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Polarity Effects for Small Volume Ionization Chambers in Cobalt-60 Beams

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A Xu

Y Xu*, J Bhatnagar , M Saiful Huq , Department of Radiation Oncology, University of Pittsburgh Cancer Institute and UPMC Cancer Center, Pittsburgh, PA


SU-E-T-623 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the polarity effects for small volume ionization chambers in ⁶⁰Co gamma-ray beams using the Leksell Gamma Knife Perfexion.

Methods: Measurements were made for 7 small volume ionization chambers (a PTW 31016, an Exradin A14, 2 Capintec PR0-5P, and 3 Exradin A16) using a PTW UNIDOSwebline Universal Dosemeter and an ELEKTA solid water phantom with proper inserts. For each ion chamber, the temperature/pressure corrected electric charge readings were obtained for 16 voltage values (±50V, ±100V, ±200V, ±300V, ±400V, ±500V, ±600V, ±700V). For each voltage, a five-minute leakage charge reading and a series of 2-minute readings were continuously taken during irradiation until 5 stable signals (less than 0.05% variation) were obtained. The average of the 5 reading was then used for the calculation of the polarity corrections at the voltage and for generating the saturation curves.

Results: The polarity effects are more pronounced at high or low voltages than at the medium voltages for all chambers studied. The voltage dependence of the 3 Exradin A16 chambers is similar in shape. The polarity corrections for the Exradin A16 chambers changes rapidly from about 1 at 500V to about 0.98 at 700V. The polarity corrections for the 7 ion chambers at 300V are in the range from 0.9925 (for the PTW31016) to 1.0035 (for an Exradin A16).

Conclusion: The polarity corrections for certain micro-chambers are large even at normal operating voltage.

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