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Investigation of radiation quality with differential ionization chambers Yuen, Kenneth Fei Kam


In radiotherapy, half value layer has been accepted as a practical specification of the quality of primary x-rays. For the degraded radiation in a scattering medium, however, it is neither an applicable nor an adequate specification of the quality. This thesis describes a practical method of investigating the radiation quality in an irradiated medium. The method is based essentially on differences in emission and on differences in absorption of different materials. Two ratios were measured: (a) the ratio of the ionization current in a copper-lined ionization chamber to the ionization current in a lucite-wall chamber, (b) the ratio of the ionization current in the copper-lined chamber to the ionization current in the same lucite-wall chamber with the latter shielded by a copper absorber. These ratios, which depend on the spectrum of the x-radiation to which the chambers are exposed, change differently with change in spectrum and, therefore, give two indices of quality. For calibration purposes, the above ratios were measured for primary x-rays generated at different kilovoltages and with different filters. To investigate the quality of radiation in a scattering medium, the same ratios were measured and were compared with the results obtained for primary spectra. Radiation quality at different depths in water exposed to different qualities of primary x-rays was investigated. The results are in qualitative agreement with those obtained by previous workers.

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