Change of Ionization Chamber Correction Factors (Ppol , Pion, Kwall) with Chamber Walls of Different Materials in Continuous and Pulsed Beams
G Aldosary*, J Seuntjens, A Sarfehnia, McGill University, Montreal, QCSU-E-T-82 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To study the effect of wall material on correction factors for polarity effect Ppol , collection efficiency Pion and scatter and attenuation in the chamber wall/central electrode Kwall , in continuous and pulsed beams.
Methods: An Exadin A12 ionization chamber was modified to have geometrically identical chamber walls built from aluminum and copper. Measurements were performed using the different walls in both 60Co, and Varian-Clinac 21EX 6 MV beams. Ppol was measured using AAPM TG51 protocol. Pion was obtained form measurement data, where the collected charge was measured at 10 voltage settings and used to form Jaffe-plots. Measurements were compared against Monte Carlo simulated data (egs++/egs_chamber). Kwall values were also calculated using CAVRZnrc.
Results: For all beams and all wall materials, Ppol was found to be less than the 0.3 % limit recommended in TG-51. Saturation charges (extrapolated from the Jaffe-plots) were observed to increase with wall materials of increasing atomic number Z. The breakdown of the Boag-predicted linearity of Jaffe-plots in the near-saturation region was observed for all beam types and wall materials. The ratio of the saturation charges (relative to the C552 wall) were predicted by cavity theory, and agreed with simulations to within 9.8 % for 60Co and 5.2 % for 6 MV. Pion measured by the two-voltage technique and from Jaffe-plots were all found to be less than the 1.05 accepted upper limit recommended by TG-51. Kwall showed that attenuation and scatter for the Cu wall influences the measured signal by 1.9 % for 60Co.
Conclusion: Ppol and Pion were measured for an Exradin A12 chamber with C552, Al, and Cu wall materials. The basic behavior of Jaffe-plots is independent of wall material. As expected, Kwall increases drastically for high Z walls with decreasing beam energies.