Polarity effect for commercially available plane-parallel ionization chambers
The polarity effect was investigated for three different commercially available plane-parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30-329 chamber manufactured by PTW, Frieburg, and the Capintec PS-033 thin-window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6-, 10-, 18-, and 24-MV x-ray beams, and 9- and 22-MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane-parallel ionization chambers that we studied, we found a polarity effect of only 1%--2% for electron beams at the depth of d/sub max/. At depths greater than d/sub max/, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high-energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.
- Research Organization:
- Department of Therapeutic Radiology, University of Minnesota Hospital and Clinic, Minneapolis, Minnesota 55455
- OSTI ID:
- 7071168
- Journal Information:
- Med. Phys.; (United States), Journal Name: Med. Phys.; (United States) Vol. 14:2; ISSN MPHYA
- Country of Publication:
- United States
- Language:
- English
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655003 -- Medical Physics-- Dosimetry
CALIBRATION
DOSIMETRY
ELECTRON DOSIMETRY
IONIZATION CHAMBERS
MEASURING INSTRUMENTS
MEDICINE
MOCKUP
NUCLEAR MEDICINE
PHANTOMS
RADIATION DETECTORS
RADIOLOGY
RADIOTHERAPY
STRUCTURAL MODELS
THERAPY
X-RAY DOSIMETRY