Extrapolation Chamber Measurement and Monte Carlo simulation of the build-up region of a 60Co source and Truebeam 6 MV FFF Photon Beam
Abstract
This thesis investigates the properties of a extrapolation chamber using a 60Co source as well as a Truebeam 6 MV FFF beam. An automated white water phantom was constructed which had the ability to change the extrapolation chamber’s plate separation. The phantom could accurately twist the chamber in half rotations using a motor and belt with Arduino coding. A steel and lead radiation storage unit was designed and built for the storage of the 60Co. Correction factors for the 60Co experiments were large. The largest polarity, 1.365, was at the smallest plate separation, 0.3 mm. The recombination correction factors were all under 2% for these experiments. There was a strong agreement between the 60Co - measured results and the Monte Carlo simulations. Monte Carlo simulations showed smaller polarity correction factors than the measured results. The Truebeam had large polarity values but trended in the opposite direction to 60Co. Because of the high polarity, only the temperature and pressure corrected data was used to extrapolate the measurements. There was a weak agreement among the extrapolation chamber, Advanced Marcus and Truebeam commissioning data. There was a strong agreement between the Advanced Marcus and the Truebeam commissioning data. The extrapolation chamber results showed a higher surface dose in the build-up region to that of the Advanced Marcus and the Truebeam commissioning data. This is the opposite trend to that of theory. There are two possible explanations…
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