Calibration of ionization chambers in air kerma length

Pencil ionization chambers are, employed for the direct determination of th e computed tomography dose index (CTDI), which is used for patient dosimetr y in CT examinations. The chambers are calibrated in air kerma length in a reference x-ray field. The precision of calibration is influenced by severa l factors which are investigated in the present work. It is shown that the best irradiation length is 50% of the chamber rated length. The reading mus t be corrected for a residual signal due to the irradiation of the sensitiv e chamber volume by scattered radiation. The origin of this radiation is id entified mainly at the edges of the diaphragm limiting the useful field. Th e residual signal is determined by a linear fit of measurements performed a t minimum of three diaphragm apertures. The optimum geometric conditions ar e also discussed.