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Polarity effect of plane-parallel ionization chambers in electron radiation
J M Havercroft1, S C Klevenhagen
1Department of Medical Physics, The Royal London Hospital, UK.
Physics in Medicine and Biology
|February 1, 1994
Summary
This study quantifies the polarity effect in plane-parallel ionization chambers used for electron beam dosimetry. Several chambers require a correction of approximately 0.2% for accurate measurements.
Area of Science:
- Medical Physics
- Radiation Dosimetry
Background:
- Plane-parallel ionization chambers are essential for electron beam dosimetry.
- Limited data exists on the polarity effect in these chambers, particularly for electron beams.
Purpose of the Study:
- To investigate and quantify the polarity effect in various plane-parallel ionization chambers.
- To assess the impact of electron beam energy and depth on the polarity effect.
Main Methods:
- Irradiation of PTW Markus, NACP, Calcam, Vinten-631, and NE 2571 chambers.
- Use of electron beams with nominal energies from 4 MeV to 18 MeV.
- Analysis of polarity effect dependence on beam energy, depth, and spectrum.
Main Results:
- NACP, Markus, and Vinten chambers showed a polarity effect requiring ~0.2% correction between 4.5 MeV and 18 MeV.
- Calcam chamber exhibited similar behavior, except at energies below 4 MeV.
- Polarity effect depth dependence correlated with mean beam energy; potential dependence on beam angular spread and spectrum noted.
Conclusions:
- The polarity effect in common ionization chambers necessitates corrections for accurate electron dosimetry.
- Proposed expressing the polarity effect as a correction factor for specific chamber biases.
- Further research needed on beam spectrum and angular spread influence.