Rosser, Karen Elizabeth; (1996) Measurement of absorbed dose to water for medium energy X-rays. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of this thesis is to determine the factors. required to convert the air kerma measured using an ionization chamber to absorbed dose to water when irradiated with medium energy X-rays. This principally involved the determination of the ratio of the mass energy absorption coefficients of water to air and the chamber correction factor for an NE2561 ionization chamber. The ratio of mass energy absorption coefficients of water to air was calculated by averaging the monoenergetic values over the photon fluence spectrum at 2 em deep in water. The photon fluence spectrum was calculated using the EGS4 Monte Carlo code based on measured spectra in air. The chamber correction factor for a NE2561 chamber was determined using two methods. First the absorbed dose to water was measured using a NE2561 chamber and compared with that using a NE2571 chamber. The chamber correction factor varied from (1.022 ± 0.03) to (1.016 ± 0.01) for HVL between 0.15 and 4 mmCu. Secondly the air kerma in water was measured using the NE2561 chamber and compared with the absorbed dose to water determined using a calorimeter. Initial measurements revealed that the water calorimeter was too insensitive when using medium energy X-rays. An ice calorimeter was built to overcome this problem. The validity of the absorbed dose to water measured using the ice calorimeter was checked by comparison with the primary standard graphite calorimeter irradiated with 60Co gamma rays. The resultant calibration factor for a NE2561 chamber determined using the ice calorimeter was less than that using the graphite calorimeter by (1.8 ± 0.08)o/o. The ice calorimeter was then used to determine the chamber correction factor for the NE2561 chamber when irradiated with X-rays generated at 134 kV. The chamber correction factor was (0.98 ± 0.05).

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