orientations (0 and 90 degrees).
• Theoretical calculations to predict chamber response under specific conditions.

Main Results:

• A larger ionization chamber showed signal current peaks at 0 and 90 degrees in both gamma ray fields.
• A smaller chamber exhibited a peak at 0 degrees but not 90 degrees experimentally, though calculations predicted a 90-degree peak.
• Signal peaks are attributed to gamma ray attenuation along chamber walls at specific angles.

Conclusions:

• Signal current peaks in cylindrical ionization chambers are influenced by chamber geometry and incident gamma ray direction.
• Gamma ray attenuation along side or end walls causes these peaks when the beam aligns with wall planes.
• Care is required when measuring and calculating responses for cylindrical ionization chambers, especially when fixed perpendicular to gamma ray beams.