The Effect of Secondary Electron Emission On Ionization Chamber Dosimety
M McEwen*, C Cojocaru, C Ross, National Research Council, Ottawa, ONSU-E-T-110 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
To investigate the contribution to the reading obtained using an air-filled ionization chamber due to secondary electron emission from the chamber walls.
The ion chamber under investigation was placed in a vacuum vessel and the pressure within the vessel and chamber was varied from atmospheric pressure down to ~ 0.05 atm (5 kPa). High energy photon and electron beams from an Elekta Precise linac were used to investigate the deviation of the ionization current from what was expected after applying the standard pressure correction. Four ionization chamber types were used - two parallel-plate and two cylindrical designs - and influence quantities such as stem and cable irradiation, leakage currents and electron scatter were investigated.
For the simplest situation, the parallel-plate design, the ion current showed the theoretically-predicted deviation at low pressures and analysis of the data yielded a 0.3% correction to the signal at atmospheric pressure to account for these 'non-cavity' electrons. This correction was independent of incident energy and beam modality. For a Farmer-type cylindrical chamber the correction was of a similar magnitude, but with a measurable polarity dependence due to the concentric geometry of the electrodes. The effect for a small-volume cylindrical chamber (< 0.01 cm³) was very large, approaching 10% of the atmospheric cavity signal when collecting negative charge (the situation where secondary electrons emitted from the chamber wall reach the collecting electrode).
Secondary electron emission has been demonstrated in typical therapy-level ionization chambers and the contribution to the overall ion chamber signal (at atmospheric pressure) is around 0.3-0.5 %, although it can be significantly larger for small-volume cylindrical chambers. For standard reference-class chambers the effect should be taken into account during calibration but further work is required to determine how secondary electrons impact the performance of small-volume cylindrical chambers.
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